Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Anisotropic thermal expansion and hydrogen bonding behavior of portlandite: A high-temperature neutron diffraction study

The structure of deuterated portlandite, Ca(OD)₂, was investigated using time-of-flight neutron diffraction in the temperature range 308-643 K. Rietveld analysis reveals that with increasing temperature, the c dimension expands at a rate ∼4.5 times larger than that for a. This anisotropy of thermal expansion is due to rapid increase in the interlayer thickness along c with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, α=α₀+α₁T, where α₀=5.966×10⁻⁵ K⁻¹ and α₁=3.333×10⁻⁸ K⁻². On heating, hydrogen-mediated interatomic interactions within the interlayer become weakened, as reflected by increases in the interlayer D?O and D?D distances with increasing temperature. Correspondingly, the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at high temperatures.     

Studies on lattice thermal expansion and XPS of ThO2NdO1.5 solid solutions

The lattice parameter changes with respect to temperature (T) have been measured by high temperature X-ray diffraction (HTXRD) technique for ThO₂NdO_1.5 solid solutions containing 23.8 and 42.5 mol% NdO_1.5 in the temperature range from 298 to 2000 K. The temperature versus lattice parameter data have been made use of in calculating the lattice thermal expansivity. The values of thermal expansion of the solid solutions were found to be increased with increase in neodymium oxide content and temperature. The mean linear thermal expansion coefficients in this temperature range for ThO₂NdO_1.5 solid solutions are 12.28 × 10⁻⁶ and 12.90 × 10⁻⁶ K⁻¹, respectively. The binding energies of Th 4f_7/2 and Nd 3d_5/2 energy levels of the solid solutions containing 13.1, 23.8, 31.9, 37.2 and 42.5 mol% NdO_1.5 and two-phase mixtures containing 47.6 and 51.8 mol% NdO_1.5 were experimentally determined by X-ray photoelectron spectroscopy (XPS).     

Mixed conductivity and Mössbauer spectra of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (x=0-0.05, y=0-0.30)

Aluminum incorporation in the rhombohedrally distorted perovskite lattice of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ (x=0-0.05, y=0-0.30) decreases the unit cell volume and partial ionic and p-type electronic conductivities, while the oxygen nonstoichiometry and thermal expansion at 900-1200 K increase on doping. The creation of A-site cation vacancies has an opposite effect on the transport properties of Al-substituted ceramics. The maximum A-site deficiency tolerated by the (La,Sr)(Fe,Al)O_3−δ structure is however limited, close to 3-4%. The Mössbauer spectroscopy revealed progressive localization of electron holes and a mixed charge-compensation mechanism, which results in higher average oxidation state of iron when Al³⁺ concentration increases. The average thermal expansion coefficients of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ are (12.2-13.0)×10⁻⁶ K⁻¹ at 300-900 K and (20.1-30.0)×10⁻⁶ K⁻¹ at 900-1200 K in air. The steady-state oxygen permeability (OP) of dense Al-containing membranes is determined mainly by the bulk ionic conductivity. The ion transference numbers at 973-1223 K in air, calculated from the oxygen permeation and faradaic efficiency (FE) data, vary in the range 1×10⁻⁴-3×10⁻³, increasing with temperature.     

Thermophysical properties of SrHfO3 and SrRuO3

Polycrystalline samples of strontium series perovskite type oxides, SrHfO₃ and SrRuO₃ were prepared and the thermophysical properties were measured. The average linear thermal expansion coefficients are 1.13×10⁻⁵ K⁻¹ for SrHfO₃ and 1.03×10⁻⁵ K⁻¹ for SrRuO₃ in the temperature range between 423 and 1073 K. The melting temperatures T_m of SrHfO₃ and SrRuO₃ are 3200 and 2575 K, respectively. The longitudinal and shear sound velocities were measured by an ultrasonic pulse-echo method at room temperature in air, which enables to evaluate the elastic moduli and Debye temperature. The heat capacity was measured by using a differential scanning calorimeter, DSC in high-purity argon atmosphere. The thermal diffusivity was measured by a laser flash method in vacuum. The thermal conductivities of SrHfO₃ and SrRuO₃ at room temperature are 5.20 and 5.97 W m⁻¹ K⁻¹, respectively.     

