Cs Site Symmetry Research Articles

Synthesis, structural, morphology, spectroscopic and optical study of new metal orthophosphate MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba) compounds

This paper reports the synthesis, structure, BFDH crystal morphology, Raman and Infrared spectroscopic studies of three novel metal orthophosphate salts MII(Ga0.5Sb0.5)(PO4)2 (MII= Sr, Pb, Ba), abbreviated [Sr], [Pb], [Ba]. The structures of these phases are refined from X-ray powder diffraction data by the Rietveld method. The yavapaiite [Ba] phase is monoclinic of C2/m space group [Z=2; a= 8.106(4) Å; b= 5.178(3) Å c= 7.806(1) Å, β=94.79(4)°, V= 327(1) Å3]. Its structure consists of layers, running along the (a, b) plane, and built up of corner-connected Ga(Sb)O6 octahedra and PO43− tetrahedra. The two isotypic phases [Sr]/[Pb] of distorted yavapaiite structure crystallize in the monoclinic C2/c space group (Z=4) with parameters: a=16.455(4)/16.622(7) Å; b=5.158(1)/5.163(2) Å c= 8.005(1)/8.067(3) Å; β=115.49(1)/114.85(2)°, V= 613(1)/628(5) Å3. Their structures consist of MO10 (M= Sr, Pb), PO43− tetrahedra and Sb(Ga)O6 octahedra. In [Pb] phase, the Pb is surrounded by eight oxygen atoms (Pb-O: 2.65-2.77 Å) and two oxygen atoms forming abnormal bond distances Pb-O(4) [3.447(3)Å]. The BFDH crystal morphology predicts hight (h k l) facets for each compound, the center-to-plane distances dhkl are slightly strong in the distorted Yavapaiite [Pb/Sr] rather than in [Ba]. The IR and Raman study is performed based on theoretical group analysis considering Cs site symmetry of PO43− groups in [Ba], and Ci symmetry in [Pb]/[Sr] phases. The assignment and discussion of the observed bands were done by comparison with other yavapaiite materials. The UV–visible investigation reveals direct optical gap energies of values 4.16 eV for [Ba] and 4.24 eV for [Sr], indicating a semi-conducting nature of the prepared materials.

Ferromagnetic and antiferromagnetic orderings in wurtzite diluted magnetic nanostructures

Using a new approach based on symmetry analysis, we have determined the magnetic symmetry groups (Shubnikov groups) of ferro- and antiferromagnetic wurtzite nanostructures doped with magnetic atoms periodically distributed at cation sites as well as the direction of the spontaneous magnetic field. Quantum wells, nanorods (nanowires), nanotubes, and quantum dots have been considered. The direction of the spontaneous magnetic field is determined by magnetic atoms with higher C3v (3m) or Cs (m) site symmetry group (the site symmetry group is defined with respect to the doped nanostructure). When the magnetic-atom distribution becomes more or less disordered (Diluted Magnetic Semiconductors), it seems reasonable to assume that, in most of the cases, the symmetry in regions with a size of some wurtzite unit-cells remains close to that of a periodic one, hence possibly inducing local ferro- or antiferromagnetic properties. The regions can induce overall ferro- or antiferromagnetic states when they have common spontaneous magnetic field direction or, possibly, induce easy-magnetization direction or plane (direction or plane of maximal magnetization intensity for a given external magnetic field strength) when disorder imposes a spread of spontaneous magnetic field direction over the various regions. Of course, such an effect adds itself to magnetic properties eventually induced by grain boundaries, other crystalline phases, and defects such as vacancies, dislocations or interstitial atoms.

Electron paramagnetic resonance study of gadolinium in Czochralski-grown yttrium fluoride single crystals

Single crystals of YF3 grown by the Czochralski method have been investigated by electron paramagnetic resonance (EPR). In this paper we present an EPR analysis of residual Gd3+ impurities. We found that the Gd3+ rare-earth ion substitutes for Y3+ in YF3. Detailed analysis of the EPR spectra allowed us to determine the fine structure parameters of Gd3+ in monoclinic Cs site symmetry. The EPR angular dependence is dominated by the electronic quadrupole tensor; the g-factor is slightly axial. The b02 parameter found for Gd3+ in YF3 is unexpectedly high when compared to Gd3+ in the hosts of LaF3 structure where the coordination number is the same. The results let us conclude that YF3 is a promising laser host crystal.

