The heat capacity and thermodynamic properties of potassium perrhenate and ammonium perrhenate from 8 to 304 K

Abstract

The heat capacities of KReO4 and NH4ReO4 from 8 to 304 K have been measured. The data show no obvious order–disorder phase transition but the curve for NH4ReO4 has a broad bump extending from about 100 to 250 K with a maximum around 200 K. This bump is absent from KReO4. For each salt, however, there is a tiny reproducible peak between 265–275 K, which is the result of impurity, probably water. An attempt has been made to calculate the normal curves of heat capacity against temperature for each salt, making use of spectroscopic information available for these crystals about the librational frequencies. For KReO4, the maximum difference between the experimental and calculated Cp values below 265 K is less than 0.8%. This excellent agreement is taken as evidence of an absence of any anomaly below 265 K. The case of NH4ReO4 is very different. Because the librational frequency ν6 of the NH4+ disappears from the spectrum at T≳180 K, the difference between the experimental and calculated Cp values was expressed as ΔCp=Clib NH4++Cextra, where the cextra refers to any other unaccounted contribution. When plotted against temperature, ΔCp rises steeply from 100 K to a sharp peak at 190 K (ΔCp=45 JK−1 mol−1) and then drops to 3 R (24.9 JK−1 mol−1) as 260 K is reached. Various models of restricted rotation of the ammonium ion are tested including that of a tetrahedron in a tetrahedral environment. All models tested so far fail to produce the peak in ΔCp. Our data are consistent with recent x-ray measurements suggesting that the hydrogen bonding present at 135 K disappears at room temperature and with NMR data showing a large amount of rotational motion of the NH4+ ion about 100 K.