Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

Jayaraman, A.; Singh, Anil K.; Chatterjee, Aritra; Devi, S. Usha

doi:10.1103/physrevb.9.2513

Cited by 277 publications

(152 citation statements)

References 21 publications

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“…In view of the limited pressure range of previous studies, together with conflicting reports on the pressure dependence of its lattice parameter [10,16], we carried out high-precision XRD measurements to determine the pressure-volume relation in EuO up to 92 GPa (Fig. 1).…”

Section: +mentioning

confidence: 99%

“…After reaching about 92 GPa, the pressure was released and the structural changes were observed to be reversible. XRD results were previously used to argue, based on bondvalence sum rules [11,12], that the valence of Eu in EuO dramatically increases as a function of pressure with a first order valence transition towards a Eu 3+ state taking place at the modest isostructural volume collapse resulting in a 2.5+ valence at about 35 GPa [10,17]. While a volume collapse could in principle be a signature of a sizable increase in valence, just the opposite takes place at the structural phase transition in EuO, as discussed below.…”

Section: +mentioning

confidence: 99%

“…At higher pressures we observe a novel reentrant valence transition in Eu ions into a lower valence state despite a concomitant 7% volume collapse. Oftentimes, valence state is derived from structural data alone via bond valence sum rules [10][11][12], which relate interatomic distances and coordination number to valence. We demonstrate that bond-valence models fail to quantitatively describe the pressure-dependent mixed-valent states of EuO in a broad pressure range.…”

Section: +mentioning

confidence: 99%

“…The general approximation "bond length is a unique function of bond valence" [11] is widely used to determine a material's valence based on crystal structure. The prediction of valence by these methods [1,[10][11][12] is done using a linear combination of lattice constants as a function of pressure considering the ionic radius of, for example, Eu 2+ and Eu 3+ ions (which differ by about 10%). We used the parameterization method of [11] to calculate the valence based on bond distances from XRD data (see supplemental material [15] for details).…”

Section: +mentioning

confidence: 99%

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Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model

Souza-Neto

Zhao

et al. 2012

Phys. Rev. Lett.

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The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at ≈ 45 GPa is accompanied by a reentrant Eu valence transition into a lower valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.The phenomenon of mixed-valency in f -electron systems occurs when otherwise localized f -orbitals of RareEarth or Actinide elements hybridize in the solid with s,p,d electrons. A quantum superposition of differently occupied (valence) f -states emerges when the energy difference between competing single-valent states is smaller than the f -electron bandwidth, usually termed fluctuating valence state [1]. The onset of mixed-valency under applied pressure, chemical substitutions, or finite temperature has dramatic consequences on the macroscopic properties of f -electron systems including volume collapse [2], quenched magnetism [3], onset of superconductivity [4,5], Kondo physics [6], and quantum criticality [4,5]. Despite mixed-valency being central to f -electron physics, our ability to directly probe this peculiar quantum electronic state at high pressures is limited.EuO with its simple NaCl (B1) crystal structure is a model system to study valence effects upon lattice compression [7]. Eu-containing compounds are prototypical mixed-valent systems because Eu can display both trivalent (as most Rare-Earths ions do) and divalent electronic states at ambient conditions, the latter stabilized by a half-filled 4f orbital occupation ([Xe]4f 7 5d 0 6 sAdditionally, the ferromagneticsemiconductor character of EuO [7] coupled with perfect spin-polarization of electronic states near the Fermi level generated interest for possible applications of EuO in Spintronics [8]. A dramatic, three-fold increase in magnetic ordering temperature is observed under applied pressures of up to ≈14 GPa (or at strained interfaces), reaching a maximum T C ≈ 200 K but decreasing at higher pressures [9]. The relationship between crystal structure, electronic structure and magnetic ordering temperature has fueled much of the research in EuO over the last two decades, with the question of Eu valency remaining key for a complete understanding of this and other f -electron mixed-valent systems. In this Letter we report direct measurements of electronic valence and crystal structure in Europium monoxide (EuO) at pressures up to 90 GPa using x-ray absorption spectroscopy, nuclear forward scattering and x-ray diffraction techniques. Below 40 GPa a complex, pressuredependent valence is observed to fluctuate between Eu 2+and Eu 3+ states at a frequency f > ∼ ∆E/h ∼ 0.15 PetaHertz where ∆E is the 4f bandwidth. At higher pressures we obse...

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Section: +mentioning

confidence: 99%

Section: +mentioning

confidence: 99%

Section: +mentioning

confidence: 99%

Section: +mentioning

confidence: 99%

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Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model

Souza-Neto

Zhao

et al. 2012

Phys. Rev. Lett.

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“…Experimentally, the transition of EuO (at room temperature) is continuous, which the present theory cannot describe. For EuS the experiments show no anomalous compression curve 68 , but the band gap closes at 160 kbar, just before the structural transition to the CsCl structure (at 200 kbar) 70 . However due to the LMTO-ASA approximations, significant uncertainty persists in the values of the total energy differences between different crystal structures.…”

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confidence: 99%

Pressure induced valence transitions inf-electron systems

et al. 2007

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A review is given of pressure induced valence transitions in f -electron systems calculated with the self-interaction corrected local spin density (SIC-LSD) approximation. These calculations show that the SIC-LSD is able to describe valence changes as a function of pressure or chemical composition. An important finding is the dual character of the f -electrons as either localized or band-like. A finite temperature generalisation is presented and applied to the study of the p-T phase diagram of the α → γ phase transition in Ce.

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Pressure-induced Oxidation State Change of Ytterbium in YbGa2

Schwarz

Giedigkeit

Niewa

et al. 2001

Z. anorg. allg. Chem.

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The crystal structure and the electronic properties of YbGa 2 realising a CaIn 2 type atomic arrangement were characterised at ambient conditions using single crystal X-ray diffraction data and magnetic susceptibility measurements at ambient pressure. Pressure-induced changes of structural and electronic properties of YbGa 2 were measured by means of angle-dispersive X-ray powder diffraction and XANES at the Yb L III threshold. At pressures above 22(2) GPa, YbGa 2 undergoes a structural phase transition into a high pressure modification with a UHg 2 type crystal structure. Parallel to the pressure-induced structural alterations, ytterbium in YbGa 2 undergoes an increase of the oxidation state from +2 at ambient conditions to +3 in the high-pressure phase. Quantum chemical calculations of the Electron-Localisation-Function confirm that the phase transition is associated with a conversion of the three-dimensional gallium network of the low-pressure crystal structure into two-dimensional gallium layers in the high-pressure modification.

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Pressure-volume relationship and pressure-induced electronic and structural transformations in Eu and Yb monochalcogenides

Cited by 277 publications

References 21 publications

Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model

Reentrant Valence Transition in EuO at High Pressures: Beyond the Bond-Valence Model

Pressure induced valence transitions inf-electron systems

Pressure-induced Oxidation State Change of Ytterbium in YbGa2

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