The present invention relates to a process for producing a layer of crystalline A/M/X material, wherein the process comprises disposing on a substrate a precursor composition comprising: (a) a first precursor compound comprising a first cation (M), which first cation is a metal or metalloid cation; and (b) a solvent, wherein the solvent comprises; (i) acetonitrile, propionitrile, acetone or a mixture thereof; and (ii) an alkylamine. The invention also relates to a composition comprising: (i) a compound of formula MXn, (ii) a compound of formula AX, (iii) acetonitrile, propionitrile, acetone or a mixture thereof; and (iv) an alkylamine of formula RANH2, wherein RA is a C1-8 alkyl group.