Pressure-Induced Polymorphic Transitions in Crystalline Diborane Deduced by Comparison of Simulated and Experimental Vibrational Spectra

We investigate and assign the pressure-induced structural transformations in crystalline diborane (B2H6) observed spectroscopically by Song and co-workers (J. Phys. Chem. B2009, 113, 13509; J. Chem. Phys.2009, 131, 174506) between 3.5 and 24 GPa at room temperature. The assignment is made by calculating the Raman and infrared vibrational spectra of 10 candidate structures at various pressures and comparing the results to experiment. We find that solid diborane undergoes a polymorphic transition at about 6 GPa from β-diborane (P21/n) to a P21/c diborane and possibly a second transition near 14 GPa to another P21/c diborane structure. We conclude that no cyclic oligomers or chains of the composition (BH3)n (n > 2) are formed from diborane up to at least 24 GPa under the experimental conditions employed by the Song group, even when such structures are thermodynamically favored. This suggests that pressure-induced chemical transformations of molecular crystals of diborane are kinetically hindered.