Acta Crystallographica Section B

Structural Science

Volume 57, Part 6 (December 2001)

research papers

Acta Cryst. (2001). B57, 747-758 [ doi:10.1107/S0108768101013647 ]

Temperature-dependent neutron powder diffraction study of the Ba(OD)₂ polymorphs: a new low-temperature phase

A. Friedrich, M. Kunz and E. Suard

Abstract: The structural behaviour of both low-temperature [beta] - and high-temperature [alpha] -Ba(OD)₂, barium dihydroxide-d, was investigated at temperatures between 10 and 552 K by neutron powder diffraction. While the [beta] phase (P2₁/n) remains stable to the lowest temperature investigated, the quenchable [alpha] phase (Pnma) shows a reversible orthorhombic-to-monoclinic phase transition between 100 and 150 K. The structure of the new [alpha] _m phase (P2₁/n) is quite similar to that of the [alpha] phase. This behaviour is unusual as a metastable phase transforms to another metastable phase. The Pnma [left right arrow] P2₁/n low-temperature phase transition is driven by an order-disorder mechanism, mainly caused by one of the D atoms, which is disordered on positions off the mirror plane, mimicking a special position on the mirror plane in the orthorhombic phase. Refinements of the [alpha] phase above the phase transition indicate this disorder across the mirror plane through a conspicuously high isotropic displacement parameter if compared to the other D atoms. At low temperature the energy of the vibration is lowered and the D atom is frozen at a general position in a correlated way, thus violating the mirror plane and reducing the space-group symmetry.

Keywords: polymorphism; low-temperature; phase transitions; deuterium.