Transition Metal Ions

BY ALESSANDRO BENCINI AND CLAUDIA ZANCHINI

1 Introduction

After some years the task of preparing the Specialist Periodical Reports on Transition Metal Ions returns back to Firenze. It is with great pleasure that we are now starting to prepare this issue.

The basic format of the chapter will be left unchanged, but in order to cover some recent applications of the e.s.r. spectroscopy, we add some new paragraphs. In the Extended Systems section we will review application of e.s.r. spectroscopy to compounds which contain transition metal ions in three and lower dimensional magnetic lattices. In the Semiconductors section we will resume the main information that e.s.r. spectroscopy can give in semiconducting solids containing transition metal ions either as main constituents or as impurities. In another section, called Superconductors, we will try to resume the main applications of e.s.r. in a research field which is largely attracting the attention of many researchers after the discovery of high Tc superconductors formed by copper oxides.

Due to the extremely large number of papers reporting the application of e.s.r. in many fields of physics and chemistry we cannot be exhaustive, but we will mention here the most relevant applications. Particular emphasis will be given to single crystal measurements and to works in which the physical properties of the compounds are investigated with the largest number of techniques. Papers written in languages different from English, in general, will not be referred, since we, like many other researchers, have a lot of difficulties in moving through the babel of human languages.

Review articles covering particular aspects of the applications of the e.s.r. spectroscopy will be reported in each section.

Without any doubt some overlap will occur with other chapters of this book, but this already occurred in the past and did not cause any damage to the reader. This time, perhaps, the reader will be much more involved in understanding our English, which, we hope, will not be too italian-like.

Two books dealing with e.s.r. spectroscopy in exchange coupled systems appeared. The first oneby Yablokov, Voronkova and Mosina, in russian, is entitled "Paramagnetic Resonance of Exchange Clusters"; the second one by Bencini and Gatteschi , entitled "Electron Paramagnetic Resonance of Exchange Coupled Systems", is intended to collect in one place as much information as possible on the use of e.s.r. spectroscopy in the analysis of systems in which two or more spins are magnetically coupled.

2 General

The conventional technique of detection of an e.s.r signal makes use of the modulation of the magnetic field. To achieve the optimum signal one must then use a modulation amplitude comparable to the linewidth, which leads, however, to lineshape distortion or to a decrease in the signal to noise ratio especially for broad resonances. Alternative methods to field modulation in e.s.r. spectroscopy have been reviewed by Hyde, Sczaniecki and Froncisz. In particular the use of Loop Gap Resonators and the Electron Paramagnetic Rotary Resonance is emphasized. This latter technique uses the resonance that occurs when the difference of two incident microwave frequencies matches the precession frequency of the rotating frame, in the Bloch formalism.

In the field of pulsed e.s.r. a detection technique based on the measurement of the longitudinal magnetization, Mz, is described (LOD-PESR). This technique can be used to measure the rapid changes of Mz during short microwave pulses of duration tp ≤ T1, T2, and flip angles β ≤ π. Using this technique, data acquisition of the echo envelope modulation can be initiated after much shorter time than in a conventional electron spin echo experiment.

Other experiments using pulsed techniques can be found in references 5,6.

The double modulation electron spin resonance spectroscopy (DOMESR) has been analyzed both theoretically and experimentally.

Apparatus for high pressure measurements and for low temperature Q-band measurements have been described.

A review on the future developments of the e.s.r. instrumentation has appeared.

The measurement of e.s.r. spectra of ions having large zero field splitting of the ground state is attracting the attention of researchers. For this purpose submillimeter wave e. s. r. in high magnetic fields and ODMR techniques should be applied. On this subject we cite the measurements on CsFeC13 at 4.2 K using FIR laser at 103.6 cm-1 and 84.2 cm-l in pulsed magnetic fields up to 17 T, and the measurement of the spin hamiltonian parameters of the triplet luminescent state of a Ba3(VO4)2 crystal. Spectrometers working with frequencies up to 16 cm-l and with magnetic fields up to 20 T have been described in references 13,14.

An apparatus which can be derived from a conventional e.s.r. spectrometer has been used to monitoring the superconducting transition in high Tc oxides. The experiment consists in the measure of the imbalance of the bridge when the sample is loaded in the cavity arm of a four arm microwave bridge X-band e.s.r. spectrometer.

