• PDF / 449,337 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 40 Downloads / 195 Views

DOWNLOAD

REPORT

---

3 G. JOMARD"' , T. PETIT', L. MAGAUD , A. PASTUREL 1 CEA-Grenoble/ DRN/DTP/SECC/LA3C, Av. des Martyrs 38000 Grenoble FRANCE 2 Laboratoire d'Etude des Propridtds des Solides CNRS B.P. 166, 38042 Grenoble-Cedex FRANCE 3 Laboratoire de Physique et mod6lisation des Milieux Condensds CNRS B.P. 166, 38042 Grenoble-Cedex, FRANCE

ABSTRACT The structural and electronic properties of four different structures of zirconia (Zr02) are studied using ab initio total energy calculations. The calculations are made in the framework of density functional (DFT) and pseudopotential theory. We compare results given within the LDA (Local Density Approximation) and including Generalized Gradient Corrections (GGCs) in the Perdew Wang and Perdew Becke formalisms. We present results for pure and defective zirconia (oxygen vacancies and Zr substitution by Fe) showing the effects of such point defects on the relative structural stabilities of the different pseudopolymorphs. INTRODUCTION Zirconia is an important material in ceramic engineering combining high temperature and high strength. Particularly important is the transformation toughening materials associated with the tetragonal to monoclinic phase transition. In that way, some experiments report a tetragonal structure stabilization of zirconia in the first stages of Zr oxidation process which could be related to the presence of oxygen vacancies and iron [1,2]. Thus the knowledge of the crystal structure of zirconia polymorphs and the mechanisms of transitions between them are of considerable interest in view of their connection with the properties of advanced zirconia-based ceramics. Recent research has revealed the existence of several polymorphs of Zr02 in different ranges of temperature and pressure. At atmospheric pressure, it is established that Zr0 2 displays three polymorphs. At low temperatures the monoclinic C2h phase (space group P21/c) is stable [3]. Around 1400 K there is a first-order displacive martensitic phase transition to tetragonal D" (P42/nrmc) structure [4]. At a temperature of about 2600 K, the tetragonal phase transforms into the cubic fluorite O (Fm3m) structure [5]: this cubic structure is a special case of the tetragonal structure and can be obtained from the latter by making the ratio of the lattice constants c/a equal to v2 and by shifting pairs of oxygen atoms in the z direction to their central positions in the unit cell. At high pressures, a variety of phase transitions have been observed but there is yet no general agreement about the high pressure phase diagram. Preliminary results [6] give two phases with orthorhombic symmetry, a low pressure orthorhombic phase closely related to the monoclinic one and a higher pressure phase with a distorted cotunnite (PbCl2) structure. A more recent study [7] gives the occurrence of the orthorhombic phases up to about 50GPa but does not confirm the space groups previously reported. Several attempts have been made to modelize and to understand the energetics and bonding in zirconia. One of the main goals of these apppro