  LiOsO3 is one of the first materials identified in a recent literature as a
'polar metal', a class of materials that are simultaneously noncentrosymmetric
and metallic. In this work, the linear and nonlinear optical susceptibility of
LiOsO3 is studied by means of ellipsometry and optical second harmonic
generation (SHG). Strong optical birefringence is observed using spectroscopic
ellipsometry. The nonlinear optical susceptibility extracted from SHG
polarimetry reveals that the tensor components are of the same magnitude as in
isostructural insulator LiNbO3, except the component along the polar axis d33,
which is suppressed by an order of magnitude. Temperature-dependent SHG
measurements in combination with Raman spectroscopy indicate a continuous
order-disorder type polar phase transition at 140 K. Linear and nonlinear
optical microscopy techniques reveal 109 deg/71 deg ferroelastic domain walls,
like in other trigonal ferroelectrics. No 180 deg polar domain walls are
observed to emerge across the phase transition.
