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The optical spectral band positions and spin-Hamiltonian parameters (g factors g_‖, g_⊥ and zero-field splitting D) of CdS: Ti²⁺ and CdSe: Ti²⁺ crystals are calculated from the complete diagonalizaion (of energy matrix) method based on a two-spin-orbit parameter model for 3d² ions in trigonal symmetry. In the model, both the contribution to spin-Hamiltonian parameters due to the spin-orbit parameter of central 3d² ions and that of ligand ions are included. The crystal field parameters used in the calculations are obtained from the superposition model which enables correlation of the optical and EPR spectral data with the defect structure of the studied paramagnetic impurity centers in crystals. From the calculations, the defect structures of Ti²⁺ centers in CdS: Ti²⁺ and CdSe: Ti²⁺ are acquired, the signs of zero-field splittings D are suggested, and the optical band positions and spin-Hamiltonian parameters are explained. The results are discussed.

Zr:Nd:LiNbO₃ crystals codoped with 0.1 mol% of Nd₂O₃ and three concentrations of ZrO₂ (0, 2 and 4 mol%) were grown by the Czochralski method from the congruent melt. The X-ray diffraction (XRD) patterns, UV–visible absorption and infrared (IR) spectra were measured to analyze the crystal composition and defect structure. The blue and green upconversion emissions of Nd³⁺ ions under 598 nm excitation were observed. The intensity of upconversion emissions was increased by the introduction of 2 mol% zirconium ion (Zr⁴⁺) and decreased by the introduction of 4 mol% Zr⁴⁺ ions. The luminescence decay measurement indicated that the ⁴G_7/2 state of Nd³⁺ ion was mainly populated by excited state absorption process. It was proposed that Nd:LiNbO₃ crystals doped with approximately 2 mol% Zr⁴⁺ ions could be applied as laser materials at 522/535 nm.

The effects of differently charged Ge impurities on the local atomic structure and lattice dynamics of $\alpha$-quartz were studied. We have determined the equilibrium structures and calculated the symmetrized local density of vibrational states for the Ge-doped $\alpha$-quartz. The frequencies of localized vibrations of $A$- and $B$-symmetries induced by Ge impurities were obtained. Besides, we have analyzed what contribution the vibrations of atoms located around the Ge impurities make to the localized symmetrized vibrations.

A series of In:Mn:Fe:LiNbO₃ crystals were prepared with fixed concentrations of Fe₂O₃, MnCO₃ and various concentrations of In₂O₃. Their ultraviolet-visible (UV-Vis) absorption spectra were measured in order to investigate their defect structure. The optical damage resistance of In:Mn:Fe:LiNbO₃ crystals was characterized by the transmitted beam pattern distortion method. It increases remarkably when the concentration of In₂O₃ is over its threshold concentration. The optical damage resistance of In(3.0 mol%):Mn:Fe:LiNbO₃ crystal is two orders of magnitude higher that of the Mn:Fe:LiNbO₃ crystal. The dependence of the defects on the optical damage resistance of the In:Mn:Fe:LiNbO₃ crystals was discussed.

Mg:Er:LiNbO₃ crystals were grown by the Czochralski method. Their UV-Vis absorption edges and OH^- transmission spectra were measured in terms of the defect structure. The absorption edges shift towards a shorter wavelength with increasing Mg concentration. The dependence of the shift mechanism of the absorption edge on the defect structure was discussed in detail. The OH^- absorption bands shift from 3486 cm^-1 to 3535 cm^-1 when the Mg concentration exceeds a threshold value. It is considered that the 3535 cm^-1 absorption band corresponds to the (MgNb)^3-–OH^- complexes.

The local structure and the spin Hamiltonian parameters (the zero-field splitting D, the g factors g_//, g_⊥ and the hyperfine structure constants A_// and A_⊥) for the trigonal Mn²⁺ center in Bi₄Ge₃O₁₂ are theoretically studied from the perturbation formulas of these parameters for a 3d⁵ ion in trigonal symmetry. The impurity Mn²⁺ replacing host Bi³⁺ is not found to occupy the exact Bi³⁺ site but to suffer a large off-center displacement by about 0.36 Å towards the center of the oxygen octahedron along the C₃-axis due to the size and charge mismatching substitution. The calculated spin Hamiltonian parameters based on the above displacement show good agreement with the observed values. The results and the mechanism of the impurity displacement are discussed.

Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various ratios of Li/Nb = 0.946, 1.05, 1.20 and 1.38 in the melt. The crystal composition and defect structure were analyzed by XRD, UV-Vis and IR spectroscopy. The results show that the threshold concentration of Hf in LiNbO₃ crystals decrease with the increasing of the Li/Nb ratio; when the Li/Nb ratio is 1.05, the threshold concentration of Hf is less than 2 mol%, largely under the threshold concentration of Hf ions in congruent Hf:Fe:LiNbO₃ crystal (4 mol).^1–3 With the increase of Li/Nb, Hf ions first replace the ; when the concentration of Hf ions is higher than the threshold value, Hf ions occurs on normal Nb and Li sites.

A series of Hf, Er co-doped LiNbO₃ crystals were grown by Czochralski technique with 1 mol% of Er₂O₃ and with 2, 4, 6 and 8 mol% of HfO₂, respectively. The optical damage resistance of Hf:Er:LiNbO₃ crystals was studied by the transmitted beam pattern distortion method. The optical damage resistance of Hf (6 mol%): Er:LiNbO₃ crystals is about two orders of magnitude higher than that in Hf:Er:LiNbO₃. The X-ray power diffraction, the ultraviolet-visible absorption spectra and the infrared absorption spectrum were measured and discussed in terms of the spectrometric characterization and the defect structure of crystals. The results showed that with mild co-doping with HfO₂, Er³⁺ substitutes Nb⁵⁺, whereas with heavy co-doping, a part of Er³⁺ substitutes Li⁺. The structure defects were discussed in this paper to explain the improvement of the optical damage resistance in the Hf:Er:LiNbO₃.

Six crystal field energy levels obtained from the photoluminescence spectra of Yb³⁺-doped CuInS₂ semiconductor crystal are calculated from the diagonalization (of energy matrix) method. The root-mean-square (r.m.s) deviation σ is 4.5 cm^-1 and so the calculated results are in good agreement with the experimental values. The local tetragonal distortion angle θ for the Yb³⁺ center in CuInS₂ (which is different from the corresponding angle θ_h in the host crystal) is obtained from the calculation.

Congruent Ho³⁺ (1 mol.%): LiNbO₃ crystals codoped with MgO (X mol.%, X = 1, 3, 5 and 7) were grown by the Czochralski technique. The ultraviolet-visible (UV-Vis) and infrared (IR) spectra were measured in order to analyze the defect structure of the crystals. The concentrations of Mg, Ho, Li and Nb in the crystals were carried out with an inductively coupled plasma atomic emission spectrometer. Experimental results indicates as Mg²⁺ doping concentration increases in melt, the distribution coefficients of Mg and Ho ions decrease, the Li/Nb ratio in the crystals decreases first and then increases, and the absorption edge shifts to a shorter wavelength. The light-induced scattering of Mg:Ho:LiNbO₃ crystals was quantitatively scaled via the incident exposure energy. The results demonstrated that Mg (7 mol.%): Ho:LiNbO₃ crystal had the weakest light-induced scattering and the mechanism related to their defect structures was discussed.

Point defect structure of B2 TrSc (Tr$=$Cd, Ru) alloys was investigated using supercell and special quasi-random structure (SQS) approaches. According to our results, Tr and Sc anti-sites are the constitutional point defects in Tr-rich and Sc-rich B2 TrSc, respectively. To investigate the thermal defect concentrations at finite temperatures, we adopted the Wagner–Schottky model using point defect formation enthalpies obtained from supercell and SQS approaches. The present results suggest that the predominant thermal defects in B2 CdSc are of exchange type, and in B2 RuSc are of interbranch Sc type. The calculated results show an agreement with the available theoretical and experimental data.

