Introduction　(K,　Na)NbO3　(KNN)-based　lead-free　piezoelectric　ceramics　are　considered　to　be　one　of　the　important　candidates　to　replace　lead-based　piezoelectric　ceramics　due　to　their　high　Curie　temperature　and　excellent　comprehensive　performance.　To　further　improve　its　performance,　the　strategy　of　element　doping　is　employed.　The　doping　behavior　of　the　Mn　element　in　pressure-sintered　KNN　ceramics　has　been　extensively　studied.　However,　there　are　significant　differences　in　the　density　of　Mn-doped　KNN　ceramics　in　different　works,　which　poses　a　challenge　to　the　analysis　of　the　underlying　mechanism　of　Mn　doping　and　its　modification　effect.　Therefore,　this　study　takes　hot-pressed　KNN　ceramics　as　the　research　object　due　to　their　high　densification,　and　systematically　explores　the　impact　of　Mn　doping　on　the　microstructure　and　electrical　properties　of　KNN　ceramics.　Methods　In　this　study,　KNN　ceramics　with　MnO2　doping　were　prepared　by　hot-pressed　sintering.　The　specific　composition　is　(K0.5Na0.5)NbO3–x%　MnO2　(abbreviated　as　KNN–x%　MnO2,　x　=　0,　0.5,　1.0,　1.5).　The　raw　materials　include　K2CO3　(99.0%),　Na2CO3　(99.8%),　Nb2O5　(99.99%),　and　MnO2　(98.8%),　all　of　which　were　purchased　from　Sinopharm　Chemical　Reagent　Co.,　Ltd.　First,　carbonate　and　Nb2O5　were　placed　in　a　nylon　ball　mill　with　zirconia　balls　according　to　the　formula　ratio,　and　ball　milled　for　24　h　using　anhydrous　ethanol　as　the　medium;　the　ball-milled　slurry　was　dried　and　calcined　at　730　℃　for　4　h,　and　then　ball-milled　again　and　calcined　again　at　930　℃　for　4　hours.　Subsequently,　different　amounts　of　MnO2　were　added　to　the　calcined　powder,　mixed　and　ball-milled　for　24　h,　and　then　dried　and　ground　to　obtain　Mn-doped　KNN　ceramic　powder.　Finally,　the　ceramic　block　was　prepared　by　hot　pressing　and　sintering　at　950　℃　and　30　MPa　for　2　h　under　argon　atmosphere.　After　cutting,　grinding,　and　annealing　at　different　temperatures,　the　sample　surfaces　were　subsequently　coated　with　silver　electrode　for　electrical　measurements.　The　poling　process　was　performed　at　3.5　kV/mm　in　120　℃　silicone　oil　for　30　min.　The　surface　micromorphology　of　the　ceramics　was　characterized　by　Merlin　scanning　electron　microscope　(SEM).　The　crystal　structure　was　analyzed　by　D/Max　2500　X-ray　diffractometer　(XRD).　The　density　of　the　samples　was　measured　using　the　Archimedean　drainage　method.　The　dielectric　properties　were　tested　using　a　TH2827　impedance　analyzer.　The　ferroelectric　properties　were　obtained　using　a　TF　Analyzer　2000　ferroelectric　analyzer.　The　piezoelectric　constant　d33　was　measured　using　a　ZJ–3A　quasi-static　d33　tester.　The　mechanical　quality　factor　Qm　was　measured　using　a　TH2839　impedance　analyzer.　The　local　domain　structure　and　domain　switching　behavior　were　tested　and　analyzed　using　an　MFP–3D　atomic　force　microscope　with　piezoelectric　force　microscopy　(PFM).　Results　and　discussion　High-density　(relative　density　＞　98%)　KNN–x%　MnO2　ceramics　were　successfully　prepared　by　hot　pressing　sintering　process.　XRD　results　show　that　MnO2　doping　does　not　change　the　perovskite　phase　structure　of　KNN.　With　the　increase　of　Mn　doping　amount,　the　Pr　of　the　ceramic　shows　a　trend　of　first　increasing　and　then　slightly　decreasing.　At　x　=　1.0–1.5%,　Pr　reaches　14.92–18.32　μC/cm2,　d33　can　reach　up　to　106　pC/N,　and　both　positive　and　negative　strains　are　improved,　reflecting　enhanced　ferroelectric　and　piezoelectric　responses　after　Mn　doping.　PFM　test　shows　that　the　switching　voltage　of　the　local　ferroelectric　domain　gradually　increases　from　5　V　to　20　V,　indicating　that　Mn　doping　enhances　the　domain　wall　pinning　effect.　At　the　same　time,　the　mechanical　quality　factor　Qm　of　the　ceramic　is　significantly　improved,　with　the　highest　value　reaching　175,　reflecting　the　typical　＇hard＇　doping　behavior.　Conclusions　This　study　found　that　Mn-doped　KNN　ceramics　exhibit　an　atypical　＇hard＇　doping　effect　for　the　high-density　hot-pressed　ceramics　that　is　different　from　the　traditional　doping　mechanism　in　lead-based　ceramics,　in　which　Qm　and　Pr　are　simultaneously　improved　and　d33　is　maintained　at　a　high　value.　This　atypical　＇hard＇　doping　effect　induced　by　Mn　doping　provides　a　new　perspective　for　understanding　the　mechanism　of　element　doping　in　the　KNN　system,　and　also　provides　significant　guidance　for　the　optimized　design　of　high-performance　lead-free　piezoelectric　ceramics.　©　2025　Chinese　Ceramic　Society.　All　rights　reserved.