Valence Instability Research Articles

Chemical approach to neutral–ionic valence instability, quantum phase transition, and relaxor ferroelectricity in organic charge-transfer complexes

Neutral–ionic (NI) phase transition is a reversible switching of organic charge-transfer complexes between distinct valence states by external stimuli. This phase transformation in the low-dimensional system is demonstrated to provide a variety of novel dielectric, structural, and electronic properties. Importantly, ionization of the electron donor–acceptor pairs is usually accompanied by a ferroelectric or antiferroelectric order of the molecular lattice, leading to huge dielectric response near the transition point. Although these characteristics are potentially useful for future electronic and optical applications, the thermally accessible NI transition (TINIT) is still an extremely rare case. The TINIT compounds including some new materials are overviewed in order to provide convenient guides to their design and experimental identifications. The phase transition and dielectric properties can be closely controlled in various ways depending on chemical and physical modifications of the crystals. Among them, a quantum phase transition and relaxor ferroelectricity, both of which are currently attracting subjects from both scientific and practical perspectives, are highlighted as the first achievements in organic charge-transfer complexes.

Anisotropy, disorder, and superconductivity inCeCu2Si2 under high pressure

Resistivity measurements were carried out up to 8 GPa on single-crystal and polycrystalline samplesof CeCu2Si2 from differing sources in the homogeneity range. The anisotropic response to current directionand small uniaxial stresses was explored, taking advantage of the quasi-hydrostatic environmentof the Bridgman anvil cell. It was found that both the superconducting transition temperatureTc and the normal state properties are very sensitive to uniaxialstress, which leads to a shift of the valence instability pressurePv and a small butsignificant change in Tc for different orientations with respect to the tetragonalc-axis. The coexistence of superconductivity and residual resistivity close tothe Ioffe–Regel limit around 5 GPa provides a compelling argument for theexistence of a valence fluctuation mediated pairing interaction at high pressure inCeCu2Si2.

Valence instability of cerium under pressure in the Kondo-like perovskiteLa0.1Ce0.4Sr0.5MnO3

Effect of hydrostatic pressure and magnetic field on electrical resistance of the Kondo-like perovskite manganese oxide, La$_{0.1}$Ce$_{0.4}$Sr$_{0.5}$MnO$_3$ with a ferrimagnetic ground state, have been investigated up to 2.1 GPa and 9 T. In this compound, the Mn-moments undergo double exchange mediated ferromagnetic ordering at $T_{\rm C}$ $\sim$ 280 K and there is a resistance maximum, $T_{\rm max}$ at about 130 K which is correlated with an antiferromagnetic ordering of {\it cerium} with respect to the Mn-sublattice moments. Under pressure, the $T_{\rm max}$ shifts to lower temperature at a rate of d$T_{max}$/d$P$ = -162 K/GPa and disappears at a critical pressure $P_{\rm c}$ $\sim$ 0.9 GPa. Further, the coefficient, $m$ of $-logT$ term due to Kondo scattering decreases linearly with increase of pressure showing an inflection point in the vicinity of $P_{\rm c}$. These results suggest that {\it cerium} undergoes a transition from Ce$^{3+}$ state to Ce$^{4+}$/Ce$^{3+}$ mixed valence state under pressure. In contrast to pressure effect, the applied magnetic field shifts $T_{\rm max}$ to higher temperature presumably due to enhanced ferromagnetic Mn moments.

Valence and magnetic instabilities in Sm compounds at high pressures

We report on the study of the response to high pressures of the electronic and magneticproperties of several Sm-based compounds, which span at ambient pressure the whole range ofstable charge states between the divalent and the trivalent. Our nuclear forward scatteringof synchrotron radiation and specific heat investigations show that in both golden SmS andSmB6 the pressure induced insulator to metal transitions (at 2 and –7 GPa, respectively) are associated with the onset of long-range magnetic order, stable up toat least 19 and 26 GPa, respectively. This long-range magnetic order, which is characteristic ofSm3+, appears already for a Sm valence near 2.7. In contrast to these compounds, metallic Sm,which is trivalent at ambient pressure, undergoes a series of pressure induced structuralphase transitions which are associated with a progressive decrease of the ordered 4fmoment.

Electronic structure, magnetic, and transport studies of single-crystallineUCoGa5

The magnetic susceptibility, electrical resistivity in zero and in magnetic fields up to $8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, thermopower and thermal conductivity measurements in a wide temperature range have been performed on ${\mathrm{UCoGa}}_{5}$ single crystals. On the basis of the susceptibility maximum at $650\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the phonon part of the thermal conductivity, a mixed-valence state of uranium in this compound has been postulated. On the other hand, a normal positive behavior of magnetoresistivity rules out the possibility of spin fluctuation as a mechanism driving the susceptibility through the maximum. In turn, the thermopower results may support both kinds of many-body behaviors. The electronic structure has also been studied by combining x-ray photoemission spectroscopy results with those obtained in the band structure calculations. In the latter the tight-binding linear muffin-tin orbital method in the atomic sphere approximation has been applied. A very good agreement between the experimental and calculated data has been achieved. A complex satellite structure of the core level spectra supports an idea of the presence of the valence instability in this compound. This idea is also concluded from the comparison of obtained experimental data with some similar rare earth and uranium ternary compounds.

