The　effects　of　La　doping　on　the　ferroelectric　properties　of　0.92Na(0.5)Bi(0.5)TiO(3)-0.08BaTiO(3)　(NBT-BT)　solid　solution　have　been　studied　both　experimentally　and　theoretically.　The　experimental　results　show　that　an　abnormal　ferro-to-antiferroelectric　phase　transition　is　induced　by　La　doping　in　NBT-BT.　The　first-principles　calculations　indicate　that　La3+　cations　selectively　substitute　for　the　A　site　in　NBT-BT　as　donors.　Furthermore,　the　computed　binding　energy　reveals　that　La　cations　is　most　likely　to　substitute　Ba2+　or　Na+,　not　Bi3+,　at　A　site　as　donors　in　NBT-BT,　as　supported　by　our　Raman　spectra.　The　ferro-to-antiferroelectric　phase　transition　of　La-doped　NBT-BT　is　believed　to　originate　from　the　lattice　aberrance　and　redistribution　of　valence　electrons,　thus　strengthening　the　bonding　of　A-O,　enhancing　the　hybridization　between　the　A　cation　d　orbital　and　O　2p　orbital,　and　resulting　in　the　deflection　of　the　polar　direction　of　NBT-BT　lattice.