Origin and nature of valence transition of Eu-based compounds

本课题旨在研究铕基化合物的价态跃迁起源和价态涨落的本质。在以前的研究中，我们首次发现Eu的价态跃迁是由强磁场引起的。我们用组态间涨落（ICF）模型讨论了这种场诱导的价态跃迁。为了阐明ICF模型的微观起源，研究了价态跃迁的压力效应。结果表明：（1）EuPd_2Si_2经900℃退火后，样品质量明显提高。这抑制了在低温下磁化率的上升，这在以前的研究中都或多或少地观察到，我们认为磁化率的上升不是来自氧化物，而是来自Pd/Si位的无序，而这种无序在高于900℃的温度下退火后可以恢复。2）在ICF模型中， ...更多信息 Eu^&lt;3+&gt;和Eu^&lt;2+&gt;的激发态由E_<ex>=E_0（1-αp_2）给出，其中p_2为Eu^&lt;2+&gt;态的占据几率，E_0和α为常数。在<1-x>E_0随浓度线性变化的假设下，解释了EuNi_2（Ge_ Si_x）_2的价态跃迁性质。另一方面，压力效应表明，对于EuNi_2Ge_2，静压等效于掺杂Si。这些结果表明E_0与体积有很强的依赖关系。注意到上述方程，我们认为，α的起源，因此价跃迁是非常大的体积依赖性的激发能在ICF模型的框架。3)将该体系视为Eu^&lt;2+&gt;_ Eu^&lt;3+&gt;_的虚拟合金，讨论了电阻率ρ的温度依赖性<p2><p3>，其中p_2和p_2分别为Eu^&lt;2+&gt;和Eu^&lt;3+&gt;态的占据几率。结果表明，杂质在虚拟合金中的随机散射是ρ随温度变化的原因。4）用助熔剂法获得了EuNi_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