A Naturally Induced Hole Center in the Mineral Hypersthene

In natural hypersthene a paramagnetic center in triclinic site symmetry was detected by electron paramagnetic resonance with the following spin Hamiltonian parameters: g(xx) = 2.0104, g(yy) = 2.0256, g(zz) = 2.0090. This center is most likely formed as a hole on an oxygen of the SiO(4) unit. No hyperfine splitting was detected at room temperature. The EPR signal disappears after heating the crystal at 773 K for 1 h although the colour does not bleach

An EPR Study of Fe(3+) in Enstatite

The paramagnetic resonance spectrum of light-green natural enstatite, (Mg, Fe(2+)) [SiO(3)] shows evidence of Fe(3+) in triclinic site symmetry. Only one transition of sufficient intensity could be evaluated with the following Hamiltonian parameters: g(xx)=4.774, g(yy)=4.003, g(zz)=3.723. No hyperfine splitting was detected. It is suggested that Fe(3+) should occupy the more distorted M(2) site. A number of spread signals of very low intensity from 50 to 500 mT can be attributed to other Fe(3+) transitions or to Mn(2+). The occurrence of a small portion of iron in a trivalent state could be due to either oxidizing conditions during crystal growth or to subsequent ionizing radiation

Polarizing-field orientation and thermal treatment effects on the dielectric behavior of fluorapatite

A thermally stimulated depolarization currents (TSDC) study in natural fluorapatite single crystals has established different relaxation mechanisms for two polarization orientations (E-p parallel and perpendicular to the crystallographic c axis), which are discussed in relation to the defect chemistry and the specific columnar structure in apatite. The intensities of the thermostimulated current signals between the two poling field orientations demonstrate a difference of at least one order of magnitude, with the higher one recorded for the electric field parallel to the c axis. The TSDC thermogram appearing with the electric field parallel to c axis, in the 10-320 K range, consists of a broad and complex band (HT), with a maximum around 300 K. The relative intensity of associated current signals is indicative of extensive dipole-like ionic motions along c axis with a distribution in their activation energies ranging between 0.14 and 0.85 eV. The microdomain structure of fluorapatite along c axis permits the formation of charge layers at the interfaces. After annealing, the induced changes of size and/or shape of the interfaces could explain the observed changes of band intensity and location. With the electric field perpendicular to c axis, the spectrum consists of at least five well-defined relaxation bands, the high temperature ones (HT1, HT2, HT3) decreasing after heating at 673-873 K. The most dramatic change was recorded for an intermediate LT2 single-relaxation band located around 185 K, with a high activation energy of 1.06 eV, which manifested a significant growth after annealing. Rietvelt analysis of the x-ray diffraction patterns of the original and annealed apatite powders, indicates change in the unit cell parameters of the hexagonal structure (i.e., a increases from 9.3921 to 9.3940 Angstrom after annealing), which can be related to the establishment of a new equilibrium distribution of the abundant trivalent rare-earth (Ce, La, Nd, Pr,...) impurity ions. The origin of the TSDC bands is discussed and tentative correlations are suggested, in terms of substitute aliovalent ions-vacancy dipoles. The thermal response of the high temperature relaxation bands in the case of E-p perpendicular to c axis, is characteristic of dipole clustering phenomena - although an explanation based on localized changes in the structural environment of the pertinent dipoles/ions cannot be disregarded. (C) 1999 American Institute of Physics. [S00218979(99)10001- X]

TSDC probe of anisotropic polarizability in fluorapatite single crystals

A study of single phase hexagonal fluorapatite crystals from Durango (Mexico), by means of the TSDC technique, has demonstrated a variety of dielectric relaxation mechanisms for different poling orientations, parallel and perpendicular to the crystallographic c-axis. In the first case, the TSDC spectrum consists of a broad complex current band, featuring a distribution of the pertinent energy barriers. In the case of perpendicular electric stimulus (Ep ⊥ c) the overall intensity of the relaxation spectrum is reduced by at least one order of magnitude, compared to that with Ep ∥ c axis, with at least five highly overlapping dielectric bands. A tentative attribution of the anisotropic polarizability observed in the two different orientations to specific micromolecular mechanisms takes into account the creation and selective partitioning of several microstructural defects (e.g. limited Cl- and REE3+ substitution for F- and Ca2+ respectively, ionic vacancies and dislocations). These types of defects produce several kinds of (re)orientable dipolar units and permit short range motions of electric charges, which are both directly detectable by dielectric relaxation spectroscopy methods