New High-pressure Polymorph of Sodium Fluoride

Abstract

RECENTLY, Piermarini and Weir¹ found a high-pressure transition (from the face-centred cubic to the cæsium chloride-type structure) between 9 and 15 kbar in anhydrous rubidium fluoride. Somewhat later transitions were also found for cæsium fluoride and potassium fluoride². Bridgman³ found no evidence for high-pressure polymorphs in any of the alkali fluorides and noted that the fluorides appeared to be anomalous in this respect. For the case of sodium fluoride Bridgman⁴ stated that “the shearing curve definitely has a maximum near the end of the pressure range, leading to the expectation of a transition. Because of the interest which any transition in this particularly simple compound would have, unusually careful exploration was made with the volume apparatus. Runs were made at 25°, 125°, 150°, and 175° C. It is certain that there is no transition with the large volume change associated with the change from face centred to body centred, which is the transition anticipated”.