Showing papers on "Superexchange published in 2004"

Ferromagnetism in Fe-doped SnO2 thin films

Abstract: Thin films grown by pulsed-laser deposition from targets of Sn0.95Fe0.05O2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m2 kg−1, respectively. The 57Fe Mossbauer spectra show the iron is all high-spin Fe3+ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 μB, is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is proposed to explain the high Curie temperature.

Making a molecular wire: charge and spin transport through para-phenylene oligomers.

Abstract: Functional molecular wires are essential for the development of molecular electronics Charge transport through molecules occurs primarily by means of two mechanisms, coherent superexchange and incoherent charge hopping Rates of charge transport through molecules in which superexchange dominates decrease approximately exponentially with distance, which precludes using these molecules as effective molecular wires In contrast, charge transport rates through molecules in which incoherent charge hopping prevails should display nearly distance independent, wirelike behavior We are now able to determine how each mechanism contributes to the overall charge transport characteristics of a donor−bridge−acceptor (D−B−A) system, where D = phenothiazine (PTZ), B = p-oligophenylene, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), by measuring the interaction between two unpaired spins within the system's charge separated state via magnetic field effects on the yield of radical pair and triplet recombination product

Long-range electron transfer across Peptide bridges: the transition from electron superexchange to hopping.

Abstract: Long-range electron transfer rate constants for complexes of the type [(bpy)2RuIIL−Pron−apyRuIII)(NH3)5]5++ proline residues (n) varying from 0 to 9 were determined by complementary electron pulse radiolysis and flash photolysis techniques from the picosecond to the millisecond time scales. The activationless kmax values from both techniques coalesce into one data set. The distance dependence of the reactions is consistent with a smooth transition from a superexchange mechanism with attenuation constant β = 1.4 A-1 to a hopping mechanism with attenuation constant β = 0.17 A-1. The transition occurs between n = 3 and 4 prolines, and the virtual hopping rate constant at the shortest distance is about 1 x 106 times slower than that observed for the superexchange value.

Thermally Activated Conduction in Molecular Junctions

Abstract: We report temperature dependence measurements on the conductance of individual molecular wires. The results show for the first time in a molecular junction the theoretically predicted transition from coherent superexchange tunneling conductance to an activated hopping mechanism as temperature is increased.

Spin-controlled Mott-Hubbard bands in LaMnO3 probed by optical ellipsometry.

Abstract: Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of LaMnO3 in the range 0.5-5.6 eV at temperatures 50

Spectroscopic demonstration of a large antisymmetric exchange contribution to the spin-frustrated ground state of a D3 symmetric hydroxy-bridged trinuclear Cu(II) complex: ground-to-excited state superexchange pathways.

Abstract: The magnetic and electronic properties of a spin-frustrated ground state of an antiferromagnetically coupled 3-fold symmetric trinuclear copper complex (TrisOH) is investigated using a combination of variable-temperature variable-field magnetic circular dichroism (VTVH MCD) and powder/single-crystal EPR. Direct evidence for a low-lying excited S = (1)/(2) state from the zero-field split ground (2)E state is provided by the nonlinear dependence of the MCD intensity on 1/T and the nesting of the VTVH MCD isotherms. A consistent zero-field splitting (Delta) value of approximately 65 cm(-1) is obtained from both approaches. In addition, the strong angular dependence of the single-crystal EPR spectrum, with effective g-values from 2.32 down to an unprecedented 1.2, requires in-state spin-orbit coupling of the (2)E state via antisymmetric exchange. The observable EPR intensities also require lowering of the symmetry of the trimer structure, likely reflecting a magnetic Jahn-Teller effect. Thus, the Delta of the ground (2)E state is shown to be governed by the competing effects of antisymmetric exchange (G = 36.0 +/- 0.8 cm(-1)) and symmetry lowering (delta = 17.5 +/- 5.0 cm(-1)). G and delta have opposite effects on the spin distribution over the three metal sites where the former tends to delocalize and the latter tends to localize the spin of the S(tot) = (1)/(2) ground state on one metal center. The combined effects lead to partial delocalization, reflected by the observed EPR parallel hyperfine splitting of 74 x 10(-4) cm(-1). The origin of the large G value derives from the efficient superexchange pathway available between the ground d(x2-y2) and excited d(xy) orbitals of adjacent Cu sites, via strong sigma-type bonds with the in-plane p-orbitals of the bridging hydroxy ligands. This study provides significant insight into the orbital origin of the spin Hamiltonian parameters of a spin-frustrated ground state of a trigonal copper cluster.

