Normal modes of the MoO2−4 ion in Tb1.8Eu0.2(MoO4)3 single crystal

Polarized Raman spectra (single crystal) at 300 K and infrared spectra (powder) at 300 and 77 K in the region 250–1000 cm^?1 of a binary molybdate of terbium and europium have been recorded. Based on C_2v symmetry, group theoretical analysis has been carried out and a vibrational assignment is proposed.     

Infrared and Raman spectra of alkali halide/NO−3 systems

Detalled Raman spectra of the NO^?₃ ion substitutionally isolated in Rbl, Kl and KCl have been investigated at various temperatures between 77 and 300 K. These spectra, together with their corresponding infrared spectra, have been used to study the effective symmetry of these systems. Changes in the infrared and Raman activities of some of the modes of the NO^?₃ ions have been observed as the samples were cooled. Two different symmetries, one at room and the other at low temperature, have been identified as D₃ and C_3v respectively.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Resonance Raman and luminescence studies on the quasi-one dimensional crystals of [Pt(en)z][Pt(en)2Cl2](ClO4)4

Resonance Raman scattering, infrared luminescence and absorption spectra of [Pt(en)₂][Pt(en)₂Cl₂](ClO₄)₄ single crystals have been measured for polarized light at 2 K and room temperature. Contrary to the previous works on polycrystals, the incident photon energy dependence of the Raman cross section due to the symmetric ClPt^IVCl stretching mode has a resonance peak at the charge-transfer absorption edge, which is well explained with the two-band model in a one-dimensional system. Two characteristic structures observed below the absorption edge are concluded not to be related to the charge-transfer transition by measurements of the excitation spectra both of the Raman scattering and of the luminescence. The origin of this luminescence is also discussed.     

Synthesis, crystal structure and spectroscopy properties of Na3AZr(PO4)3 (A=Mg, Ni) and Li2.6Na0.4NiZr(PO4)3 phosphates

Na₃AZr(PO₄)₃ (A=Mg, Ni) phosphates were prepared at 750 °C by coprecipitation route. Their crystal structures have been refined at room temperature from X-ray powder diffraction data using Rietveld method. Li_2.6Na_0.4NiZr(PO₄)₃ was synthesized through ion exchange from the sodium analog. These materials belong to the Nasicon-type structure. Raman spectra of Na₃AZr(PO₄)₃ (A=Mg, Ni) phosphates present broad peaks in favor of the statistical distribution in the sites around PO₄ tetrahedra. Diffuse reflectance spectra indicate the presence of octahedrally coordinated Ni²⁺ ions.     

The crystal structure, vibrational and luminescence properties of the nanocrystalline KEu(WO4)2 and KGd(WO4)2:Eu obtained by the Pechini method

The luminescent nanocrystalline KEu(WO₄)₂ and KGd_0.98Eu_0.02(WO₄)₂ have been prepared by the Pechini method. X-ray diffraction, infrared and Raman spectroscopy as well as optical spectroscopy were used to characterise the obtained materials. The crystal structure of KEu(WO₄)₂ was refined in I2/c space group indicating the isostructurality to KGd(WO₄)₂. The size of the crystalline grains depended on the annealing temperature, increasing with the increase of the temperature. The average size of crystallites of both crystals formed at 540 °C was about 50 nm. Vibrational spectra showed noticeable changes as a function of size due to, among others, phonon confinement effect. Luminescence studies did not reveal significant changes for the nanocrystallites with the lowest grain size in comparison with the bulk material. The differences observed in luminescence spectra in form of slight inhomogeneous broadening of the spectral lines and increase of the hypersensitive I_0-2/I_0-1 ratio point to very low symmetry of Eu³⁺ ions and change of the polarisation of the local vicinities of Eu³⁺. X-ray diffraction, vibrational and optical studies showed that the structure of the synthesised nanocrystalline KEu(WO₄)₂ and KGd(WO₄)₂:Eu is nearly the same as that found for the bulk material. The size-driven phase transitions were established for both compounds.     

Lattice instabilities in Cs2MFe(CN)6 (M = Mg2+, Ca2+, and Sr2+): The crystal structure of Cs2BaFe(CN)6·2H2O

The room temperature Raman spectra of Cs₂MFe(CN)₆ (M = Mg²⁺, Ca²⁺, and Sr²⁺) suggest that these salts undergo phase transformations similar to those found in Cs₂LiCr(CN)₆ where the distortion from the high-symmetry phase proceeds primarily along two modes of vibration. The distortion involves an antiferroelectric rotation of the hexacyanide moiety and a cesium translation. On the basis of the spectra a correlation has been made between the size of M and the apparent transition temperature. In going down the alkaline earths, the apparent transition temperature increases. The structure of the barium salt determined at room temperature shows the crystal latitce contains two waters of hydration. Many similarities have been found between Cs₂BaFe(CN)₆·2H₂O and the low-symmetry phase structure of Cs₂LiCr(CN)₆.     

