(Sm_1/3Ca_2/3)_2.75C₆₀ is a member of the family of non-stoichiometric strongly-correlated rare-earth fullerides, (Sm_1−xCa_x)_2.75C₆₀ (0 ≤ x ≤ 1), in which an orthorhombic 2 × 2 × 2 supercell of the face-centred cubic (fcc) unit cell of stoichiometric A₃C₆₀ (A = alkali metal) fullerides is stabilized by the long-range ordering of partially-occupied metal sites. At ambient temperature and pressure, it is a mixed valence compound with an average Sm valence of +2.33(2) implying a formal charge of −5.78 for the C₆₀ anions. Here we study its electronic response to the application of pressure in the range 0–9 GPa. Synchrotron X-ray absorption measurements in the high-resolution partial fluorescence yield mode (PFY-XAS) at ambient temperature show the onset of an abrupt strongly hysteretic (width ∼2.5 GPa) first-order reversible phase transition at ∼4 GPa, accompanied by a drastic increase in the bulk Sm valence by ∼20% to +2.71(3). This is coincident with the huge lattice contraction and concomitant insulator-to-metal transition encountered before for Sm_2.75C₆₀ in the same pressure range and provides a possible explanation of the physical properties in terms of the strong coupling between the lattice and electronic degrees of freedom. The behaviour is reminiscent of the electronic and lattice response to pressure of highly correlated Kondo insulators like SmS and its ternary derivatives, Sm_1−xR_xS (R = Ca, Y, etc.). However, a distinguishing feature of the fulleride systems is that the C₆₀ anionic sublattice can act as an electron reservoir due to the availability of a close-lying band derived by the t_1g orbitals and can accept excess charge as the 4f-electron occupation number decreases. The observed electronic response as a function of pressure is thus opening new possibilities for accessing metallic fullerides at elevated pressures.

中文翻译：

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混合价（Sm1 / 3Ca2 / 3）2.75C60富勒德中的压力诱导价转变

（SM _1/3的Ca _2/3）_2.75 Ç ₆₀是家庭非化学计量的强相关性稀土类富勒烯中的一个成员，（SM _{1- X}的Ca _X）_2.75 Ç ₆₀（0≤ X ≤1），其中，化学计量A ₃ C ₆₀（A =碱金属）富勒烯的面心立方（fcc）晶胞的正交晶体2×2×2超级电池通过部分占据的金属位点的远距离排序而得以稳定。在环境温度和压力下，它是一种混合化合价化合物，平均Sm化合价为+2.33（2），这意味着C ₆₀的形式电荷为-5.78阴离子。在这里，我们研究了它对0-9 GPa压力施加的电子响应。在环境温度下以高分辨率部分荧光产率模式（PFY-XAS）进行的同步加速器X射线吸收测量显示，在〜4 GPa处出现了突然的强磁滞（宽度约2.5 GPa）一阶可逆相变，并伴随着整体Sm价急剧增加约20％至+2.71（3）。这与Sm _2.75 C ₆₀之前遇到的巨大晶格收缩和伴随的绝缘体到金属的转变相吻合。在相同的压力范围内，并根据晶格和电子自由度之间的强耦合为物理性质提供了可能的解释。该行为让人联想到高度相关的近藤绝缘子（例如SmS及其三元导数Sm _{1- x} R _x S（R = Ca，Y等））对压力的电子和晶格响应。但是，富勒分子系统的一个显着特征是，由于存在由t _1g推导的紧密谱带，C ₆₀阴离子亚晶格可以充当电子储库。轨道，并随着4f电子占据数的减少而接受过量电荷。因此，观察到的作为压力的函数的电子响应为在高压下进入金属富勒化物开辟了新的可能性。