Calcium zinc manganites, Ca_4Mn_7Zn_3O_(21-δ) (0.5 < δ < 2.5) with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties

摘要

Phase-pure calcium zinc manganite, Ca_4Mn_7Zn_3O_(21-δ) has been synthesized by the solid-state reaction between CaMnO_3, ZnO and Mn_3O_4 at ～ 1300℃. Conversion of Ca_4Mn_7Zn_3O_(21-δ) to Ca_4Mn_7Zn_3O_(19±δ) occurs at 1330-1350℃ which is influenced by the oxygen nonstoichiometry associated with increasing Mn~(3+)/Mn~(4+) ratio. The Ca_4Mn_7Zn_3O_(21-δ) phase is rhombohedral (space group=R3m) and has the β"-alumina (BA)-type structure consisting dominantly of Mn~(4+) (3d~3) ions. The Ca_4Mn_7Zn_3O_(19±δ) is hexagonal (space group=P6_3/mmc) having the magnetoplumbite (MP)-type structure containing dominantly Mn~(3+) (3d~4) ions. The electrical transport properties of Ca_4Mn_7Zn_3O_(21-δ) and Ca_4Mn_7Zn_3O_(19±δ) are indicative of the activated-type small polaron hopping conductivity below 366 and 392 K respectively. The anisotropic charge transport is influenced only in a limited way by the mixed-valency of Mn prevailing in the manganite spinel blocks. Highly nonlinear current-voltage (Ⅰ-Ⅴ) curves with nonlinearity coefficient, α, up to 64 was achieved at the turn-on field strength E_t～ 110-160 V/mm. Magnetic properties of Ca_4Mn_7Zn_3O_(21-δ) and Ca_4Mn_7Zn_3O_(19±δ) have been investigated which showed the antiferromagnetic-insulating (AFI) behavior with T_N=103 and 80 K and with an AF-ordered effective magnetic moment, μ_(eff)=3.54 and 4.28 μ_B/Mn respectively. The Ca_4Mn_7Zn_3O_(19±δ) phase showed the ferrimagnetic interaction below T_N (at ～20 K).