Several experimental thermochemical techniques-adiabatic calorimetry, differential scanning calorimetry, drop calorimetry, and drop solution calorimetry-were employed to obtain the heat capacity dependences and the enthalpy of formation of the single-phase complex-composition ("high-entropy") oxide (MgCoNiCuZn)0.2O with the rock-salt structure. The enthalpy of formation from the binary oxides MO (M is either Mg, Co, Ni, Cu, or Zn) was found to be (6.92 +/- 0.65) kJ mol-1, which is quite different from the previously reported estimates. The heat capacity data for (MgCoNiCuZn)0.2O between 0 K and 1420 K were successfully described by the sum of the Debye and Einstein terms. The resulting function was used to derive the standard Gibbs function, G degrees(T), that is valid up to at least 1420 K - almost in the whole T range where (MgCoNiCuZn)0.2O is near oxygen-stoichiometric in air, as the thermogravimetric analysis demonstrated that in air (MgCoNiCuZn)0.2O exhibits oxygen exchange above approximate to 1450 K. Based on the obtained G degrees(T) we confirmed the primary role of configurational entropy in stabilizing the (MgCoNiCuZn)0.2O phase relative to the constituent binary oxides. However, the nonconfigurational entropy change, which comprises sizeable magnetic and vibrational contributions with opposite signs, constitutes more than 15% of the positive entropy of formation from the binary oxides. The thermodynamic description of (MgCoNiCuZn)0.2O allowed us to demonstrate that solid solution equilibria must be taken into account when the real-life stability of complex-composition oxides is assessed, highlighting the need for more experimental thermodynamic data and pointing out the dangers of using computation-based thermodynamic quantity estimates when it is not known how accurate they are with respect to the actual experimental values.

Due to their unique magnetic and electronic properties, materials based on rare-earth perovskite-type cobaltites are promising for electronic and energy conversion applications. In this study, several single-phase cobaltites (5R0.2)CoO3-s with five different rare-earth elements R in the A-sublattice were synthesized via the glycerol-nitrate technique. X-ray diffraction (XRD) confirmed the orthorhombic (Pnma) perovskite structure of (5R0.2) CoO3-s, and energy-dispersive microanalysis showed a uniform cation distribution in these oxides. The heat capacity of (5R0.2)CoO3-s was investigated using differential scanning calorimetry, and the enthalpy of formation was determined via reduction reaction calorimetry and drop solution calorimetry. The heat capacity curves, dominated above room temperature by the broad peak associated with the cobalt spin state transition, closely resemble those of the "simple" rare-earth cobaltites RCoO3-s. Similarly, trends in the enthalpies of reduction in hydrogen and the enthalpies of formation from binary oxides (R2O3 and CoO) are identical for (5R0.2)CoO3-s and RCoO3-s. This indicates enhanced stability for the oxides with larger average radius of the rare-earth ions (i.e., the Goldschmidt tolerance factor closer to 1). The calculated enthalpies of mixing in the A-sublattice of (5R0.2) CoO3-s are close to zero, suggesting that these solid solutions can be regarded as near-ideal ones. The absence of significant enthalpic destabilization in the (5R0.2)CoO3-s oxide systems indicates the feasibility of synthesizing multiple perovskite cobaltites that are thermodynamically stable in wide temperature ranges, down to room temperature. This offers the potential to finely tune A-site ion sizes and dependent properties by varying the complex composition.

Thermodynamics of the formation of Cs4PbX6 (X = Cl, Br, I) halides, promising for various optoelectronic applications, was studied at 298.15 K via solution calorimetry and analysis of solution equilibria. The compounds in question were synthesized by a solid-state reaction technique from binary halides and characterized by powder X-ray diffraction. They were shown to be intrinsically stable with respect to decomposition to a mixture of either binary halides or CsPbX3 and CsX (X = Cl, Br, I) under standard conditions at 298.15 K. The interaction with gaseous components of the ambient atmosphere, such as O2, CO2 and H2O, was shown to be thermodynamically unfavorable, whereas the presence of liquid water is detrimental since the latter will degrade all the Cs4PbX6 (X = Cl, Br, I) halides leaching the CsX salts. The change in thermodynamic stability depending on the composition of the halides Cs4PbX6 is similar to that observed for the perovskites CsPbX3, i.e., the chlorides are the most stable, and the iodides are the least stable to interaction with external gaseous agents. At the same time, complex halides Cs4PbX6 (X = Cl, Br, and I) are significantly more stable than the corresponding perovskite counterparts. Also, based on the results obtained, the thermodynamic guidelines for growing high-quality single crystals of various cesium-lead halides were discussed.

