AN INVESTIGATION FOR THE Cd2+-VACANCIES IN RbCdF3 and CsCdF3 CRYSTALS DOPED WITH Fe3+ IMPURITIES

This paper presents high-order perturbation calculations for the zero-field splitting parameters D, F, and a of Fe³⁺-V_Cd pairs in RbCdF₃ and CsCdF₃ crystals doped with Fe³⁺ impurities. In comparison with experimental data, the present results are better than those obtained in previous work by using the phenomenological su-perposition model of spin-Hamiltonian. Moreover, the crystal fields arising from the Cd²⁺-vacancy and their induced crystal lattice distortion are analysed and calculated. It is found that the contribution from the Cd²⁺-vacancies cannot be ignored. Mena-while, we obtain that a ligand F^- along the [001]-axis moves towards the central Fe³⁺ ion by Δ= 0.0020nm in RbCdF₃ and 0.0028nm in CsCdF₃. These results are quite different from Δ = 0.0104nm and Δ = 0.0112nm, respectively, obtained in previous work by using the superposition model of spin-Hamiltonian and ignoring the effect of the vacancy.     

ESR of Fe3+ -Rhombic,Fe2+ -Vo and Fe1+ -Vo complexes in SrTiO3

Three new Fe-defect complexes have been identified by ESR in Fe doped crystals of SrTiO₃. These are an Fe³⁺ center with rhombic fine structure in the “as-grown” crystals, and the vacancy associated complexes Fe²⁺ -V_o and Fe¹⁺ -V_o in “reduced” crystals. Photo-excited charge transfer effects are observed for these centers.     

ANISOTROPY OF OPTICAL ABSORPTION INTENSITY IN Tm3+ DOPED YVO4 CRYSTALS

Seven absorption group-bands (¹D₂, ¹G₄, ³F₂, ³F₃, ³H₄, ³H₅, ³F₄) of Tm³⁺ in YVO₄ single crystals have been observed in the orientation absorption spectra recorded in the spectral range from 200 to 4000 nm at 300K. The integrated absorption cross section for each group-band was accurately evaluated. On the assumption that the anisotropy of this uni-axial crystal is small, the Judd-Ofelt theory was extended for the calculation of 4f-4f transition intensities of Tm³⁺ in YVO₄. Two sets of phenomenological intensity parameters were derived from a least-squares-fit procedure. For c-axis cut sample we have Ω₂=10.18 (10^-20cm²), Ω₄=1.96 (10^-20cm²), Ω₆=0.75 (10^-20cm²).For a-axis cut sample we have Ω₂=8.20 (10^-20cm²), Ω₄=2.47 (10^-20cm²), Ω₆=0.91 (10^-20cm²). The anisotropy of the optical absorption intensities of Tm³⁺:YVO₄ was theoretically analyzed in detail.     

SPECTROSCOPIC INVESTIGATION ON THE PROPERTIES OF VISIBLE AND NEAR INFRARED EMISSIONS IN Tm3+, Ho3+ Co-DOPED YVO4 CRYSTALS

With the help of absorption and fluorescence spectra, the spectroscopic properties of Tm³⁺, Ho³⁺:YVO₄ crystals have been investigated under different dopant concentrations. The absorption results show that the maximum absorption in the spectral range of currently available powerful laser diodes (LD) is at 796 nm, corresponding to the transition ³H₆→³H₄ of Tm³⁺ in the crystals. Upon the excitation with a LD at 808 nm, we have detected ten fluorescence bands in the spectral range spanning from visible to infrared. In visible region 4 upconversion bands have been observed centered at 475, 547, 660, and 701 nm. Discussions of the energy transfer processes suggest that cross relaxations and excited absorptions with the involvement of Tm^{3+ 3}F₄, ³H₅ and Ho^{3+ 7}I₅ states are responsible for the four upconversion emissions. Comprising the relative integrated intersities of 2 μm (Ho³⁺: ⁵I₇→⁵I₈) and 1.8 μm (Tm³⁺: ³F_4→³H₆) bands we have observed an efficient energy transfer from Tm³⁺ (³F₄) to Ho³⁺ (⁵I₇). Examination of the ratios of the intensity of 2 μm band to that of 1.8 μm band as a function of the ratios of Tm³⁺ concentration (c_Tm) to Ho³⁺ concentration (c_Ho) indicates that small concentration ratios (c_Tm/c_Ho) lead to high efficient energy transfers of Tm³⁺(³F₄)→Ho³⁺(⁵I₇).     

