${\mathrm{O}}_3$stilt and the Pb/(Zr/Ti) displacement order parameters in Zr-rich ${\mathrm{PbZr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ti}}_{\mathrm{x}}$ ${\mathrm{O}}_3$sfrom 20 to 500 K

Neutron diffraction and dielectric data have been collected from a polycrystalline sample of ${\mathrm{PbZr}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Ti}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ (x=0.035) to determine the temperature dependence of the two-order parameters associated with the oxygen octahedral tilt and the Pb/(Zr/Ti) displacement, respectively, through the sequence of phase transitions ${\mathrm{F}}_{\mathrm{RL}}$ -${\mathrm{F}}_{\mathrm{RH}}$ -${\mathrm{P}}_{\mathrm{C}}$ present in this mixed system. The weak coupling between tilt and polarization (displacements) is satisfactorily described within the framework of a two-order-parameters statistical theory in which the respective effective fields involve two higher-order terms in addition to a linear term.