Vibrational properties and magnetic specific heat of the covalent chain antiferromagnet RbFeSe2

Abstract

The magnetic specific heat of $\mathrm{RbFeS}{\mathrm{e}}_{2}$ and the spin state of $\mathrm{F}{\mathrm{e}}^{3+}$ ions in the compound have been studied. Phonon dispersion and phonon density of states (PDOS), element specific and total, were evaluated from first-principles calculations. It is shown that iron atoms in quasi-one-dimensional chains have dramatically different vibrational properties against Rb and Se atoms: the Fe PDOS is mostly concentrated within two Einstein-like optical phonon peaks at high frequencies. Analysis of our M\ossbauer data for $\mathrm{RbFeS}{\mathrm{e}}_{2}$, utilizing the calculated Fe PDOS as well as our optical absorption measurements, have shown full agreement with the location of the high-frequency optical-type lattice vibrations within the $\mathrm{FeS}{\mathrm{e}}_{4}$ tetrahedra. The calculated PDOS was utilized to evaluate the lattice contribution to the specific heat. The phonon heat capacity has been used to evaluate the magnetic specific heat of the quasi-one-dimensional antiferromagnetically correlated $\mathrm{F}{\mathrm{e}}^{3+}$ ion chains in $\mathrm{RbFeS}{\mathrm{e}}_{2}$. An intermediate spin state $S=3/2$ has been found most closely relevant to our magnetic entropy analysis for $\mathrm{F}{\mathrm{e}}^{3+}$ ions in $\mathrm{RbFeS}{\mathrm{e}}_{2}$.