UC Irvine

TWO crystal structures, both cubic, are found in the alkali halide group of compounds: (1) the 'sodium chloride type' of interpenetrating face-centred cubes, and (2) the 'cæsium chloride type' of interpenetrating simple cubes. All the group except cæsium chloride, cæsium bromide and cæsium iodide resemble sodium chloride at ordinary temperatures and pressures1. Measurements of the breakdown strength of halides of the sodium chloride type have shown2 that there exists a simple relationship between the breakdown field strength, F, and the interionic distance, a, given by Fa3 = 28(±25 per cent) where F is measured in MV./cm. and a in Å. No previous work on the cæsium chloride type of halide has come to our notice. © 1961 Nature Publishing Group.