Pressure-induced transformation processes in ferroelastic $Pb_{3}(P_{1–x}As_{x}O_{4})_{2}$, x = 0 and 0.80

The high-pressure behaviour of two represent- ative palmierite-type Pb$_3$(P$_x$As$_{1–x}$O$_4$)$_2$ ferroelastic com- pounds with x = 0 and 0.80 was analyzed by combined single-crystal X-ray diffraction and Raman spectroscopy. Single-crystal diffraction measurements on the As-rich compound Pb3(P0.20As0.80O4)2 show that it undergoes the same macroscopic monoclinic to trigonal phase transi- tion as previously observed in pure Pb$_3$(PO$_4$)$_2$ but with a significantly lower transition pressure, p$_c$ = 0.90(4) GPa for x = 0.80 as opposed to p$_c$ = 1.81(1) for x = 0.0. Syn- chrotron X-ray diffraction experiments reveal that both compounds exhibit significant diffuse scattering in a pressure interval of ~1.5 GPa above the corresponding pressure-induced transitions, indicating the persistence of monoclinic nanodomains within the macroscopically trigonal phase above the phase-transition point, similar to the high-temperature structural state. Raman spectros- copy reveal quite distinct lattice dynamics for x = 0 and x = 0.80, indicating different pathways of pressure-driven structural alteration. The pure phosphate compound shows a displacive phase transition of thermodynami- cally second-order type, whereas the As-rich compound exhibits an order-disorder phase transition with multistep structural changes on the mesoscopic scale. The pressure evolution of the Pb phonon modes as well as the broadening of the X-ray diffraction peaks suggests a further pressure-induced phase transition occurring in the range 5–7 GPa for Pb$_3$(P$_{0.20}$As$_{0.80}$O$_4$)$_2$, whilst no indication for a second phase transition up to 10 GPa was observed for Pb$_3$(PO$_4$)$_2$