Jahn-Teller phase transition in Cu0.50TiO(PO4):: Powder structural characterization of the β-variety and thermal study

The thermal study of Cu0.50TiO(PO4), by X-ray diffraction and DSC, shows a phase transition alpha -> beta with a hysteresis (similar to 600 degrees C during heating; similar to 300 degrees C during cooling). Single crystals have been obtained for the alpha-phase but the beta-phase can only be stabilised at room temperature as a powder mixture with alpha. Structural characterization of the beta-variety has been done with diffraction data (X-ray Cu K alpha(1) and neutrons) using a powder rich in beta-phase (alpha(20%)+beta(80%)). A monoclinic cell (a = 7.1134(7) angstrom; b = 7.7282(7) angstrom; c = 7.3028(7) angstrom; beta = 119.30(1)degrees; V = 350.1(1) angstrom(3)) has been found for beta-phase, space group P2(1)/c. An "ab initio" structure determination has been done, and the Rietveld refinement leads to cR(wp) = 0. 150 and R-B = 0.041. The results from the X-ray data were confirmed by refinements from neutron data. Similarly to the alpha-phase, the structure of beta-Cu0.50TiO(PO4)) can be described as a TiOPO4 framework constituted of chains of tilted corner-sharing [TiO6] octahedra running parallel to the c axis and cross linked by [PO4] tetrahedra. Ti atoms are displaced from the centres of the octahedral units, leading to long (2.27 angstrom) and short (1.73 angstrom) Ti-O(1) bonds. The [CuO6] octahedra exhibit a typical Jahn-Teller distorted coordination with four short equatorial Cu-O bonds (2 x 1.93 angstrom and 2 x 2.06 angstrom), and two longer apical Cu-O bonds (2 x 2.33 angstrom). The two longer Cu-O bonds are almost parallel to the b axis. The transition from the alpha to the beta-phase is characterized by a "rocking" of the Jahn-Teller elongation from the (a,c) plane to the b direction accompanied by a relatively strong expansion of the cell volume. (C) 2007 Elsevier Masson SAS. All rights reserved.