The structure of KAlSiOo with P6tmc symmetry

Hexagonal KAlSiO. produced from nepheline by alkali exchange in molten KCI shows different diffraction effects from different crystals, even from the same exchanged batch. These crystals show variable degrees ofdiffuse streaking parallel to c*, and the observed iffraction symmetries range from that of the kalsilite space group, P6r, to that of space group P6smc. Study of the crystal structure of this latter symmetry phase reveals a kalsilite-like arrangement but with positionally disordered oxygen atoms. The disorder is attributed to a domain structure. Transformation of nepheline to kalsil i te proceeds by tetrahedral rotation such that successive (00. l) layers undergo opposite-sense rotations. Where this sequence is interrupted, out-of-phase domain boundaries occur