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Investigation of high‐pressure planetary ices by cryo‐recovery. II. High‐pressure apparatus, examples and a new high‐pressure phase of MgSO 4 ·5H 2 O
Author(s) -
Wang Weiwei,
Fortes A. Dominic,
Dobson David P.,
Howard Christopher M.,
Bowles John,
Hughes Neil J.,
Wood Ian G.
Publication year - 2018
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/s1600576718003977
Subject(s) - diffraction , liquid nitrogen , high pressure , phase (matter) , powder diffraction , brine , analytical chemistry (journal) , crystallography , ambient pressure , chemistry , materials science , mineralogy , thermodynamics , chromatography , optics , organic chemistry , physics
An apparatus is described for the compression of samples to ∼2 GPa at temperatures from 80 to 300 K, rapid chilling to 80 K whilst under load and subsequent recovery into liquid nitrogen after the load is released. In this way, a variety of quenchable high‐pressure phases of many materials may be preserved for examination outside the high‐pressure sample environment, with the principal benefit being the ability to obtain high‐resolution powder diffraction data for phase identification and structure solution. The use of this apparatus, in combination with a newly developed cold‐loadable low‐temperature stage for X‐ray powder diffraction (the PheniX‐FL), is illustrated using ice VI (a high‐pressure polymorph of ordinary water ice that is thermodynamically stable only above ∼0.6 GPa) as an example. A second example using synthetic epsomite (MgSO 4 ·7H 2 O) reveals that, at ∼1.6 GPa and 293 K, it undergoes incongruent melting to form MgSO 4 ·5H 2 O plus brine, contributing to a long‐standing debate on the nature of the high‐pressure behaviour of this and similar highly hydrated materials. The crystal structure of this new high‐pressure polymorph of MgSO 4 ·5H 2 O has been determined at 85 K in space group Pna 2 1 from the X‐ray powder diffraction pattern of a sample recovered into liquid nitrogen and is found to differ from that of the known ambient‐pressure phase of MgSO 4 ·5H 2 O (pentahydrite, space group ), consisting of corner‐sharing MgO 6 –SO 4 ion pairs rather than infinite corner‐sharing chains.