Crystal-field levels in the distorted perovskite PrGaO3

Inelastic neutron scattering has been employed to study the crystalline-electric-field interaction of the distorted perovskite PrGaO3. The observed energy spectra show several well defined inelastic peaks of magnetic origin in an energy transfer range up to 86 meV which are attributed to crystalline-electric-field transitions within the ground-state J multiplet 3H4 of the Pr3+ ions. The spectra are analysed in terms of single-particle crystal-field theory based on geometrical coordinations associated with the Cs site symmetry of the Pr3+ ions. J mixing as well as the imaginary part of the crystal-field Hamiltonian are found to be important to reproduce the experimental data. The resulting crystal-field parameters are close to the values extrapolated from a similar study of NdGaO3. Our approach turns out to constitute a useful extrapolation scheme for other structurally related perovskite-type compounds.

ChemInform Abstract: Laser‐Induced Optical Emission Studies of Eu3+ Sites in Polycrystalline Powders of Monoclinic and Body‐Centered Cubic Eu2O3.

AbstractFor polycrystalline powders of monoclinic Eu2O3, the f‐f emission spectrum is found to result from three distinct Eu3+ sites of Cs site symmetry (5D0 → 7FJ for J = 0, 1, 2).

Infrared spectroscopic study of site symmetry effects for sulphite and sulphate anions in solid sodium sulphite oxidized by air oxygen

The mutual replacement of SO32- and SO42- anions was studied in Na2SO3.7 H2O crystals where sulphite is slowly oxidized to sulphate by air oxygen. In a series of Na2(SO3, SO4).7 H2O samples, the C1 or Cs site symmetry appears for the SO32- anion and the site symmetry of the SO42- anion is lowered gradually from Td through D2d or C3v to C1. The replacement was also studied for anhydrous sodium sulphite obtained by dehydration or by evaporation from aqueous solution. Despite the different structures of Na2SO3 and Na2SO4 crystals (C3i1 and D2h24, respectively), the favourable dimensions of the sterically different anions allow the SO42- ions, with their own symmetry Td, to be located in the C3 sites of the SO32- ions in Na2SO3.

Infrared spectra of 15NH3⋅H2O, 2 15NH3⋅H2O, and deuterated forms of ammonia hemihydrate at 90 °K

The infrared spectra of various deuterated samples of ammonia hemihydrate are reported with some infrared absorption frequencies of 2 15NH3⋅H2O and 15NH3⋅H2O. The spectra of the 15N-substituted samples confirm the previous general assignments, and reveal a third component of ν3(NH3) in NH3⋅H2O. The absorption by 2ND3⋅D2O, NDH2 isolated in 2NH3⋅H2O, and ND2H isolated in 2ND3⋅D2O, show, with the aid of normal coordinate calculations, that the primary splitting of the bands due to ν2 and ν3 of ammonia is multiple site splitting, due to the existence of two types of ammonia molecule in the hemihydrate. The Cs site symmetry of the ammonia molecules is revealed by the stretching vibrations of NH2D in 2NH3⋅H2O but not by the symmetric deformation vibrations of NH2D or ND2H. The stretching vibrations of NH2D in 2NH3⋅H2O at 90 °K also indicate that the type II ammonia molecules occupy both of the orientations which preserve the plane of symmetry. The uncoupled O–D stretching frequencies of HDO in 2NH3⋅H2O confirm the assignment of the two νOH(H2O) bands in the spectrum of 2NH3⋅H2O as largely due to site splitting. The low-frequency νOD(HDO) stretching band is very broad and the corresponding νOH(HDO) band is even broader, as has been seen previously for analogous bands in hexamethylenetetramine hexahydrate. Similarly, one set of isolated νND(NH2D) stretching bands is sharp while the corresponding νNH(ND2H) bands are very broad, as has also been reported previously for solid ammonia. The assignment to rotational vibrations of features below 850 cm−1 in 2NH3⋅H2O is confirmed, but no evidence indicates the degree of mixing of ammonia and water displacements in these vibrations.

The phonon spectrum of crystalline tellurophene-d0 and tellurophene-d4

The phonon spectra of tellurophene-d0 and tellurophene-d4 at 75 K have been studied and the exciton splittings of the fundamental modes have been assigned. The spectra are in accord with centrosymmetric factor group predictions, assuming a Cs site symmetry and a [Formula: see text] space group. In the circumstances, crystals of tellurophene have four molecular units per unit cell and become isomorphous with other aromatic heterocyclic crystals. Two-phonon processes, revealed for thick samples, are associated with internal + internal and internal + external combinations. Some of these last are accounted for by K ≠ 0 critical points selection rules.