The role of trapping levels of nonequilibrium electrons during the formation of pinning centers for domain walls in the magnetic semiconductor CdCr2Se4

A complex study of electrical conductivity and photoconductivity in constant and variable electric fields, thermally stimulated conductivity, and the photoferromagnetic effect are carried out in the temperature range 10–300 K for a CdCr2Se4 magnetic semiconductor with various concentrations of Ga impurity and Se vacancies (V Se). The phenomenon of hopping photoinduced conduction is observed for the first time. It is shown that shallow donor levels that exchange photoelectrons trapped by them with Cr3+ magnetic ions may be responsible for the photoferromagnetic effect. As a result of this exchange, the Cr3+ ions acquire valence and spin instabilities, which lead to nonequilibrium pinning of the domain wall by these ions.

Valence Instabilities, Phase Transitions, and Abrupt Lattice Expansion at 5 K in the YbGaGe System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Enhanced dielectric response in (BEDT-TTF) (ClMeTCNQ) at the neutral-ionic phase transitions

We have studied dielectric properties of (BEDT-TTF)(CIMeTCNQ) around the neutral-ionic (NI) transitions, whose critical temperatures can be tuned from 300 K to 0 K by variation of pressure in the range between 0.25 ∼ 1.0 GPa. We found that thermal-activation-type dielectric response becomes predominant around the transitions higher than 50 K, while Curie-Weiss type enhancement was not detected above the transitions. Here we feature the dielectric properties in the range above 50 K, so as to contrast another type of dielectrical enhancement observed around the quantum critical point of the NI valence instabilities in (BEDT-TTF)(ClMeTCNQ).

Valence instabilities, phase transitions, and abrupt lattice expansion at 5 K in the YbGaGe system.

The powder synchrotron X-ray diffraction technique was used to study the thermal expansion behavior of the mixed valence layered compound, YbGa1.05Ge0.95 in the temperature range 3-1123 K. A surprising abrupt isosymmetric phase transition, accompanied by a dramatic volume increase (negative thermal expansion), was found at 5 K induced by a sudden Yb valence transition from +(2 + epsilon) toward +2. At high temperatures, the material undergoes a transformation to a highly disordered structure until it eventually collapses at 1123 K to a structure with isovalent Yb ions and flat Ga/Ge planes (AlB2 type).

Valence instability in a dimer of two-orbital system: possible charge separation due to `negative U' effect

A novel mechanism of charge separation in a dimer of two-orbital molecular system is proposed. The effective electron–electron repulsive energy in the dimer, U eff, has been estimated by means of valence-bond (VB) model calculations for the ground state of the dimer. It has been found that the HOMO–LUMO interplay leads to negative U eff in the strong dimerization condition, indicating spontaneous separation of charges on dimers (2 dimer − → dimer 0 + dimer 2−). Possible relevance to experimental features in some [Pd(dmit) 2] salts (dmit=1,3-dithiol-2-thione-4,5-dithiolate) is discussed.

Signatures of valence fluctuations inCeCu2Si2under high pressure

Simultaneous resistivity and a.c.-specific heat measurements have been performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies were observed around the pressure coinciding with a maximum in the superconducting critical temperature, $T_c^{max}$. These anomalies can be linked with an abrupt change of the Ce valence, and suggest a second quantum critical point at a pressure $P_v \simeq 4.5$ GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability, and associated superconductivity, is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f-electrons. We explain the T-linear resistivity found at $P_v$ in this picture, while other anomalies found around $P_v$ can be qualitatively understood using the same model.

Correlation between transport properties and valence instability states of Eu ions in LuxEu1−xCu2Si2

Results of measurements of thermopower, electrical resistance, dc magnetic susceptibility, X-ray absorption and diffraction are presented for alloys of the solid solution LuxEu1−xCu2Si2. The temperature dependencies of the thermopower and electrical resistance are similar to those observed for other systems, based on Ce, Eu, and Yb with strong electron–electron interactions. As in the LaxEu1−xCu2Si2 system, the coexistence of Kondo behavior and valence instability states of Eu ions is observed. The Kondo-type state appears on increasing the value of the crystalline lattice parameter a. In the LuxEu1−xCu2Si2 system, the a parameter reaches its maximum values in the composition range 0.2≤x≤0.3 (a=0.4096 and 0.4098nm for x=0.2 and 0.3, respectively).

Phonon dispersions of fcc delta-plutonium-gallium by inelastic x-ray scattering.

We report an experimental determination of the phonon dispersion curves in a face-centered cubic (fcc) delta-plutonium-0.6 weight % gallium alloy. Several unusual features, including a large elastic anisotropy, a small-shear elastic modulus C', a Kohn-like anomaly in the T1[011] branch, and a pronounced softening of the [111] transverse modes, are found. These features can be related to the phase transitions of plutonium and to strong coupling between the lattice structure and the 5f valence instabilities. Our results also provide a critical test for theoretical treatments of highly correlated 5f electron systems as exemplified by recent dynamical mean field theory calculations for delta-plutonium.

