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 local distortion in PO4 and AsO4 molecules. Well defined XAFS signals were obtained up to 18A-1 in KDA and 14A-1 in KDP and DKDP. Extraordinary temperature dependence of the higher frequencies in k3 χ (k) of KDA was found, but nothing happened at Tc. Obtained bond lengths P-O and As-O are consistent with structural analyses. Present experiments suggest that the AsO4 molecule already distorts at the high temperature phase, while it is hardly to conclude that a PO4 molecule is in a disordered state because of the insufficient resolution.