Pulsed ESR of conducting alkali-DCNQI salts: a general analysis of the electronic spin relaxation rates

Abstract

We have determined the temperature and angular dependence of the electronic spin relaxation rates 1/ T 1e and 1/ T 2e in the radical anion salts of M(2,5-DMe-DCNQI) 2 with metallic counterions (MLi, Na, K and Rb), originating from the dipolar interaction between the spins of the conduction electrons (FS) and from contributions by spin orbit coupling (SOC). Taking into account the known crystal structures, the FS contribution could be determined quantitatively by (i) calculating the spin density distribution on the DCNQI molecule, (ii) summing over all neighboring molecules within two lattice constants, and (iii) introducing a 1D spectral density function for the electronic mobility. Using semiclassical relaxation theory for SOC interaction we introduced a new model for 1D systems based on the following considerations. Each ‘out-of-chain scattering event’ of the conduction electrons leads to an angular-dependent relaxation contribution. This depends on the relative mutual arrangement of the DCNQI molecules, i.e., the crystal structure, and possesses a spectral density resulting from a pulse-like interaction. The quantitative analysis of the experimental data gave spin diffusion constants D for the four salts of 0.05–0.4 cm 2 s −1, anisotropies σ ‖/ σ τ of (3.8–6.5)×10 3, and, by comparison of the conductivity data, additionally allowed the separation between the activation energies of the charge carrier concentration (about 17 meV) and that of the ‘effective mobility’.