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The Post‐Cotunnite Phase in BaCl 2 , BaBr 2 and BaI 2 under High Pressure
Author(s) -
Léger J. M.,
Haines J.,
Atouf A.
Publication year - 1995
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889895001580
Subject(s) - monoclinic crystal system , coordination number , crystallography , intermetallic , diffraction , phase (matter) , barium , powder diffraction , chemistry , diamond anvil cell , materials science , crystal structure , inorganic chemistry , metallurgy , organic chemistry , ion , physics , alloy , optics
Angle‐dispersive powder diffraction patterns have been obtained as a function of pressure in a diamond‐anvil cell for BaCl 2 , BaBr 2 and BaI 2 ( P max = 40 GPa). These cotunnite‐structure compounds ( Pnam , Z = 4) at ambient conditions transform to a common monoclinic ( P 112 1 / a , Z = 8) high‐pressure structure between 5 and 15 GPa at room temperature. The low‐pressure cotunnite cell is doubled along a and slightly distorted, γ = 87–89°; the coordination number has increased from 9 to 10. This monoclinic structure is a distortion of the Co 2 Si type ( Pnam , Z = 4) found in intermetallic AB 2 compounds with the same coordination number; it is the same as that already found for PbCl 2 and SnCl 2 . Simulations of the diffraction patterns of the three barium dihalides with the same atomic positions give excellent agreement with the observed diffraction patterns although the atomic diffusion factors vary greatly. A new structure type is thus added to the structures of the AX 2 compounds and this is now the final structure that has been reached in the high‐pressure sequence of phase transformations in these materials.