Structure and band gap of the La2NiMnO6 double perovskite: synthesis, structure characterization and DFT+U ab initio simulations

La₂NiMnO₆ is a near-room-temperature ferromagnetic insulator whose reported band gap scatters widely: about 1.0 to 1.5 eV experimentally, at least 0.8 to 1.7 eV in DFT, with optical extractions often stopping short of the absorption edge and the ordering state and phase composition of the measured samples frequently left undetermined. We address both sides of this uncertainty with a single specimen batch: solid-state synthesis; laboratory XRD; time-of-flight neutron powder diffraction on GEM (ISIS), whose Ni/Mn contrast and oxygen sensitivity give access to site occupancies, oxygen positions and P2₁/n / R-3 phase fractions; and DFT+U calculations at a published 4.2 K neutron structure yielding an indirect Kohn-Sham gap of 0.7 to 1.0 eV for U_Ni = 5.0 to 7.1 eV. Near-infrared diffuse reflectance with controlled Kubelka-Munk analysis (in progress) will close the structure-electronic-optical loop on the same batch.