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Blue-emitting Cs 3 Cu 2 I 5 has attracted attention owing to its near-unity PL quantum yield and applications in DUV photodetectors and scintillators. Its PL properties originate from the unique local structure around the luminescent center, the [Cu 2 I 5 ] 3- polyhedron iodocuprate anion consisting of the edge-shared CuI 3 triangle and the CuI 4 tetrahedron dimer, which is isolated by Cs + ions. We found that solid-state reactions between CsI and CuI occur near room temperature (RT) to form Cs 3 Cu 2 I 5 and/or CsCu 2 I 3 phases. High-quality thin films of these phases were obtained by the sequential deposition of CuI and CsI by thermal evaporation. We elucidated that the formation of interstitial Cu + and the antisite of I - at the Cs + site in the CsI crystal through Cu + and I - diffusion results in the RT synthesis of Cs 3 Cu 2 I 5 . The unique structure formation of the luminescent center was revealed using a model based on the low packing density of the CsCl-type crystal structure, similar sizes of Cs + and I - ions, and the high diffusivity of Cu + . The self-aligned patterning of the luminous regions on thin films was demonstrated.