Advances in Chemistry

The following sections are included:

• FOREWORD

• Contents

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There have been major advances in solid state and materials chemistry in the last two decades and the subject is growing rapidly. In this account, a few of the important aspects of materials chemistry of interest to the author are presented. Accordingly, transition metal oxides, which constitute the most fascinating class of inorganic materials, receive greater attention. Metal-insulator transitions in oxides, high temperature superconductivity in cuprates and colossal magnetoresistance in manganates are discussed at some length and the outstanding problems indicated. We then discuss certain other important classes of materials which include molecular materials, biomolecular materials and porous solids. Recent developments in synthetic strategies for inorganic materials are reviewed. Some results on metal nanoparticles and nanotubes are briefly presented. The overview, which is essentially intended to provide a flavour of the subject and show how it works, lists references to many crucial reviews in the recent literature.

A wide range of condensed matter systems traverse the metal–nonmetal transition. These include doped semiconductors, metal–ammonia solutions, metal clusters, metal alloys, transition metal oxides, and superconducting cuprates. Certain simple criteria, such as those due to Herzfeld and Mott, have been highly successful in explaining the metallicity of materials. In this article, we demonstrate the amazing effectiveness of these criteria and examine them in the light of recent experimental findings. We then discuss the limitations in our understanding of the phenomenon of the metal–nonmetal transition.

Transition-metal oxides at the metal–insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator–metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR). Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the ‘metallic’ state and show no significant density of states at the Fermi level. Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.

A fascinating phenomenon, recently found to occur in certain transition-metal oxides, is phase separation wherein pure, nominally monophasic oxides of transition metals with well-defined compositions separate into two or more phases over a specific temperature range. Such phase separation is entirely reversible, and is generally the result of a competition between charge-localization and -delocalization, the two situations being associated with contrasting electronic and magnetic properties. Coexistence of more than one phase, therefore, gives rise to electronic inhomogeneity and a diverse variety of magnetic, transport, and other properties, not normally expected of the nominal monophasic composition. An interesting feature of phase separation is that it covers a wide range of length scales anywhere between 1–200nm. While cuprates and manganates, especially the latter, provide excellent examples of phase separation, it is possible that many other transition-metal compounds with extended structures will be found to exhibit phase separation.

The science and technology of nanomaterials has created great excitement and expectations in the last few years. By its very nature, the subject is of immense academic interest, having to do with very tiny objects in the nanometer regime. There has already been much progress in the synthesis, assembly and fabrication of nanomaterials, and, equally importantly, in the potential applications of these materials in a wide variety of technologies. The next decade is likely to witness major strides in the preparation, characterization and exploitation of nanoparticles, nanotubes and other nanounits, and their assemblies. In addition, there will be progress in the discovery and commercialization of nanotechnologies and devices. These new technologies are bound to have an impact on the chemical, energy, electronics and space industries. They will also have applications in medicine and health care, drug and gene delivery being important areas. This article examines the important facets of nanomaterials research, highlighting the current trends and future directions. Since synthesis, structure, properties and simulation are important ingredients of nanoscience, materials chemists have a major role to play.

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Nebulized spray pyrolysis of metal–organic precursors in methanol solution has been employed to prepare powders of TiO₂, ZrO₂ and PbZr_0.5Ti_0.5O₃(PZT). This process .ensures complete decomposition of the precursors at relatively low temperatures. The particles have been examined by scanning and transmission electron microscopy as well as X-ray diffraction. As prepared, the particles are hollow agglomerates of diameter 0.1–1.6 μm, but after heating to higher temperatures the ultimate size of the particles comprising the agglomerates are considerably smaller (0.1 μm or less in diameter) and crystalline.

A structural investigation of cubic oxides (space group I23) of the formula Bi_26−XM_XO_40−δ (M = Ti, Mn, Fe, Co, Ni and Pb) related to γ-Bi₂O₃ phase has been carried out by the Rietveld profile analysis of high-resolution X-ray powder diffraction data in order to establish the cation distributions. Compositional dependence of the cation distribution has been examined in the case of Bi_26−XCo_XO_40−δ (1 < x < 16). The study reveals that in Bi_26−XM_XO_40−δ with M = Ti, Mn, Fe, Co or Pb, the M cations tend to occupy tetrahedral (2a) sites when X < 2 while the octahedral (24f) sites are shared by the excess Co or Ni cations with Bi atoms when X > 2. Also experimental magnetic moments of Mn, Co and Ni derivatives have been used to establish the valence state and distribution of these cations.

The structure of Ln_0.5Sr_0.5CoO₃ is rhombohedral when Ln = La, Pr, or Nd, but orthorhombic (Puma) when Ln = Gd. The Ln_0.5Ba_0.5CoO₃ compounds, except for Ln = La, are orthorhombic (Pmmm). The ferromagnetic Curie temperature, Tc, of Ln_0.5A_0.5CoO₃ increases with the average size of the A-site cation up to an 〈r_A〉 of 1. 40 Å, and decreases thereafter due to size mismatch. Disorder due to cation-size mismatch has been investigated by studying the properties of two series of cobaltates with fixed 〈r_A〉 and differing size variance, σ². It is found that Tc decreases linearly with σ², according to the relation, Tc = T°c − pσ². When σ² is large (>0.012 Ų), the material becomes insulating , providing evidence for a metal–insulator transition caused by cation-size disorder. Thus, Gd_0.5Ba_0.5CoO₃ with a large σ² is a charge-ordered insulator below 340 K. The study demonstrates that the average A-cation radius, as well as the cation-size disorder, affects the magnetic and transport properties of the rare earth cobaltates significantly.

