Electronic Configuration of SmB6

Abstract

The unusual magnetic and electrical properties of the semiconductor SmB₆ have been closely investigated recently. Primarily from magnetic susceptibility measurements, it has been hypothesized that the Sm valence changes from 3 + (4f⁵, ⁶H_5/2) at high temperatures to 2 + (4f⁶, ⁷F₀) at low temperatures. We have used the Mössbauer effect in ¹⁴⁹Sm to make a direct determination of the valence state by means of the ``isomer shift,'' which measures the s‐electron density at the Sm nucleus. This density changes with the 4f configuration because of coulombic shielding effects. Unfortunately, the change between 4f⁵ and 4f⁶ is too small to resolve resonance lines from the two charge states. The observed isomer shift of −0.4 mm/sec lies approximately halfway between the values (∼−0.9 and ∼0. mm/sec⁻¹ respectively) obtained for well characterized ionic di‐ and trivalent Sm compounds, and is not observed to vary between room temperature and 1.1°K. A new model is presented to explain both the magnetic susceptibility data and the Mössbauer results.