Antisymmetric exchange in two tricopper(II) complexes containing a [Cu3(μ3-OMe)]5+ core

Abstract: Reaction of CuX2 (X− = Cl− or Br−) with 2 molar equivalents of 3{5}-(2,4,6-trimethylphenyl)pyrazole (HpzMes) in MeOH in the presence of NaOH yields [Cu3X(HpzMes)2(μ-pzMes)3(μ3-OMe)]X (X− = Cl− or Br−). Crystal structures of these compounds show almost identical triangles of Cu(II) ions, centred by a triply bridging methoxide ligand and with three edge-bridging pyrazolide groups. The mesityl substituents on the bridging pyrazolide ligands are arranged in HT, HH, TT fashion. χMT for both compounds decreases steadily with decreasing temperature, reaching 0.40 cm3 mol−1 K at 70 K before decreasing further below 40 K. This low temperature behaviour could not be interpreted using conventional superexchange Hamiltonians, but was reproduced by an alternative model that incorporated an additional antisymmetric exchange term. This interpretation was confirmed by the Q-band EPR spectra of the two compounds. NMR experiments show that the structures of these compounds are not retained in solution, in contrast to other closely related tricopper compounds. These are the first examples of triangular Cu(II) compounds bearing a [Cu3(μ3-OR)]5+ (R ≠ H) core motif, and the first triangular compounds showing antisymmetric exchange to have been analysed by both susceptibility and EPR measurements.

DNA electron transfer processes: Some theoretical notions

Abstract: Charge motion within DNA stacks, probed by measurements of electric conductivity and by time-resolved and steady-state damage yield measurements, is determined by a complex mixture of electronic effects, coupling to quantum and classical degrees of freedom of the atomic motions in the bath, and the effects of static and dynamic disorder. The resulting phenomena are complex, and probably cannot be understood using a single integrated modeling viewpoint. We discuss aspects of the electronic structure and overlap among base pairs, the viability of simple electronic structure models including tight-binding band pictures, and the Condon approximation for electronic mixing. We also discuss the general effects of disorder and environmental coupling, resulting in motion that can span from the coherent regime through superexchange-type hopping to diffusion and gated transport. Comparison with experiment can be used to develop an effective phenomenological multiple-site hopping/superexchange model, but the microscopic understanding of the actual behaviors is not yet complete.

Temperature effects on conduction through a molecular junction

Abstract: The measured effect of temperature on charge transport through individual chemisorbed 1-nitro-2,5-di(phenylethynyl--mercapto)benzene molecules spanning a gold electrode gap is reported for the range of 10–300 K. Conduction is dominated by coherent superexchange at low temperatures and shifts to an incoherent hopping process above bias-dependent temperature thresholds. The results are consistent with proposed theoretical mechanisms in which bias-dependent heat dissipation from inelastic vibrational scattering within the molecule raises the temperature of the junction above that of the surrounding system, which in turn leads to the shift in the prevailing conduction process with increasing bias.

Electron Delocalization in Nickel Metallic Wires: A DFT Investigation of Ni3(dpa)4Cl2 and [Ni3(dpa)4]3+ (dpa = Dipyridylamide) and Extension to Higher Nuclearity Chains

Abstract: The electronic structure of Ni3(dpa)4Cl2 (1) has been investigated within the framework of the density functional theory (DFT), using two types of exchange-correlation functionals and various basis sets. The “broken-symmetry” approach proposed by Noodleman for the characterization of electronic states displaying an antiferromagnetic coupling has been applied to 1. All calculations lead to the conclusion that the ground state results from an antiferromagnetic coupling between the terminal Ni atoms, both displaying a high-spin electronic configuration. The central Ni atom is in a low-spin configuration, but is involved in a superexchange interaction connecting the two magnetic centers. These results are in agreement with the assignments recently proposed by the group of F. A. Cotton on the basis of magnetic measurements. It is shown that the ground state electronic configuration calculated for 1 provides the trinickel framework with some delocalized σ bonding character. The observed geometry of 1 is accurat...

