EPR study of low spin domains formation in the process of spin transition in Na[Fe(Th-Sa)2]

The high spin (HS) <-> low spin (LS) transition in a polycrystalline sodium salicylalthiosemicarbazonatoferrate(III) (Na[Fe(Th-Sa)2]) is studied by X-band EPR under atmospheric and hydrostatic pressure up to 600 MPa in the temperature range 80-330 K, when the effective value of the magnetic moment μeff changes gradually from 5.9 down to 5.7 B.M. Its value diminishes rapidly below 50 K reaching 3.4 B.M. at 1.8 K. Some Q-band EPR measurements were performed in the range 150-236 K. The EPR linewidth analysis has shown that LS complexes formed are not statistically distributed among HS ones but aggregate in domains. In the gradual process of the HS → LS transition, the EPR linewidth of the LS complexes reveals abrupt changes in two temperature ranges (240-236 and 195-191 K) related to rearrangement of complexes in domains. In the lower temperature range, the rearrangement is accompanied by a rapid increase in the LS complexes content. The application of external pressure stimulates the HS → LS transition but the character of the process depends on temperature. An analysis of the process of the domain formation and destruction confirms the existence of the short- and long-range correlations between the LS complexes