The application of e. s. r. spectroscopy in microscopy, dating and dosimetry is reviewed in reference 16. In particular the use of e. s. r. microscopy and imaging are evidenced. The construction of a portable e.s.r. spectrometer to be used for microscopy, dating and dosimetry is described.

In the field of the analysis of defect states in semiconductors some applications of photo-e.s.r. techniques and the effects of non-resonant absorption due to free carriers has been reviewed. The two techniques have been found useful in the identification of the nature of the defects in semiconductors and in the determination of the free carrier concentration.

Pilbrow reviewed the use and possibilities of the electron paramagnetic resonance spectroscopy of transition metal ions.

Theory. The calculation of the spin hamiltonian parameters from molecular orbital functions obtained from some SCF procedure is certainly the best way to check the quality of the function describing the ground state of the molecule. Among the theoretical models available up to date for computing the electronic structure of open shell transition metal complexes the MS-Xa model still receives much attention. Ruiz-Lopez and Natoli have computed optical excitations, g values, and XANES in vanadyl porphyrins. Here the principal g values are computed using the standard perturbation solution for a D4h d1 system and taking the covalency parameters from the ground state Xa-wavefunction. Barriuso, Aramburu and Moreno computed the electronic structure of a Ni+ impurity into LiF. The calculations give a reasonable correlation between the Ni-F bond distance and optical transitions and hyperfine values.

Other molecular orbital methods have been applied to compute the g tensor in ferricytochrome c and azidomyoglobin and the g tensor and the anisotropic part of the hyperfine tensor in vanadyl complexes.

The use of approximate d orbitals of double zeta quality allows one to compute the Racah parameters and to be used in Ligand or Crystal Field calculations. Although some criticism appeared on the choice of the basis function the method has been applied with some success, for example, to iron(II).

Ligand Field calculations have been also applied to the e.s.r. spin hamiltonian parameters in a number of complexes.

Some troubles seem to arise among Chinese researchers on the applicability of e.s.r. techniques to the determination of crystal structure parameters in particular from zero field splitting parameters and from hyperfine and superhyperfine coupling constants. It is our opinion that, apart from the accuracy with which these parameters are measured, a serious lack in obtaining precise measurements of the crystalline parameters is the theoretical model (s) one has to use to establish the required correlation. Ligand Field models, either including vibronic effects, are always overparameterized, except for cubic site symmetries, and Molecular Orbital calculations, at any level of sophistication, often fail to give accurate descriptions of the radial function of the ground state needed to evaluate hyperfine terms. As a matter of fact, however, e.s.r. spectroscopy is invaluable in individuating impurities or defect sites in solids and the correlation between geometry of the coordination site and e.s.r. parameters must be used to guess, with all its uncertainty, the local site symmetry of the sites, having in the mind that X-ray absorption or EXAFS spectroscopy cannot be used in these cases. Of course much more information should be obtained in all the cases in which endor spectra can also be measured.

Analysis of the Spectra and Computing. Misra has developed a least-squares-fitting procedure to analyze single crystal e.s.r. and endor spectra with the spin hamiltonian

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

allowing for the non coincidence of the g, D, Ai, gi, and Pi tensors. The procedure is based on a second order calculation of the transition energies, AEi, and minimisation of the function

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where the summation runs over all the observed (e.s.r., endor) lines and δi is the effective weight factor for the ith line position, and it is related to the uncertainty in the measure of the static magnetic field and position of the crystal with respect to the field. This procedure is a generalization of a previously reported one and based on first order calculations. As an example of application, the spectra of VO2+ doped K2C2O4•H2O were fitted using the spin hamiltonian parameters obtained from the first order formulae as starting parameter values. Unfortunately no evaluation of the standard deviations of the measured spin hamiltonian parameters was done, thus making difficult any meaningful test.

Rudowicz developed the expressions which relate the g and D tensors for s = 2 ions at sites of various symmetries to the spin-orbit coupling constant, spin-spin coupling constant, and the energy level splitting of the 5v ground multiplet. Applications to Fel-xAxF2 (A = Mn, Zn, Mg) and FeCl2•2H2O have been discussed.

Expressions for the position of forbidden hyperfine lines, ΔM=±1, Δm=±1, where M and m are the electron and nuclear azimuthal spin quantum numbers, have been developed to third-order in perturbation theory.

A high order perturbation formula has been worked out for computing the g values of d8 ions in trigonal symmetry and the spin hamiltonian parameters of d3 ions possessing C2 symmetry.