The ferromagnetic structure of BiMnO₃, T_c = 105 K, has been determined from powder neutron-diffraction data collected at 20 K on a sample synthesized at high pressures using a cubic anvil press. BiMnO₃ is a distorted perovskite that crystallizes in the monoclinic space group C2 with unit-cell parameters a = 9.5317(7) Å, b = 5.6047(4) Å, c = 9.8492(7) Å, and β = 110.60(1)° (Rp = 6.78 %, wRp = 8.53%, reduced X² = 1.107). Data analysis reveals a collinear ferromagnetic structure with the spin direction along [010] and a magnetic moment of 3.2 μB. There is no crystallographic phase transition on cooling the polar room-temperature structure to 20 K, lending support to the belief that ferromagnetism and ferroelectricity coexist in BiMnO₃. Careful examination of the six unique Mn-O-Mn superexchange pathways between the three crystallographically independent Mn³⁺ sites shows that four are ferromagnetic and two are antiferromagnetic, thereby confirming that the ferromagnetism of BiMnO₃ stems directly from orbital ordering.

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Colossal magnetoresistance (CMR) and related properties of perovskite manganates of the general formula Ln_1−xA_xMnO₃ (Ln = rare-earth; A = divalention) are discussed in detail. The manganates are ferromagnetic at or above a certain value of x (or Mn⁴⁺ content) and become metallic at temperatures below the curie temperature, T_c, This behavior is commonly attributed to double-exchange. CMR is generally a maximum close to T_c or the insulator-metal (I-M) transition temperature, T_im. The T_c and %MR are markedly affected by the size of the A site cation, 〈r_A〉, thereby affording a useful electronic phase diagram when T_c or T_im is plotted against 〈r_A〉 or pressure. The commonalities and correlations found in the properties of manganates are examined along with certain unusual features in the electron-transport properties of these materials. Recent studies clearly indicate that double-exchange alone cannot explain the many fascinating features of the manganates. Some of the Ln_l−xA_xMnO₃ compositions exhibit charge-ordering and related effects. Charge ordering is crucially dependent on 〈r_A〉 or the e_g bandwidth and the charge-ordered insulating state transforms to a metallic ferromagnetic state on the application of a magnetic field, charge-ordering and double-exchange being competing interactions. The importance of Jahn-Teller interaction in determining the various properties of manganates is highlighted. CMR found in a few inorganic solids other than the perovskite manganates is pointed out.

Charge ordering occurs in some mixed-valent transition metal oxides. The perovskite manganates of the formula Ln_1−xAxMnO₃ (Ln = Rare Earth, A = Ca Sr) are especially interesting because long-range ordering of the and ions in these materials is linked to antiferromagnetic spin ordering, and also to the long-range ordering of the Mn³⁺ (e_g) orbitals and the associated lattice distortions. Charge ordering occurs at a higher temperature than spin ordering in some of the manganates (Tco > T_N), whereas in some others T_co = T_N. Orbital ordering occurs without charge ordering in the A-type antiferromagnetic manganates, but in the manganates where charge ordering occurs, antiferromagnetism of CE-type is found along with orbital ordering. The subtle relations between charge, spin, and orbital ordering are discussed in the article, with special attention to the effects of cation size, chemical substitution, dimensionality, pressure, and magnetic and electric fields. Unusual features such as phase separation and electron–hole asymmetry are also examined.

Properties of the hole-doped Ln_1−xA_xMnO₃ (Ln = rare earth, A = alkaline earth, x < 0.5) are compared with those of the electron-doped compositions (x > 0.5). Charge ordering is the dominant interaction in the latter class of manganates unlike ferromagnetism and metallicity in the hole-doped materials. Properties of charge-ordered (CO) compositions in the hole- and electron-doped regimes, Pr_0.64Ca_0.36MnO₃ and Pr_0.36Ca_0.64MnO₃, differ markedly. Thus, the CO state in the hole-doped Pr_0.64Ca_0.36MnO₃ is destroyed by magnetic fields and by substitution of Cr³⁺ or Ru⁴⁺ (3%) in the Mn site, while the CO state in the electron-doped Pr_0.36Ca_0.64MnO₃ is essentially unaffected. It is not possible to induce long-range ferromagnetism in the electron-doped manganates by increasing the Mn—O—Mn angles up to 165 and 180° as in La_0.33Ca_0.33Sr_0.34MnO₃; application of magnetic fields and Cr/Ru substitution (3%) do not result in long-range ferromagnetism and metallicity. Application of magnetic fields on the Cr/Ru-doped, electron-doped manganates also fails to induce metallicity. These unusual features of the electron-doped manganates suggest that the electronic structure of these materials is likely to be entirely different from that of the hole-doped ones, as verified by first-principles linearized muffin-tin orbital calculations.

Films of charge-ordered Nd_0.5Ca_0.5MnO₃, Gd_0.5Ca_0.5MnO₃, Y_0.5Ca_0.5MnO₃, and Nd_0.5Sr_0.5MnO₃ show insulator-metal transitions on the passage of small electrical currents. That such an electric-field-induced transition occurs even in Y_0.5Ca_0.5MnO₃ where the charge-ordered state is not affected by magnetic fields is noteworthy. The transition is attributed to the depinning of the randomly pinned charge solid. These materials also exhibit an interesting memory effect probably due to the randomness of the strength as well as the position of the pinning centers.

