Department of Materials Science and Technology, Tokyo University of Science, Niijuku, Tokyo 125-8585, Japan.
J Phys Condens Matter. 2013 Jun 12;25(23):235403. doi: 10.1088/0953-8984/25/23/235403. Epub 2013 May 17.
The low-temperature (LT) superstructure and the phase transition temperature have been investigated for a series of Cd6M crystalline approximants by transmission electron microscopy as well as electrical resistivity measurements. Except for M = Lu, Cd6M is found to undergo a phase transition to a monoclinic phase at a low temperature and the transition temperature (Tc) scales well with the size of the M atom. For M = Ca, Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm the LT superstructure is explained by a √2a × a × √2a lattice with the space group C2/c, and for M = Sr and Yb a √2a × 2a × √2a monoclinic lattice with P2/m. On the other hand, no phase transition is observed for M = Lu, indicating that a Cd4 tetrahedron at the cluster center remains disordered down to the lowest temperature, i.e. 16 K. It is shown that the volume inside the Cd20 dodecahedron plays a crucial role in the occurrence of the phase transition, and long-term aging in particular promotes the phase transition for late rare-earth elements such as Ho, Er and Tm, suggesting that the transition is sensitive to and is even hindered by disorder such as atomic vacancies. The absence of the transition for M = Lu is attributed to the highest activation energy for the transition due to the smallest volume inside the Cd20 dodecahedron.
通过透射电子显微镜和电阻率测量研究了一系列 Cd6M 晶态近似物的低温（LT）超结构和相变温度。除了 M = Lu 之外，Cd6M 被发现会在低温下发生向单斜相的相变，并且相变温度（Tc）与 M 原子的大小很好地相关。对于 M = Ca、Y、Pr、Nd、Sm、Gd、Tb、Dy、Ho、Er 和 Tm，LT 超结构可以用空间群 C2/c 的 √2a×a×√2a 晶格来解释，而对于 M = Sr 和 Yb，则可以用 P2/m 的 √2a×2a×√2a 单斜晶格来解释。另一方面，对于 M = Lu，没有观察到相变，这表明在最低温度 16 K 时，簇中心的 Cd4 四面体仍然是无序的。结果表明，Cd20 十二面体内部的体积在相变的发生中起着至关重要的作用，特别是长期老化会促进 Ho、Er 和 Tm 等后期稀土元素的相变，这表明相变对无序（如原子空位）敏感，甚至受到其阻碍。M = Lu 没有发生相变归因于 Cd20 十二面体内部体积最小导致的相变的最高激活能。
