A neutron scattering study on dynamical structure in β-pyrochlore oxide

在氧化β-焦绿盐KOs_2O_6中，在其超导转变温度以下，热容出现λ形异常。该异常起源于结构相变，但其成因尚不清楚。我们对高质量的小晶体KOs_2O_6进行了高分辨率的中子粉末衍射测量，以研究跃迁前后的晶体结构。发现相变是一个不对称破缺的同构相变。此外，在低温阶段，笼体体积扩大，K离子密度分布变宽，表明在过渡阶段嘎嘎运动的基态发生了变化。

Inβ-pyrochlore oxide KOs_2O_6, a λ-shaped anomaly in heat capacity is observed below its superconducting transition temperature. This anomaly originates in a structure phase transition, but its origin is unknown. We ha ve carried out high -resolution neutron powder diffraction measurements on high quality and small crystals of KOs_2O_6 in order to investigate the crystal structure below and above the transition. It is found that the phase transition is an isomorphic one without symmetry breaking. Besides, in the low-temperature phase, the cage volume expands and the density distribution of K ions becomes broader, indicating that the ground state of rattling motion changes at the transition.