Peritectic Process Research Articles

Selezen А.О., Piskach L.V. INTERACTION IN THE QUASI-TERNARY Tl2Se–CdSe–SnSe2 SYSTEM

Selezen А.О., Piskach L.V. INTERACTION IN THE QUASI-TERNARY Tl2Se–CdSe–SnSe2 SYSTEM

Tl2Se–TlInSe2–Tl4P2Se6 QUASITERNARY SYSTEM

Complex chalcogenide compound are widely used as working elements for semiconductor optical technology, thermal generation, solar power. Special attention is paid to compounds of the M2P2Se6 type (M – Ag, Cu) which due to its layer crystal structure possess promising ferroelectric, thermoelectric and electro-optical properties. Heterovalent substitutions of cations 2М2+ ® 4M1+ in the composition of M2P2Se6 type compounds must leads to deformation of the crystal structure, changing in the value of the dipole moment and, accordingly, to change the electro-physical properties.
 The Tl2Se–In2Se3–“P2Se4” system characterized by the formation of intermediate complex compounds which melts congruently TlInSe2 (1023 К), Tl4P2Se6 (758 К), TlInP2Se6 (875 К) and TlIn5Se8 (melts incongruently L+In2Se3«TlIn5Se8 at 1029 К), In4(P2Se6)3 (formed by syntactic reaction at 880 К). Triangulation of the Tl2Se–In2Se3–“P2Se4” system was shown that then divided on secondary quasiternary systems, one of them is Tl2Se–TlInSe2–Tl4P2Se6.
 Phase equilibria in the Tl2Se – TlInSe2 – Tl4P2Se6 quasiternary system were studied using classical methods of physicochemical analysis DTA (chromel-alumel thermocouple, with an accuracy of ±5 K), XRD (DRON-3-13 diffractometer, Cu Ka radiation, Ni filter, Guinier Huber G670 diffractometer, CuKα1 radiation), MSA (metallographic microscope Lomo Metam R1) in combination with the simplex method of mathematical modeling of phase equilibria in multi-component systems. Crystal structure calculation was carried out with program WinCSD.
 Investigation of physical-chemical interaction allowed to constructed perspective view of phase state diagram and liquidus surface projection of the Tl2Se–TlInSe2–Tl4P2Se6 ternary system. In the ternary system formed the boundary solid solution: a- on the basis of Tl2Se, b- on the basis of TlInSe2, g-, d-, e- on the basis of ltm-, mtm- and htm-Tl4P2Se6 (ltm, mtm, htm – low, middle and high temperature modification, respectively). The liquidus of the ternary system consists of primary crystallization areas: Tl2Se-е1-Е1-е2-Tl2Se (a phase), TlInSe2-е3-U2-U1-E1-TlInSe2 (b phase), m1-U1-E1-e2-m1 (g phase), m2-U2-U1-m1-m2 (d phase) and Tl4P2Se6-e5-U2-m2-Tl4P2Se6 (ε phase). The Tl2Se–TlInSe2–Tl4P2Se6 quasiternary system is characterized by the processes: monovariant eutectic L«htmTl4P2Se6+TlInSe2 (e5-U2, 776-693 К), monovariant eutectic L«Tl2Se+TlInSe2 (e1-Е1, 614-539 К), monovariant eutectic L«Tl2Sе+ltmTl4P2Se6 (e2-Е1, 610-539 К); monovariant peritectic L+mtmTl4P2Se6«ltmTl4P2Se6 (m1-U1, 640-620 К); monovariant peritectic L+htmTl4P2Se6« mtmTl4P2Se6 (m2-U2, 747-693 К); monovariant peritectic L+mtmTl4P2Se6«TlInSe2 (U2-U1, 693-620 К); monovariant peritectic L+ltmTl4P2Se6«TlInSe2 (U1-E1, 620-539 К). Lines of the monovariant equilibria are crossed in three point: U2 – invariant peritectic process L+htmTl4P2Se6«TlInSe2+mtmTl4P2Se6 (12 mol.% Tl2Se, 20 mol.% TlInSe2, 68 mol.% Tl4P2Se6, 693 К), U1 – invariant peritectic process L+mtmTl4P2Se6«TlInSe2+ltmTl4P2Se6 (38 mol.% Tl2Se, 9 mol.% TlInSe2, 53 mol.% Tl4P2Se6, 620 К), E1 – invariant eutectic process L « Tl2Se+TlInSe2+ltmTl4P2Se6 (47 mol.% Tl2Se, 7 mol.% TlInSe2, 46 mol.% Tl4P2Se6, 539 К). New complex compounds were not observed in the ternary system. Limited solid solutions on the basis of TlInSe2, Tl4P2Se6 initial compounds are not up to 5–8 mol%.
 Crystal-structure studies of Tl2Se, TlInSe2 and Tl4P2Se6 complex chalcogenides were carried out by a powder method, refinement of the structural parameters – by the Rietveld method. The lattice parameters are: Tl2Se – Р4/n, а=8,540; с=12,380 Å, TlInSe2 – I4/mcm, a=8.064, c=6.833 Å, Tl4P2Se6 – P121/c1, a=12.239, b=9.055, c=12.328 Å, b=98.83. Crystal-chemical analysis of the compounds showed that they are characterized by a mixed ion-covalent type of chemical bond. During the transition from the binary Tl2Se to TlInSe2 ternary compound the covalent component of the In–Se bond is enhanced, the opposite change is observed for Tl4P2Se6, an increase in the ion component of the Tl–Se bond. The study of the mechanisms of formation of solid solutions showed that with the reciprocal solubility of the TlInSe2, Tl4P2Se6 ternary compounds characterized by the formation of substitution structure, the dissolution of Tl2Se in ternary selenides follows the substitution and subtraction mechanism.

