Acta Crystallographica Section B

Structural Science

Volume 55, Part 5 (October 1999)

research papers

Acta Cryst. (1999). B55, 712-720 [ doi:10.1107/S010876819900395X ]

Structure of and electron density in RbTiOAsO₄ at 9.6 K

J. Almgren, V. A. Streltsov, A. N. Sobolev, B. N. Figgis and J. Albertsson

Abstract: Structure factors for rubidium titanyl arsenate, RbTiOAsO₄, were measured with Mo K radiation ( [lambda] = 0.71069 Å) at 9.6 and 295 K. The data show that there is no phase transition between room temperature and 9.6 K. The space group is Pna2₁. Unit-cell parameters are a = 13.218 (1), b = 6.6500 (9) and c = 10.761 (1) Å at 9.6 K, and a = 13.261 (2), b = 6.6791 (8) and c = 10.769 (1) Å at 295 K. As the temperature was lowered from 295 to 9.6 K the Rb atoms moved along the c axis in the direction of the polarization vector, while no significant change was noted for the Ti-O-As network. Strong accumulation and polarization of the difference electron density ( [Delta] [rho] ) in exceptionally short covalent Ti-O bonds alternates with the depleted density in long Ti-O bonds. The [Delta] [rho] near the Ti atoms is polarized and aligned in the negative c direction in accordance with the ferroelectric properties of this material. However, the electron density near the Rb atoms is depleted in this direction and the excess [Delta] [rho] is moved further away from the nuclei along the c vector.

Keywords: Rubidium titanyl arsenate; Electron density distributions; Low-temperature diffractometry; Ferroelectrics.