Low-spin high-spin equilibria in $1T-{\mathrm{Fe}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2 \left(x\le \frac{1}{3}\right)$ and the temperature dependence of the associated energy gap

Measurements of magnetic susceptibility and Mössbauer isomer shift are reported for $1T-{\mathrm{Fe}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2 (x<~\frac{1}{3})$ and indicate the existence of a gradual transition from low-spin to high-spin ${\mathrm{Fe}}^{2+}$ as a function of increasing temperature. A quantitative analysis of the data reveals that the energy gap between the ground ${}_{}{}^1{}_{}{}^{}A_1^{}{}_{}{}^{}$ state and the lowest of the spin orbitally split ${}_{}{}^5{}_{}{}^{}T_2^{}{}_{}{}^{}$ levels is temperature dependent. The detailed behavior, which is dependent on concentration $x$, can be quantitatively understood in terms of a modulation of the cubic crystal field and Hund's-rule exchange energies by the motion of the local ligand environment and includes important contributions both from a static striction mechanism and from the thermal excitation of the relevant local mode.