Tetragonal Site Symmetry Research Articles

Site-selective time resolved laser spectroscopy and DFT studies of Nd3+ optical centers in BaF2 doped crystals

Site-selective time-resolved laser spectroscopy techniques, which provide high spectral resolution at low temperatures (4.2 K and 6.5 K), were used to clarify the formation mechanism, spectral and kinetic properties of the Nd3+ ion single-site optical centers formed in the fluorite-type BaF2 single crystals doped with Nd3+. This formation occurs by heterovalent substitution of Ba2+ host cations with additional charge compensation by F− anions. To determine the lifetimes of the Stark 4F3/2(1) levels of the Nd3+ impurity centers, the fluorescence decay kinetics was measured using the gated photon counting technique. The data from spectroscopic experiments were combined with density functional theory (DFT) calculations to identify and analyze the local structure of Nd3+ cations in the Nd3+ ions doped BaF2 crystal. Our studies have shown that several types of single-site Nd3+ optical centers can be distinguished in BaF2:Nd3+. We confirmed the existence of an optical center with high C4v site symmetry and a long radiative lifetime of the 4F3/2(1) state τ = 14.3 ms. We also describe two other configurations of centers with local structure characterized by low site symmetry of Cs, which were identified and described for the first time. The decrease in the tetragonal site symmetry from C4v to Cs was explained by the fact that the Nd3+ dopants, distributed over the equivalent cationic positions of the fluorite-type crystal lattice, coordinate to anionic motifs that essentially differ in the position of the additional charge compensator F−. This structural variability induced in the local coordination environment of the optical center affects its spectroscopic properties through a shift in the energy of the 4F3/2(1) Stark level of the Nd3+ ion and results in a different radiative lifetime τ.

Structural and electrical properties of copper ferrite (CuFe2O4) NPs

Spinel ferrites are the important class of soft magnetic material composed of iron oxide and metal oxide as their main compound. Copper ferrite (CuFe2O4) is one such spinel ferrite with great potential technological applications. It is well-known about ‘Jahn-Teller distortion’ that the copper ferro-spinel are at room temperature has a co-operative distortion to the tetragonal site-symmetry which depends only upon the copper ion distribution. The structural and electrical properties of Copper ferrite were investigated by X-ray diffraction and DC-resistivity studies. The room temperature XRD data revealed that the formation of tetragonal structure of (CuFe2O4) NPs. The crystallite size (t) nm was obtained from Debye-Sherrer’s formula indicated that the (CuFe2O4) NPs shows nanocrystalline nature with crystallite size 14 nm. The lattice constant (a) was found to be in the reported range (∼8.380 Ǻ). The DC-resistivity parameters were drawn from the two point probe device. An activation energy AE of the copper ferrite nanoparticles was found to be 0.01 eV.

Structural, optical, and luminescence properties of Cu2+-doped Ca-Li hydroxyapatite nanopowders prepared by mechanochemical synthesis

Novel luminescent phosphor materials have potential applications in lighting, display devices, and optical communication. In this study, novel Cu2+-doped calcium-lithium hydroxyapatite (CLHA) nanopowders were successfully fabricated by a simple mechanochemical technique. The synthesized samples were characterized by several microscopic and spectroscopic techniques. The hexagonal phase of the prepared sample was confirmed by x-ray diffraction results, and the average crystallite sizes were evaluated. Fourier transform infrared spectra demonstrated the formation of numerous vibrational modes ascribed to phosphate molecules and other hydroxyl groups. The morphologies of the Cu2+-doped CLHA nanopowders were investigated by scanning electron microscopy and transmission electron microscopy analyses. Optical absorption and electron paramagnetic resonance data indicated that the distortion of Cu2+ in the host lattice corresponds to the tetragonal site symmetry. Photoluminescence (PL) spectra of the Cu2+-doped CLHA nanopowders revealed strong blue and weak green emission peaks. From the PL emission spectra, the chromaticity CIE parameters were evaluated, and the results indicated that the prepared phosphor materials may be useful for lighting and display devices.