Vibrational studies of BH−4 and BD−4 isolated in alkali halides

Single crystals of alkali halides doped with BH^?₄ and BD^?₄ were grown from the melt. Previously unreported bands in the infrared spectra of BH^?₄ and BD^?₄ isolated in different alkali halides are interpreted in terms of summation bands of internal and external modes of vibration. This has allowed the torsional and translational modes of the impurity ion to be identified. The tetrahedral symmetry of the borohydride ion is retained when it is isolated within alkali halides with the NaCl structure. A reduction of symmetry towards C_3v was observed when BH^?₄ (or BD^?₄) was isolated within lattices with CsCl structure.Raman and far infrared spectra of alkali halide/BH^?₄ systems will be reported for the first time, and high pressure infrared studies of these systems will be described. The effects of pressure in the internal mode, external modes, Fermi resonance and NaCl to CsCl structural phase changes will be discussed.     

Vibrational spectra of solid tri-μ-carbonyl-(hexacarbonyl)di-iron(O), Fe2(CO)9

Laser Raman spectra of solid tri-μ-carbonyl(hexacarbonyl)di-iron(0), Fe₂,(CO)₉, have been recorded at 295, 100 and 15 K using a surface scanning technique to avoid sample decomposition. The data indicate that no phase change occurs throughout the temperature range studied. The room temperature IR spectrum of the solid was also investigated in the 700-350 cm^?1 region. The vibrational assignments proposed are in good agreement with the factor group predictions for the known P6_3/m (C_6h²) space group symmetry of the crystal.     

Neutron powder diffraction, and solid-state deuterium NMR analyses of Yb2RuD6 and spectroscopic vibrational analysis of Yb2RuD6 and Yb2RuH6

The crystal structure of Yb₂RuD₆ has been determined by neutron powder diffraction and the results were consistent with the Fm3m (#225) space group, a=7.2352(18) Å, with the atoms arranged according to the well-known K₂PtCl₆ structure. No structural phase transition was observed in going from room temperature to 4 K. Raman spectra were not available due to fluorescence, but all fundamental bands and combination bands were assigned from FTIR and PAIR spectra only following previous studies for other alkaline earth and europium ruthenium ternary metal hydrides and deuterides. The deuterium nuclear quadrupole coupling constant, 40.9 kHz, leads to an ionic character of the Ru-D bond of 82%.     

Synthesis, Crystal Structure, Magnetism, and Optical Properties of Gd3[SiON3]O—An Oxonitridosilicate Oxide with Noncondensed SiON3 Tetrahedra

The novel oxonitridosilicate oxide (sion oxide) Gd₃[SiON₃]O was obtained by the reaction of gadolinium metal with its carbonate oxide and silicon diimide in a radiofrequency (r.f.) furnace at a temperature of 1400°C. The crystal structure of Gd₃[SiON₃]O (I4/mcm, a=649.1(2) pm, c=1078.8(6) pm, Z=4, R₁=0.0411, wR₂=0.0769, 405 F² values, 19 parameters, 123 K) is isotypic with that of Ba₃[SiO₄]O and Cs₃[CoCl₄]Cl. It can be derived from the perovskite structure type by a hierarchical substitution: Ti⁴⁺→O^2-, O^2-→Gd³⁺, Ca²⁺→[SiON₃]^7- resulting in the formation of large [OGd₆]¹⁶⁺ octahedra, which are twisted by ξ=16.47(1)° around [001]. The low-temperature single-crystal data investigation led to a crystallographic splitting of the central O atom which could not be resolved at room temperature. The UV-Vis absorption spectra in reflection geometry of the yellow title compound revealed two overlaying broad bands, one peaking at almost the same wavelength as observed in gadolinium oxide (340 nm) and a second red-shifted band at approximately 400 nm indicating a strong influence of nitrogen on the ligand field splitting of the 5d states of Gd³⁺. Temperature-dependent magnetic susceptibility measurements of Gd₃[SiON₃]O show Curie-Weiss behavior from 2 to 300 K with an experimental magnetic moment of 7.68(5) μ_B/Gd, indicating trivalent gadolinium. There is no evidence for magnetic ordering down to 2 K. According to the paramagnetic Curie temperature of −7(1) K, the exchange between the gadolinium magnetic moments is supposed to be only weak. The vibrational spectroscopic data (IR and Raman) are reported.     