Electronic and vibrational spectra of ferricyanide ions doped in NaCl and KCl crystals

Ten absorption bands are observed in the electronic spectra of NaCl and KCl crystals doped with Fe(CN)63- ions in the range 50000-20000 cm-1. Six of these bands are assigned to the ligand to metal charge transfer transitions (t1u6t2g5 to t1u5t2g6 and t1u5t2g5eg1 and a1g2t2g5 to a1g1t2g6). The positions of these bands agree with those calculated using Dq=1944 cm-1, B=623 cm-1 and C=3270 cm-1 for d6 configuration. The remaining four bands are assigned to crystal field transitions for d5 configuration and agree with the calculated peak positions for Dq=3450 cm-1, B=700 cm-1 and C=2400 cm-1. Vibration spectrum in the (C identical to N) stretch region shows that the Fe(CN)63- ion is present predominantly in the Cs site symmetry.

Fluorescence zeeman effects of the KC1 compensated lattice: The Cs site symmetry and the validity of the “S = 1 representation” of Sm2+ in KC1

The anisotropy patterns, polarizations and field dependence of Zeeman effects in several prominent 4.2°K fluorescence lines of KC1 : Sm2+ zre described. The effective Hamiltonian in the “S = 1 representation”, which was previously employed in the calculation of Zeeman anisotropy patterns, is shown to be inappropriate for large compensation fields at which Zeeman effects can originate from closely spaced non-degenerate representations. The basic concept of site distribution in a compensated alkali halide lattice is discussed. The presence of a hitherto unreported Cs site, which is predicted by distribution calculations, has been established. At high dopant concentrations at which Sm2+ : K+ vacancy pairs are expected to aggregate through dipolar attractive forces, new fluorescence lines have been observed.

Optical Absorption and Fluorescence Spectra of Single-Crystal TbF3

Absorption and fluorescence measurements on a single crystal of TbF3 have been used to prepare an energy-level scheme for the Stark components of the 7F multiplet and the 5D4 level of Tb3+ in this crystal. The results are consistent with Cs site symmetry for the rare-earth ion. In addition a limited set of measurements indicate that the 5D4 level decays with lifetimes of 3.9 ± 0.4, 0.46 ± 0.05, and 0.30 ± 0.05 msec at 4.2, 77, and 300°K, respectively.

Optical Absorption and Fluorescence Spectra of EuF3

The absorption spectrum of EuF3 powder has been obtained in the region from 1.8–10μm. By means of a diffuse reflection technique, the crystal sublevels of the 5D0,1,2 states were obtained at liquid-nitrogen temperature. Fluorescence from the 5D0 state and from some of the crystal sublevels of the 5D1,2 states to the 7FJ(J = 0, 1, 2, 3, 4, 5, 6) states has been observed. From an analysis of the observed transitions, it was possible to assign 2J+1 Stark components to the observed states, in agreement with the Cs site symmetry for Eu3+ in EuF3. The intermediate coupling free-ion energy levels for Eu3+ have been calculated, including the interactions of all the multiplets of the f6 configuration. Values of F2=351.5 cm−1, F4=57.64 cm−1, F6=5.625 cm−1, and ζ=1328 cm−1 are found to give the best over-all agreement between the observed and calculated energy levels.

Optical Studies of Electronic Transitions in Hexa- and Tetracoordinated Mn2+ Crystals

The polarized absorption spectra of Mn2+ in (CH3)4NMnCl3 and [(CH3)4N]2MnCl4 have been studied at 298°, 77°, and 2°K. Marked optical anisotropy was observed in both complexes. Details of the spectra indicate that the surroundings of the hexacoordinated Mn2+ ion are trigonally distorted from octahedral symmetry. Unlike the chemically similar alkali metal manganese halides, (CH3)4NMnCl3 undergoes no paramagnetic—antiferromagnetic transition between 298° and ≈4°K. Abnormal intensities in the spectrum appear to be unrelated to exchange mechanisms. The surroundings of the tetracoordinated Mn2+ ion very nearly approximate tetrahedral symmetry. However, the true Cs site symmetry is evident in the very sharp line absorption near 23 000 cm−1. The polarization behavior of the ion in both coordinations is discussed qualitatively in terms of the selection rules for the appropriate double group. The green luminescence of the tetrahedral complex and the pink luminescence of the octahedral complex have also been measured. The emission is shown conclusively to involve the transition 6A1g(6S) ←4T1g(4G) in the (CH3)4NMnCl3 complex.