Structural aspects of the antiferroelectric-paraelectric phase transition in double perovskite Pb$_2$MgWO$_6$ at high pressures and temperatures

The crystal structure of antiferroelectric Pb$_2$MgWO$_6$ has been studied using neutron diffraction at high pressures to 5.4 GPa at room temperature and energy-dispersive X-ray diffraction at high pressures to 4 GPa in the temperature range 300–400 K. At normal conditions, in Pb$_2$MgWO$_6$, there is an antiferroelectric phase with the crystal structure described by the orthorhombic symmetry with space group Pnma. At temperature $T$ = 313 K and normal pressure or at room temperature and pressure $P\sim$ 0.9 GPa, the crystal under-goes a structural phase transition to the cubic phase with space group $Fm\bar{3}m$ (paraelectric phase). The temperature and pressure dependences of the lattice parameters, unit cell volume, and interatomic bond lengths have been obtained, and the thermal expansion coefficients and the bulk moduli have been calculated for the antiferroelectric and paraelectric phases of Pb$_2$MgWO$_6$.