Conductance mechanisms and the thermal transition in caesium chloride

Abstract

Experiments on the abnormal conductance of caesium chloride (Harpur, Moss & Ubbelohde 1955) show that this can be altered in various ways. Methods described include heating the salt at 400° C in atmospheres with controlled partial pressures of oxygen, or in nitrogen containing gaseous hydrogen chloride. The conductances of samples of caesium chloride treated with hydrogen chloride show a pre-transition increase of lattice defects on approaching the transitionbcc→fccat 469° C. Micro-analyses show a ratio ([Cs+] — [Cl–])/[CsCl] in the salt as ordinarily obtained, ranging from about 0·000018 to 0·00013. After heating in oxygen this can rise to as much as 0·00066. It is suggested that some of the vacant chloride-ion sites are occupied by the O2-ion. After prolonged heating in oxygen caesium chloride exhibits oxidizing properties which are tentatatively attributed to a proportion of divalent caesium ions of the order of magnitude [Cs2+]/[Cs+] = 0·0007. After treatment with a mixture of hydrogen chloride and nitrogen, this oxidizing power and the excess caesium ratio fall below the limit of detection. Possible conductance mechanisms are discussed in the light of these experiments. The results also give additional information about the nature of the thermal transition. This shows marked hysteresis, suggesting some smearing of the free-energy curves of the two phases due to co-existence, within one crystal, of subcrystalline regions of the two crystal phases over a narrow range of temperatures, with contributions to the free energy from strain and internal surface energy.