Temperature Dependence of the Nuclear Quadrupole Frequencies of 23Na and 187Re in NaRe04

The quadrupole coupling constants in solids usually depend on temperature (see f.e. Ref. [1 — 8 ]). This can be explained by the change of vibration amplitudes of molecules, but the influence of the volume expansion on the electric field gradient has also to be taken into account [4], For 185Re and 187Re in some peirhenates NQR measurements have been reported [9 — 16]. With the exception of NH 4Re0 4 the NQR signals of 185Re and 187Re in perrhenates yield decreasing quadrupole coupling constants, if the temperature of the solid sample is increased. This is the normal behaviour [9, 11 — 14, 16], whereas for NH 4ReC>4 in the temperature range from 100 K to 300 K an inverse effect has been found [1 1 , 1 2 , 16]. To clear this peculiar^ of the perrhenates a series of different measurements have been reported: The pressure coefficients of the quadrupole reso­ nance frequencies in perrhenates at various tem­ peratures [15, 16] and also lattice parameters [16, 17] have been determined. Recently some NMR measurements of proton relaxation rates have been published for NH 4Re0 4 [18—20]. It seems, that the anomalous temperature dependence of the 187Re NQR signal in NÜ 4Re0 4 can be explained by reorientation of the ammonium ion and by re­ arrangement of the lattice [17], For NaReC>4 and KReC>4 a detailed study of the temperature dependence of the NQR signal of 187Re has also been reported [13]. Neglecting effects of volume expansion Johnson and Rogers [13] calculated the quadrupole frequency of 18 "Re as a function of temperature. They realized that the change in the field gradient with expansion of the lattice is important and cannot be neglected.