Effects of ionizing radiations on the optical properties of ionic copper-activated sol-gel silica glasses

Studying the impact of radiations on doped silica glasses is essential for several technological applications. Herein, bulk silica glasses, activated with various concentrations of luminescent monovalent copper (Cu+), have been prepared using the sol-gel technique. Thereafter, these glasses were subjected to X- or γ−rays irradiation at 1 MGy(SiO2) accumulated dose. The effect of these ionizing radiations on the optical properties of these glasses, as a function of the Cu-doping content, were investigated using optical absorption and photoluminescence spectroscopies. Before any irradiation, the glass with the lowest copper concentration exhibits blue and green luminescence bands under UV excitation, suggesting that Cu+ ions occupy both cubic and tetragonal symmetry sites. However, at higher Cu-doping level, only the green emission band exists. Moreover, we showed that the hydroxyl content decreases with increasing copper doping concentration. Both X and γ radiation exposures induced visible absorption due to HC1 color centers in the highly Cu-doped glasses. In the case of the lower Cu-doped glass, the Cu+ sites with a cubic symmetry are transformed into sites with tetragonal symmetry.

Electron magnetic resonance data on high-spin Mn(III; S = 2) ions in porphyrinic and salen complexes modeled by microscopic spin Hamiltonian approach

The spin Hamiltonian (SH) parameters experimentally determined by EMR (EPR) may be corroborated or otherwise using various theoretical modeling approaches. To this end semiempirical modeling is carried out for high-spin (S=2) manganese (III) 3d4 ions in complex of tetraphenylporphyrinato manganese (III) chloride (MnTPPCl). This modeling utilizes the microscopic spin Hamiltonians (MSH) approach developed for the 3d4 and 3d6 ions with spin S=2 at orthorhombic and tetragonal symmetry sites in crystals, which exhibit an orbital singlet ground state. Calculations of the zero-field splitting (ZFS) parameters and the Zeeman electronic (Ze) factors (g||=gz, g⊥=gx=gy) are carried out for wide ranges of values of the microscopic parameters using the MSH/VBA package. This enables to examine the dependence of the theoretically determined ZFS parameters bkq (in the Stevens notation) and the Zeeman factors gi on the spin-orbit (λ), spin-spin (ρ) coupling constant, and the ligand-field energy levels (Δi) within the 5D multiplet. The results are presented in suitable tables and graphs. The values of λ, ρ, and Δi best describing Mn(III) ions in MnTPPCl are determined by matching the theoretical second-rank ZFSP b20(D) parameter and the experimental one. The fourth-rank ZFS parameters (b40, b44) and the ρ (spin-spin)-related contributions, which have been omitted in previous studies, are considered for the first time here and are found important. Semiempirical modeling results are compared with those obtained recently by the density functional theory (DFT) and/or ab initio methods.

Synthesis, structure and biological activities of mixed ligand copper(II) and nickel(II) complexes of N′-(1E)-[(5-bromo-2-hydroxyphenyl)methylidene]benzoylhydrazone

We have made an effort to synthesize ternary complexes [Cu2(L)2(Him)2]·CH3OH 1, [Cu(L)(bipy)] 2 and [Ni(HL)(PMDT)]BF43 (H2L=N′-(1E)-[(5-bromo-2-hydroxyphenyl)methylidene]benzoylhydrazone; Him=imidazole; bipy=2,2′-bipyridine and PMDT=N,N,N′,N″,N″-pentamethyldiethylenetriamine). Their crystal structures were determined by X-ray diffraction method. Single crystal X-ray of 1 revealed the geometry of phenoxo bridged copper center and the distance between the two Cu(II) ion is 3.245Å. The two copper ions are penta coordinated with distorted square pyramidal geometries. Magnetic measurements of dicopper complex 1 exhibited the presence of antiferromagnetic exchange interaction. The observed trend in the spin Hamiltonian parameters in complexes 1 and 2g||>g⊥>2.0023 and |A⊥|≪|A|||≈163–173×10−4cm−1) indicates dx2-y2 band ground state with tetragonal site symmetry for copper(II). Complex 3 was crystallized in orthorhombic crystal system having six coordination numbers. Superoxide dismutase activities of all these complexes have been revealed to catalyze the dismutation of superoxide (O2-) and IC50 values were evaluated. DNA cleavage properties and antibacterial activity have been measured and discussed.

