Chromium(III) phosphate describes inorganic compounds with the chemical formula CrPO4.(H2O)n, where n = 0, 4, or 6. All are deeply colored solids. Anhydrous CrPO4 is green. The hexahydrate CrPO4•6H2O is violet. Synthesis Chromium phosphate is prepared by treating a phosphoric acid solution of chromium(VI) oxide with hydrazine. Hexahydrated chromium(III) phosphate Hexahydrate chromium phosphate, CrPO4•6H2O, is prepared by reducing chromium trioxide, CrO3, with ethanol in the presence of orthophosphoric acid, H3PO4, at temperatures ranging from −24 °C to +80 °C. Mesoporous phase Gel-like chromium(III) phosphate is prepared through the reduction of ammonium dichromate, (NH4)2Cr2O7, using ethanol, CH3COOH, and nitric acid, HNO3. This process is done in the presence of ammonium dihydrogen phosphate and urea at an elevated temperature where tetradecyltrimethylammonium bromide (TTBr) is used as structure directing agent. Films Preparation of textured chromium phosphate is carried out by mixing equimolar solutions of aqueous chromium nitrate and diammonium phosphate in a dish placed in a sealed chamber with the low temperature ammonia vapor catalyst diffusing into the solution at a constant rate. After 24h, the resulting purple film grows out from the liquid through the hydrolysis and polycondensation occurring in the reaction environment at the air/liquid and film/liquid boundary. Surface tension makes the film compact making it easy to insert a microscope slide and lift the film from underneath the solution surface. Once obtained the solution is washed with deionized water and ethanol, then dried in a vacuum. Amorphous phase The preparation of anhydrous chromium(III) phosphate begins by grinding a mixture of 75 mol% of chromium(III) oxide, Cr2O3, and 25 mol% of pure ammonium hydrogen phosphate, (NH4)2HPO4. This mixture is pressed into pellets and heated under air pressure at 400 °C for 24h in order to remove ammonia and water. After this, a heating sequence of 450 °C (24 h), 700 °C (3⋅24 h), 800 °C (24 h) and 850 °C (2⋅24 h) occurs. The pellet mixture is gradually cooled thereafter. Physical properties Crystal structure Chromium(III) phosphate can exist as two isomorphs. Its β-isoform is orthorhombic with the Cmcm space group (a = 0.5165, b = 0.7750, c = 0.6131 nm). The structure consists of infinite chains of trans edge-sharing CrO6 octahedra, which run parallel to the c-axis, and are linked by PO4 tetrahedra. Above 1175 °C, β-CrPO4 converts to α-CrPO4. α-CrPO4 is orthorhombic as well, with the Imma space group (a = 1.0380, b = 1.2845, c = 0.6278 nm). The