The calorimetry and thermodynamic functions of Nd Mg<Stack><Subscript>3</Subscript><Superscript>I</Superscript></Stack>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> (Me<Superscript>I</Superscript>-Li,Na, K) manganites in the range from 298.15 to 673 K

The ceramic technology is employed for synthesizing manganites of composition Nd Mg ₃ ^I Mg₃Mn₄O₁₂(Me^I-Li, Na, K). The X-ray technique is used to find that the compounds crystallize in tetragonal syngony. The parameters of their crystal lattices are determined. Their heat capacities are experimentally determined in the range from 298.15 to 673 K, which enables one to reveal second-order phase transitions. In view of these transitions, equations describing the C _p ^° ～ f(T) dependence are derived, and the thermodynamic functions C _p ^° (T), H°(T)-H°(298.15), S°(T), and Φ ^xx (T) are calculated.     

Heat Capacity and Thermodynamic Functions of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> from 14 to 320 K

The heat capacity of Ga₂Se₃ is measured from 14 to 320 K by adiabatic calorimetry. The smoothed heat capacity data are used to evaluate temperature-dependent thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) of gallium selenide. Under standard conditions, the thermodynamic properties of Ga₂Se₃ are C _p ⁰ (298.15 K) = 120.8 ± 0.2 J/(K mol), S⁰(298.15 K) = 180.4 ± 0.4 J/(K mol), H⁰(298.15 K) - H⁰(0) = 25.32 ± 0.05 kJ/mol, and Φ⁰(298.15 K) = 95.52 ± 0.19 J/(K mol). The Debye characteristic temperature of Ga₂Se₃ evaluated from heat capacity data is 340 ± 10 K.     

The Heat Capacity and Thermodynamic Functions of Ternary Manganites DyM<Superscript>I</Superscript>MgMn<Subscript>2</Subscript>O<Subscript>6</Subscript> (M<Superscript>I</Superscript> − Na,K, Cs) in the Temperature Range from 223 to 673 K

The solid-phase method is used to synthesize ternary manganites of composition DyM^IMgMn₂O₆ (M^I ? Na, K, Cs); their X-ray investigation is performed to demonstrate that they all crystallize in orthorhombic system. Their heat capacity is determined experimentally in the range from 223 to 673 K, equations are derived which describe the dependences C _p ^o ～ ?(T), and the thermodynamic functions C _p ^o , H^o(T)?H^o(298.15), S^o(T), and Φ**(T) are calculated. Second-order phase transitions are observed in the process of investigation of heat capacity.     

A physicochemical study of the perovskites M<Superscript>II</Superscript>(In<Subscript>2/3</Subscript>U<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> (M<Superscript>II</Superscript> = Sr,Ba) at low temperatures

The heat capacity of crystalline Sr(In_2/3U_1/3)O₃ and Ba(In_2/3U_1/3)O₃ in the range 80–350 K was determined by adiabatic vacuum calorimetry, and the thermodynamic functions of these compounds in the range from T → 0 to 350 K were calculated. The standard entropies of formation of these compounds at 298.15 K were calculated. The absolute entropies and standard entropies of formation of perovskites M^II(A^III _2/3U_1/3)O₃ (M^II = Sr, A^III = Sc, In, Fe; M^II = Ba, A^III = Sc, In, Y, Nd-Lu) were estimated.     

Critical Behavior of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>Mn<Subscript>0.95</Subscript>Fe<Subscript>0.05</Subscript>O<Subscript>3</Subscript> Manganite

The critical behavior of perovskite manganite La_0.67Ba_0.33Mn_0.95Fe_0.05O₃ at the ferromagnetic–paramagnetic has been analyzed. The results show that the sample exhibited the second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various such as modified d’Arrott plot Kouvel–Fisher method and critical magnetization M(T _C, H). The critical exponents values for the La_0.67Ba_0.33Mn_0.95Fe_0.05O₃ are close to those expected from the mean field model β = 0.504 ± 0.01 with T _C = 275661 ± 0.447 (from the temperature dependence of the spontaneous magnetization below T _C ), γ = 1.013 ± 0.017 with T _C = 276132 ± 0.452 (from the temperature dependence of the inverse initial susceptibility above T _C ), and δ = 3.0403 ± 0.0003. Moreover, the critical exponents also obey the single scaling equation of M(H, ε) = |ε| ^β f _±(H/|ε| ^β+γ ).     

