High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.     

Synthesis and high-temperature heat capacity of Yb<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Lu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Yb₂Sn₂O₇ and Lu₂Sn₂O₇ have been prepared by solid-state reactions, by firing mixtures of Yb₂O₃ or Lu₂O₃ and SnO₂ at 1473 K, and the molar heat capacity of these compounds (pyrochlore structure) has been determined by differential scanning calorimetry. The C _p (T) data have been used to evaluate the thermodynamic properties of the stannates: enthalpy increment, entropy change, and reduced Gibbs energy.     

Synthesis and high-temperature heat capacity of Gd<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Gd₂Sn₂O₇ gadolinium stannate with the pyrochlore structure has been prepared by solid-state reaction and its high-temperature heat capacity has been determined by differential scanning calorimetry in the temperature range 350–1020 K. The C_p(T) data are shown to be well represented by the classic Maier–Kelley equation. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of gadolinium stannate: enthalpy increment H°(T)–H°(339 K), entropy change S°(T)–S°(339 K), and reduced Gibbs energy Ф°(Т).     

Heat capacity and thermodynamic properties of crystalline SrB<Subscript>4</Subscript>O<Subscript>7</Subscript>

The heat capacity of a strontium tetraborate (SrB₄O₇) single crystal has been determined in the temperature range 55–300 K by adiabatic calorimetry, and its Debye characteristic temperature, entropy change, enthalpy increment, and phonon mean free path have been calculated as functions of temperature.     

High-temperature heat capacity and thermodynamic properties of TbBiGeO<Subscript>5</Subscript> and DyBiGeO<Subscript>5</Subscript>

Polycrystalline TbBiGeO₅ and DyBiGeO₅ samples have been prepared by solid-state reactions, by firing stoichiometric mixtures of Tb₂O₃ (Dy₂O₃), Bi₂O₃, and GeO₂ in air at 1003, 1073, 1123, 1143, 1173, and 1223 K. The molar heat capacity of the bismuth terbium and bismuth dysprosium germanates has been determined by differential scanning calorimetry. The experimental C _p (T) data obtained in the range 350–1000 K have been used to evaluate the thermodynamic functions of the synthesized oxide compounds: enthalpy increment, entropy change, and reduced Gibbs energy.     

High-temperature thermodynamic properties of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> system

High-temperature mass spectrometry is used to investigate vaporization processes and determine SiO₂ activity in the Al₂O₃-SiO₂ system between 1850 and 1970 K. The results are consistent with the known phase equilibria in this system.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 434–441.Original Russian Text Copyright © 2005 by Bondar, Lopatin, Stolyarova.     

High-temperature phase transition of Tm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The high-temperature (>1600°C) order—disorder phase transition of Tm₂Ti₂O₇ is shown to be irreversible. The 740°C ionic conductivity of nanocrystalline Tm₂Ti₂O₇ ceramics synthesized at 1670°C is 2 × 10^-3 S/cm and remains unchanged upon heat treatment in air at 860°C for 240 h. The conductivity of the high-temperature (disordered pyrochlore) phase of Tm₂Ti₂O₇ is independent of grain size in the range 20–30 nm.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1495–1500.Original Russian Text Copyright © 2004 by Shlyakhtina, Knotko, Larina, Borichev, Shcherbakova.     

Hydrothermal synthesis and photoluminescence properties of Y<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript>:Tb<Superscript>3+</Superscript> phosphors

This article presents the synthesis and photoluminescence (PL) properties of Y₂Zr₂O₇:Tb³⁺. The Tb³⁺-doped Y₂Zr₂O₇ zirconates were successfully synthesized by a hydrothermal process at 200 °C for 20 h. X-ray diffractometer (XRD) patterns revealed that all of the products were phase-pure with the fluorite structure. PL study showed that the Y₂Zr₂O₇:Tb³⁺ phosphors exhibited obvious PL emission peaks which located at 490, 545, 585, and 623 nm; the dominant emission located at 545 nm is assigned to ⁵D₄ → ⁷F₅ transition. Furthermore, Tb³⁺-doping concentration strongly affected the PL properties, and the quenching concentration is 5 at.%.     

High-temperature thermodynamic properties of LuPO<Subscript>4</Subscript>

The high-temperature enthalpy of lutetium orthophosphate has been determined as a function of temperature in the range 432.92–1744.58 K using drop calorimetry. The present and earlier experimental data have been used to calculate temperature-dependent heat capacity of LuPO₄ in the range 1–1750 K.     

