Absorption spectra of thin layers of solid solutions Cs<Subscript>2</Subscript>(Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)I<Subscript>4</Subscript>

The excitonic absorption spectra of thin films of ferroelectric Cs₂CdI₄ and Cs₂ZnI₄ solid solutions are studied for the first time. It is found that, within the whole range of molar concentrations x, the spectra of Cs₂(Cd_{1 − x} Zn _x )I₄ comprise two bands that originate from the bands of the two compounds. It is shown that the exciton transfer occurs most efficiently between the tetrahedrons CdI₄ and ZnI₄ along the b axis of the crystal. Unusual behavior of the concentration with regards to the maxima of long-wavelength excitonic bands E _m (x) agrees well with the developed theory that takes into account dependence on x of the matrix element of the intertetrahedron exciton transfer and that is similar to the theory of Davydov’s splitting in molecular crystals.     

Interaction of dopant cations with 4:1 defect clusters in non-stoichiometric 3d transition metal monoxides: A theoretical study

The ASED-MO theory and a normalized ion energy model have been used to study the energetics of substitutional dopants in Mn, Fe, Co, and Ni non-stoichiometric monoxides; we find that certain dopants stabilize 4:1 defect clusters (4 octahedral M²⁺ vacancies surrounding an M³⁺ tetrahedral interstitial). For example, Fe³⁺-based 4:1 clusters in Mn_1−xO and Mn³⁺-based 4:1 clusters in Fe_1−xO have comparable stabilities to those in the undoped material, while Mn³⁺- and Fe³⁺-based 4:1 clusters in Co_1−xO and Ni_1−xO are significantly more stable than those in the undoped oxides. Cr³⁺ has a strong octahedral site preference energy in all cases, which prevents formation of doped 4:1 defect clusters. Despite a strong tetrahedral site preference energy, Zn²⁺ does not cause formation of 4:1 clusters because they would destabilize oxygen 2p orbitals.     

Octahedral vs tetrahedral coordination of the co(II) ion in layer compounds: CoxZn1−xIn2S4(O⩽x⩽0.46) solid solution

Large crystals of Co_xZn_1−xIn₂S₄ (0.00 ⩽ x ⩽ 0.46) solid solution were grown from the vapour phase by chemical transport modified by the time variation of the temperature profile procedure. Co²⁺ ions were found, by XPS analysis, to occupy tetrahedral as well as octahedral sites of the ZnIn₂S₄-type structure. The magnetic properties of Co_0.46Zn_0.54In₂S₄, crystallizing in space group R 3m, were explained in terms of the short-range magnetic order arising from the presence of isolated clusters of magnetic ions.     

Preparation and properties of disordered NaBi(XO4)2, X=W or Mo, crystals doped with rare earths

NaBi_1−xRE_x(XO₄)₂, X=W or Mo and RE=Pr, Nd, Ho, Er and Yb single crystals have been grown by the Czochralski technique. Rare earth concentrations about 3.5×10²⁰ cm⁻³ have been achieved in crystals with good optical quality. Melt stability is obtained by synthesising NaBi(XO₄)₂ from the precursor Na₂X₄O₁₃ phase and minimising Mo volatility. The strength of W and Mo compounds to chemical attack and thermal annealing in several atmospheres is reported. Mo compound is etched by inorganic acids and becomes coloured after vacuum annealing. The optical absorption, photoluminescence and refractive indices of the hosts are characterised and show a dichroic character. The lattice disorder induces broadening of the 10 K optical absorption of the rare earth impurities.     

Vibrational spectroscopic and force field studies of copper(II) chloride and bromide compounds,and crystal structure of KCuBr3

Vibrational spectroscopic and force field studies have been performed of 15 related copper(II) chloride and copper(II) bromide compounds, including hydrated salts crystallizing in ternary aqueous systems with alkali and ammonium halides. For halocuprates with distorted octahedral coordination characteristic stretching Raman wavenumbers, corresponding to symmetric stretching Cu^II X modes in the equatorial plane, were found in the ranges 247–288 cm⁻¹ for X = Cl, and 173–189 cm⁻¹ for X = Br, while the low‐wavenumber stretching modes for the weaker axial Cu X interactions varied considerably. The tetrahedral coordination for Cs₂CuCl₄ and Cs₂CuBr₄ leads to somewhat lower Cu X symmetric stretching wavenumbers, 295 and 173 cm⁻¹, respectively. The assignments of the copper–ligand stretching vibrations were performed with the aid of normal coordinate calculations. Correlations between force constants, averaged Cu X stretching wavenumbers and bond distances have been evaluated considering the following aspects: (1) Jahn–Teller tetragonal distortion (axial elongation) of the octahedral copper(II) coordination environment, (2) differences between terminal and bridging halide ligands (3) effects of coordinated water and the influence of outer‐sphere cations. Force constant ratios for terminal and bridging metal–halide bonds reveal characteristic differences between planar and tetrahedrally coordinated M₂X₆ species. In the hydrated copper(II) halide complexes, the halide ligands are more strongly bound than coordinated water molecules. The crystal structure of KCuBr₃ (K₂Cu₂Br₆), which was determined to provide structural information for the force field analyses, contains stacks of planar dimeric [Cu₂Br₆]²⁻ complexes held together by weak axial Cu Br interactions. Copyright © 2007 John Wiley & Sons, Ltd.     

