High-Resolution Study of Isomer Shifts of the 6.2-keVγRays of Tantalum-181

Isomer shifts of the 6.2-keV $\gamma$ rays of ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ are reported for dilute impurities of tantalum in transition-metal hosts and for several tantalum compounds. The experimental line-shift-to-linewidth ratios represent an improvement of the resolution by more than an order of magnitude relative to other Mössbauer resonances. For ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ impurities in transition-metal hosts a systematic variation of the electron density at the nucleus is found. From a comparison of the isomer shifts of ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$ impurities in transition-metal host with those observed for impurities of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{99}{}_{}{}^{}\mathrm{Ru}$, ${}_{}{}^{193}{}_{}{}^{}\mathrm{Ir}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$, a value of $\Delta 〈r^2〉=-5\mathrm{}\times {10}^{-2\mathrm{}}$ ${\mathrm{fm}}^2$ can be derived for the change of the mean-squared nuclear-charge radius. The negative sign of $\Delta 〈r^2〉$ is in agreement with the observed variation of the isomer shifts of LiTa${\mathrm{O}}_3$, NaTa${\mathrm{O}}_3$, and KTa${\mathrm{O}}_3$, as well as with the isomer shift found for TaC.