Structure and thermal properties associated with some hydrogen bonds in crystals VII. Behaviour of KH 2 PO 4 and KH 2 A sO 4 on cooling

Abstract

A low-temperature micro-thermostat is described, suitable for single-crystal X-ray work. Thermal expansions of KH 2 PO 4 (and in less detail of KH 2 AsO 4 ) have been measured by X-ray methods from room temperatures down to about 80° K. Both these crystals differ from some previously investigated organic compounds with short hydrogen bonds, in that maximum thermal contraction on cooling occurs along the c axis, which is approximately perpendicular to the planes containing the hydrogen bonds in these two crystals. This axis becomes electrically polarized below a thermal ‘transition point’. Remarkable effects are observed in the neighbourhood of the transition point, similar to those previously described for Rochelle salt. When the tetragonal single crystals are cooled below the respective Curie points (123° K for KH 2 PO 4 and 96° K for KH 2 AsO 4 ) they change into crystal ‘hybrids’, in which subcrystalline units of lower symmetry pack so as to preserve the c axis and either the (100) or (010) planes of the original tetragonal structure, and to maintain electrical neutrality on the average. When a ‘hybrid ’ crystal is warmed above the Curie point, the subcrystalline units merge after a varying lapse of time into the original tetragonal lattice. This change from single crystal to hybrid and back can be repeated at will by cooling and heating. The structure of these crystal hybrids has been investigated by both Bragg and Laue methods. On the basis of the new experimental evidence and of previous work, an explanation is offered for phase transitions ‘of the second kind’, preferably termed ‘continuous’, in terms of crystal structure. It is known that ‘discontinuous’ phase changes at a transition ‘point’ involve a rearrangement of molecules within the single crystal of one structure, to give a powder of crystals with a different structure. But if the new crystals can pack as sub­crystalline units into the original crystal lattice, as is possible when the difference in structure is small and not too much strain is involved, the phase change will occur continuously over a range of temperatures instead of discontinuously at a transition point. The X-ray studies described give new information about the thermodynamics of crystals, and promise to throw light on certain problems of imperfect crystals. Possible biological analogues of hybrid crystals are briefly referred to.