Molecules into Materials

The following sections are included:

• Dedication

• Why this book?

• Contents

Please refer to full text.

The wavenumbers of charge-transfer bands in 1 : 1 complexes between an oxidizing metal and a series of reducing ligands are plotted against those for other metals with the same ligands. The resulting straight line plots are used to discuss the stereochemistry of copper, redox potentials, the spectra of thiocyanate complexes, and solvent effects.

The charge-transfer spectra of 10 tetrahalide complexes of the first transition series have been measured at 77°K in the form of evaporated films or rigid glassy solutions. To assign the spectra in detail, theoretical expressions are developed, using vector coupling methods, to describe the first-order effect of spin–orbit coupling at both the metal and the ligand nucleus on the terms arising from each of the excited charge-transfer configurations. Theoretical expressions for the intensities of charge-transfer transitions are also derived by the same method and tested by substituting approximate LCAO eigenvectors. The assignments established by these arguments lead to the conclusion that the highest occupied orbitals of mainly ligand character occur throughout in the order 1e<3t₂<t₁ with a total energy spread of about 15 kK.

The electronic absorption spectra of tetrahalide complexes of Zn^II, Cd^II, Hg^II, and Tl^III have been measured either in solution or as thin films. The effects of second-order spin–orbit coupling account satisfactorily for the observed band intensities and splittings. The one-electron transitions responsible for the lowest-energy groups of absorption bands are all of the type t₂→a₁. There is no evidence for transitions from the non-bonding t₁, or e molecular orbitals, and an explanation is advanced for their non-appearance.

No abstract received.

A theoretical treatment is given of the charge-transfer bands of ferrous, ferric, and cuprous phenanthroline complexes. Methyl substituents are shown to cause additive shifts in the band frequency. The best values of the shifts associated with substitution are extracted statistically and interpreted by Hückel molecular orbital theory. It is necessary to assume that the methyl groups perturb the molecule both by changing the energy of the π-molecular orbital involved in charge-transfer, and by changing the energy of the d-orbitals. The latter effect can be accounted for as a purely inductive (σ) effect, produced by the change in the σ-donor power of the ligands. which can be assessed theoretically or experimentally. This treatment provides an assignment of the molecular orbital involved in charge-transfer. Furthermore, it is shown that the methyl group acts both inductively and hyperconjugatively: appropriate parameters are obtained from the data and compared with results from other sources. Finally, the relations between d-orbital energy and ligand σ-donor power are discussed.

Measurement of the charge-transfer band of tris(phenanthroline)iron(II) at low temperature provides support for the suggestion that its fine structure arises from a vibrational progression in the excited state.

A one-electron theory of charge-transfer is applied to the charge-transfer bands of ferrous, ferric, and cuprous phenanthroline complexes. The energies of the d-orbitals are calculated, and possible reasons for the involvement of unexpected ligand molecular orbitals are discussed. The origin of the intensities is then considered, and it is suggested that the main source of intensity is the ransition moment resulting from the transfer of charge itself, though borrowing from internal ferrous and cuprous (not ferric) transitions, and borrowing from internal ligand transitions may be appreciable. Metal-ligand resonance and overlap integrals are estimated from the data.

The effects of substitution on extinction coefficients are additive for cuprous complexes. The effects of substitution in each position are extracted statistically; the results are not in perfect agreement with the predictions derived from any of the sources of intensity considered, but do not contradict the assignment mentioned above.

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The following sections are included:

• Introduction

• Ligand Field Spectra
  • Exchange Intractions in Magnetically Ordered Crystals; Transition Metal Fluorides
  • Environmental Effects; Magnus' Green Salt

• Charge Transfer Spectra
  • Intramolecular transitions
    • Magnus' Green Salt
    • Nickel(II) Dimethylglyoximate
    • Cyanopalladites and -platinites
    • Oxalatoplatinites
  • Intermolecular transitions
    • Hexahalides and Tetroxo-anion Salts
    • Hexacyanides
    • Mixed valence Salts

• Conclusion

• References

Compounds containing chains of metal atoms are found with four distinct classes of structure, each of which, as a result of the widely varying strength of the metal-metal interaction, has associated with it its own characteristic pattern of physical properties. Thus one may have either single or mixed valency phases with near neighbor contact between metal ions formed either directly or through anion bridges. The optical, X-ray photo electron, magnetic and transport properties of examples of each class are surveyed, with particular emphasis on work carried out at Oxford, to highlight the relative importance of single center and collective excited states in each category, in relation to their structures and the magnitude of the metal–metal interaction. It is pointed out that a collective description may be appropriate for magnetic and electronic excitations even without electron delocalization.

No abstract received.

