# Calcium zinc manganites, $Ca_4Mn_7Zn_3O_{21 -\delta} (0.5<\delta <2.5)$ with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties

Vijayanandhini, K and Kutty, TRN (2007) Calcium zinc manganites, $Ca_4Mn_7Zn_3O_{21 -\delta} (0.5<\delta <2.5)$ with beta-alumina or magnetoplumbite-type structure and their nonlinear electrical transport and magnetic properties. In: Materials Letters, 61 (17). pp. 3652-3657.

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## Abstract

Phase-pure calcium zinc manganite, $Ca_4Mn_7Zn_3O_{21 -\delta}$ has been synthesized by the solid-state reaction between $CaMnO_3$, ZnO and $Mn_3O_4$ at $\sim 1300^oC$. Conversion of $Ca_4Mn_7Zn_3O_{21-\delta}$ to $Ca_4Mn_7Zn_3O_{19\pm \delta}$ occurs at 1330–1350°C which is influenced by the oxygen nonstoichiometry associated with increasing $Mn^{3+}/Mn^{4+}$ ratio. The $Ca_4Mn_7Zn_3O_{21-\delta}$ phase is rhombohedral (space group=R3¯m) and has the $\beta"$-alumina (BA)- type structure consisting dominantly of $Mn^{4+} (3d^3)$ ions. The $Ca_4Mn_7Zn_3O_{19 \pm \delta}$ 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 $Ca_4Mn_7Zn_3O_{21 -\delta}$ and $Ca_4Mn_7Zn_3O_{19 \pm \delta}$ 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, $\alpha$, up to 64 was achieved at the turn-on field strength $E_t \sim 110–160 V/mm$. Magnetic properties of $Ca_4Mn_7Zn_3O_{21 -\delta}$ and $Ca_4Mn_7Zn_3O_{19 \pm \delta}$ have been investigated which showed the antiferromagnetic-insulating (AFI) behavior with TN=103 and 80 K and with an AF-ordered effective magnetic moment, $\mu_{eff}=3.54$ and $4.28 \mu _B/Mn$ respectively. The $Ca_4Mn_7Zn_3O_{19 \pm \delta}$ phase showed the ferrimagnetic interaction below $T_N (at \sim 20 K)$.

Item Type: Journal Article Copyright of this article belongs to Elsevier Science. Electronic ceramics;Electron microscopy;Crystal structure;Electrical and magnetic properties Division of Chemical Sciences > Materials Research Centre 10 Sep 2007 19 Sep 2010 04:39 http://eprints.iisc.ernet.in/id/eprint/11797