Mixed valency in organic charge transfer complexes

Mixed-valence (partial charge transfer state) and segregated stacking are the key factors for constructing organic metals. Here, we discuss the ionicity phase diagrams for a variety of charge transfer systems to provide a strategy for the development of functional organic materials (Mott insulator, semiconductor, superconductor, metal, complex isomer, neutral-ionic system, etc.).     

Mixed valency character of bismuth in ferrite lattices

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Mixed valency and magnetism in cyanometallates and Prussian blue analogues

Prussian blue (PB) is a well-known archetype of mixed valency systems. In magnetic PB analogues {CxAy[B(CN)6]z}.nH2O (C alkali cation, A and B transition metal ions) and other metallic cyanometallates {Cx(AL)y[B(CN)8]z}.nH2O (L ligand), the presence of two valency states in the solid (either A-B, or A-A' or B-B') is crucial to get original magnetic properties: tunable high Curie temperature magnets; photomagnetic magnets; or photomagnetic high-spin molecules. We focus on a few mixed valency pairs: V(II)/V(III)/V(IV); Cr(II)/Cr(III); Fe(II)-Fe(III); Co(II)-Co(III); Cu(I)-Cu(II); and Mo(IV)/Mo(V), and discuss: (i) the control of the degree of mixed valency during the synthesis, (ii) the importance of mixed valency on the local and long-range structure and on the local and macroscopic magnetization, and (iii) the crucial role of the cyanide ligand to get these original systems and properties.     

Electron spin-lattice relaxation in oxide superconductors and fullerides

Using the original technique of measuring very short spin-lattice relaxation times T₁, the electron-spin relaxation has been investigated in a number of HTSC materials and related phases. The relaxation of Gd³⁺ ions in GdBa₂Cu₃O_6+x is shown to be affected by both quasiparticle and antiferromagnetic fluctuations, the latter being increased at lower oxigen content. The pseudo-spin gap of about 200 K is found for both contributions. For Cu²⁺ EPR in YBaCuO, the T₁ values reveal several types of paramagnetic centers including the green phase Y₂BaCuO₅. In the fulleride RbC₆₀, the measuring of T₁ and T₂ yielded additional support for 1D conductivity and allowed to monitor the metal-insulator phase.     

Superconductivity and electrical resistivity in alkali metal doped fullerides: Phonon mechanism

We consider a two- peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies from the dynamical matrix for the intermolecular alkali- C₆₀ phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random phase approximation. Transition temperature,T _c, is obtained in a situation when the free electrons in lowest molecular orbital are coupled with alkali-C₆₀ phonons as 5 K, which is much lower as compared to reportedT _c (≈ 20 K). The superconducting pairing is mainly driven by the high frequency intramolecular phonons and their effects enhance it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It is inferred from the two- peak model for phonon density of states that high frequency intramolecular phonon modes play a major role in pairing mechanism with possibly some contribution from alkali-C₆₀ phonon to describe most of the superconducting and normal state properties of doped fullerides.     

Electrical transport in rare-earth orthochromites

The electrical conductivity (σ) and Seebeck coefficient (

) measurements on pressed pellets of La, Nd and Sm orthochromites are reported for the temperature range 300 to 1000K. These orthochromites exhibit extrinsic, p-type conduction in the whole temperature range. In both measurements, breaks define a transition temperature (T_B), which has been predicted to be due to some dielectric anomaly. Seebeck coefficient measurements show that thermal generation of charge carriers begins above T_B.     

