Quasi-binary System Research Articles

Optimization of Hydride–Ion Conductivity of Perovskite–Type Structure in SrLiH3–CaLiH3–NaLiH2 Quasi–Ternary System

Introduction Solid electrolytes with high ionic conductivities are essential for the development of all–solid–state batteries. Our prior research has identified perovskite–type AELiH3 (AE = Ca, Sr, Ba) as an emerging hydride–ion conductor.[1] Using the nudged elastic bandmethod revealed that the smaller A–site cation offers the lower migration energy. Experimentally, the synthesis of AE 1–x Na x LiH3–x was attempted, and a similar trend in conductivity was observed. However, it was found that CaLiH3 could not be synthesized, possibly due to an unfavorable tolerance factor (t = 0.83) for the cubic perovskite structure. In this study, we explored the compositional space in the SrLiH3–CaLiH3–NaLiH2 quasi–ternary diagram to investigate the formation range of the perovskite-type structure. Inspired by the NEB calculation results noted above, the A-site cations of the SrLiH3–NaLiH2 quasi-binary system were substituted by smaller Ca2+ ions, aiming to reduce the migration energy. Furthermore, the association energy between dopants and hydrogen vacancies, which is a constituent of the activation energy in addition to the migration energy, was calculated through DFT calculations. The relationship between the composition, the activation energy, and the ionic conductivity was investigated to establish material designing principle for perovskite-type hydride-ion conductors. Methodology The synthesis was carried out via the mechanochemical milling at 600 rpm for 12 h. The obtained samples were subjected to X–ray diffraction (XRD) measurement to identify the constituent phases. Synchrotron XRD (BL02, SPring–8, Japan), and neutron powder diffraction (NPD) data (BL09 SPICA, J–PARC, Japan) were obtained. The crystal structure was refined by Rietveld analysis using the Z–Rietveld software. The ionic conductivities of the uniaxially compressed powder samples were measured by alternating current impedance. Density functional theory calculations were performed using the projector-augmented wave method and PBEsol functional as implemented in the VASP code. Results and discussion As the first step, we examined the synthesis of Sr1–x Ca x LiH3 solid solutions. In the range of 0 ≦ x≦ 0.45, the single-phase of cubic perovskite-type structure were obtained. For x ≧ 0.5, secondary phases were detected in XRD patterns, suggesting the solubility limit is x ≈ 0.45. The ionic conductivity enhanced upon increasing the Ca amount, and reached the maximum ionic conductivity, σ 100 ºC = 1.1 × 10–6 S cm–1, for x = 0.5. The activation energy decreased upon Ca-substitution, and reached minimum of 47.2 kJ mol–1 for x= 0.45. These results suggest the activation energy can be mitigated by employing a smaller A-site cation, in accordance with NEB calculation results. Next, perovskite-type hydride-ion conductors within the SrLiH3–CaLiH3–NaLiH2 quasi-ternary system were explored. The figure shows heat map of the ionic conductivities in the SrLiH3–CaLiH3–NaLiH2 quasi-ternary system. The composition of Sr0.75Ca0.1Na0.15LiH2.85 exhibited a maximum ionic conductivity of 1.2 × 10–4 S cm–1 at 100ºC, and Sr0.825Ca0.025Na0.15LiH2.85exhibited a minimal activation energy of 41.8 kJ mol–1, superior over the previously reported hydride-ion conductor Sr0.925Na0.075LiH2.925 (6.4 × 10–5 S cm–1, 46.3 kJ mol–1) [1]. Structural refinement using NPD data of Sr0.7Ca0.1Na0.2LiH2.8confirmed a larger atomic displacement parameter (0.0277) of hydrogen than that of Sr0.8Na0.2LiH2.8 (0.0231(3)), revealing that the A–site substitution by Ca offer more flatten energy landscape of hydrogen in terms of the average structure. In contrast, more substitution by Ca did not provide enhancement of the ionic conductivity. The association energy between the dopant (Na) and a hydrogen–vacancy in AE 63Na1Li64H191 supercell (AE = Ca, Sr, Ba) was calculated to be 19, 14, and 22 kJ mol–1, respectively. The association energy in CaLiH3 was higher than that of SrLiH3, in other words, the local trapping of hydrogen–vacancy by Na is more prominent for CaLiH3. These results suggest the trade-off between the migration energy and the association energy in Sr1–x Ca x Na y LiH3–y . The optimal balance between the migration energy and association energy was achieved with small amounts of Ca (y = 0.025 ~ 0.1), leading to reduced activation energy and enhanced ionic conductivity compared to the previously reported perovskite–type hydride–ion conductors.