The following sections are included:

• Introduction

• Experimental

• Results and Discussion

• Conclusions

• References

No abstract received.

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Reflection spectra of Sb(III), Sb(V); Bi(III), Sb(V); In(III), Sb(V); and Tl(III), Sb(V) hexahalide complexes diluted in hexahalostannate(IV) crystals are reported for a number of cations. The pure compounds Cs₂Bi_0.5Sb_0.5Cl₆, Cs₂In_0.5Sb_0.5Cl₆, and Cs₂Tl_0.5Sb_0.5Cl₆ also have been prepared and characterized from their X–ray powder photographs. The Bi and In compounds show evidence of superlattice formation which is assumed to exist, undetected, in the Sb(III), Sb(V) compounds. On this evidence the solid spectra have been assigned to electron transfer transitions from the ns² or (n + 1)s² shell of the trivalent ion to the ns₀ shell of the pentavalent ion. The abnormally deep colors of the In and Tl compounds also are discussed.

No abstract received.

Four series of hexachloroantimonate(III,v) salts, A^I₂Sb_xSn_1−xCl₆ (A^I = NH_4⁺, Cs⁺, and MeNH_3⁺). and , have been prepared and characterised by chemical analysis. X-ray powder diffraction, and far-i.r. measurements. Visible and u.v. spectra of all the compounds were measured at room temperature by diffuse reflectance and. in the case of the MeNH_3⁺ salts, by single-crystal transmission between room temperature and 6°K. The concentration-dependence of the spectra showed that the intensity of the visible mixed-valence band was proportional to the concentration of Sb^III–Sb^V pairs in the hexachlorostannate(IV) lattice. From the absolute intensity of the mixed-valence transition it was found that delocalisation of the optical (5s) electrons of SbCl₆³⁻-on to the neighbouring SbCl_6⁻ was less than 0.1% in the ground state, in agreement with the far-i.r. results. The band, therefore, is almost purely charge-transfer in character. The temperature-dependence of the half-width of the band between 300 and 6°K could be accounted for in terms of broadening by a single effective frequency of 210 cm.⁻¹. The relation between this optically derived frequency and the ground-state vibrational frequencies of SbCl₆³⁻ and SbCl₆⁻ is considered.

Two-probe D.C. conductivity measurements are reported for the three series of hexachloroantimonates(III,v) whose optical properties were examined in the preceding paper: A^I₂Sb_xSn_1−xCl₆ (A^I = NH₄⁺, Cs⁺), . For values of x and y greater than about 0-1, the compounds behave as ohmic semiconductors. The specific conductivities at room temperature are proportional to x² and y respectively, while the activation energies are independent of the antimony concentration. Seebeck coefficients, determined at room temperature, indicate that for x and y > 0-1 the majority carriers are holes, but below 0-1. electrons. It is suggested that the conductivity of the compounds containing sufficient antimony to form a continuous path through the lattice is electronic while that of the dilute materials and single-valence host lattices is ionic. In the former, the magnitude of the conductivity is consistent with a diffusion mechanism in which carrier formation by electron transfer between SbCl₆⁶⁻ and SbCl₆⁻ is followed by migration of the resulting SbCl₆²⁻ among the SbCl₆³⁻. The charge-carrier formation step is the adiabatic analogue of the Franck–Condon intermolecular charge transfer process studied in the preceding paper. Relaxation frequencies calculated from the observed conductivity by use of the diffusion model are in the range of Sb–Cl vibrations. The relation between the semiconductor activation energy and the optical charge-transfer energy is discussed.

The following sections are included:

• Introduction

• Theory of Mixed Valence Effects
  • The Wave Functions and Mixed Valence Classification
  • Mixed Valence Spectra
  • Magnetism and Electron Transport
  • Molecular Geometry

• Survey of the Elements
  • Titanium
  • Vanadium
  • Chromium
  • Manganese
  • Iron
  • Cobalt
  • Nickel
  • Copper
  • Zirconium and Hafnium
  • Niobium and Tantalum
  • Molybdenum and Tungsten
  • Technetium and Rhenium
  • Ruthenium and Osmium
  • Rhodium and Iridium
  • Palladium and Platinum
  • Silver
  • Gold
  • Gallium
  • Indium
  • Thallium
  • Tin
  • Lead
  • Phosphorus and Arsenic
  • Antimony
  • Bismuth
  • Lanthanides
  • Actinides
  • Miscellaneous

• Conclusions

• References

No abstract received.

The contribution of mixed-valence electron delocalisation to the ferromagnetic exchange between the iron(III) ions in Prussian Blue {Fe^III₄[Fe^II(CN)₆]₃·14H₂O} has been estimated theoretically. Agreement between the calculated and observed values of the Curie temperature is quite good.

