Pressure-dependent susceptibility of singlet-ground-state paramagnets: Pr and Tm monopnictides and Pr monochalcogenides

Measurements of the pressure dependence (up to ∼ 6 kbar) of the magnetic susceptibility $\chi$ at 4.2 K are presented for three Pr monopnictides (PrAs, PrSb, and PrBi), two Tm monopnictides (TmAs and TmSb), and three monochalcogenides (PrS, PrSe, and PrTe). The results for ($\frac{1}{\chi }$) ($\frac{d\chi }{\mathrm{dP}}$) in units of ${10}^{-3\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$ are: PrAs +10; PrSb, +7; PrBi, +9; TmAs, +6; TmSb, +3; PrSe, -2; and PrTe, -1; for PrS, $\left(\frac{1}{\chi }\right) \left(\frac{d\chi }{\mathrm{dP}}\right)$ is nonmonotonic, but negative. Independent inelastic-neutron-scattering data, which show that for all the materials studied the crystalline electric field (CEF) of the surrounding ions produces a nonmagnetic (singlet) ground state with a magnetic (triplet) first excited state, are employed for analysis of the present experiments. None of the compounds studied orders magnetically; the susceptibility is Van Vleck like, and is nearly temperature independent at 4.2 K. For both sets of monopnictides, the susceptibility increases under pressure, whereas the point-charge model, which has been used previously to explain the CEF splittings, always predicts a decrease. For the monochalcogenides, the susceptibility decreases slightly, consistent with the predictions of the point-charge model. The results are discussed in terms of the predictions of the point-charge model, the pressure dependence of the enhancement, and other possible mechanisms.