Unequivocal heteroatom insertion into a 24-ring channel gallophosphate and its photoluminescence

Five different transition metal elements have been successfully inserted into a unique 24-ring (24R) channel structure, NTHU-1, which is the first neutral framework gallophosphate displaying visible photoluminescence without added metal activators. Incorporation of heteroatoms into the intraframework sites has intricately tuned luminescence properties and made subtle changes in channel size as well as imposed magnetism to NTHU-1. The resultant phases, designated as NTHU-1M with M = Cr, Mn, Fe, Co, CrCo, and Zn, were respectively characterized by X-ray diffraction, thermal analyses, photoluminescence, UV-vis spectra, and/or magnetic studies. Our reaction conditions enabled the formation of large crystals with higher quality and a more definite degree of incorporation, which allowed unambiguous elucidation of the location, population, and environment of each of the heteroatoms in one 24R channel structure from single crystals, than ever reported before. In addition, the successful insertion of Cr produced unprecedented Cr and Ga mixed-metal phosphates with different optical properties from Cr-doped aluminophosphates or zeolites. Two distinct metal sites in different geometries were observed in NTHU-1, where the tetrahedral site was occupied by divalent heteroatoms and the octahedral site by the trivalent. The two distinct sites could even be occupied simultaneously, resulting in a unique CrCo dual insertion. Factors concerning the incorporation and the influence of heteroatoms on the luminescence property are discussed.