Pressure-induced Valence Transition Research Articles

Pressure-induced large valence transitions in Yb-Cu binary intermetallic systems

Pressure-induced large valence transitions in Yb-Cu binary intermetallic systems

Variation of Pressure-Induced Valence Transition with Approximation Degree in Yb-Based Quasicrystalline Approximants

Variation of Pressure-Induced Valence Transition with Approximation Degree in Yb-Based Quasicrystalline Approximants

Pressure-Induced Valence Transition and Heavy-Fermion State in Europium Compounds

Pressure-Induced Valence Transition and Heavy-Fermion State in Europium Compounds

Valence Transitions in Yb1+xIn1−xCu4 Studied by High-resolution X-ray Absorption Spectroscopy, X-ray Diffraction, and Photoelectron Spectroscopy

Temperature- and pressure-induced valence transitions in the Yb-rich compounds Yb1+xIn1−xCu4 were studied directly by the high-resolution x-ray absorption spectroscopy. We observed a correlation between the temperature dependence of χT and the Yb valence which could be described by the Anderson model, where χ and T are magnetic susceptibility and temperature, respectively. The valence transition temperature of Yb1+xIn1−xCu4 was found to be insensitive to the chemical pressure compared to to the case of YbIn1−xAgxCu4. This difference could be attributed to the difference in the electronic structure above the Fermi level. The shift of Tv to higher temperatures with increasing x in Yb1+xIn1−xCu4 is interpreted to be caused by the stronger hybridization contributed by the additional Yb site at the In site. Hydrostatic pressure dependence of the Yb valence in Yb1.06In0.94Cu4 showed an anomaly. The Yb valence decreased at the pressure range of 5–20 GPa where an anomaly was also observed in the pressure dependence of the lattice constant.

Direct Observation of Pressure-induced Yb Valence Transition in YbInCu4

The Yb valence state of YbInCu4 was measured at 300 and 13 K under pressure by high-resolution x-ray absorption spectroscopy (XAS). The electronic structure of Cu was also measured by XAS at the Cu-K absorption edge. Pressure-induced disappearance of the valence transition was observed directly at 13 K. The present results suggest a charge transfer from Yb to Cu sites at 13 K after the disappearance of the valence transition under pressure.

Pressure evolution of the electronic structure of non-centrosymmetric EuRhGe3

Among europium compounds, pressure induced valence transitions and/or intermediate valence states are often observed. In such systems, applying pressure of several GPa can drive a Eu valence from divalent to almost trivalent. Non-centrosymmetric EuRhGe3 possesses magnetic Eu2+ ions and exhibits antiferromagnetic ordering at ∼11 K at ambient pressure. Pressure resistant magnetic ordering and stable divalent Eu state have been reported in EuRhGe3. Here, we study the pressure evolution of the Eu valence of EuRhGe3 by high resolution x-ray absorption spectroscopy using the partial fluorescence yield method. Our study reveals a successive increase of the Eu valence with increasing pressure without any valence transition. The obtained mean Eu valence approaches ∼2.4 around 40 GPa at 300 K. The experimental data are also analyzed by a full multiplet configuration interaction calculation based on the single impurity Anderson model. The analysis reveals a decrease of the Eu 4f orbital occupation by applying pressure. Pressure evolution of the electronic structure studied by density functional theory suggests that the Rh ions have little contribution to the pressure evolution of the Eu valence, while it implies an active involvement of the Ge ions.

Reentrant valence transition in YbCu4.5 under pressure

The electronic structure of ${\mathrm{YbCu}}_{4.5}$ under pressure has been studied with x-ray emission spectroscopy. Pressure-induced first-order valence transition to the divalent Yb state is found around 0.6--2.7 GPa, accompanied by the structural transition at the same pressure range of 0--1.0 GPa suggested by the change in the x-ray diffraction pattern. We also measured temperature dependencies of the Yb valence and magnetic susceptibility, which are compared with the single impurity Anderson model.

Pressure-induced valence change and moderate heavy fermion state in Eu-compounds

A pressure-induced valence transition has attracted much attention in Eu-compounds. Among them, EuRh2Si2, EuNi2Ge2, and EuCo2Ge2 reveal the valence transition around 1, 2, and 3GPa, respectively. We have succeeded in growing single crystals of EuT2X2 (T: transition metal, X: Si, Ge) and studied electronic properties under pressure. EuRh2Si2 indicates a first-order valence transition between 1 and 2GPa, with a large and prominent hysteresis in the electrical resistivity. At higher pressures, the first-order valence transition changes to a cross-over regime with an intermediate valence state. Tuning of the valence state with pressure is reflected in a drastic change of the temperature dependence of the electrical resistivity in EuRh2Si2 single crystals. Effect of pressure on the valence states on EuRh2Si2, EuIr2Si2, EuNi2Ge2, and EuCo2Ge2, as well as an isostructural related compound EuGa4, are reviewed.

