“…First, the cuprous chromite CuCrO 2 , also named “MacConnelite” in honor of the geologist Richard Bradford McConnell and occasionally noted as Cu 2 Cr 2 O 4 in older works, crystallizes in a rhombohedral structure (delafossite type, space group R 3̅ m ; Figure a) and it is one of the most encouraging candidates as transparent p-type conducting oxide in optoelectronic devices. − This phase also presents interest in thermoelectric applications, − catalytic/photocatalytic, − photocathode in dye-sensitized solar cells (DSSCs), − water splitting/H 2 production, , and multiferroic behavior. , Second, the copper chromite CuCr 2 O 4 presents a tetragonally distorted spinel structure (hausmannite type, space group I 4 1 / amd ; Figure b) at room temperature and transforms into a cubic structure (spinel type, space group Fd 3̅ m ; Figure c) through a transition involving the cooperative Jahn–Teller distortion at 580 ± 5 °C. , This spinel phase has wide commercial application as catalysts in organic synthesis process, − H 2 production, and photovoltaic. , Third, the copper chromate CuCrO 4 compound (space group Cmcm ; Figure d) is also stable under p O 2 = 1 atm . This phase contains chromium at oxidation degree +VI, which is toxic and prohibited in some major safety regulations, e.g., European Union regulation REACH (Registration, Evaluation, Authorisation, and Restriction of Chemicals).…”