 Samarium hexaboride is a typical intermediate @-@ valence compound where samarium is present both as Sm2 + and Sm3 + ions at the ratio 3 : 7 . It belongs to a class of Kondo insulators , that is at high temperatures ( above 50 K ) , its properties are typical of a Kondo metal , with metallic electrical conductivity characterized by strong electron scattering , whereas at low temperatures , it behaves as a non @-@ magnetic insulator with a narrow band gap of about 4 – 14 meV . The cooling @-@ induced metal @-@ insulator transition in SmB6 is accompanied by a sharp increase in the thermal conductivity , peaking at about 15 K. The reason for this increase is that electrons themselves do not contribute to the thermal conductivity at low temperatures , which is dominated by phonons , but the decrease in electron concentration reduced the rate of electron @-@ phonon scattering . 