Divalent, trivalent, and heavy fermion states in Eu compounds

Most of the Eu compounds are in the divalent (Eu) electronic state and order magnetically. On the other hand, the Eu-trivalent (Eu) compounds exist but are small in number. An energy difference between the Eu and Eu states is, however, not extremely large. Therefore, the valence transition occurs in some Eu compounds. We present the characteristic properties of the Eu compounds from several viewpoints: a canting magnetisation in the Eu-antiferromagnets, the Fermi surface property in the Eu-compounds of EuPd and EuCoSi, the heavy fermion state in EuNiP, the temperature-induced valence transition in EuPdSi, the pressure-induced valence transition in EuRhSi and EuRuP, and the heavy fermion state approaching to the quantum critical point with increasing pressure in EuNiGe .

Pressure-Induced Valence Transition and Characteristic Electronic States in EuRh2Si2

EuRh2Si2 is well known to show a valence transition at a pressure of about 1 GPa from a Eu-divalent (antiferromagnetic) state to a nearly Eu-trivalent (paramagnetic) state, which was clarified using polycrystalline samples. We have succeeded in growing single crystals of EuRh2Si2 by the Bridgman method and studied their electronic properties measuring the electrical resistivity under pressure. EuRh2Si2 indicates a first-order valence transition in the pressure range from 1 to 2 GPa, distinguished by a sharp transition and a prominent hysteresis in the temperature dependence of the electrical resistivity. A critical end point in the valence transition is estimated as \(P_{\text{CEP}} \simeq 2.05\) GPa and \(T_{\text{CEP}} \simeq 170\) K. In the pressure range from 2 to 3 GPa, the electrical resistivity is found to exhibit a characteristic behavior for a moderately heavy-fermion compound such as EuIr2Si2. At pressures higher than 3 GPa, the resistivity reveals a normal metallic behavior in a nearly trivalen...

Pressure-induced structural and valence transition in AgO.

The pressure-induced evolution of AgO crystal structures and the oxygen environment of Ag atoms were investigated by means of density functional theory with a hybrid functional and a structure prediction method. Under ambient conditions, AgO has two nonequivalent Ag1 and Ag2 sites that adopt linear and square planar oxygen environment configuration, respectively, corresponding to Ag mixed-valence states. The results show that both the coordination environment and the valence state of the Ag1 site are sensitive to pressure and will gradually approach those of the Ag2 site as it increases. The band gap also decreases significantly and at 75 GPa AgO experiences a pressure-induced semiconductor-to-metal transition. At ∼77 GPa, there is a structural transition from monoclinic (P21/c) to trigonal (R3[combining macron]m), accompanied by a valence state transition from the mixed-valence state to a single-valence state.

Comment on “Pressure-induced structural and valence transition in AgO” by C. Hou, J. Botana, X. Zhang, X. Wang and M. Miao, Phys. Chem. Chem. Phys., 2016, 18, 15322

The recent article by Hou et al. has focused on a theoretical study of mixed valence compound AgO in order to elucidate the nature of the electronic structure of this system as a function of external pressure. The authors claim that '…the effects of pressure on the Ag valence state are not investigated yet.' This statement is incorrect in view of the theoretical study published in 2015 by Włodarska et al., which answers most of the questions posed by these authors, including the key one: '…does the Ag cation exhibit similar behaviour to the Au cation in M2Au2X6halides, experiencing mixed-valence to single-valence transition under pressure? If so, what is the mechanism?'.

Origin of the black-golden transition in Sm1-xYxS

We report angle-resolved photoemission spectroscopy, electrical resistivity and Hall effect measurements on Sm1-xYxS. At the smallest doping concentration x = 0.03, the system changes from a semiconducting to metallic conductivity, whereas it is at a critical concentration xc ≊ 0.19 that there occurs the volume collapse accompanied by a color change from black to golden. It appears that the band gap between 4f and 5d bands vanishes at xc. From these results, we suggest that the black-to-golden transition of Sm1-xYxS has the same mechanism as the pressure-induced valence transition of SmS, but the semiconductor-to-metal transition has a different origin for Sm1-xYxS and SmS.