Theory of optical spectral weights in Mott insulators with orbital degrees of freedom

Abstract: Introducing partial sum rules for the optical multiplet transitions, we outline a unified approach to magnetic and optical properties of strongly correlated transition metal oxides. On the example of ${\mathrm{LaVO}}_{3}$ we demonstrate how the temperature and polarization dependences of different components of the optical multiplet are determined by the underlying spin and orbital correlations dictated by the low-energy superexchange Hamiltonian. Thereby the optical data provide deep insight into the complex spin-orbital physics and the role played by orbital fluctuations.

Orbital-only models: ordering and excitations

Abstract: We consider orbital-only models in Mott insulators, where the orbital–orbital interactions are either due to Jahn–Teller distortions or due to the Kugel–Khomskii superexchange. This leads to highly anisotropic and frustrated orbital Hamiltonians. For two-fold degenerate eg systems, both types of orbital interactions lead to the same form of the Hamiltonian—the 120° model. In both cases, the predicted symmetry of the orbital ordering is the same, although different from the one observed experimentally. The orbital operators that appear in the two kinds of orbital-only Hamiltonians are different. In the case of superexchange, the orbital degrees of freedom are represented by quantum pseudo-spin 1/2 operators. But when the interactions are Jahn–Teller mediated and the coupling with the lattice is strong, the orbital operators are essentially classical pseudospins. Thus as a function of the relative coupling strengths, a quantum-to-classical crossover is expected. For three-fold degenerate t2g orbitals, the Jahn–Teller coupling gives rise to a particular type of orbital compass models. We point out that fluctuations—whether due to quantum effects or finite temperature—are of prime importance for ordering in the 120° and orbital compass models. The fluctuations generally generate a gap in the orbital excitation spectrum. These orbital excitations—orbitons—are hybrid excitations that carry both a lattice Jahn–Teller and a magnetic Kugel–Khomskii character.

Orbital polarons versus itinerant e g electrons in doped manganites

Abstract: We study an effective one-dimensional (1-D) orbital $t\text{\ensuremath{-}}J$ model derived for strongly correlated ${e}_{g}$ electrons in doped manganites The ferromagnetic spin order at half filling is supported by orbital superexchange $\ensuremath{\propto}J$ which stabilizes orbital order with alternating ${x}^{2}\ensuremath{-}{y}^{2}$ and $3{z}^{2}\ensuremath{-}{r}^{2}$ orbitals In a doped system it competes with the kinetic energy $\ensuremath{\propto}t$ When a single hole is doped to a half-filled chain, its motion is hindered and a localized orbital polaron is formed An increasing doping generates either separated polarons or phase separation into hole-rich and hole-poor regions, and eventually polarizes the orbitals and gives a metallic phase with occupied $3{z}^{2}\ensuremath{-}{r}^{2}$ orbitals This crossover, investigated by exact diagonalization at zero temperature, is demonstrated both by the behavior of correlation functions and by spectral properties, showing that the orbital chain with Ising superexchange is more classical and thus radically different from the 1-D spin $t\text{\ensuremath{-}}J$ model At finite temperature we derive and investigate an effective 1-D orbital model using a combination of exact diagonalization with classical Monte Carlo for spin correlations A competition between the antiferromagnetic and ferromagnetic spin order was established at half filling, and localized polarons were found for antiferromagnetic interactions at low hole doping Finally, we clarify that the Jahn-Teller alternating potential stabilizes the orbital order with staggered orbitals, inducing the ferromagnetic spin order, and enhancing the localized features in the excitation spectra Implications of these findings for colossal magnetoresistance manganites are discussed

Synthesis, Crystal Structures, and Magnetic Properties of a Mono- and a Dinuclear Copper(II) Complex of the 2,4,6-Tris(2-pyridyl)-1,3,5-triazine Ligand

