  Density functional theory (DFT) based first-principles investigations of
structural, elastic, electronic band structure, and optical properties of
superconducting ternary phosphides (BaM2P2; M = Ni, Rh) have been carried out
in this study. This is the first detailed pressure dependent study of these
properties for the titled compounds. The calculated ambient condition
properties are compared with existing experimental and theoretical results,
where available. The pressure dependent variations of the electronic density of
states at the Fermi level, N(EF), and the Debye temperature, thetaD, have been
studied and their effect on superconducting transition temperature have been
explored. N(EF) shows nonmonotonic pressure dependence in BaNi2P2. The pressure
dependence of N(EF) for BaRh2P2, on the other hand, is monotonic; decreasing
with increasing pressure up to 15 GPa and saturating at higher pressure.
Pressure dependence of N(EF) is reflected in the pressure dependent
superconducting transition temperature. The Debye temperature increases with
increasing pressure. The variation of the optical parameters (real and
imaginary parts of the dielectric constant, refractive indices, reflectivity,
absorption coefficient, and loss function) with photon energy show metallic
behavior complementing the features of electronic band structure calculations.
The absorption spectra of BaNi2P2 show strong optical absorption in the
ultraviolet region, while BaRh2P2 absorbs photons over a wider energy band
including the entire visible range. The reflectivity spectra for both BaNi2P2
and BaRh2P2 reveal that these materials are very strong reflectors of visible
spectrum and particularly BaNi2P2 have significant potential to be used as
coating material to reduce solar heating.