In this paper, a series of Yb (0.5mol.%):Ho (0.5mol.%):LiNbO₃ crystals doped with various concentration of Mg${}^{2+}$ (1, 3, 5 and 7mol.%) were grown by the Czochralski technique. The ability of optical damage resistance of Mg:Yb:Ho:LiNbO₃ crystals increases with increasing the Mg${}^{2+}$ doping concentration. The optical homogeneity of Mg:Yb:Ho:LiNbO₃ crystals doped with different concentration of Mg${}^{2+}$ was detected using the birefringence gradient method. The results demonstrated that the optical homogeneity is getting better with the increase of the Mg${}^{2+}$ doping concentration. The studies on the infrared transmission spectra indicated that Mg${}^{2+}$ ions first replaces anti-site Nb${}_{\mathrm{\text{Li}}}^{4+}$ in the form of Mg${}_{\mathrm{\text{Li}}}^{+}$ defect, once the concentration of Mg${}^{2+}$ reaches or exceeds the threshold concentration, it begins to substitute Li-site and Nb-site of normal lattice and form defect of Mg${}_{\mathrm{\text{Nb}}}^{3-}-$3Mg${}_{\mathrm{\text{Li}}}^{+}$. Therefore, the change of Mg${}^{2+}$ doping concentration is the fundamental reason leading to a violet shift of the OH${}^{-}$ absorption peak.

Zr:Yb:Tm:LiNbO₃ crystals with various [Li]/[Nb] ratios (0.946, 1.05, 1.20 and 1.38) were grown by the Czochralski technique. Distribution coefficients of Zr${}^{4+}$, Yb${}^{3+}$ and Tm${}^{3+}$ ions were analyzed by the inductively coupled plasma-atomic emission spectrometer (ICP-AES). The influence of [Li]/[Nb] ratio on the composition and defect structure of Zr:Yb:Tm:LiNbO₃ crystals was investigated by X-ray diffraction and IR transmission spectrum. The results show that as the [Li]/[Nb] ratio increases in the melt, the distribution coefficients of Yb${}^{3+}$ and Tm${}^{3+}$ ions both increase while that of Zr${}^{4+}$ ion deceases. When the [Li]/[Nb] ratio increases to 1.20 in the melt, Zr:Yb:Tm:LiNbO₃ crystal is nearly stoichiometric. In addition, when the [Li]/[Nb] ratio reaches up to 1.38, Nb${}_{\mathrm{\text{Li}}}^{4+}$ are completely replaced and Li${}^{+}$ starts to impel the Zr${}^{4+}$, Yb${}^{3+}$ and Tm${}^{3+}$ into the normal Li sites.

Lithium niobate is a crystal with excellent electric-optic and nonlinear optical properties. Double-doped and multi-doped lithium niobate are widely used in a wide variety of optical devices. A series of Zn:Ru:Fe:LiNbO₃ crystals with different lithium-niobium molar ([Li]/[Nb]) ratios were grown by the Czochralski method. The structures of the crystals with different [Li]/[Nb] ratios are analyzed using X-ray diffraction patterns and infrared spectra. The effect of the [Li]/[Nb] ratio dependence of the doped element concentration and segregation coefficient was studied using an inductive coupled plasma-atomic emission spectrometer (ICP-AES). It was found that the doping elements do not change the crystal structure, and the change of the [Li]/[Nb] ratio will affect the concentration and the occupancy of the doped ions in the crystal. The segregation coefficient and defect structures can be controlled by different [Li]/[Nb] ratios in Zn:Ru:Fe:LiNbO₃ crystal.

Zn (1, 3, 5, 7 mol.%): Dy: LiNbO₃ crystals were prepared by the traditional vertical pull-up method. The effective segregation coefficients of ${\text{Zn}}^{2+}$ and ${\text{Dy}}^{3+}$ were found to decrease with increasing ${\text{Zn}}^{2+}$ concentration using ICP-AES analysis test. The lattice size was found to increase first and then decrease using XRD analysis test. The absorption spectrum was found blue shifted and produced two peaks using IR. Research shows that when the Zn content reaches or exceeds the threshold value, ${\mathrm{\text{Nb}}}_{\mathrm{\text{Li}}}^{4+}$ is replaced completely, and Zn occupies the Li and Nb positions of lithium niobate, thus introducing the non-intrinsic defect ${\mathrm{\text{Zn}}}_{\mathrm{\text{Li}}}^{+}$ and ${\mathrm{\text{Zn}}}_{\mathrm{\text{Nb}}}^{3-}$. Both defects are able to form ${\mathrm{\text{3Zn}}}_{\mathrm{\text{Li}}}^{+}-{\mathrm{\text{Zn}}}_{\mathrm{\text{Nb}}}^{3-}$ charge self-compensating defect groups and inhibit the charge transport process and thus will greatly increase the photodamage resistance.

