Phase constitution and structure Rietveld refinement in the system La_1-xZr_xMnO_3+δ (0<x≤0.3)

E.Syskakis, A. Gantis and M. Calamiotou

University of Athens, Solid State Physics Dept., Panepistimiopolis Zografos 157 84, Athens, Greece

LaMnO_3+δ manganites doped with tetravalent ions at the La site are possible candidates for n-type conductivity. They belong to the family of rotationally distorted mixted valence perovkites with the lattice distortions playing significant role in their physical properties [1]. We report here the synthesis and structural study of the nominally composed La_1-xZr_xMnO_3+δ (0<x≤0.30). Polycrystalline samples have been prepared by the solid state reaction method with thermal treatments in air at 1200-1670K. The XRD Rietveld refinement analysis (Fig.1) showed that all samples contain the main orthorhombic (Pnma) phase La_1-zZr_zMnO3_+δ, the cubic (Fd3m) pyroclore La₂Zr₂O₇ as well as a cubically stabilized (Fm3m) ZrO2 phase. The amount of the latter two phases, depending on the final treatment temperature, increases in general with increasing Zr content. No manganese oxide phase could be detected. This indicates that the solubility of Zr is limited in the perovskite structure. Moreover it is found that monoclinic ZrO₂ can be stabilised to cubic (Fm3m) by the presence of La and Mn.

Figure 1. Experimental and calculated XRD pattern for sample with 30mole % Zr.

The lattice constants and cell volume behaviour on Zr content of the main phase,

La_1-zZr_zMnO_3+δ, depends on the final treatment temperature (1570 and 1670K respectively). The refinement of the occupation of the La/Zr site could not give clear results about the actual substitution level of Zr, due to the overlapping of peaks in the multiphase system La_1-xZr_xMnO_3+δ. However characteristic changes at the La site have been observed by the Bond Valence Sum (BVS) values at the La site which are indicative of the stress of the perovskite cell around the La atom. BVS values have been calculated according to from the La-O refined interatomic distances [2]. They exhibit a minimum, close to the nominal value 3 for an unstressed La⁺³ ion, at x=0.15 (x=0.2) for samples annealed at 1570K ( 1670K) (fig.2). This suggests that the stresses of the perovskite cell are relaxed at those nominal compositions by the substitution of the smaller Zr⁺⁴ for the bigger La⁺³ ion and/or by the La deficiency. The MnO₆ octahedron distortions show also a minimum at x=0.2. Electrical conductivity and ac susceptibility data are consistent with the structural analysis showing limited solubility of Zr in the perovskite structure. They however stronger indicate that the incorporation of Zr into the orthorhombic

La_1-zZr_zMnO_3+δ structure is prohibiting the double exchange interaction mechanism.

Figure 2. Bond Valence Sum values at the La site. Doted line: the nominal value 3 for La

Acknowledgment

This work is financially supported by the Grant EPEAEK “Phytagoras” from the Greek Ministry of Education. Thanks are due to Ms. G. Apostolopoulou for assistance during the experiments.

1. E. Liarokapis, Th. Leventouri, D. Lampakis, D. Palles, J.J. Neumeier,D.H. Goodwin, Phys.Rev.B 60 (1999),12758; P.G. Radaelli, M.Marezio, H.Y. Hwang, S-W Cheong, B, Batlogg, Phys.Rev.B 54 (1996) 8992

2. I.D. Brown and D. Altermatt, Acta Cryst. Sect.B, B41(1985), 244