ANOMALOUS INCOMMENSURABILITY AND EMBRYONIC FLUCTUATIONS IN LEAD PHOSPHATE PB3(PO4)2

We report the results of precise x-ray- and neutron-scattering experiments on lead phosphate in the critical-temperature region of the ferroelastic transition. These results show the existence of anomalous incommensurate satellite reflections above T(c), where the incommensurability changes from one Brillouin zone to another. It is shown that the incommensurate satellites are located at the commensurate reciprocal-lattice points of the monoclinic cell, which is stabilized below T(c). Similar phenomena are known to occur in the precursor regime of the martensitic phase transitions of metals and alloys such as Ti-Ni. The present investigation provides an observation of an anomalous incommensurability in a nonmetallic compound. The observed results are interpreted in terms of embryonic fluctuations, which are theoretically predicted to exist at a first-order phase-transition point when there is strong coupling between the order parameter and the bulk strain.