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 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 Tb<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

Tb₂Sn₂O₇ has been prepared by solid-state reaction in air at 1473 K over a period of 200 h and its isobaric heat capacity has been studied experimentally in the range 350–1073 K. The C _p(T) data for this compound have no extrema and are well represented by the classic Maier–Kelley equation. The experimental C _p(T) data have been used to evaluate the thermodynamic properties of terbium stannate (pyrochlore structure): enthalpy increment H°(T)–H°(350 K), entropy change S°(T)–S°(350 K), and reduced Gibbs energy Ф°(Т).     

Crystal structure of the perovskites Ba<Subscript>2</Subscript>A<Superscript>II</Superscript>UO<Subscript>6</Subscript> (A<Superscript>II</Superscript> = Zn,Cd, Pb)

The compounds Ba₂ZnUO₆, Ba₂CdUO₆, and Ba₂PbUO₆ were prepared by high-temperature solidphase reactions. Their structures (space group Fm \(\bar 3\) m) were refined by the Rietveld method. In the morphotropic series Ba₂A^IIUO₆, correlations were found between the A^II-O, U-O, and Ba-O bond lengths and the crystal-chemical radius of A^II.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

Synthesis and structure of (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>[UO<Subscript>2</Subscript>(CH<Subscript>3</Subscript>COO)<Subscript>3</Subscript>]<Subscript>2</Subscript>(NCS)

The compound (NH₄)₃[UO₂(CH₃COO)₃]₂(NCS) (I) was synthesized and examined by single crystal X-ray diffraction analysis. The compound crystallizes in the rhombic system with the unit cell parameters a = 11.5546(4), b = 18.5548(7), c = 6.7222(3) Å, V = 1441.19(10) ų, space group P2₁2₁2, Z = 2, R = 0.0345. The uranium-containing structural units of crystals of I are isolated mononuclear groups [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes. The specific features of packing of the uranium-containing complexes in the crystal structure are considered.     

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.     

Reduction of U(VI) in POCl<Subscript>3</Subscript>-lewis acid solutions

Features of the behavior of uranyl ions in POC1₃-MCl _x -²³⁵UO ₂₊ ² solutions (M = Ti, Si, Zr, Sn, Sb) were considered. Irreversible accumulation of U(IV) in the course of synthesis of POCl₃-SnCl₄-²³⁵UO ₂₊ ² solutions prepared from water-containing U(VI) compounds, excluding UO₂(ClO₄)₂ · 5H₂O, was found. The reaction rate increases with increasing uranyl concentration, k _l[U(IV)] ～ (1.6±0.2) × 10^?6 s^?1 (T = 380 K). The U(IV) accumulation was also observed on heating (T = 360–380 K) POCl₃-SnCl₄-²³⁵UO ₂₊ ² and POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions hermetically sealed in glass cells and on irradiating them by the light of a xenon lamp. In POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions prepared from UO₂(C1O₄)₂ · 5H₂O, U(IV) disappears within several days after stopping the irradiation. The reduction of U(VI) is caused by formation of uranyl dichlorophosphate complexes and by deactivation of uranyl excitation with chlorine-containing agents.     

Thermal Dissociation of RMnO<Subscript>3</Subscript> (R = Dy,Yb, Lu)

The phase equilibria involved in the thermal dissociation of RMnO₃ (R = Dy, Yb, Lu) were studied in the range 973–1173 K by a static method in a vacuum circulation unit and by x-ray diffraction analysis of quenched solid phases. The RMnO₃ manganites were shown to dissociate by the reaction RMnO₃ = 1/2R₂O₃ + MnO + 1/4O₂. The temperature dependences of the equilibrium oxygen pressure and Gibbs energy change in this reaction were determined for the three compounds. The experimental data were used to evaluate the standard thermodynamic functions of formation of RMnO₃ from R₂O₃ and Mn₂O₃: ΔH⁰(T) = ?88.93 kJ/mol, Δ S⁰(T) = 46.56 J/(mol K) for DyMnO₃; ΔH⁰(T) = ?130.95 kJ/mol, Δ S⁰(T) = 86.25 J/(mol K) for YbMnO₃; ΔH⁰(T) = ?142.94 kJ/mol, Δ S⁰(T) = 102.87 J/(mol K) for LuMnO₃.     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Superconducting Properties of NdO<Subscript>0.5</Subscript><Emphasis Type="Bold">F</Emphasis><Subscript>0.5</Subscript>BiS<Subscript>2</Subscript> Single Crystals

