Thermal expansion of ammonium pertechnetate and ammonium perrhenate

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The thermal expansion of NH4TcO4 and NH4ReO4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the BO4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH4TcO4 and NH4ReO4 is a consequence of differences in the H-BO4 interactions due to changes in the B-O bond lengths within the BO4 groups. Both ammonium salts decompose when heated above ∼500 K. ©2019 Elsevier Inc.
Technetium oxides, Technetium compounds, Crystal structure, Ammonium compounds, Rhenium compounds, Sodium compounds, Salts, Tungstates, Pertechnetates
Reynolds, E. M., Yu, M., Thorogood, G. J., Brand, H. E. A., Poineau, F. & Kennedy, B. J. (2019). Thermal expansion of ammonium pertechnetate and ammonium perrhenate. Journal of Solid State Chemistry, 274, 64-68. doi:10.1016/j.jssc.2019.01.036