Mössbauer and X-ray studies for Ni0.2ZnxMg0.8−xFe2O4 ferrites

The spinel ferrites of Ni_0.2Zn_xMg_0.8−xFe₂O₄, 0⩽x⩾0.8, were studied at room temperature using X-ray diffraction and Mössbauer patterns. The analysis of the X-ray diffraction patterns proved that the samples have a single phase cubic spinel structure. The calculated values of the theoretical, true and average lattice constants, tetrahedral bond, tetrahedral edge and unshared octahedral edge were found to increase while the shared octahedral edge and octahedral bond decrease as the Zn²⁺ ion substitution increases. Mössbauer studies showed that the samples for x=0, 0.2 and 0.4 are magnetic and show rather broad lines, while for x=0.6 and 0.8 are paramagnetic. The hyperfine parameters of the tetrahedral and octahedral sites were determined as functions of composition x. The cation distributions were deduced and supported by X-ray studies. The B-site pattern was composite and has been fitted into multicomponents and the deduced hyperfine parameters have been discussed as a function of x.     

Solid solution formation in the systems CuMIIIX2-AgMIIIX2 where MIII=Al,Ga, In and X2=S,Se

A study of solid solution formation in the systems CuAlX₂-AgAlX₂, CuGaX₂-AgGaX₂ and CuInX₂-AgInX₂, where X=S, Se, has shown that when M^III=Al and Ga, CuM^IIIX₂ and AgM^IIIX₂ were not completely miscible. The CuInX₂-AgInX₂ system, however, showed complete solid solubility. The limits of solid solubility are explained in terms of the c/a ratio, and the internal atomic coordinate u. The lattice constant a is a linear function of composition, while c bows upwards. This behavior is also discussed in terms of trends in 2-c/a.     

Solid-State NMR Studies on Ionic <Emphasis Type="Italic">closo</Emphasis>-Dodecaborates

Alkali metal dodecahydro-closo-dodecaborates M₂[B₁₂H₁₂] (M = K, Rb, Cs, NH₄, N(CH₃)₄) and the perhalogenated cesium salts Cs₂[B₁₂X₁₂] (X = Cl, Br, I) are studied by solid-state ¹¹B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B₁₂X₁₂]²⁻ icosahedra which is examined by variable-temperature ¹¹B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M⁺. All alkali metal dodecahydro-closo-dodecaborates M₂ [B₁₂H₁₂] exhibit at elevated temperatures ¹¹B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic ¹¹B NMR spectra with a dominant homonuclear ¹¹B–¹¹B dipolar splitting. The per-halogenated cesium salts Cs₂[B₁₂X₁₂] behave differently. Hence, from the experimental ¹¹B NMR spectra at room temperature a substantial mobility is only seen for the [B₁₂Cl₁₂]²⁻ anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B₁₂X₁₂]²⁻ clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental ¹¹B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M₂ [B₁₂H₁₂] is achieved by line shape simulations, considering [B₁₂H₁₂]²⁻ ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M⁺, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction. Authors' address: Klaus Müller, Institut für Physikalische Chemie, Universit?t Stuttgart, Pfaffen-waldring 55, Stuttgart 70569, Germany     

New compounds with incommensurate phases: Cs2CdI4 and Cs2ZnI4

Abstract

In this paper the results of birefringence studies and of optical observations in polarized light in a wide temperature region are presented for crystals Cs₂CdI₄ and Cs₂ZnI₄. There is the following sequence of phases: commensurate orthorhombic Pnma ? incommensurate ? monoclinic ferroelastic P2₁/n ? triclinic ferroelastic PT. A correlation was observed between the peculiarities of birefringence and NQR spectra temperature dependence. An assumption is made, that in Cs₂ZnI₄ crystal in a broad pre-transition region (T - T _i = 100 K) precursor clusters exist, which manifest themselves as coexistence of NQR spectra of two phases and as deviation of birefringence from the linear temperature dependence (“tail”).     

