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

A thermally stimulated depolarization currents (TSDC) study in natural fluo rapatite single crystals has established different relaxation mechanisms fo r two polarization orientations (E-p parallel and perpendicular to the crys tallographic c axis), which are discussed in relation to the defect chemist ry and the specific columnar structure in apatite. The intensities of the t hermostimulated current signals between the two poling field orientations d emonstrate 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 therm ogram 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 30 0 K. The relative intensity of associated current signals is indicative of extensive dipole-like ionic motions along c axis with a distribution in the ir activation energies ranging between 0.14 and 0.85 eV. The microdomain st ructure of fluorapatite along c axis permits the formation of charge layers at the interfaces. After annealing, the induced changes of size and/or sha pe of the interfaces could explain the observed changes of band intensity a nd location. With the electric field perpendicular to c axis, the spectrum consists of at least five well-defined relaxation bands, the high temperatu re ones (HT1, HT2, HT3) decreasing after heating at 673-873 K. The most dra matic change was recorded for an intermediate LT2 single-relaxation band lo cated around 185 K, with a high activation energy of 1.06 eV, which manifes ted a significant growth after annealing. Rietvelt analysis of the x-ray di ffraction patterns of the original and annealed apatite powders, indicates change in the unit cell parameters of the hexagonal structure (i.e., a incr eases from 9.3921 to 9.3940 Angstrom after annealing), which can be related to the establishment of a new equilibrium distribution of the abundant tri valent rare-earth (Ce, La, Nd, Pr,...) impurity ions. The origin of the TSD C bands is discussed and tentative correlations are suggested, in terms of substitute aliovalent ions-vacancy dipoles. The thermal response of the hig h 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 d ipoles/ions cannot be disregarded. (C) 1999 American Institute of Physics. [S00218979(99)10001- X].