A process for producing a crystalline A/M/X material, which crystalline A/M/X material comprises a compound of formula: a b c wherein: comprises one or more A cations selected from (H 2 N-C =NH 2) +, an alkali metal cation, C 1-10 alkylammonium, C 2-10 alkenylammonium, C 1-10 alkyliminium, C 3-10 cycloalkylammonium and C 3-10 cycloalkyliminium, wherein the C 1-10 alkylammonium, C 2-10 alkenylammonium, C 1-10 alkyliminium, C 3-10 cycloalkylammonium and C 3-10 cycloalkyliminium are each optionally substituted with one or more substituents selected from amine, C 1-6 alkylamine, imine, C 1-6 alkylimine, C 1-6 alkyl, C 2-6 alkenyl, C 3-6 cycloalkyl and C 6-12 aryl; comprises one or more M cations which are metal or metalloid cations selected from Pd 4+, W 4+, Re 4+, Os 4+, Ir 4+, Pt 4+, Sn 4+, Pb 4+, Ge 4+, Te 4+, Bi 3+, Sb 3+, Ca 2+, Sr 2+, Cd 2+, Cu 2+, Ni 2+, Mn 2+, Fe 2+, Co 2+, Pd 2+, Ge 2+, Sn 2+, Pb 2+, Yb 2+ and Eu 2+; comprises one or more X anions selected from F -, Cl -, Br - and I -; a is a number from 1 to 4; b is a number from 1 to 3; and c is a number from 1 to 8, provided that the compound of formula a b c may be a double perovskite of formula 2 [B +][B 3+] 6 or a layered double perovskite of formula 4 [B +][B 3+] 8, wherein and are as defined above, [B +] comprises one or more M cations selected from Li +, Na +, K +, Rb +, Cs +, Cu +, Ag +, Au + and Hg +, and [B 3+] comprises one or more M cations selected from Bi 3+, Sb 3+, Cr 3+, Fe 3+, Co 3+, Ga 3+, As 3+, Ru 3+, Rh 3+, In 3+, Ir 3+ and Au 3+; the process comprising: a) contacting an aqueous solution comprising an A precursor and an aqueous solvent with an organic solution comprising an M precursor and an organic solvent, wherein the aqueous solution and/or the organic solution comprises an X precursor, and wherein the organic solution comprises a hydrohalic acid; and b) allowing a precipitate to form when the said aqueous and organic solutions are contacted, and wherein the process further comprises producing an optoelectronic device comprising the crystalline A/M/X material, wherein the optoelectronic device is a light-emitting device. comprises one or more A cations selected from (H 2 N-C =NH 2) +, an alkali metal cation, C 1-10 alkylammonium, C 2-10 alkenylammonium, C 1-10 alkyliminium, C 3-10 cycloalkylammonium and C 3-10 cycloalkyliminium, wherein the C 1-10 alkylammonium, C 2-10 alkenylammonium, C 1-10 alkyliminium, C 3-10 cycloalkylammonium and C 3-10 cycloalkyliminium are each optionally substituted with one or more substituents selected from amine, C 1-6 alkylamine, imine, C 1-6 alkylimine, C 1-6 alkyl, C 2-6 alkenyl, C 3-6 cycloalkyl and C 6-12 aryl; comprises one or more M cations which are metal or metalloid cations selected from Pd 4+, W 4+, Re 4+, Os 4+, Ir 4+, Pt 4+, Sn 4+, Pb 4+, Ge 4+, Te 4+, Bi 3+, Sb 3+, Ca 2+, Sr 2+, Cd 2+, Cu 2+, Ni 2+, Mn 2+, Fe 2+, Co 2+, Pd 2+, Ge 2+, Sn 2+, Pb 2+, Yb 2+ and Eu 2+; comprises one or more X anions selected from F -, Cl -, Br - and I -; a is a number from 1 to 4; b is a number from 1 to 3; and c is a number from 1 to 8, provided that the compound of formula a b c may be a double perovskite of formula 2 [B +][B 3+] 6 or a layered double perovskite of formula 4 [B +][B 3+] 8, wherein and are as defined above, [B +] comprises one or more M cations selected from Li +, Na +, K +, Rb +, Cs +, Cu +, Ag +, Au + and Hg +, and [B 3+] comprises one or more M cations selected from Bi 3+, Sb 3+, Cr 3+, Fe 3+, Co 3+, Ga 3+, As 3+, Ru 3+, Rh 3+, In 3+, Ir 3+ and Au 3+; a) contacting an aqueous solution comprising an A precursor and an aqueous solvent with an organic solution comprising an M precursor and an organic solvent, wherein the aqueous solution and/or the organic solution comprises an X precursor, and wherein the organic solution comprises a hydrohalic acid; and b) allowing a precipitate to form when the said aqueous and organic solutions are contacted,