Spin-lattice relaxation in some iridium salts I. Relaxation of the isolated (IrCl6)2- complex

Spin-lattice relaxation times have been measured at frequencies from 9·5 to 24 GHz for isolated (IrCl 6 ) 2- complexes in (NH 4 ) 2 PtCl 6 and K 2 PtCl 6 . The relaxation rate for the ammonium salt can be expressed as a sum of terms corresponding to a direct process, a Raman process and a concentration-dependent process as follows: for the ratio Ir:Pt = c less than about three per cent ( f in Hz, T in °K). The value of Δ(similar 6·5 °K) agrees with the previously measured exchange interaction between nearest-neighbour (IrCl 6 ) 2- complexes. At higher values of c , a faster variation than c 2 is found. Data for the potassium salt are similar except for the strongly enhanced Raman process, 1/ T 1 = 3·5 × 10 -2 T 9 . Theoretical estimates for the direct and Raman states indicate that a simple Debye model is not sufficient to explain all experimental data. All data regarding the concentration-dependent process agree with a mechanism in which single complexes cross relax to exchange-coupled clusters of three or more complexes, and relaxation of such clusters is discussed briefly. In particular, exchange-coupled pairs relax much too slowly to affect relaxation of single complexes (see also II).