Defects in inorganic photorefractive materials and their investigations

16 B. Briat et al.
trigonal symmetry and offer similar chemical backgrounds, oxygen octahedra,
for incorporation (see Fig. 6). It is only by identifying the next cation neighbors
of a paramagnetic extrinsic ion by careful ENDOR measurements that
conclusive evidence can be obtained. With this method e.g. the behavior of
Cr 3+ has been studied in great detail [48, 85]. In Mg-doped and stoichiometric
crystals it replaces Nb, and enters on Li sites in Li-deficient crystals. Fig. 7
shows as examples that the EPR spectra of the extrinsic defects Cr 3+ and
Fe 3+ are quite different for congruent and stoichiometric crystals, indicating
different sites of the ions.
The interrelation of aliovalent extrinsic defects with their intrinsic compensators
is demonstrated with Fig. 8. It shows one EPR line of isolated Cr 3+
Li
as depending on the Li fraction x c of the host crystal [88]. It is seen that
the signal becomes considerably sharper with increasing Li content. Due to
these smaller linewidths many satellite lines could be resolved with a crystal
grown from a melt with x m =0.4985 (Fig. 8). For this composition the lines
resulting from Cr 3+
Li
associated with intrinsic defects at various discrete closer
lattice distances were identified in addition to axial spectra related to Cr 3+
Li
compensated by distant defects. It was possible to identify intrinsic compensator
positions up to the ninth cation shell around the central Cr 3+
Li
[88]. By
comparison of the EPR and optical spectra it could furthermore be shown that
the low symmetry short distance associations have higher oscillator strengths
than the isolated Cr 3+
Li .
This study of Cr 3+
represents a sample case for other extrinsic cation
dopants, usually also leading to low symmetry satellite lines in the EPR spectra
such as Mn 2+ ,Fe 3+ ,Gd 3+ ,Yb 3+ ,Nd 3+ [89] and Er 3+ [77]. ENDOR
investigations of Cr 3+
Nb
in stoichiometric crystals revealed that in this case
protons in the nearest neighborhood are present to compensate the effective
negative charge of Cr 3+
Nb
. They are located asymmetrically between two O2−
ions of the plane perpendicular to the c-axis of the oxygen octahedron next to
Fig. 8. Strong decrease of EPR
linewidth of the main Cr 3+
Li
transition
with increasing Li content
[88]. Small satellite lines can be
seen, caused by complexes consisting
of Cr 3+
Li
and compensating intrinsic
defects.