Polarized neutron diffraction has been used to investigate the spin delocalization from the high-spin Fe(III) sites to the low-spin Fe(II) in deuteriated Prussian Blue, Fe₄[Fe(CN)₆]₃ · xD₂O. Measurements of the 111, 200, and 400 reflections were made on a powdered sample at 3 K and 4.8 T using a neutron wavelength of 1.074 Å. The expectation value of S at the Fe(II) site is −0.008±0.028 corresponding to an upper limit of about 5% of an electron for the spin delocalization.

The following sections are included:

• INTRODUCTION: HISTORICAL BACKGROUND

• THEORIES OF MIXED VALENCY
  • Preliminaries
  • The Robin–Day model
  • A perturbation model for bridging groups
  • Dynamics of electron transfer: adiabatic and Franck–Condon processes
  • Class II or class III dimers: V₀₁ versus E_Ad
  • The Piepho–Krausz–Schatz model

• TYPES OF MIXED VALENCY COMPOUNDS
  • Discrete dimers and oligomers
    • Evidence from structures
    • Preparation and thermodynamics
    • The Creutz-Taube complex: class II or III?
  • One-dimensional complexes
    • Class II complexes with bridging anions: Wolfram's Red Salt
    • Class III chains with directly interacting metal atoms: KCP

• CONCLUSION

• REFERENCES

The two-site one-electron vibronic coupling model of Piepho, Krausz, and Schatz (PKS) for mixed-valence systems is extended to two-site two-electron dimeric systems. The full dynamic problem is solved and characterization of the system reduces to a simple matrix diagonalization. For strong vibronic coupling, a perturbation treatment is also presented which leads to simple analytical expressions for the eigenvalues and eigenfunctions. The model is used to fit the absorption, resonance Raman, and luminescence spectra of the one-dimensional chlorine-bridged mixed-valence compound [Pt^IIL₄][Pt^IVL₄Cl₂]Cl₄·4H₂O (L = ethylamine), Wolffram's Red Salt.

The intensity of the mixed-valence absorption in a series of mixed-valence chlorocuprates has been measured as a function of the mole fraction of copper(I). The low intensity of the band agrees with its assignment as an intermolecular charge transfer from a chlorocuprate(I) anion to a chlorocuprate(II).

The bandshape of the intervalence absorption of and has been measured from 300 to 4 K in a crystal of (CH₃NH₃)₂Sb_xSn_1−xCl₆ in order to study the variation of the zeroth, first and second moments. At all temperatures the bandshape is Gaussian, as required by the vibronic model of Piepho, Krausz and Schatz (P.K.S.) in the weak-interaction limit. From the temperature dependence of the second moment, we estimate the electron–phonon coupling constant (and hence the displacement in vibrational coordinate from the ground state to the intervalence charge-transfer state), together with the effective phonon frequency coupled to the transition. The latter is very close to the mean of the ground-state totally symmetric Sb—Cl stretching modes of and , and the displacement in the vibrational coordinate is also about half the difference between the Sb—Cl crystallographic bond lengths in the two complex ions. To explain the temperature dependence of the zeroth and first moments anharmonicity must be involved. A simple model for the variation of intervalence excitation energy with interionic distance, combined with an isotropic model of the thermal lattice expansion, gives a quantitative account of the change in first moment with temperature and a qualitative description of the change in zeroth moment.

Please refer to full text.

The following sections are included:

• Preparation and Crystal Growing

• Crystal and Magnetic Structures

• Magnetic Susceptibility and Magnetization

• Inelastic Neutron Scattering

• Optical Properties

The title compounds have been prepared and characterised by chemical analysis and X-ray diffraction. By magmetic-susceptibility and magnetisation measurements they are shown to order ferromagnetically at T_c = 58 ± 2 and 55 ± 2 K respectively. Up to ca. 0.7 T_c the magnetisation M(T) obeys the equation 1 − [M(T)/M(0)] = CTγ where γ = 1.5 ± 0.1. The optical spectra of both compounds contain sharp absorption bands at 533 and 626 nm, assigned as quintet–triplet ligand-field transitions. Up to 0.9 T_c their intensities vary with temperature as T², as predicted for ‘hot’ exciton–magnon combination bands in a two-dimensional easy-plane ferromagnet.

A simple correlation exists between the intensity of a quintet-to-triplet transition in the visible absorption spectrum of (MeNH₃)₂CrCl₄ (T_c 58 K) and the magnetic order; from 6 to 50 K the oscillator strength of the optical transition is proportional to T² and from 70 to 125 K it is nearly constant.