Preparation, crystal structure and thermal expansion of a new bismuth barium borate, BaBi2B4O10

Single crystals of a new compound, BaBi₂B₄O₁₀ were grown by cooling a melt with the stoichiometric composition. The crystal structure of the compound has been solved by direct methods and refined to R₁=0.049 (wR=0.113) on the basis of 1813 unique observed reflections (|F_o|>4σ|F_o|). It is monoclinic, space group P2₁/c, a=10.150(2), b=6. 362(1), c=12.485(2) Å, β=102.87(1)^o, V=786.0(2) Å³, Z=4. The structure is based upon anionic thick layers that are parallel to (001). The layers can be described as built from alternating novel borate [B₄O₁₀]⁸⁻_∞ chains and bismuthate [Bi₂O₅]⁴⁻_∞ chains extended along b-axis. The borate chains are composed of [B₃O₈]⁷⁻ triborate groups of three tetrahedra and single triangles with a [BO₂]⁻ radical. The borate chains are interleaved along the c-axis with rows of the Ba²⁺ cations so that the Ba atoms are located within the layers. The layers are connected by two nonequivalent Ba-O bonds as well as by two equivalent Bi-O bonds with bond valences in the range of 0.2-0.3 v.u.Thermal expansion of BaBi₂B₄O₁₀ studied by high-temperature X-ray powder diffraction in the temperature range of 20-700 °C (temperature step 30-35 °C) is highly anisotropic. While the b and c unit-cell parameters increase almost linearly on heating, temperature dependencies of a parameter and β monoclinic angle show nonlinear behavior. As a result, on heating orientation of thermal expansion tensor changes, and bulk thermal expansion increases from 20×10⁻⁶ °C⁻¹ at the first heating stage up to 57×10⁻⁶ °C⁻¹ at 700 °C that can be attributed to the increase of thermal mobility of heavy Bi³⁺ and Ba²⁺ cations.     

Structural changes accompanying negative thermal expansion in Zr2(MoO4)(PO4)2

Zr₂(MoO₄)(PO₄)₂ is orthorhombic (Sc₂W₃O₁₂ structure) from 9 to at least 400 K, and shows anisotropic volume negative thermal expansion (α_a=−8.35(4)×10⁻⁶ K⁻¹; α_b=3.25(3)×10⁻⁶ K⁻¹; α_c=−8.27(5)×10⁻⁶ K⁻¹ in the range 122-400 K) similar in magnitude to A₂M₃O₁₂ (M—Mo or W) with large A³⁺. The contraction on heating is associated with a pattern of Zr-O-Mo/P bond angle changes that is somewhat similar, but not the same as that for Sc₂W₃O₁₂. On heating, the most pronounced reductions in the separation between the crystallographic positions of neighboring Zr and P are not associated with significant reductions in the corresponding Zr-O-P crystallographic bond angles, in contrast to what was seen for Sc₂W₃O₁₂.     

Lattice crossover and mixed valency in the LaCo1−xRhxO3 solid solution

The full LaCo_1−xRh_xO₃ solid solution was investigated utilizing structural, electrical transport, magnetic, and thermal conductivity characterization. Strong evidence for at least some conversion of Rh³⁺/Co³⁺ to Rh⁴⁺/Co²⁺ is found in both structural and electrical transport data. The crystal structure is that of a rhombohedrally distorted perovskite over the range 0.0≤x≤0.1. The common orthorhombic distortion of the perovskite structure is found over the range 0.2≤x≤1.0. A crossover of all three orthorhombic cell edges occurs at x=0.5 giving the appearance of a cubic structure, which actually remains orthorhombic. The octahedra in the orthorhombic structure must be distorted for x values less than 0.5, and the observed distortion suggests orbital ordering for Co²⁺. Electrical resistivity measurements as a function of temperature show semiconducting-like regions for all compositions. There is a steady increase in electrical resistivity as the Rh content increases. Large positive thermopower values are generally obtained above 475 K. With increasing Rh substitution there is a decrease in thermal conductivity, which slowly rises with increasing temperature due to increased electrical conductivity. The electronic part of the thermal conductivity is suppressed significantly upon Rh substitution. A thermoelectric figure-of-merit (ZT) of about 0.075 has been achieved for LaCo_0.5Rh_0.5O₃ at 775 K, and is expected to reach 0.15 at 1000 K.     

