Vibrational spectroscopy identifies the bond asymmetry of hexagonal diamond
Abstract
Bulk hexagonal diamond has been synthesized by independent routes, but its structure remains contested: the two recent refinements disagree even on the sign of the difference between its two inequivalent bond lengths, 238~mÅ apart, and both depart from an earlier 2003 refinement. Here we test the competing structures with first-principles lattice dynamics. Relaxed hexagonal diamond has an interlayer bond \emph{longer} than the intralayer bonds by 24~mÅ in both functionals, an effect of its eclipsed conformation that scales with polytype hexagonality. The bright zone-center A1g mode gauges the interlayer bond at ≈ −2,100~\icm~Å−1, and neither refined coordinate reproduces the full pattern of measured modes. The only structure matching the twinned sample's three bands requires tens-of-gigapascals confining stress and lattice constants excluded by its own diffraction. Raman spectroscopy and diffraction jointly select a small positive bond asymmetry: inverting the spectrum of the phase-pure sample gives OB−OA=24±3~mÅ (95\% interval), and two determinations on separate samples give +33±8 and +60±45~mÅ. The 1{,}529~\icm{} feature cannot be assigned to homogeneous ideal 2H diamond, and the local HRTEM observation remains an open puzzle.
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BibTeX
@misc{zhu2026vibrational,
title = {{Vibrational spectroscopy identifies the bond asymmetry of hexagonal diamond}},
author = {Li Zhu},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.00138},
eprint = {2608.00138},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.00138},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Vibrational spectroscopy identifies the bond asymmetry of hexagonal diamond
AU - Li Zhu
PY - 2026
DA - 2026/09/16
PB - arXiv
DO - 10.48550/arxiv.2608.00138
UR - https://arxiv.org/abs/2608.00138
AB - Bulk hexagonal diamond has been synthesized by independent routes, but its structure remains contested: the two recent refinements disagree even on the sign of the difference between its two inequivalent bond lengths, 238~mÅ apart, and both depart from an earlier 2003 refinement. Here we test the competing structures with first-principles lattice dynamics. Relaxed hexagonal diamond has an interlayer bond \emph{longer} than the intralayer bonds by 24~mÅ in both functionals, an effect of its eclipsed conformation that scales with polytype hexagonality. The bright zone-center A_1g mode gauges the interlayer bond at ≈ −2,100~\icm~Å^−1, and neither refined coordinate reproduces the full pattern of measured modes. The only structure matching the twinned sample's three bands requires tens-of-gigapascals confining stress and lattice constants excluded by its own diffraction. Raman spectroscopy and diffraction jointly select a small positive bond asymmetry: inverting the spectrum of the phase-pure sample gives OB−OA=24±3~mÅ (95\% interval), and two determinations on separate samples give +33±8 and +60±45~mÅ. The 1{,}529~\icm{} feature cannot be assigned to homogeneous ideal 2H diamond, and the local HRTEM observation remains an open puzzle.
ER -