Investigations of the spin-Hamiltonian parameters and tetragonal compression due to the static Jahn–Teller effect for Cr 5+ center in cubic SrTiO 3 crystal

By using the distortion model that the tetragonal site symmetry of Cr 5+ in cubic phase of SrTiO 3 originates from the static Jahn–Teller effect, the spin-Hamiltonian parameters ( g factors g ||, g ⊥ and hyperfine structure constants A ||, A ⊥) of this tetragonally compressed Cr 5+ center in SrTiO 3 are calculated from the high-order perturbation formulas based on a two-mechanism model. In the model, not only the contribution to spin-Hamiltonian parameters due to the crystal-field mechanism, but also that due to the charge-transfer mechanism (which is omitted in the extensively used crystal-field theory) are included. The calculated results are in reasonable agreement with the experimental values. The tetragonal compression (characterized by Δ R= R ⊥− R ||, where R ⊥ and R || denote the metal–ligand distances perpendicular to and parallel with the C 4 axis) of Cr 5+ center is also obtained from the calculation. The results (including the tetragonal compression and the relative importance of the charge-transfer mechanism) are discussed.

Superposition model analysis of the zero-field splitting parameters of Fe 3+ doped in TlInS 2 crystal – Low symmetry aspects

In this study, using the superposition model (SPM) and crystallographic data, the second- and fourth-rank zero-field splitting (ZFS) parameters (ZFSPs) have been calculated for Fe 3+ ions at two In sites in TlInS 2 single crystal. Since crystallographic data indicate C 1 site symmetry, a full triclinic ZFS Hamiltonian has been employed. Model parameters were adjusted so to obtain best agreement between the SPM calculated ZFSPs and the second-rank ZFSPs measured by electron magnetic resonance (EMR). The effects of site symmetry on the theoretically predicted ZFSPs and interpretation of the available experimental ZFSPs have been considered. The experimental second-rank ZFSPs were obtained using approximated orthorhombic spin Hamiltonian. Hence, to facilitate comparison with the SPM calculated ZFSPs, the ascent in symmetry method has been applied to the crystallographic data to quantify the structural approximation from triclinic to orthorhombic and to tetragonal site symmetry. Our considerations provide additional structural information, especially concerning the low site symmetry aspects, pertinent for Fe 3+ impurity centers in TlInS 2 and related crystals. Our SPM analysis of ZFSPs indicates that satisfactory so tentative agreement can be achieved between the theoretical predictions and the experimentally measured values of the second-rank ZFSPs. The procedure proposed here may serve as a general framework for modelling of ZFSPs for other ion-host systems. More accurate modelling for Fe 3+ ions in TlInS 2 may be carried out when better quality EMR results taking into account the low symmetry effects in EMR spectra and the values of the fourth-rank ZFSPs become available. The results of SPM calculations support earlier suggestions that Fe 3+ ions substitute for In 3+ ions in TlInS 2.

Study of the Kramers rare earth ions ground multiplet with a large orbital contribution by multifrequency EPR spectroscopy: in scintillator

Abstract A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3 + impurities in PbWO 4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g ‖ and g ⊥ , are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g ⊥ -parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S 4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce 3 + ions with tetragonal symmetry S 4 in the PbWO 4 scintillator.

Synthesis, structure, magnetism, and spectroscopic properties of heterobinuclear copper(II)-zinc(II) complexes and their copper(II)-copper(II) analogues in asymmetric ligand environments.

