On the High-Temperature Phase Transition of Gd5Si2Ge2

The first-order monoclinic-to-orthorhombic (β→γ) phase transition of the giant magnetocaloric
material Gd5Si2Ge2 was studied using in situ high-temperature single-crystal X-ray diffraction. A special
crystal mounting procedure was developed to avoid crystal contamination by oxygen or nitrogen at high
temperatures. The elastic β→γ transformation occurs at 300−320 °C during heating, and it is reversible
during fast and slow heating and slow cooling but irreversible during rapid cooling. Contrary to theoretical
predictions, the macroscopic distribution of the Si and Ge atoms remains the same in both the orthorhombic
γ-polymorph and the monoclinic β-phase. It appears that interstitial impurities may affect stability of both
the monoclinic and orthorhombic phases. In the presence of small amounts of air, the β→γ transformation
is complete only at 600 °C. The interslab voids, which can accommodate impurity atoms, have been located
in the structure, and an effect of partially filling these voids with oxygen or nitrogen atoms on the β−γ
transition is discussed.

利用原位高温单晶X射线衍射技术，对巨型磁热材料Gd5Si2Ge2的一阶单斜晶系到正交晶系（β→γ）相变进行了研究。为避免高温下晶体被氧气或氮气污染，开发了一种特殊的晶体安装程序。在加热过程中，弹性的β→γ转换发生在300−320°C之间，并且在快速加热、慢速加热以及慢速冷却过程中具有可逆性，但在快速冷却过程中则不可逆。与理论预测相反，Si和Ge原子的宏观分布在这两种晶系（正交晶系的γ多形性和单斜晶系的β相）中均保持不变。似乎间隙杂质可能影响单斜晶系和正交晶系的稳定性。在存在少量空气的情况下，β→γ转换仅在600°C时才完全完成。能够容纳杂质原子的层间空隙已被定位在结构中，并讨论了通过氧或氮原子部分填充这些空隙对β−γ转变的影响。