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Sergey I. Troyanov, E. V. Kosterina, A. Loose, M. Reehuis, Erhard Kemnitz

Single crystal neutron diffraction study of the phase transition in (NH₄)H₅(PO₄)₂

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.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 218, 07/2003
Pages: 470

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