Microstructure and magnetic properties of electrospun one-dimensional Al-substituted SrFe12O19 nanofibers

SrAl_xFe_12−xO₁₉ (x=0-3.0) nanofibers with diameters about 100 nm have been prepared by electrospinning and subsequent heat treatment. With Al³⁺ ion content ranging from 0 to 3.0, the lattice parameters decrease due to Fe³⁺ ions substituted by smaller Al³⁺ ions and the average grain size calculated by the Scherrer's equation reduces from 65 to 37 nm. The magnetization shows a continuous reduction with the Al content and its value measured at 77 K is higher than at room temperature, which can be explained by Bloch's law. For the coercivity, its value initially increases, reaching a maximum value of 617 (298 K) and 547 kA m⁻¹ (77 K) at x=2.0, and then reduces with the Al content further increase largely arising from the substituted Al³⁺ ion arrangement in different interstitial sites of the strontium ferrite unit cell.     

Heat capacity and thermodynamic properties of crystalline ornidazole (C7H10ClN3O3)

The molar heat capacities of 1-(2-hydroxy-3-chloropropyl)-2-methyl-5-nitroimidazole (Ornidazole) (C₇H₁₀ClN₃O₃) with purity of 99.72 mol% were measured with an adiabatic calorimeter in the temperature range between 79 and 380 K. The melting-point temperature, molar enthalpy, Δ_fusH_m, and entropy, Δ_fusS_m, of fusion of this compound were determined to be 358.59±0.04 K, 21.38±0.02 kJ mol⁻¹ and 59.61±0.05 J K⁻¹ mol⁻¹, respectively, from fractional melting experiments. The thermodynamic function data relative to the reference temperature (298.15 K) were calculated based on the heat capacities measurements in the temperature range from 80 to 380 K. The thermal stability of the compound was further investigated by DSC and TG. From the DSC curve an intensive exothermic peak assigned to the thermal decomposition of the compound was observed in the range of 445-590 K with the peak temperature of 505 K. Subsequently, a slow exothermic effect appears when the temperature is higher than 590 K, which is probably due to the further decomposition of the compound. The TG curve indicates the mass loss of the sample starts at about 440 K, which corresponds to the decomposition of the sample.     

High-temperature transport properties, thermal expansion and cathodic performance of Ni-substituted LaSr2Mn2O7−δ

The substitution of manganese with nickel in LaSr₂Mn₂O_7−δ, where the solubility limit corresponds to approximately 25% Mn sites, enhances the Ruddlesden-Popper phase stability at elevated temperatures and atmospheric oxygen pressure. The total conductivity of LaSr₂Mn_2−yNi_yO_7−δ (y=0-0.4) decreases with nickel additions, whilst the average thermal expansion coefficients calculated from dilatometric data in the temperature range 300-1370 K increase from (11.4-13.7)×10⁻⁶ K⁻¹ at y=0 up to (12.5-14.4)×10⁻⁶ K⁻¹ at y=0.4. The conductivity and Seebeck coefficient of LaSr₂Mn_1.6Ni_0.4O_7−δ, analyzed in the oxygen partial pressure range 10⁻¹⁵-0.3 atm at 600-1270 K, display that the electronic transport is n-type and occurs via a small polaron mechanism. Reductive decomposition is observed at the oxygen pressures close to Ni/NiO boundary, namely ∼2.3×10⁻¹¹ atm at 1223 K. Within the phase stability domain, the electronic transport properties are essentially p(O₂)-independent. The steady-state oxygen permeability of dense LaSr₂Mn_1.6Ni_0.4O_7−δ membranes is higher than that of (La,Sr)MnO_3−δ, but lower if compared to perovskite-like (Sr,Ce)MnO_3−δ. Porous LaSr₂Mn_1.6Ni_0.4O_7−δ cathodes in contact with apatite-type La₁₀Si₅AlO_26.5 solid electrolyte exhibit, however, a relatively poor electrochemical performance, partly associated with strong cation interdiffusion between the materials.     

New high permittivity tetragonal tungsten bronze dielectrics Ba2LaMNb4O15: M=Mn, Fe

The new phases Ba₂LaMNb₄O₁₅: M=Mn, Fe were prepared by solid state reaction at 1100 °C. They have the tetragonal tungsten bronze structure, space group P4/mbm, at room temperature. The two octahedral sites show partial order of M and Nb with preferential occupancy of the smaller B(1) sites by M. Both phases have high permittivities 90±15 over the range 10-320 K. Ba₂LaFeNb₄O₁₅ is highly insulating with bulk conductivity ?10⁻⁸ ohm⁻¹ cm⁻¹ at 25 °C and tan δ?0.001 over the range 100-320 K and at 10⁵ Hz. Solid solutions between these new phases and the compositionally and structurally related relaxor ferroelectric Ba₂LaTi₂Nb₃O₁₅ show gradual loss of ferroelectric behaviour attributed to replacement of polarisable Ti⁴⁺ by a mixture of (Mn, Fe)³⁺ and Nb⁵⁺.     