Modeling of the Electronic Properties of M-Doped Supercells (М = Zr, Nb) with a Monoclinic Structure For Lithium-Ion Batteries

The T–x phase diagram of the quasi-binary system Li2O–TiO2 was refined and the isothermal cross section of the ternary Li–Ti–O system at 298 K was constructed. The equilibrium phase regions of Li–Ti–O in the solid state are determined with the participation of boundary binary oxides and four intermediate ternary compounds , , and . Using the density functional theory (DFT LSDA) method, the formation energies of the indicated ternary compounds of the Li2O–TiO2 system were calculated and the dependence of on the composition was plotted. Ab initio modeling of supercells based on M-doped anode material based on the (LTO) compound with a monoclinic structure () was carried out. It has been shown that partial substitution of cations and oxygen in the m-LTO–M structure increases the efficiency of a lithium-ion battery (LIB) both by stabilizing the structure and by increasing the diffusion rate of . Due to the contribution of d-orbitals (Zr4+-4d, Nb3+-4d orbitals) to the exchange energy, partial polarization of electronic states occurs and the electronic conductivity of m-LTO–M increases. The formation of oxygen vacancies in the m-LTO–M crystal lattice, as in binary oxides, can create donor levels and improve the transport of and electrons. M-doping of the m-LTO structure by replacing cations, in particular lithium, with Zr or Nb atoms noticeably reduces the band gap (Eg) of m-LTO–M supercells. In this case, in the m-LTO–M band structure, the Fermi level shifts to the conduction band and the band gap narrows. Decreasing the Eg value increases the electronic and lithium-ion conductivity of m-LTO–M supercells.

The crystal structures and magnetic properties of YbMg0.75In1.25 and Yb6Mg6.41In5.59

Abstract The quasi-binary system YbMg2-YbIn2 was studied around the equiatomic composition. In contrast to the ordered rare earth (RE) phases REMgIn (ZrNiAl type), ytterbium forms phases with different structures and pronounced Mg/In mixing (M sites). The structures of YbMg0.75In1.25 (CaLiSn type, P3m1, a = 501.95(7), c = 1087.3(2) pm, wR2 = 0.0490, 790 F 2 values, 32 variables) and Yb6Mg6.41In5.59 (Yb6Ir5Ga7 type, P63/mcm, a = 1060.77(14), c = 970.27(16) pm, wR2 = 0.0484, 701 F 2 values, 26 variables) were refined from single-crystal X-ray diffractometer data. YbMg0.75In1.25 is an AlB2 superstructure with a tripling of the subcell. The magnesium and indium atoms form three differently puckered layers of M 6 hexagons. The Yb6Mg6.41In5.59 structure is derived from the hexagonal Laves phase YbMg2 (MgZn2 type, P63/mmc). A klassengleiche symmetry reduction leads to four crystallographically independent M sites for the rows of corner- and face-sharing tetrahedra which allow a composition close to the equiatomic one. The M–M distances in both structures cover the broad range from 289 to 331 pm, comparable to the sums of the covalent radii. Temperature dependent magnetic susceptibility studies of the polycrystalline YbMg0.75In1.25 and Yb12Mg13In11 samples indicate Pauli paramagnetism with room temperature values of 2.8(1) × 10−3 emu mol−1 (YbMg0.75In1.25) and 5.2(1) × 10−3 emu mol−1 (Yb12Mg13In11).

