Temperature dependence of the 59Co quadrupole coupling constant in K3Co(CN)6

Abstract

Expressions are presented for calculating the effects of bond-stretching and rotational modes of vibration on the temperature dependence of quadrupole coupling constants in crystals. Contributions to the motional averaging of the electric field gradient tensor arising from point charges, point dipoles, and overlap of closed wave functions are evaluated and the resulting expressions applied to analyse the temperature variation of the 59Co quadrupole coupling constant in K3Co(CN)6. Both stretching and rotational internal modes of vibration of the Co(CN)6 octahedron had to be taken into account to analyse the data. In particular, an intermediate frequency mode of wavenumber 414 cm−1 has been found to make a substantial and positive contribution to the shift in coupling constant arising from the low (~ 100 cm−1) and intermediate frequency (~ 400 cm−1) modes. The remainder of the shift (~ 90%) has been attributed to a rotary mode of wavenumber 26 cm−1. The extracted temperature dependence of this mode points to the possible existence of a structural phase transition in K3Co(CN)6 below 100 K.