Pressure-induced amorphization and collapse of magnetic order in the type-I clathrate Eu8Ga16Ge30
Pressure-induced amorphization and collapse of magnetic order in the type-I clathrate Eu8Ga16Ge30
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I 型包合物 Eu8Ga16Ge30 中压力诱导的非晶化和磁序塌陷
DOI：
10.1103/physrevb.88.144105
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发表时间：
2013
影响因子：
3.7
通讯作者：
C. Giles
中科院分区：
文献类型：
--
作者：
J. Mardegan;G. Fabbris;L. Veiga;C. Adriano;M. Avila;D. Haskel;C. Giles
我们利用 X 射线粉末衍射 (XRD)、X 射线吸收近边结构 (XANES) 和 X 射线磁圆二色性 (XMCD) 技术研究了 I 型包合物 Eu${}_{8}$Ga${}_{16}$Ge${}_{30}$ 在压力下的低温结构和电子特性。 XRD 测量显示在 18 GPa 以上时转变为非晶相。与之前关于其他笼形化合物的报道不同，当晶胞体积减小至其环境压力值的 81$%$ 时，在晶体-非晶相变之前没有观察到体积塌陷。将压力相关的相对体积与 Murnaghan 状态方程拟合，得到体积模量 ${B}_{0}=65\ifmmode\pm\else\textpm\fi{}3$ GPa 和压力导数 ${B}_{0}^{\ensuremath{'}}=3.3\ifmmode\pm\else\textpm\fi{}0.5$。 Eu ${L}_{2}$-边缘 XMCD 数据显示在与晶体-非晶相变一致的压力下磁序的淬灭。该信息以及 XANES 光谱中观察到的 Eu${}^{2+}$ 价态的持续性直至最高压力点 (22 GPa) 表明，XMCD 强度的抑制是由于长程磁序的损失。与其他包合物相比，结果表明客体离子笼相互作用在确定框架结构的机械稳定性和非晶化的临界压力方面的重要性。最后，发现压力释放后晶体结构不会恢复，从而产生在环境压力和温度下至少是亚稳态的非晶材料。
We investigate the low temperature structural and electronic properties of the type-I clathrate Eu${}_{8}$Ga${}_{16}$Ge${}_{30}$ under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81$%$ of its ambient pressure value. Fits of the pressure-dependent relative volume to a Murnaghan equation of state yield a bulk modulus ${B}_{0}=65\ifmmode\pm\else\textpm\fi{}3$ GPa and a pressure derivative ${B}_{0}^{\ensuremath{'}}=3.3\ifmmode\pm\else\textpm\fi{}0.5$. The Eu ${L}_{2}$-edge XMCD data shows quenching of the magnetic order at a pressure coincident with the crystalline-amorphous phase transition. This information along with the persistence of an Eu${}^{2+}$ valence state observed in the XANES spectra up to the highest pressure point (22 GPa) indicates that the suppression of XMCD intensity is due to the loss of long range magnetic order. When compared with other clathrates, the results point to the importance of guest ion-cage interactions in determining the mechanical stability of the framework structure and the critical pressure for amorphization. Finally, the crystalline structure is not found to recover after pressure release, resulting in an amorphous material that is at least metastable at ambient pressure and temperature.