Electrical transport in heavy rare-earth vanadates

This paper reports measurements of electrical conductivity () and Seebeck coefficient (S) between 300 and 1250 K and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) between 300 and 1200 K, together with X-ray diffraction studies of heavy rare-earth vanadates (RVO₄ with R=Tb, Dy, Ho, Er and Yb). All these vanadates have been found to have a tetragonal unit cell. The DTA study shows a flat dip in the temperature interval 1075 to 1300 K, indicating a possible structural phase transition of these compounds. Practically no weight loss has been observed in TGA from 300 to 1200 K in any of the vanadates. All RVO₄ are semiconducting materials with the room-temperature value lying in the range 10^–12 to 10^{–3 –1} m^–1, becoming of the order of 10^{–2 –1} m^–1 around 1000 K. The electrical conductivity of all vanadates exhibits an exponential increase in the temperature intervals 420 K toT ₁ andT ₁ toT ₂, with different values of the activation energy. A log againstT ^–1 plot shows a peak aroundT ₃ and drops to a minimum value aroundT ₄, before increasing again with temperature.T ₄ >T ₃ >T ₂ >T ₁ are different for different vanadates and these are termed break temperatures.T ₄ lies well within the temperature range of the DTA peak and can be termed the phase transition temperature. In the lower temperature interval the electrical conduction is essentially extrinsic. The localized charge carriers on defect centres conduct by a hopping mechanism. The defect centres are V⁴⁺ ions in all vanadates with R⁴⁺ centres in some of them. It is concluded that in the temperature intervalT ₁ <T <T ₂ the conduction mechanism is of the intrinsic band type, with oxygen 2p and vanadium 3d as the valence and conduction bands, respectively. Related parameters like the energy band gap and the mobilities of the charge carriers have also been evaluated. The low values of mobility suggest that large polarons with intermediate coupling are the charge carriers rather than bare electrons in the intrinsic region. All these vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller.     

Electrical transport in light rare-earth vanadates

This paper reports electrical transport studies of rare-earth vanadates (RVO₄ with R = Ce, Pr, Nd, Sm, Eu and Gd), prepared by solid state reaction and characterized by X-ray diffraction studies. TGA study (300 to 1200 K) shows no weight loss; possible phase transitions in the range 1075 to 1300 K have been indicated by DTA studies. All these vanadates are typical semiconducting compounds with room temperature electrical conductivity (σ) lying between 10⁻⁴ and 10⁻²Ω⁻¹m⁻¹. Measurements of σ and the Seebeck coefficient (S) are reported in the temperature interval 400 to 1200 K. Two linear regions 400 to T ₁K and T ₁ to T ₂K have been obtained from the log σ against T ⁻¹ as well as the S against T ⁻¹ plots followed by a peak around T ₃ and minima around T ₄K. T ₄>T ₃>T ₂>T ₁, are different for different vanadates. It has been concluded that in the interval 400 to T ₁K, conduction is of the extrinsic hopping type with Ce⁴⁺ in CeVO₄, Pr⁴⁺ in PrVO₄ and V⁴⁺ in Nd to Gd vanadates as dominant defect centres. In the temperature interval T ₁, to T ₂K, the conduction has been shown to be of the intrinsic band type in all vanadates with polarons of intermediate coupling strength as the dominant charge carriers. Above T ₂ all vanadates tend to become metallic, but before this is achieved the phase change makes the conductivity smaller. T ₄ is close enough to the temperature corresponding to the DTA peak to be termed the phase transition temperature.     

Electrical transport in light rare-earth molybdates

Rare-earth molybdates of the type R₂(MoO₄)₃ with R=La, Ce, Pr, Nd, Sm and Eu were prepared and characterized, and the electrical conductivity, and Seebeck coefficient, S in the temperature range 450–1200 K were measured. These molybdates are concluded to be insulating solids with a band gap which increases slowly going down the series from 2.30 eV for La molybdate to 3.20 eV for Eu molybdate. The plots of log and S versus T ^–1 show, in general, three linear regions with two break temperatures T ₁ and T ₂ occurring due to a change in the conduction mechanism. At higher temperatures the intrinsic conduction in these solids occurs via a band mechanism. The O^2– 2p and Mo⁶⁺ 4d orbitals form the valence and conduction bands, respectively. These bands are the main support of conduction in La, Sm and Eu molybdates; however, for Ce, Pr and Nd molybdates 4f ⁿ levels fall within the band gap and become very effective in electrical conduction. The main charge-carrying entities seem to be electrons in Ce, Pr and Nd molybdates and holes in La, Sm and Eu molybdates. On the basis of mobility calculations of charge carriers it is concluded that the charge carriers in these bands become polarons which are, in fact, the charge carrying entities. At lower temperatures electrical conduction is mainly extrinsic. Cerium molybdate shows a semiconductor-semimetal transition around 940 K.     

