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Single crystals of K5Bi0.9Nd0.1 (MoO4)4 and K5Bi0.97Nd0.03(MoO4)4 were grown from sto-ichiometric melts by the usual Czochralski technique. The crystal (Nd = 0.1) belongs to trigonal system, with space group R3m, Z = 1.5, and cell parameters a = 6.023 ?, c=20.887?.The room temperature absorption spectra of K,Bi(MoO4)4-Nd3+(3 % or 10 % Nd3+) consist of several bands characteristic of the rare earth Nd ions in crystals. The substitution of Nd3+ by Bi3+ does not affect the surrounding of the active ions. Details of room temperature emission spectra corresponding to the 4F3/2→4I13/2 and 4F3/2→4I11/2 transitions are also given.

Optical absorption spectra, OH infrared absorption bands, ESR spectra of Fe3+ ions, and photoconductivities of a group of LiNbO3 crystals, including nominaly undoped LiNbO3, LiNbO3:6 mol% MgO and LiNbO3:6 mol% MgO + 0.5 mol% Nd2O3 crystals, are measured at room temperature. OH band and ESR measurements show that Nd doping will not directly influence the Mg doping concentration threshold effect and the site of incorporation of Fe ions in LiNbO3: Mg. This conclusion is demonstrated by experimental photoconduction results. When no external electric field is applied the photovoltaic current density in LiNbO3: Mg, Nd is larger than that in undoped and Mg‐doped LiNbO3 at some wavelengths owing to the larger optical absorption coefficient. Besides, the effect of UV irradiation on the photoconduction in doped LiNbO3 crystals is observed experimentally. The photoconduction in LiNbO3:6mol% MgO will increase after the UV irradiation, but no change is observed in LiNbO3: Mg, Nd crystals. By means of defect chemistry calculations of the threshold effect in Mg‐doped LiNbO3 crystals according to the nature of substitution of Nd ions in the Mg‐doped crystals, the experimental results are discussed.