Origin and nature of valence transition of Eu-based compounds

本课题旨在研究铕基化合物价跃迁的起源和价波动的性质。在之前的研究中，我们首次发现了强磁场诱导的Eu价跃迁。我们用构型间涨落（ICF）模型讨论了这种由场引起的价跃迁。为了阐明ICF模型的微观起源，研究了价态跃迁的压力效应。得到了以下结果：1)在900℃下对EuPd_2Si_2进行退火处理，极大地提高了样品的质量。这抑制了在低温下磁化率的上升，这在以前的所有研究中或多或少都观察到。我们得出结论，磁化率的上升不是由氧化物引起的，而是由Pd/Si位的无序引起的，在高于900℃的温度下退火可以恢复。2)在ICF模型中，Eu^<3+>基态与Eu^<2+>激发态之间的激发能由E_<ex>=E_0 （1-αp_2）给出，其中p_2为Eu^<2+>态的占据概率，E_0和α为常数。假设EuNi_2 （Ge_<1-x>Si_x)_2)的E_0随浓度线性变化，解释了EuNi_2 (Ge_<1-x>Si_x)_2）的价跃迁性质。另一方面，压力效应表明静水压力与掺杂Si对EuNi_2Ge_2的影响相当。这些结果表明，E_0与体积密切相关。注意到上述方程，我们认为，在ICF模型的框架内，α的起源，因此价跃迁是异常大的体积依赖于激发能。3)将该体系视为Eu^<2+>_<p2>Eu^<3+>_<p3>的虚合金，讨论了电阻率ρ的温度依赖性，其中p_2和p_2分别为Eu^<2+>和Eu^<3+>态的占据概率。我们已经证明，在虚合金中由于随机性引起的杂质散射是ρ的温度依赖性的原因。4)用通量法得到了euuni_2p_2和EuRu_2P_2的大单晶。少

This project has been performed to study the origin of valence transition and the nature of valence fluctuation of Eu-based compounds. In the previous study, we have found that the Eu valence transition is induced by strong magnetic field for the first time. We have discussed this field-indued valence transition in terms of the interconfigurational fluctuation (ICF) model. In order to clarify the microscopic origin of the ICF model, the pressure effect of the valence trasition has been studied. The following results were obtained.1) Annealing EuPd_2Si_2 at 900℃ strongly improves the quality of the sample. This suppresses the uptum of magnetic susceptibility at low temperatures, which was observed in all the previous studies more or less. We concluded that the uptum of magnetic susceptibility comes not from the oxide but from disordering of Pd/Si site, which is recovered by annealing at higher temperatures than 900℃.2) In the ICF model, the excitation energy between the ground state of … More Eu^<3+> and the excited state of Eu^<2+> is given by E_<ex>=E_0 (1-αp_2), where p_2 is the occupation probability of Eu^<2+> state E_0 and α are constants. The nature of valence transition of EuNi_2 (Ge_<1-x>Si_x)_2) is explained by assuming that E_0 varies linearly with the concentraiton. On the other hand, the pressure effects have revealed that hydrostatic pressure is equivalent to doping Si for EuNi_2Ge_2. These results suggest that E_0 is strongly depends on volume. Noting the above equation, we believe that the origin of α, hence the valence transition is anomalously large volume dependence of the excitation energy in the framework of the ICF model..3) The temperature dependence of electrical resistivity, ρ, is discussed by regarding the system as a virtual alloy of Eu^<2+>_<p2>Eu^<3+>_<p3>, where p_2 and p_2 are the occupation probabilities of Eu^<2+> and Eu^<3+> states, respectively. We have shown that impurity scattering due to randomness in a virtual alloy is responsible for the temperature dependence of ρ.4) Large single crystals of EuNi_2P_2 and EuRu_2P_2 were obtained by the flux method. Less