QUASIBINARY SYSTEM Ag7PSe6– Ag2Se

Semiconductor binary and ternary argentum selenides are of considerable practical interest, particularly as promising thermoelectric materials. These include Ag2Se, a narrow-band semiconductor, and Ag7PSe6, with a disordered cationic sublattice and a rigid anionic framework that causes efficient phonon scattering. Doping of Ag7PSe6 with the narrow bandgap semiconductor Ag2Se should increase its conductivity and Ag2Se with a more disordered Ag7PSe6 argyrodite phase to a decrease in thermal conductivity. In both cases, this should increase the thermoelectric figure of merit of the respective phases. At the same time, establishing the conditions for obtaining doped phases requires studying the conditions of their existence in the Ag7PSe6-Ag2Se system. Present work studies phase equilibria in the Ag7PSe6-Ag2Se system in the entire concentration range. A corresponding phase diagram was constructed based on the X-ray phase and differential thermal analysis results. It was found that the Ag7PSe6-Ag2Se system is quasi-binary. It is characterized by four processes: two eutectoid processes at 405 K and 433 K, caused by the presence of two polymorphic modifications in Ag2Se and Ag7PSe6, respectively; eutectic process at 893 K (the coordinates of the eutectic point are 893 K and ~8 mol% Ag2Se) and monotectic process at 977 K. Above this temperature, in the concentration range of ~9 - ~87 mol%, a region of immiscibility in the liquid is observed. The homogeneity regions of the initial components are less than 5 mol%.
 Keywords: quasibinary system; selenides; phase equilibria; phase diagram.

The Phase Еquilibrium in the HgS–Ga2S3–Bi(Sb)2S3 Systems

Phase equilibria in the quasi-ternary systems HgS–Ga2S3–Bi(Sb)2S3 were studied by physico-chemical analysis methods on 177 alloys that were synthesized by direct single-temperature method. Phase diagrams of the quasi-binary systems HgS–Bi2S3 and Ga2S3–Bi2S3, six vertical sections (HgGa2S4–HgBi2S4, HgGa2S4–Bi2S3, HgGa6S10–Bi2S3, HgGa6S10–HgBi2S4, HgGa2S4–Sb2S3, and HgS–“GaSbS3”), and liquidus surface projections were investigated. Due to large primary crystallization region of mercury thiogallate, particularly at the HgGa2S4–Bi2S3 and HgGa2S4–HgBi2S4 sections, and low temperature (950-1050 K), the growth of single crystals of mercury thiogallate is possible using solution-melt method.

Phase equilibrium investigations and thermodynamic modelling of the ZrO2-Ta2O5 system

Phase equilibria in the ZrO2-Ta2O5 system were studied in the temperature range from 1673 K to 1973 K using equilibration technique and in the temperature range up to 2473 K using differential thermal analysis (DTA). Phase and chemical composition of the samples was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-Ray analysis (EDX). Melting relations were studied using DTA followed by microstructure investigation by means of SEM/EDX. The eutectic reaction was determined at 1994 K and composition of 51 ± 2 mol% Ta2O5. It was found that the intermediate compound Zr6Ta2O17 melts by peritectic reaction at 2166 K. The heat capacity of this compound was measured using differential scanning calorimetry (DSC) in three temperature ranges from 100 K to 230 K, from 220 K to 570 K and from 330 K to 1295 K. Based on the obtained results and literature data thermodynamic parameters of the quasibinary ZrO2-Ta2O5 system were optimized using the CALPHAD approach.

