"114"-Type Nitrides LnAl(Si4-x Alx )N7 Oδ with Unusual [AlN6 ] Octahedral Coordination

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1-xPbxZn2Ga2O7

Lone pair cations like Pb²⁺ are extensively utilized to modify and tune physical properties, such as nonlinear optical property and ferroelectricity, of some specific structures owing to their preference to adopt a local distorted coordination environment. Here we report that the incorporation of Pb²⁺ into the polar “114”-type structure of CaBaZn₂Ga₂O₇ leads to an unexpected cell volume expansion of CaBa_1-xPb_xZn₂Ga₂O₇ (0 ≤ x ≤ 1), which is a unique structural phenomenon in solid state chemistry. Structure refinements against neutron diffraction and total scattering data and theoretical calculations demonstrate that the unusual evolution of the unit cell for CaBa_1-xPb_xZn₂Ga₂O₇ is due to the combination of the high stereochemical activity of Pb²⁺ with the extremely strained [Zn₂Ga₂O₇]⁴⁻ framework along the c-axis. The unprecedented cell volume expansion of the CaBa_1−xPb_xZn₂Ga₂O₇ solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba²⁺ is replaced with smaller Pb²⁺.

Lone pair cations can impart interesting features in some structures, such as noncentrosymmetry. Here the authors show unexpected cell volume expansion in a polar “114”-type oxide upon replacing Ba²⁺ with a smaller Pb²⁺, and attribute it to high stereochemical activity of Pb²⁺ with the strained framework.