The single-phase Pr1-xBaxCoO3-delta (0 < x <= 1/3) ceramic oxides were successfully synthesized using the standard ceramic technique. They possess an orthorhombic Pbnm structure at room temperature after cooling in air. However, an orthorhombic (Pbnm) -> rhombohedral (R<(3)over bar>c) phase transition occurs in Pr1-xBaxCoO3-delta upon heating in air. The spin state transition of cobalt ions contributes significantly to the thermal expansion of Pr1-xBaxCoO3-delta with low barium content (x <= 0.1) between room temperature and around 450 degrees C. The temperatures of structural and spin state transitions depend on the Ba content. In addition, oxygen exchange at high temperatures causes noticeable chemical expansion of Pr1-xBaxCoO3-delta. By analyzing the thermochemical expansion and nonstoichiometry data for Pr1-xBaxCoO3-delta, both measured in air, we proposed and verified a novel approach to evaluating the defect structure and thermodynamics of defect equilibria in oxide ceramic materials. The obtained defect structure model was shown to predict accurately the experimental pO(2)-T-delta data of Pr1-xBaxCoO3-delta perovskites.

For three perovskites from the Sr(Ti,Fe)O3-delta (STF) system-SrTi0.7Fe0.3O3-delta (STF30), SrTi 0.5 Fe 0.5 O 3-delta (STF50), and SrTi 0.3 Fe 0.7 O 3-delta (STF70)-the enthalpy of oxidation, Delta H ox , measured by Calvet calorimetry, was found to be an almost temperature-independent function of delta . The data on Delta H ox ( delta ) allowed subtracting the redox contribution from the drop calorimetric measurement results, yielding the enthalpy increments of STF with fixed oxygen content. The heat capacities of these STF oxides were successfully described by the single Einstein term, with the fitted Einstein temperatures depending almost linearly on titanium content. The additional dilatometric and thermogravimetric measurements were used to estimate the thermal (TEC) and chemical expansion coefficients of STF. While TEC somewhat decreases with titanium content, the chemical expansion remains rather high for all STF. Finally, the obtained thermochemical data were used to compare the energy and oxygen storage capacities of different STF with those of undoped strontium ferrite; both were found to decrease noticeably with titanium substitution. Hence, in applications such as chemical looping, e.g., in solar energy conversion and storage, the STF with lower titanium content would possess better functional properties, although they might be less chemically stable than titanium-rich STF oxides.

Standard enthalpies of formation at 298.15 K, Delta fH degrees 298.15, for RCoO3-delta (R = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y) perovskite-type rare-earth cobaltites were obtained from the combined results of drop and reduction calorimetry. The relationship between the oxygen nonstoichiometry, delta, and thermodynamics of RCoO3-delta was discussed. The fact that RCoO3-delta with smaller ionic radius of R3+ tend to be less stable, having more positive enthalpy of formation from binary oxides (CoO and R2O3), Delta f,oxH degrees 298.15, was confirmed for all the studied RCoO3-delta. Drop calorimetry results were also used in discussing, comparing and assessing the literature heat capacity, Cp(T), data for RCoO3-delta (R /= Y). For YCoO3-delta, both Delta fH degrees 298.15 and Cp(T) were obtained for the first time solely from the drop calorimetric results; Delta fH degrees 298.15 values determined by two different methods perfectly agree with each other. The reference data and Cp(T) of RCoO3-delta were used to calculate Delta fH degrees 298.15 from the published EMF measurement results so as to compare the available Delta f,oxH degrees 298.15(RCoO3-delta) values. An unusual dependence of Delta fH degrees 298.15(LaCoO3-delta) on its heat treatment history was identified and supported by the additional isoperibolic solution calorimetry measurements.

The crystal structure of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3–δ was studied depending on temperature in dry and wet atmosphere using in situ high temperature X-ray powder diffraction. Phase transition from tetragonal I4/mcm to cubic Pm-3m structure was shown to occur in Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3–δ upon heating from 25 up to 1000 °C irrespective of air humidity. The cubic Pm m and tetragonal I4/mcm phases possess comparable coefficients of linear thermal expansion (CTEs), 10.6·10–6 K–1. Even in very dry atmosphere (10–4 atm H2O), chemical expansion caused by hydration contributes significantly to the observed structural transformations and the variation of the unit cell parameters of Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3–δ.

