Solid State and Nuclear Results from Mössbauer Studies withI129

The Mössbauer effect of the 26.8-keV transition in ${\mathrm{I}}^{129}$ has been studied in a number of iodine compounds. The following results have been obtained: (a) The isomeric shifts for the alkali iodides were found to be linearly dependent on the number of $5p$ holes in the iodine ion. From this dependency, a calibration of the isomeric shift scale in terms of the $5s$-electron density was obtained. The $s$ and $p$ populations inferred from these data are in sharp disagreement with those predicted by the simple theory of electronegativity. An explanation for this discrepancy is given. (b) From the calibration mentioned under (a), a relative change of nuclear radius $\frac{\Delta R}{R}=\frac{(R_{26.8}-R_{\mathrm{gnd}})}{R_{\mathrm{gnd}}}=3\mathrm{}\times {10}^{-5\mathrm{}}$ is computed. (c) The isomeric shifts for some iodates indicate that their $5s$-electron density is about 18% larger than that of the iodides. This increase is caused by the partial removal of $5p$ electrons from iodine in the I-O bonds, which results in reduced screening of the $5s$ electrons. The shift for potassium periodate implies that the $5s$-electron density is decreased by 33% compared to the iodate. This decrease is attributed mainly to the $\mathrm{sp}$ hybridization of the I-O bonds in KI${\mathrm{O}}_4$. The average charge removed per I-O bond, calculated from the isomeric shifts of the iodates and periodate, is $0.61 e$. (d) The quadrupole splitting of KI${\mathrm{O}}_3$ was used to determine the ${\mathrm{I}}^{129}$ quadrupole ratio, $\frac{Q_{26.8}}{Q_{\mathrm{gnd}}}=1.23\mathrm{}\pm 0.02$. The asymmetry of the iodate spectrum and the sign of the quadrupole moment show that the sign of the iodate field gradient is negative. The values of the quadrupole coupling constant, $\mathrm{eqQ}$, at 80°K for KI${\mathrm{O}}_3$ and N${\mathrm{H}}_4$ I${\mathrm{O}}_3$ are in agreement with nuclear-magnetic-resonance results and $\mathrm{eq}{Q}_{127}=1030\mathrm{}\pm 20$ Mc/sec for Ba${\left(\mathrm{I}{\mathrm{O}}_{\mathrm{a}}\right)}_2$. (e) The recoilless fraction $f$ was found to deviate from 0.26 by less than 25% for all alkali iodides at 80°K.