Optical properties and lattice dynamics of a novel allotrope of orthorhombic elemental germanium

: Optical and vibrational properties of a novel allotrope of elemental germanium Ge(oP32), which crystallizes in the structure corresponding to the orthorhombic space group Pbcm, are studied experimentally by means of absorption and polarized Raman scattering measurements and theoretically using the first principles density functional theory. Material is found to be a direct band gap semiconductor with E g   =  0.33 eV. Out of theoretically predicted 48 Raman-active modes, 27 are observed in the spectra and assigned to the specific lattice eigenmodes of the crystal based on their symmetry and a comparison with the results of first principles lattice dynamics calculations. Remarkably, the highest frequency vibration is observed at 316 cm-1, that is higher than the cubic crystalline [Formula: see text]-Ge mode at 300 cm-1. Exceptional sharpness of observed phonon lines (between 0.8 and 2.5 cm-1 at T  =  10 K) implies excellent crystallinity of Ge(oP32) crystals.