Abstract: The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) with Cu(BF4)(2)(.)3H(2)O and N,N-bis(3-aminopropyl)methylamine (Medpt) yielded the monomeric complex [(tptz)Cu-II(Medpt)](BF4)(2)(.)2MeOH (1) whereas reaction of tptz with CuCl2-2H(2)O yielded the dinuclear complex [{(CuCl2)-Cl-II}(tptz){(CuCl2)-Cl-II(MeOH)}] (2). The molecular structures of 1 and 2 were established by single-crystal X-ray diffraction studies. The copper ion in complex I is elongated octahedral with the nitrogen atoms of Medpt and the triazine nitrogen atom of tptz in the equatorial positions, and the two pyridyl nitrogen atoms of tptz in axial positions. The two Cu-II ions in complex 2 are bridged by tptz, coordinating to Cu1 in a terpyridine-like fashion and to Cu2 in a bipyridine-like coordination mode. The triazine nitrogen atom bound to Cu1 is coordinated in an equatorial position whereas the triazine nitrogen atom bound to Cu2 is situated in the apical position. The equatorial plane of Cu1 is coplanar with the 1,3,5-triazine plane whereas the equatorial plane of Cu2 is perpendicular to the 1,3,5-triazine plane. Intermolecular interactions in the solid state result in both compounds in "dimeric units" due to hydrogen bonding. Both complexes were investigated by variable-temperature magnetic susceptibility measurements. Through-space dipolar couplings and/or weak through-bond interactions in complex I result in a small intermolecular antiferromagnetic interaction (J = -0.09 cm(-1)). The intramolecular exchange coupling in the dinuclear complex 2 is antiferromagnetic (J = -2.5 cm(-1)). This antiferromagnetic interaction was analyzed in view of the molecular structure considering spin-polarization, superexchange and orbital-orthogonality. A spin-polarization pathway seems to be the leading contribution which does not involve the 1,3 bridging pathway. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Magnetic ordering in La1-xSrxMnO3-x/2 anion-deficient manganites

Abstract: The structural, magnetic, and electrotransport properties of La1−xSrxMnO3− x/2(0≤x≤0.30) manganites with perovskite structure are investigated experimentally as a function of oxygen deficiency. In the solid solutions La1−xSrxMnO3, a change in the type of symmetry of the unit cell is observed at x=0.125. Samples with x≤0.125 are characterized by an O′-orthorhombic unit cell, whereas samples with x>0.125 are characterized by a rhombohedral unit cell. The structural properties of the anion-deficient solid solutions La1−xSrxMnO3−x/2 are analogous to those of the stoichiometric system. It is assumed that, as the oxygen content decreases, La1− xSrxMnO3−x/2 anion-deficient solid solutions experience a series of successive magnetic phase transformations in the ground state: from an A-type (x=0) antiferromagnet to a cluster spin-glass-type inhomogeneous magnetic state (0.175>x≤0.30) through a two-phase (antiferromagnetic and ferromagnetic) state (0>x≤0.175). The anion-deficient solid solution with x=0.175 has the maximal value of the ferromagnetic component. As the oxygen deficiency increases, the resistivity of La1− xSrxMnO3−x/2 samples first decreases (up to a value of x=0.175), acquiring an activation character, and then increases (up to a value of x=0.30). In this case, none of the anion-deficient solid solutions exhibits a metal-semiconductor transition in the whole range of concentrations considered. A peak of magnetoresistance at a temperature below the point of magnetic ordering is observed only in the sample with x=0.175. The results of experiments carried out with a series of La1−xSrxMnO3−x/2 anion-deficient solid solutions are summarized in the concentration diagrams of the spontaneous magnetic moment and the critical temperature of magnetic phase transitions. Hypothetical magnetic phase states are pointed out. The experimental results obtained can be interpreted in terms of the phase-separation model and the competition between ferromagnetic and antiferromagnetic indirect superex-change interactions. It is assumed that Mn3+-O-Mn3+ indirect superexchange interactions in the orbitally disordered phase are positive in the case of octahedral coordination of manganese ions and are negative when the coordination of at least one Mn3+ ion is pentahedral.

