Quantum-fluctuation-stabilized orthorhombic ferroelectric ground state in lead-free piezoelectric (Ba,Ca)(Zr,Ti)O3

We numerically investigate the phase diagram of the giant-piezoelectric $(1\ensuremath{-}x)\mathrm{Ba}({\mathrm{Zr}}_0.2{\mathrm{Ti}}_0.8){\mathrm{O}}₃\ensuremath{-}x({\mathrm{Ba}}_0.7{\mathrm{Ca}}_0.3){\mathrm{TiO}}₃$ system, treating the ions either as classical objects (via classical Monte Carlo or CMC simulations) or quantum mechanically (via path-integral quantum Monte Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram, but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that ``only'' yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.