On the microstructure and symmetry of apparently hexagonal BaAl2O4

The P63 (a=2ap, b=2bp, c=cp) crystal structure reported for BaAl2O4 at room temperature has been carefully re-investigated by a combined transmission electron microscopy and neutron powder diffraction study. It is shown that the poor fit of this P63 (a=2ap, b=2bp, c=cp) structure model for BaAl2O4 to neutron powder diffraction data is primarily due to the failure to take into account coherent scattering between different domains related by enantiomorphic twinning of the P6322 parent sub-structure. Fast Fourier transformation of [0 0 1] lattice images from small localized real space regions (∼10 nm in diameter) are used to show that the P63 (a=2ap, b=2bp, c=cp) crystal structure reported for BaAl2O4 is not correct on the local scale. The correct local symmetry of the very small nano-domains is most likely orthorhombic or monoclinic.