Cs2XF6 (X=Si,Ge) compounds: Common and different features as uncovered by the first-principles calculations

Results of ab initio calculations of the electronic, optical and elastic constants for Cs₂GeF₆ and Cs₂SiF₆ are reported for the first time. Both compounds are the direct band gap dielectrics, with the calculated band gaps 6.926 eV (Cs₂SiF₆) and 6.417 eV (Cs₂GeF₆). Analysis of the calculated elastic constants and Cauchy condition shows both crystals as slightly covalent compounds. However, with increased pressure chemical bonds in Cs₂GeF₆ turn to be more ionic, whereas in Cs₂SiF₆ the covalent character of chemical bonds is enhanced. Pressure dependence of the chemical bond lengths and lattice constants was determined and represented as the second power functions of external pressure. Different trends in the values of the Mulliken charges for both compounds were found. The obtained results are applicable to the analysis of the luminescence properties of impurity ions at varying pressure or to the microscopic studies of crystal field effects in these crystals.     

Infrared laser magnetic resonance detection of SD at 1885·76 cm-1

The visible and near infrared luminescence spectra of MoCl₆ ^3- in Cs₂NaMCl₆ (M=Sc, Y, In) and MoBr₆ ^3- in Cs₂NaYBr₆ have been measured between 15 000 cm^-1 and 3000 cm^-1 at liquid helium temperatures. Comparison with new electronic absorption, infrared and Raman spectra have enabled five luminescence transitions between the states derived from the t _2g ³ configuration to be assigned unambiguously. Each electronic transition shows extensive vibronic structure which can be analysed to yield the vibrational frequencies of the MoX ₆ ^3- ion in each state. The spectra are strongly influenced by resonant interactions between the MoX ₆ ^3- ion and the internal and lattice modes of the host lattices and there is an enormous variation in the intensities of the vibronic origins.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

Study on charge transfer interaction and valence state of layered perovskite-type mixed valence complex [NH3(CH2) n NH3]2[(AuII2)(AuIIII4)(I3)2] (n = 7, 8), by means of 197Au Mössbauer spectroscopy

Cs₂[Au^I X ₂][Au^III X ₄](X = Cl, Br, and I) is well known for the three-dimensional perovskite-type gold mixed valence system. Recently, layered perovskite-type gold mixed valence complexes, [NH₃(CH₂) _n NH₃]₂[(Au^II₂)(Au^IIII₄)(I₃)₂] (n = 7 and 8), have been synthesized. We have investigated the relationship between the structural dimensionality and the Au^I–Au^III charge transfer interaction for Cs₂[Au^II₂][Au^IIII₄] and [NH₃(CH₂) _n NH₃]₂[(Au^II₂)(Au^IIII₄)(I₃)₂] (n = 7 and 8) by means of ¹⁹⁷Au Mössbauer spectroscopy.     

Effect of Co substitution on the structural and magnetic properties of Zn–W hexaferrite

The polycrystalline samples of the composition Zn_2−XCo_XBaFe₁₆O₂₇ were prepared by the usual ceramic technique with X=0.4, 0.6, 0.8, 1, 1.2, 1.4 and 1.6. The samples were sintered at 1300 °C for 8 h. The X-ray diffraction patterns confirm the presence of single W-hexagonal phase. The lattice parameters c and a was found to increase by increasing Co content whereas the X-ray density decreases and porosity increases by increasing Co content. The value of saturation magnetization (σ) decreases by increasing Co content due to the decrease of super-exchange interaction. The magnetic moment of the sample were calculated and it showed that in Zn_2−XCo_XBaFe₁₆O₂₇ nearly all Co²⁺ ions are distributed among the octahedral sites of spinel S block. The M–H hysteresis loop for all the samples are clearly showing low coercivity, indicating that all the samples belongs to the family of soft ferrite. The values of coercivity increases by increasing Co content. The far infrared absorption spectrum showed the presence of three absorption bands corresponding to tetrahedral, octahedral and hexahedral sites.     

