Breakdown of the "white-noise approximation" in the Mössbauer relaxation spectra: The case ofCs2NaYbCl6

There are many restrictions on using the white-noise approximation to describe the line shape of radiation emitted by a Mössbauer atom experiencing paramagnetic relaxation. Using a resolvent operator method we have obtained an expression for the line shape without making such an approximation. As a result the relaxation matrix depends on the hyperfine frequencies as well as on correlation functions of the bath variables. The situation has been illustrated for the case of spin-spin relaxation of a "rigid lattice" using an exponential form for the correlation function. The calculations have been utilized to analyze the Mössbauer relaxation spectra of ${\mathrm{Cs}}_2$NaYb${\mathrm{Cl}}_6$ measured using the 84.6-keV transition in ${}_{}{}^{170}{}_{}{}^{}\mathrm{Yb}$. The analysis permits us to deduce the dipolar field and the correlation time for the spin bath in this material.