Synchrotron X-ray analysis of RbTiOAsO4

Structure factors for rubidium oxotitanium arsenate, RbTiOAsO4, were measur ed at 293 K with focused synchrotron X-radiation [lambda = 0.7500 (9) Angst rom] using a fast avalanche photodiode counter. The accurate synchrotron si ngle-crystal data are of sufficient quality and resolution to detect the sp litting positions of the Rb cations at room temperature. Strong accumulatio n of the similar to Delta rho difference charge density near the Rb atoms a t a distance of similar to 0.5 Angstrom in the -c direction can be attribut ed to the partial occupancy of additional sites related by pseudosymmetry. This type of static and/or dynamic disorder is temperature-dependent and se ems to be universal for the KTiOPO4 family of compounds. The best modelling of the experimental data was obtained with the Rb atom in split positions described within the harmonic approximation and the multipole functions for the other atoms. The Delta rho density features in the Ti-O and As-O coval ent bonds can be related to the linear and non-linear susceptibility using bond-polarization theory. The charge-density maps reflect the anisotropy of non-linear susceptibility, which is larger for directions where locally an tisymmetric components of Delta rho are strong.