Yttria-stabilized　zirconia　(YSZ)　has　been　used　as　a　thermal　barrier　coating　(TBC)　material　in　gas　turbines　for　several　decades.　Although　continuous　efforts　have　been　made　to　develop　novel　TBC　materials　that　can　work　at　a　higher　temperature,　no　single　material　other　than　YSZ　has　all　the　desired　attributes　for　the　TBCs.　In　this　paper,　we　report　the　in-situ　synthesis　of　quasi-binary　GdNbO4/Gd3NbO7　composites　based　on　the　simple　Gd2O3-Nb2O5　binary　phase　diagram.　The　fracture　toughness　of　these　quasi-binary　composites　is　remarkably　enhanced　compared　with　the　value　predicted　by　the　rule　of　mixtures　because　the　ferroelastic　domain　switching　is　more　activated　due　to　the　residual　stress　in　the　quasi-binary　composites,　which　triggers　more　crack　defections　due　to　the　enlarged　process　zone.　Additionally,　the　Gd3NbO7　phase　provides　a　low　thermal　conductivity　due　to　the　substantial　chemical　inhomogeneity,　which　diffuses　phonons.　Gd3NbO7/GdNbO4　exhibits　a　balanced　thermal　conductivity　of　1.6　W/(m·K)　at　1073　K　and　a　toughness　value　of　2.76　MPa·m0.5,　and　these　values　are　among　the　best　comprehensive　properties　that　have　been　obtained　for　new　TBC　materials.　The　work　demonstrates　a　feasible　approach　of　designing　a　new　TBC　material　with　balanced　properties　and　can　be　easily　fabricated.　[Figure　not　available:　see　fulltext.]　©　2022,　The　Author(s).