Electrical conduction in light rare-earth tungstates

This paper reports the results of electrical conductivity (σ) and thermoelectric power (H) of light rare-earth tungstates in the temperature range 600–1200K. Holes are the dominant charge carriers over the whole studied temperature range for Nd, Sm and Gd tungstates. However, in the case of La, Ce and Pr tungstates, the conduction is dominated by electrons at lower temperatures, but above 950K in La, 800K in Ce and 950K in Pr the dominant charge carriers become holes. A sharp break and change in the log σ vs 1/T slope occurs in La, Ce and Pr tungstates around the same temperature at which the dominant charge carrier changes from electrons to holes. In the case of Nd, the conductivity anomaly occurs around 1020K without any change in the nature of the charge carrier. The data have been analysed using band theory.     

Electrical transport in light rare-earth orthochromites

Electrical conductivity, , Seebeck coefficient, S, and dielectric constant, , measurements on the pressed pellets of six light rare-earth orthochromites, RCrO₃, where R = La, Pr, Nd, Sm, Eu and Gd, have been carried out in the temperature range 300 to 1000 K. These are essentially electronic conductors, exhibiting p-type extrinsic semiconducting nature in the studied temperature range. The extrinsic charge carriers (holes) originate from Cr⁴⁺ centres which are present due to native defects in these solids. Their room-temperature electrical conductivities lie in the range 10^–7 to 10^{–5 –1} cm^–1, which become of the order of 10^{–2 –1} cm^–1 near 1000 K. The conductivity is a maximum in LaCrO₃ and drops across the RCrO₃ series, with SmCrO₃ being an exception. The mechanism involved in the electrical transport is the hopping of holes from Cr⁴⁺ centres to neighbouring Cr³⁺ ions. The activation energy of transport is nearly 0.3 eV. Typical hopping mobility of the holes is of the order of 10² cm^–2 V^–1 sec^–1 at 325 K and of the order of 10 cm² V^–1 sec^–1 at 1000 K. The mobility activation energy of the holes in a typcial RCrO₃ decreases with temperature due to the smoothing of the potential barriers between Cr⁴⁺ and Cr³⁺ sites. Several discontinuities are observed in the T against T ^–1 and S against T ^–1 plots of RCrO₃. The anomalies which these discontinuities reflect here have also been indicated.     

Mixed valency in polynuclear MnII/MnIII, MnIII/MnIV and MnII/MnIII/MnIV clusters: a foundation for high-spin molecules and single-molecule magnets

Mixed-valent Mn/O dinuclear and polynuclear molecular compounds containing MnIII are almost without exception trapped valence. Large differences between the strengths of the exchange interactions within MnIIMnIII, MnIIIMnIII and MnIIIMnIV pairs lead to situations where MnIIIMnIV interactions, the strongest of the three mentioned and antiferromagnetic in nature, dominate the intramolecular spin alignments in trinuclear and higher nuclearity mixed-valent complexes and often result in molecules that have large, and sometimes abnormally large, values of molecular spin (S). When coupled to a large molecular magnetoanisotropy of the easy-axis-type (negative zero-field splitting parameter, D), also primarily resulting from individual Jahn-Teller distorted MnIII centres, such molecules will function as single-molecule magnets (molecular nanomagnets). Dissection of the structures and exchange interactions within a variety of mixed-valent Mnx cluster molecules with metal nuclearities of Mn4, Mn12 and Mn25 allows a ready rationalization of the observed S, D and overall magnetic properties in terms of competing antiferromagnetic exchange interactions within triangular subunits, resulting spin alignments and relative orientation of MnIII JT axes. Such an understanding has provided a stepping stone to the identification of a 'magnetically soft' Mn25 cluster whose groundstate spin S value can be significantly altered by relatively minor structural perturbations. Such 'spin tweaking' has allowed this cluster to be obtained in three different forms with three different groundstate S values.     

