Electrooptic single crystals having a room temperature tetragonal tungsten bronze-type structure were grown from the pseudosystem KNbO3LiNbO3BaNb2O6 using covered platinum crucibles cooled at 2°C/hr. The largest and best quality crystals were grown from molten solutions with compositions approximately between K3Li2Nb5O15 and KBa2Nb5O15. The K3Li2Nb5O15 functions as a flux and the compositions of the grown crystals lie close to KBa2Nb5O15. These tungsten bronze-type crystals, which can be represented as KBa2Nb5O15 solid solutions with LiNbO3 and K3Li2Nb5O15, are not microtwinned and a large range of the bronze compositions phase match the Nd:YAG laser (1.06 μ) for second harmonic (0.53 μ) generation without double refraction above room temperature. © 1970.