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Temperature-induced valence transition and associated lattice collapse in samarium fulleride

机译：富勒sa中的温度诱导价转变和相关的晶格塌陷

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摘要

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-T-c) copper oxide superconductors. Metal fulleride salts(1)-metal intercalation compounds of C-60- and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system-they contain highly correlated cation (rare-earth) and anion (C-60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm-2.75 C-60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + epsilon) + to nearly 2+. The negative thermal expansion-heating from 4.2 to 32 K leads to contraction rather than expansion-occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour(2). We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm2+ towards the smaller Sm(2+epsilon)+, analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3). [References: 15]
机译：给定系统的不同自由度通常彼此独立，但在某些材料中可以牢固耦合，从而引起相位平衡，对外部扰动敏感。这样的系统通常表现出异常的物理特性，这在理论上是很难处理的，例如强价相关的电子系统，例如中价稀土重费米子和近藤绝缘子，巨大的磁阻锰矿和高转变温度（高Tc）氧化铜就是例证。超导体。金属富勒酸盐（1）-C-60的金属插层化合物以及基于稀土金属的材料也表现出很强的电子相关性。因此，稀土富勒分子构成了一个特别吸引人的系统-它们包含高度相关的阳离子（稀土）和阴离子（C-60）亚晶格。在这里，我们使用高分辨率的同步加速器X射线衍射和磁化率测量结果表明，冷却稀土富勒分子Sm-2.75 C-60会诱导32 K附近的等对称相变，并伴随着各向同性的体积显着增加和sa价从（2 +ε）+过渡到近2+。从4.2 K到32 K的负热膨胀-加热导致收缩而不是膨胀，其发生率比通常表现出这种行为的三元金属氧化物大40倍（2）。我们将大的负热膨胀（在富勒烯或其他分子系统中前所未有）归因于从Sm2 +到较小的Sm（2 + epsilon）+的准连续化合价转变，类似于中价近藤绝缘子中遇到的化合价或构型转变就像短信（参考3）。 [参考：15]

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