1. Thermal expansion of ammonium pertechnetate and ammonium perrhenate.
- Author
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Reynolds, Emily M., Yu, Michelle, Thorogood, Gordon J., Brand, Helen E.A., Poineau, Frederic, and Kennedy, Brendan J.
- Subjects
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THERMAL expansion , *SYNCHROTRONS , *X-ray powder diffraction , *PERTECHNETATE , *CHEMICAL bond lengths , *AMMONIUM salts - Abstract
The thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 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 NH 4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the B O 4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO 4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH 4 TcO 4 and NH 4 ReO 4 is a consequence of differences in the H B O 4 interactions due to changes in the B O bond lengths within the B O 4 groups. Both ammonium salts decompose when heated above ∼500 K. Variable temperature synchrotron X-ray diffraction revels that the thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K is highly anisotropic, including negative thermal expansion along the c -axis. Whereas NH 4 ReO 4 exhibits negative volume thermal expansion over a narrow temperature range NH 4 TcO 4 does not exhibit negative volume thermal expansion. Both oxides have the tetragonal Scheelite type structure and the observed anomalous thermal expansion is associated with disorder in the orientation of the NH 4 groups and the presence of both equatorial and axial H-bonding to the B O 4 tetrahedral. Image 1 • Structures of NH 4 TcO 4 and NH 4 ReO 4 determined using synchrotron X-ray powder diffraction. • Thermal expansion of NH 4 TcO 4 and NH 4 ReO 4 between 97 and 500 K determined. • Anisotropic thermal expansion, including NTE, related to disorder of NH 4 groups. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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