Mössbauer Study of Magnetic States of KFeF3and Implications for RbFeF3

Mössbauer absorption spectra of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in an unstrained {100} single-crystal platelet of KFe${\mathrm{F}}_3$ have been measured between 297 and 4.2 °K. The transition to the trigonal antiferromagnetic state occurred at $T_N=112.5\mathrm{}\pm 0.5$ °K. The relaxation effects reported near $T_N$ in a previous study were absent, but spectra of a strained crystal provided evidence of strong magnetoelastic interactions. A transition to a lower-symmetry weak ferromagnetic phase was found at $T_R=36.6\mathrm{}\pm 0.5$ °K. At 4.2 °K the magnetic hyperfine field is $H_{\mathrm{hf}}=182.4\mathrm{}\pm 0.5$ kOe, and the magnitude of the quadrupole splitting is $\left|\Delta E_Q\right|=1.97\mathrm{}\pm 0.05$ mm/sec. Analysis of the data in the antiferromagnetic phase indicates the spin alignment is parallel to the 〈111〉 distortion. The temperature dependence $H_{\mathrm{hf}}$ and $\Delta E_Q$ in this phase is compared with predictions of self-consistent molecular-field-theory calculations. Only fair agreement is found with the magnetically induced electric-field-gradient (EFG) model, which describes the ${\mathrm{Fe}}^{2+}$ ions by a Hamiltonian incorporating free-ion terms, a cubic crystal field, and an exchange interaction. A substantially better fit is obtained with a second model which adds to this Hamiltonian an axial distortion term $\left(\frac{\delta }{3}\right)(L_z^2-2)$, where $\delta$ is zero above $T_N$ and varies below $T_N$ as ${(1-\frac{T}{T_N})}^{\gamma }$, with $\gamma =\frac{1}{2} or \frac{1}{3}$. Application of this second model to the tetragonal antiferromagnetic state of RbFe${\mathrm{F}}_3$ indicates that the spins align parallel to a 〈100〉 direction rather than the 〈111〉 direction predicted previously by use of the magnetically induced EFG model. The sign obtained for the axil-distortion parameter $\delta$ in RbFe${\mathrm{F}}_3$ is inconsistent with an analysis which attributes the cubic-to-axial transformation to Jahn-Teller stabilization effects.