Red Ca(0.99)Al(1-4 delta/3-x)Si(1+delta+)xN(3-x)C(x):Eu-0.01(2+) (delta = 0.345; x = 0-0.2) nitride phosphors exhibit a blue-shifted emission with increased eye sensitivity function and excellent thermal stability. The variations in the photoluminescence in the Ca(0.99)Al(1-4 delta/3-x)Si(1+delta+x)N(3-x)Cx:Eu-0.01(2+) (delta = 0.345; x = 0-0.2) system are thoroughly investigated. The enhanced emission energy and the improved thermal stability with increasing x are dominated by the second-sphere shrinkage effect via the substitution of small Si4+ for large Al3+ with simultaneous charge compensation. Related proofs of the second-sphere shrinkage effect control for photoluminescence are confirmed via high-resolution neutron powder diffraction, EXAFS, and Si-29 solid-state NMR techniques.