{
  "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 𝑛𝑚𝑚, 𝑍 = 2), neutral helium atoms reside in square antiprismatic cavities (2𝑏 sites) without covalent charge transfer (𝑞 = +0.0395𝑒, 1𝑠1.962𝑝0.00). The phase is thermodynamically protected against binary decomposition (2 BaFClHe → 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 (417.9cm−1[𝐵1𝑔], 443.0cm−1[𝐴2𝑢], 548.3cm−1[𝐸𝑢], 567.9cm−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 𝑚𝑚𝑐, 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.",
  "abstract_shown": true,
  "affiliations": [
    "Independent Researcher"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 1,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Evgeniy Agafonov"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/0cs9kkepgtfbd.bib",
  "cited_by": 0,
  "doi": "10.26434/chemrxiv.15010485/v1",
  "fulltext_url": null,
  "id": "0cs9kkepgtfbd",
  "issns": [],
  "landing_url": "https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010485/v1",
  "language": null,
  "licence": "CC-BY",
  "licence_label": "CC BY",
  "licence_source": "journal",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "1908|High-pressure geophysics and materials"
    ]
  ],
  "openalex": "W7222113430",
  "pdf_url": "https://chemrxiv.org/doi/pdf/10.26434/chemrxiv.15010485/v1",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-14",
  "publisher": "American Chemical Society (ACS)",
  "ris": "https://research.draconitic.com/paper/0cs9kkepgtfbd.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:24Z",
      "native_id": "10.26434/chemrxiv.15010485",
      "source": "chemrxiv",
      "url": "https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010485/v1"
    }
  ],
  "subjects": [
    {
      "field": "earth",
      "origin": "openalex:1908|High-pressure geophysics and materials",
      "subfield": "geophysics"
    }
  ],
  "title": "Helium Inclusion in a Mixed-Anion Framework Under Pressure: Discovery of Tetragonal BaFClHe and the Diamond Anvil Cell Sensor Paradox",
  "title_text": "Helium Inclusion in a Mixed-Anion Framework Under Pressure: Discovery of Tetragonal BaFClHe and the Diamond Anvil Cell Sensor Paradox",
  "type": "preprint",
  "updated_at": "2026-10-30T12:17:24Z",
  "url": "https://research.draconitic.com/paper/0cs9kkepgtfbd",
  "venue": "ChemRxiv",
  "versions": []
}