6Ag2Se + Ag8GeTe6 ↔ 6Ag2Te + Ag8GeSe6 Reciprocal System

Here, we present the results of DTA and XRD studies of phase equilibria in the 6Ag2Se + Ag8GeTe6 ↔ 6Ag2Te + Ag8GeSe6 reciprocal system (system A). A Т–х diagram of the Ag8GeSe6–Ag8GeTe6 boundary system, several inner polythermal sections, isothermal sections at 300 and 1000 K, and the liquidus surface projection were plotted. The Ag8GeSe6–Ag8GeTe6 system is a partially quasi-binary system; it features continuous substitutional solid solutions between Ag8GeTe6 and the high-temperature cubic Ag8GeSe6 phase (the δ phase). Once solid solutions are formed, the polymorphic transition temperature in Ag8GeSe6 decreases, thereby stabilizing the ion-conducting cubic phase in the range of ≥40 mol % Ag8GeTe6 compositions at room temperature and below it. System A is shown to be a reversible reciprocal system; its liquidus surface is comprised of three fields, which relate to the primary crystallization of the solid solutions between the high-temperature Ag2Se and Ag2Te (α phase) phases, IT-Ag2Te-base solid solutions (β phase), and the δ phase. The subsolidus portion of system A features complex interactions related to polymorphism in the terminal compounds and in phases based on them

Sol-Gel-Syntheses and Structural as well as Electrical Characterizations of Anatase- and Rutile-Type Solid Solutions in the System IrO2 -TiO2.

This paper describes solid solutions in the quasibinary oxide system iridium-titanium IrO2 -TiO2 with rutile and anatase crystal structures. Based on X-ray diffraction evaluations using Rietveld refinements, changes of lattice parameters were determined within the composition series of 0-100 mol % iridium. These changes prove the existence of a complete solid solution series in the rutile structure type. The solubility limit for iridium in the anatase lattice was found to be 6.0(8) mol % iridium for the underlying sol-gel process. In addition, iridium is a promoter for the conversion from anatase to rutile type. Furthermore, the X-ray diffraction results of a calcination temperature series for the composition with 5 mol % iridium are shown, which confirm the findings of the composition series and allow conclusions on the phase segregation behavior. The results are complemented by 2-point conductivity measurements at different pressures in a piston press to investigate the question of the conductivity mechanism.

Study of Structure and Phase Transformations in Rejuvenated Rapidly Quenched TiNiCu Alloys

Alloys of the quasibinary TiNi-TiCu system manufactured by melt quenching in the form of thin 20–50 μm ribbons have proven to show good potential as materials for the fabrication of micromechanical devices. At high cooling rates (about 106 K/s), this method allows producing high-copper (more than 20 at.%) amorphous alloys which exhibit an excellent shape-memory effect after crystallization. Their properties are known to largely depend on the crystallization conditions and the structure of the initial amorphous material acting as a precursor for the formation of crystal phases. It has been shown recently that the rejuvenation procedure (cryogenic thermocycling) of metallic glasses is one of the most promising methods of improving their properties. In this study, we investigated for the first time the effect of cryogenic thermocycling of rapidly quenched amorphous TiNiCu on the initial state, as well as on structure formation and the phase transformation patterns of subsequent crystallization conducted using various methods. The effect was analyzed utilizing the methods of scanning and transmission electron microscopy, X-ray diffraction analysis, and differential scanning calorimetry. The results show that rejuvenation treatment slightly reduces the glass transition and crystallization onset temperatures and moderately changes the sizes of structural features (grains, martensite plates), the quantity of the martensite phase, and the characteristic temperatures and enthalpy of the martensitic transformation.

Formation energy crossings in Ga2O3-Al2O3 quasibinary system: ordered structures and phase transitions in (Al x Ga1−x )2O3

A quasibinary system of Ga2O3-Al2O3 offers a range of applications in wide bandgap semiconductor engineering. Different polymorphs and concentrations of (Al x Ga1−x )2O3 manifest a variety of structural and electronic properties, paving the way for tunability of (Al x Ga1−x )2O3 for specific functions. In this work, we investigate the energetics of alpha (α) and beta (β) polymorphs of Ga2O3 and Al2O3 by considering all possible configurations in a conventional unit cell. Using density functional theory, we show that the formation energies of (Al x Ga1−x )2O3 in α and β configurations start to coincide at 50% concentration (Al0.5Ga0.5)2O3. The corundum configuration then becomes more dominant (lower in energy) than its monoclinic counterpart at around 80% Al concentration. The lowest formation energy configurations for 50% concentration in both α and β polymorphs also manifest a preference towards an ordered phase. These show that the stability of Ga2O3-Al2O3 and its phase transitions are significantly influenced by the relative arrangements of Ga and Al within the quasibinary semiconducting crystal.

