Transition Metal Ions

BY ALESSANDRO BE NCI NI AND CLAUDIA ZANCHINI

1 Introduction

E. s. r. spectroscopy has been applied in a number of research fields mainly as a diagnostic tool for determining the spin and/or the oxidation state of the transition metal ion. A large number of papers therefore appeared, dealing with straightforward applications of e.s.r., while in other cases e.s.r. was applied to systems which were found to be too complicated to permit a complete spectroscopic characterization. An example of these latter systems are the transition-ion containing clusters found in living organisms (usually formed with iron and manganese) and their synthetic analogues, whose synthesis and crystallographic characterization have received much attention in the last few years. In order to rationalize this harvest of data we follow (with minor modifications) the classification scheme adopted in issue 12B. In part 1 we will report review articles of general interest or books; in part 2 attention will be given to "areas of interest" like studies on phase transitions, mixed valence systems, superconductors, etc.; in part 3 and subsequent sections the systematic of e.s.r. is presented classifying the systems according to the spin of the transition metal ion. Only articles written in English will be generally reviewed and we will try to put emphasis on papers reporting on rather uncommon spin systems or papers in which a characterization as detailed as possible has been performed, especially when data coming from more than one spectroscopic and physico-chemical technique were interpreted together. In order to avoid as much as possible the overlap among different areas of interest we have separated complexes in different sections. Oligonuclear complexes containing mixed valences (generally delocalized) have been grouped in the Mixed Valence Systems section, while those exhibiting long range exchange interactions have been reported in the Extended Systems section. Magnetic exchange interactions will be represented with the spin hamiltonian H = Jij Si • Sj. A negative value of Jij indicates a ferromagnetic interaction. Readers chiefly interested in the nature of the transition ion or in high nuclearity systems must therefore check more than one section..

The book from J.R. Pilbrow entitled Transition Ion Electron Paramagnetic Resonance gives an up to date account of the applications of the e.s.r. spectroscopy to transition metal ions. The book covers the fundamental aspects of e.s.r. of transition metal ions and also presents the principles of both CW and pulsed techniques and zero-field e.s.r. One chapter is also devoted to the principles and techniques of simulation of e.s.r. spectra.

Several review articles dealing with various applications of e.s.r. spectroscopy and related techniques appeared. Particularly interesting is the article by Trautwein et al. in which Mossbauer, e.s.r., and magnetic susceptibility studies on iron-containing proteins and their synthetic analogues are reviewed. Applications of e.s.r. to study the structure of impurities in insulators and in antiferromagnetic solids and to investigate the surface structure of catalysts. An extensive review article outlines the nature of the information which can be obtained from the subset of spin-spin interactions that yield additional lines in the e.s.r. spectra. Ferromagnetic e.s.r. is considered in ref. and in ref. NMR and e.s.r. in two-dimensional magnets has been presented. Optically detected magnetic resonance has been discussed in ref. 10 and applied to clarify the structure of intrinsic and impurity centres in ionic solids.

The principles and applications of pulse e.s.r. and pulse electron-nuclear multiple resonance have been reviewed. A recently developed technique based on pulsed e.s.r. methods for dynamic polarisation of nuclear spins in semiconductors is reviewed.

Modern techniques in electron paramagnetic resonance, such as two-dimensional and Fourier-transform e.s.r., far infrared e.s.r, imaging, spin-echo, and high field e.s.r. are presented in three review articles.

2 General

A number of new e.s.r. spectrometers have been described and the apparatuses are under patent licence. Some instruments, however, are rather peculiar and will be mentioned here. A CW e.s.r spectrometer working at the magnetic earth-field with a frequency of 1.845 MHz with optimized sensitivity is described. Only very narrow lines can of course be detected (maximum sweep 125 µT). A modular low frequency spectrometer with sensitivity of 8x1021 spins T-1 has been constructed using commercially available components. Coupling spins to other degrees of freedom in order to obtain dynamical polarization and/or greater detection sensitivity constitutes rather new approaches to magnetic resonance. Several of these techniques have been described. A reduction of the overall receiver noise figure of 24.6 dB has been obtained for a Varian E-110 spectrometer working at Q-band frequency by the use of a low-noise Gunn diode oscillator. Simple modification of a Varian E-line series of spectrometers is suggested in order to perform fast direct detection of the e.s.r. signaf. An e.s.r. signal from a sample of irradiated quartz which can be easily saturated was used to monitor the performance of an automatic frequency control (AFC) circuit, measuring the different saturation behaviour of the absorption and dispersion signals. Modulation of the magnetic field to detect the e.s.r. signal is still under development, while electron paramagnetic rotary resonance is being studied as an alternative method.

A software package containing routines for data acquisition, processing and graphic representation of e.s.r. (and other) spectra has been described. The data acquisition system allows one to perform kinetic studies as well and includes spectra accumulation.

Pulse e.s.r. technique is receiving much attention and we mention in particular the following advances from Arthur Schweiger and co-workers: suppression of artefacts and undesired echoes by selection of coherence-transfer pathways; detection of small hyperfine and quadrupole interactions; generation of electron-spin-echo-envelope modulations (ESEEM) in paramagnetic systems that do not contain nonsecular hyperfine interactions; resolution of an e.s.r. spectrum in a second dimension.

Electron-spin transient nutation spectroscopy has been studied by Astashkin and Schweiger in order to find a way of simplifying complicated solid state e.s.r. spectra. The technique is based on the physical fact that the nutation frequency of a spin system precessing around a magnetic field is given by the product of the transition matrix element and the strength of the magnetic field causing the nutation. In solid state spectra these matrix elements may vary from line to line and the nutation frequency is therefore different from line to line. In a transient nutation experiment the microwave...