Abstract: There is disclosed herein novel organic-inorganic layered perovskites of the general formula A.sub.2 MX.sub.4 where A is an organic ammonium cation, M is a divalent rare earth metal and X is a halogen. These compounds can be made by a low temperature (about 100.degree.-160.degree. C.) solid state reaction between the organic ammonium salt with the appropriate hydrogen halide and MX.sub.2 where M is a divalent rare earth metal and X is a halogen). A specific example is (C.sub.4 H.sub.9 NH.sub.3).sub.2 EuI.sub.4 which has been made through a reaction at about 140.degree.-160 C. between C.sub.4 H.sub.9 NH.sub.2. HI and EuI.sub.2. This new compound produces intense blue photoluminescence at room temperature, with a peak wavelength of 460 nm and a fairly narrow peak width (FWHM=24 nm). In addition to the simple aliphatic ammonium cations of the form C.sub.n H.sub.2n+1 NH.sub.3.sup.+, e.g., propylammonium (C.sub.3 H.sub.7 NH.sub.3) and butylammonium (C.sub.4 H.sub.9 NH.sub.