Dependence of the mixed alkali effect on temperature and total alkali oxide content in y[xLi2O·(1−x)Na2O]·(1−y)B2O3 glasses

The complex conductivity spectra of mixed alkali borate glasses of compositions y [xLi₂O·(1−x)Na₂O]·(1−y)B₂O₃ (with x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0; y=0.1, 0.2, 0.3) in a frequency range between 10⁻² Hz and 3 MHz and at temperatures ranging from 298 to 573 K have been studied. For each glass composition the conductivities show a transition from the dc values into a dispersive regime where the conductivity is found to increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. Mixed alkali effects (MAEs) in the dc conductivity and activation energy are identified and discussed. It has been for the first time found that the strength of the MAE in the logarithm of the dc conductivity linearly increases with the total alkali oxide content, y, and the reciprocal temperature, 1/T.     

Cu2Sc2Ge4O13, a novel germanate isotypic with the quasi-1D compound Cu2Fe2Ge4O13 between 100 and 298 K

The germanate compound Cu₂Sc₂Ge₄O₁₃ has been synthesized by solid-state ceramic sintering techniques between 1173 and 1423 K. The structure was solved from single-crystal data by Patterson methods. The title compound is monoclinic, a=12.336(2) Å, b=8.7034(9) Å, c=4.8883(8) Å, β=95.74(2), space group P2₁/m, Z=4. The compound is isotypic with Cu₂Fe₂Ge₄O₁₃, described very recently. The structure consists of crankshaft-like chains of edge-sharing ScO₆ octahedra running parallel to the crystallographic b-axis. These chains are linked laterally by [Cu₂O₆]⁸⁻ dimers forming a sheet of metal-oxygen-polyhedra within the a-b plane. These sheets are separated along the c-axis by [Ge₄O₁₃]¹⁰⁻ units. Cooling to 100 K does not alter the crystallographic symmetry of Cu₂Sc₂Ge₄O₁₃. While the b, c lattice parameter and the unit cell volume show a positive linear thermal expansion (α=6.4(2)×10⁻⁶, 5.0(2)×10⁻⁶ and 8.3(2)×10⁻⁶ K⁻¹ respectively), the a lattice parameter exhibits a negative thermal expansion (α=−3.0(2)×10⁻⁶ K⁻¹) for the complete T-range investigated. This negative thermal expansion of a is mainly due to the increase of the Cu-Cu interatomic distance, which is along the a-axis. Average bond lengths remain almost constant between 100 and 298 K, whereas individual ones partly show both significant shortages and lengthening.     

Heat capacity, enthalpy and entropy of ternary bismuth tantalum oxides

Heat capacity and enthalpy increments of ternary bismuth tantalum oxides Bi₄Ta₂O₁₁, Bi₇Ta₃O₁₈ and Bi₃TaO₇ were measured by the relaxation time method (2-280 K), DSC (265-353 K) and drop calorimetry (622-1322 K). Temperature dependencies of the molar heat capacity in the form C_pm=445.8+0.005451T−7.489×10⁶/T² J K⁻¹ mol⁻¹, C_pm=699.0+0.05276T−9.956×10⁶/T² J K⁻¹ mol⁻¹ and C_pm=251.6+0.06705T−3.237×10⁶/T² J K⁻¹ mol⁻¹ for Bi₃TaO₇, Bi₄Ta₂O₁₁ and for Bi₇Ta₃O₁₈, respectively, were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S°_m(298.15 K)=449.6±2.3 J K⁻¹ mol⁻¹ for Bi₄Ta₂O₁₁, S°_m(298.15 K)=743.0±3.8 J K⁻¹ mol⁻¹ for Bi₇Ta₃O₁₈ and S°_m(298.15 K)=304.3±1.6 J K⁻¹ mol⁻¹ for Bi₃TaO₇, were evaluated from the low-temperature heat capacity measurements.     