Temperature behavior of the protonic conductor K4LiH3(SO4)4

The results of the X-ray structural study for the K₄LiH₃(SO₄)₄ single crystal are presented at a wide temperature range. The thermal expansion of the crystal using the X-ray dilatometry and the capacitance dilatometry from 8 to 500 K was carried out. The crystal structures data collection, solution and refinement at 125, 295, 443 and 480 K were performed. The K₄LiH₃(SO₄)₄ crystal has tetragonal symmetry with the P4₁ space group (Z=4) at room temperature as well as at the considered temperature range. The existence of a low-temperature, para-ferroelastic phase transition at about 120 K is excluded. The layered structure of the crystal reflects a cleavage plane parallel to (001) and an anisotropy of the protonic conductivity. The superionic high-temperature phase transition at T_S=425 K is isostructural. Nevertheless, taking into account an increase of the SO₄ tetrahedra libration above T_S, a mechanism of the Grotthus type could be applied for the proton transport explanation.     

Cation distribution and particle size effect on Raman spectrum of CoFe2O4

Mössbauer and Raman spectroscopic studies were carried out on CoFe₂O₄ particles synthesized with size ranging from 6 to 500 nm (bulk). Cation distribution studies were carried out on the high temperature and room temperature phases of the microcrystalline CoFe₂O₄ by Mössbauer and Raman spectroscopic methods. The high temperature phase of CoFe₂O₄ showed a decreased inversion parameter of 0.69 as compared to the value of the room temperature phase of 0.95, indicating that the structure gradually transforms towards a normal spinel. Corresponding Raman spectra for these two phases of CoFe₂O₄ showed a change in relative peak intensity of the vibrational mode at 695 cm⁻¹(A_1g(1)) to 624 cm⁻¹ (A_1g(2)). The relative peak intensity ratio, I_v between the A_1g(1) and A_1g(2) vibrational mode was decreasing with lowering of inversion parameter of the CoFe₂O₄ spinel system. A variation of laser power on the sample surface was reflected in the cation distribution in ferrite phase. Superparamagnetic, single domain CoFe₂O₄ particles (6 nm) showed a 20 cm⁻¹ red shift and broadening of phonon modes when compared to the macro-crystalline CoFe₂O₄ (500 nm). Variation of Raman shift with particle size was studied by considering the bond polarization model. Raman spectroscopic studies clearly indicate the variation in the cation distribution in nano-sized particles and distribution tending to a normal spinel structural configuration.     

Structural aspects of the metal-insulator transition in V4O7

V₄O₇ has a transition with decreasing temperature at 250 K and the structure has been refined at 298 and 200 K. The triclinic structure (A$\text{1}$) consists of rutile-like layers of VO₆ octahedra extending indefinitely in the a-b plane and four octahedra thick along the c-axis. The average VO distances for the four independent V atoms are 1.967, 1.980, 1.969, and 1.984 Å at 298K and 1.948, 1.992. 1.961, and 2.009 Å at 200K. At 200K there is a clear separation into strings of V³⁺ or V⁴⁺ ions running parallel to the pseudorutile c-axis. In addition, all of the 3+ and half of the 4+ sites are paired to form short VV bonds. The remaining V⁴⁺ atom is displaced toward one oxygen so as to balance its electrostatic charge. The distortion at the metal-insulator transitions in V₄O₇, Ti₄O₇, VO₂ + Cr, and NbO₂ are compared.     

Crystal chemistry peculiarities of Cs2Te4O12

The Raman and IR-absorption spectra of the Cs₂Te₄O₁₂ lattice are first recorded and interpreted. Extraordinary features observed in the structure and Raman spectra of Cs₂Te₄O₁₂ are analyzed by using ab initio and lattice-dynamical model calculations. This compound is specified as a caesium-tellurium tellurate Cs₂Te^IV(Te^VIO₄)₃ in which Te^IV atoms transfer their 5p electrons to [Te^VIO₄]₃⁶⁻ tellurate anions, thus fulfilling (jointly with Cs atoms) the role of cations. The Te^VI-O-Te^VI bridge vibration Raman intensity is found abnormally weak, which is reproduced by model treatment including the Cs⁺ ion polarizability properties in consideration.     

Phase transitions in the A2BX4-compound: Tetramethylammonium tetrachlorozincate tetrachlorocuprate, [(CH3)4N]2Zn0.5Cu0.5Cl4, and room temperature crystal structure determination

Preparation, crystal structure, and infra-red absorption spectra are reported for the first material in an A₂BX₄ compound with metal transition substitution in tetrahedral anion, tetramethylammonium tetrachlorozincate tetrachlorocuprate: [(CH₃)₄N]₂Zn_0.5Cu_0.5Cl₄. The calorimetric study shows five endothermic peaks at 248.75, 271.75, 278.6, 286.7, and 293.7 K. The determination of unit cell in the 240–298 K temperature range confirms those observed by the DSC technique. At room temperature, the compound crystallizes in an orthorhombic system (P2₁ cn space group) with Z = 4 and the following unit cell dimensions: a = 8.988 (3), b = 15.527 (2) and c = 12.269 (4) ?. The structure was solved by using 1986 independent reflections down to an R value of 0.048. The crystal structure consists of alternating organic-inorganic [(TMA)⁺/(Cu)ZnCl₄²⁻] layers and organic sheets (TMA)²⁺. All Organic groups and (Cu)ZnCl₄²⁻ are not disordered. Their main geometrical features are those commonly observed in the atomic arrangements of (TMA)₂ZnCl₄ and (TMA)₂CuCl₄. The text was submitted by the authors in English.     

