Evidence for a new molecular packing at low temperatures in vapor-deposited indomethacin

Thin films of the low molecular weight organic glassformer indomethacin were prepared at different substrate temperatures using physical vapor deposition. When analyzed by wide angle x-ray scattering, samples prepared at T$_{g}$-50 K showed a broad high intensity peak that is not present in samples prepared at T$_{g}$. When such samples were annealed at T$_{g}$+4 K the extra peak eventually vanished but only after 24,000 s (more then 500 $\tau _{\alpha })$. At low deposition rates the WAXS signature of this new molecular packing is only observed for substrate temperatures below T$_{g}$-20 K. Based on WAXS data on supercooled indomethacin, the new WAXS peak is unexpected. These results suggest that ordinary indomethacin glasses aged to equilibrium 20 K below the conventional T$_{g}$ would undergo a first order transition to a new amorphous phase.