Structural Order Reversed: Local and Average Structure Across the Ammonia Borane Phase Transition

Abstract

Ammonia borane (NH3BH3) is a hydrogen rich molecular solid whose structural behaviour is governed by molecular orientation and N–H· · ·H–B dihydrogen interactions. Its temperature-dependent phase transition is conventionally described as an order–disorder transition from a disordered tetragonal phase to a more ordered orthorhombic phase. However, this description does not fully capture the underlying local structure. Single crystal X-ray diffraction, X-ray total scattering, Pair Distribution Function (PDF) analysis and Reverse Monte Carlo (RMC) modelling reveal an unexpected inversion of local structural order across the phase transition. At 300 K, despite pronounced disorder in the average tetragonal structure, the local structure exhibits three preferential NH3 orientations, whereas the BH3 groups remain substantially disordered. In contrast, at 100 K, modelling reveals substantial local disorder of NH3 despite the more ordered average orthorhombic structure. Single crystal diffraction additionally reveals weak satellite reflections, while the 100 K powder diffraction pattern is better reproduced by a trial commensurately modulated model with threefold periodicity than by the conventional average structure. Together, these results indicate that the low temperature phase combines periodic structural modulation with substantial local disorder, demonstrating that average and local structural order do not evolve in parallel with temperature.

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BibTeX RIS

BibTeX

@misc{piekara2026structural,
  title = {{Structural Order Reversed: Local and Average Structure Across the Ammonia Borane Phase Transition}},
  author = {Anna Piekara and Damian Trzybiński and Ewelina Zaorska and Wojciech A. Sławiński and Wojciech Slawinski},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15010462/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010462/v1},
  note = {CC BY},
}

RIS

TY  - UNPB
TI  - Structural Order Reversed: Local and Average Structure Across the Ammonia Borane Phase Transition
AU  - Anna Piekara
AU  - Damian Trzybiński
AU  - Ewelina Zaorska
AU  - Wojciech A. Sławiński
AU  - Wojciech Slawinski
PY  - 2026
DA  - 2026/10/14
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15010462/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010462/v1
AB  - Ammonia borane (NH3BH3) is a hydrogen rich molecular solid whose structural behaviour is governed by molecular orientation and N–H· · ·H–B dihydrogen interactions. Its temperature-dependent phase transition is conventionally described as an order–disorder transition from a disordered tetragonal phase to a more ordered orthorhombic phase. However, this description does not fully capture the underlying local structure. Single crystal X-ray diffraction, X-ray total scattering, Pair Distribution Function (PDF) analysis and Reverse Monte Carlo (RMC) modelling reveal an unexpected inversion of local structural order across the phase transition. At 300 K, despite pronounced disorder in the average tetragonal structure, the local structure exhibits three preferential NH3 orientations, whereas the BH3 groups remain substantially disordered. In contrast, at 100 K, modelling reveals substantial local disorder of NH3 despite the more ordered average orthorhombic structure. Single crystal diffraction additionally reveals weak satellite reflections, while the 100 K powder diffraction pattern is better reproduced by a trial commensurately modulated model with threefold periodicity than by the conventional average structure. Together, these results indicate that the low temperature phase combines periodic structural modulation with substantial local disorder, demonstrating that average and local structural order do not evolve in parallel with temperature.
ER  -

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