Crystalline-electric-field level scheme of NdB6

We have determined the crystalline-electric-field (CEF) level scheme of NdB6 by means of Raman spectroscopy. We could resolve two CEF-transitions at 92 cm−1 and 98 cm−1 due to the spectral resolution of 2 cm−1 (=0.25 meV). The temperature dependent Raman scattering reveals the two transitions to be on top of one another, in agreement with fits of the temperature dependence of the magnetic susceptibility. The spectroscopically determined CEF level scheme yields a good fit of the elastic constants and hence the mean field prediction of a ferroquadrupolar ordering near 6 K. This result sheds new light on the phase transition in NdB6 nearT N =8.6 K which has so far been interpreted only in terms of antiferromagnetic ordering.