Origin of the generally defined absorption edge of non-stoichiometric lithium niobate crystals

The ultraviolet absorption edges of LiNbO3 crystals with different Li2O contents and MgO doping concentrations were investigated. The generally defined absorption edges at absorption coefficient α=15α=15 or 20 cm−1 of all these crystals fit the Urbach rule perfectly. The origin of this absorption edges in non-stoichiometric LiNbO3 crystals is attributed to the presence of Li vacancies.