  Ternary pnictides semiconductors with II-IV-V2 stoichiometry hold potential
as cost effective thermoelectric materials with suitable electronic transport
properties, but their lattice thermal conductivities ($\kappa$) are typically
too high. Gaining insight into their vibrational properties is therefore
crucial to finding strategies to reduce $\kappa$ and achieve improved
thermoelectric performance. We present a theoretical exploration of the lattice
thermal conductivities for a set of pnictide semiconductors with ABX2
composition (A = Zn, Cd; B = Si, Ge, Sn; and X = P, As), using machine-learning
based regression algorithms to extract force constants from a reduced number of
density functional theory simulations, and then solving the Boltzmann transport
equation for phonons. Our results align well available experimental data,
decreasing the mean absolute error by ~3 Wm-1K-1 with respect to the best
previous set of theoretical predictions. Zn-based ternary pnictides have, on
average, more than double the thermal conductivity of the Cd-based compounds.
Anisotropic behaviour increases with the mass difference between A and B
cations, but while the nature of the anion does not affect the structural
anisotropy, the thermal conductivity anisotropy is typically higher for
arsenides than for phosphides. We identify compounds, like CdGeAs2, for which
nanostructuring to an affordable range of particle sizes could lead to values
low enough for thermoelectric applications.
