CRYSTAL STRUCTURE AND THERMAL EXPANSION OF NiSi

M.Kh. Rabadanov

Daghestan State University, ul. M.Gadjieva, 43A, Makhachkala, Russia. rabmur@sinol.ru

Keywords: NiSi, X-ray analysis, thermal expansion

The huge anisotropy of the thermal expansion was found in NiSi monocrystals and what's more this crystal is compressed along crystallographic b-axis under the rise of the temperature. At the same time the information about the crystal structure of this monocrystal in the literature is rather poor and does not allow to understand the nature of such behaviour.

The results of X-ray researches of NiSi monocrystals are presented under the temperature of 295 and 418 K. On the basis of the experiments' results carried out on the ENRAF-NONIUS diffractometer the models of structure with the isotropic and anisotropic thermal vibrations of atoms and the anisotropic model with the account of anharmonicity were refined. The relative coordinates and parameters of the thermal vibrations of atoms were obtained. It was shown that the coordinative poliedrons of the both atoms were the strongly deformed octaedrons; the bond lengths Ni-Si have disorder from 2.280 up to 2.398 Ä.

It is obtained that the thermal vibrations of Ni atoms are more anisotropic than Si atoms thermal vibrations; the thermal ellipsoids of Si atoms are close to two-axes ones; for Ni atoms the root-mean square displacements on the crystallographic b-direction along which the coefficient of the thermal expansion is negative are the greatest and the thermal ellipsoids are extended along this axis.

The explanation of the huge anisotropic of this crystal's thermal expansion including negative "expansion" along an b-axis is given. It is significant, that the considerable thermal expansion along c-axis is solely connected with the changes of geometry in Si-Si zigzag "chains", whereas the both "chains" Si-Si and Ni-Ni are extended along a-axis. It is obtained that the significant thermal expansion in the planes of symmetry on which all atoms are located leads to their rapproachement and causes the compression along b-axis. For all this all bond lengths in unit cell grow with the increase of the temperature. It is shown that such thermal expansion is defined by geometry of the atoms' arrangement and the chemical bonds between them (Ni-Ni, Si-Si, Ni-Si).