POSSIBLE ORIGIN OF THE ISOTOPE EFFECT ON THE PHASE-TRANSITION OF K3(D, H)(SO4)2

Structural data of K3H(SO4)2 and K3D(SO4)2 reported by Noda et al. [J. Phys. Soc. Jpn. 59 (1990) 2809 and ibid. 3249] are precisely examined in order to investigate the possible origin of the isotope effect in a hydrogen bond material. The SO4 molecule is slightly distorted from a regular tetrahedron. The distortion is explained by the displacement of the sulfur atom as well as that of the oxygen atom which is in charge of forming the hydrogen bond. The size of the SO4 molecule in the D-compound is slightly larger than that in the H-compound, but the difference seems to be marginally small. The major geometrical difference is only seen in the hydrogen bond length R(OO). In addition, electron transfer between hydrogen and oxygen atoms with opposite directions was found. The difference of the ionicity of H and D atoms is considered to be the origin of the difference of the bond length R(OO).