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.     

Thermodynamic properties of Dy<Subscript>2</Subscript>O<Subscript>3</Subscript> · 2ZrO<Subscript>2</Subscript> and Ho<Subscript>2</Subscript>O<Subscript>3</Subscript> · 2ZrO<Subscript>2</Subscript> in the range 10–340 K

The low-temperature heat capacity of Dy₂O₃ · 2ZrO₂ and Ho₂O₃ · 2ZrO₂ has been determined by adiabatic calorimetry in the temperature range 10–340 K. The results have been used to calculate the entropy, enthalpy increment, and reduced Gibbs energy of the zirconates without taking into account their low-temperature magnetic transformations.     

Subsolidus Phase Relations in the System Dy<Subscript>2</Subscript>O<Subscript>3</Subscript>-Rh<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Dy₂O₃-Rh₂O₃ system is studied by thermal analysis, x-ray diffraction, and chemical analysis of annealed and quenched samples. The results are used to construct a schematic subsolidus phase diagram of the system. Only one double oxide, DyRhO₃, is obtained. Some of its physicochemical properties are reported.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 8, 2005, pp. 960–965.Original Russian Text Copyright © 2005 by Skrobot, Ugolkov, Grebenshchikov, Gusarov.     

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 electrical conductivity of Ln<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> and LnYTi<Subscript>2</Subscript>O<Subscript>7</Subscript> (Ln = Dy,Ho)

Ho₂Ti₂O₇ and LnYTi₂O₇ (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho₂Ti₂O₇ is 4 × 10^?4 S/cm, and that of HoYTi₂O₇ is 1 × 10^?3 S/cm, with activation energies E _a = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi₂O₇ (3 × 10^?4 S/cm at 740°C, E _a = 1.09 eV) is almost one order of magnitude lower than that of HoYTi₂O₇ (1 × 10^?3 S/cm at 740°C, E _a = 1.17 eV).     

Thermodynamic functions of germanium oxide molecules in the gaseous phase: GeO<Subscript>2</Subscript>(g), Ge<Subscript>2</Subscript>O<Subscript>2</Subscript>(g), and Ge<Subscript>3</Subscript>O<Subscript>3</Subscript>(g)

Critical analysis of experimental and theoretical data on the structure and vibrational frequencies of GeO₂, Ge₂O₂, and Ge₃O₃ molecules is performed. The values of molecular constants are chosen, and thermodynamic functions in the rigid rotator–harmonic oscillator approximation are calculated in temperature interval Т = 298.15–3000 K. The values obtained are introduced into the data base of the IVTANTERMO program complex.     

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).     

The Heat Capacity and Electrophysical Properties of Neodymium and Lithium Chromite NdLiCr<Subscript>2</Subscript>O<Subscript>5</Subscript>

The method of dynamic calorimetry is used to investigate the heat capacity of chromite NdLiCr₂O₅ in the temperature range from 298 to 673 K. A λ-like peak is observed at 523 K.The equations of temperature dependence of the heat capacity of this chromite are derived.The temperature dependences of the thermodynamic functions are calculated.The results of electrophysical investigations indicate that this compound may be classed with semiconductors.     

High-Temperature Heat Capacity and Thermodynamic Properties of HoBiGeO<Subscript>5</Subscript> and ErBiGeO<Subscript>5</Subscript>

Polycrystalline HoBiGeO₅ and ErBiGeO₅ samples have been prepared by solid-state reactions, by firing stoichiometric mixtures of Ho₂O₃ (Er₂O₃), Bi₂O₃, and GeO₂. The effect of temperature on the heat capacity of the synthesized compounds has been investigated by differential scanning calorimetry in the range 350–1000 K. The experimental C_p(T) data have been used to evaluate the thermodynamic functions of bismuth holmium and bismuth erbium germanates: enthalpy increment, entropy change, and reduced Gibbs energy.     

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.     