Solidification process and effect of thermal treatments on Ni–Co–Mn–Sn metamagnetic shape memory alloys

The solidification process of three metamagnetic Ni–Co–Mn–Sn alloys has been investigated by DTA and DSC calorimetry, X-ray diffraction, optical and electron microscopy. The as-cast microstructures are composed of three phases: austenite L21/6-layered martensite, D03 phase and γ phase (disordered fcc). Subsequent annealing treatments at 900°C completely dissolve the D03 phase in the three alloys and also the γ phase in alloys with low Co content (6.4 and 7.2at.%). However, for 9at.% Co the stability of γ-phase extends to temperatures close to the melting point and becomes very sensitive to the Mn and Sn content. No traces of the B2–L21 order–disorder transition have been obtained in the alloys investigated. The multiphase solidification can be interpreted as a peritectic process where D03 is the primary solidified phase with dendritic growth, followed by the peritectic reaction of this phase with the residual liquid to form the austenite L21, whereas the γ-phase precipitates from the austenite L21 upon further cooling.

The reciprocal CuInS 2+2CdSe⇔CuInSe 2+2CdS system. Part I. The quasi-binary CuInSe 2–CdSe system: Phase diagram and crystal structure of solid solutions

The type of interaction in quasi-binary system CuInSe 2 (CIS)–CdSe was investigated using differential thermal and X-ray phase analysis methods. The limits of existence of solid solutions based on low-temperature ( α) and high-temperature ( γ) CIS modifications and CdSe ( β) with chalcopyrite, sphalerite and wurtzite structures, respectively, were established in sub-solidus region at 620 K and 870 K. For certain compositions of solid solutions, the structure was refined using powder X-ray diffraction. A phase diagram of the CIS–CdSe system was constructed. A peritectic process L+ β⇔γ takes place in the system at 1260 K.

Control of purity and crystallinity of MVO4 compounds (M=Al, Cr, Fe, In, Y, Nd) by DTA

Two aqueous methods were followed for the synthesis of MVO4 (M=trivalent element). The first, with vanadium pentoxide and M nitrate yielded crystalline InVO4 III and amorphous FeVO4. The second method, using ammonium metavanadate and M nitrate, led to amorphous compounds with M=In, Fe, Cr or Al, and semi-amorphous compounds with M=Nd or Y. The crystallinity and purity of these products were examined by DTA. Starting with amorphous compounds, two irreversible effects appeared; the first, at about 100°, was endothermic and corresponded to a considerable loss of water; the second was exothermic and corresponded to the crystallization of the compounds in the temperature range from 250 to 550°. Only InVO4 presented two more irreversible exothermic peaks, corresponding to allotropic transformations, as proved by the X-ray diagrams. A reversible effect then appeared, showing an invariant equilibrium corresponding to a peritectic process for InVO4 III, FeVO4, CrVO4 and AlVO4 at 1134, 870 and 765°, respectively; these temperatures agreed with the known M2O3-V2O5 phase diagrams. The DTA curves of the semi-amorphous Nd and Y compounds showed only a loss of water at about 100°; crystallization then occurred gradually during heating; X-ray analysis confirmed these results.