In-situ nanoscale imaging of charge transfer of BiNiO3 under high pressure

Over last decades, both synchrotron radiation techniques and high pressure research have made great progress. Advanced synchrotron capabilities with high spatial resolution, high flux, and high energy resolution provides us many new avenues to conduct advanced high pressure researches. In this talk, we will focus on the new developments of the nanoscale imaging techniques on the pressure induced phase separation in three dimensions. BiNiO3 under goes a charge transfer induced phase transition under high pressure or temperature, which shows excellent colossal negative thermal expansion effect [1]. Co-exist of both high density and low density phases over a wide range pressure or temperature plays the key roles on the negative thermal expansion behavior. We utilized a newly developed X-ray absorption near edge spectroscopy tomography method, and successfully resolved the mixture of high/low pressure phases as a function of pressure at tens of nanometer resolution. By choosing incident x-ray energy near Ni absorption edge, the pressure induced valence transition can be mapped at tens of nanometer scale in 3d, which provides crucial information on the HP-LP phase boundary [2]. As temperature driven grain growth upon heating, we can draw fundamental information on the pressure-induced phase growth mechanism.

Valence transition induced by pressure and magnetic field in antiferromagnet EuRh2Si2

We report the effects of a magnetic field on a pressure-induced valence transition in EuRh2Si2 by using magnetization measurements under high pressure. The M-H curves at 2 K under various pressures up to 1.18 GPa exhibit no anomalies associated with a field-induced valence transition. On the other hand, the M-H curves under a pressure of ∼ 1 GPa, where a temperature-induced valence transition occurs at around 30 K, show a clear metamagnetic transition with a large hysteresis at around 30 K. This behavior corresponds to a field-induced valence transition. The M-T curves in magnetic fields indicate that with increasing magnetic field, the temperature-induced valence transition shifts toward lower temperature and that its thermal hysteresis becomes larger. Cooling the sample under a pressure of ∼ 1 GPa in a magnetic field of 7 T collapses the valence transition and stabilizes the magnetic high-temperature phase down to 2 K. Besides, with decreasing magnetic field, three sharp valence transitions toward the nonmagnetic Eu3+ direction are observed: 4.65, 2.65 and 1.93 T.

Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduc- tion states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.

Pressure-Induced Valence Transition in Antiferromagnet EuRh2Si2

Considering the unique properties of EuRh 2 Si 2 from the viewpoint of the Eu valence, we have examined its physical properties under external pressure. At ambient pressure, EuRh 2 Si 2 is an antiferromagnet with a Neel temperature T N of 25 K, and the Eu ion is in the divalent state. The application of pressure up to 0.84 GPa slightly shifts T N toward higher values. Under pressures higher than 1.00 GPa, an abrupt first-order valence transition emerges simultaneously with the disappearance of antiferromagnetism. For P =1.17 GPa, the valence change associated with valence transition is roughly estimated to be ∼0.19 from the thermal expansion anomaly. The valence transition temperature T v increases rapidly with increasing pressure. The temperature–pressure phase diagram of EuRh 2 Si 2 is very similar to those of the other systems showing pressure-induced valence transition.

Temperature and pressure-induced valence transitions inYbNi2Ge2andYbPd2Si2

We have measured the temperature and pressure-induced Yb valence transitions in tetragonal ${\text{YbNi}}_{2}{\text{Ge}}_{2}$ and ${\text{YbPd}}_{2}{\text{Si}}_{2}$ using x-ray absorption spectroscopy in the partial fluorescence yield mode and resonant x-ray emission spectroscopy. A temperature dependence of the Yb valence on the order of 0.1 has been measured, consistent with the magnetic-susceptibility study. The crossover from the low-temperature state having a stronger mixed valence to a high-temperature local moment behavior is analyzed within the Anderson impurity model. Pressure-induced second-order valence transitions are observed for both compounds with a more gradual transition in ${\text{YbPd}}_{2}{\text{Si}}_{2}$ than that of ${\text{YbNi}}_{2}{\text{Ge}}_{2}$. The mean valences are slightly less than $3+$ at ambient pressure but increase with applying pressure. Small variations in the Yb valence on the order of 0.03--0.05 can result in drastic change in the physical properties such as magnetic order and transport properties. Our results show that the Yb valence is noninteger around the quantum critical point.

Pressure induced valence transitions in the Anderson lattice model

Abstract We apply the equation of motion method to the Anderson lattice model, which describes the physical properties of heavy fermion compounds. In particular, we focus here on the variation of the number of f electrons with pressure, associated to the crossover from the Kondo regime to the intermediate valence regime. We treat here the non-magnetic case and introduce an improved approximation, which consists of an alloy analogy based decoupling for the Anderson lattice model. It is implemented by partial incorporation of the spatial correlations contained in higher-order Green's functions involved in the problem that have been formerly neglected. As it has been verified in the framework of the Hubbard model, the alloy analogy avoids the breakdown of sum rules and is more appropriate to explore the asymmetric case of the periodic Anderson Hamiltonian. The densities of states for a simple cubic lattice are calculated for various values of the model parameters V , t , E f , and U .

