Single crystal neutron diffraction study of the phase transition in (NH4)H5(PO4)2
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Sergej I. Troyanov
Abstract
The crystal structure of NH4H5(PO4)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(4), b = 7.554(3), c = 9.507(4) Å, β = 100.71(4)°, Z = 4. It is characterized by disorder of the NH4+ cations and short symmetrical hydrogen bonds (2.422(8) Å) connecting the layers of H2PO4 tetrahedra. The low temperature modification I is monoclinic, (space group P21/c), a = 10.040(4), b = 7.628(2), c = 9.558(3) Å, β = 102.51(4)°, Z = 4. Below the phase transition the NH4+ cations become ordered and the layers of PO4 tetrahedra are connected by asymmetrical hydrogen bonds (2.427(7) Å) allowing to formulate the phase I as (NH4)(H2PO4)(H3PO4). The changes in the thermal ellipsoids and a possible mechanism of the phase transition are discussed.
© 2003 Oldenbourg Wissenschaftsverlag GmbH
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