High-pressure crystal structure and phase transitions in B ₂ O ₃

A high-pressure polymorph of boron oxide, B₂O₃-P2₁2₁2₁, has been obtained in a laser-heated diamond anvil cell at 50(3) GPa after heating boron trioxide to a temperature of Tmax ≤ 2500(300) K. The crystal structure was determined from synchrotron single crystal X-ray diffraction and was confirmed by a combination of density functional theory (DFT) calculations and experimental Raman spectroscopy. Experimental- as well as DFT-data confirm the non-centrosymmetric/non-polar space group P2₁2₁2₁. The structural model of B₂O₃-P2₁2₁2₁ is in agreement with an earlier DFT-based crystal structure prediction. The crystal structure is characterized by corner-sharing [BO₄] tetrahedra with covalent B-O bonds. Our experimental Raman data show that B₂O₃-P2₁2₁2₁ can be recovered under ambient conditions.