Configuration of Sm in SmB6

Abstract

SmB₆ has been found to show no magnetic ordering down to a temperature of 0.35°K, in spite of the fact that the other rare earth hexaborides showed ordering in the neighborhood of 10–20°K.^1,2 We interpret this to mean that the Sm ion is in its nonmagnetic divalent configuration at low temperatures. Direct evidence for the changing electronic configuration of Sm from trivalent to divalent with decreasing temperature has now been found by the observation that the size of the unit cell goes through a minimum near 150°K. The magnetic susceptibility, which has been measured between 1° and 800°K shows evidence for the presence of primarily Sm⁺³ above 300°K and Sm⁺² below 100°K. Below 40°K the resistivity of SmB₆ increases as T decreases with an activation energy of about 0.002 eV. Such semiconducting behavior is to be expected from divalent Sm in the hexaboride lattice.