Phase transformations in non-stoichiometric vanadium carbide

Abstract

X-ray diffraction, differential scanning calorimetry and reflected light microscopy studies are made of disorder-order phase transformations in the region of homogeneity of a non-stoichiometric cubic vanadium carbide, (0.66 < y < 0.88). It is found that according to the composition of the carbide, both an ordered phase with monoclinic or trigonal symmetry and a cubic ordered phase may form in this carbide at a temperature below 1450 K. Consideration is given to the effect that non-stoichiometry and the ordering of structural vacancies have on the heat capacity of the carbide. Temperatures and heats of reversible equilibrium disorder-order transitions are determined. The ordering transformations and are shown to be first-order phase transitions. An equilibrium phase diagram of the V-C system is constructed which allows for the formation of ordered phases in a non-stoichiometric vanadium carbide. The order parameter functional method is used for calculation of phase equilibria in the region of ordering of non-stoichiometric cubic carbide.

Order–disorder transformations and phase equilibria in strongly nonstoichiometric compounds

“…ì ÍÑÐ×ËÅÖÓÂÙËÑÐÐÂâ àÐÕÓÑÒËâ ÍÂÓÃËAE VC 0,875 ; DH trans 3 괃 ÏÑÎß À1 Ë T trans 1380 K ì ÕÇÒÎÑÕÂ Ë ÕÇÏÒÇÓÂÕÖÓ ÒÇÓÇØÑAE V 8 C 7 3 VC 0Y875 , ÑÙÇÐÇÐÐÞÇ Ð ÑÔÐÑÄÇ àÍÔÒÇÓËÏÇÐÕÂÎßÐÞØ AEÂÐÐÞØ [51,52,121,167].…”

“…ÓËÔ. 11), ÄÔÇ ÒÓÇÄÓÂÜÇÐËâ ÃÇÔÒÑÓâAEÑÍ ë ÒÑÓâAEÑÍ Ä ÍÂÓ-ÃËAEÇ ÄÂÐÂAEËâ VC y âÄÎâáÕÔâ ×ÂÊÑÄÞÏË ÒÇÓÇØÑAEÂÏË ÒÇÓÄÑÅÑ ÓÑAEÂ.´ÂÍÑÌ ÉÇ ÄÞÄÑAE ÔAEÇÎÂÐ Ä [51,52] . 170, å 1] ±²¦£²¡»¦¯ªÁ ¢¦³±°²Á¥°¬ ë ±°²Á¥°¬ ª ¶¡©°£½¦ ²¡£¯°£¦³ªÁ ¶ÂÊÑÄÞÇ ÅÓÂÐËÙÞ AEÄÖØ×ÂÊÐÑÌ ÑÃÎÂÔÕË (VC y V 8 C 7 ) ÒÓË ÐÂÎËÚËË Ö ×ÂÊÞ V 8 C 7 ÑÃÎÂÔÕË ÅÑÏÑÅÇÐÐÑÔÕË ÓÂÔÔÚË-ÕÞÄÂÎË Ô ÒÑÏÑÜßá ÖÓÂÄÐÇÐËÌ ÐÂÎßÐÞÌ ÍÂÓÃËAE a- Ta [76].…”

Влияние нестехиометрии на период решетки кубического карбида ванадия VC-=SUB=-y-=/SUB=-

“…Это хорошо согласуется со скачкообразным изменением периода базисной решетки карбида вана-дия от 0.4202 до 0.4200 nm в результате разупоря-дочения фазы V 8 C 7 при нагреве выше температуры превращения порядок−беспорядок T trans = 1413 ± 10 K. Этот эффект был обнаружен при высокотемператур-ном рентгеновском исследовании [16]. В [20] показа-но, что при комнатной температуре период базисной решетки упорядоченной фазы V 6 C 5 во всей области гомогенности больше, чем период a B1 неупорядоченного карбида VC y .…”

Phase Formation of Nanolaminated Transition Metal Carbide Thin Films

“…In group 4 carbides, x is usually 0.6 -0.95 but it becomes narrower in range in group 5 and even narrower in group 6. For the group 5 elements (V, Nb and Ta), non-stoichiometric rocksalt cubic phase tends to undertake order transformation during cooling from higher temperature, leading to the formation of chemically ordered phases, such as V8C7, V6C5, Nb6C5 and possibly V4C3 and Ta6C5, depending on the composition [85][86][87]. These ordered phases retain a basis of the rock-salt structure with ordered vacancies on the C-interstitial sites, resulting in a reduced symmetry of the entire crystal.…”

“…The hexagonal M2C phase is often recognized in literature as β-phase, and sometimes written as β-M2Cx to emphasize its homogeneity region, e.g., x = 0.88 -1.00 for β-V2Cx. Structurally, the metal atoms in the β-phase can be described in an 'ABABAB…' stacking sequence along the c-axis of the hexagonal lattice, as compared to an 'ABCABC…' stacking sequence along the [111]-direction of the NaCl-type TMCs [85,88,89].…”