Local Distortion of AsO 4 and PO 4 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 PO 4 and AsO 4 molecules. Well defined XAFS signals were obtained up to 18Å -1 in KDA and 14Å -1 in KDP and DKDP. Extraordinary temperature dependence of the higher frequencies in k 3 χ (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 AsO 4 molecule already distorts at the high temperature phase, while it is hardly to conclude that a PO 4 molecule is in a disordered state because of the insufficient resolution.