High-Frequency Conductivity of Amorphous and Crystalline Sb2Te3 Thin Films

The results of study of charge transfer processes in thin amorphous and crystalline Sb2Te3 films in a wide range of frequencies and temperatures are presented. The frequency spectra of conductivity were obtained by the dielectric spectroscopy method. The authors analyzed the frequency dependences of the conductivity in the electric field and the temperature dependences of the exponent s. A transition from the classical correlated barrier hopping (CBH) to quantum mechanical tunneling (QMT) was observed at a certain temperature Tt. The CBH model allowed the authors to calculate the conductivity parameters of two phases. Two areas with different types of conductivity were revealed on the conduction spectra, and the activation energies of charge transfer processes for amorphous and crystalline films were determined. The following features were discovered: the difference in the temperatures of the change of the charge transfer mechanism and the transition from the semiconductor region to the metal region on the temperature dependence of conductivity. They can help to identify the amorphous phase in the quasi-binary chalcogenide Sb2Te3-GeTe system.

PHASE EQUILIBRIUM IN THE QUASI-TERNARY SYSTEM Y2O2S–Ga2S3–Tb2O2S

The results of the study of quasibinary (lateral) and non-quasibinary polythermal sections in quadrilateral systems Y2O2S–Ga2S3–Tb2O2S show the prognosis of the established liquidity surface, where the nature of the chemical interaction is relatively simple. Thus, the Ga2S3–Y2O2S və Ga2S3–Tb2O2S sides of a quasi-triangular system are quasi-binary eutectic systems with a limited solubility based on both components. On the Y2O2S–Tb2O2S side, a continuous solid solution area is formed. Thus, in the Y2O2S–Ga2S3–Tb2O2S quasi-triangular system, crystallization ends not at the non-variant triple point, but at the low-temperature binary non-variant (eutectic e1).

Crystallization and Decomposition of Compounds with the Aurivillius Crystal Structure in the Bi2GeO5–Bi2SiO5 Pseudobinary Metastable System

The possibility of partial substitution of Si for Ge in the structure of Bi2GeO5 during synthesis from the melt and the effect of the substitution on the structure of the synthesized material have been studied. The microstructure of the synthesized compounds after their complete decomposition has been investigated. Using the phase powder X-ray diffraction- and optical microscopy methods, it has been found that metasta-ble phases of bismuth silicate and germanate with structures of the Aurivillius type form a continuous seriesof solid solutions. It is shown that a mixed structure composed of large Bi4Si(Ge)3O12 crystals and the Bi12Si(Ge)O20 + Bi4Si(Ge)3O12 point eutectic is obtained after the decomposition of compounds of the Bi2SiO5–Bi2GeO5 quasi-binary system, regardless of the percentage of substitution of Si for Ge. Upon slow heating up to annealing temperatures (at a rate of 13.5°C/min), a structure of decomposition that is more finely dispersed and uniform than the structure obtained upon rapid heating (when charg the material into an already preheated furnace), which is more coarsely grained and inhomogeneous. Moreover, regions with a large structure similar to a dendritic one can appear in the material when the contents of silicon and germa-nium oxides in the alloy are close to each other (20/30–30/20 mol %). Such regions differ slightly in chemical composition from the surrounding material and appear during both slow and rapid heating of the material to annealing temperatures.

Experimental studies of the quasi-binary system Na2SO4–NiSO4 by differential thermal analysis, calorimetry and X-ray diffraction

This work is dedicated to experimental studies of the quasi-binary sub-system Na2SO4–NiSO4 with Differential Thermal Analysis (DTA) with simultaneous Thermal Gravimetry (TG), Differential Scanning Calorimetry (DSC), and X-ray Diffraction (XRD) to obtain the transition temperatures, the phase compositions, and thermodynamic data of Na2Ni(SO4)2, (Fp: 982 ± 3 K, ΔHfus = 59 ± 3 kJ mol−1), compound in this sulfate system. A compound Na6Ni(SO4)4) with the decomposition temperature 682 ± 3 K was confirmed. The lack in the data of the NiSO4-rich region was clarified. The entire composition range could be measured and the structure change at 1243 ± 3 K (“dkostruct”) that enables to describes corrosion processes on Ni-containing alloys in a sulphatic-rich environmental, as well as the melting point of NiSO4 at 1483 ± 3 K could be confirmed in this work. A new and complete phase diagram was determined.