Ferromagnetism in non-metallic solids is quite a rare property. In solids containing molecular building blocks it is even rarer. One strategy for preparing ionic ferromagnets is to combine a low-dimensional continuous lattice containing transition-metal ions with organic molecular ions. In this article we review the synthesis, structures and magnetic and optical properties of halogenochromate(II) salts with substituted ammonium cations. The shape of the latter has a striking influence on the bulk magnetic behaviour of the solids. Monosubstituted cations yield tetrahalogenochromates(II) with layer structures which are ferromagnets with Curie temperatures up to 60 K. On the other hand, tetramethylammonium trihalogenochromates(II) are one-dimensional antiferromag-nets. The striking difference in the intensities of the visible absorption spectra of the two classes of materials can be explained by an exchange-induced electric-dipole mechanism.

This paper reports an experimental and theoretical study of the temperature dependence of the oscillator strengths of exciton–magnon absorption bands in the optical spectra of FeCl₂, CoBr₂ and NiBr₂, representative examples of the class of metamagnetic transition-metal layer compounds in which strong intralayer ferromagnetic exchange interactions are combined with much weaker antiferromagnetic exchange between layers. FeCl₂ and CoBr₂ have large anisotropy, the former being an easy-axis system, the latter easy-plane, while in NiBr₂ the anisotropy is small and of easy-plane type. In all three compounds at low temperatures the form of the temperature dependence of the exciton–magnon absorption bands, associated with spin-forbidden ligand-field transitions, is dominated by coupling of the electronic excited states with magnons propagating within the ferromagnetic planes. In FeCl₂ the intensity of the exciton–magnon band at 4270 Å varies as T²exp(−A/kT) over the temperature range 4.0–7.2 K, extending the range previously examined by Schnatterly and Fontana. We have also improved their resolution and found a band which we assign as the pure exciton. In CoBr₂, bands at 4940 Å have intensities varying approximately as T³, in agreement with theory, while in NiBr₂ the band at 6070 Å varies as T². The effect of zero-point spin fluctuations on the limiting form of the exciton–magnon combinations at low temperature is also discussed.

The following sections are included:

• Noncollinear Magnetic Structures

• Neutron Diffraction

• The Magnetic Phase Diagram

• Chemical Substitution

• Conclusion

The optical absorption spectrum of the ferromagnetic insulator K₂CrCl₄ has been measured from 8000 to 25 000 cm⁻¹ and for temperatures between 4.2 and 300°K.

High-temperature (300—80 K) susceptibility data are reported for polycrystalline samples of the ionic ferromagnets M₂CrCl₄ (M = K, Rb, or Cs) and interpreted using the high-temperature series-expansion method for the quadratic-layer Heisenberg S = 2 ferromagnet to yield estimates of 7.0, 8.5, and 6.5 cm⁻¹ for the nearest-neighbour exchange integrals. Low-temperature (80—4·2 K) magnetization measurements are also reported for polycrystalline Rb₂CrCl₄ and Cs₂CrCl₄. In both compounds the magnetization obeys the equation {1 − [M(T)/ M(0)]} = CT^γ with γ = 1.54 up to 60 K, i.e. ca. 0·9 T_c. Saturation magnetizations are ca. 0·4 B.M. lower than expected for S = 2 ions.

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The synthesis and structural and magnetic characterization of 16 compounds AM^IIFe^III(C₂O₄)₃ (A = N(n-C₃H₇)₄, N(n-C₄H₉)4, N(n-C₅H₁₁)₄, P(n-C₄H₉)₄, P(C₆H₅)₄, N(n-C₄H₉)₃(C₆H₅CH₂), (C₆H₅)₃PNP(C₆H₅)₃, As(C₆H₅)₄; M^II = Mn, Fe) are reported. X-ray powder diffraction profiles are indexed in R3c or its subgroup P6₅22 or P6lmmm to derive unit cell constants. The structures of all the compounds consist of two-dimensional honeycomb networks [M^IIFe^III(C₂O₄)_3⁻]_∞. The M^II = Fe compounds behave as ferrimagnets with T_c between 33 and 48 K, but five exhibit a crossover from positive to negative magnetization near 30 K when cooled in a field of 10 mT. The compounds exhibiting this unusual magnetic behavior are those that have the highest T_c. Within the set N(n-C_nH_2n+1)₄Fe^IIFe^III(C₂O₄)₃ (n = 3–5), T_c increases with interlayer separation and the low-temperature magnetization changes from positive (n = 3) to negative (n = 4, 5). In the M = Mn^II compounds, the in-plane cell parameter a₀ is ∼0.03 Å greater than in the corresponding M = Fe^II ones while the interlayer separation (c₀/6) is on average 0.08 Å smaller. All members of the M^II = Mn series have magnetic susceptibilities showing broad maxima at 55 K characteristic of two-dimensional antiferromagnetism, but the magnetization of several of the salts increases sharply below 27 K due to the onset of spin canting, the magnitude of which varies significantly with A.

