Amorphization of crystalline orthoboric acid on a vitreous B₂O₃ substrate

Time-resolved x-ray diffraction measurements were carried out during and after the exposure of anhydrous boron oxide glass films to humid air. B(OH)₃ crystals grew on the glass substrate with planar sheets of B(OH)₃ molecules aligned parallel to the surface. After the space above the sample was evacuated, the crystal peaks began to disappear. During the decay, new crystals of orthoboric acid grew with the orientation of the B(OH)₃ sheets perpendicular to the substrate. Then crystals with all orientations decayed completely, and an amorphous scattering pattern remained. The water acquired by the sample during the exposure eventually reacted with the anhydrous B₂O₃ glass to produce amorphous BO_(3+y)/2H_y. The decay of the crystals took from one to several days, depending on the length of the exposure to humid air. Measurements of the metastable equilibrium phases in the B₂O₃-H₂O binary system showed that at room temperature the amorphous phase could reach the composition BO_1.85H_0.7. At higher water concentrations this saturated amorphous phase coexisted with crystalline B(OH)₃. This result is consistent with the instability of the crystalline B(OH)₃ film in contact with anhydrous glass, and we present a thermodynamic model for this process. In the present case, amorphization is driven by the outdiffusion of water from a crystalline phase rather than interdiffusion in conventional solid-state amorphization.