A novel Sn<Subscript>2</Subscript>Sb<Subscript>2</Subscript>O<Subscript>7</Subscript> nanophotocatalyst for visible-light-driven H<Subscript>2</Subscript> evolution

A novel pure cubic-phase pyrochlore structure tin(II) antimonate nanophotocatalyst, stoichiometric Sn₂Sb₂O₇, has been prepared by a modified ion-exchange process using an antimonic acid precursor, and employed in visible-light-driven photocatalytic H₂ evolution for the first time. The physicochemical properties (crystal phase, chemical composition and state, textural properties, and optical properties) of the material were investigated by different instrumental techniques. Compared with the antimonic acid precursor, the as-prepared Sn₂Sb₂O₇ had a narrower bandgap, smaller crystal size, and larger BET surface area. The as-prepared Sn₂Sb₂O₇ was validated as a promising candidate for visible-light-driven photocatalytic H2 evolution with a constant rate of 40.10 μmol·h⁻¹·g_cat ⁻¹.      

Synthesis and thermoluminescence properties of CdB<Subscript>4</Subscript>O<Subscript>7</Subscript>:Tb<Superscript>3+</Superscript> and CdB<Subscript>4</Subscript>O<Subscript>7</Subscript>:Mn<Superscript>2+</Superscript>

Polycrystalline samples of undoped, terbium-doped (CdB₄O₇:Tb³⁺), and manganese-doped (CdB₄O₇:Mn²⁺) cadmium tetraborate have been prepared by solid-state reactions at 850°C. Using differential scanning calorimetry and X-ray diffraction, we have determined the melting point of CdB₄O₇ (t _m = 976°C) and shown that this compound melts incongruently. The observed monotonic decrease in the orthorhombic cell parameters of the doped materials indicates the formation of substitutional solid solutions (sp. gr. Pbca). The thermoluminescence intensity of the doped materials has been shown to depend on the nature and concentration of the activators and the irradiation time.     

Growth and properties of LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript>-NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> crystals

Platelike Li_{1 ? x} Na _x Cu₂O₂ single crystals up to 2 × 10 × 10 mm in dimensions have been grown by slowly cooling (1 ? x)Li₂CO₃·xNa₂O₂·4CuO melts in alundum crucibles in air. Li_{1 ? x} Na _x Cu₂O₂ solid solutions in the LiCu₂O₂-NaCu₂O₂ system have been shown to exist in the composition range 0.78 < x < 1. The temperature stability ranges of NaCu₂O₂ and LiCu₂O₂ are 780–930 and 890–1050°C, respectively. The Mössbauer spectra and electrical conductivity of the crystals have been measured.     

Characterization and electrical properties of semiconducting Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-K<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> glasses

Semiconducting glasses of the Fe₂O₃-Bi₂O₃-K₂B₄O₇ system were prepared by the press-quenching method and their dc conductivity in the temperature range 223–393 K was measured. The glass transition temperature values (T_g) of the present glasses were larger than those of tellurite glasses. This indicates a higher thermal stability of the glass in the present system. The density for these glasses was consistent with the ionic size, atomic weight and amount of different elements in the glasses. Mössbauer results revealed that the relative fraction of Fe increases with increasing Fe₂O₃ content. Electrical conductivity showed a similar composition dependency as the fraction of Fe. The glasses had conductivities ranging from 10 to 10 Scm at temperatures from 223 to 393 K. Electrical conduction of the glasses was confirmed to be due to non-adiabatic small polaron hopping and the conduction was primarily determined by hopping carrier mobility.     

Electrical and thermal properties of PTFE-Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> composites

A new polymer-ceramic composite was prepared using PTFE and low loss Sr₂ZnSi₂O₇. The dielectric properties of the composite were studied in the microwave and radiofrequency ranges. The relative permittivity (ε_r) and dielectric loss (tan δ) increased with the filler loading from 0.10 to 0.50 volume fractions (v_f). The observed values of ε_r, thermal conductivity and coefficient of thermal expansion (CTE) were compared with the corresponding theoretical predictions. The ability of the composite towards moisture absorption resistance was studied as a function of filler loading. It was also found that the variation of ε_r was less than 2% in the temperature range 25–90 °C, at 1 MHz. For a filler content of 0.50 v_f, the PTFE/Sr₂ZnSi₂O₇ composite exhibited ε_r = 4.4_, tan δ = 0.003 (at 4–6 GHz), CTE = 38.3 ppm/°C, thermal conductivity = 2.1 W/mK and moisture absorption = 0.09 wt%.     

Synthesis and High-Temperature Heat Capacity of Dy<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Ho<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates have been prepared by solid-state reactions in several sequential firing steps in the temperature range 1237–1473 K using stoichiometric mixtures of Dy₂O₃ (or Ho₂O₃) and GeO₂. The heat capacity of the synthesized germanates has been determined as a function of temperature by differential scanning calorimetry in the range 350–1000 K. The experimentally determined C _p (T) curves of the dysprosium and holmium germanates have no anomalies and are well represented by the Maier–Kelley equation. The experimental C _p (T) data have been used to evaluate the thermodynamic functions of the Dy₂Ge₂O₇ and Ho₂Ge₂O₇ pyrogermanates: enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(T).     

