The first-principles calculation and Pauling's second rule predict that an O2- ion coordinated with two Al3+ and one Si4+, or Si4+-O-[3]-2Al(3+), is stabilized by cross-substitution to form 2Si(4+)-N-[3]-Al3+ configuration. Motivated by this idea, this work has attempted to synthesize a new phosphor by partial nitridation of Sr3Al10SiO20:Eu2+, which contains one set of Si4+-O-[3]-2Al(3+) per twenty O2- ions. Synthesis condition of Sr3Al10-xSi1+xO20-xNx:Eu2+ with x = 1 was investigated by variation of Si3N4/SiO2 ratio in the starting materials. Neutron diffraction analysis has shown that nitridation as much as x = 0.6-2 was attained by supply of Si3N4 in excess of the stoichiometry, because Si3N4 is partly oxidized even by firing in reduced atmosphere. Luminescence intensity is increased by seven times at the best by the nitridation, though little change is observed in luminescence spectra and X-ray diffraction patterns. The increased luminescence intensity results mainly from more efficient reduction of Eu3+ by Si3N4. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.058205jes]