Some of the compositions of the half-doped rare-earth manganates, La_0.5−xLn_xCa_0.5MnO₃ (Ln = Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ with relatively small A-cation radii, 〈r_A〉, show an unusual behavior wherein they become ferromagnetic (FM) on cooling the charge ordered (CO) state (T_co > T_c). With increase in 〈r_A〉, however, the T_c becomes greater than T_co. Thus, plots of T_c and T_co against〈r_A〉 for La_0.5−xLn_xCa_0.5MnO₃ (Ln = Nd, Pr) and Nd_0.5Ca_0.5−xSr_xMnO₃ show cross-over from the T_co > T_c regime to the T_c > T_co regime around 〈r_A〉 values of 1.195 ± 0.003 and 1.200 ± 0.005 Å, respectively. Between Tc and Tco, the CO and FM phases are likely to coexist. In Nd_0.5Ca_0.5Mn_1−xM_xO₃ (M = Cr, Ru), T_co > T_c when x ≤ 0.10, suggesting the re-entrant nature of the FM transition.

Electron transport and magnetic properties of three series of manganates of the formula (La_1−xLn_x)_0.7Ca_0.3MnO₃ with Ln = Nd, Gd and Y, wherein only the average A-site cation radius 〈r_A〉 and associated disorder vary, without affecting the Mn⁴⁺/Mn³⁺ ratio, have been investigated in an effort to understand the nature of phase separation. All three series of manganates show saturation magnetization characteristic of ferromagnetism, with the ferromagnetic T_c decreasing with increasing x up to a critical value of x, x_c (x_c = 0.6, 0.3, 0.2 respectively for Nd, Gd, Y). For x > x_c, the magnetic moments are considerably smaller, showing a small increase around T_M, the value of T_M decreasing slightly with increase in _x or decrease in 〈r_A〉. The ferromagnetic compositions (x ≤ x_c) show insulator-metal transitions, while the compositions with x > x_c are insulating. The magnetic and electrical resistivity behaviour of these manganates is consistent with the occurrence of phase separation in the compositions around x_c corresponding to a critical average radius of the A-site cation, , of 1.18 Å. Both T_c and T_IM increase linearly when or x ≤ x_c as expected of a homogeneous ferromagnetic phase. Both T_c and T_M decrease linearly with the A-site cation size disorder as measured by the variance σ². Thus, an increase in σ² favours the insulating AFM state. Percolative conduction is observed in the compositions with . Electron transport properties in the insulating regime for x > x^c conform to the variable-range hopping mechanism. More interestingly, when x > x_c, the real part of dielectric constant (ε′) reaches a high value (10⁴−10⁶) at ordinary temperatures dropping to a very small (∼500) value below a certain temperature, the value of which decreases with decreasing frequency.

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The time evolution of colloidal gold particles in the nanometric regime has been investigated by employing electron microscopy and electronic absorption spectroscopy . The particle sir distributions are essentially Gaussian and show the same time dependence for both the mean and the standard deviation, enabling us to obtain a time-independent universal curve for the particle size. Temperature dependent studies show the growth to be an activated process with a barrier of about 18 kJ mol⁻¹. We present a phenomenological equation for the evolution of particle size and suggest that the growth process is stochastic.

Properties of materials determined by their size are indeed fascinating and form the basis of the emerging area of nanoscience. In this article, we examine the size dependent electronic structure and properties of nanocrystals of semiconductors and metals to illustrate this aspect. We then discuss the chemical reactivity of metal nanocrystals which is strongly dependent on the size not only because of the large surface area but also a result of the significantly different electronic structure of the small nanocrystals. Nanoscale catalysis of gold exemplifies this feature. Size also plays a role in the assembly of nanocrystals into crystalline arrays. While we owe the beginnings of size-dependent chemistry to the early studies of colloids, recent findings have added a new dimension to the subject.

Superlattices formed by arrays of Pt or Au nanoparticles have been obtained by layer-by-layer deposition by using dithiols as cross-linkers. The superlattices have been characterized by X-ray diffraction, photoelectron spectroscopy, and scanning tunneling microscopy. The core-level intensities of the metal and of the dithiol in the X-ray photoelectron spectra show the expected increase with successive depositions. The formation of such structures has been confirmed by depositing Pt and Au layers alternatively. Layers of metal and CdS nanoparticles have been deposited alternatively to obtain heterostructures.

Core-level binding energies of the component metals in bimetallic clusters of various compositions in the Ni–Cu, Au–Ag, Ni–Pd, and Cu–Pd systems have been measured as functions of coverage or cluster size, after having characterized the clusters with respect to sizes and compositions. The core-level binding energy shifts, relative to the bulk metals, at large coverages or cluster size, ΔE_a, are found to be identical to those of bulk alloys. By substracting the ΔE_a values from the observed binding energy shifts, ΔE, we obtain the shifts, ΔE_c, due to cluster size. The ΔE_c values in all the alloy systems increase with the decrease in cluster size. These results establish the additivity of the binding energy shifts due to alloying and cluster size effects in bimetallic clusters.

