Pressure Induced Valence Transitions in f-Electron Systems (original) (raw)
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First-Principles Theory of Intermediate-Valence f-electron Systems
Physical Review Letters, 2004
We propose a first-principles based method for calculating the electronic structure and total energy of solids in an intermediate-valence configuration. The method takes into account correlation effects (d ÿ f Coulomb interaction) and many-body renormalization of the effective hybridization parameter of the f system. As an example, the formation of a pressure-induced intermediate-valence state in Yb is considered and its electronic structure and equation of state are calculated and compared to experimental data. The agreement is found to be excellent for both properties, and we argue that the developed method, which applies to any element or compound, provides for the first time a quantitative theoretical treatment of intermediate-valence materials.
Physical Review B, 2012
We report systematic ab initio calculations of the electronic band structure, phonon dispersion relation, and the structural characterization of FeF 2 in the rutile (P 4 2 /mnm) structure as well as in several high-pressure phases by means of the generalized gradient approximation (GGA) + U approximation. Using the phonon dispersion relations, we calculated the Gibbs free energy and evaluated the phase transitions at 300 K, at which most experimental measurements are performed. Calculated Raman and infrared vibrational modes, lattice parameters, and electronic structure for all considered crystalline structures are compared with available experimental data. Our calculations show that at 5.33 GPa, the FeF 2 undergoes a second-order proper ferroelastic phase transition, rutile → CaCl 2 -type structure. This result is supported by the softening of the elastic shear module C s in the rutile phase, the softening (hardening) of the B 1g (A g ) Raman active mode in the rutile (CaCl 2 -type) structure near the transition pressure, and the decrease of the square of the spontaneous strain e ss from the CaCl 2 -type structure. This demonstrates that the rutile → CaCl 2 -type phase transition is driven by the coupling between the Raman active B 1g mode and shear modulus C s . At 8.22 GPa, the CaCl 2 -type structure undergoes a first-order phase transition to the P bca phase, a distorted fcc P a3 phase with a volume reduction of V ≈ 7%, as reported in experiments. Upon further increase of the pressure, the P bca phase transforms to a F mmm phase othorhombic center-type structure at ∼20.38 GPa, with V ≈ 2.5%. Finally, at 25.05 GPa, there is a phase transition to the orthorhombic cotunnite structure (P nma space group), with V ≈ 5.8%, which is stable up to 45 GPa, the largest considered pressure. The coordination number for the Fe ion in each phase is 6, 6, 6, 8, and 9 for rutile, CaCl 2 -type, P bca, F mmm, and cotunnite structures, respectively. The evolution of the band gap, phonon frequencies, and magnetic moment of Fe ion as a function of the applied pressure is reported for all studied phases. The exchange constants J 1 , J 2 , and J 3 , calculated for rutile and the lowest Gibbs free-energy high-pressure phases, are reported.
1998
Bc and Cf may occur under pressure [22,23]. 1.2 Atomic and band approaches in the transition element theory Most important peculiarity of transition metals from the theoretical point of view is an important role of electron correlations. There exist a number of approaches to the problem of treating many-electron (ME) systems with strong correlations. The first one was the self-consistent field (Hartree-Fock) approximation at solving the Schroedinger equation for ME atoms, which yielded satisfactory quantitative results. The Hartree-Fock method allows to take into account ME atomic terms, but its full version requires the solution of a complicated system of integro-differential non-linear equations [20], so that its direct generalization on solids (systems of large number of atoms) is hardly possible. Main successes of the solid state theory were connected with first-principle one-electron band structure calculations. Modern versions of this approach, which are based on the spin-density functional method (2.3), enable one to obtain a precise description of ground state characteristics [24]. At the same time, the band theory is wittingly insufficient for strongly localized f-states, and also at treating some physical phenomena, e.g. magnetism (especially at finite temperatures) and metal-insulator (Mott-Hubbard) transition [25]. Such oversimplified versions of the Hartree-Fock method, as the Stoner mean-field theory of itinerant magnetism, turned out to be not too successful. Later, some shortcomings of the Stoner theory were improved by semiphenomenological spin-fluctuation theories [26], which, however, do not take into account in most cases correlation effects in the ground state. To treat electron correlations, various perturbation approaches in the electronelectron interaction (e.g., diagram techniques) were used. Besides that, the Fermi-liquid theory was proposed to describe systems with strong electron interactions [27]. However, this theory is violated in the cases where the correlations result in a reconstruction of the ground state, e.g. for systems with a Mott-Hubbard gap [25]. Another approach to the problem of electron correlations was proposed by Hubbard [28-31] who considered the simplest microscopic model including the strong on-site Coulomb repulsion. Starting from the atomic limit,
Condensed Matter, 2021
AsF3E as a representative of a molecular crystal has been chosen to find the precise localization of the lone pair (LP) E centroid 4s2 of As3+ and to enlighten the behavior of lone pair triplets of fluorine atoms. Starting from stereochemistry rationale, Density Functional (DFT) electronic structure calculations yielding the electron localization (ELF) mapping led to precise large crystal structure evolutions from basic X-rays data (V = 267.2Å3 at 193K), to (V = 230.5Å3) and under Van der Waals forces (DEW) V = 206.4Å3, and then under pressure P, all illustrated with ELF maps and band structures. Calibrated pressures up to 100 GPa exhibit the remarkable shrinking of all inter-atomic distances including As-E from 0.94Å down to 0.46Å, while the major three bonds As-F1, As-F2 and As-F3 are continuously expanding. The resulting picture of the application of pressure on AsF3 molecular structure leads to the progressive immersion of the 4s2 doublet within the electronic cloud with an orig...
