Crystal structure of H 3 NCH 2 COOHÁH 2 PO 3 was studied in the paraelectric phase (P phase) at 298 K and in the ferroelectric phase (F phase) at 170 K by means of the single crystal neutron diffraction. Two independent protons in the hydrogen bonds forming the H(HPO 3 ) À chains are disordered over two sites with the separations of 0.486(3) and 0.474 (5) A in the P phase. In the F phase, they are ordered at a position nearly equal to one of two sites related by the disorder. With the proton order, lengths of the P-O bonds with the oxygens involved in the hydrogen bonds forming the chain deviate from their values determined in the P phase. A possible mechanism for the appearance of the spontaneous polarization is discussed in connection with the deformation of the HPO 2À 3 anions in the F phase.

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