We report on superconducting properties of high-quality single crystals of F-substituted NdOBiS₂ using low-temperature magnetization and transport measurements. Using the mixture of CsCl and KCl as the flux, we have synthesized our single crystals. This compound exhibits bulk superconductivity with a transition temperature of about T _c～4.6 K. The critical current density J _c as a function of temperature has been derived and decreases with the increasing temperature. We construct the phase diagram H _c2(T). The zero-temperature value for \(H_{\mathrm {c2}}^{B\parallel c}\) for value for \(T_{c}^{90~\%}\) and \(T_{c}^{0~\%}\) is estimated to be approximately 2.17 and 1.72 T respectively by using Werthamer-Helfand-Hohenberg model.     

Temperature effect on the photoluminescence intensity and Eu<Superscript>2+</Superscript> excited state lifetime in EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript> and EuGa<Subscript>2</Subscript>S<Subscript>4</Subscript>:Er<Superscript>3+</Superscript>

The photoluminescence (PL) spectra and Eu²⁺ excited state lifetime of EuGa₂S₄ and EuGa₂S₄:Er³⁺ have been studied in the range 78–500 K. The spectra show a band at 545 nm, due to the 4f ⁶5d → 4f ⁷(⁸ S _7/2) transition. With increasing temperature, the full width at half maximum Γ(T) of the PL band of EuGa₂S₄ and EuGa₂S₄:Er³⁺ crystals increases from 0.15 to 0.22 and from 0.13 to 0.19 eV, respectively. Over the entire temperature range studied, Γ(T) is a linear function of T ^1/2. The 545-nm emission intensity and Eu²⁺ excited state lifetime in EuGa₂S₄ and EuGa₂S₄:Er³⁺ vary exponentially with temperature. The luminescence quenching energies evaluated from the Arrhenius plots of I(10³/T) and τ(10³/T) coincide (0.10 eV) within the error of determination.     

Critical Temperature of Smart Meta-superconducting MgB<Subscript>2</Subscript>

Enhancing the critical temperature (T _C ) is important not only to widen the practical applications but also to expand the theories of superconductivity. Inspired by the meta-material structure, we designed a smart meta-superconductor consisting of MgB₂ microparticles and Y₂O₃/Eu³⁺ nanorods. In the local electric field, Y₂O₃/Eu³⁺ nanorods generate an electroluminescence (EL) that can excite MgB₂ particles, thereby improving the T _C by strengthening the electron–phonon interaction. An MgB₂-based superconductor doped with one of four dopants of different EL intensities was prepared by an ex situ process. Results showed that the T _C of MgB₂ doped with 2 wt% Y₂O₃, which is not an EL material, is 33.1 K. However, replacing Y₂O₃ with Y₂O₃/Eu³⁺II, which displays a strong EL intensity, can improve the T _C by 2.8 to 35.9 K, which is even higher than that of pure MgB₂. The significant increment in T _C results from the EL exciting effect. Apart from EL intensity, the micromorphology and degree of dispersion of the dopants also affected the T _C . This smart meta-superconductor provides a new method to increase T _C .     

Magnetic,magnetocaloric properties,and critical behavior in a layered perovskite La<Subscript>1.4</Subscript>(Sr<Subscript>0.95</Subscript>Ca<Subscript>0.05</Subscript>)<Subscript>1.6</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La_1.4(Sr_0.95Ca_0.05)_1.6Mn₂O₇. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature T _C = 248 K, a Neel transition at T _N = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??S _M) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T _C = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M _S(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??S _M) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??S _M) vs. M ²] and the classical extrapolation from the Arrott curves (µ₀H/M vs. M ²), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well.