  The properties of liquid Si-Ge binary systems at melting conditions deviate
from those expected by the ideal alloy approximation. Particularly, a
non-linear dependence of the dielectric functions occurs with the reflectivity
of liquid Si-Ge reaching a maximum at 50\% Ge content, being 10\% higher than
in pure Si or Ge. Using \textit{ab initio} methodologies, we modelled liquefied
Si-Ge alloys, unveiling very high coordination numbers and poor symmetry in the
first coordination shell with respect to Si and Ge, related to different
bonding properties. We simulated optical functions, quantitatively replicating
the aforementioned reflectivity trend and we highlighted a direct relationship
between atomic structure and optical properties, indicating that the unusual
optics arises from Si-Ge higher local coordination characterized by low
symmetry. We forecast further implications for the overall class of these
alloys. These findings expand our comprehension of liquefied semiconductors and
are essential for implementing controlled laser melting procedures to highly
dope these materials for advanced transistors, superconductors, sensors and
plasmonic devices.
