Semi-ab initio calculations of superposition model and crystal field parameters for Co2+ ions using the exchange charge model

This paper attempts for the first time to establish a reliable linkage between the two well-known and independent models of crystal field (CF), namely the exchange charge (ECM) and superposition models (SM). Our approach aims to show that the SM parameters can be reliably extracted from the distance dependence of the CF invariants for Co2+ as derived from the ECM through some semi-ab initio calculations which involved a single fitting parameter and a set of newly constructed procedures. Complete sets of the numerical values of SM parameters B¯k and tk for Co2+ in its own host lattices of Li2Co3(SeO3)4, CoSO4·H2O, CoSeO4·H2O, and Co(OH)2 are obtained and they are found to be around 13,000–16,000 cm−1 for B¯2, 4100–5700 cm−1 for B¯4, 4.1–5.0 for t2 and 6.2–6.5 for t4. The present results generally agree with but should be much better than those incomplete sets of results found by previous researchers using the conventional fitting approach. Plausible explanations for some noticeable discrepancies are also discussed together with the effects of different CF contributions on values of the SM parameters.