Paramagnetic resonance studies of radicals produced by radiation damage in inorganic solids.

The theory of electron spin resonance in the solid state and the experimental techniques involved are briefly outlined. A detailed description of the evaluation of the principal magnetic parameters from single crystal data is included. A single crystal study of KC103 ?-irradiated at 295K is discussed. The radicals produced by ?-irradiation and subsequent optical bleaching are C102, (C1-C102)-, C1-2 and 0-3. The stuctures, the configuration in the lattice, the magnetic parameters, and the optical properties of these species are described. A mechanism is suggested whereby the formation of these centres is accounted fore. The products of u.v. photolysis of C102 in frozen aq. H2SO4 or KC104 are C100 and/or C10 depending on the matrix. C100 has been detected by e.s.r. below 140K. The principal values and the direction cosines, of the g- and A-tensors of C100 in a single crystal of KC104, have been obtained, and interpreted in terms of a model in which there is approximately 15% unpaired spin-density on the chlorine atom, and in which the C100 bond angle is 112. Optical and i.r. studies of C100 are also discussed. In KC104, C100 slowly decays at 295 to C102, but can be regenerated by subsequent photolysis. A mechanism for this interconversion is suggested. PART II The centres (WO4-WO4)3- and (MoO4-MoO4)3-, produced by ?-irradiation of pure CaWO4 and CaMoO4 at 77K, are discussed. However, the presence of group V elements in these scheelite crystals results in almost complete replacement of these hole-centres by centres associated with the impurities. A paramagnetic niobium species formed by ?-radiolysis of CaMoO4:Sm:Nb is described. The radical is NbO42-, occupying four magnetically distinct sites at 77K with completely anisotropic g- and A-tensors, but only one site, having axially symmetric g- and A-tensors, at 195K. This temperature variation has been interpreted in terms of a mobile distortion which sets in at temperatures above 150K. A superhyperfine interaction with adjacent MoO42- ions can be explained by a charge-transfer process. The unpaired spin-density in NbO42- resides almost entirely on oxygen, and hyperfine coupling to niobium arises from spin-polarisation of the electrons in the Nb-O ?-bonds. Similarly, XO42- centres are formed in CaWO4 and CaMoO4 doped with a group V element, X (P, As, V and Ta). The structure and stability of these species are discussed. It is proposed that these tetroxy-anion hole-centres occupy covalent WO42- or MoO42- sites.