Structure of and electron density in RbTiOAsO4 at 9.6 K.

Structure factors for rubidium titanyl arsenate, RbTiOAsO(4), were measured with Mo Kalpha 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 Pna2(1). 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 (Deltarho) in exceptionally short covalent Ti-O bonds alternates with the depleted density in long Ti-O bonds. The Deltarho 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 Deltarho is moved further away from the nuclei along the c vector.