A new complex compound, catena-triglycinium-µ-chlorido-tetrachloridocuprate(II) glycine co-crystal (or glycine-triglycinium pentachlorocuprate), (C2H6NO2)3CuCl5·(C2H5NO2)n, was synthesized and studied by single crystal X-ray diffraction (SC XRD) and mid-range infrared spectroscopy. The compound crystallizes in a non-centrosymmetric triclinic P1 space group with lattice parameters a = 5.1277(2) Å, b = 9.1412(6) Å, c = 12.2023(5) Å, α = 101.407(4)°, β = 97.460(3)°, γ = 105.832(4)°, Z = 1. The unit cell contains four glycine ions—three glycinium cations and single zwitter-ion—linked through hydrogen bonds network. The anionic part of the compound is presented by infinite chains [CuCl6]n of distorted (elongated) octahedra, connected by vertices and alternating direction of elongated axis. Positions of hydrogen atoms were refined using geometry optimization via density functional theory (DFT) approach. Thermogravimetric analysis (TGA) showed the title compound to be stable in air atmosphere up to ∼ 388‒393 K and decomposes upon further heating. Unit cell content of the glycine-triglycinium pentachlorocuprate, determined by the SC XRD analysis

Perovskite-type oxides BaZr1–xYxO3−x/2 (x = 0.1, 0.2) were synthesized and their enthalpy increments were measured by means of high-temperature drop calorimetry in the temperature range of (373–1273) K in air. The data obtained were used for estimating the high-temperature thermodynamic functions (constant pressure heat capacity and entropy increments) of the zirconates BaZr1–xYxO3−x/2 (x = 0.1, 0.2). They were found to be only weakly dependent on the concentration of Y-dopant. Thermal expansion coefficient of zirconates BaZr1–xYxO3−x/2 (x = 0.1, 0.2) was successfully estimated by Grüneisen equation. Also, Neumann-Kopp rule was shown to be inapplicable for accurate estimation of heat capacities of the studied oxides. Thermodynamic analysis showed that BaZr1–xYxO3−x/2 (x = 0.1, 0.2) oxides are prone to chemical interaction with CO2 at typical working temperatures of proton-conducting solid oxide fuel cells. Some possibilities to overcome this issue have been discussed.

Proton-conducting electrolytes (PCEs) are extensively researched materials utilized in solid oxide electrochemical cells with various operating principles. The high ionic (protonic) transport of PCEs offers superior performance of such cells at reduced temperatures (400-600 degrees C). However, chemical-related deformations (strains) caused by the materials' hydration/dehydration pose serious technological challenges for fabricating multilayered (cer-cer, cer-met, and cer-glass) assemblies. Therefore, studying both thermal and chemical expansions of PCEs is crucial. This work presents in-depth high-temperature characterization of Sc-doped BaSnO3 compounds (BaSn1-xScxO3-delta) in terms of their thermochemical expansion behavior. This characterization includes high-temperature X-ray diffraction and dilatometry analyses under various measuring conditions. On one hand, the collected data can serve as technological characteristics for the selection of appropriate functional materials. On the other hand, the thermochemical expansion data can be used to evaluate the thermodynamic parameters (enthalpy and entropy of hydration) and transport parameters (water chemical diffusion coefficient and chemical surface exchange constant) of PCEs. The latter was shown in the present work for the first time. This paves a new way to reveal the relationships between the composition, structure, thermochemical response, and transport properties of complex oxides capable of reversible water uptake.

Perovskite-type oxides BaZr1–xYxO3−x/2 (x = 0.1, 0.2) were synthesized and their enthalpy increments were measured by means of high-temperature drop calorimetry in the temperature range of (373–1273) K in air. The data obtained were used for estimating the high-temperature thermodynamic functions (constant pressure heat capacity and entropy increments) of the zirconates BaZr1–xYxO3−x/2 (x = 0.1, 0.2). They were found to be only weakly dependent on the concentration of Y-dopant. Thermal expansion coefficient of zirconates BaZr1–xYxO3−x/2 (x = 0.1, 0.2) was successfully estimated by Grüneisen equation. Also, Neumann-Kopp rule was shown to be inapplicable for accurate estimation of heat capacities of the studied oxides. Thermodynamic analysis showed that BaZr1–xYxO3−x/2 (x = 0.1, 0.2) oxides are prone to chemical interaction with CO2 at typical working temperatures of proton-conducting solid oxide fuel cells. Some possibilities to overcome this issue have been discussed.

