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]
