Pseudo-binary　layered　compound　IVVI-V2VI3　families　show　great　promise　for　application　in　thermoelectrics.　Herein,　through　introducing　iodine　in　GeSb2Te4,　several　synergistic　effects　come　into　being　and　contribute　to　outstanding　thermoelectric　performance.　The　I　Te　donor-like　defects　suppress　the　hole　carrier　concentration　from　5.72　x　1020　cm-3　to　2.80　x　1020　cm-3　.　First-principles　calculations　reveal　that　iodine　doping　increases　the　band　gap　from　0.253　eV　to　0.302　eV　and　contributes　to　valence　band　convergence.　Seebeck　coefficient　value　reaches　up　to　135.7　mV/K　at　773　K,　and　the　power　factor　values　are　entirely　boosted　in　the　whole　temperature　region,　reaching　a　maximum　value　of　12.4　mW　center　dot　cm-1　center　dot　K-2　in　GeSb　2　Te　3.96　I　0.04　.　Moreover,　iodine　doping　simultaneously　reduces　the　lattice　and　electronic　thermal　conductivity,　leading　to　the　greatly　reduced　total　thermal　conductivity　from　2.89　W　center　dot　m-1　center　dot　K-1　in　pristine　sample　to　0.89　W　center　dot　m-1　center　dot　K-1　in　GeSb　2　Te　3.84　I　0.16　at　323　K.　Finally,　a　maximum　zT-1.12　at　773　K　and　an　average　zT-0.62　over　323-773　K　are　achieved　in　GeSb　2　Te　3.88　I　0.12　.　This　work　puts　forward　an　effective　strategy　to　synergistically　optimize　phonon　and　carrier　transport　properties　of　pseudo-binary　compounds　through　halogen　doping,　which　may　be　effective　in　other　similar　material　systems.　(c)　2024　The　Authors.　Published　by　Elsevier　B.V.　on　behalf　of　The　Chinese　Ceramic　Society.　This　is　an　open　access　article　under　the　CC　BY-NC-ND　license　(http://creativecommons.org/licenses/by-nc-nd/4.0/).