Diamonds grown at high pressure and high temperature from melts of rare-earth metals (a new class of solvent–catalyst demonstrated in 2021) are nitrogen-free and rich in group-IV color centers, but they exhibit unexplained optical and growth phenomena whose microscopic origins remain unknown: a narrow 448 nm photoluminescence line unique to light rare-earth catalysts, an anomalously intense 720 nm emission (with a weaker 837 nm line) specific to heavy rare-earth catalysts, an untested hypothesis that rare-earth melts purify the diamond by forming stable nitrides (nitrogen gettering), and an unexplained dependence of crystal habit on rare-earth identity. The problem is to determine the microscopic defect structures behind the 448 nm and 720 nm emissions, to directly demonstrate or refute the nitride-based nitrogen-getter mechanism in the actual growth cell, and to establish the surface-chemistry and kinetic basis of the light-versus-heavy rare-earth habit dichotomy — jointly constituting the mechanism-level understanding needed for predictive control of defect incorporation and growth morphology in this catalyst family.

Background

Diamond is synthesized industrially at high pressure and high temperature (HPHT, typically 5–8 GPa and 1400–2100 °C) by dissolving carbon in a molten metal solvent–catalyst — classically Fe, Ni, or Co — from which diamond crystallizes. Conventional metal catalysts let atmospheric nitrogen dissolve into the growing crystal; producing nitrogen-free 'type II' diamond (type classification: type I contains detectable nitrogen, type II does not) requires additives called nitrogen getters, such as Ti, Al, or Zr, which chemically bind nitrogen into stable nitrides in the melt. Optical characterization of synthetic diamond relies mainly on photoluminescence (PL), in which a laser excites crystal defects (impurity–vacancy complexes, called optical centers or color centers) that emit at characteristic wavelengths; a sharp zero-phonon line (ZPL) marks each center's electronic transition. Important examples include the negatively charged silicon-vacancy center (SiV−, ZPL at 737 nm) and nitrogen-vacancy centers (H3, N3), all of interest for quantum-photonics and sensing applications.

A separate line of work established magnesium as an 'ultra-reducing' HPHT catalyst that also acts as a nitrogen getter: diamonds grown from Mg–C and Mg–Si–C melts are nitrogen-free, contain SiV centers (Palyanov et al., CrystEngComm 17, 4928, 2015; Crystals 7, 119, 2017), and show a weak, never-identified PL line at 720 nm — roughly ten times weaker than the SiV line. An EPR (electron paramagnetic resonance, a magnetic-resonance method that resolves the symmetry and structure of paramagnetic defects) study of Mg–Si–C diamonds found only split-vacancy silicon centers (KUL1, KUL8) and left the 720 nm PL line unattributed (Nadolinny et al., 2015). Diamond growth in carbon–rare-earth-metal (REM) binary systems was first shown by Ekimov et al. in 2017 (Materials Letters 193, 130).

In 2021, Palyanov, Borzdov, Kupriyanov, Khohkhryakov, and Nechaev (Scientific Reports 11, 2021) systematically surveyed diamond synthesis from melts of 15 REMs (Sc, Y, and the lanthanides La through Lu) at 7.8 GPa and 1800–2100 °C. They determined the crystallization windows, found that all grown diamonds are nitrogen-free type II (nitrogen below IR-absorption detection), demonstrated incorporation of Si, Ge, and Sn as SiV, GeV, and SnV color centers, and observed unusual crystal habits: diamonds from light REMs (La, Ce, Pr, Nd) and from the heavy-series exceptions Yb and Tb are pure octahedra, whereas the remaining heavy REMs of the Gd–Lu series produce crystals with rare tetragon-trioctahedron faces ({311}, {411}, {211}) and trigon-trioctahedron faces ({311}, {661}) (trioctahedral habits: cubic crystal forms in which each octahedral face is replaced by three faces of the indicated {hkl} orientation) — a diversity of growth faces never seen in conventional Fe/Ni/Co systems, and one that contradicts the usual empirical rules linking such faces to growth rate or getter addition. Their PL spectroscopy (80 K, 395 nm excitation) revealed two unexplained phenomena: diamonds from light REMs (La, Ce, Pr) show a narrow 448 nm line, stated to be previously unreported for synthetic diamonds, while diamonds from heavy REMs (Tm, Tb, Er, Lu) show an unusually intense 720 nm line with a weaker 837 nm companion — in some crystals exceeding the 737 nm SiV line in intensity and varying strongly between crystals from the same run — with no 448 nm line at all.

The authors proposed that nitrogen-free growth is 'most likely' enabled by the strong affinity of REMs for nitrogen — REM nitrides are thermodynamically comparable in stability to Ti and Zr nitrides — making the melt itself the getter; but this remained a plausibility argument from handbook thermodynamics, with no measurement of nitrogen partitioning, no nitride phase identified in the quenched products, and no cell-specific thermodynamic or kinetic analysis. Thus each phenomenon — the new 448 nm center, the anomalous 720 nm intensity, the nitride-getter mechanism, and the composition-dependent habit — was documented but not explained, and the paper explicitly states that the defect natures are 'not clear at the moment' and 'deserve further investigations'.

Problem Statement

For diamonds synthesized in rare-earth-metal–carbon (REM–C) melt systems at 7.8 GPa and 1800–2100 °C (the growth envelope established by Palyanov et al. 2021), resolve the four linked, currently unexplained aspects of REM-catalyzed growth, whose joint resolution constitutes defect- and mechanism-level understanding sufficient for predictive control of defect incorporation and crystal habit in this catalyst family: (1) identify the microscopic defect structure and electronic nature of the optical center producing the narrow 448 nm photoluminescence line observed in diamonds grown from light rare-earth catalysts (La, Ce, Pr) — a line unreported for synthetic diamond before this work — including why it forms only in light-REM growth; (2) determine the origin of the 720 nm emission and its weaker 837 nm companion in diamonds grown from heavy rare-earth catalysts (observed for Tm, Tb, Er, and Lu among the heavy Gd–Lu group), in particular why this line reaches intensities comparable to or exceeding the 737 nm SiV line in these crystals — an order of magnitude stronger than in Mg-based systems — and why its strength varies between crystals from the same growth run; (3) directly demonstrate, or refute, the proposed mechanism whereby the REM melt acts as a nitrogen getter through formation of stable REM nitrides — e.g. by measurements of nitrogen partitioning between diamond and quenched catalyst, identification of nitride phases in the quenched products, or thermodynamic/kinetic analysis specific to the actual growth-cell conditions — thereby establishing why all REM-grown diamonds are nitrogen-free type II; and (4) establish the surface-chemistry and growth-kinetics basis of the composition-dependent crystal habit — pure octahedra from light REMs (La–Nd) and from the heavy-series exceptions Yb and Tb versus tetragon-trioctahedron ({311}, {411}, {211}) and trigon-trioctahedron ({311}, {661}) faces from the remaining heavy REMs — sufficient to predict which face families develop from a given REM composition and growth conditions. All four parts are mandatory, and the task is resolved only when each is answered as follows. Parts (1), (2), and (4) each require a mechanism-level account of the stated phenomenon, without further experimental proof: a defect-structure assignment that accounts for the observed line(s) and their rare-earth dependence for (1) and (2), and a surface-chemistry/growth-kinetics account from which the light- and heavy-REM face families can be predicted for (4). Part (3) requires a demonstration or refutation of the nitride-getter mechanism under growth-cell conditions; a direct experimental demonstration that establishes the mechanism is by itself sufficient.

References

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