Single crystal neutron diffraction study of the phase transition in (NH_4)H_5(PO_4)_2

摘要

The crystal structure of NH_4H_5(PO_4)_2 was investigated at 298 and 80 K by single crystal neutron diffraction. The room temperature modification II crystallizes monoclinic (space group C2/c), a = 10.156(04, b = 7.554(3), c = 9.507(4)A, β = 100.71(4)~°, Z = 4. It is characterized by disorder of the NH_4~+ cations and short symmetrical hydrogen bonds (2.422(8)A) connecting the layers of H_2PO_4 tetrahedra. The low temperature modification I is monoclinic (space group P2_1/c), a = 10.040(4), b = 7.628(2), c = 9.558(3)A, β = 102.51(4)~°, Z = 4. Below the phase transition the NH_4~+ cations become ordered and the layers of PO_4 tetrahedra are connected by asymmetrical hydrogen bonds (2.427(7)A) allowing to formulate the phase I as (NH_4)(H_2PO_4). The changes in the thermal ellipsoids and a possible mechanism of the phase transition are discussed.