A comparatively simple method for estimating the mixing enthalpy of melts by differential scanning calorimetry using standard equipment is proposed. The enthalpies of mixing of CsX–PbX2 (X = Cl, Br) melts are determined by this method. The measured values of mixing enthalpy in the CsCl–PbCl2 system are in good agreement with those obtained by means of independent measurements. For the CsBr–PbBr2 system, the enthalpy of mixing was measured for the first time. The similar values of mixing enthalpy were found for both studied systems.

The differential scanning calorimetry study showed that the double perovskite HoBaCo2O6-δ (HBC), depending on its oxygen content, undergoes three phase transitions in the temperature range 298–773 K. Their origin was tentatively explained using the relevant literature data. For the single-phase tetragonal HBC, the oxygen nonstoichiometry dependences on the oxygen partial pressure were investigated by thermogravimetric and flow reactor methods in the intermediate-temperature range of 573–773 K. The proposed defect structure of HBC was successfully verified using the obtained data on its oxygen nonstoichiometry combined with those reported earlier. As a result, the values of the thermodynamic parameters (∆Hi∘, ∆Si∘) of the defect reactions proceeding in HBC were determined. The defect structure of HBC was shown to be similar to that of YBaCo2O6-δ (YBC) likely due to similar ionic radii of Ho3+ and Y3+.

Three different tetrachlorocuprates A2CuCl4 containing 3-aminopropionic, 4-aminobutyric and 6-aminohexanoic acid cations in A-sublattice were synthesized. The temperatures of the decomposition onset for A2CuCl4 were found to increase slightly with the aliphatic chain length in A cation. The standard reaction enthalpies for the formation from organic ammonium chloride and anhydrous copper (II) chloride at T = 298.15 K, Delta rH degrees hal,298 ("hal" denotes the formation from the source halides), were determined by isoperibolic solution calorimetry. The layer shift parameter S was introduced for the sake of comparison of thermochemical and structural data. Both original and literature thermochemical data indicate that simultaneous increase in S and decrease in the molar volume, Vmol, leads to a decrease in the Delta rH degrees hal,298.

Chemical compatibility and cation interdiffusion between the double perovskite cobaltites RBaCo2O6−δ (R = Gd, Pr) and proton-conducting electrolyte BaZr0.8Y0.2O3−δ were studied. Chemical interaction was found to occur already at 1100 °C as a result of the partial dissolution of RBaCo2O6−δ (R = Gd, Pr) in BaZr0.8Y0.2O3−δ. Analysis of the element distribution along the cross sections of diffusion couples RBaCo2O6−δ(R = Gd, Pr)|BaZr0.8Y0.2O3−δ showed strong interdiffusion of cations, with cobalt being the most mobile one. Its diffusion depth in the electrolyte reaches up to several hundreds of micrometers. The addition of NiO as a sintering aid to BaZr0.8Y0.2O3−δ promotes cation diffusion especially through the grain boundary mechanism, increasing the diffusion depth of Co. The possible implications of cation interdiffusion on the performance of proton-conducting SOFCs are discussed based on the results obtained.

Differential scanning calorimetry studies of the complex oxide YBaCo2O6-δ (YBC), combined with the literature data, allowed outlining the phase behavior of YBC depending on the oxygen content and temperature between 298 K and 773 K. The oxygen nonstoichiometry of single-phase tetragonal YBC was measured at different temperatures and oxygen partial pressures by both thermogravimetric and flow reactor methods. The defect structure of YBC was analyzed. As a result, the thermodynamic functions (∆Hi○, ∆Si○) of the defect reactions in YBC were determined. Experimental data on the oxygen content and those calculated based on the theoretical model were shown to be in good agreement. Standard enthalpies of formation at 298.15 K (∆Hf○) were obtained for YBC depending on its oxygen content using solution calorimetry. It was found that ∆Hf○ = f(6-δ) function is linear in the range of (6-δ) from 5.018 to 5.406 and that its slope is close to the value of the enthalpy of the quasichemical reaction describing oxygen exchange between the oxide and ambient atmosphere, which confirms the reliability of the suggested defect structure model.