X-ray structural study of ferroelectric KH2AsO4 and KD2AsO4

In order to understand systematically the relationship between the crystal structure and phase transition in tetragonal KH2PO4 (KDP) family, the struc tures of KH2AsO4 (KDA) and KD2AsO4 (DKDA), which are the end members concer ning the transition temperature T-c have been studied by X-ray diffraction at room temperature and at T-c + 5 K (110 K and 171 K. respectively). It wa s revealed that the hydrogen-bond distances R-OO for KDA and DKDA are longe r than those for KDP and DKDP, respectively. These results smear the linear R-OO vs T-c relation found earlier in the tetragonal KDP family, contrary to our expectation. The temperature dependence of the bond lengths and angl es in KDA and DKDA are compared with those in other members in the family a nd similarity and difference are discussed.