Effect of Fe on the Hydrogen Production Properties of Al-Bi-Sn Composite Powders.

Fe additives may play an important role in the preparation of aluminum-based hydrolysis hydrogen powder, with high hydrogen yield, low cost, and good oxidation resistance. Therefore, it is necessary to ascertain the effect of Fe on the hydrogen production performance of Al-Bi-Sn composite powders. According to the calculated vertical cross-section of the Al-10Bi-7Sn-(0~6)Fe (wt.%) quasi-binary system, Al-10Bi-7Sn-xFe (x = 0, 0.5, 1.5, 3) wt.% composite powders for hydrogen production were prepared by the gas-atomization method. The results showed that the Al-10Bi-7Sn-1.5Fe (wt.%) powder exhibited an extremely fast hydrogen generation rate at 50 °C, which reached 1105 mL·g−1 in 27 min in distilled water, 1086 mL·g−1 in 15 min in 0.1 mol·L−1 NaCl solution, and 1086 mL·g−1 in 15 min in 0.1 mol·L−1 CaCl2 solution. In addition, the antioxidant properties of these powders were also investigated. The results showed that the hydrogen production performance of the Al-10Bi-7Sn-1.5Fe (wt.%) powder could retain 91% of its hydrogen production activity, even though the powder was exposed to 25 °C and 60 RH% for 72 h. The addition of Fe not only promoted the hydrogen generation rate of the Al-Bi-Sn composite powders, but also improved their oxidation resistance. The Al-10Bi-7Sn-1.5Fe (wt.%) composite powder shows great potential for mobile hydrogen source scenarios with rapid hydrogen production.

QUASIBINARY SECTIONS FORMATIO IN THE Ag – Sb – P – Se SYSTEM

The possibility of combining different properties of materials within Ag – Sb – P – Se system prompted the study of the nature of physicochemical interactions in it. The main attention was paid to systems based on AgSbP2Se6 with the participation of stable binary and ternary compounds. It was assumed that the cross sections involving stable compounds would be quasi-binary. However, the nonquasibinarity of the sections Ag2Se – AgSbP2Se6 and Ag7PSe6– AgSbP2Se6 was proved, which indicated a complex interaction in the Ag – Sb – P – Se system. Difficulties were caused by the formation in the process of synthesis of multicomponent mixtures, the determination of the phase composition of which was complicated by the presence of reflexes on the powder patterns of different phases at the same angles. In this regard, it was expedient to summarize the experimental data on the nature of physicochemical interactions in the cross sections of the Ag – Sb – P – Se system. This would allow the establishment of quasi-binary, quasi-ternary and quasi-quaternary systems within the Ag – Sb – P – Se system, which was the aim of this work.
 It was established the nonquasibinarity of Ag7PSe6 – AgSbP2Se6 system. It intersects two quasi-ternary systems Ag4P2Se6 – AgSbSe2 – Se and Ag4P2Se6 – Sb2Se3 – Se. Other samples consist four-phases, in each of which, in addition to three complex compounds, selenium was detected. The quasiternary and quasiquaternary systems were formed by following quasibinaries: Ag7PSe6 – Ag4P2Se6, Ag7PSe6 – AgSbSe2, Ag7PSe6 – Se, Ag4P2Se6 – AgSbSe2, Ag4P2Se6 – Se, AgSbSe2 – Se, Ag4P2Se6 – Sb2Se3, AgSbSe2 – Sb2Se3, Sb2Se3 – Se, AgSbP2Se6 – Ag4P2Se6, AgSbP2Se6 – Sb2Se3, AgSbP2Se6 – Se.
 In the study of the cross section Ag2Se – AgSbP2Se6 it was found that it is also not quasi-binary and intersects one quasi-ternary (Ag7PSe6 – AgSbSe2 – Ag) and one quasi-binary system (Ag4P2Se6 - AgSbSe2). However, it is not taken into account that the Ag2Se – AgSbP2Se6 system also intersects the Ag4P2Se6 – Sb2Se3 system, which is quasi-binary. A refining analysis of the data confirmed that the cross section of Ag2Se - AgSbP2Se6 intersects the above quasi-ternary and two quasi-binary systems. In addition to the identified twelve, the study of the nature of the interaction in the cross sectionAg2Se – AgSbP2Se6 confirmed the quasibinarity of four systems (Ag7PSe6 – Ag2Se, Ag2Se – Ag, Ag2Se – AgSbSe2 and Ag – AgSbSe2) and additionally a system AgSbP2Se6 – Sb4(P2Se6)3.
 Thus, were established seventeen quasi-binary sections. It should be noted that some of them were known in the literature. Ag2Se – Ag and Sb2Se3 – Se systems are partial of the Ag – Se and Sb – Se systems, respectively, the Ag2Se – AgSbSe2 and AgSbSe2 – Sb2Se3 systems are subsystems of the quasi-binary Ag2Se – Sb2Se3 system. So, by fact thirteen quasi-binary systems in the quaternary system are established.
 It is noteworthy that the quaternary compound AgSbP2Se6 does not form quasi-binary systems with silver-containing complex compounds. The exception is Ag4P2Se6, which is often the final, occasionally intermediate product of the interaction of AgSbP2Se6 with other silver-containing compounds.

