Determination of the crystal structures of NdBaMn2O6 : Coexisting polar and nonpolar phases

The crystal structures of coexisting polar and nonpolar phases in $A$-site ordered ${\mathrm{NdBaMn}}_{2}{\mathrm{O}}_{6}$ were investigated by a combination of convergent-beam electron diffraction (CBED) and density functional theory (DFT) calculations. A symmetry analysis of the CBED patterns from several segments of $A$-site ordered ${\mathrm{NdBaMn}}_{2}{\mathrm{O}}_{6}$ at room temperature ($\text{RT}\ensuremath{\approx}293$ K) revealed the presence of two crystallographic distinct phases with polar $C2mm$ (No. 38) and nonpolar $Cmmm$ (No. 65) space groups. The combined method was used successfully to determine the crystal structures of the coexisting phases with the same chemical composition and similar lattice parameters. DFT calculations revealed that the comparable total energies of the two phases could be the origin of their coexistence.