The following sections are included:

• Experimental Section

• Results

• Conclusion

Detailed bulk magnetization and magnetic susceptibility measurements are reported for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFe^IIFe^III(C₂O₄)₃ (A = quaternary ammonium, phosphonium, or arsonium). The compounds all crystallize in two-dimensional hexagonal honeycomb lattices, but while some show conventional low field positive magnetization at low temperature (Néel type Q), others show a large negative magnetization below a compensation temperature T_comp (Néel type N) under the same measurement protocol. Isothermal and temperature-dependent magnetizations are measured after zero field and field cooling. Temperature-dependent hysteresis measurements are also presented. In compounds that exhibit negative magnetization a discontinuity in the magnetization is observed at T_t (T_comp < T_t < T_c) which correlates with the onset of a giant anisotropy and is indicative of a magnetostrictive transition. In compounds that exhibit positive magnetization, hysteresis behavior and frequency-dependent ac susceptibility data indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a characteristic blocking temperature or imbalance and disorder of Fe(II) and Fe(III).

Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh₄) [Fe^IIFe^III(ox)₃] and (NBu₄) [Fe^IIFe^III(ox)₃] by neutron polarization analysis and Mössbauer spectroscopy. Polarized neutron diffraction profiles obtained between 2 and 50 K on (d₂₀-PPh₄) [Fe^IIFe^III(ox)₃] show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca. 0.8 Å⁻¹ is attributed to two-dimensional short-range magnetic correlations, which are described by a Warren function. The correlation length is ca. 50 Å between 2 and 30 K and then decreases to ca. 20 Å at 50 K. The Mössbauer spectra of (PPh₄) [Fe^IIFe^III(ox)₃] and (NBu₄) [Fe^IIFe^III(ox)₃] have been measured between 1.9 and 293 K and 1.9 and 315 K, respectively, and are very similar. The paramagnetic spectra exhibit both high-spin Fe^II and Fe^III doublets with relative areas which indicate a 5% and 2% excess, respectively, of Fe^III. The coexistence in (PPh₄) [Fe^IIFe^III(ox)₃] between 10 and 30 K of broad sextets and doublets in the Mössbauer spectra and the paramagnetic scattering observed in the polarized neutron measurements indicate the coexistence of spin-correlated and spin-uncorrelated regions in the layers of this compound. The polarized neutron scattering profiles and the Mössbauer spectra yield the magnetic exchange correlation length and lifetime, respectively, and the combined results are best understood in terms of layers composed of random frozen, but exchange correlated domains of ca. 50 Å diameter at the lowest temperatures, of spin-correlated domains and spin-uncorrelated regions at intermediate temperatures, and of largely spin-unconrelated regions above the Néel temperature as determined from magnetometry. The similarity of the Mossbauer spectra of (PPh₄) [Fe^IIFe^III(ox)₃] and (NBu₄) [Fe^IIFe^III(ox)₃] leads to the conclusion that similar magnetic exchange correlations are present in the latter compound.

Long-range ferrimagnetic order in which one magnetic sub-lattice is formed from partly-occupied p-orbitals and the other from d-orbitals can be achieved in molecular ion-radical salts where the cations are oxidised organo-chalcogen donors and the anions are 3d-metal complexes containing both N-bonded NCS and planar N-heterocyclic ligands. Overlaps between p-orbitals of the radical cations and those of the heterocycle are implicated in the cation-anion exchange pathway.

Three new charge transfer salts of BEDT-TTF and TTF with the counter ions [M(NCS)₄(C₉H₇N)₂]⁻ (M = Cr, Fe; C₉H₇N = isoquinoline) are described. The materials are prepared by standard electrocrystallisation techniques. The nature of the anion is verified in the crystal structure of the salt [C₉H₈N][Cr(NCS)₄(C₉H₇N)]·C₁₂H₂₄O₆·H₂O which is used as the electrolyte when M = Cr. All of the charge transfer salts display long range ferrimagnetic order originating from the interaction between M (S = 3/2 or S = 5/2) and the donor (S = 1/2). The measured critical temperatures are 4.2 K (BEDT-TTF, M = Cr), 4.5 K (BEDT-TTF, M = Fe) and 8.9 K (TTF, M = Cr). Each of the compounds also shows a modest magnetic hysteresis of 338,18 and 75 Oe for BEDT-TTF salts of M = Cr, Fe and the TTF salt of Cr, respectively.

Please refer to full text.

