Relationship between the rotation angles of octahedra and bond-strength energy in crystals with perovskite structure
- PDF / 285,356 Bytes
- 9 Pages / 612 x 792 pts (letter) Page_size
- 82 Downloads / 205 Views
RY OF CRYSTAL STRUCTURES
Relationship between the Rotation Angles of Octahedra and Bond-Strength Energy in Crystals with Perovskite Structure N. M. Olekhnovich Joint Institute of Solid-State and Semiconductor Physics, National Academy of Sciences of Belarus, ul. Petrusya Brovki 17, Minsk, 220072 Belarus e-mail: [email protected] Received April 24, 2006
Abstract—The dependence of the strain energy of anion–cation bonds on the sum Ψ2 of squares of the angles of ordered rotations of octahedra around the coordinate axes of the reduced cell has been determined for ABX3 compounds with perovskite structure in the two-particle approximation. A relation determining the change in the thermodynamic Gibbs potential with allowance for the change in the bond-strain energy in the dependence on Ψ2 is proposed. An expression for the equilibrium value of Ψ2 as a function of the bond-strain energy and temperature is obtained. Analysis of Ψ2 is performed for a homologous series of LnVO3, LnFeO3, and LnAlO3 compounds (Ln are rare earth elements) and a series of CaBO3 and SrBO3 compounds has been performed. It is shown that the calculated values of Ψ2 for each of these series of compounds are in agreement with the values experimentally obtained from the structural data. The transition temperatures of the compounds belonging to the series studied into the cubic phase are estimated and compared with the experimental data. PACS numbers: 61.50.Ah, 61.50.Ks DOI: 10.1134/S106377450705001X
INTRODUCTION It is known that binary oxide and halides (ABX3) and similar systems of more complex composition contain a large class of compounds with perovskite structure. Temperature phase transitions related to ordered lattice distortions are observed for most crystals of such type. In the high-temperature phase existing above some temperature TC, ordered distortions are not observed, and the lattice is cubic. A transition to the range below TC is accompanied by the formation of ordered lattice distortions, which decrease the lattice symmetry. Depending on the nature of the elements entering the composition of the above-mentioned compounds, ordered distortions can be related to the rotation of BX6 octahedra with respect to the coordinate axes, to displacement of cations, or simultaneously to both these effects. Such phase transitions are assigned to displacive ones. It is believed that the interatomic interaction potential in crystals with displacive phase transitions, in contrast to the systems exhibiting order–disorder phase transitions, either above or below TC, is singlewell. However, there are experimental data according to which the phase transitions in crystals with perovskite structure show signs of order–disorder transitions [1–4]. On the basis of the data on the X-ray absorption fine structure, it was established that the interatomic interaction potential in perovskites is multiple-well [4]. The nature of the multiple-well potential in such crystals is related to the interatomic bond strain [5], which origi-
nates from the inconsistency
Data Loading...
