  The pressure-induced polymorphism of binary octect compounds has long been
considered a settled problem although the possible atomic disordering of some
phases remains a puzzling observation. Taking GaP as a case study, we conclude,
through x-ray microdiffraction and first-principles calculations, that its
high-pressure phase II (previously reported as being disordered) adopts in fact
an ordered base-centered monoclinic structure previously unknown in this class
of compounds. The formation of layered patterns with variable degrees of
interlayer dimerization, as observed in GaP, marks a paradigm shift of our
understanding of ordering in octect high-pressure phases which calls for a more
extensive re-examination. A rich polymorphism with fine tuning of chemical and
physical properties can be envisioned.