The synthesis and physical characterization of a complex of BEDT-TTF [bis(ethylenedithio)tetrathiafulvalenium] with Cu^IICl₄²⁻ are reported. The structure of (BEDT-TTF)₃CuCl₄·H₂O (I) (crystal data: triclinic, space group PI, a = 16.634 Å, b = 16.225 Å, c = 8.980 Å, α = 90.72°, β = 93.24°, γ = 96.76°, V = 2402 ų, Z = 2) consists of alternating layers of organic cations and inorganic anions. The organic layers contain three crystallographically independent molecules, each 2/3⁺, and two distinct stacks alternating with each other. The anion is dimerized as [CuCl₄·H₂O]₂, with the Cu^II in a distorted tetrahedral geometry. I is metallic down to 200 mK with a conductivity anisotropy at 300 K of 7:1 ((σ ∥ a−c):(σ ⊥ a−c) = 140:20 S cm⁻¹) within the layers and an EPR spectrum characterized by two lines, one from each spin system (conduction and Cu²⁺ localized moments). The g value of the Cu resonance (g_∥ = 2.29 and g_⊥ = 2.05) decreases at a gradient of −4 × 10⁻⁵ K⁻¹ from 4 to 300 K. while that of the conduction electron (g = 2.03–2.05) is temperature independent. The EPR line width increases by 25% and 90% for the Cu and the conduction electrons, respectively, over the same temperature range. The spin susceptibility derived from the Cu resonance fits to a Curie–Weiss law (θ ≈ 1 (1) K) or to a Bleaney–Bowers model for a dimer (J = 4 (1) K), and that of the free carriers shows a Pauli type temperature-independent behavior between 30 and 300 K. Below 30 K, the intensity of the free-carrier resonance falls to 30% of the room-temperature value. From the EPR, there is indication of weak interaction between the two spin systems. From the optical reflectivity data, the width of the conduction band is estimated to be 0.6 eV. A band-structure calculation indicates several zero-transfer integrals, and the Fermi surface has very anisotropic pockets of both electrons and holes, suggesting a semi-metal.

We report the crystal and electronic band structures at 295 K and measurements of temperature-dependent magnetic susceptibility, electron paramagnetic resonance, optical reflectivity, conductivity, and thermopower for two copper tetrahalide salts of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF), (BEDT-TTF)₃CuBr₄ and (BEDT-TTF)₃CuCl₂Br₂. The two salts are isostructural with layers of BEDT-TTF having charges of 0 and +e separated by layers of pseudo-square-planar tetrahalides of copper (II). At ambient pressure these salts show conductivities near 1 S cm⁻¹, and the magnetic properties indicate coupled localized spins present on both BEDT-TTF and the d⁹ copper layers. At 60 K, there is a discontinuous drop in susceptibility, a sharpening of the electron paramagnetic resonance linewidth, and an increase in g value that we attribute to the loss of the contribution from the BEDT-TTF sheets. This may be associated with a Jahn-Teller distortion of the planar copper complexes. Below the transition temperature the susceptibility can be fitted to a quadratic layer antiferromagnet with J = 15 K (CuBr₄) and 8 K (CuCl₂Br₂) and one spin per formula unit. Under pressure there is a very rapid increase in conductivity, to 500 S/cm at 24 kbar, the largest increase of conductivity under pressure in any molecular solid yet studied. There is a sharp transition from metal to insulator at temperatures rising to 111 K near 5 kbar, and falling to 80 K at 20 kbar. We consider that these salts are nearly metallic at ambient pressure, with strongly enhanced susceptibilities, but are brought to a fully metallic state under pressure as a result of increased intermolecular contacts.

The relation between crystal structure and bulk magnetic properties is investigated in the molecular charge transfer salts (BEDT-TF)₂MCl₄ (M = Ga, Fe). (BEDT-TTF)₂GaCl₄ crystallizes in the triclinic system. Its crystal structure consists of pairs of BEDT-TTF molecules arranged in layers with intermolecular S···S interactions. Band structure calculations predict semimetallic behavior contrary to the semiconductivity observed even under a pressure of 6 kbar (σ(300 K, 1 bar) = 10⁻¹ S cm⁻¹ and E_A = 0.2eV). The static (Faraday and SQUID magnetometry) and spin (EPR) susceptibilities indicate low-dimensional Heisenberg antiferromagnetic behavior with the susceptibility tending to zero as the temperature approaches zero. The data are analyzed using several low-dimensional magnetic models and are best fitted to a model consisting of two different spin dimers (Δ1 = 108 K and Δ₂ = 212 K). The static magnetic susceptibility of (BEDT-TTF)₂FeCl₄ is modeled by a sum of Curie–Weiss (S = 5/2 for Fe(d⁵) and θ = −4 K), χ_dp, and single dimer (Δ = 45 K) parameters. The BEDT-TTF layers in these compounds thus behave as Mott—Hubbard-localized systems, and the interaction between the magnetic moment on the Fe with those on the organic layer is negligible.

