Combining the Cu 3d and P 3p valence orbitals is another viable technique for achieving high valence band dispersion, and was investigated in the ternary phosphides MCuP, where M = Mg, Ca, Ba, and Sr, by Williamson et al. in 2017. MgCuP crystallises in the Pnma space group, resulting in a Cu atom tetrahedrally coordinated to four P atoms, while CaCuP, BaCuP and SrCuP crystallise in the P6 3 /mmc space group, where the Cu adopts a trigonal planar configuration with the P atoms forming hexagonal layers of Cu-P sheets (Figure ). The trigonal planar coordination of Cu allows for greater spatial overlap and mixing of the Cu 3d states and P 3p states at the valence band maximum compared to tetrahedrally coordinated Cu in MgCuP, at the cost of more anisotropic behaviour. Of the three P6 3 /mmc structures studied computationally, CaCuP emerged as the most promising candidate for p-type TCM applications, and when synthesised as a nominally undoped powder sample it displayed p-type conductivity of around 500 S cm -1 (±50 S cm -1 ). This is extremely high for a pressed pellet subject to grain boundary and interface scatter-ing. These samples also showed a strong optical absorption onset at around 2.7 eV, in good agreement with predicted behaviour. However, the optical absorption coefficient of a powder sample is not easily accessible. These preliminary results justify a more systematic exploration of thin-film CaCuP to investigate potential deposition methods, optical properties, the origin of its electrical conductivity, and the dependence of the latter on growth conditions.