Mechanochemical synthesis of noncentrosymmetric compounds in oxide systems

We study the influence of anomalous anisotropy in the elastic constants of paratellurite (α-TeO₂) on the rate of the following mechanochemical reactions: Bi₂O₃ + SeO₂ = BiSeO₅; Bi₂O₃ + TeO₂ = Bi₂TeO₅; SeO₂ + TeO₂ = TeSeO₄. The formation of the following compounds was registered by means of X-ray analysis and thermal annealing: Bi₆SeO₁₁, Bi₂TeO₅, and TeSeO₄.     

ESR evidence of a low-temperature phase transition in Na2SeO4

Abstract

ESR investigations of gamma-irradiated sodium selenate in powder form indicates a number of paramagnetic species as SeO^? ₂, SeO^? ₃, and SeO^? ₄, identified on the basis of their g-tensors. The appearance of a new ESR line at g = 2.0260 attributed to the SeO^? ₄ radical indicates a possible structural phase transition at 165 K in Na₂SeO₄.     

Anharmonic phonons and central peaks at structural phase transitions

Recent light scattering studies of solids undergoing structural phase transitions clearly demonstrate that the “central peak” phenomenon is not one, but several processes (both static and dynamic) contributing in different degrees to the spectra observed for different types of transition. Our own work on Pb₅Ge₃O₁₁, SrTiO₃, K₂SeO₄ and BaMnF₄ is reviewed and the studies by other on KDP, KH₃(SeO₃)₂, KD₃(SeO₃)₂ are discussed briefly.     

Structure determination of Cs0.864Rb1.136SeO4

The present structure determination of Cs _x Rb_2-x SeO₄ shows that the crystal belongs to the β - K₂SO₄ family, like the related compounds Cs₂SeO₄ and Rb₂SeO₄. In β - K₂SO₄ compounds the cations occupy two different sites; one of these sites, which is surrounded by 11 anionic atoms, is occupied preferentially by Cs, while the other one with 9 neighbours by Rb. A slight excess of Rb has been determined by X-ray structure analysis in a single crystal prepared at 292 K; its formula is Cs_0.864(9)Rb_1.136(9)SeO₄.

Crystals were grown by evaporation from equimolar water solutions of Cs₂SeO₄ and Rb₂SeO₄ at 292 and 301 K. No phase transition was detected by DSC in the range 103-323 K for either of the samples. Lattice parameters of different crystals isolated at various stages of crystallization indicate formation of crystals with a different proportion of Cs/Rb cations. The lattice parameters as well as their ratios differed significantly from those given by Endo et al., (1983).     

Stress dependence of Tc for the incommensurate phase transition in K2SeO4

We have measured the uniaxial stress dependence of the frequency of the zone center A₁ soft mode in the low temperature phase of K₂SeO₄. The frequency shows a strong decrease with stress. We infer from this a similar decrease of the incommensurate phase transition temperature T_c. This behavior is explained by detailed lattice dynamical calculations for K₂SeO₄ which reveal the unusual balance between Coulomb and short-range forces in this structure. Our calculations show that this balance is changed by stress in such a way that the quasi-harmonic normal mode frequencies harden drastically. T_c is reduced by this purely volume dependent effect. This unusual balance between Coulumb and short-range forces also provides a natural explanation for the occurence of an incommensurate phase transition in K₂SeO₄. We infer that similar transitions are likely in isomorphous structures.     

Symmetry determination of commensurate and incommensurate phases of K2SeO4 by EPR

From a reinterpretation of EPR results on γ-irradiated K₂SeO₄ crystals, previously reported by Aiki et al., the space group symmetry for commensurate (ferroelectric) phase of K₂SeO₄ is determined to be Pna2₁. The EPR results support the modulation of polarization as the reason for disappearance of ferroelectricity in incommensurate phase.     

Nonempirical calculation of linear and nonlinear polarizability of TeO<Subscript>2</Subscript>-based molecular clusters and piezoelectric properties of crystalline tellurium oxide

Polarization characteristics of the paratellurite TeO₂ crystal were studied in the model of the molecular cluster represented by the (TeO₂)₃ molecular chain. Nonempirical calculations of linear and nonlinear polarizability reveal a steep growth of the second hyperpolarizability when passing on from the single TeO₂ molecule to the molecular chain (TeO₂)₃, which may be related to intermolecular contacts. Linear polarizability of isostructural molecules SO₂, SeO₂, and TeO₂ have an equal order of magnitude, while the second hyperpolarizability of TeO₂ is somewhat higher than that of SeO₂ and exceeds the SO₂ polarizability by an order of magnitude.     