Three new molecular charge transfer salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), (BEDT-TTF)₄AFe(C₂O₄)₃C₆H₅CN (A = H₂O, K, NH₄), have been prepared, and their crystal structures and physical properties determined. The structures of all three salts consist of successive layers of BEDT-TTF and layers of approximately hexagonal geometry containing alternating A and Fe(C₂O₄)₃³⁻, with C₆H₅CN lying within the hexagonal cavities. When A = K or NH₄ the BEDT-TTF layers consist of daimers (BEDT-TTF)₂²⁺ separated by isolated (BEDT-TTF)⁰, the charge difference being estimated from the C=C and C–S bond lengths. These salts are semiconductors (σ ~ 10⁻⁴ S cm⁻¹, E_A = 0.14 eV), and their magnetic susceptibilities are dominated by S = 5/2Fe^III. The EPR spectra accordingly show only one sharp signal. When A = H₂O the BEDT-TTF adopt the β″ packing, and the salt is a superconductor (T_c 7.0(3) K). The magnetic susceptibility above the critical temperature is the sum of a Pauli component (2 × 10⁻³ emu mol⁻¹) and a Curie—Weiss term. Below the transition the susceptibility depends on the field penetration according to the London penetration depth, increasing with increasing field until the critical field. The Meissner effect is almost complete, indicating absence of pinning. The EPR spectra of the A = H₂O compound are characterized by two resonances, one of Dysonian shape due to the conduction electrons on the organic cations and the other of Lorentzian shape arising from the 3d electrons of Fe^III. Electronic band structure calculations suggest that the A = K compound is semiconducting (E_g = 03 eV) in good agreement with that result obtained from electrical measurement and the A = H₂O is metallic (W = 1.1 eV) with both electron and hole pockets in the Fermi surface. Optical reflectivity of the latter gives an electronic bandwidth of 1.0 eV, fully consistent with the band structure calculation.

Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)₄[A^IM^III(C₂O₄)₃]·PhCN, where A^I = H₃O, NH₄, K; M^III = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene. Attempts to substitute M^III with Ti, Ru, Rh, or Gd are also described. New compounds with M = Co and Al are prepared and detailed structural comparisons are made across the whole series. Compounds with A = H₃O⁺ and M = Cr, Fe are monoclinie (space group C2/c), at 150, 120 K a = 10.240(1) Å, 10.232(12) Å; b = 19.965(1) Å, 20.04(3) Å; c = 34.905(1) Å, 34.97(2) Å;β = 93.69(1)°, 93.25(11)°, respectively, both with Z = 4. These salts are metallic at room temperature, becoming superconducting at 5.5(5) or 8.5(5) K, respectively. A polymorph with A = H₃O⁺ and M = Cr is orthorhombic (Pbcn) with a = 10.371(2) Å, b = 19.518(3) Å, c = 35.646(3) Å, and Z = 4 at 150 K. When A = NH₄⁺, M = Fe, Co, Al, the compounds are also orthorhombic (Pbcn), with a = 10.370(5) Å, 10.340(1) Å, 10.318(7) Å; b = 19.588(12) Å, 19.502(1) Å, 19.460(4) Å; c = 35.790(8) Å, 35.768(1) Å, 35.808(8) Å at 150 K, respectively, with Z = 4. All of the Pbcn phases are semiconducting with activation energies between 0.15 and 0.22 eV. For those compounds which are thought to contain H₃O⁺, Raman spectroscopy or C=C and C—S bond lengths of the BEDT-TTF molecules confirm the presence of H₃O⁺ rather than H₂O. In the monoclinie compounds the BEDT-TTF molecules adopt a β″ packing motif while in the orthorhombic phases (BEDT-TTF)₂ dimers are surrounded by monomers. Raman spectra and bond length analysis for the latter confirm that each molecule of the dimer has a charge of +1 while the remaining donors are neutral. All of the compounds contain approximately hexagonal honeycomb layers of [AM(C₂O₄)₃] and PhCN, with the solvent occupying a cavity bounded by [M(C₂O₄)₃]³⁻ and A. In the monoclinie series each layer contains one enantiomeric conformation of the chiral [M(C₂O₄)₃]³⁻ anions with alternate layers having opposite chirality, whereas in the orthorhombic series the enantiomers form chains within each layer. Analysis of the supramolecular organization at the interface between the cation and anion layers shows that this difference is responsible for the two different BEDT-TTF packing motifs, as a consequence of weak H-bonding interactions between the terminal ethylene groups in the donor and the [M(C₂O₄)₃]³⁻ oxygen atoms.