Magic, nuclearity giant clusters formed by the mesoscale self-assembly of Pd nanocrystals of 2.5 nm diameter (nuclearity, ∼561) have been identified by transmission electron microscopy. The clusters have discrete diameters, corresponding to those expected for magic nuclearity of 13, 55, 147, 309, 561, and 1415 and corresponding to closed shells of 1, 2, 3, 4, 5, and 7, respectively. Imaging at different tilt angles has provided confirmation of the spherical nature of these giant clusters. Giant clusters of magic nuclearity have also been found with Pd nanocrystals of ∼ 3.2 nm diameter (nuclearity, ∼ 1415).

The effects of the interactions of metal ions with lipoic acid-capped Ag and Au nanoparticles have been studied by the combined use of electronic absorption spectroscopy and transmission electron microscopy. Three types of effects that are dependent on the metal ion concentration can be distinguished. First, in the dilute regime, there is reversible chelation of the metal ions, causing a marked dampening of the plasmon resonance band of the nanoparticles, but there is no aggregation. The magnitude of plasmon dampening depends on the nature as well as the concentration of the metal ions. In the intermediate concentration regime, aggregation occurs, but in the high concentration regime, there is precipitation. These different regimes are clearly evidenced in the changes in the electronic spectra and in the electron micrographs.

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Multiwalled as well as single-walled carbon nanotubes are conveniently prepared by the pyrolysis of organometallic precursors such as metallocenes and phthalocyanines in a reducing atmosphere. More importantly, pyrolysis of organometallics alone or in mixture with hydrocarbons yields aligned nanotube bundles with useful field emission and hydrogen storage properties. By pyrolysis of organometallics in the presence of thiophene, Y-junction nanotubes are obtained in large quantities. The Y-junction tubes have a good potential in nanoelectronics. Carbon nanotubes prepared from organometallics are useful to prepare nanowires and nanotubes of other materials such as BN, GaN, SiC, and Si₃N₄.

Pyrolysis of acetylene over iron or cobalt nanoparticles well dispersed on silica substrates gives copious yields of aligned carbon nanotube bundles. By carrying out the pyrolysis of pyridine over these catalyst surfaces, good quantities of aligned carbon–nitrogen nanotube bundles have been produced. The composition of the carbon–nitrogen nanotubes varies between C₁₀N and C₃₃N, depending on the catalyst.

B–C–N, C–N and B–N nanotubes, prepared by the pyrolysis of the appropriate precursor compounds around 1000°C in an atmosphere of argon, have been examined by electron microscopy and other techniques. The compositions of the nanotubes have been analysed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. There is significant compositional variation in B–C–N and C–N nanotubes. Properties of the B–C–N and C–N nanotubes have been compared with those of carbon nanotubes, particularly with respect to the I–V characteristics as obtained from UHV-STM studies. The near absence of B–N nanotubes on pyrolysing the appropriate precursor compound, and other observations made in the present study, indicate the crucial role played by carbon in the initial nucleation and growth of nanotubes.

Boron-doped carbon nanotubes have been prepared by the pyrolysis of acetylene–diborane mixtures in a stream of helium and hydrogen. The nanotubes so obtained have compositions around C₃₅B . The presence of boron favours nanotube formation and the boron content does not vary with the depth. Considering that the nitrogen-doped nanotubes, reported recently (Chem. Phys . Lett. 287 (1997) 671) have the average composition C₃₅N, it may be possible to use a combination of these p-type and n-type nanotubes for certain applications.

Carbon nanotubes have been opened using a variety of oxidants including HF/BF₃ and OsO₄ at room temperature. The opened nanotubes have been filled with metals such as Ag, Au, Pd and Pt by simple chemical means. The opened nanotubes, containing a high concentration of acidic functional groups, get covered or closed by reaction with reagents such as ethylene glycol. The opened nanotubes can also be closed by treatment with benzene vapour in a reducing atmosphere of argon and hydrogen at high temperatures. Refluxing carbon nanotubes in a H₂SO₄–HNO₃ mixture results in a clear colourless solution, which on removal of the solvent and excess acid gives a white solid containing functionalized nanotubes. Neutralization of the acidic solution by alkali results in the precipitation of a brown solid containing nanotubes.

Acid-treated carbon nanotubes have been decorated with nanoscale Au, Pt and Ag clusters by employing several procedures. The nature and extent of metal coverage can be varied by changing the concentration of the metal compound or by mild sonication (treatment with ultrasound). The metal clusters seem to get deposited on the acidic surface sites of the nanotubes.

Adsorption of N₂, benzene and methanol have been studied on as-prepared single-walled carbon nanotubes (SWNT) as well as SWNTs treated with HC1 and HNO₃. These nanotubes are good microporous materials with total surface areas well above 400 m²/g and internal surface areas of 300 m²/g or higher. Benzene molecules are shown to be adsorbed within the pores of the SWNTs.

We report here an in situ x-ray diffraction investigation of the structural changes in carbon single-wall nanotube bundles under quasihydrostatic pressures up to 13 GPa. In contrast with a recent study [Phys. Rev. Lett. 85, 1887 (2000)] our results show that the triangular lattice of the carbon nanotube bundles continues to persist up to ∼ 10 GPa. The lattice is seen to relax just before the phase transformation that is observed at ∼ 10 GPa. Further, our results display the reversibility of the two-dimensional lattice symmetry even after compression up to 13 GPa well beyond the 5 GPa value observed recently. These experimental results explicitly validate the predicted remarkable mechanical resilience of the nanotubes.

