  Quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and
theoretically in the present paper. Investigations of polarized fundamental
absorption spectra demonstrated a high sensitivity to external light
illumination. The photoinduced changes were studied using the cw 532 nm green
laser with energy density about 0.4 J/cm2. The spectral maximum of the
photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV
(energy gap equal to about 1.85 eV) corresponding to maximal density of the
intrinsic defect levels. Spectroscopic measurements were performed for
polarized and un-polarized photoinducing laser light to separate contribution
of the intrinsic defect states from the pure states of the valence and
conduction bands. To understand the origin of the observed photoinduced
absorption near the fundamental edge, the benchmark first-principles
calculations of the structural, electronic, optical and elastic properties of
Cu2CdGeS4 were performed in the general gradient approximation (GGA) and local
density approximation (LDA) approaches. The calculated dielectric function and
optical absorption spectra exhibit some anisotropic behavior within the
0.15...0.20 eV energy range near the absorption edge; the optical anisotropy
was also found in the deep inter-band transition spectral range. Peculiar
features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron
density distribution. Possible intrinsic defects are shown to affect
considerably the optical absorption spectra. Pressure effects on the structural
and electronic properties were modeled by optimizing the crystal structure and
calculating all relevant properties at elevated hydrostatic pressure. The first
estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure
derivative for Cu2CdGeS4 are reported.
