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

在β-焦绿石型氧化物KOs_2O_6中，在超导转变温度以下观察到λ形的热容反常。这种异常起源于结构相变，但其起源尚不清楚。为了研究KOs_2O_6相变点以下和以上的晶体结构，我们对高质量的小晶体进行了高分辨中子粉末衍射测量。发现相变是一个同构的相变，没有对称性破缺。此外，在低温阶段，笼体积扩大，K离子的密度分布变得更宽，表明在过渡的拍击运动的基态变化。

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.