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).     

Conductivity of Sm<Subscript>2</Subscript>TiO<Subscript>5</Subscript> and Sm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The oxygen-ion conductivity of porous materials, the coarse-grained pyrochlore-like Sm₂Ti₂O₇ and fine-grained Sm₂TiO₅ compounds, produced by mechanical activation of initial oxides is studied at 400–1000 °C. The Sm₂TiO₅ samples contain ～15 wt % of the nanosized pyrochlore-like Sm₂TiO₅ phase in addition to the rhombic phase. As determined by impedance spectroscopy, the ionic conductivities of Sm₂TiO₅ and Sm₂Ti₂O₇ at 1000°C are 1.3 × 10^?3 and 1.8 × 10^?4 S cm^?1, and the activation energies of the bulk and grainboundary conductivities of the materials are 1.04 and 1.24 eV for Sm₂TiO₅ and 1.69 and 1.80 eV for Sm₂Ti₂O₇.     

Paramagnetism and Superconductivity in Eu<Subscript>0.7</Subscript>Sm<Subscript>0.3</Subscript>Ba<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript>

Magnetic properties of sintered Eu_0.7Sm_0.3Ba₂Cu₃O_7?δ were investigated both in dc and ac magnetic fields. The dc response reflects the interplay between the rare earth ion paramagnetic and the superconducting charge carrier subsystems, respectively. The harmonic susceptibilities exhibit special features: the second harmonic is anomalously high and the third harmonic in zero dc-field has reversed temperature dependence with respect to the theoretical models. The magnetic relaxation at low fields is monotonous and occurs as a two-stage relaxation, each stage obeying logarithmical time dependence with different rates. At high fields, the relaxation is nonmonotonous with a peak at intermediate time suggesting a temporary re-entrance of irreversibility when the flux-line density increases in the center of the sample because of the redistribution of the vortices toward that region.     

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.     

Structure effect on the absorption and circular dichroism spectra of Sm<Subscript>14</Subscript>B<Subscript>6</Subscript>Ge<Subscript>2</Subscript>O<Subscript>34</Subscript> crystals

The spectroscopic and chiroptical properties of rare-earth-rich borate germanate crystals of composition Sm₁₄B₆Ge₂O₃₄ have been studied for the first time. The absorption and circular dichroism spectra of the crystals have been measured. For some of the f-f transitions involved, we have calculated the dipole strength and optical rotatory power.     

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.     

Effect of Eu<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on the crystallization behavior of BaO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

We gave studied the crystallization behavior of 30BaO · 25Bi₂O₃ · 45B₂O₃ glasses doped with Eu₂O₃ to different levels. At a Eu₂O₃ content of 7 mol % or higher, the glasses undergo volume crystallization. The only precipitating phase is a solid solution between europium and bismuth oxides. With increasing europium concentration in the glass, the structure of the crystallites changes from cubic to rhombohedral. We have investigated the morphology, physicochemical properties, and luminescence spectra of the glasses and glass-ceramics.     

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).     

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.     

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.     

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.     

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.     

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.     

Synthesis,characterization and formation mechanism of monodispersed Gd<Subscript>2</Subscript>O<Subscript>2</Subscript>S:Eu<Superscript>3+</Superscript> nanocrystals

Monodispersed Gd₂O₂S:Eu³⁺ nanostructures with tunable morphologies have been selectively fabricated by solvothermal method in the presence of stable inorganic precursors avoiding metalorganic precursors. The size and morphology of the products were controlled successfully by adjusting the reaction conditions. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). The corresponding UV absorption and photoluminescence excitation spectra show a significant blue-shift confirming the quantum confinement effect. A possible growth mechanism for the formation of monodispersed Gd₂O₂S:Eu³⁺ nanocrystals has been proposed. The luminescence mechanism and the size dependence of their fluorescence properties are also 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.     

Intense blue and violet luminescence in the (B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>):Eu<Superscript>2+</Superscript> system

Glassy-crystalline samples of compositions (B₂O₃-Al₂O₃-SiO₂):Eu²⁺ (3 at %) and (B₂O₃-2SiO₂):Eu²⁺ (3 at %) were obtained by sintering the initial powdered mixtures at 1300°C in air. Being excited by laser radiation at a wavelength of 325 nm, the former samples exhibit intense blue photoluminescence with a maximum at 434–448 nm, while the latter samples emit in the violet spectral interval with a maximum at 409 nm. An increase in the content of B₂O₃ leads to a shift of the emission maximum toward a shorter wavelength, while additional annealing at 1300°C C in vacuum shifts the spectrum toward longer wavelengths.     

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.