Studies of single-step electrodeposition of CuInSe2 thin films with sodium citrate as a complexing agent

The influences of various parameters in a single-step electrodeposition of CuInSe₂ from aqueous solution containing CuCl₂, InCl₃, and SeO₂, with sodium citrate as the complexing agent, are investigated. Co-deposition of CuInSe₂ from a room temperature, aqueous bath of these electrolytes is accomplished by the aid of sodium citrate. In this work the optimum potential for deposition of CuInSe₂ is found to be −0.5 V vs. Ag/AgCl, the deposition time is 800 s, the concentration ratio of CuCl₂, InCl₃, and SeO₂ is 9 mM:22 mM:22 mM in aqueous solution, and the annealing temperature is 225 °C. Under the optimum conditions, crystalline layers of CuInSe₂ having the chalcopyrite structure can be successfully synthesized. Cyclic voltammetry (CV), scanning electron microscope (SEM), X-ray diffractometer (XRD), and energy dispersive X-ray spectrometer (EDX) were used to examine the electrochemistry, morphologies, structures, and compositions of CuInSe₂ thin films deposited on ITO glass.     

Part II: Effect of high energy proton beam fluence on the electrical studies of lithium gallium phosphate glass electrolyte doped with selenium ions

A series of glass samples, according to the formula (100 ? x) (0.5Li₂O ? 0.2Ga₂O₃ ? 0.3P₂O₅) + xSeO₂ (x?=?0, 2, 4, 6, 8, 10) and 12 mol.%, labeled as LGPS _x (x is the mole percent of SeO₂), were synthesized through melt quenching technique. All the LGPS _x samples were irradiated by a high energy proton (H⁺) beam of 3 MeV at fluence of 10¹⁴, 5 × 10¹⁴, and 10 × 10¹⁴ ions/cm². FTIR and Raman spectra indicated that SeO₂ acts either as a glass modifier (SeO ₃ ^2? ) or a glass former (SeO ₄ ^2? ), as the dose rate of beam fluence changes. The bulk conductivity of the highest conducting sample LGPS₁₀ irradiated at a beam fluence of 5 × 10¹⁴ ions/cm² was determined as 5.87 × 10^?04 S/cm at 303 K. The super curve in the normalized spectra of electrical modulus at different temperatures confirms that the LGPS samples follow the temperature-independent, frequency-dependent relaxation time before and after irradiation.     

Structure of the low-temperature phase of Rb3D(SeO4)2 by single-crystal neutron diffraction

Crystal structure of Rb₃D(SeO₄)₂ has been investigated at 25 K (below the transition temperature T_c=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO₄ groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO₄ groups was identified to be in the state closer to a HSeO₄⁻ ion or to a SeO₄²⁻ ion and the dipole arrangement of SeO₄-D-SeO₄ dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.     

Raman spectroscopic study of the uranyl selenite mineral marthozite Cu[(UO2)3(SeO3)2O2]·8H2O

The mineral marthozite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 K. The bands at 812 and 797 cm⁻¹ were assigned to the symmetric stretching modes of the (UO₂)²⁺ and (SeO₃)²⁻ units, respectively. These values gave the calculated U O bond lengths in uranyl of 1.799 and/or 1.814 Å. Average U O bond length in uranyl is 1.795 Å, inferred from the X‐ray single crystal structure analysis of marthozite by Cooper and Hawthorne. The broad band at 869 cm⁻¹ was assigned to the ν₃ antisymmetric stretching mode of the (UO₂)²⁺ (calculated U O bond length 1.808 Å). The band at 739 cm⁻¹ was attributed to the ν₃ antisymmetric stretching vibration of the (SeO₃)²⁻ units. The ν₄ and the ν₂ vibrational modes of the (SeO₃)²⁻ units were observed at 424 and 473 cm⁻¹. Bands observed at 257, and 199 and 139 cm⁻¹ were assigned to OUO bending vibrations and lattice vibrations, respectively. O H···O hydrogen bond lengths were inferred using Libowiztky's empirical relation. The infrared spectrum of marthozite was studied for complementation. Copyright © 2008 John Wiley & Sons, Ltd.     

A comparative study of the modulated structures of some A2BX4 compounds

The structures of the modulated phases of K₂SeO₄, Rb₂ZnCl₄, Rb₂ZnCl₄ and Cs₂CdBr₄ are compared. Both commensurate and incommensurate modulations are considered. Clear common features can be derived between the polarization vectors of their primary harmonic modulations. Even distortions with very different modulation wavevectors present a clear correlation.     

Parametric approach to centrifugal distortion constants: Application to bent symmetric XY2-type molecules

A method to generate all the mathematically possible values of the centrifugal distortion constants of a molecule compatible with its vibration frequencies is developed. Application of the method to the XY₂-bent symmetric-type molecules enables mapping of the τ_αβγδ constants as a function of a single parameter. The use of the method in defining the limits as well as in refining the experimental values of τ_αβγδ constants is discussed. The method also opens up a route to fix the force constants unambiguously employing the τ_αβγδ constants as additional data. The numerical example of SeO₂ molecule is given to illustrate the general procedure.     

