Mixed valencies: Structure of phase diagrams, lattice properties and the consequences of electron hole symmetry (original) (raw)
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Mixed valencies: Symmetries and phonon phenomena
Journal of Magnetism and Magnetic Materials, 1979
The consequences of electron-hole symmetry, the structure of phase diagrams and lattice properties of mixed valence compounds are discussed on the basis of the periodic Anderson model which was extended to include the interaction of 4f electrons and lattice vibrations.
Natural Science, 2019
The scope of solid-state transitions, from melting temperatures down to 4.2 K, is described for six systems: K x VF 3 , Rb x VF 3 , Cs x VF 3 , K x CrF 3 , Rb x CrF 3 , and Cs x CrF 3 (for x = 0.0 to 1.0). Connections are drawn between the compounds' compositions and structures with the various transitions and ordering events. Upon solidification from the melt and gradual cooling to room temperature, a sequential descent of symmetry appears to occur, from high-symmetry perovskite phases, through possible reconstructive transitions, to phases designated α, β, and δ, within which ionic ordering finally sets in, forming many new lower-symmetry structures. Many stable new structures are seen at room temperature. Finally, at cryogenic temperatures, magnetic ordering sets in. Other anomalies for these systems are also described. The analysis underscores the overall correspondence of structure, composition, and magnetic properties in these compounds. This lowering of symmetry mirrors what has been chronicled for oxygen-bearing perovskites that have yielded so many high-temperature ceramic superconductors.
Anomalous behaviour of the coefficient of thermal expansion in intermediate valence systems
Solid State Communications, 1993
The electronic contribution to the coefficient of thermal expansion for intermediate valence systems is related to the temperature derivative of 4f-level occupancy. Using the Green's function technique, the occupancy of the 4f-level and its temperature derivative have been evaluated self-consistently with the periodic Anderson model in the limit of infinite Coulomb repulsion and upto quadratic terms in hybridization interaction. The influence of the 4flevel position with respect to the conduction band and the strength of hybridization interaction on the temperature dependence of the coefficient of thermal expansion is investigated. It has been noted that a displays a peak-like behaviour as a function of temperature and also can be both positive and negative as observed in some experiments.
Natural Science, 2019
The scope of solid-state transitions, from melting temperatures down to 4.2 K, is described for six systems: K x VF 3 , Rb x VF 3 , Cs x VF 3 , K x CrF 3 , Rb x CrF 3 , and Cs x CrF 3 (for x = 0.0 to 1.0). Connections are drawn between the compounds' compositions and structures with the various transitions and ordering events. Upon solidification from the melt and gradual cooling to room temperature, a sequential descent of symmetry appears to occur, from high-symmetry perovskite phases, through possible reconstructive transitions, to phases designated α, β, and δ, within which ionic ordering finally sets in, forming many new lower-symmetry structures. Many stable new structures are seen at room temperature. Finally, at cryogenic temperatures, magnetic ordering sets in. Other anomalies for these systems are also described. The analysis underscores the overall correspondence of structure, composition, and magnetic properties in these compounds. This lowering of symmetry mirrors what has been chronicled for oxygen-bearing perovskites that have yielded so many high-temperature ceramic superconductors.
Thermal expansion of mixed valence compounds
Solid State Communications, 1991
The anamolous thermal expansion of mixed valent systems is studied using the periodic Anderson model (PAM). The coefficient of thermal expansion is related to the temperature derivative of the 4f-level occupancy. The 4f-level occupancy and its temperature derivative have been evaluated self-consistently using the PAM in the mean-field approximation for a wide range of temperatures. Thereby, the interpolation, the earlier authors employed to obtain a peak from the low and high temperature results, is avoided. The order of magnitude and the qualitative behaviour obtained here are in agreement with earlier results.
Phonons of the anomalous element cerium
Proceedings of the National Academy of Sciences, 2011
Many physical and chemical properties of the light rare-earths and actinides are governed by the active role of f electrons, and despite intensive efforts the details of the mechanisms of phase stability and transformation are not fully understood. A prominent example which has attracted a lot of interest, both experimentally and theoretically over the years is the isostructural γ − α transition in cerium. We have determined by inelastic X-ray scattering, the complete phonon dispersion scheme of elemental cerium across the γ → α transition, and compared it with theoretical results using ab initio lattice dynamics. Several phonon branches show strong changes in the dispersion shape, indicating large modifications in the interactions between phonons and conduction electrons. This is reflected as well by the lattice Grüneisen parameters, particularly around the X point. We derive a vibrational entropy change ΔS γ−α vib ≈ ð0.33 AE 0.03Þk B , illustrating the importance of the lattice contribution to the transition. Additionally, we compare first principles calculations with the experiments to shed light on the mechanism underlying the isostructural volume collapse in cerium under pressure.
Phonon softening and high-pressure low-symmetry phases of cesium iodide
Physical Review Letters, 1992
The relative stability of various high-pressure phases of CsI is studied from first principles and analyzed using the Landau theory of phase transitions. We demonstrate that the cubic-to-orthorhombic transition recently observed to occur slightly below 20 Gpa is driven by the softening of an acoustic phonon at the M point of the Brillouin zone. The coupling between this mode and anisotropic strain makes the transition slightly first order (with a volume variation of the order of 0. 1%), and stabilizes the experimentally observed orthorhombic phase with respect to other competing symmetry-allowed structures.
Low temperature features of the local structure of Sm1 − x Y x S
Journal of Experimental and Theoretical Physics, 2007
The first-order semiconductor-metal isostructural phase transition that occurs at room temperature in SmS under the pressure 0.65 GPa [1] continues to attract close attention of researchers. Under normal pressure, SmS is a nonmagnetic semiconductor with the lattice of the NaCl type. In a semiconductor, "black," phase, the valence of Sm is 2+. However, as the pressure increases and, correspondingly, the volume of the unit cell decreases, the 4 f level overlaps with the 5 d band, the bandgap becomes zero, and the transition to a metal, "gold," phase occurs. In this case, because the 4 f electrons are partially delocalized, the degree of occupation of the 4 f level becomes noninteger, and samarium passes to a mixed-valence state.
Anomalous lattice dynamics in intermediate-valence CeNi
Physical Review B, 1998
The low-energy phonon modes of the intermediate-valence system CeNi and the isostructural reference compound LaNi have been measured at room temperature by inelastic neutron scattering in the scattering planes ͑001͒ and ͑100͒. In general, the frequencies of the acoustic and lower optical-phonon branches are found to be appreciably lower in CeNi. This effect is especially dramatic for the acoustic modes propagating along the a and b directions. Maximum softening was observed near the zone boundary, where it can exceed 20% for some of the branches. These data represent the most pronounced effect of electron-lattice coupling in a Ce-based intermediate-valence compound that we are aware of.