Neutron-Scattering Study of TmSb: A Model Crystal-Field-Only Metallic Paramagnet

Inelastic-neutron-scattering studies have been carried out on the paramagnetic metallic compound TmSb. The ${\mathrm{Tm}}^{3+}$ ground-state multiplet ${}_{}{}^3{}_{}{}^{}H_6^{}{}_{}{}^{}$ is split by the octahedral crystal field into six levels ${\Gamma }_1$, ${\Gamma }_4$, ${\Gamma }_5^{\mathrm{}\left(2\right)\mathrm{}}$, ${\Gamma }_2$, ${\Gamma }_5^{\mathrm{}\left(1\right)\mathrm{}}$, ${\Gamma }_3$. Transitions between these levels have been observed and the parameters $A_4〈r^4〉=(6.86\mathrm{}\pm 0.10)$ meV, $A_6〈r^6〉=(0.44\mathrm{}\pm 0.04)$ meV have been deduced. The fourth-order term is consistent with that expected on the basis of a simple nearest-neighbor point-charge model, whereas the sixth-order term is an order of magnitude too large; this is in marked contrast with previous results in PrSb and other praseodymium pnictides and chalcogenides where the effective-point-charge model is quantitatively correct for both terms. It is found that the actual spectra can be reproduced in detail using ordinary crystal-field theory together with a simple approximation to the instrumental-resolution function. Furthermore, no appreciable exchange broadening of the transitions is observed at low temperatures, thus confirming TmSb as a model crystal-field-only metallic paramagnet.