O. R. Gilliam

Low temperature intrinsic defects in x‐irradiated hydroxyapatite synthetic single crystals

ESR studies of radiation‐induced defects have been conducted on synthetic calcium hydroxyapatite single crystals. For a room temperature x‐irradiation a major defect (labeled A) was reported to be an O− ion. X irradiation at 6 K shows defect A, trapped atomic hydrogen, and a nonaxial holelike center (labeled I). These new centers are stable at 6 K but anneal near 77 K. Observations at 9 and 35 GHz indicate that the I center is a spin‐1/2 defect located in six inequivalent sites. In the ab plane, spectra exhibit an isotropic hyperfine doublet (∼13 G splitting) and an anisotropic doublet (17–27 G splitting) in three symmetry‐related sites. For other orientations additional site splitting and ’’forbidden transitions’’ make the spectra very complex. The hyperfine coupling tensor for the anisotropic doublet has diagonal elements −29.5, −19.2, and +3.11 G. The g tensor for this defect has diagonal elements 2.0068, 2.0032, and 2.0148. The sets of directional cosines associated with the minimum g value and the in...

Superhyperfine interactions and their origins for nd1 ions in rutile‐structure oxides

Paramagnetic resonance spectra of 4d1 (Nb4+ and Mo5+) and 5d1 (W5+) ions at substitutional cation sites in single crystals of tetragonal GeO2 have been studied at 92 K. Spin‐Hamiltonian parameters obtained from hyperfine and superhyperfine (shf) structures are reported and compared with previously reported results for these ions and for V4+ (3d1), in rutile‐structure GeO2, TiO2, and SnO2. Analysis of superhyperfine interactions with these nd1 impurity cations in GeO2 indicates that the unpaired spin is highly delocalized into nearest‐neighbor ligand cations. On the basis of an LCAO‐MO treatment, delocalization probabilities are inferred from measured shf interactions for these impurities; for each, the isotropic shf interaction is attributed to admixture of the unoccupied ligand‐cation 4s orbital into the outer d orbital of the central impurity ion. The anisotropic contribution to the shf interaction is attributed to admixture of 4p ligand‐cation orbitals. The results suggest an increase in the probabilit...

Electron Spin Resonance of the CN Molecule Trapped in Gamma‐Ray Irradiated Potassium Cyanate

Electron spin resonance (ESR) studies have been made on single crystals of potassium cyanate (KNCO) that have been irradiated with gamma rays at room temperature. Two ESR spectra resulted. One of these consisted of an S = 12 center displaying a three‐line hyperfine structure. This spectra is assigned to a CN molecule. The molecule occupies two sites in the lattice in which the molecular axis is parallel to the [110] and the [110] directions of the unit cell. The parameters of the spin Hamiltonian deduced from the spectra are | A‖ | = 5.0 ± 0.5 G, | A⊥ | = 2.3 ± 0.5 G, | Aiso | = 3.2 ± 0.5 G, and g⊥ = 1.999 ± .002. The g‖ is not determined.

Electron Paramagnetic Resonance in Gamma‐Ray‐Irradiated Niobium‐Doped Calcium Molybdate

Calcium molybdate (CaMoO4) doped with niobium impurity ions was studied by electron paramagnetic resonance (EPR). Single crystals were irradiated with 60Co gamma rays at 77°K which caused three paramagnetic species. One of these is associated with the niobium impurity ion substitutional for molybdenum; the major part of its EPR spectrum showed an S = 12 defect with a 10‐line hyperfine structure for each of four equally probable defect sites. Normally forbidden δMI = ± 1 transitions were also observed. The spectra were fit to a rhombic spin Hamiltonian with parameters: gxx = 2.006 ± 0.002, gyy = 2.011 ± 0.002, gzz = 2.053 ± 0.002, | Axx | = 23 ± 2 G, | Ayy | = 30 ± 2 G, and | Azz | = 30 ± 2 G where zz lies approximately along a normal Mo–O bond direction in the lattice. A partially resolved six‐line ligand hyperfine structure appeared on the major resonances with the intensity expected for overlapping 95Mo and 97Mo hyperfine interactions. The model proposed for this defect is a hole shared between adjacent...