Electrical conduction in non-metallic rare-earth solids

Schematic energy band diagrams for the genesis of charge carriers in non-metallic rare-earth solids have been presented. It has been shown that positions of 4f bands have significant effect on the genesis and nature of charge carriers, their conduction mechanism and magnitude of electrical conductivity () and Seebeck coefficient (S) of the solid. Relevant relations have been given for both and S in different situations. Experimental data on rare-earth sesquioxides (R₂O₃), rare-earth tungstates [R₂(WO₄)₃] and rare-earth molybdates [R₂(MoO₄)₃] in the intrinsic range have been explained as examples for the validity of energy band diagrams.     

Structural peculiarities of AC60 (A=K,Rb) fullerides

The strain of the C₆₀ molecule in the polymeric-like phase was described. It is shown that a small rotation of the C₆₀ molecule is possible at the phase transition. Numerical calculation of AC₆₀ powder neutron diffraction spectra for the Fm3m phase was made.     

Electrostatic interaction forces in aqueous salt solutions of variable concentration and valency

We use atomic force microscopy (AFM) to determine electrostatic interactions between Si tips and Si wafers in aqueous electrolytes of variable composition. We demonstrate that dynamic force spectroscopy (DFS) in the frequency modulation (FM) mode with stiff cantilevers and sharp tips allows for a continuous detection of the tip-sample interactions without mechanical jump-to-contact instability and with substantially higher lateral resolution than standard colloidal probe measurements. For four different species of salt (NaCl, KCl, MgCl(2), CaCl(2)) we find repulsive electrostatic forces at the lowest salt concentrations (1 mM) that become progressively screened until they are dominated by attractive van der Waals forces at the highest concentration (100 mM). For the divalent cations the crossover from repulsive to attractive forces occurs at lower concentrations than for monovalent cations. Surface charges extracted from fits to standard Poisson-Boltzmann double layer theory indicate a rather weak dependence of the surface charge on the ion concentration. The high lateral resolution of our approach is illustrated by a 2D force field measurement over a patterned surface of a supported lipid bilayer on a mica substrate.     

Electrical transport in heavy rare-earth iron garnets

The measurements of electrical conductivity () in the temperature range 450 to 1250 K and thermoelectric power (S) in the temperature range 600 to 1200 K of sintered pressed pellets of rare-earth iron garnets (REIG) with a general chemical formula RE₃Fe₅O₁₂ (where RE=Y, Gd, Dy, Ho, Er and Yb) are reported. Values corresponding to the crystalline state have been evaluated employing pore fraction correction. It is observed that plots of log T againstT ^–1 are linear with breaks in the slopes of temperatureT ₁ (lying between 560 and 578 K) andT ₂ (~ 1000 K). However, plots ofS againstT ^–1 are linear over the entire temperature range. The results have been discussed using the usual electrical transport theories and it has been concluded that electrical conduction in these solids up to a temperature of 1250 K is extrinsic in which holes localized on Fe³⁺ sites (Fe⁴⁺ centres created by native defects) conduct via a thermally activated hopping mechanism. Mobility activation energy, mobility and the number of such centres in each garnet are also evaluated.     

Luminescence in the rare-earth ferroelectric-ferroelastic gadolinium molybdate

The present paper discusses a new result of photoluminescence in gadolinium molybdate and its behaviour in the temperatural region near the phase transition. The special temperature dependence of photoluminescence showed that the luminescence spectra are highly congested between $\text{λ = 4700}\text{A}\text{?}$ and 8000 Å which do not exhibit temperature dependence. Only the peak at $\text{λ = 5470}\text{A}\text{?}$ has a temperature dependence. We have also studied photoconductivity in this material. Our results indicate that the peak at $\text{λ = 5470}\text{A}\text{?}$ can only be explained as an optical transition for a free electron between bands. The electrons are captured at a recombination center. All experimental dependences are discussed and found to be in agreement with existing theories for such processes.