Crystallography of the litharge to massicot phase transformation from neutron powder diffraction data.

Abstract

The crystallographic phase change from tetragonal litharge (α-PbO; P4/nmm) to orthorhombic massicot (β-PbO; Pbcm) has been studied by full-matrix Rietveld analysis of high-temperature neutron powder diffraction data collected in equal steps from ambient temperature up to 925 K and back down to 350 K. The phase transformation takes place between 850 and 925 K, with the coexisting phases having equal abundance by weight at 885 K. The product massicot remains metastable on cooling to near ambient temperature. Both structures are layered networks of OPb₄ tetrahedra and PbO₄ square pyramids, with the space between alternate layers of Pb atoms (in approximate cubic close packing) occupied either by O atom layers or Pb atom lone pairs. In massicot, the symmetric Pb and O layers of litharge become deeply corrugated parallel to [001], with the O-atom layers splitting into two layers, although these are still sandwiched between the approximately cubic close-packed Pb atom layers. The crystallite size of the initial litharge component decreases from around 3500 Å to around 1100 Å at the midpoint of the phase change at 885 K, whereupon the size of the massicot crystallites increases from a similar value to around 2500 Å at 350 K during the cool-down stage. The unit-cell dimensions and atomic coordinates of litharge change smoothly throughout the phase change, but there is a rapid expansion of the c-axis immediately prior to the recrystallization to massicot, and the one free coordinate (Pb z) decreases significantly to producer a thinning of the layers. Increases in the O displacment parameters suggest that this atom is `on the move' as the transformation approaches. Changes in the massicot parameters as its crystallites emerge from the transformation are largely unremarkable. The formation of the heavily corrugated layers in massicot requires significant movements of the O atoms (∼1.2 Å) from their positions in litharge and thus for Pb-O bonds to be broken during the phase change. The requirement for these bonds to be re-broken in a conversion back to litharge is likely to be the reason why massicot is metastable at ambient temperature. Evidence of a temporary intermediate `amorphous' phase in the phase transformation from which the massicot grows is provided in the form of broad, very low amplitude `peaks' in the high-temperature diffraction patterns.