Microwave spectra of HPO and DPO: molecular structure

Microwave spectra of the unstable phosphorus containing molecule, HPO, and its deuterated species were measured in the frequency range of 70-380 GHz. The molecule was produced by a DC-glow discharge of a gas mixture of PH₃, CO₂, and H₂(D₂). Rotational constants and centrifugal distortion constants for HPO and DPO were determined accurately. Harmonic force constants were evaluated from the centrifugal distortion constants determined in the present study, and with vibrational frequencies reported previously. The zero-point average structure for HPO was obtained by taking the isotopic difference of the PH bond length into consideration: r_z(PH)=1.473(7) Å, r_z(PO)=1.4843(9) Å, and θ_z=104.57(16)°. The errors were estimated from the residual inertial defect. Equilibrium bond lengths for the PH and PO bonds were derived as 1.455(7) and 1.4800(9) Å, respectively, by assuming anharmonic constants of the corresponding diatomic molecules.     

The microwave spectra and vibration-rotation interaction constants for CD3I in the excited vibrational states

The microwave spectra in the J = 1 → 3 region for CD₃I has been observed and six excited vibrational states assigned. The vibration rotation interaction constants, α^B and nuclear quadrupole coupling constants, eQq, have been determined for the states: ν₂, ν₃, ν₅, ν₆, 2ν₃, and ν₃ + ν₆. For the degenerate vibrational states, the l type doubling constants, q_t were determined.     

The 313 nm band system of SeO2. Part 2: rotational structure

Rotational analyses of the first four bands in the ν₁ progression of the transition of SeO₂ at 312.7 nm are reported. The gas phase sample, which contained selenium isotopes in natural abundance, was formed in a free jet expansion from a heated nozzle source. The rotational constants for the state show a rather erratic dependence on v₁^′, consistent with the identification of small, local perturbations in the rotational structure. The A-rotational constant shows a particularly irregular dependence on v₁^′. The r₀ structure was therefore determined from the B and C values of ⁸⁰SeO₂ to be:

     

Analysis of the fundamental asymmetric stretching mode of T2O

The ν₃ mode of T₂O, observed at 0.04 cm⁻¹, has been analyzed. By an iterative process of fitting and assignment 210 lines were assigned; 167 of these were used in the final fitting. The standard deviation of the fit is 0.023 cm⁻¹. The rotational constants, A, B, and C, as well as the quartic distortion constants, have been evaluated for the excited state.     

Anomalous birefringence and dilatation in the Rb(H,D)3(Se0)3))2 near normalincommensurate phase transition: Fluctuations and defects

An attempt is made to extend the method of experimental analysis of fluctuation behaviour at normal (N)—incommensurate (INC) phase transitions in the case of narrow INC-phases. Values of fluctuation parameters (the “effective Ginzburg Numbers”) for RbH₃(SeO₃)₂ and RbD₃(SeO₃)₂ crystals are suggested and compared with those of some other crystals from the A₂BX₄ family (G = 0.01 ÷ 0.036). In addition, in the crossover region the role of defects is clearly demonstrated by comparing the behaviour of incompletely deuterated crystal Rb(H, D)₃(SeO₃)₂ with that of RbH₃(SeO₃)₂.     

Force Constants,Coriolis Coupling Constants,and Mean Amplitudes of Vibration of InCl6 3-

Abstract

The general quadratic force constants, coriolis coupling constants and mean amplitudes of vibration have been evaluated for InCl₆ ^3- using recent vibrational data. The results are employed to study the trend of variation in the isoelectronic sequences e.g. InCl₆ ^3-, SnCl₆ ^2- and SbCl₆ ^?.

In the present communication we report the force constants, coriolis coupling constants and mean amplitudes of vibration for InCl₆ ^3- which have not been reported so far. The results are used to study the trend of variation of the molecular parameters in the isoelectronic sequence e.g. InCl₆ ^3-, SnCl₆ ^2- and SbCl₆ ^?. The fundamental frequencies of InCl₆ ^3-, employed in this computation, have been reported on the basis of octahedral symmetry, from infrared and Raman Spectral Studies¹.     

Studies of molecular relaxation by infrared and microwave coherent transients

Novel techniques to observe coherent transients are successfully demonstrated on microwave transitions of NH₃, H₂O, OCS, and O₂. The relaxation rate constants in thev ₂ excited state of NH₃ are obtained by infrared experiments. Paper was prepared for presentation at FICOLS.