Rapid synthesis of dual-element isotope-enriched α − Mo O 3 crystals by reactive vapor transport

In this work, we develop a rapid reactive vapor transport technique to efficiently utilize limited isotopically pure precursors, particularly gaseous $^{18}\mathrm{O}_{2}$, and synthesize mm-scale, high-quality isotope-enriched crystals within few-minute growth durations. We unlock this capability by using metallic molybdenum precursors with high source temperatures ($900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) and total pressures ($\ensuremath{\sim}1$ atm) to maximize precursor efficiency and yield. Subsequently, we grow $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$ single crystals with high and uniform enrichment levels of $^{98}\mathrm{Mo}$ and $^{18}\mathrm{O}$ isotopes in several different permutations. As probed by Raman spectroscopy, modest and significant phonon energy redshifts occur following $^{98}\mathrm{Mo}$ and $^{18}\mathrm{O}$ enrichment, respectively. By demonstrating control over both molybdenum and oxygen isotopic enrichments, we establish a powerful tool to advance nanophotonics and thermal management goals using $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Mo}{\mathrm{O}}_{3}$. This work is motivated by the possibility to enhance and engineer lattice vibrational mode phenomena including thermal conduction and hyperbolic phonon polariton dispersion---with particular interest in comparing the effects of light and heavy element enrichment.