  The phosphorescence of boric acid at room temperature is a puzzling
phenomenon subject to controversial interpretations although the role of
structural defects has not yet been considered. Heat treatments of boric acid
cause its transformation into the metaboric phase and amorphous boron oxide.
The structural changes after thermal processing can create defects that become
centers of luminescence and recombination channels in the visible range. In the
present work, we have thermally processed commercial boric acid at different
temperatures. Samples treated between 200 and 400{\deg}C exhibit remarkable
phosphorescence in the visible range. At around 480 and 528 nm, two distinct
phosphorescent emissions occur, associated with trapped charge carriers
recombinations identified by thermoluminescence and electron paramagnetic
resonance spectroscopy. Our structural and optical studies suggest that the
activation of boric acid phosphorescence after heat treatment is correlated
with the presence of defects. The afterglow results from a trapping and
detrapping process, which delays the recombination at the active optical
centers. Time-dependent density functional study of defective BOH molecules and
clusters shows the emergence of near UV and blue optical transitions in
absorption. These defects trigger the photoluminescence in thermally processed
boric acid samples.
