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Abstract
In the improper ferroelastic palmierite-type lead phosphate order parameter coupling with a defect induced conjugate field leads to the renormalization of the two different critical temperatures of three order parameter components {Q
3} and {Q
1
Q
2}. The influence of the lead dilution by barium on the ferroelastic domain pattern, the critical temperature of the ferroelastic transformation R
m–C2/c and the development of the intermediate regime in (Pb1–x
Ba
x
)3(PO4)2 is studied using optical birefringence measurements, Raman and infrared spectroscopy. At the ferroelastic transition temperature T
c
the orientational contribution of the three-states Potts model becomes critical. T
c
is reduced from 453 K (x = 0) to zero K at x⋍ 0.12. Modifications of the shape of zigzag needle domains as well as the angle between the monoclinic binary axis and the W walls along [031] with temperature and increasing Ba-content are reported. Above the ferroelastic transition point the component Q
3, which corresponds to the displacive part in the Gibbs free energy, leads to dynamic short-range monoclinic deformation in the trigonal matrix. The temperature where Q
3 shows critical behaviour is renormalized to 720 K (x = 0.12) as compared with 563 K in pure lead phosphate. For x>0.13 no monoclinic precursor clusters were found.
