Nickel(II) chromate (NiCrO4) is an acid-soluble compound, red-brown in color, with high tolerances for heat. It and the ions that compose it have been linked to tumor formation and gene mutation, particularly to wildlife.[1]
Nickel(II) chromate can be formed in the lab by heating a mixture of chromium(III) oxide and nickel oxide at between 700 °C and 800 °C under oxygen at 1000 atm pressure. It can be produced at 535 °C and 7.3 bar oxygen, but the reaction takes days to complete.[2] If the pressure is too low or temperature too high but above 660 °C, then the nickel chromium spinel NiCr2O4 forms instead.[2]
Karin Brandt also claimed to make nickel chromate using a hydrothermal technique.[3]
Precipitates of Ni2+ ions with chromate produce a brown substance that contains water.
The structure of nickel chromate is the same as for chromium vanadate, CrVO4. Crystals have an orthorhombic structure with unit cell sizes a = 5.482 Å, b = 8.237 Å, c = 6.147 Å. The cell volume is 277.6 Å3 with four formula per unit cell.[4] [5]
Nickel chromate is dark in colour, unlike most other chromates which are yellow.[2] The infrared spectrum of nickel chromate show two sets of absorption bands. The first includes lines at 925, 825, and 800 cm−1 due to Cr-O stretching, and the second has lines at 430, 395, 365 (very weak) due to Cr-O rock and bend and 310 cm−1 produced from Ni-O stretching.[6]
When heated at lower oxygen pressure around 600 °C, nickel chromate decomposes to the nickel chromite spinel, nickel oxide and oxygen.[2]
4 NiCrO4 → 2 NiCr2O4 + 2 NiO + 3 O2 (gas)
Nickel chromates can also crystallize with ligands. For instance, with 1,10-phenanthroline it can form triclinic olive-colored crystals of [Ni(1,10-phenanthroline)CrO<sub>4</sub>•3H<sub>2</sub>O]•H2O, orange crystals of Ni(1,10-phenanthroline)3Cr2O7•3H2O, and yellow powdered Ni(1,10-phenanthroline)3Cr2O7•8H2O.[7]