The Ag2S–GeS2–P2S5 system at 500 ​K

Phase equilibria in the quasi-ternary system Ag2S–GeS2–P2S5 at 500 ​K (227 ​°C) were investigated by physico-chemical analysis methods on 55 (plus 7 glass samples) alloys that were synthesized by direct single-temperature method. Phase diagram of the quasi-binary system Ag7PS6–Ag8GeS6 was also investigated. The limited solid solution ranges of the Ag7PS6 and Ag8GeS6 compounds are formed in the Ag7PS6–Ag8GeS6 system. The crystal structure of the two compositions was investigated by Rietveld method. The unit cell periods of the phases are: а ​= ​1.5108 ​nm, b ​= ​0.74609(7) nm, c ​= ​1.0571(1) nm for Ag7.9P0.1Ge0.9S6, and а ​= ​1.0403 ​nm for Ag7.1P0.9Ge0.1S6. Optical band gap was determined from the energy position of optical absorption edge at 293 ​K (20 ​°C) at a fixed value of the absorption coefficient α ​= ​300 ​cm−1.

REGULARITIES OF PHYSICO-CHEMICAL INTERACTION IN THE QUASIBINARY SYSTEMS BASED ON THE TERNARY HALIDES Rb3(Cs3)Sb2(Bi2)Br9(I9) AND K2(Rb2,Cs2,Tl2)TeBr6(I6): EXPLANATION WITHIN THE FRAMEWORK OF THE BOND VALENCE MODEL

