LOCAL DISTORTION OF ASO4 AND PO4 MOLECULES IN KDP-FAMILY CRYSTALS

XAFS experiments around the K-absorption edge of P atoms in KDP and As atoms in KDA were carried out in order to study the existence of loca l distortion in PO4 and AsO4 molecules. Well defined XAPS signals were obtained up to 18 angstrom-1 in KDA and 14 angstrom-1 in KDP and DKDP . Extraordinary temperature dependence of the higher frequencies in k3 chi (k) of KDA was found, but nothing happened at T(c). Obtained bond lengths P-O and As-O are consistent with structural analyses. Present experiments suggest that the AsO4 molecule already distorts at the hig h temperature phase, while it is hardly to conclude that a PO4 molecul e is in a disordered state because of the insufficient resolution.