Local structure about Er and Hf defects in LiNbO3: possibility of meta-stable distributions

The non-linear optical properties of LiNbO₃ (LNO) are often tuned by doping on the Li or Nb sites. Zn and In suppress the photorefractive response, important for second harmonic generation applications, while Fe and Hf enhance the photorefractive response. LNO can also be grown in two forms, congruent (cLNO, slight Nb excess) and stoichiometric (sLNO), which have somewhat different properties. An important question still under debate is the substitution site for various dopants. EXAFS data for Er(3+) (cLNO and sLNO) and Hf(4+) (sLNO) are reported and compared. A signature for occupation on the Li site is a large peak in the EXAFS r-space plot near 2.9 A, whereas the largest peak above 2 A for Nb site substitution is near 3.5 A. We show that Er is mostly on the Li site with about 30 % on the Nb site, while Hf substitutes roughly equally on both the Li and Nb sites. In contrast another 3+ defect In(3+) is primarily on the Li site. Some limited theoretical calculations differ from these results but most are carried out for low temperatures. Since LNO crystallizes near 1500 K, are the dopants and their charge compensating centers quenched in place at very high T and not in thermal equilibrium? A more difficult question is whether these meta-stable distributions can be controlled.