Size Dependence of the Tetragonal to Orthorhombic Phase Transition of Ammonia Borane in Nanoconfinement

We have investigated the thermodynamic property modification of ammonia borane via nanoconfinement. Two different mesoporous silica scaffolds, SBA-15 and MCM-41, were used to confine ammonia borane. Using in situ Raman spectroscopy, we examined how pore size influences the phase transition temperature from tetragonal (<i>I4mm</i>) to orthorhombic (<i>Pmn21</i>) for ammonia borane. In bulk ammonia borane, the phase transition occurs at around 217 K; however, confinement in SBA-15 (with ~8 nm pore sizes) reduces this temperature to approximately 195 K, while confinement in MCM-41 (with pore sizes of 2.1-2.7 nm) further lowers it to below 90 K. This suppression of the phase transition as a function of pore size has not been previously studied using Raman spectroscopy. The stability of the <i>I4mm</i> phase at a much lower temperature can be interpreted by incorporating the surface energy terms to the overall free energy of the system in a simple thermodynamic model, which leads to a significant increase in the surface energy when transitioning from the tetragonal phase to the orthorhombic phase.