Native point defects in rock-salt and wurtzite MnSe: formation energies across the growth window and spin splitting in the A-type order
Manganese selenide grows both as rock-salt $α$-MnSe, a type-II antiferromagnet, and as metastable wurtzite w-MnSe, whose magnetic order has not been measured. We compute the neutral Mn and Se vacancies and the two antisites of both polymorphs with density functional theory plus a linear-response Hubbard $U$, between the Se-rich limit bounded by MnSe$_2$ and the Mn-rich limit set by the measured formation enthalpy. In $α$-MnSe the Mn vacancy is lowest over most of the window (0.92 eV under Se-rich conditions), consistent with its p-type conduction. In w-MnSe, the Se antisite is lowest under Se-rich conditions, narrowly in the A-type order and clearly in the stripe order, and the Se vacancy is lowest under Mn-rich conditions. The Se vacancy has nearly the same formation energy in both polymorphs, whereas defects that change the number of Mn atoms differ by 0.4-1.1 eV, although DFT+$U$ places wurtzite below rock salt, contrary to experiment. A spin-degenerate stripe order lies 14 meV per formula unit below the altermagnetic A-type order of w-MnSe, and the formation energies shift between these orders by up to 0.24 eV, most for the Mn vacancy. Three of the four defects leave an uncompensated moment of 3-5 $μ_\mathrm{B}$. In the A-type order, band unfolding shows that the sign-reversing splitting of the most strongly split band, about 230 meV and 3 eV below the valence-band maximum, keeps its peak height within the 8% scatter of the analysis at 3% defects, while points on its shoulders move by up to about 70 meV. Band-edge states are not resolved.