Structure of and electron density in RbTiOAsO4 at 9.6 K

Structure factors for rubidium titanyl arsenate, RbTiOAsO4, were measured w ith Mo K alpha radiation (lambda = 0.71069 Angstrom) 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) Angstrom at 9.6 K, and a = 13.261 (2), b = 6.6791 (8) and c = 10.769 (1) Angstrom at 295 It. As the temperature was l owered from 295 to 9.6 K the Rb atoms moved along the c axis in the directi on of the polarization vector, while no significant change was noted for th e Ti-O-As network. Strong accumulation and polarization of the difference e lectron density (Delta p) in exceptionally short covalent Ti-O bonds altern ates with the depleted density in long Ti-O bonds. The Delta rho near the T i atoms is polarized and aligned in the negative c direction in accordance with the ferroelectric properties of this material. However, the electron d ensity 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.