Adsorption of hydrogen at 300 K has been investigated on well -characterized samples of carbon nanotubes, besides carbon fibres by taking care to avoid many of the pitfalls generally encountered in such measurements. The nanotube samples include single- and multi-walled nanotubes prepared by different methods, as well as aligned bundles of multi-walled nanotubes. The effect of acid treatment of the nanotubes has been examined. A maximum adsorption of ca. 3.7 wt% is found with aligned multi-walled nanotubes. Electrochemical hydrogen storage measurements have also been carried out on the nanotube samples and the results are similar to those found by gas adsorption measurements.

Carbon nanotubes were discovered in 1991. It was soon recognized that layered metal dichalcogenides such as MoS₂ could also form fullerene and nanotube type structures, and the first synthesis was reported in 1992. Since then, a large number of layered chalcogenides and other materials have been shown to form nanotubes and their structures investigated by electron microscopy. Inorganic nanotubes constitute an important family of nanostructures with interesting properties and potential applications. In this article, we discuss the progress made in this novel class of inorganic nanomaterials.

Nanowires of Au, Ag, Pt, and Pd (1.0–1.4 nm diam) have been produced in the capillaries of single-walled carbon nanotubes (SWNTs). The nanowire is single-crystalline in some cases. Dispersions of the nanowires in alcohol show longitudinal plasmon absorption bands at different wavelengths, suggesting the presence of a distribution of aspect ratios. A novel phenomenon involving the intercalation of metal layers (∼0.5 nm thick) in the intertubular space of SWNT bundles has been observed. SWNTs decorated by metal nanoparticles are formed in some of the preparations.

Nanorods of several oxides, with diameters in the range of 10–200 nm and lengths upto a few microns, have been prepared by templating against carbon nanotubes. The oxides include V₂O₅, WO₃, MoO₃ and Sb₂O₅ as well as metallic MoO₂, RuO₂ and IrO₂. The nanorods tend to be single-crystalline structures. Nanotube structures have also been obtained in MoO₃ and RuO₂.

By carrying out the reaction of Ga₂O₃ powder with carbon nanotubes around 1100 °C, nanowires, nanobelts and nanosheets of Ga₂O₃ have been obtained, the diameter and proportion of the nanowires depending on the flow rate of argon through the reaction zone. Reaction of Ga₂O₃ powder with activated carbon mainly gives rise to nanosheets and nanorods. The procedures employed in this study are attractive since they give high yields of nanowires and nanobelts. The nanowires are single crystalline with the growth direction perpendicular to the (102) planes. The Ga₂O₃ nanowires exhibit good photoluminescence characteristics.

Several methods have been employed to synthesize SiC nanowires. The methods include heating silica gel or fumed silica with activated carbon in a reducing atmosphere, the carbon particles being produced in situ in one of the methods. The simplest method to obtain β-SiC nanowires involves heating silica gel with activated carbon at 1360 ° C in H₂ or NH₃. The same reaction, if carried out in the presence of catalytic iron particles, at 1200 ° C gives α-Si₃N₄ nanowires and Si₂N₂O nanowires at 1100 ° C. Another method to obtain Si₃N₄ nanowires is to heat multi-walled carbon nanotubes with silica gel at 1360 ° C in an atmosphere of NH₃. In the presence of catalytic Fe particles, this method yields α-Si₃N₄ nanowires in pure form.

Mn-doped GaN nanowires have been prepared by reacting a mixture of acetylacetonates with NH₃ at 950 ° C in the presence of multi-walled (MWNTs) and single-walled (SWNTs) carbon nanotubes, the nanowires prepared with SWNTs being considerably smaller in diameter. GaMnN nanowires with 1 %, 3% and 5% Mn so obtained have been characterized by X-ray diffraction, EDAX analysis and photoluminescence (PL) spectra. The GaMnN nanowires are all ferromagnetic even at 300 K, exhibiting magnetic hysteresis. The PL spectra of the GaMnN nanowires prepared with SWNTs show a large blue-shift of the Mn²⁺ emission.

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The nature of diamond–graphite hybrids has been studied by molecular mechanics, by examining the structures of species such as C₈₄H_x wherein the sp³ to sp² carbon ratio is varied progressively. The dependence of the average coordination number on the atom fraction of hydrogen has been examined in the light of the random covalent network model. The HOMO–LUMO gap has been estimated in a graphite-like C₁₁₀H_x and in a diamond-like C₁₂₀H_x as a function of the sp³/sp² carbon atom ratio. The gap increases exponentially with the fraction of sp³ carbon. Shapes of fullerene-like structures with 7-membered rings, in addition to 6- and 5-membered rings, have been investigated along with structures of bent nanotubes having similar ring systems.

Scanning tunneling microscopy of solid films of C₆₀ and C₇₀ clearly demonstrate the occurrence of photochemical polymerization of these fullerenes in the solid state. X-ray diffraction studies show that such a polymerization is accompanied by contraction of the unit-cell volume in the case of C₆₀ and expansion in the case of C₇₀. This is also evidenced from the STM images. These observations help to understand the differences in the amorphization behavior of C₆₀ and C₇₀ under pressure. Amorphization of C₆₀ under pressure is irreversible because it is accompanied by polymerization associated with a contraction of the unit cell volume. Monte Carlo simulations show how pressure-induced polymerization is favored in C₆₀ because of proper orientation as well as the required proximity of the molecules. Amorphization of C₇₀, on the other hand, is reversible because C₇₀ is less compressible and polymerization is not favored under pressure.

