Red-emitting　Mn4+　activated　oxide　phosphors　with　a　cheap　price　and　excellent　physical/chemical　stability　have　become　a　hot　research　topic　for　their　potential　applications　in　white　LEDs　(w-LEDs).　Herein,　we　report　a　novel　double-perovskite　Gd2ZnTiO6:Mn4+　(GZT:Mn4+)　red　phosphor.　The　material　microstructures　were　characterized　with　the　aid　of　XRD　Rietveld　refinement　and　HRTEM　observations.　The　luminescence　properties　and　dynamics　of　Mn4+　in　GZT　were　studied　in　detail　using　low/room　temperature　steady/transient　spectroscopic　techniques.　The　crystal　field　strength　and　nephelauxetic　effect　influencing　the　Mn4+　emission　energy　were　also　analyzed.　It　is　revealed　that　the　special　crystal　structure　of　GZT　featuring　alternately　slant-wise　arranged　[TiO6]/[ZnO6]　octahedrons　with　the　[-Mn4+-O2--Zn2+-]　bond　angle　deviating　from　180°　is　beneficial　to　achieving　efficient　Mn4+:　2Eg　→　4A2　transition　in　the　deep-red　region.　After　mixing　the　red-emitting　GZT:Mn4+　with　the　commercial　blue　and　green　phosphors　in　various　ratios,　and　then　coupling　the　mixture　with　a　365　nm　UV　chip　to　build　a　w-LED,　the　white　light　was　found　to　evolve　from　cool　to　warm　with　a　tunable　correlated　color　temperature　(CCT)　from　6977　K　to　4742　K,　a　color　rendering　index　(CRI)　up　to　82.9,　and　an　improved　R9　value　to　43,　which　validates　that　GZT:Mn4+　is　a　promising　red　color　converter　for　UV-based　w-LEDs.　©　The　Royal　Society　of　Chemistry　2016.