Phase transitions, structural changes and molecular motions in [Zn(NH3)4](BF4)2 studied by neutron scattering, X-ray powder diffraction and nuclear magnetic resonance

Nuclear magnetic resonance (¹H NMR and ¹⁹F NMR) measurements performed at 90-295 K, inelastic incoherent neutron scattering (IINS) spectra and neutron powder diffraction (NPD) patterns registered at 22-190 K, and X-ray powder diffraction (XRPD) measurements performed at 86-293 K, provided evidence that the crystal of [Zn(NH₃)₄](BF₄)₂ has four solid phases. The phase transitions occurring at: T_C3=101 K, T_C2=117 K and T_C1=178 K, as were detected earlier by differential scanning calorimetry (DSC), were connected on one hand only with an insignificant change in the crystal structure and on the other hand with a drastic change in the speed of the anisotropic, uniaxial reorientational motions of the NH₃ ligands and BF₄⁻ anions (at T_C3 and at T_C2) and with the dynamical orientational order-disorder process (“tumbling”) of tetrahedral [Zn(NH₃)₄]²⁺ and BF₄⁻ ions (at T_C1). The crystal structure of [Zn(NH₃)₄](BF₄)₂ at room temperature was determined by XRPD as orthorhombic, space group Pnma (No. 62), a=10.523 Å, b=7.892 Å, c=13.354 Å and Z=4. Unfortunately, it was not possible to determine the structure of the intermediate and the low-temperature phase. However, we registered the change of the lattice parameters and unit cell volume as a function of temperature and we can observe only a small deviation from near linear dependence of these parameters upon temperature in the vicinity of the T_C1 phase transition.     

The vibrational spectra and crystallographic properties of CsPF6

Indications are that CsPF₆(I) at ambient conditions is cubic with a possible space group of Fm3m-O⁵_h. A slight distortion of the unit cell cannot, however, be ruled out. Assuming Fm3m symmetry the Raman spectra of CsPF₆(I) are consistent with a disordered model in which the PF^?₆ ions are tilted away from the crystallographic axes. The phase transition which occurs below 90 K in CsPF₆ is reflected in the vibrational spectra and further significant changes occur below 60 K particularly in the Raman bands. It is not yet clear whether these changes represent the establishment of long-range order or whether a further phase of CsPF₆ exists below 60 K. A possible structure for CsPF₆ at very low temperatures is discussed.     

Single crystal Raman study of potassium oxalate monohydrate,K2C2O4. H2O,and potassium oxalate monodeuterate,K2C2O4. D2O

Raman spectra of poly crystalline and single crystal K₂C₂O₄. H₂O and K₂C₂O₄. D₂O have been recorded at room temperature. From an earlier neutron diffraction study it is known that the space group is C⁶₂h. The water molecule occupies a C₂ site and the oxalate ion a C₁ site. The assigned water vibrations show small factor group splitting between g modes (Raman active) and u modes (IR active). The internal oxalate vibrations are found to have wavenumbers in good agreement with those reported from Raman studies of other oxalates.     

Absorption spectrum and crystal-field analysis of U4+ ions in CsCdBr3 single crystals

Single crystals of U⁴⁺:CsCdBr₃ were grown by the Bridgman–Stockbarger technique. It has been assumed, that U⁴⁺ ions are substituting two Cd²⁺ ions and possess the C_3v site symmetry. Thirty seven energy levels, located between 4000 and 25,000 cm⁻¹ and encompassing all but the ¹S₀ multiplet, were assigned from 7 K absorption spectra. The symmetry of the levels were determined on the basis of the observed small splitting of the Γ₃ doublets as well as by a comparison of low temperature absorption spectra of the U⁴⁺:CsCdBr₃ with that previously reported for U⁴⁺ in Cs₂UBr₆ and Cs₂ZrBr₆ single crystals. A crystal-field analysis was performed by fitting eight atomic (in the orthogonal formalism) and 6 crystal-field parameters to the experimental Stark levels with an r.m.s. deviation of 100 cm⁻¹. The obtained values of the Hamiltonian parameters are discussed and compared with those reported in previous analyses of U⁴⁺ ions. The relatively strong crystal field, resulting in N_v = 8530 cm⁻¹ proves that in the CsCdBr₃ crystals the U⁴⁺ ions are located at a high symmetry site.