Atomic model for phase transitions in KH 2PO 4 like crystals

Abstract

Abstract Results of inelastic scattering of neutrons from KH 2 PO 4 and isomorphous crystals, indicate a low frequency hydrogen mode (400 cm −1 ). This is supported by i.r. absorption measurements and by Raman scattering. A theoretical treatment shows that the protons lie in almost symmetric double minimum potential wells in the 0–0 direction. Tunneling through the potential barrier gives rise to 400 cm −1 splitting of the ground level, equal to the splitting of the 2400–2800 cm −1 i.r. doublet. Thermal agitation of protons to the 400 cm −1 level (which penetrates the barrier more than the ground level and therefore gives stronger 0–0 attraction) will tend to shorten the 0–0 distance as the temperature is raised-contrary to the thermal expansion of the lattice. There are two temperatures where the two effects balance. At each of them a regenerative expansion or contraction of 0–0 distance is triggered by the dominating thermal expansion causing melting or the ferro-electric transition. One may thus compute the Curie temperature from the melting point and vice-versa. The exact nature of the structure changes at the Curie point is explained as well as many other data as: Behaviour of isomorphic salts, deuteration effects, thermal annomalies. One gets predictions about the temperature behaviour of the H-vibration spectrum.