Heterobinuclear copper(II)-zinc(II) complexes and their homobinuclear dicopper(II) counterparts (1-4) of two asymmetric ligands (H2L1 and H2L2), based on 2-aminocyclopent-1-ene-1-dithiocarboxylate, are reported. The ligands are capable of providing both donor set and coordination number asymmetry in tandem. Metal centers in these complexes are connected by a micro-alkoxo and a bridging pyrazolate moiety, as confirmed by X-ray structure analyses of 1, 3, and 4. The Cu(1) site in the dicopper complex (1) is square planar and so are the copper sites in the Cu-Zn complexes 3 and 4. The pentacoordinated Zn sites in the latter complexes have distorted TBP geometry (tau = 0.74), while the corresponding Cu site in 1 has a highly distorted square pyramidal structure (tau = 0.54). The Cu...Zn separations in 3 and 4 are 3.3782 and 3.3403 angstroms, respectively, while the Cu...Cu distance in 1 is 3.3687 angstroms. The dicopper complexes are EPR silent at 77 K, in which the copper(II) centers are coupled by strong antiferromagnetic coupling (J = ca. -290 cm(-1)) as confirmed by variable-temperature (4-300 K) magnetic measurements. These compounds (1 and 2) undergo two one-electron reductions and a single step two-electron oxidation at ca. -0.26, -1.40, and 1.0 V vs Ag/AgCl reference, respectively, as indicated by cyclic and differential pulse voltammetry done at subambient temperatures. EPR spectra of 3 and 4 display axial anisotropy at 77 K with the gperpendicular region being split into multiple lines due to N-superhyperfine coupling (AN = 15.3 x 10(-4) cm(-1)). The observed trend in the spin-Hamiltonian parameters, gparallel > gperpendicular > 2.04 and |Aperpendicular| << |Aparallel| approximately (120-150) x 10(-4) cm(-1), indicates a d(x2-y2)-based ground state with tetragonal site symmetry for the Cu(II) center in these molecules.

Importance of the chromophore orientation to the ligand-to-metal energy transfer in lanthanide complexes with pendant-arm fitted cyclen derivatives

Comparative solid state structural studies of three lanthanide macrocyclic complexes derived from 1,4,7,10-tetraazacyclododecane with pendant arms bearing amide co-ordinating groups have been performed in order to evaluate the parameters influencing the co-ordination polyhedron and to assess the importance of the geometric factor in energy transfer processes. In all the investigated structures the co-ordination geometry is a mono-capped twisted square antiprism, a situation commonly observed for similar compounds. High resolution luminescence spectra of the europium complexes are consistent with a tetragonal site symmetry for the metal ion. An analysis of the presented crystal structures and of previously reported ones indicates that (i) the relative orientation of the O4 and N4 planes is not determined by the co-ordinated solvent molecule and (ii) the twist angle between them is mainly dictated by the flexibility of the pendant arms. Interpretation of the luminescence properties of the complexes of Sm(III), Eu(III) and Tb(III) can be made from the structural parameters found in the solid state and in solution (by NMR spectroscopy). Both the energy parameter (i.e. the gap between the ligand triplet state and the metal ion excited state) and the geometric parameter (i.e. the donor–acceptor distance and the orientation of the choromophore) have to be taken into account to explain the results obtained in terms of the efficiency of the L→Ln intra-molecular energy transfer. Furthermore, the correlative comparison between structural and luminescent properties shows how inter-molecular interactions in the solid state can be a prominent factor in the effectiveness of this transfer.

Luminescence properties of Pr3+ and Ce3+ in KCaF3 single crystals

The optical excitation and luminescence spectra of (Pr3+,Ce3+) ions in KCaF3 are presented. High-resolution excitation and emission spectra of Pr3+ and Ce3+ were measured at temperatures between 77 and 293K. First measurements of KCaF3:Ce3+ crystal show that there are nonequivalent Ce3+ centers which radiate either in UV or visible range. The major Ce3+ nonequivalent center in KCaF3 is Ce3+ ion in the site with tetragonal site symmetry (S4). Energy levels for the 4f2 ground configuration of Pr3+ in KCaF3 were established from excitation and fluorescence spectra. Energy levels assignments were made assuming electric-dipole transition selection rules for S4 site symmetry. Twenty-seven energy levels in the lowest six multiplets of the ground configuration were established. In the analysis of experimental data, the Oh magnetic-dipole selection rules were also considered to explain missing or weak transitions which are allowed in S4. Spectra of Pr3+ in KCaF3 have been analyzed with a crystal-field Hamiltonians of S4 or Oh symmetries including J mixing effects. Crystal-field parameters in these Hamiltonians have been determined by the minimization the RMS deviation between calculated and experimental energy Stark levels. We conclude that both model calculations in the S4 and Oh site symmetries for Pr3+ in KCaF3, provide the basis for a consistently good correlation with data.

