STRUCTURE OF HIGH TEMPERATURE PHASES OF THALLOUS NITRATE. P. U. M. Sastry, A. Sequeira, Solid State Physics Division, B.A.R.C., Mumbai 400085, India

TlNO₃ exhibits structural transitions from orthorhombic phase (III) at R. T. to hexagonal phase (II) (T_c=79deg. C) and then to cubic phase (I) (T_c=147deg.C). These transitions are believed to be associated with reorientation of planar nitrate ions. The structure of phase III is reported in an earlier single crystal neutron study¹. In the present work, the structures of phase II (at 115deg.C) and phase I (at 170deg. C) are determined using powder neutron diffraction. Refinements using Rietveld technique indicate that the structures of phase II (Sp.Gr. P3₁; a=10.435(1) Å, c=7.451(1) Å, Z=9) and phase I (Sp.Gr. Pm3m, a=4.307(1) Å,Z=1) are similar to those of RbNO₃ (IV) and RbNO₃(III) respectively. The final R-values (R_p) for phases II and I are 3.8% and 5.1%. In all the three phases, nitrate ions are oriented perpendicular to the three edges of surrounding Tl-cubes with equal proportions in phases II and I. Transformation to phase II from phase III involves an out-of-plane flip of 90deg. for one of the eight nitrate ions in the cell. While the nitrate ions are ordered in phases III and II, they exhibit a 12-fold orientational disorder in phase I with one of the N-O bonds oriented parallel to an edge of Tl-cube. Refinements rule out the earlier models proposed in literature for phases II and I in which the nitrate ions are oriented normal to the diagonal of Tl-cube.

1. P. U. M. Sastry, H. Rajagopal and A. Sequeira, Acta Cryst., C50, 1854 (1994).