Photoluminescence of Si<Subscript>0.9</Subscript>Ge<Subscript>0.1</Subscript>O<Subscript>2</Subscript> and GeO<Subscript>2</Subscript> films irradiated with silicon ions

We have studied the photoluminescence (PL) of GeO₂ and 90 mol % SiO₂-10 mol % GeO₂ films synthesized by method of RF magnetron sputtering and then irradiated with silicon ions and annealed. The PL of silicon-implanted GeO₂ films, related to the presence of Si nanocrystals (nc-Si), was observed for the first time. It is established that the transformation of the defect centers responsible for the PL in the spectral range 350–600 nm, as well as the formation of nc-Si emitting in the region of 700–800 nm, significantly depend on the matrix type. In particular, the PL intensity at 700–800 nm in 90 mol % SiO₂-10 mol % GeO₂ films is weak. The role of the isovalent substitution of Si and Ge atoms in the transformation of defect centers and the formation of nc-Si is discussed.     

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.     

Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.     

Electret effect in Pb<Subscript>5</Subscript>Ge<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The short-circuit thermally stimulated depolarization current (TSDC) through lead germanate single crystals has been measured as a function of temperature (100–800 K) and time. The results indicate that poling of the crystals at elevated temperatures (t ≥ 150°C) produces an electret state related to the spacecharge mechanism of polarization. Relatively mild poling conditions (E = 0.75 kV/cm, t = 150°C) produce a well-defined, long-lived electret with an abnormally large electret charge (1400 μC/cm²). The electret discharge leads to heating of the crystal, in the form of a peak in the range 520–570 K, whose height exceeds that of the pyroelectric peak near T _C by two to three orders of magnitude. From the TSDC data, we evaluated the stored electret charge, its density, and the activation energy of traps for electret charges, and made preliminary conclusions as to the nature of the traps. We demonstrate that neglect of the electret discharge current (which depends on a number of factors) when determining the pyroelectric coefficient p ^σ above 280 K leads to a significant scatter in data.     

Twinning modes of Er<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript>

Theory of deformation twinning by Bilby and Crocker is applied to calculate the twinning elements for all possible twinning modes in monoclinic Er₂Si₂O₇ TYPE C and TYPE D. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning mode for monoclinic Er₂Si₂O₇ TYPE C and TYPE D. These predications of the theory are compared with the available experimental information and may act as guideline for future experimental work.     

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.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders

A procedure has been developed for the synthesis of MgAl₂O₄ nanopowders with a characteristic particle size of 10–40 nm. Translucent hydrous xerogels have been synthesized as precursors to MgAl₂O₄. The synthesized magnesium aluminum spinel nanopowders are promising for the fabrication of optical ceramics.     

Synthesis,crystal structure,and proton conductivity of KFe(H<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>)<Subscript>2</Subscript>

KFe H₂P₂O7)₂ is synthesized at 443 K in molten polyphosphoric acids containing K and Fe ions, and its crystal structure is determined: triclinic unit cell with a = 4.9974(6) Å, b = 7.4766(9) Å, c = 7.8185(9) Å, = 82.29(2)°, = 83.37(2)° , = 74.13(2)° ; Z = 1, sp. gr. P . The structure is made up of infinite ribbons formed by corner-shared PO₄ tetrahedra and FeO₆ octahedra, with the K atoms in between. Neighboring ribbons are linked by hydrogen bonds. The proton conductivity of potassium iron(III) dihydrogen diphosphate is rather low.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 74–77. Original Russian Text Copyright © 2005 by Chudinova, Murashova, Ilyukhin, Tarnopolskii, Yaroslavtsev.     

Plasma-chemical Synthesis of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles for Doping of High-Temperature Superconductors

Ferrite nanoparticles (Fe₃O₄) have been produced by the direct low-pressure plasma-chemical synthesis. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), vibration magnetometry (VSM), and Mössbauer spectroscopy (NGR) were used for measurements, showing that the produced nanoparticles have an average size of 9.4 nm, a crystalline phase of magnetite, possess a property of superparamagnetism at room temperature, and have a blocking temperature of 89 K. The peculiarities of nanoparticle behavior in the magnetic field, related to a large specific surface area, are discussed.