  We perform ab initio calculations that indicate that the relative stability
of antiphase boundaries (APB) with armchair and zigzag chiralities in monolayer
boron nitride (BN) is determined by the chemical potentials of the boron and
nitrogen species in the synthesis process. In an N-rich environment, a zigzag
APB with N-rich core is the most stable structure, while under B-rich or
intrinsic growth conditions, an armchair APB with stoichiometric core is the
most stable. This stability transition is shown to arise from a competition
between homopolar-bond (B-B and N-N) and elastic energy costs in the core of
the APBs. Moreover, in the presence of a carbon source we find that a
carbon-doped zigzag APB becomes the most stable boundary near the N-rich limit.
The electronic structure of the two types of APBs in BN is shown to be
particularly distinct, with the zigzag APB depicting defect-like deep
electronic bands in the band gap, while the armchair APB shows bulk-like
shallow electronic bands.
