The room-temperature structure of antiferroelectric lead zirconate, PbZr${\mathrm{O}}_3$, has been studied by x-ray and neutron diffraction. The symmetry is orthorhombic (pseudotetragonal) with lattice parameters $a=5.88\mathrm{}$ A, $b=11.77\mathrm{}$ A, $c=8.22\mathrm{}$ A. The space group is $\mathrm{Pba}2\mathrm{}$ and there are 8 formula units in the unit cell. The structure results from slight distortions of the cubic perovskite lattice, which the crystal assumes above 230°C.

With respect to the ideal perovskite structure, Pb atoms suffer antiparallel shifts along the former cubic [110] direction; oxygen atoms also suffer antiparallel shifts within the (001) plane and, in addition, unbalanced antiparallel shifts along the $c$ direction.

The noncentric symmetry is in accord with the presence of a small piezoelectric effect as reported by Roberts. The oxygen octahedra surrounding the Zr atoms appear to be distorted, and it is possible to explain the strong optical anisotropy within the (001) plane as well as the pseudotetragonality ($b=2a$).

• Received 22 October 1956

DOI:https://doi.org/10.1103/PhysRev.105.849