Path-integral Monte-Carlo simulations for electronic dynamics on molecular chains: I. Sequential hopping and super exchange

Abstract: An improved real-time quantum Monte Carlo procedure is presented and applied to describe the electronic transfer dynamics along molecular chains. The model consists of discrete electronic sites coupled to a thermal environment which is integrated out exactly within the path integral formulation. The approach is numerically exact and its results reduce to known analytical findings (Marcus theory, golden rule) in proper limits. Special attention is paid to the role of superexchange and sequential hopping at lower temperatures in symmetric donor-bridge-acceptor systems. In contrast to previous approximate studies, superexchange turns out to play a significant role only for extremely high lying bridges where the transfer is basically frozen or for extremely low temperatures where for weaker dissipation a description in terms of rate constants is no longer feasible. For bridges with increasing length an algebraic decrease of the yield is found for short as well as for longer bridges. The approach can be extended to electronic systems with more complicated topologies including impurities and in presence of external time dependent forces.

Ligand K-edge X-ray absorption spectroscopy and DFT calculations on [Fe3S4]0,+ clusters: delocalization, redox, and effect of the protein environment.

Abstract: Ligand K-edge XAS of an [Fe3S4]0 model complex is reported. The pre-edge can be resolved into contributions from the μ2Ssulfide, μ3Ssulfide, and Sthiolate ligands. The average ligand−metal bond covalencies obtained from these pre-edges are further distributed between Fe3+ and Fe2.5+ components using DFT calculations. The bridging ligand covalency in the [Fe2S2]+ subsite of the [Fe3S4]0 cluster is found to be significantly lower than its value in a reduced [Fe2S2] cluster (38% vs 61%, respectively). This lowered bridging ligand covalency reduces the superexchange coupling parameter J relative to its value in a reduced [Fe2S2]+ site (−146 cm-1 vs −360 cm-1, respectively). This decrease in J, along with estimates of the double exchange parameter B and vibronic coupling parameter λ2/k-, leads to an S = 2 delocalized ground state in the [Fe3S4]0 cluster. The S K-edge XAS of the protein ferredoxin II (Fd II) from the D. gigas active site shows a decrease in covalency compared to the model complex, in the same o...

Effect of Dipolar Interaction on the Antiferromagnetic Resonance Spectra of NiO

Abstract: We have investigated the antiferromagnetic (AF) resonance modes (AFMR) of NiO, theoretically using a model that includes the effects of exchange, dipolar coupling, and a small cubic anisotropy, and experimentally using Brillouin scattering. Using only superexchange between next nearest Ni atoms the model accounts for the observed AF structure with a $[11\overline{2}]$ spin orientation. The model predicts that there are four, weakly coupled, AF lattices that should therefore exhibit eight AFMR modes. Because of degeneracies, only five distinct frequencies are predicted by the model. Three of these frequencies are consistent with the doublet observed by Raman scattering and the central peak reported in Brillouin experiments. Using Brillouin scattering we report the observation of the two missing modes.

Effect of spin disorder on magnetic properties of nanostructured Ni-ferrite

Abstract: This work presents a structural and magnetic study of NiFe2O4 nanoparticles. Powder samples with different particle sizes were made by annealing the fine powder at different temperatures. X-ray diffraction, transmission electron microscopy, and low temperature SQUID magnetometry experiments were carried out to characterize the samples. Results support a previously proposed model that the NiFe2O4 nanoparticle has a core/shell structure due to surface disorder. However, a detailed analysis of the data reveals that although the structural disorder in the surface of the particle is the major cause for forming a shell, the superexchange coupling is the driving force for the magnetic configuration, which creates a magnetically immobilized region near the surface region with thicknesses significantly larger than that of the structurally inhomogeneous layer on the surface. Also, the ease of magnetization of the core can be significantly affected. Since the crystalline and magnetic disorders commonly exist in the ...

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs†

Abstract: Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay constants (β) of the squared electronic coupling matrix element (VDA2) in transient, solv...

Nanoscale phase coexistence and percolative quantum transport.

Abstract: We study the nanoscale phase coexistence of ferromagnetic metallic and antiferromagnetic insulating (AFI) regions by including the effect of AF superexchange and weak disorder in the double exchange model. We use a new Monte Carlo technique, mapping on the disordered spin-fermion problem to an effective short range spin model, with self-consistently computed exchange constants. We recover ``cluster coexistence'' as seen earlier in exact simulation of small systems. The much larger sizes, $\ensuremath{\sim}32\ifmmode\times\else\texttimes\fi{}32$, accessible with our technique, allow us to study the cluster pattern for varying electron density, disorder, and temperature. We track the magnetic structure, obtain the density of states, with its ``pseudogap'' features, and, for the first time, provide a fully microscopic estimate of the resistivity in a phase coexistence regime, comparing it with the ``percolation'' scenario.

