Luminescence of hexagonal boron nitride (hBN) has puzzled researchers for long time.
In standard solid state physics textbooks direct band gap semiconductors are considered efficient light emitters while indirect ones are regarded as inefficient.
Hexagonal boron nitride seems to defy this rule. In fact hBN emits light in the ultraviolet with an efficient comparable to direct band gap materials. In this study we show that this is due to the strong electron-phonon coupling, the is enhanced  by the presence of strong bond excitons.

The paper is available here: https://arxiv.org/abs/1807.11797

and published here: PRB 99, 081109(R)