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Abstract
The potassium niobyl cyclotetrasilicate K2(NbO)2Si4O12 crystallizes in the non-centrosymmetrical polar space group P4bm with a = 8.7404(8) Å, c = 8.136(1) Å, Z = 2. The acentric character of the structure is mainly due to a sequence of short (niobyl) and long Nb-O bonds along the 4-fold axes. Optical studies on small crystals have shown that the transparency range extends from 0.3 to 4.5 μm and that they are positively uniaxial.
Second harmonic generation tests, as a function of temperature, have been performed on a polycrystalline sample with use of the 1.06 μm line of a YAG-Nd laser. They show that phase matching conditions are still obtained at 1080[ddot]C (limit of the equipment).
K2(NbO)2Si4O12 melts congruently at 1180[ddot]C and appears as rather inert and chemically stable.