LiNbO 3 : A Paradigm for Photorefractive Materials
- PDF / 985,435 Bytes
- 7 Pages / 576 x 792 pts Page_size
- 62 Downloads / 217 Views
32
in LiNbO3 has recently been gathered.14"16 This interesting topic of materials science will constitute the focal point of this article. Particular attention will be paid to the inhibition of the PR effect by Mg doping and the underlying physical mechanisms. Recent developments in the understanding of PR fixing17 will also be described. Main Structural and Physical Properties of LiNbO3 The following summarizes the main structural and physical properties of LiNbO3. Detailed accounts can be found in several references.618
Structural Features The room-temperature ferroelectric structure of LiNbO3 is related to that of the perovskite structure.19 It consists of distorted oxygen octahedra sharing faces and forming a planar hexagonal arrangement, Figure la. The pile-up of the octahedra along the perpendicular direction, c axis, follows the cation sequence: Li, Nb, and vacant site (Figure lb). The point-symmetry group is C3 (3), with the trigonal axis along the cation rows. It shows only a very slight deviation from the C3i, (3m) group due to a small differential rotation of the oxygen triangles around the c axis. In fact, many analyses use the C3!, instead of the correct C3 symmetry. An important feature of LiNbO3, as with many other oxides, is the strong departure from nominal stoichiometry. In accordance with its phase diagram, LiNbO3 is normally grown with the congruent composition (48.38 mol% of Li2O) assuring homogeneous and good quality crystals.2" Other stoichiometries have also been grown, although the homogeneity, composition, and crystalline quality are often doubtful. Anyhow, it appears that congruent and stoichiometric LiNbO3 present remarkably different PR behavior.21 The Li deficiency in congruent crystals is accommodated by means of Nb u antisites and Nb vacancies in a relative con-
(b)
(a)
o Li • Nb
Oo Figure 1. Structure of the LiNbO3 lattice, (a) Projection on the ab plane. Large circles with different patterns represent oxygen on different planes parallel to the paper, (b) Perspective of the arrangement of the oxygen octahedra along the c axis.
MRS BULLETIN/MARCH 1994
LiNbO3: A Paradigm for Photorefractive Materials
centration that guarantees overall electrical neutrality.22 Many physical properties of LiNbO 3 markedly depend on stoichiometry, e.g., lattice parameter, Curie temperature, absorption spectra, and luminescence yield. For example, the absorption edge shifts to lower wavelengths for decreasing Li deficiencies, and this behavior is often used to measure the crystal stoichiometry.23 Electrooptic Behavior For a Pockels electrooptic crystal such as LiNbO3, the change in refractive index is related to the applied electric field E through the linear relation24 /M2),7 = r,jkEk
(1)
r,j being the electrooptic coefficients and A(l/n2);J the field-induced change of the /;' component of the index tensor. Since this tensor is symmetric, it is usual to adopt a second-rank 6 X 3 matrix rmk to represent the r,jk tensor. The subindex in designates all six distinct ij pairs. From Equation 1 one
Data Loading...
