Origin and nature of valence transition of Eu-based compounds

本项目研究了Eu基化合物价态转变的起源和价态起伏的本质。在以前的研究中，我们首次发现Eu的价态转变是由强磁场引起的。我们用组态间涨落(ICF)模型讨论了这种场诱导的价态转变。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)在900℃下对EuPd_2Si_2进行退火处理，显著提高了样品的质量。这或多或少地抑制了低温下磁化率的上升，这在所有先前的研究中都有观察到。我们的结论是，磁化率的上升不是来自氧化物，而是Pd/Si位的无序，这种无序可以通过高于900℃的温度来恢复。2)在ICF模型中，…基态之间的激发能更多的Eu^&lt；3&gt；和Eu^&lt；2&gt；的激发态由E_&lt；ex&gt；=E_0(1-αp_2)给出，其中p_2是Eu^&lt；2&gt；态的占据几率E_0和α为常数。通过假设E0随浓度线性变化，解释了EuNi2(Ge&lt；1-x&gt；Six)2的价态转变的本质。另一方面，压力效应表明，对于EuNi2Ge_2，静水压相当于掺硅。这些结果表明，E0强烈地依赖于体积。注意到上面的方程，我们认为α的起源，因此价跃迁是在ICF模型框架下激发能的反常大体积依赖的。3)通过将该体系视为Eu^&lt；2&gt；_&lt；P3&gt；其中p2和p2分别是Eu^&lt；2&gt；2&gt；和Eu^&lt；3&gt；态的占据几率，讨论了ρ与温度的关系。4)用助熔剂法合成了EuNi2P2和EuRu2P2大单晶。4)用熔剂法制备了ρ单晶。较少

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