Gigantic Effect of Pressure in CeFeAsO1-y

It was an astonishing discovery that LaFeAs(O,F) (La1111) shows a high superconducting transition temperature TC of 26K. 1) The enhancement of TC by substituting lanthanoid elements was also amazing as well as the discovery of La-1111. TC comes up to 56K presently in Sm-1111, and it is a refreshing surprise that such a variety of superconductors with high TC containing a typical magnetic element, i.e., iron, appeared in a short period of time. The first pressure study was performed by Takahashi et al. in La-1111. The most interesting result in this study was the marked increase in TC. They show that the onset of TC is rapidly enhanced to 43K in optimally doped LaFeAsO0:89F0:11 at 4GPa. Further application of pressure causes a gradual decrease in TC, and consequently the onset temperature of superconducting transition is suppressed to 9K at 30GPa. They also investigated the pressure dependence of underdoped LaFeAsO0:95F0:05. According to this study, underdoped La-1111 also shows TC enhancement by pressure. However, the increase in TC is smaller than that observed in optimally doped LaFeAsO0:89F0:11. This result indicates that there is a doping dependence in TC shift under high pressure; in other words, pressure is a parameter independent of doping level. We can refer to other highpressure studies in Ln-1111 (Ln; lanthanoid) except La-1111 by Zocco et al. (Ce-1111) and Yi et al. (Sm-1111, Nd1111). In these studies, TC shows a monotonic decrease under pressure. When we think of the TC of optimally doped oxygen-deficient Ln-1111, TC is almost constant at approximately 52K when we choose Ln 1⁄4 Nd, Sm, Eu, Gd, Tb, and Dy. According to an earlier study, Ce-1111, which has a TC of 41K 7) intermediate between those of La-1111 and Pr-1111, shows a monotonic negative pressure dependence of TC in fluorine-substituted CeFeAsO0:88F0:12. 4) Unfortunately, this result is not clear below 4GPa where La-1111 shows a marked TC enhancement. Moreover, the Ce ion is known as a candidate that induces to pressure-induced valence transition. CeFePO, which has a ZrCuSiAs type crystal structure, the same as Ln-1111, also presents the behavior of a heavy-fermion metal. For these reasons, we have carefully investigated the pressure dependence of TC in oxygen-deficient Ce-1111, especially below 4GPa. We prepared two oxygen-deficient Ce-1111 samples in this study. Both samples are polycrystalline and synthesized by a high-pressure and high-temperature method. One sample (CeFeAsO1 y, ]1) shows superconductivity at TC 1⁄4 40K and the other (]2) shows no superconductivity because of the difference in oxidization between their starting materials. By considering these characteristic features of these two samples, it is reasonable that ]1 is optimally doped and ]2 has a close-to-stoichiometric parent composition. The hydrostaticity of pressure is very important to observe the intrinsic behaviors of physical properties under pressure because Ln-1111 has inhomogeneous compressibility, which originates from its layered crystal structure. Therefore, we employed a cubic anvil apparatus with Daphne 7474 oil as pressure-transmitting medium in this study. Electrical resistivity was measured by a conventional DC 4-wire method to determine the transition temperature TC under a high pressure of up to 15GPa. Pressure always shifts upward 0.2 or 0.3GPa during warming above approximately 200K because of inside friction. We can see systematic resistivity anomalies in our study at approximately 200K caused by such an increase in pressure, which are not intrinsic. Figure 1 shows the resistivity of superconductive CeFeAsO1 y (]1) under high pressures. First, TC decreases very rapidly with pressure below 4GPa with a large transition width. In Nd-1111, we see no broadening and TC decreases much more slowly under pressure. Carrier doping level is not affected by pressure in underdoped Nd1111, TC decreases apparently with crystal structural factor under pressure in Nd-1111. Thus, the superconductivity of Ce-1111 is suppressed principally not by crystal structural factor but by the other reason. At 4.5GPa, the resistivity drop still remains; however, no zero resistivity is observed. Therefore, bulk superconductivity disappears at this pressure. One possible reason for such a distinct behavior is the valence transition of the Ce ion. The Ce ion is known for valence transition from Ce3þ to Ce4þ, which is sometimes induced by pressure, for example, the pressure-induced enhancement of superconductivity takes place in CeCu2Si2. 8,9) This speculation also suggests a possible means of continuous carrier doping using pressure0 50 0 0.1 0.2 0.3