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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 KH 2 PO 4 (KDP) family, the structures of KH 2 AsO 4 (KDA) and KD 2 AsO 4 (DKDA), which are the end members concerning 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 was revealed that the hydrogen-bond distances R OO for KDA and DKDA are longer 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 angles in KDA and DKDA are compared with those in other members in the family and similarity and difference are discussed.