Sintering Phase Transition And Thermoelectromagnetic Properties Of Cu-doped Ordered Sr₃YCo₄O_10.5+? Material System

Tetragonal superstructure Sr₃YCo₄O_10.5+?material has been widely studied due to its complex ordered structure and high T_c?335K,A-site(Sr/Y)ordering and oxygen vacancy ordering are critical for its room-temperature ferromagnetism.Besides,the structure and physical properties of Sr₃YCo₄O_10.5+?can be modulated by doping.In this paper,Sr₃YCo₄O_10.5+?and Cu doping series polycrystals were prepared by solid-state reaction.The phase transformation of raw materials powders in solid-state sintering,especially the ordered phase transformation,were systematically studied.The crystal structure,morphology,thermoelectric and magnetic transport performance,and Cu doping effect were characterized and discussed deeply.Tetragonal superstructure Sr₃YCo₄O_10.5+?polycrystals and hexagonal-2H Sr CoO_2.5polycrystals were prepared in air by solid-state reaction at 1180?/24 h+1180?/24 h,hexagonal-2H Sr₃YCo₄O₁₀ was prepared in nitrogen at 1180?/24 h.The solid-state sintering in air,Sr₃YCo₄O_10.5 crystallizes at 968?,the exothermic phase transition of oxygen absorbing(?)takes place at 1042?,it is speculated that Sr₃YCo₄O_10.5+?is ordered,which is verified by(103)and(215)peaks of XRD,indicating that enough oxygen absorption above1000? is key to tetragonal superstructure formation.Sr₃YCo₄O_10.5+?polycrystal shows semiconductor electric transport behavior,and has room-temperature ferromagnetism,T_c(ZFC)=328-332 K,T_c(FC)=332 K,which is attributed to the ordered structure results in the G-type arrangement of CO³⁺ions moments with different spin states.The Hopkinson peak appears at T_c in ZFC curves,indicating polycrystal has magnetocrystal and domain anisotropy,induces the M-H curves are unsymmetrical while T=350?280?200?4 K,appearing exchange bias phenomenon,H_EB=-0.56 KOe?11.36 KOe.After double sintering,the structure and ferromagnetism ordered improving,resistivity decreases and the thermopower increases.Single sintering Sr₃YCo_4-xCu_xO_10.5+?(x=0,0.2,0.4,0.6,0.8,1.0,2.0,3.0)polycrystals were prepared in air by solid-state reaction at 1100?/24 h.and then double sintering Sr₃YCo_4-xCu_xO_10.5+?(x=0,0.2,0.4)were prepared at 1100?/24 h.When x=0?0.4,single-phase tetragonal superstructure Sr₃YCo_4-xCu_xO_10.5+?form at 1042?,grain size and density increase.When x=0.6?3.0,low melting point copper-rich phase with monoclinic structure form at 978?,melting point in 1092?1120?,wherein,the space group of x=0.6?1.0monoclinic copper-rich phase is Cc,a=2.91?b=6.92?c=3.94(?),x=2.0 and 3.0 monoclinic copper-rich phase's is Pc,a=2.91?b=6.92?c=3.94(?).The solid solubility of Cu is between0.4?0.6.The polycrystals in the solution range show semiconductor thremoelectric transport behavior.With the increase of Cu doping,the mobility and concentration of hole carr iers increase,leading to significantly decrease in resistivity.Carrier concentration and spin entropy improving make thermopower of x=0?0.2 polycrystals unchanged,and thermopower of x=0.4 decreases.Magnetization and Tc decrease with Cu doping,T_c=320 K?271 K,magnetic structure transits to ferromagnetism from G-type antiferromagnetism,which is attributed to the decrease of Co³⁺spin state.