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H.L. Keller, D. Kucharczyk, H. Küppers

The ferroelastic monoclinic low temperature modification of ammonuim hydrogen oxalate hemihydrate

NH₄HC₂0₄ * ¹/₂ H₂0, which at room temperature belongs to space group Pmnb, undergoes at 145.6 K a phase transition to the monoclinic space group P2₁/n with a = 12.331(1), b = 11.2241(1), c = 6.7608(3) Å and β = 91.64(6)° (at 128 K). The lattice parameters were measured very accurately in order to reveal the anomalous behaviour in the vicinity of the phase transition. The low temperature phase turns out to be twinned and constituted of ferroelastic domains (mmmF2/m species). Diffraction effects and observations with the polarizing microscope indicate lamellae which lie predominantly parallel to (100). Refinement with 2738 independent reflections measured at 128 K led to a weighted R_w = 0.075. The structural change is mainly characterized by the NH₄ ions and the water molecules moving offtheir special position on mirror planes which they occupy above the transition temperature. This leads to a strengthening of hydrogen bonds and rearrangements within the hydrogen bond network.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 158, 03/1982
Pages: 221 - 232

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