Cu(II), Mn(II) in tetragonal site in chrysocolla

Chrysocolla samples from two regions are studied spectroscopically to ascertain the site symmetries of the transition metal ions present in them. The results indicate the tetragonal site symmetry for Cu(II) and Mn(II). The crystal field and spin Hamiltonian parameters are evaluated.

EPR measurements of the spin-hamiltonian zero-field splitting parameters as a function of temperature in MN 2+-doped Cs 2NaLaCl 6

The spin-Hamiltonian zero-field splitting parameter b 2 0 is measured as a function of temperature in Mn 2+ doped Cs 2NaLaCl 6 single crystals (elpasolite type). The experiments showed that the Mn 2+ ion occupies two different tetragonal symmetry sites in the cubic crystalline lattice of Cs 2NaLaCl 6. Both sites possess principal Z-axes along 〈100〉 crystallographic directions. Electron spin resonance measurements were made at X-band frequencies (9.2 GHz) in the temperature range from 12.8 to 300 K. The present results are taken into account in order to establish a possible relation between the spin-Hamiltonian parameter b 2 0 and the temperature. A tentative determination of the Debye temperature is derived from the experimental data.

Computer package for microscopic spin Hamiltonian analysis of the 3 d4 and 3 d6 (spin S = 2) ions at orthorhombic and tetragonal symmetry sites

A FORTRAN package based on the microscopic spin Hamiltonian expressions derived by computer algebra (ALTRAN) for the high spin ( S = 2) 3 d 4 and 3 d 6 ions with an orbital singlet ground state at orthorhombic (point groups: C 2 v , D 2, D 2 h ) and tetragonal (point groups: C 4 v , D 4, D 4 h , D 2 d) symmetry sites is presented. The spin—orbit (λ) and the spin—spin ( ρ) coupling contributions up to the fourth-order perturbation theory are taken into account within the 5 D approximation. The package enables efficient numerical calculations of the Zeeman electronic (Ze) parameters and the zero-field splitting (ZFS) parameters for the S = 2 3 d 4 and 3 d 6 ions. The following terms are included: λ and λ 2 for the Ze parameters, λ 2 , λ 3 λ 4 , ρ, ρ 2 and λ 2ρ for the second-rank ZFS ones, and λ 4 , ρ 2 and λ 2ρ for the fourth-rank ZFS ones. The program is applicable to all possible energy level schemes with a ground orbital singlet arising from the 5 D multiplet due to orthorhombic or tetragonal symmetry crystal fields. The input parameters are λ, ρ, the energy levels Δ j ( j = 1, 2, 3, 4) and the mixing coefficient s, which can be obtained from other spectroscopic data. The ZFS parameters output in the extended Stevens notation B k q and b k q , as well as the conventional notation ( D, E; a, F, K), is provided.

AN INVESTIGATION FOR THE ZERO FIELD SPLITTING PARAMETERS OF Fe3+ IONS AT THE DEFECT SITES IN THE FLUOROPEROVSKITES AM F3:Fe3+

The zero field splitting (ZFS) of Fe3+ ions at the tetragonal symmetry sites in the fluoroperovskites AMF3 doped with Fe3+ is analysed and shown the existence of the crystal defects and lattice distortion.The ZFS parameters are investigated as a function of the defect induced lattice distortion parameters.The distortion parameters are obtained by adjusting them to fit the experimental ZFS data.

Neutral manganese acceptor in GaP: An electron-paramagnetic-resonance study.