Mg:Fe:Cu:LiNbO₃ crystals with different [Li]/[Nb] ratios (0.946, 1.05, 1.20, 1.38) were prepared by the Czochralski method. The crystal structure and the occupancy of impurity ions were analyzed by X-ray diffraction (XRD). The effective segregation coefficient was analyzed by an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The optical uniformity of the crystal is analyzed by birefringence gradient. The results show that the dopant ions do not change the crystal structure, and the concentration of dopant ions changes with the ratio of [Li]/[Nb]. The greater the ratio of [Li]/[Nb], the better the optical uniformity. Finally,we conclude that when the [Li]/[Nb] ratio approaches 1.20, the intrinsic defects of Mg:Fe:Cu:LiNbO₃ crystals almost disappear and the crystals approach the stoichiometric ratio. When the [Li]/[Nb] ratio is 1.38, the optical uniformity of the crystal is the best.

Zn:Yb:Tm:LiNbO₃ crystals with a range of [Li]/[Nb] ratios (0.94, 1.05, 1.20, and 1.38) were grown by Czochraski technique. The influence of [Li]/[Nb] ratio on the concentration of doped elements in grown crystal was investigated by inductively coupled plasma atomic emission spectrometry (ICP-AES). The defect structures formed at different [Li]/[Nb] ratios were studied by X-ray diffraction and INFRARED spectroscopy. The results show that, when the [Li]/[Nb] ratio increases, the segregation coefficients of ${\text{Yb}}^{3+}$ and ${\text{Tm}}^{3+}$ ions increase, while the segregation coefficient of ${\text{Zn}}^{2+}$ ions decreases. When the [Li]/[Nb] ratio in the melt increases to 1.20, the Zn:Yb:Tm:LiNbO₃ crystal approaches the stoichiometric ratio.

The defect structure resulting from copper doping of potassium niobate (KNbO₃) ceramics was investigated by a combined analysis of electron paramagnetic resonance (EPR) spectroscopy and first-principles calculations based on density functional theory (DFT). The results indicate that under atmospheric oxygen partial pressure, Cu preferentially substitutes on the Nb sites where it can trap one or two oxygen vacancies. Correspondingly, for 0.25 mol% Cu doped KNbO₃ ceramics, two types of defect associates are formed — and . The association of Cu impurities and oxygen vacancies lowers the defect formation energy by 1.0 eV to 2.7 eV, depending on the Fermi level. Owing to the opposite charges of these two types of defects, overall charge neutrality is theoretically possible by mutual compensation of the defect associates, without formation of additional non-associated oxygen vacancies.

The defect structure of barium titanate (BaTiO₃) heat-treated under reduced p_O2 has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The results confirm the formation of -centers. Based on the relative size of the principal values of the electron g-matrix, g_⊥ > g_‖, the local distortion of the oxygen octahedron about the -centers was also analyzed and the formation of defect associates unambiguously established.

Eu²⁺-doped CsBr in form of needle image plates is a promising alternative to the granular Eu²⁺:BaFBr X-ray storage phosphor with respect to photo-stimulated luminescence, yield and spatial resolution. However, in order to understand the corresponding mechanisms on an atomic scale, the defect structure of Eu²⁺-doped CsBr photostimulable X-ray storage phosphors has to be analyzed. By means of high-frequency electron paramagnetic resonance (EPR) spectroscopy, three types of defects have been observed — an electrically neutral dimeric defect complex and a neutral trimeric defect complex (F_Z-center), as well as acceptor-type F-centers that are generated by the trapping of electrons at initially neutral bromine vacancies.