Synthesis and High-Temperature Heat Capacity of Sm<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript> and Eu<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Sm₂Ge₂O₇ and Eu₂Ge₂O₇ germanates have been prepared by solid-state reactions via multistep firing of stoichiometric mixtures of Sm₂O₃ (Eu₂O₃) and GeO₂ in air at temperatures from 1273 to 1473 K. The molar heat capacity of the samarium and europium germanates has been determined by differential scanning calorimetry in the range 350–1000 K and the C _p (T) data have been used to evaluate their thermodynamic properties.     

Anharmonicity in the V<Subscript>2</Subscript>O<Subscript>3</Subscript> molecule and thermodynamic properties of V<Subscript>2</Subscript>O<Subscript>3</Subscript> in the gas phase

A quantum-mechanical calculation of the relative stability, structural parameters, and vibrational frequencies of V₂O₃ molecule isomers for different spin states was carried out using the BPW91/6-311+G(d, p) method. It was shown that the isomer with the C _s structure (nonplanar VOVO rectangle with an O atom attached to it) in the X ⁵ A″ electronic state possesses the maximum stability. The energy of the C _2v symmetry structure was higher than the lowest energy by just 23 cm⁻¹. It definitely indicated the impossibility of usage of the harmonic model in order to calculate the thermodynamic functions of V2O3 (g). A model is proposed based on which the energy levels and vibrational sums of states for this type of motion were calculated for the C _s → C _2v → C _s transition coordinate. These data, as well as results obtained from quantum-mechanical calculations, were used to calculate the thermodynamic functions of V₂O₃ (g) in the temperature range of T = 100–6000 K. The calculations were performed with the five excited electronic states with energies from 1000 to 9000 cm⁻¹ taken into account. A comparison with the data calculated in the “rigid rotator-harmonic oscillator” approximation was performed.     

Thermodynamic properties of SrAl<Subscript>12</Subscript>O<Subscript>19</Subscript> and SrAl<Subscript>4</Subscript>O<Subscript>7</Subscript>

Strontium aluminates are important compounds with interesting properties such as long-duration phosphorescence and elastico-deformation luminescence. They have potential application in flexible light emitting panels. Since there are serious discrepancies in available thermodynamic data for these compounds, a redetermination of their Gibbs energies of formation was undertaken using solid-state electrochemical cells incorporating single-crystal SrF₂ as the electrolyte in the temperature range from 1000 to 1300 K. However, the measurements were restricted to SrAl₁₂O₁₉ and SrAl₄O₇ because of the formation of strontium oxyfluoride phase between SrAl₂O₄ and SrF₂. For the reactions, SrO + 6 Al₂O₃ → SrAl₁₂O₁₉, ΔG ^o/J mol^?1 (± 280) = ?83386 ? 25.744 (T/K), and SrO + 2Al₂O₃ → SrAl₄O₇, ΔG ^o/J mol^?1 (± 240) = ?80187 ? 25.376 (T/K). The high entropy of SrAl₄O₇ and SrAl₁₂O₁₉ can be partly related to their complex structures. The results of this study are consistent with calorimetric data on enthalpy of formation of other Sr-rich aluminates and indicate only marginal stability for SrAl₄O₇ relative to its neighbours, SrAl₁₂O₁₉ and SrAl₂O₄. The thermodynamic data explain the difficulty in direct synthesis of phase pure SrAl₄O₇ and the formation of SrAl₂O₄ as the initial ternary phase when reacting SrO and Al₂O₃ or crystallizing from amorphous state, irrespective of composition.     

Thermochemical and luminescent properties of the K<Subscript>2</Subscript>MgV<Subscript>2</Subscript>O<Subscript>7</Subscript> and M<Subscript>2</Subscript>CaV<Subscript>2</Subscript>O<Subscript>7</Subscript> (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.     

Luminescence studies on Eu<Superscript>2+</Superscript> and Tb<Superscript>3+</Superscript> doped Ca<Subscript>2</Subscript>MgSi<Subscript>2</Subscript>O<Subscript>7</Subscript> phosphors

Eu²⁺ and Tb³⁺ doped Ca₂MgSi₂O₇ phosphors were synthesized by conventional solid-state reaction. The phase formation was confirmed by X-ray powder diffraction technique and refined lattice parameters were calculated by rietveld refinement process using Celref v3. The photoluminescence (PL) excitation and emission spectra were investigated. The phosphors exhibited broaden green emitting luminescence peaking at 520 nm when excited at 374 nm source. Morphological studies were carried out using Scanning electron microscopy (SEM) images of the sample with optimum PL emission. The dependence of photoluminescence intensity on co-dopant concentration and the kinetic parameters were also reported. Time resolved fluorescence spectroscopy (TRFS) is used to investigate the decay in luminescence signals with respect to time. The sample proved to be a good long lasting material, which makes it useful in emergency signs, textile printing, textile exit sign boards and electronic instrument dial pads etc.