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PreprintCC BYChemRxiv

Helium Inclusion in a Mixed-Anion Framework Under Pressure: Discovery of Tetragonal BaFClHe and the Diamond Anvil Cell Sensor Paradox

  • Evgeniy Agafonov1
  1. 1Independent Researcher
Open at sourcePDFDOICite

Abstract

Helium has the highest ionization potential among all elements (24.59 eV) and is notoriously chemically inert. Here, utilizing density functional theory (DFT-PBE-PAW) and density functional perturbation theory (DFPT), we report the discovery of a stable quaternary helium insertion compound in a mixed-anion matrix, BaFClHe, synthesized under moderate pressures of 35 GPa. In the tetragonal matlockite-derived framework (𝑃 4 π‘›π‘šπ‘š, 𝑍 = 2), neutral helium atoms reside in square antiprismatic cavities (2𝑏 sites) without covalent charge transfer (π‘ž = +0.0395𝑒, 1𝑠1.962𝑝0.00). The phase is thermodynamically protected against binary decomposition (2 BaFClHe β†’ BaF2 + BaCl2 + 2 He) by a substantial enthalpy barrier of +67.22 kJ/mol, while DFPT calculations confirm complete dynamical stability with zero imaginary phonon modes across the Brillouin zone. The four isolated high-frequency optical modes (417.9cmβˆ’1[𝐡1𝑔], 443.0cmβˆ’1[𝐴2𝑒], 548.3cmβˆ’1[𝐸𝑒], 567.9cmβˆ’1[𝐸𝑔]) provide distinct spectroscopic fingerprints for in situ detection. Strikingly, while pure host BaFCl undergoes a transition to an ultra-dense hexagonal phase (𝑃 63 π‘šπ‘šπ‘, ZrBeSi-type) at 35 GPa, helium insertion completely reverses the polymorph stability by over 186.2 kJ/mol, demonstrating guest-induced structural templating. Because Sm2+:BaFCl crystals have served as standard luminescence pressure sensors with helium as the hydrostatic pressure-transmitting medium for four decades, this discovery demonstrates that helium is not an inert spectator fluid, resolving an overlooked fundamental interaction in experimental high-pressure physics.

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Cite

BibTeXRIS
BibTeX
@misc{agafonov2026helium,
  title = {{Helium Inclusion in a Mixed-Anion Framework Under Pressure: Discovery of Tetragonal BaFClHe and the Diamond Anvil Cell Sensor Paradox}},
  author = {Evgeniy Agafonov},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15010485/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010485/v1},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - Helium Inclusion in a Mixed-Anion Framework Under Pressure: Discovery of Tetragonal BaFClHe and the Diamond Anvil Cell Sensor Paradox
AU  - Evgeniy Agafonov
PY  - 2026
DA  - 2026/10/14
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15010485/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010485/v1
AB  - Helium has the highest ionization potential among all elements (24.59 eV) and is notoriously chemically inert. Here, utilizing density functional theory (DFT-PBE-PAW) and density functional perturbation theory (DFPT), we report the discovery of a stable quaternary helium insertion compound in a mixed-anion matrix, BaFClHe, synthesized under moderate pressures of 35 GPa. In the tetragonal matlockite-derived framework (𝑃 4 π‘›π‘šπ‘š, 𝑍 = 2), neutral helium atoms reside in square antiprismatic cavities (2𝑏 sites) without covalent charge transfer (π‘ž = +0.0395𝑒, 1𝑠1.962𝑝0.00). The phase is thermodynamically protected against binary decomposition (2 BaFClHe β†’ BaF2 + BaCl2 + 2 He) by a substantial enthalpy barrier of +67.22 kJ/mol, while DFPT calculations confirm complete dynamical stability with zero imaginary phonon modes across the Brillouin zone. The four isolated high-frequency optical modes (417.9cmβˆ’1[𝐡1𝑔], 443.0cmβˆ’1[𝐴2𝑒], 548.3cmβˆ’1[𝐸𝑒], 567.9cmβˆ’1[𝐸𝑔]) provide distinct spectroscopic fingerprints for in situ detection. Strikingly, while pure host BaFCl undergoes a transition to an ultra-dense hexagonal phase (𝑃 63 π‘šπ‘šπ‘, ZrBeSi-type) at 35 GPa, helium insertion completely reverses the polymorph stability by over 186.2 kJ/mol, demonstrating guest-induced structural templating. Because Sm2+:BaFCl crystals have served as standard luminescence pressure sensors with helium as the hydrostatic pressure-transmitting medium for four decades, this discovery demonstrates that helium is not an inert spectator fluid, resolving an overlooked fundamental interaction in experimental high-pressure physics.
ER  -