Phosphosulfides with P/S < 1. A closer look at the known PSs indicates that they mainly consist of ternary and quaternary compositions, and that the preferred (molar) stoichiometric P/S ratios are 1, 1/3, 2/7 and 1/4, (Fig ). PSs exhibiting the 1/3, 2/7 and 1/4 ratios are usually called thiophosphates, due to their similarity to their oxide-based counterparts (phosphates). Rather than being true multi-anion materials, thiophosphates are polyanionic compounds in which P and S form a well-defined structural unit (the polyanion) with P in the center. The metal cations only bond to the S atoms of the polyanions, and therefore P is in a positive oxidation state, as it tends to donate electrons to S within the polyanion (Fig. ) (visualized with VESTA ). This is a completely opposite behavior with respect to the well-known III-phosphide semiconductor (In,Ga)P, and also to other emerging phosphide semiconductors such as ZnGeP 2 and CuP 2. In these single-anion semiconductors, P is always in a negative oxidation state (usually -3, unless P-P bonds are present). Instead, thiophosphates have more similarities with polyanionic semiconductors based on other elements, such as the well-studied PEC material BiVO 4 . The most common polyanions in thiophosphates are [PS 4 ] 3- in materials with P/S = 1/4 such as Cu 3 PS 4 ; [P 2 S 6 ] 2-, and [P 2 S 6 ] 4-in materials with P/S = 1/3 such as KPS 3 and NiPS 3 ; and [P 2 S 7 ] -4 in materials with P/S = 2/7 such Ag 4 P 2 S 7 .