Identification of Fe4+ and Fe5+ charge-transfer photochromic absorption bands in SrTiO3

Optical absorption and the enhancement and bleaching of Fe³⁺ and Fe⁵⁺ electron paramagnetic resonance in SrTiO₃: Al measured as a function of wavelength and time show that the photochromic absorption bands are due to electron transfer from O^2- valence states to Fe⁴⁺ and Fe⁵⁺. They occur at 2.09 and 2.82 eV for Fe⁴⁺ and at 1.99 and 2.53 eV for Fe⁵⁺.     

DIELECTRIC AND PYROELECTRIC CHARACTERISTICS AND FERROELECTRIC SOFT MODES IN PbTiO3 POLYCRYSTALS DOPED WITH Ca2+

The effects of Ca²⁺ ions on the structure and dielectric and pyroelectric characteristics of PbTiO₃ polycrystals were investigated. With increasing content of Ca, a decrease in tetragonal axial ratio (c/a) was observed as a consequence of shrinkage of the lattice parameter c. The dielectric constant and pyroelectric coefficient increase and Curie temperature (T_C) shifts toward lower temperature with increasing content of Ca. The theory of ferroelectric soft mode has been developed to illuminate the relationship between the structure and properties of the polycrystal.     

Far infrared absorption of Fe2+ in KMgF3

Far infrared absorption in cubic KMgF₃ doped with Fe²⁺ is reported . A line is observed at 87cm^?1, which is assigned to the (Γ_5g → Γ_3g, Γ_4g) transition in the Fe²⁺. The reduction in the spin-orbit coupling from the free ion value has been predicted by Ham, Schwarz and O'Brien.     

Third and fourth order elastic constants of fluoperovskites CsCdF3, TlCdF3, RbCdF3, RbCaF3

The hydrostatic dependence of the second order elastic constants of several fluoperovskites has been measured and studied from a macroscopic point of view and through a force field analysis. The first part of this paper is devoted to the experimental determination of some combinations of the third and fourth order elastic constants. The second part analyses the pressure dependence of the second order elastic constants from a macroscopic point of view. The large values of the second order anharmonicity coefficients $\text{b}{}_{\mathrm{ij}}\text{=}\text{1}\text{2}\text{(}\text{?}{}^2\text{C}{}_{\mathrm{ij}}\text{?P}{}^2\text{)}{}_0\text{C}{}^{\mathrm{(0)}}{}_{\mathrm{ij}}$ are connected to the occurrence of a phase transition in some compounds, whereas the first order coefficients $\text{(}\text{?C}{}_{\mathrm{ij}}\text{?P}\text{)}{}_0$ have an order of magnitude which agrees with a quasiharmonic hypothesis. The third part is devoted to the microscopic interpretation of the third order elastic constants. A rigid ion model is used with either classical Born Mayer potentials or a modification of these potentials with an axial term (Cowley and Rousseau's models).     

Near-infrared absorption of MgF2:Fe2+

The multiphonon sideband of the spin allowed transition 6T_2g:A₁ ? 2〉 → 6E_g:B₃ ? 2〉 within the 3d⁶ orbital of Fe²⁺ in D_2h symmetry was analyzed with respect to the one phonon absorption and density of states of the coupled phonon modes. Temperature dependence of the zero-phonon line shows, that the axial anisotropy of the S = 2 spin multiplet in the excited B₃ state is about 0.9 cm^-1.     

Theoretical study of local lattice structure of Fe-V cd and Fe-Li systems in RbCdF3 and CsCdF3 crystals

A theoretical method for investigating the inter-relation between the electronic and the molecular structures of a 3d⁵ ion in a tetragonal ligand-field has been established on the basis of a 252×252 complete energy matrix. By means of this method, the local structure of the Fe³⁺-V _cd and Fe³⁺-Li⁺ systems in RbCdF₃:Fe³⁺ and CsCdF₃:Fe³⁺ crystals are determined by the experimental EPR spectrum. Our calculation show that the local lattice structure around an octahedral Fe³⁺ center has a compression distortion along the crystalline axis in RbCdF₃ as well as in CsCdF₃ crystals, and that the compression magnitude of a tetragonal Fe³⁺-Li⁺ system is larger than that of the Fe³⁺-V _cd system. This may be ascribed to the fact that a Fe³⁺ ion replaces a Cd²⁺ ion and a Li⁺ ion substitutes for another Cd²⁺ ion next to the Fe³⁺ ion in the Fe³⁺-Li⁺ system, and the Li⁺ ion will shift to the Fe³⁺ ion, which pushes the F₁ ion toward the Fe³⁺ ion. Using this method, the experimental EPR parameters , and are also interpreted simultaneously.     

