  Inorganic semiconductors based on heavy pnictogen cations (Sb3+ and Bi3+)
have gained significant attention as potential nontoxic and stable alternatives
to lead-halide perovskites for solar cell applications. A limitation of these
novel materials, which is being increasingly commonly found, is carrier
localization, which substantially reduces mobilities and diffusion lengths.
Herein, the layered p\v{r}\'ibramite CuSbSe2 is investigated and discovered to
have delocalized free carriers, as shown through optical pump terahertz probe
spectroscopy and temperature-dependent mobility measurements. Using a
combination of theory and experiment, it is found that the underlying factors
are: 1) weak coupling to acoustic phonons due to low deformation potentials, as
lattice distortions are primarily accommodated through rigid inter-layer
movement rather than straining inter-atomic bonds, and 2) weak coupling to
optical phonons due to the ionic contributions to the dielectric constant being
low compared to electronic contributions. This work provides important insights
into how pnictogen-based semiconductors avoiding carrier localization could be
identified.