The work is the first attempt to employ the bond valence model (BVM) in the explanation and interpretation of the phase diagrams observed for the quasibinary systems based on the ternary halides Rb3(Cs3)Sb2(Bi2)Br9(I9) (A3B2X9) and K2(Rb2,Cs2,Tl2)TeBr6(I6) (A2BX6) belonging to the structural family of perovskite. The most characteristic features of the phase diagrams of such systems are (a) a monotonic increase in the melting and crystallization temperatures when the smaller A atom is replaced by a larger chemical analogue; (b) a gradual decrease in the melting and crystallization temperatures when the bromine atoms are replaced by iodine atoms, reaching the temperature minimum at a molar ratio of ~1 : 1, and then an increase of the temperatures to the pure iodine component of the system. With the isostructurality of the starting components, in case (a) a continuous series of solid solutions (CSSS) of substitution is formed, without a temperature minimum; if the starting components belong to different structural types, then a peritectic interaction with the wide concentration limits of solid solutions is observed. Under the same conditions concerning the isostructurality/non-isostructurality of starting components, in case (b), correspondingly, either a CSSS of substitution with a temperature minimum is formed or eutectic interaction with the wide concentration limits of solid solutions is observed.
 Within the framework of the BVM, in case (a) a monotonic increase of the melting and crystallization temperatures when replacing a smaller atom A with a larger analogue can be interpreted as a consequence of reducing the deficit of the bond valence sum (BVS) for cations A, resulting in the stabilization of the structure as a whole. In case (b), partial replacement of one halogen ion by a halogen ions of another sort leads either to a deficit (if a larger anion is replaced by a smaller one) or to an excess (if a smaller anion is replaced by a larger one) of the BVS values for the cations A and for the anion that replaced the main halogen sort. An increase in the excess or deficit of the BVS values destabilizes the structure of ternary halides, reducing the melting and crystallization temperatures; the maximum destabilization is achieved at a ~1 : 1 molar ratio of the components, which is expressed in reaching the characteristic temperature minimum of the respective systems.
 The approach described in the work can expectedly be employed in explanation and interpretation of another phase diagrams based on the structures featuring the closest packing formed by alkaline metals and halogens.
 Keywords: ternary halides; crystal structures; phase diagrams; bond valence model.

Phase equilibria, glass formation and optical properties of glasses in the Ag2S–BIVS2–CV2S3 systems (BIV–Ge, Sn; CV–As, Sb)

Phase equilibria of the quasi-ternary systems Ag2S–BIVS2–CV2S3 (BIV– Ge, Sn; CV–As, Sb) were investigated using phase analysis based on XRD. Isothermal sections at 500 Kand glass formation regions in the respective systems were determined. The quasi-binary sections GeS2–As2S3 and GeS2–Sb2S3 of the studied systems exhibit glass formation in the entire concentration range. Large glass formation regions were also found at the Ag2S–GeS2 (0–55 mol.% Ag2S) and Ag2S–As2S3 (0–75 mol.% Ag2S) sections. As the starting compounds As2S3, GеS2 and Sb2S3 are in the glassy state, they act as the glass-forming agents in the quasi-ternary systems. Glass formation regions in the tin-containing systems are significantly smaller than the analogous germanium-containing ones which is due to metal nature resulting in the change from tetrahedral to octahedral surrounding. Glassy materials may possess interesting optical properties, therefore optical absorption spectra at 297 K were measured. Bandgap energy Eg of the glasses of the quasi-binary system GeS2–As2S3 was estimated from the data on the spectral distribution of the absorption coefficient at the fundamental absorption edge. It was found that the addition of As2S3 to germanium (IV) sulfide results in shifting the absorption edge to higher energies.

Effect of thermal history on the properties of efficient thermoelectric alloys Ge-=SUB=-0.86-=/SUB=-Pb-=SUB=-0.1-=/SUB=-Bi-=SUB=-0.04-=/SUB=-Te

In this work, we study thermoelectric properties of GeTei-based alloys, doped with bismuth, with partial substitution of lead for germanium: Ge0.86Pb0.1Bi0.04Te. The aim of the study is to explore the possibility of increasing the thermoelectric efficiency of a compound by combining optimal doping and isovalent substitution to improve the electronic properties with a simultaneous decrease of the lattice thermal conductivity. We studied alloy samples prepared in two different research laboratories using similar, but not completely identical procedures. It is shown that the electronic (thermoelectric power and electrical conductivity) properties of the samples of the two groups are in good agreement with each other. The properties of alloys depend on the thermal history of the samples due to the presence at temperatures of 600-800 K of a phase transition from a low-temperature rhombohedral to a high-temperature cubic structural modification and missibility gap in GeTe-PbTe quasibinary system below 870 K. The thermoelectric figure of merit of alloys reaches a maximum value of 1.5 at a temperature of about 750 K. Keywords: thermoelectric alloys, thermoelectric power, electrical conductivity, thermal conductivity