Polycrystalline powder and solution spectra are still more easy to handle than single crystal ones and a number of papers deal with the development of computer programs to simulate them in order to extract the principal values of the spin hamiltonian parameters.

E.s.r. and endor powder spectra for S = 1/2 spin systems including hyperfine and superhyperfine coupling from nitrogen nuclei have been considered by Greiner and Baumgarten.

A fitting procedure of polycrystalline powder spectra which uses a least-squares method has been developed by She, Chen and Yu and applied to the fitting of copper(II) spectra with and without hyperfine splitting. The parameters used in the calculations are the principal values of the g and A tensors and the linewidths. The methods can use energy levels computed using perturbation solutions of the spin hamiltonian or complete matrix diagonalization. Also in this case, however, no estimate of the standard deviations on the parameters neither of the correlation between them has been reported. In particular it is this latter quantity which gives any physical reality to the best fitting parameters.

Multifrequency e.s.r. spectra of solutions of copper(II) complexes have been studied with the purpose of minimizing the uncertainty in the determination of motional and magnetic parameters. Always for solutions, an analysis of the effect of the temperature on the e. s. r. spectra of hexaaquoions of several transition metal ions has been reported.

Phase Transitions. -The nature of the phase transitions in a series of dipropionates of formulae Ca2M(C2H5COO)6 M = Pb, Ba, Sr have been investigated using single crystal measurements on Mn2+ doped samples. In particular Ca2Pb(C2H5COO)6 undergoes a paraelectric-ferroelectric second order phase transition around Tc1 = 343 K and a first order phase transition around Tc2 180 K. The e.s.r. spectra revealed that the structure is still tetragonal both above and below Tc2. In the ferroelectric phase below Tel the critical behaviour was followed by measuring the temperature dependence of the angular splitting of the maximum field along the z axis of one manganese ion. The high field resonance separates into two maxima below Tel, and the variation with temperature of the high field resonance follows the equation

Θ = Θ0(Tc1-T)β (3)

where Θ0 = 1.148° and β = 0.51(3). Θ being the angle with the c direction. The ordering of the propionate molecules around the manganese ion in the series of propionates is discussed.

The interaction between the paramagnetic probe and the lattice has been studied, within the Crystal Field theory, for chromium(III) doped RbCdF3 below the phase transition.

The phase transition in the (Cd,Mn)S, a semimagnetic semiconductor, was also studied. In this and in the (Zn,Mn)S system a transition from a paramagnetic state into a spin-glass-like phase, determined by the antiferromagnetic interaction between the manganese(II) ions, occurs at a Neel temperature which depends on the manganese(II) concentration, TN(xMn). The temperature dependence of the linewidth of the observed signal was found to follow the law

B(T) = A[T-TN(xMn)]-1 (4)

for small differences, with l/A = 0.60(4) K-1T-1. An equation showing the variation of TN with xMn is also given.

Studies of the phase transitions in K2(Cr,Cd)F4, KSc(MoO4)2, (Zn, Mn)TiF6•6H2O, (NH4)2Cd2(SO4)3, and Ba(Ti,Fe)O3 were also reported.

Jahn-Teller Effect. The theory of the Jahn-Teller interactions in metallocenes has been developed to include orthorhombic Ligand Field components and applied to the analysis of cobaltocene, chromocene and ferricenium cation.

The e.s.r. spectra of copper(II) doped Cd(NH4)(SO4)2•6H2O have been studied both on single crystals and on polycrystalline powder down to 4.2 K. The principal values of the g and A tensors were found to be temperature dependent while the principal directions of the two tensors remained parallel within experimental error. The temperature dependence of the spectra was attributed to a pseudo-Jahn-Teller distortion experienced by the Cu(H2O)62+ chromophore and rationalized using a model with three adiabatic potential valleys. The relative ordering of these three valleys arising from the Jahn-Teller distortion was found to be similar to that observed in the Tutton salt but with the higher energy distortion at significantly smaller energy.

Resonance Raman spectroscopy studies performed on radical cations of nickel(II), copper(II), and iron(III) chloride complexes of octaethyl-and tetraphenyl-porphyrins have been interpreted by a pseudo-Jahn-Teller mixing of the a1u and a2u molecular orbitals (D4h labelling) via the A2g vibrational mode. These results are in agreement with previously reported e.s.r. spectra of a cobalt(II) nitrosyl complex in which the observed hyperfine coupling with both the cobalt nucleus and protons of the saturated ring were rationalized through a a1u/a2u admixture.