Fine details of crystal structure and atomic vibrations in YbB₁₂ with a metal-insulator transition

The crystal structure of single-crystal Kondo insulator YbB₁₂ was studied at nine temperatures in the range 85-293 K based on X-ray diffraction data. Very weak Jahn-Teller distortions of the cubic lattice were detected at all temperatures, but did not require a revision of the structural model. Heat capacity and electrical conductivity of YbB₁₂ single crystals were studied in the temperature range 1.9-300 K. It is found that both the structural parameters and the indicated physical properties have some specific features in the temperature range 120-160 K. The unit cell of YbB₁₂ contracts when cooled below 160 K and expands at around 120 K. The temperature dependences of the equivalent atomic displacement parameters U_eq(T) are no longer monotonic around 140 K and should be modeled by two Einstein curves for Yb and two Debye curves for boron atoms above and below this temperature. As follows from the temperature behavior of the specific heat, coupled oscillations of Yb ions in a double-well potential lead to the appearance of a charge gap in the density of states and gradual deterioration in conductive properties of the crystal below 150 K. This metal-insulator phase transition is accompanied by a kink in the U_eq(T) curves and changes in the unit-cell values.