An E.P.R. study of some transition metal impurities in silver chloride and silver bromide

• Main Author: Cook, Frances Barbara Irene
• Published: University of Warwick 1972
• Subjects: 530; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.594985

The electron paramagnetic resonance (E.P.R.) and photosensitivity of silver chloride and silver bromide doped with a number of transition metal ions" has been investigated. The valent state, lattice position and site symmetry of the impurities are discussed in relation to the E.P.R. spectra and the chemical properties of the ions. The E.P.R. spectra of trivalent chromium in AgCl and AgBr are analysed. The principal spectrum at 95K in both halides has orthorhombic symmetry with axes in the < 100> directions. Subsidiary spectra are also observed with tetragonal and orthorhombic symmetries. The possible arrangements of the two charge compensating silver ion vacancies around the trivalent ion are discussed. The principal spectrum is shown to be consistent with a centre in which the Cr3+ ion is associated with a nearest and a next nearest cation vacancy. The lines joining the vacancies to the Cr3+ ion are at an angle of 1350 • Other centres are proposed to explain the subsidiary spectra. The Cr3+ resonances are studied at temperatures up to 500K. The broadening of the fine structure lines at high temperatures is used to . determine the activation energy for vacancy motion around the C~ ion in each of the centres observed. A set of lines which increase in intensity as the sample is warmed above room temperature is assigned to the Cr3+ ion associated with a single next nearest cation vacancy. The spin Hamiltonian parameters of the spectra of Cr3+ in the chloride and the bromide are compared. The substitutional incorporation of the Cr3+ ion is contrasted with the interstitial incorporation of the Fe 3 + ion which has a similar radius. A dewar and cavity for use at 4.2K are described. The E.P.R. spectra of ytterbium, erbium and dysprosium in AgCl are presented. The resonances are assigned to the trivalent state of these ions and the ground states are derived from the experimental g-values. The symmetries of the spectra are used to derive the probable arrangements of vacancies about the rare earth ions. Preliminary investigations of the E.P.R. and optical properties of the remaining rare earths and the 4d and 5d series ions in AgCI are presented. The rare earths other than europium are thought to be present only in the trivalent state in AgCI. The stable valent state is discussed in relation to the chemical properties of the rare earths. The E.P.R. and electrical measurements available to date on the lattice site and vacancy association of transition ions in AgCI and AgBr are summarised. The results are used to show that association with nearest neighbour vacancies is most likely for ions which are relatively large compared with the Ag+ ion. Small impurity ions are shown to be more likely to be associated with next nearest cation vacancies.