Pressure-induced onset of long-range magnetic order in two-dimensional spin-frustrated CuFeO 2

Abstract: Magnetic properties of two-dimensional spin-frustrated CuFeO 2 have been studied up to 19 GPa by means of 5 7 Fe Mossbauer spectroscopy. The partially disordered spin arrangement at ambient pressure in the 11-16 K range, transforms with pressure to a long-range ordered "5-sublattice" phase with a distinct T N , a similar role played by external magnetic field in neutron studies. This phase gradually substitutes for the "4-sublattice" magnetic ground state present at ambient pressure, reaching 100% at 19 GPa. Despite the presence of long-range order, this high pressure phase exhibits magnetic relaxation above 20 K attributed to the notwithstanding weak interplanar superexchange interaction. The dramatic twofold increase of T N at 19 GPa is explained in terms of the unusual increase of the intraplanar direct exchange J L caused by the anomalous anisotropic compression of CuFeO 2 in which c/a increases with pressure.

Electron-density studies of molecular magnetic materials

Abstract: For more than forty years, the experimental determination and analysis of electron densities have played a fundamental role in advances in the chemical bond concept. The present paper illustrates the application of this approach to the field of molecular magnetism with examples that recently appeared in the literature. Particular attention is attached to several classes of materials, purely organic free radicals, coordination compounds and organometallic complexes, which exhibit specific magnetic behaviors. It is shown to what extent the electron-density analysis can shed light on bonding aspects that are closely related to magnetic couplings. Relations between spin delocalization, spin polarization, superexchange and the characteristics of the electron density are described. The use of the topological theory of 'atoms in molecules' allows the possible magnetic interaction pathways to be located and defined, especially through weak intermolecular contacts. The complementarity with polarized neutron diffraction and spin-density modeling techniques is particularly evident from the chosen examples.

A temperature-dependent single-crystal Raman spectroscopic study of fayalite: evidence for phonon-magnetic excitation coupling

Abstract: The polarized single-crystal Raman spectrum of synthetic fayalite, Fe(2)SiO(4), was recorded between 5 and 773 K in order to investigate its lattice dynamic behavior. A broad absorption envelope is observed at wavenumbers between 800 and 960 cm(-1) and it contains two intense bands at 816 and 840 cm(-1) at 293 K in the (cc) spectrum. The integral area of the envelope decreases upon cooling from 293 K and reaches a minimum around 55 K. It then increases again with a further decrease in temperature down to 5 K. It is proposed that the envelope in the (cc) spectra consists of seven different modes, some of which are symmetry-forbidden, that arise from combination scattering of nonsymmetric internal SiO(4)-stretching modes of B(ig) symmetry (i = 1, 2, 3) and low-energy excitations. The individual modes can be observed under different polarizations and agree in number and wavenumber with those obtained by fitting the broad envelope with Lorentzians. An analysis of the Raman spectrum as a function of temperature, using the known magnetic properties of fayalite, allows the assignment of the low-energy excitations to short-range magnetic interactions. Modulation of the Fe(2+)(1)-Fe(2+)(2) exchange energy leads to phonon-magnetic excitation coupling and the main role in the Fe(2+)(1)-Fe(2+)(2) magnetic interaction occurs via superexchange through the oxygens. The magnetic excitations are not magnons in the usual sense, that is as quasiparticles having a long wavelength in an ordered system. The degree of observed broadening of the SiO(4)-stretching modes is consonant with a Fe(2+)(1)-Fe(2+)(2) exchange energy of 4.7 cm(-1) presented by Schmidt et al. (1992). At temperatures above 300 K the line width of the mode at 840 cm(-1) decreases slightly, whereas those of low energy lattice modes increase. This suggests that a decrease in mode broadening due to weakened magnetic interactions compensates any thermally related broadening. Complete Fe(2+) spin disorder may not be reached until at least 530 K. Results from this study show that estimates of third-law entropies for silicates using simple crystal-chemical considerations that do not account for magnetic properties cannot give accurate values for many transition-metal-containing phases.