The crystal structure of KH 2 AsO 4 (KDA) is refined at room temperature by neutron single-crystal structural analysis. The distance R OO between two oxygen atoms bonded by a hydrogen atom is slightly longer than that in KH 2 PO 4 (KDP). A double-peak distribution of H with the splitting distance d is visualized directly in the nuclear density map obtained by the maximum entropy method. The parameter d of KDA is also larger than that of KDP, though the transition temperature of KDA is lower than that of KDP.

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