In order to clarify the nature of the neutral Mn acceptor in GaP, we have carried out optical-absorption and electron-paramagnetic-resonance (EPR) experiments using both conventional and thermally detected EPR on semi-insulating GaP:Mn. In thermal equilibrium at low temperatures, all the manganese occurs in the charged acceptor state ${\mathrm{Mn}}_{\mathrm{Ga}}^{2+}$(${\mathit{A}}^{\mathrm{\ensuremath{-}}}$). By illumination with photon energies greater than 1.2 eV, it can be partially converted into the neutral charge state. The arising photostimulated EPR spectrum shows the characteristic of a tetragonally distorted center with an integer spin. The resonance lines are detectable only at temperatures below 7 K, and their linewidth of about 50 mT is due to the unresolved Mn-hyperfine splitting. We interpret the experimental data in terms of ${\mathrm{Mn}}_{\mathrm{Ga}}^{3+}$ ions on strain-stabilized sites of tetragonal symmetry due to a strong T\ensuremath{\bigotimes}\ensuremath{\epsilon} Jahn-Teller coupling within the $^{5}$${\mathit{T}}_{2}$ ground state. Such a behavior is expected for a 3${\mathit{d}}^{4}$ defect, as observed for the isoelectronic impurity ${\mathrm{Cr}}^{2+}$ in GaAs, and other tetrahedrally coordinated semiconductors. The analysis of the EPR spectra thus verifies that, in GaP, the neutral charge state of the Mn acceptor is ${\mathrm{Mn}}_{\mathrm{Ga}}^{3+}$(${\mathit{A}}^{0}$) in contrast to its behavior in GaAs and InP. \textcopyright{} 1996 The American Physical Society.

Nonlocal Coulomb interactions and the magnetic anisotropies in the low-temperature phases of the cuprates.

We have extended and generalized earlier studies of the magnetic anisotropies of the Cu spins in the copper-oxide planes generated by the spin-orbit coupling on the coppers and by the nonlocal Coulomb interactions between neighboring ions. Our derivation of the anisotropic magnetic Hamiltonian includes the contributions from general matrix elements of the Coulomb interaction both between the copper ions of a single bond and between the adjacent copper and oxygen sites. The results apply to a general rotation axis of the oxygen octahedra, with the assumption of tetragonal site symmetry of the copper. This mechanism leads to a form of the anisotropic spin interaction between the Cu ions which is identical to the one generated by kinetic superexchange in the presence of purely local Coulomb interactions. Together with the numerical estimates for the resulting spin couplings, this indicates that the Coulomb direct exchange leads to significant contributions to the isotropic spin interaction and the out-of-plane anisotropy, but does not affect qualitatively the magnetic order in the ground state. \textcopyright{} 1996 The American Physical Society.

Zeeman and zero-field splitting of 3d 4 and 3d 6 ions with orbital singlet ground state at orthorhombic and tetragonal symmetry sites

The spin Hamiltonian (SH) parameters for 3 d 4 and 3 d 6 ions with the spin S = 2 in the first- and second-kind orthorhombic crystal field (CF) are studied within the 5 D approximation. Four possible energy level schemes with a distinct orbital singlet ground state with S = 2 have been established due to our previous CF studies. For each scheme the microscopic spin Hamiltonian (MSH) formulas for the zero-field splitting (ZFS) parameters in the extended Stevens operator notation B q k as well as the electronic Zeeman parameters g i are derived by computer using the ALTRAN algebraic program. The microscopic approach enables one to express the parameters B q k and g i in terms of the spin-orbit (λ) and spin-spin (ρ) coupling parameters, the orthorhombic energy level splittings (Δ i) and mixing coefficients (q,s). The present perturbation calculations extend to higher orders of perturbation theory as well as being more comprehensive than the earlier ones, since two new energy level schemes not considered before are now dealt with. Due to the ascent in symmetry method, the MSH results are also applicable to tetragonal symmetry cases involving the real ZFS terms. Some inconsistencies in the previous MSH derivations carried out by hand are clarified. Conversion relations between the ZFS parameters B 0 2, B 2 2, B 0 4, B 2 4, B 4 4 and those expressed in several other notations used in the EPR literature are provided. Specific applications of the MSH formulas derived here provide guidelines for the millimeter/submillimeter and high magnetic field EPR studies of 3 d 4 and 3 d 6 ions at orthorhombic and tetragonal symmetry sites.

Divalent manganese in two different tetragonal sites of Cs2NaLaCl6

Electron paramagnetic resonance studies performed at room temperature in Mn2+ doped Cs2NaLaCl6 single crystals (elpasolite type) showed that the Mn2+ ion occupies two different tetragonal symmetry sites in a cubic crystalline lattice. Both sites possess principal Z-axes along 〈100〉 crystallographic directions with large splittings (≊1.0 cm−1 and ≊0.8 cm−1) of the spectra. Several facts are taken into account in order to establish tentative models for the structural configuration of the two sites.