Eu2+-activated　alkali-lithosilicate　phosphors　exhibit　narrow-band　emis-sions　that　are　attractive　to　high　color-rendition　and　wide　color-gamut　displays.　Themicroscopic　mechanism　behind　the　small　emission　bandwidth　is　not　presentlyunderstood.　Here,　we　report　an　explicit　calculation　of　the　vibronic　process　occurringin　the　narrow-band　green　emission　of　Rb2Na2[Li3SiO4]4:Eu2+.　We　show　that　due　to　thehigh　rigidity　of　the　host　material,　the　structural　strain　induced　by　the　localized　Eu2+4f-5d　excitation　is　distributed　among　the　atoms　far　beyond　thefirst　coordination　shell　andhence　reduces　the　local　structural　relaxation　around　Eu2+.　The　emission　bandshape　isthus　mainly　controlled　by　the　coupling　of　the　electronic　transition　with　the　phononmodes　associated　with　motions　of　host　constituent　atoms,　which　was　further　validatedby　the　good　agreement　of　the　calculated　bandshape　with　the　experiment.　The　resultsprovide　insights　into　the　generation　of　narrow-band　emission　and　improve　ourknowledge　on　electron-phonon　coupling　of　4f-5d　transitions　in　phosphors