Novel type of phase transition to incommensurate structure in quartz and in berlinite

It is shown that the -incommensurate transitions which take place in quartz at TS846 K and in berlinite at TS 870 K are of a novel type which has recently been discussed theoretically. This transition has features in common with both instability and nucleation types. The wave vector q of the incommensurate phase rotates continuously away from the symmetry axis as the temperature is lowered below TS. Renormalization-group and scaling arguments are presented, suggesting that the perpendicular component of the wave vector should vary as qt, where t=(TS-T)/TS and is the critical exponent associated with the correlation length. © 1987 The American Physical Society.