Cubic structure and canted antiferromagnetism of CaMn 7 O 12 doped with trivalent cations (Fe, Al, Cr) Md. Motin Seikh 1 , V. Caignaert, O.I. Lebedev, B. Raveau n Laboratoire CRISMAT, UMR 6508 CNRS ENSICAEN, 6 bd Maréchal Juin, 14050 Caen, France article info Article history: Received 16 November 2013 Accepted 20 November 2013 by F. Peeters Available online 27 November 2013 Keywords: A. Double perovskite A. Spin glass D. Canted antiferromagnet D. Charge ordering abstract In this study, we show the dramatic effect of the doping of the octahedral sites with M 3 þ cations (Fe 3 þ , Al 3 þ and Cr 3 þ ) upon the structure and magnetism of the rhombohedral double perovskite CaMn 7 O 12 . In the oxides CaMn 7x M x O 12 , charge ordering between Mn 3 þ and Mn 4 þ octahedral sites is destroyed leading to the cubic structure (Im-3), whereas the initial magnetic properties (T N 90 K) have disappeared leading to canted antiferromagnetism (T N E50–70 K) for small x values (x 0.2–1). A spin glass like behaviour is also observed for larger values (x 1) in the case of Fe substitution. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Discovered in the 80s by Marezio et al. [1] the manganate CaMn 7 O 12 has been investigated in a large number of works (see for a review Ref. [2]) for its complex interrelated magnetic and structural transitions [3–7] and very recently for its magneto- electric behaviour [8] and its multiferroic properties [9–10]. In fact, this oxide belongs to the CaCu 3 Ti 4 O 12 structural family [11], derived from the cubic perovskite by a tilting of the MnO 6 /TiO 6 octahedra, so that the Jahn–Teller (J–T) cations, Cu 2 þ /Mn 3 þ , form square planar coordinated units. However, differently from CaCu 3 - Ti 4 O 12 , which is cubic (a ¼ 2a p , Im-3), CaMn 7 O 12 is rhombohedral (R-3) at room temperature. We believe that this structural distor- tion originates from the presence of particular charge ordering of Mn 3 þ and Mn 4 þ in the octahedral sites, which are at 75% occupied by the J–T Mn 3 þ cation. Based on these considerations we have studied the possibility of substitution of trivalent cations for Mn 3 þ in the oxide CaMn 7 O 12 , bearing in mind that the latter should sit in octahedral sites in normal pressure conditions of synthesis. Herein, we show that the substitution of small amounts of trivalent cations in CaMn 7 O 12 , mainly Fe 3 þ , Cr 3 þ , Al 3 þ , destroys the charge ordering in the rhombohedral R-3 structure at the benefit of the cubic Im-3 symmetry. The complex magnetic transitions previously observed at 90 K and 50 K for CaMn 7 O 12 [12–16] are suppressed by these substitutions leading for the cubic oxides CaMn 7 x M x O 12 to a canted antiferromagnetic state below 50–70 K for low x values and to a spin glass behaviour for higher content. 2. Experimental The synthesis of the oxides CaMn 7 x M x O 12 was carried out by the conventional solid state technique. The oxides CaO, MnO 2 , Mn 2 O 3 , Fe 2 O 3 or Cr 2 O 3 or Al 2 O 3 were intimately mixed in stoichio- metric proportions, pressed under  3 t in the form of parallele- pipedic bars and first heated at 900 1C in evacuated quartz tubes for 24 h. The resultant powders were ground, pressed again as bars and heated at 900 1C for 48 h. This latter operation was repeated several times, with intermediate grindings in order to eliminate traces of impurities, mainly Mn 2 O 3 and CaMnO 3 . The crystallinity of the samples and the possible presence of a secondary phase were checked by X-ray powder diffraction (XRPD) using Cu-K α radiation. All the samples that were showing a dissymmetry of the diffraction peaks, suggesting a possible mixture of the rhombohe- dral and cubic phases or a distortion of the cubic cell, were reground and annealed again, in order to reach the cubic sym- metry. For an accurate determination of the cell parameters and especially for the detection of an eventual rhombohedral distor- tion the XRPD patterns were registered with a Panalytical X'Pert Pro diffractometer under a continuous scanning mode in the 2θ range 51–1201 and step size Δ2θ ¼ 0.0171. The electron diffraction (ED) study was carried out with a transmission electron micro- scopy (TEM) using a Tecnay G2 30 UT microscope operated at 300 kV. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ssc Solid State Communications 0038-1098/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ssc.2013.11.024 n Corresponding author. Tel.: þ33 2 31 45 26 16; fax: þ33 2 31 95 16 00. E-mail address: bernard.raveau@ensicaen.fr (B. Raveau). 1 On leave from Department of Chemistry, Visva-Bharati University, Santiniketan-731235, India. Solid State Communications 180 (2014) 52–55