The syntheses, crystal structures, and physical properties of two new crystalline charge-transfer salts of BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, containing tris(oxalato)metallate(III) anions of 3d elements are reported. Electrochemical oxidation of BEDT-TTF in the presence of (NH₄)₃[Fe(C₂O₄)₃]·3H₂O or (NH₄)₃[Cr(C₂O₄)₃]·3H₂O in C₆H₅NO₂, yields crystals of β″-(BEDT-TTF)₄[A·Fe(C₂O₄)₃]·C₆H₅NO₂ [1] or β″-(BEDT-TTF)₄[A·Cr(C₂O₄)₃]·C₆H₅NO₂ [2] (A = H₃O⁺ or NH₄⁺). The crystal structure of [1] has been solved at 120 K in the monoclinic space group C2/c, and that of [2] in the same space group at 298 and 120 K. For [1], a = 10.273 Å, b = 19.949 Å, c = 35.030 Å, β = 92.97°, V=7169.6(2) ų, Z=8. For [2], at 298 K: a= 10.304 Å, b = 20.091 Å, c=35.251 Å, β = 92.70°, V= 7289.3(2) ų, Z=8, and at 120 K a= 10.283 Å, b = 19.917 Å, c = 34.939 Å, β = 93.30°, V= 7144.4(1) ų, Z=8. The crystal structures of both compounds consist of alternating layers of BEDT-TTF cations and layers containing [M(C₂O₄)₃]³⁻, H₃O⁺ or NH₄⁺, and PhNO₂. The BEDT-TTF molecules are arranged in the β″ packing motif and the tris(oxalato)metallate(III) ions form the well-known honeycomb motif found in many molecular based magnets. SQUID magnetometry, Raman spectroscopy and electron paramagnetic resonance (EPR) measurements were performed on crystals of [1]. SQUID magnetometry, single-crystal four-probe conductivity measurements, Raman spectroscopy, EPR and polarised infrared reflectance were performed on crystals of [2]. Both compounds have metal to superconducting transitions with T_c = 6.2K for [1] and for [2], T_c = 5.8 K.

No abstract received.

Making new compounds that combine different physical properties in the same crystal lattice is a significant goal for chemists working on synthetic metals. As part of our ongoing programme to synthesise molecular conductors containing embedded magnetic moments we have investigated charge transfer salts of BEDT-TTF with tris-oxalatometallate (III) ions M(C₂O₄)₃³⁻ (M = Cr, Fe). The series (BEDT-TTF)_x [AM(C₂O₄)₃]_y(C₆H₅CN)_z is especially rich and interesting since it contains semiconductors (A = K, NH₄; M = Cr, Fe) and the first molecular superconductor containing paramagnetic ions (A = H₂O; M = Fe; x = 4, y = z = 1). The steric and charge factors determining the geometry of cation and anion layers in these compounds are surveyed, as well as the relation between the crystal structures and physical properties.

The intramolecular bond lengths of the donor BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene, also ET) are sensitive to the charge carried by the molecule. By considering a large number of ET salts, we have developed a method for determining experimentally the charges of the ET molecules. The standard deviation in charge is only 0.1 for high quality structures.

Two new charge-transfer salts [bettf]₂[FeCl₄] (1) and [bettf] [FeBr₄] (2) have been prepared and crystallised electrochemically [bettf = bis(ethylenedithio)tetrathiafulvalene]. The crystal structures of both salts have been refined in the PĪ space group, the unit-cell parameters being: (1), a = 6.626(2), b = 15.025(2), c = 17.805(2), α = 82.80(2), β = 89.53(2), γ = 88.15(2), Z = 2, R = 0.044; (2), a = 8.634(2), b = 10.980(2), c = 11.773(2), Å, α = 91.91 (2), β = 102.84(2), γ = 93.73(2), Z = 2, R = 0.054. The structure of (1) consists of stacks of dimerised bettf molecules with short S · · · S contacts, forming layers separated by sheets of FeCl₄⁻. In (2) there are no stacks of bettf molecules: the structure consists of discrete dimers separated by FeBr₄⁻. Compound (1) shows semi-conducting behaviour from 160 to 300 K with ɛ_a = 0.21 eV and σ ≈ 10⁻² S cm⁻¹ at 300 K while (2) is a quasi-insulator at room temperature with σ ≈ 10⁻⁶S cm⁻¹. The magnetic susceptibility of both salts from 5 to 300 K is dominated by the with small Weiss constants [−6(1) for (1) and −5(1)K for (2)].

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