Pressure-Induced Valence Transitions in Rare Earth Chalcogenides and Pnictides
physica status solidi (b), 2001
The electronic structure of rare earth chalcogenides and pnictides is calculated with the ab-initio self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both an atomic-like description of the rare earth f-electrons and an itinerant description of other electronic degrees of freedom. Specifically, different formal valencies of the rare earth atom, corresponding to different f-shell occupancies, can be studied and their energies compared, leading to a first-principles theory for pressure-induced valence transitions. SIC-LSD calculations for cerium monopnictides and monochalcogenides, Yb monochalcogenides, and EuS are presented. The observed equilibrium lattice constants are well reproduced assuming a trivalent Ce configuration and divalent Eu and Yb configurations. The trends in the high pressure behavior of these systems are discussed. With applied pressure, isostructural phase transitions are found to occur in CeP and CeS, caused by the delocalization of the Ce f-electron, while in the heavier Ce compounds, the structural B1 ! B2 transition happens before f-electron delocalization occurs. Similarly, both Eu and Yb chalcogenides transfer to trivalent configurations with pressure, in accordance with observation.
Inorganic Chemistry, 1996
The pressure-induced iron(II) high-spin (HS) f low-spin (LS) conversion has been investigated, by near-edge X-ray absorption (XANES) spectroscopy at room temperature, in a number of parent six-coordinate complexes of the type FeL n L′ m (NCS) 2 , viz. Fe(phen) 2 (NCS) 2 form I (1) and form II (2), Fe(py) 2 bpym(NCS) 2 (3), Fe(py) 2 phen-(NCS) 2 (4) and Fe(py) 4 (NCS) 2 (5), where phen) 1,10-phenanthroline, py) pyridine, and bpym) 2,2′-bipyrimidine. The spectra of the two spin isomers are interpreted. For compounds 1-4, known to exhibit thermally-induced spin transitions, n LS vs P plots (n LS) LS fraction, P) pressure) are centered around P c) 0.80 (1), 0.65 (2), 1.00 (3) and 1.55 (4) GPa. For 5, which retains the HS form at any temperature under atmospheric pressure, the P c value is much higher than the previous ones, lying in the range 5-7 GPa. After pressure was released, the spectra were found to be quite similar to those obtained before pressure was applied. The P c values of 1-4 were analyzed in terms of spin-transition temperatures (T c), entropy (∆S) and crystal volume (∆V SC) variations associated with the spin change, crystal structures, and volumic compressibility coefficients (k v), insofar as these data were available. The alteration of the spin-conversion development, when subjecting a compound (here 2) several times to increasing then to decreasing pressures, was ascribed to a progressive decrease in the number of crystal defects (vacancies in particular).
Evidence for a common physical description of non-Fermi-liquid behavior in f-electron systems
1998
The non-Fermi-liquid (NFL) behavior observed in the low temperature specific heat C(T)C(T)C(T) and magnetic susceptibility chi(T)\chi(T)chi(T) of f-electron systems is analyzed within the context of a recently developed theory based on Griffiths singularities. Measurements of C(T)C(T)C(T) and chi(T)\chi(T)chi(T) in the systems Th1−xUxPd2Al3Th_{1-x}U_{x}Pd_{2}Al_{3}Th1−xUxPd2Al3, Y1−xUxPd3Y_{1-x}U_{x}Pd_3Y1−xUxPd3, and UCu5−xMxUCu_{5-x}M_{x}UCu5−xMx (M = Pd, Pt) are found to be consistent with C(T)/Tproptochi(T)proptoT−1+lambdaC(T)/T \propto \chi(T) \propto T^{-1+\lambda}C(T)/Tproptochi(T)proptoT−1+lambda predicted by this model with lambda<1\lambda <1lambda<1 in the NFL regime. These results suggest that the NFL properties observed in a wide variety of f-electron systems can be described within the context of a common physical picture.
Physical Review Letters, 2013
35 We discover that hcp phases of Fe and Fe 0.9 Ni 0.1 undergo an electronic topological 36 transition at pressures of about 40 GPa. This topological change of the Fermi surface 37 manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice 38 parameter ratio and Mössbauer center shift observed in our experiments. First-principles 39 simulations within the dynamic mean field approach demonstrate that the transition is 40 induced by many-electron effects. It is absent in one-electron calculations and represents 41 a clear signature of correlation effects in hcp Fe. 42 43 44 45 46 Iron is the most abundant element on our planet. It is one of the most important 47 technological materials and, at the same time, one of the most challenging elements for 48 the modern theory. As a consequence, the study of iron and iron-based alloys has been a 49 focus of experimental and computational research over the past decades. Recently, 50 investigations of phase relations and physical properties of iron and its alloys at high 51 pressure led to new exciting discoveries including evidence for a body-centred-cubic 52 (bcc) phase of iron-nickel alloy at conditions of the Earth's core [1] and the observation 53 of superconductivity in the high-pressure hexagonal close packed (hcp) phase of iron in 54 the pressure range 15-30 GPa and at temperatures below 2 K [2]. 55 While the structural properties of iron and iron-nickel alloys at pressures below 100 56 GPa are well established [3], their electronic and magnetic properties are still debated. 57