C₆₀Br₈, unlike C₆₀Br₆ and C₆₀Cl₆, forms a solid charge-transfer compound with tetrathiafulvalene (TTF), the composition being C₆₀Br₈ (TTF)₈. The unique complex-forming property of C₆₀Br₈ can be understood on the basis of the electronic structures of the halogenated derivatives of C₆₀. Molecular orbital calculations show that the low LUMO energy of C₆₀Br₈ compared with the other halogen derivatives renders the formation of the complex with TTF favourable, the four virtual LUMOs being able to accept 8 electrons. The Raman spectrum of C₆₀Br₈ (TTF)₈ shows a marked softening of the bands (–46 cm⁻¹ on average) with respect to C₆₀Br₈, suggesting that indeed 8 electrons are transferred per C₆₀Br₈ molecule, one from each TTF molecule. The complex is weakly paramagnetic and shows a magnetic transition around 80 K.

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Trithiocyanuric acid (TCA) and 4,4′-bipyridyl (BP) form hydrogen-bonded co-crystals with aromatic compounds such as benzene, toluene, p-xylene and anthracene. The TCA-BP co-crystal is composed of cavities formed by the N–H⋯N hydrogen bonds between the two molecules, and the three-dimensional structure contains channels of approximately 10 Å where aromatic molecules are accommodated. The molar ratios of TCA, BP and the aromatic compound in the co-crystals are 2:1:1 or 2:1:0.5. Benzene, toluene and p-xylene are removed from the channels around 190, 183 and 170 ° C respectively, and these aromatic guests can be reintroduced into the empty channels of the apo-hosts. The apo-hosts with empty channels have reasonable thermal stability and exhibit shape selectivity in that the empty channels accommodate p-xylene but not m- or o-xylene or mesitylene.

By the co-crystallization of trimesic acid, TMA, with molecules such as dimethylamine, N,N,N′,N′-tetramethylethylenediamine and methanol, it has been possible to generate hydrogen-bonded four-membered networks of TMA. The three-dimensional arrangement of the four-membered networks gives rise to channels occupied by the guest molecules. It has also been possible to generate a four-membered network by co-ordination of TMA with Co(II).

A novel compound of the formula Ag₂·CA (CA = cyanuric acid) possessing Ag sheets and hydrogen-bonded CA chains, exhibits anisotropic conductivity and acts as an infinite parallel plate capacitor with a high dielectric constant.

Electronic charge density distribution in molecular systems has been described in terms of the topological properties. After briefly reviewing methods of obtaining charge densities from X-ray diffraction and theory, typical case studies are discussed. These studies include rings and cage systems, hydrogen bonded solids, polymorphic solids and molecular NLO materials. It is shown how combined experimental and theoretical investigations of charge densities in molecular crystals can provide useful insights into electronic structure and reactivity.

The structure, packing, and charge distribution in molecules of nonlinear optical materials have been analysed with reference to their counterparts in centrosymmetric structures based on low temperature X-ray measurements. The systems studied are the centric and noncentric polymorphs of 5-nitrouracil as well as the diamino, dithio, and thioamino derivatives of 1,1-ethylenedicarbonitrile; the latter possesses a noncentric structure. The molecular structure of 5-nitrouracil is invariant between the two forms, while the crystal packing is considerably different, leading to dimeric N–H⋯O rings in the centric polymorph and linear chains in noncentric one. There is an additional C–H⋯O contact in the centric form with a significant overlap of the electrostatic potentials between the alkenyl hydrogen atom and an oxygen atom of the nitro group. The dipole moment of 5-nitrouracil in the noncentric form is much higher (μ=9 D) than in the centric form (≈6 D). Among the 1,1-ethylenedicarbonitriles, there is an increased charge separation in the noncentric thioamino derivative, leading to an enhanced dipole of 15 D compared to the centric diamino (5 D) and dithio (6 D) derivatives. The effect of the crystal field is borne out by semiempirical AM1 calculations on the two systems. Dipole moments calculated for the molecules in the frozen geometries match closely with those abtained fro centric crystals from the experimental charge densities. The calculated values of the dipole moment in the frozen or optimized geometries in the noncentric structures are, however, considerably lower than the observed value. Furthermore, the conformation of the S–CH₃, group in the noncentric crystal is anti with respect to the central C=C bond while the syn conformation is anti with respect to the central C=C bond while the syn conformation is predicted for the free molecule in the optimized geometry.

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Investigations of the pore expansion in mesoporous silica in the presence of n-alkanes suggest a cooperative organization of the surfactant and alkane molecules, involving additivity of chain lengths.

The kinetics of the lamellar→hexagonal→cubic transformations of mesoporous zirconia prepared by using a neutral organic amine as the amphiphile have been studied in phosphoric acid medium. The lamellar→hexagonal transformation is preceded by a loss of the template molecules and the hexagonal→cubic transformation proceeds only when the lamellar form has entirely transformed to the hexagonal phase. The kinetics of the thermal transformation of lamellar zirconia to the hexagonal form have also been examined; this transformation also occurs and is accompanied by a loss of template molecules. Accordingly, the activation energy of the transformation is comparable to the hydrogen bond energy between the amine and the oxo-zirconium species. The phase transformations of mesoporous zirconia can be understood in terms of minimum energy surfaces.

A hexagonal mesoporous phase based on SnO₂ is synthesized for the first time by using an anionic surfactant; hexagonal phases of TiO₂ are prepared with neutral amine surfactants.