Quantum phase transition in organic charge-transfer complexes.

A phase transition in an organic charge-transfer complex, which originates from the neutral-ionic valence instability, can be tuned toward zero kelvin with use of external pressure or chemical modification as a control parameter. The phase diagram and observed dielectric behaviors are typical of quantum paraelectricity, yet this zero-kelvin transition point namely, the quantum critical point, accompanies large quantum fluctuation of the molecular charge, as demonstrated by the molecular vibrational mode spectra. The result indicates that the pi-electron transfer between donor and acceptor molecules is coupled with the zero-point lattice dynamics around the quantum critical point.

Thermoelectric power in compounds with an intermediate valence of Ce: phenomenological description

Results of an investigation of the thermoelectric power component S f related to the valence instability of Ce are presented for CeNi n ( n=1, 2) and CeM 2Si 2 (M=Mn, Fe, Co) compounds in the temperature range 4–900 K. The behavior of S f satisfactorily described in a wide temperature range within the framework of a simple model of a narrow peak in the density of states g f( E) of Lorentzian shape near the Fermi level E F. The width of the peak depends on temperature and defines the characteristic single-ion Kondo temperature T K. This model describes better the behavior of S f in compounds showing a temperature instability of the intermediate valence of Ce.

Electric field effect on charge and lattice dynamics in TTF-QCl 4 and TTF-QBrCl 3

Electroreflectance (ER) spectra are measured on the mixed-stack charge transfer (CT) complexes, tetrathiafulvalane (TTF)-2-bromo-3,5,6-trichloro- p-benzoquinone (QBrCl 3) and TTF- p-chloranil (QCl 4), showing the neutral–ionic (N–I) phase transition. In the N phase of TTF-QBrCl 3, we observe a large ER signal, which originates from the electric field-induced I domains in the N stacks. In contrast, no ER signal is obtained in the N phase of TTF-QCl 4. The difference of the ER signal intensity between the two compounds is discussed by taking account of the difference in the degree of the valence instability.

NEGATIVE MAGNETORESISTANCE IN PbTe(Mn,Cr)

We have observed the negative magnetoresistance effect in the narrow-gap PbTe(Mn,Cr) semiconductor, in which the Fermi level is pinned within the gap nearby the conduction band edge. Previously the giant negative magentoresistance effect has been reported in PbTe(Mn,Yb), in which the Fermi level is pinned in the gap nearby the valence band edge. It is known that in the case of Yb doping the Fermi level pinning results from the 2+ - 3+ valence instability of an impurity. The same sort of the valence instability provides the Fermi level pinning in PbTe(Mn,Cr), but the conductivity is of the n-type, not of the p-type as in PbTe(Mn,Yb). Introduction of magnetic field leads to substantial drop of the PbTe(Mn,Cr) resistivity of about 30% at T = 4.2 K. This is however much lower than in PbTe(Mn,Yb), where the effect amplitude reached 3 orders of magnitude. The effect disappears at T = 15 K. Possible mechanisms of the effect are discussed.

Thermopower of CeNi5 with strong f-electron instability: Band effects

Numerical calculations of the thermopower component Sd that is associated with the fine structure of the density of d states near the Fermi level are carried out for CeNi5 and its La analog. The estimates, together with earlier experimental data on the transport coefficients in Ce(MxNi1−x)5 solid solutions with M=Ga, Cu (0≤x≤0.4)), are used to analyze the behavior of the thermopower component Sf originating from the Ce valence instability. It is shown that as Ce crosses over from the state of its usual intermediate valence to the saturated-valence state, the feature near the Fermi energy, which represents a Lorentzian-shaped peak of the density of f states, transforms to a double-humped structure. The possibility of formation of a strongly correlated band of f states in Ce(MxNi1−x)5 in the crossover to the Ce saturated-valence state, accompanied by the opening of a quasi-gap Δ ∼ 400 K, is discussed.

Simultaneous determination of the electronic andchemical structures in CeNixCu5-x at highpressures

Combined x-ray diffraction and x-ray absorption results arepresented on the substitutional Ce valence transition inCeNixCu5-x as a function of applied externalpressure. No significant influence of pressure on the valencein the near-transition region of x is found in the crystalstructure or in the Ce K-edge position. This result isdiscussed in the context of valence instabilities inintermetallic Ce compounds.

Dependence of the CeNi 5 thermoelectric power on strong 4f-electron instability

Results of investigation of thermoelectric power component S f related to the valence instability of Ce were presented for solid solutions Ce(M x Ni 1− x ) 5 with M=Ga, Cu (0≤ x≤0.4) in the temperature range 4–900 K. It follows from the analysis of the S f ( T) behavior that when Ce changes from a state of ordinary intermediate valence to a saturated valence, the peculiarity near the Fermi level in the form of a density of states peak of Lorentzian shape transforms into a double-humped structure with quasi-gap formation, Δ∼400 K. The possibility of a certain delocalization of f-orbitals during the transition to the saturated valence of Ce has been discussed.