title: Determination of structure and phase transition of light element nanocomposites in mesoporous silica: case study of NH3BH3 in MCM-41.
abstract: Nanocomposition of molecular crystal ammonia borane (AB) by embedding it in mesoporous silica leads to a remarkable enhancement of the hydrogen storage properties. To investigate the nature of a nanophase AB, we used atomic pair distribution function (PDF) analysis of synchrotron X-ray powder diffraction data to follow the structural evolution of AB embedded within MCM-41 at temperatures ranging from 80 to 300 K. We found that the nanophase AB residing within the mesoporous scaffold does not undergo the structural phase transition at 225 K that was observed in the neat molecular crystal. Rather, it stays in the tetragonal phase over a wide temperature range of 110 to 240 K and starts to lose structural correlation above 240 K. This finding strongly suggests that nanoconfinement of AB within mesoporous scaffolds stabilizes the high-temperature disordered tetragonal phase at a much lower temperature. PDF analyses of composite materials composed of excess AB (i.e., AB:MCM-41 > 1:1) indicates that the excess AB forms aggregates outside the mesoporous scaffold and that these aggregates have structural properties similar to neat AB, that is, the orthorhombic-to-tetragonal structural phase transition is observed at 225 K upon warming. These results may provide important insight into the mechanism behind the enhanced hydrogen storage properties of this system.
authors.affiliation: Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. hjkim@lanl.gov
authors.forename: Hyunjeong
authors.initials: H
authors.lastname: Kim
authors.forename: Abhi
authors.initials: A
authors.lastname: Karkamkar
authors.forename: Tom
authors.initials: T
authors.lastname: Autrey
authors.forename: Peter
authors.initials: P
authors.lastname: Chupas
authors.forename: Thomas
authors.initials: T
authors.lastname: Proffen