Electron microscopy study of the modulated phases in berlinite AlPO4 and quartz

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Abstract
The sequence of β → incommensurate (I) → α phases taking place in AlPO4 and SiO2 is studied by means of electron microscopy. The features of the modulated phase are in agreement with previous neutron determination of incommensurability, and as predicted by theory, the modulation wave vector is slightly rotated from the OY directions ; the thermal variation of this angle is given, and this data allows an approximate estimation of the change of discommensuration width that confirms the scheme of a gradual change from a sinusoidal modulation, close to Ti, to a domain-like structure when (Ti - T) increases (this is also confirmed by electron diffraction data showing appearance of higher order satellites). The incommensurate phase is marked by the presence of intrinsic defects corresponding to local imperfections of the superperiod, which are involved in the k-variation. In impure AlPO4 samples, a pinning of intrinsic defects by imperfections of the crystal lattice occurs. This explains the birefringence behaviour suggesting a discontinuous variation of k. The (lock-in) transition to the α phase involves an intermediate state characterized by elongated triangular microdomains, and proceeds by nucleation ; a model implying some particular defects is proposed. Dynamical (and relaxation) phenomena are also reported and qualitatively interpreted. Finally, we give a possible interpretation of memory effect and of relaxation phenomena based on intrinsic defects which differs from that of Jamet and Lederer