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.

中文翻译：

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具有金属-绝缘体转变的YbB12的晶体结构和原子振动的精细细节

基于X射线衍射数据，在85-293 K范围内的9个温度下研究了单晶近藤绝缘子YbB ₁₂的晶体结构。在所有温度下都可以检测到非常弱的立方晶格的Jahn-Teller变形，但不需要修改结构模型。在1.9–300 K的温度范围内研究了YbB ₁₂单晶的热容和电导率。发现在120–160 K的温度范围内，结构参数和所示的物理性能均具有某些特定特征。当冷却到160 K以下时，YbB _12的收缩收缩，并在120 K附近膨胀。等效原子位移参数U _eq（T）在140 K附近不再是单调的，应该用高于和低于该温度的两条爱因斯坦曲线（Yb）和两条Debye曲线（对于硼原子）进行建模。从比热的温度行为可以看出，Yb离子在双阱电势中的耦合振荡会导致在状态密度下出现电荷隙，并且在150 K以下时晶体的导电性能会逐渐下降。绝缘子相变伴随着U _eq（T）曲线的扭结和晶胞值的变化。