The structural properties and lattice instabilities of perovskites in the (Th,Pb,Bi,Y)ScO3 system are investigated by first-principles density-functional calculations with the linearized augmented plane-wave method. In all cases, Th is found to be tetravalent on the perovskite A site. The electronic structures have substantial Sc-O hybridization, as well as both Pb-O covalency, as is common in Pb based transition-metal perovskites, and significant hybridization of nominally unoccupied Th p and d states with O p states. The calculations show that the ideal cubic perovskite structure is highly unstable in (Th,Pb)ScO3. The dominant instability is against rotation of the ScO6 octahedra, as is common in perovskites with small A-site cations as defined by the perovskite tolerance factor. However, there are also substantial, though weaker, instabilities against ferroelectric distortions. These are characterized by large shifts of the A-site ions against the surrounding O, especially for the Th. The tetragonal ferroelectric state is favored over a rhombohedral state, and in addition a large tetragonal c/a ratio is found. At least for a particular ordering of the A-site ions we find that the ferroelectric and rotational distortions coexist, yielding a monoclinic symmetry ferroelectric ground state with rotated octahedra and a large Th off-centering. Calculations are also reported for BiScO3 and YScO3 in the perovskite structure. Considering only ferroelectric instabilities, these compounds show somewhat different anisotropies but both favor large c/a ratios when tetragonal. The instability against rotation of the ScO6 octahedra is similar in energy to the ferroelectric instability in BiScO3 and substantially stronger than the ferroelectric instability in YScO3. Trends in the lattice instabilities of AScO3 perovskites are discussed in terms of these results. Of the systems considered, the most likely to show a piezoelectrically active morphotropic phase boundary is Th(1-x)BixPb (1-x)/2aScO3. Implications for practical piezoelectric materials are discussed.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - May 2004|