Acta Crystallographica Section B

Structural Science

Volume 57, Part 6 (December 2001)

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 - and high-temperature -Ba(OD)₂, barium dihydroxide-d, was investigated at temperatures between 10 and 552  K by neutron powder diffraction. While the phase (P2₁/n) remains stable to the lowest temperature investigated, the quenchable phase (Pnma) shows a reversible orthorhombic-to-monoclinic phase transition between 100 and 150  K. The structure of the new _m phase (P2₁/n) is quite similar to that of the phase. This behaviour is unusual as a metastable phase transforms to another metastable phase. The Pnma 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 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.

Online 29 November 2001

	Structure factor file (CIF format) (144.6 kbytes) Contains datablock a10
	Structure factor file (CIF format) (144.7 kbytes) Contains datablock a50
	Structure factor file (CIF format) (76.4 kbytes) Contains datablock a200
	Structure factor file (CIF format) (77.9 kbytes) Contains datablock 552
	Structure factor file (CIF format) (57.8 kbytes) Contains datablock brt
	Structure factor file (CIF format) (57.8 kbytes) Contains datablock b250
	Structure factor file (CIF format) (58.1 kbytes) Contains datablock b480

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