Structural phase transitions and crystal chemistry of the series Ba2LnB′O6 (Ln=lanthanide and B′=Nb5+ or Sb5+)

The structures of 28 compounds in the two series Ba2LnSbO6 and Ba2LnNbO6 have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba2LnNbO6, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by π-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations.

Graphical abstractLattice parameters versus temperature for Ba2NdNbO6. The formation of the I4/m tetragonal phase contrasts with the antimonate series where a rhombohedral structure occurs instead. This difference is believed to be caused by the presence of π-bonding present in the niobates but absent in the antimonates.Figure optionsDownload full-size imageDownload as PowerPoint slide