**1. Photorefractive Properties and Defect Structure**

The photorefractive properties of lithium niobate (LN): Mo,Zn crystals with different doping concentrations were investigated in paper [1]. Zinc can shorten the response time and improve the photorefractive sensitivity of the LN:Mo,Zn crystal. Valence states of Mo ions were identified by XPS. Three valences (+6, +5, +4) were identified in the crystal and one (+6) in the residue. In the LN:Mo,Zn 7.2 crystal the MoNb<sup>+</sup> and MoLi<sup>3</sup>+/4<sup>+</sup> defects served as the photorefractive centre for fast photorefraction. Potential material for fast response holographic storage are 7.2 mol% Zn and 0.5 mol% Mo co-doped LiNbO3 crystals.

Vanadium and molybdenum ions are of interest in enhancing the photorefractive properties of LiNbO3. Paper [2] presents a computer modelling study of V2<sup>+</sup>, V3+, V4<sup>+</sup> and V5<sup>+</sup> as well as Mo3+, Mo4+, Mo5<sup>+</sup> and Mo6<sup>+</sup> in LiNbO3 using interatomic potentials. It was found that divalent (V2<sup>+</sup>), trivalent (V3<sup>+</sup>, Mo3<sup>+</sup>) and tetravalent (V4<sup>+</sup>) ions are incorporated at the Li and Nb sites through the self-compensation mechanism. However, the tetravalent (Mo4+) ion is more favourably incorporated at the niobium site, compensated by an oxygen vacancy. The pentavalent ions (V5<sup>+</sup>, Mo5<sup>+</sup>) and hexavalent (Mo6<sup>+</sup>) ions substitute Nb. No charge compensation is found for pentavalent ions, but there is charge compensation with a lithium vacancy for the Mo6<sup>+</sup> ion.
