Chemically induced renormalization phenomena in Pb-based relaxor ferroelectrics under high pressure

Abstract

The pressure-induced phase transition sequence in PbSc0.5Ta0.5O3 (PST) and PbSc0.5Nb0.5O3 (PSN) heavily doped with homo- and heterovalent cations on the A- or B-site of the perovskite-type structure (ABO3) was analysed by in situ synchrotron x-ray diffraction and Raman spectroscopy up to pressures of 25 GPa. We focused on the structural phenomena occurring above the first pressure-induced phase transition at pc1 from a relaxor state to a non-polar rhombohedral phase with antiphase tilting of the BO6 octahedra. The samples studied were PST doped with Nb5+ and Sn4+ on the B-site, PST doped with Ba2+ and La3+ on the A-site and PSN doped with Sr2+ and La3+ on the A-site. All of them exhibit a second pressure-induced phase transition at pc2, similar to pure PST and PSN. The second transition involves the development of either order of antiparallel Pb2+ displacements and complementary a+b−b− octahedral tilts, or a−b−b− (0 ≤ a < b) tilting alone. As in pure PST and PSN, the second phase transition is preceded by the occurrence of unequal octahedral tilts on the local scale. The substitution of Nb5+ for Ta5+ as well as the coupled substitution of Sn4+ for Sc3+ + Ta5+ on the octahedral B sites increases the second critical pressure. The doping by Nb5+ also reduces the length of coherence of antipolar Pb2+ order developed at pc2. The isovalent substitution of the larger Ba2+ for Pb2+ on the A-site suppresses the antipolar Pb2+ order due to the induced local elastic stresses and thus significantly increases pc2. The substitution of smaller cations for Pb2+ on the A-site generally favours the development of long-range order of antiparallel Pb2+ displacements because of the chemically enhanced a−a−a− octahedral tilts. However, this ordering is less when the dopant is aliovalent, due to the charge imbalance on the A-site. For all of the relaxors studied here, the dynamic compressibility estimated from the pressure derivative of the wavenumber of the soft mode associated with the first phase transition is larger in the pressure interval between pc1 and pc2 than above pc2. The dynamic compressibility of the phase above pc2 decreases if the antipolar Pb2+ order is disturbed.