Crystalline electric field of the bromo-elpasolites ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_6$ ($R$=rare earth) determined by inelastic neutron scattering

The rare-earth based bromo-elpasolites ${\mathrm{Cs}}_2\mathrm{Na}R{\mathrm{Br}}_6$ ($R$=Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, Yb) were synthesized and subsequently studied by inelastic neutron scattering. Energy splittings within the electronic ground-state $J$ multiplet due to the crystalline electric field (CEF) interaction at the $R^{3+}$ site were directly observed and rationalized in terms of CEF parameters of cubic symmetry. For some CEF transitions we have been able to resolve the splitting due to the tetragonal distortion. The resulting CEF parameters are discussed and used to predict some properties of the long-range magnetic ordering at low temperatures.