Pressure-induced delocalization in charge-ordering rare-earth compounds studied bynuclear resonant scattering

用同步辐射X射线衍射法测定了混合价Eu4As3化合物的压力-温度相图。结构相变发生在343K的常压下，体积减小，相变温度随着压力的增加而线性升高，最高可达5 GPa.根据Clausius-Clapeyron关系，估算出该转变时的熵变为41J/Molk。该值比Eu^&lt；3&gt；和Eu^&lt；2&gt；离子在Eu_4As_3的热导出价涨落态中的随机排列所期望的构型熵大两倍。在301-406K范围内，对Eu_4As_3进行了核共振非弹性x射线散射(NRIXS)实验，利用求和规则从观测到的NRIXS谱中的非弹性组分推导出热力学参数。平均无反冲分数表明，Eu离子在B…边界附近的晶格振动有软化现象价起伏状态下更多的里渊区。根据平均力常数随温度的变化关系，我们提出了Eu_4As_3中4f态与导带或价带的杂化在电荷有序温度附近和价态的起伏中起作用。我们在MoM温度下对Eu_4As_3进行了高压X射线衍射实验，从衍射数据中精确地得到了各个原子的坐标和晶格参数。我们已经证实，具有R3c空间群的三角结构在19 Gpa以下不会改变。尽管体积随压力变化不大，但畸变角的压力依赖关系在10 GPa时发生了变化。结果表明，EuAs6八面体的体积与Eu离子的价态密切相关，在常压下的电荷有序状态下，纯三价和二价Eu离子分别占据两个不等价的Eu位。我们根据在两个不等价的Eu位置上这些体积的压力依赖关系提出了重要的证据，即在10 GPa时发生了静态电荷顺序的熔化，而Eu4As3在10 Gpa以上处于混合价状态。较少

The pressure versus temperature phase diagram of the mixed-valent Eu4As3 compound was determined by means of x-ray diffraction measurements using synchrotron radiation. The structural phase transition occurs at 343 K under ambient pressure with volume reduction and the transition temperature increases linearly with pressure up to about 5 GPa. The entropy change at this transition is estimated to be 41 J/molK from the Clausius-Clapeyron relation. The value is twice lager than the configuration entropy which is expected from the random arrangement of Eu^<3+> and Eu^<2+> ions in the thermally derived valence fluctuating state of Eu_4As_3.^<151>Eu nuclear resonant inelastic x-ray scattering (NRIXS) experiments have been performed on Eu_4As_3 from 301 to 406 K. We derived the thermodynamic parameters from inelastic components in the observed NRIXS spectra using the sum rules. The mean recoilless fraction shows the softening in the lattice vibration of the Eu ion around the boundary of the B … More rillouin zone in the valence fluctuating state. We present by the temperature dependence of mean force constant that the hybridization between the 4fstates and the conduction or valence bands in Eu_4As_3 plays a role around the charge ordering temperature and in the valence fluctuating state.We have carried out high-pressure x-ray diffraction experiments on Eu_4As_3 at mom temperature, and individual atomic coordinates as well as lattice parameters were derived precisely from the diffraction data. We have confirmed that a trigonal structure with the R3c space group does not change up to 19 GPa. The pressure dependence of the distortion angle changes at 10 GPa although the volume shows no anomaly as a function of pressure. The results show that the volume of the EuAs_6 octahedron and the valence state of the Eu ion are closely correlated, and pure trivalent and divalent Eu ions occupy two nonequivalent Eu sites, respectively, in the charge-ordered state at ambient pressure. We present dear evidence on the basis of the pressure dependence of these volumes at two nonequivalent Eu sites that a melting of the static charge order occurs at 10 GPa and Eu_4As_3 is in a mixed-valence state above 10 GPa. Less