Pressure dependence of magnetic excitations in SmS

Abstract

Inelastic-neutron-scattering studies using triple-axis and time-of-flight techniques have been made in the low- and high-pressure phases of a polycrystalline sample of ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$S. There is no observable change in the energy or width of the ${}_{}{}^7{}_{}{}^{}F_0^{}{}_{}{}^{}-{}_{}{}^7{}_{}{}^{}F_1^{}{}_{}{}^{}$ excitation of the $4f^6$ configuration between ambient pressure and 0.6 GPa. In the mixed-valence phase at high pressure, there is no evidence for well-localized excitations from either the ${}_{}{}^7{}_{}{}^{}F_0^{}{}_{}{}^{}-{}_{}{}^7{}_{}{}^{}F_1^{}{}_{}{}^{}$ ($4f^6$) or the ${\Gamma }_7-{\Gamma }_8({}_{}{}^6{}_{}{}^{}H_{\frac{5}{2}}^{}{}_{}{}^{}, 4f^5)$ configurations. These results suggest that the mixed-valence phase is homogeneous. Lattice-parameter measurements versus pressure at 85 and 298 K show no evidence for pretransitional softening and establish that the phase diagram in the $P-T$ plane is reentrant. From the phase diagram and earlier heat-capacity measurements, the electronic entropy in the mixed-valence phase is estimated to saturate at $\simeq 2R$ with increasing temperature.