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The structure has been determined from three-dimensional neutron diffraction data collected at 188 K. There are four formulae in a unit cell of dimensions a = b = 7.23, c = 19.32/k. Despite the tetragonal axial symmetry, there are significant deviations towards orthorhombic symmetry in some intensities. The present structure is based on orthorhombic symmetry with space group Pccn, and has been refined by several cycles of full-matrix least squares to a final R of 4.8%. The structure shows two features: pseudosymmetry and short hydrogen bonds. These features are discussed in relation to the twinning. Explanations of the twinning are suggested. The transformation is considered to result from ordered arrangements (order-disorder) of H atoms and slight displacements (displacive) of the other atoms from the closest-packing positions. The structure is held together by an extensive system of hydrogen bonds. This phase transition is compared with that of the tetragonal form of (CH3NH3)2CdCI 4.

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