R. F. Samigullina

Structure, ionic conduction, and phase transformations in lithium titanate Li4Ti5O12

The spinel structure of lithium titanate Li4Ti5O12 is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li4Ti5O12 compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li4Ti5O12 compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)8a[Li0.33Ti1.67]16dO4 → [Li□]16c[Li1.33Ti1.67]16dO4 → [Li1.33□0.67]16c[Ti1.67□0.33]16dO4. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....

Thermochemical and luminescent properties of the K2MgV2O7 and M2CaV2O7 (M = K, Rb, Cs) vanadates

We have studied the compounds K2MgV2O7 and M2CaV2O7 with M = K, Rb, and Cs. These vanadates melt incongruently in the range 635–717°C. Cooling their decomposition products to room temperature leads to the formation of nonequilibrium phase assemblages characteristic of the corresponding oxide systems. The compounds offer broadband photo- and radioluminescence with an essentially white (to the human eye) emission spectrum. A model is proposed for luminescence centers in the vanadates, which involves the formation of defects in vanadium-oxygen groups, and an energy level diagram of the emission centers is constructed in the form of configuration curves in the harmonic oscillator approximation. The luminescent properties of these compounds suggest that they can be used as basic components of cathodo- and roentgenoluminescent screens and white-light-emitting diodes with improved color performance.

Preparation and luminescent properties of rubidium and cesium vanadates

Four rubidium and cesium vanadates, with the compositions Rb5V3O10, Rb3V5O14, Cs5V3O10, and Cs2V4O11, have been synthesized and identified, and their thermal stability ranges have been determined. Their spectroscopic properties and luminescence kinetics have been studied: we have measured their X-ray luminescence spectra at room temperature and liquid-nitrogen temperature, photoluminescence spectra, and photoluminescence excitation spectra. The spectra are compared to those of RbVO3 and CsVO3—phosphors that have optimal properties among the alkali metal vanadates.

Glycolate Ti1 − xFex(OCH2CH2O)2 − x/2 as a precursor for the preparation of quasi-one-dimensional (1D) solid solutions Ti1 − xFexO2 − 2x/2 (0 ≤ x ≤ 0.1)

Quasi-one-dimensional (1D) solid solutions Ti1 − xFex(OCH2CH2O)2 − x/2 (0 < x ≤ 0.1) with the structure of anatase were prepared by heating the glycolate Ti1 − xFex(OCH2CH2O)2 − x/2 in an atmosphere of air at a temperature of >450°C. The conditions of formation and the properties of the new glycolate Ti3Fe2(OCH2CH2O)9 were described. It was found that the synthesized Ti1 − xFexO2 − 2x/2 solid solutions exhibit photocatalytic activity in the reaction of hydroquinone oxidation in an aqueous solution on irradiation with UV light. A correlation between the rate of oxidation of hydroquinone and the concentration of iron in the catalyst was established. A procedure for the preparation of titanium dioxide with the structure of anatase doped with iron and carbon (Ti1 − xFexO(2 − x/2) − yCy) and also composites on its basis, which contain an excess amount of carbon, was proposed.

Thermochemical and Luminescent Properties of RbVO 3 , CsVO 3 , and Rb 0.5 Cs 0.5 VO 3

RbVO3, CsVO3, and Rb0.5Cs0.5VO3 have been synthesized by the Pechini process. The vanadates have an orthorhombic structure (sp. gr. Pbcm), melt congruently in the range 650–530°C, and undergo a reversible phase transition in the range 520–340°C. We have determined the onset temperatures and end points of the transformations at a temperature scan rate of 3°C/min and their enthalpies, and measured the photo-, roentgeno-, and cathodoluminescence and diffuse reflectance spectra of the vanadates. The luminescence spectra each are well fitted with three pseudo-Voigt functions. CsVO3 has the highest integrated emission intensity. The emission intensity of the Rb0.5Cs0.5VO3 solid solution is lower than that of the simple vanadates because of the optical absorption around its intrinsic luminescence band. This may be due to the presence of stable vacancy-type structural defects in Rb0.5Cs0.5VO3.

Thermal stability and cathodoluminescence of potassium strontium vanadates

We have studied the thermal stability of five potassium strontium vanadates: KSr(VO3)3, K2Sr(VO3)4, K4Sr(VO3)6, K6Sr(VO3)8, and KSrVO4. The double orthovanadate undergoes a reversible polymorphic transformation at 1117°C and is stable up to 1500°C. The double metavanadates melt peritectically in the range 490–517°C to give Sr2V2O7 crystals and peritectic melt. Pulsed cathodoluminescence studies have shown that the potassium: strontium ratio in the vanadates and their crystal structure have little effect on their optical emission properties. The performance parameters of new vanadate-based phosphors have been determined.

Thermoanalytical study of the polymorphism and melting behavior of Cu2V2O7

We have developed a procedure for the synthesis of phase-pure α- and β-Cu2V2O7. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu2V2O7 (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu2V2O7, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu2V2O7, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu2V2O7, CuVO3, and other compounds.

Spectroscopic and voltammetric characteristics of α-Zn 2 SiO 4 :V luminophor

Single-phase luminophor Zn2SiO4:V with willemite structure is prepared by hydrothermal method. It is found that the Zn: Si: V ratio between cations is 2: 0.9: 0.1. It is shown that the inclusion of vanadium in the willemite structure reduces the width of the bandgap by ΔEg ≈ 2 eV. Using a combination of spectroscopic and voltammetric techniques, it is found the zinc silicate matrix contains vanadium ions with charge states V5+, V4+, and V3+. In addition, magnetic ions form clusters.

Stabilizing the associated non-autonomous phase upon thermal expansion of Zn 2 V 2 O 7

The previously unknown effect of emergence of an associated non-autonomous phase upon heating of zinc pyrovanadate Zn2V2O7 within the region of negative volume expansion is detected. Comparison of the data of high-temperature X-ray diffraction of the Zn2V2O7 samples synthesized via solution and solid-phase routes shows that the grain size affects the stabilization of the non-autonomous phase. The presence of a non-autonomous phase results in self-dispersion of the substance upon phase transition in heating–cooling cycles.

Structure, oxygen non-stoichiometry, and phase transitions in Ca1–хPrхMnO3–δ

Manganites Ca1–хPrхMnO3–δ, where 0 ≤ х ≤ 0.15 were studied. The compounds have orthorhombic structure (space group Pnma) at ambient temperature, which consecutively transforms on heating in air into phases with tetragonal I4/mcm and cubic 000000 structure. Increase in praseodymium concentration leads to higher phase transition temperatures, lower heat of structural transformations, more narrow homogeneity range of oxygen, and a wider temperature interval of tetragonal I4/mcm phase existence.