Hexagonal, cubic and lamellar aluminoborate mesophases containing octahedral aluminium and tetrahedral boron are prepared and characterized for the first time.

Copper(II) acetate dimer and [Mn^II(bipy)₂]⁺ encapsulated in cubic Al-MCM-48 show high catalytic activity in the oxidation of phenol to catechol by oxygen activation, and of styrene to styrene oxide by singlet oxygen , respectively.

Hexagonal and lamellar nanostructured organic-metal chalcogenide composites have been prepared by the reaction of metal salt aliphatic-amine nanostructured adducts with Na₂S or Na₂Se solution; nanostructured composites of CdS, SnS₂, Sb₂S₃ and CdSe with long-chain aliphatic amines obtained in this manner have been characterized.

Hexagonal microporous phases of SiO₂ and AlPO₄ with pore sizes in the range intermediate between traditional microporous and mesoporous materials have been prepared, by making use of supramolecular organization of shortchain amine (n-C₆H₁₂NH₂) template molecules. The pore diameters are around 1.4 nm in both cases. The hexagonal microporous phase of A1PO₄ is formed only in the presence of fluoride ions. The surface area of SiO₂ after removal of the template is 800 m² g⁻¹. In the case of AlPO₄, the surface area is 190 m² g⁻¹ after removal of 60% of the template.

Ordered mesoscale hollow spheres (1000 nm diameter) of binary oxides such as TiO₂ and ZrO₂ as well as of ternary oxides such as ferroelectric PbTiO₃ and Pb(ZrTi)O₃ have been prepared by templating against colloidal crystals of polystyrene, by adopting different procedures.

Macroporous carbons of different pore sizes, containing three-dimensionally connected voids, have been prepared by an elegant method. The method involves the coating of ordered silica spheres with sucrose, followed by carbonization using sulfuric acid, and the removal of silica with aqueous hydrofluoric acid. The carbon samples show the expected optical properties. The surface area of the macroporous carbon samples varies between 120 to 550 m²g⁻¹, depending on whether nonporous or mesoporous silica spheres were used as templates.

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Open-framework metal phosphates occur as one-dimensional (ID) chains or ladders, two-dimensional (2D) layers, and complex threedimensional (3D) structures. Zero-dimensional monomers have also been isolated recently. These materials are traditionally prepared by hydrothermal means, In the presence of organic amines, but the reactions of amine phosphates with metal ions provide a facile route for the synthesis, and also throw some light on the mode of formation of these fascinating architectures. Careful studies of the transformations of monophasic zinc phosphates of well-characterized structures show that the 1D structures transform to 2D and 3D structures, while the 2D structures transform to 3D structures. The zero-dimensional monomers transform to 1D, 2D, and 3D structures. There is reason to believe that the 0D monomers, comprising four-membered rings, are the most basic structural units of the open-framework phosphates and that after an optimal precursor state, such as the ladder structure, is formed, further building may occur spontaneously. Evidence for the occurrence of self-assembly in the formation of complex structures is provided by the presence of the structural features of the onedimensional starting material in the final products. These observations constitute the beginning of our understanding of the building-up principle of such complex structures.

Acid degradation of 3D zinc phosphates primarily yields a one-dimensional ladder compound , an observation that is significant considering that the latter forms 3D structures on heating in water.

Use of tributylphosphate, an organophosphate, as the phosphorus source in place of phosphoric acid, has enabled the synthesis of several new open-framework zinc(II) and cobalt(II) phosphates, under solvothermal conditions.

A novel iron phosphate, [(C₄N₃H₁₆)(C₄N₃H₁₅)]⁵⁺⁻ [Fe₅F₄(H₂PO₄)(HPO₄)₃(PO₄)₃]⁵⁻H₂O, consisting of a Fe−O/ F−Fe network crosslinked by PO₄ groups is shown to possess unusually large elliptical voids of 24 T atoms (T = Fe, P) and to exhibit a gradual low- to high-spin transformation.

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A hierarchy of novel zinc oxalates including monomers and dimers has been prepared by reaction of amine oxalates with zinc ions, the amine oxalates having been characterized for the first time. In most of the amine oxalates one of the carboxyl groups transfers a proton to the amino nitrogen, leaving the other carboxyl group free to form hydrogen bonds. The zinc oxalates obtained are composed of a network of ZnO₆ octahedra and oxalate units, and possess zero-, one-, two- and three-dimensional structures. The monomer, dimer, and chain zinc oxalates are the first members of the hierarchy of structures. Relationships amongst these various oxalate structures are noteworthy and give indications as to the manner in which these structures are formed. Thus, the three-dimensional structure can be formed by the linking of layers, and the layer structure by condensation of linear chains. The isolation and characterization of a hierarchy of zinc oxalates of differing dimensionalities assumes significance in the light of the recent finding that low-dimensional structures transform to higher, more complex structures in the phosphate family.

Two three-dimensional open-framework metal dicarboxylates possessing channels with a hydrophobic environment have been synthesized. One is a malonate of the formula, [Cd(O₂C-CH₂-CO₂)(H₂O)]·H₂O, I, and the other a glutarate of the formula [Mn(O₂C-(CH₂)₃-CO₂)], II. The three-dimensional structure of I is attained through infinite Cd–O–Cd corner-linkages. On the other hand, II, contains Mn–O–Mn layers cross-linked by MnO, octahedra and glutarate moieties.