Estimation of 4s covalent bonding of Fe2+ and Fe3+ ions in NiO

The charge densities of the 4s electrons in Fe²⁺ and Fe³⁺ ions in the ionic compound, NiO, were measured from an internal conversion experiment and compared with some predicted values on the basis of different theoretical methods.     

ESR-analysis of the chalcopyrite structure: CuGaS2:Fe3+

The electron spin resonance spectrum of Fe³⁺ impurity ions in the ternary semiconductor CuGaS₂ has been analysed. The two magnetically inequivalent metal sites in the chalcopyrite lattice could be distinguished via fourth order terms in the spin Hamiltonian. No serious deviations from stoichiometry could be detected in transport-grown crystals. It was found that traces of iron impurities lead to strong absorption in the visible and near infrared spectral region.     

Stabilization of Fe2+ centers in LiNbO3: Ti waveguides

Holographic measurements in LiNbO₃: Ti waveguides are reported indicating a mode dependent enhancement of photoconductivity. This enhancement can be explained by a stabilization of Fe²⁺ centers in the waveguide which are responsible for optical damage effects.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

Mössbauer spectra of Fe3+ and Fe2+ ions in hexagonal ferrite with w-structure

The hexagonal ferrite Fe₂W = BaFe₂²⁺Fe³⁺₁₆O₂₇ exhibits a sharp ⁵⁷Fe Mössbauer spectrum at 300 and 78 K. All seven sublattices in this complicated crystal structure are detected. Fast electron exchange between Fe²⁺ and Fe³⁺ ions gives rise to sharp lines and makes them indistinguishable. At 5 K the exchange is slow and the Fe²⁺ ions are detected from the presence of a weak subspectrum with broadened lines separated from the main spectrum of the Fe³⁺ ions. Analysis shows that the Fe²⁺ ions reside exclusively on one of the seven sublattices, which is occupied statistically by Fe²⁺ and Fe³⁺ ions in the ratio of 2 : 1. For SrFe²⁺₂Fe³⁺₁₆O₂₇ the situation is the same.     

Electron paramagnetic resonance of Fe3+ in orthorhombic KNbO3

EPR spectra of KNbO₃:Fe single crystals are obtained in the - 10–110°C temperature range. The angular dependence of resonant lines is well reproduced by spin Hamiltonian parameters relevant to a Fe³⁺ impurity ion substituted to Nb ion in KNbO₃ crystal. The temperature behaviour of resonant lines is explained by a quadratic dependence of axial parameter D vs polarization P_s of the form D = βP²_s in the orthorhombic phase.     

Absorption spectra of Bi3+ and Fe3+ in Y3Ga5O12

The absorption spectra of Y₃Ga₅O₁₂:(Fe³⁺), Y₃Ga₅O₁₂:(Bi³⁺) and Y₃Ga₅O₁₂:(Bi³⁺, Fe³⁺) are presented to 40,000 cm^-1. Assignments are discussed and the transitions analysed in terms of their involvement in the large Faraday rotation observed in bismuth substituted iron garnets.     

Effects of Eu3+  Fe3+ exchange interaction on the spin Hamiltonian parameters of Fe3+ in EuGaG

The anisotropic exchange $\text{1}\text{3}\text{a}{}_{\mathrm{\mu \nu }}\text{Y}{}_{\mathrm{\mu \nu }}\text{(L}{}_{\mathrm{<mtext>Eu</mtext>}}\text{)S}{}_{\mathrm{<mtext>Eu</mtext>}}\text{· S}{}_{\mathrm{<mtext>Fe</mtext>}}\text{(0 < μ ? 6)}$ is incorporated with the isotropic exchange ?2 a₀₀S_Eu · S_Fe to interpret the observed spin-Hamiltonian parameters g and D of Fe³⁺ doped into EuGaG. Calculations from the observed g shifts yield a value of a₀₀ equal to 0.01 K. In order to explain the observed D shift, it is concluded that the spherical harmonics Y_μν(L_Eu) with μ > 2 are of dominant importance.     

The sign of Fe3+OCr3+ superexchange determined from Fe57 hyperfine field measurements

The sign of the super-transferred hyperfine field at an impurity site is shown to depend on the sign of the impurity-host superexchange interaction. This fact is used to determine that the Fe³⁺OCr³⁺ superexchange in YCrO₃ is antiferromagnetic.     

Electron paramagnetic resonance of Fe3+ in KNbO3 at 300 K

In the orthorhombic phase of KnbO₃ an EPR spectrum of Fe³⁺ substitutional for Nb⁵⁺ has been observed with the main axis parallel to the pseudocubic one of the orthorhombic phase. The cubic parameter a is very close to the one in BaTiO₃. A second spectrum probably due to local charge compensation is also reported.