Observation of discrete molecular structures in glassyPxSe1−xby Raman spectroscopy

We report conclusive evidence for the existence of molecular local ordering in four phosphorus-selenium glasses. The compositions were chosen to span the P-rich glass-forming region and were ${\mathrm{P}}_2$Se, ${\mathrm{P}}_3$Se, ${\mathrm{P}}_4$Se, and ${\mathrm{P}}_{0.84}$ ${\mathrm{Se}}_{0.16}$. Raman spectra show unequivocally the existence in each glass of well-defined ${\mathrm{P}}_4$ ${\mathrm{Se}}_3$ cage molecules. All nine Raman-active internal modes of this molecule have been observed, with a clear trend of reduction of their intensity as the P content is increased. We find that these materials behave as a matrix-supported assembly of ${\mathrm{P}}_4$ ${\mathrm{Se}}_3$ molecules.