THERMAL EFFECTS IN THE STRUCTURE OF AMMONIUM PERRHENATE

The structure of ND4ReO4 has been determined by neutron powder diffrac tion at 20 K intervals from 20 to 240 K and at 298 K. At 20 K the N-D bond distance is 1.025 (2) angstrom, the Re-O bond distance is 1.733 ( 2) angstrom and the D ... O interionic distance is 1.878 (2) angstrom. The thermal expansion is normal at low temperatures, but becomes anom alous above about 90 K. The anion orientation changes by 3.2-degrees o ver the whole temperature range while the cation orientation changes b y 28-degrees, and at the higher temperatures the deuterium thermal ell ipsoid becomes highly elongated in the direction perpendicular to the N-D bond. This lends support to the pseudo-spin model for the anomalou s behaviour of this salt, in which disorder between two non-equivalent orientations of the ammonium ion is assumed.