Improvement of Er emissions in oxyfluoride glass ceramic nano-composite by thermal treatment

In order to improve the 1.53 μm emission of Er³⁺-doped oxyfluoride glass ceramic containing CaF₂ nano-crystals, series of samples with same Er³⁺ doping lever thermal treated under different conditions were prepared. The UV-VIR-NIR absorption spectra, near-infrared and up-conversion emission spectra, and ⁴I_13/2 decay curves were measured. Based on Judd-Ofelt theory, the radiative transition probability, fluorescence branching ratio and radiative decay time of various metastable transitions of precursor glass and glass ceramics were evaluated. With the increasing of heating temperature, the Judd-Ofelt intensity parameter Ω₂ monotonously decreased from 4.39×10⁻²⁰ to 2.72×10⁻²⁰ cm²; the emission lifetime and quantum efficiency significantly increased from 5.9 to 8.0 ms and 70% to 98%, respectively. The wavelength dependence of gain cross-sections of oxyfluoride glass and glass ceramics were computed to be relatively flat in the range of 1530-1565 nm for population inversion from 0.7 to 1.0.     

Two highly proton-conductive molecular hybrids based on ionized water clusters and poly-Keggin-anion chains

Two proton-conductive molecular hybrid complexes, {[Zn(H₂O)₈][H(H₂O)₂](HINO)₄(PMo₁₂O₄₀)}_n (1) and {[Mn(H₂O)₈][H(H₂O)_2.5](HINO)₄(PMo₁₂O₄₀)}_n (2), were constructed by introducing protonated water clusters, transition metal ionized water clusters and [PMo₁₂O₄₀]³⁻ anions in the gallery of H-bonding networks based on isonicotinic acid N-oxide (HINO). Single-crystal X-ray diffraction analyses at 293 K revealed that both complexes presented exactly the same three-dimensional (3D) hydrogen-bonded networks with large one-dimensional (1D) channels. Interestingly, [PMo₁₂O₄₀]³⁻ anions just filled in the 1D channels and self-assembled into poly-Keggin-anion chains. Thermogravimetric analyses both show no weight loss in the temperature range of 20-100 °C, indicating that all water molecules in the unit structure are not easily lost below 100 °C. Surprisingly, the proton conductivities of 1 and 2 in the temperature range of 85-100 °C under 98% RH conditions reached high proton conductivities of 10⁻³ S cm⁻¹. A possible mechanism of the proton conduction was proposed according to the experimental results.     

Low temperature structural variations and molar heat capacity of stolzite, PbWO4

The crystal structure, thermal expansion and heat capacity of PbWO₄ (mineral name stolzite) scintillator material were comprehensively studied over a wide temperature range. No phase transitions were found down to 2 K (I4₁/a, scheelite structure type). A distinct feature of the temperature induced structural variations in PbWO₄ are the different thermal elongations of shorter and longer Pb-O distances. The low-temperature thermal expansion of PbWO₄ was parameterized on the basis of the 1st order Grüneisen approximation using a Debye function for the internal energy with a Debye temperature of 237 K, a bulk modulus of 67 GPa and a Grüneisen parameter of 1.08. The expansion along the c-axis is about 2.5-3 times higher in the range 23-290 K than along the a-direction. This pronounced anisotropy of the thermal expansion arises from the arrangements of rigid tetrahedral WO₄²⁻ units along 〈100〉-directions while Pb²⁺ cations occupy the sites between WO₄²⁻ in 〈001〉-directions.     

Structure and magnetic properties of the cubic oxide fluoride BaFeO2F

Fluorination of the parent oxide, BaFeO_3−δ, with polyvinylidine fluoride gives rise to a cubic compound with a=4.0603(4) Å at 298 K. ⁵⁷Fe Mössbauer spectra confirmed that all the iron is present as Fe³⁺. Neutron diffraction data showed complete occupancy of the anion sites, indicating a composition BaFeO₂F, with a large displacement of the iron off-site. The magnetic ordering temperature was determined as T_N=645±5 K. Neutron diffraction data at 4.2 K established G-type antiferromagnetism with a magnetic moment per Fe³⁺ ion of 3.95 μ_B. However, magnetisation measurements indicated the presence of a weak ferromagnetic moment that is assigned to the canting of the antiferromagnetic structure. ⁵⁷Fe Mössbauer spectra in the temperature range 10-300 K were fitted with a model of fluoride ion distribution that retains charge neutrality of the perovskite unit cell.