New inorganic-organic hybrid open-framework materials of the phosphate - oxalate family, [Fe₂(H₂O₂- (HPO₄)₂(C₂O₄)]·H₂O) (I), [Fe₂(H₂O₂- (HPO₄)₂(C₂O₄)]·2H₂O (II), [C₃N₂H₁₂]- [Fe₂(HPO₄)₂(C₂O₄)_1.5]₂ (III), and [C₃N₂OH₁₂][Fe₂(HPO₄)₂(C₂O₄)_1.5]₂ (IV) have been synthesized hydrothermally in the presence of structure-directing amines. The amine molecules are incorporated inIII and IV, whereas I and II are devoid of them. The oxalate units act as a bridge between the layers in all the compounds. The layers in I and II are entirely inorganic, being formed by FeO₆ and PO₄ units, whereas in III and IV oxalate units constitute the inorganic layers and act as the bridge between these layers. Such a dual role of the oxalate unit is unique and noteworthy. The formation of two types of inorganic layers in I and II consisting of four-, six-, and eight-membered rings, indicates the interconversions between the various rings in the phosphate - oxalates to be facile. All the phosphate - oxalates show antiferromagnetic ordering at low temperatures.

Four unusual inorganic–organic nanocomposites containing extended alkali halide structures, present as layers or as three-dimensional units, have been synthesized by a metathetic reaction carried out under hydrothermal conditions. These compounds, with cadmium oxalate host lattices and the alkali halide structures as guests, have the compositions [RbCl][Cd₆(C₂O₄)₆]·;2H₂O, I; 2[CsBr][Cd(C₂O₄)]·H₂O, II; 2[CsBr][Cd₂(C₂O₄)(Br)₂]·2H₂O, III; and 3[RbCl][Cd₂(C₂O₄)(Cl₂)]·H₂O, IV. Crystal data for I, rhombohedral, space group = R-3 (no. 148), a = 9.3859(3), c = 23.9086(8) Å; V = 1824 .05(1) ų, Z = 18, M = 1359.51, ρ_calcd = 3.702 g cm⁻³, μ(Mo Kα) = 7.375 mm⁻¹, R₁ = 0.04, wR₂ = 0.0904 [537 observed reflections with I > 2σ(I))]; for II, orthorhombic, space group = Pbcm (no. 57), a = 6.0854(6), b = 11.0793(11), c = 16 .889(2) Å; V = 1138 .7(2) ų, Z = 8, M = 644.06, ρ_calcd = 3.745 g cm⁻³, μ(Mo Kα) = 15.219 mm⁻¹, R₁ = 0.04, wR₂ = 0.08 [408 observed reflections with I > 2σ(I)]; for III, orthorhombic, space group = Cmcm (no.63), a = 23.6151( 14), b = 10.2528(6), c = .7.8199(6)Å, V = 1894.2(2) ų, Z = 16, M = 970.33, ρ_calcd = 3.374 g cm⁻³, μ(Mo Kα) = 14.487 mm⁻¹, R₁ = 0.03, wR2 = 0.0744 [651 observed reflections with I > 2σ(1)]; for IV, monoclinic, space group = P2₁/c (no. 14), a = 8.0648(2)(3), b = 22.9026(4), c = 9.3967(3) Å; V = 1681.10(7) ų, Z = 4, M= 782.53, ρ_calcd = 3.076 g cm⁻³, μ(Mo Kα) = 11.961 mm⁻¹, R₁ = 0.04, wR₂ = 0.093 [2071 observed reflections with I > 2σ(I)]. Compounds I-IV possess many unusual structural features. While I has interpenetrating lattices of the three -dimensional cadmium oxalate and RbCl with an expanded lattice, giving rise to a super rock-salt cell, II possesses alternating cadmium oxalate and CsBr layers, the latter comprising six-membered CsBr rings. Compounds III and IV have isolated cadmium oxalate units (zero-dimensional) linked and stabilized by the alkali halides.

Sodalite structures have not hitherto been obtained by rational design based on linking of square building units. We have, for the first time, succeeded in synthesizing sodalite networks by making use of squaric acid as the 4-membered square unit. Cobalt, manganese and zinc squarates with this architecture have been obtained (Crystal data for 1, Co(H₂O)₂(C₄O₄): cubic, space group Pn-3n (No. 222), a = 16.280(5) Å, α = β = γ = 90°, V = 4315(2) ų, Z = 48, ρ_calc = 1.912 mg m⁻³, R₁ = 0.041 for Mo Kα, λ = 0.71073 Å. Crystal data for 2, Mn(H₂O)₂(C₄O₄): trigonal, space group R-3 (No. 148), a = b = 11.607(5), c = 14.66(3) Å, α = β = 90°, γ = 120°, V = 1711(4) ų, Z = 18, ρ_calc = 1.773 mg m⁻³, R₁ = 0.029. Crystal data for 3, Zn(H₂O)₂(C₄O₄): cubic, space group Pn-3n (No. 222), a = 16.256(3) Å, α = β = γ = 90°, V = 4295.6(15) ų, Z = 48, ρ_calc = 1.980 mg m⁻³, final R₁ = 0.033. In the sodalite structures obtained by us, twelve metal(H₂O)₂O₄ units link the squares, instead of oxygen atoms as in the normal aluminosilicate sodalite.

The following sections are included:

• Brief Biodata of Professor C.N.R. Rao

• Scientific Colleagues