Neutron diffraction study of temperature dependence of atom distribution in cubic zirconium carbohydrides

Full text: Earlier we have shown that the complex distribution of interstitial atoms takes place in the crystal lattice of cubic zirconium carbohydrides prepared by self-propagating high-temperature synthesis (SHS). One of the features of SHS-synthesis of inorganic compounds is high burning temperature and fast cooling from combustion temperature after synthesis. At that, self-hardening of the prepared product takes place, and structural state of the compound can correspond to high-temperature state. But the question about distribution of interstitial atoms over the crystal structure of cubic carbohydrides at the relatively low temperatures (T< 1100 deg C) is opened. The aim of the present work is neutron and X-ray diffraction study of distribution of interstitial atoms at relatively low temperatures (T≤ 1100 deg C). Neutron diffraction patterns were obtained using the neutron diffractometer DN-500 of INP AS RUz (λ = 1.085 A), and X-ray diffraction patterns - using the X-ray diffractometer DRON-3M (λ =1.5418 A). The samples were annealed in evacuated and sealed quartz ampoules. Over the homogeneity range the samples of cubic carbohydrides were annealed at different temperatures. The annealings up to 900 deg C during 50 h at each temperature: 500, 600, 700, 800 deg C did not result to change of neither character of interstitial atoms distribution in the disordered solid solutions (space group (sp. gr.) Fm3m), nor degree of long-range order and sizes of anti-phase domains (APhD) in the ordered zirconium carbohydrides (sp. gr. Fd3m). So, at T≤ 900 deg C process of diffusion of interstitial atoms is essentially slowed down. Quenching in water after annealing at the temperature of 1000 deg C during 50 h did not lead to change of distribution of interstitial atoms in the disordered zirconium carbohydrides. The same annealing results to increasing degree of long-range order (η ) of carbon sublattice in the ordered carbohydrides. Hardening after annealing at 1100 deg C practically leads to restoration of the initial parameters of the long-range order that is to reduction them up to the values corresponding to the initial SHS-products. The obtained results on heat treatment allow one to assume: firstly, the temperature of the order - disorder transition decreases with increasing carbon concentration, and respectively, with decreasing of number of octahedral vacancies and so stoichiometry of the carbon ordered phase corresponds to the stoichiometry of Zr2C; secondly, the temperature of the order - disorder transition in zirconium carbohydrides is above 1100 deg C. It is shown that unlike in Zr-C system, in cubic zirconium carbohydrides the temperature of the order - disorder transition is much higher, and over the wide homogeneity range in the temperature interval of 1000-500 deg C the formation of trigonal ordered phase is not observed. Hence, hydrogen suppresses the formation of the ordered trigonal phase, expanding temperature range of stability of the ordered cubic phase. (author)