AN INVESTIGATION INTO THE CRYSTAL STRUCTURE AND DISORDER OF PbZrO3. Corker, D. L., Glazer, A. M., Clarendon Laboratory, Physics Department, Oxford University, England
The following study reports a reinvestigation into the structure of lead zirconate, PbZrO3. Previously reported by Glazer et al. (1) in 1993, the crystal structure was determined in space group Pbam and said to exhibit a disorder in the oxygen sites such that the two oxygen substructures A and B were related by:
XA, YA, Z[[Alpha]] XB, YB, ZB + 0.5
This disorder, although agreeing with an earlier single-crystal X-ray diffraction study, disagrees with a neutron powder diffraction analysis by Whatmore, in 1976 (2), where no indications of a disorder could be found.
To investigate this discrepancy further structural analysis is presented. This includes single-crystal X-ray diffraction, at both room and low temperature, and also neutron diffraction, although due to growth problems of large single crystals, the neutron analysis is restricted to that of powders only. If the disorder is false, then explanations of a disorder being exhibited are further cell doublings previously not recognised, twinning or possibly incorrect space group determination. Indeed, there has been much contradicting evidence over the space group determination and whether the structure is truly centrosymmetric. While there has been weak ferroelectricity reported by Dai et al. (3) which indicates a non-centrosymmetric space group, Scott and Burns (4), were unable to find neither piezoelectric or pyroelectric effects hence supporting the centrosymmetric classification Pbam.
There is also the possibility that the disorder is simply an artifact of pseudosymmetry. Whatever the space group, one of the main problems in an accurate structure determination is the strong pseudosymmetric character of the Pb and Zr sites and the relatively high scattering factors of the cations to the oxygens. Assuming nothing from previous determinations, an account is given of each of the stages used and pitfalls encountered in investigating all possibilities.
(l)Glazer, A. M., Roleder, K. & Dec, J. (1993). Acta Cryst. B49, p846
(2)Whatmore, R. PhD thesis, (1975). Univ. of Cambridge, England.
(3)Dai, X., Li, J. & Viehland, D. (1995). Cond. Matter, 51, 5, 2651
(4)Scott, B. A. & Burns, G. (1972). J. Am. Ceram. Soc.55, 333