RUBIDIUM TITANYL ARSENATE (RTA) AT 9.6 K

Jörgen Albertsson¹, Jenni Almgren¹, Victor A. Streltsov² and Alexander N. Sobolev³

¹Inorganic Chemistry, Chalmers University of Technology, S­412 96 Göteborg, Sweden;
²Crystallography Centre and
³Department of Chemistry, University of Western Australia, Nedlands, Western Australia 6907, Australia.
E-mail: jalb@inoc.chalmers.se.

RTA (RbTiOAsO₄) is isostructural with KTP (KTiOPO₄) and a non­linear optic material, which can be used for sum and difference frequency generation and optic parametric oscillation in a wide range of visible and infrared wavelengths, especially if quasi­phase matched samples can be prepared. The ferroelectric properties of RTA should be manifest in changes of the charge density polarization and in static atomic displacements when the temperature is changed. In the present study, the structural parameters and difference electron density have been determined from X­ray diffraction data at room temperature (298 K) and 9.6 K. The low-temperature experiment was conducted using a cryostat similar to the one described by Larsen [Acta Cryst. (1995). B51, 468­482]. No phase transition was detected in the investigated temperature interval. The unit cell volume decreases from 953.8(2) at room temperature to 945.9(2) A³ at 9.6 K without significant changes in the Ti­As­O framework structure. On the other hand, the rubidium ions, which are located in holes in the framework, move approximately 0.05 A in the negative c­direction, further away from their pseudo-symmetric position in the room temperature structure. Increased asymmetrical dipole-type polarization along the c­direction at the Ti and Rb sites may be detected in the 9.6 K electron density maps, whereas the density distribution in the plane perpendicular to c remains symmetrical. Since the data were collected from an as-grown crystal of size 0.58x0.54x0.36 mm using a sealed tube, the present maps have a high noise level but the study will be continued with high-flux synchrotron X­radiation and much smaller samples of RTA, to minimize absorption and extinction effects.

The room temperature structure of RTA has been published by Thomas, Mayo & Watts [Acta Cryst. (1992). B48, 401­407]. Some crystal and structure refinement data at 9.6 K are as follows: orthorhombic space group Pna2₁, a = 13.2180(10), b = 6.6500(9), c = 10.7610(10) A, Z = 8, pseudo­octahedral shape, i = 18.90 mm^­1; 15878 intensities collected to q_max = 50^o, R_int = 0.0224; 146 parameters refined using 5020 independent reflections with F_obs > 2s(F_obs), R = 0.0320, wR = 0.0263, extinction coefficient (Zachariasen) = 0.741(9), Flack parameter = 0.455(6) (the as­grown crystal is a polar twin).