AB INITIO CALCULATIONS OF OPTICAL PHONON DEFORMATION POTENTIALS IN DIAMOND AND ZINC-BLENDE SEMICONDUCTORS

The behavior of the internal lattice vibrational modes in (NH₄)₂Te_xSn_1-xCl₆ mixed crystals has been investigated by Raman scattering. All of the Raman-active [MX₄]^2- internal modes show two-mode property in the mixed crystals with compositions from x =0 to 1, and apart from the sample with x = 0.5, their energy positions are composition-independent, Space group calculations including site symmetry considera-tions and frequency fittings describe a good force field model. It has been demonstrated that the composition-independent two-mode behavior may come from the strong clustering effect, rather than from the character of the gap and local modes of impurities in mixed crystals. For the sample with x=0.5, the two component elements seem to form a pseudo-periodic lattice structure.     

The invar effect and phase transitions in Cs2ZnI4 crystals

An in situ x-ray diffraction study of Cs₂ZnI₄ crystals performed in the 4.2–300 K temperature range is reported. The lattice parameter measurements have revealed three anomalies corresponding to phase transitions. The thermal expansion coefficient along the c axis was found to vanish in the region of incommensurate and commensurate modulated phases, 120–96 K (the invar effect). A possible crystallographic model relating modulated atomic displacements to the invar effect is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 137–142 (January 1999)     

Rearrangement of the vibration spectrum of Zn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se solid solutions with varying composition

Experimental lattice reflection spectra of Zn_{1 ? x} Cd _x Se bulk crystals are interpreted and analyzed for compositions x = 0.08, 0.21, and 0.30 crystallizing in the cubic zinc blende structure and x = 0.45, 0.65, and 0.80 crystallizing in the hexagonal wurtzite structure. The lattice vibration modes for all the compositions are divided, according to their frequency, into ZnSe- and CdSe-like vibrations. On a phase transition at x > 0.3, a marked jump (7–8 cm^?1) in the frequency of lattice modes is observed, but the largest changes take places in the mode oscillator strengths. However, the summed oscillator strength of the ZnSe- and CdSe-like vibration modes changes only slightly during the phase transition, which corresponds to the double-mode type of rearrangement of the vibration spectrum of Zn_{1 ? x} Cd _x Se solid solutions with varying x. An analysis of the lattice modes is accompanied by the comparison with the parameters of the lattice modes of Zn_{1 ? x} Cd _x Se (x = 0–0.55) epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy.     

Lattice IR spectroscopy of Zn<Subscript>1 −<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se epitaxial layers grown on a GaAs substrate by molecular-beam epitaxy

Based on the results from comparison of the data on lattice reflection for Zn_{1 ?x} Cd _x Se bulk crystals (x = 0.08, 0.22) and Zn_{1 ?x} Cd _x Se thin layers of the same compositions grown on GaAs, a more adequate interpretation is given to the lattice IR reflection spectra of Zn_{1 ?x} Cd _x Se alloy films (x = 0–0.55) grown on a GaAs substrate by molecular-beam epitaxy. In this composition region, two ZnSe-and CdSe-like lattice modes are observed corresponding to a double-mode type of rearrangement of the vibration spectrum with varying x. A weak (with oscillator strength smaller than 0.25) high-frequency mode (with respect to the dominant modes) at 220 cm^?1 is also observed.     

Isotopic effect in Brillouin light-scattering spectra of a Cs5H3(SO4)4 · xH2O crystal

A Brillouin scattering study of the behavior of hypersonic longitudinal acoustic phonons in crystalline pentacesium trihydrogen tetrasulfate Cs₅H₃(SO₄)₄ · xH₂O (PCHS) and its deuterated analog Cs₅D₃(SO₄)₄ · xD₂O (DPCHS) at temperatures ranging from 420 to 120 K is reported. The effect of deuteration on the crystal lattice dynamics is investigated. The differences in the behavior of hypersonic acoustic phonons in the PCHS and DPCHS crystals suggest the existence of a hydrogen isotope effect in both the high-and low-temperature phases. A possible model of the isotopic effect in the high-temperature phase is discussed. An analysis is made of the acoustic response of the PCHS and DPCHS crystals in the region of the transition to the glasslike phase.     

Surface phonon–polaritons in rectangular quantum wires of polar ternary mixed crystals

In this paper, the dispersion relations of surface phonon–polaritons in freestanding rectangular quantum wire systems of polar ternary mixed crystals are derived. The numerical calculations for Al_xGa_1−xAs and Zn_xCd_1−xSe quantum wire systems are performed. The results reveal that the frequencies of surface phonon–polariton modes are sensitive to the geometric structures of the quantum wires, the wave-vectors in z-direction, and the compositions of the ternary mixed crystal materials. The effects of the “two-mode” and “one-mode” behaviors of the ternary mixed crystals on the surface phonon–polariton modes are also discussed.