Anisotropy of the Debye-Waller factor in cesium-graphite intercalation compounds by Mössbauer spectroscopy, and the quadrupole moment of the 81-keV state inCs133

Abstract

Mössbauer-effect measurements are reported in the lamellar compounds ${\mathrm{C}}_8$Cs and ${\mathrm{C}}_{24}$Cs. The experiments utilized the 81-keV transition from the ${5/2}^{+}$ first excited to the ${7/2}^{+}$ ground state in ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$. The ${\mathrm{C}}_8$Cs samples were made from ordered pyrolytic graphite and exhibited a highly anisotropic recoilless fraction which at 4.2 K was approximately 20 times greater along the $c$ axis than perpendicular to it. The magnitude and sign of the quadrupole coupling of the ${5/2}^{+}$ excited state was measured [+4.29(5) × ${10}^{-7\mathrm{}}$ eV] and combined with an NMR value for the ground-state coupling to give $\left|\frac{Q_{\mathrm{ex}}}{Q_{\mathrm{gr}}}\right|=88\mathrm{}\pm 6$. The sign of $\mathrm{eq}$ was determined to be negative on the basis of simple models of the cesiated-graphite lattice, resulting in the value $Q_{\mathrm{ex}}=-0.22\mathrm{}\left(5\right)\mathrm{}$ b. The negative sign and the magnitude are in accord with the trend of low-lying ${5/2}^{+}$ states in this region of neutron number. Isomer-shift values indicate that the Cs is fully ionized in ${\mathrm{C}}_{24}$Cs and approximately 50% ionized in ${\mathrm{C}}_8$Cs.