ALaMnBO 6 (A = Ca, Sr, Ba; B = Fe, Ru) double perovskites K. Ramesha a , V. Thangadurai a , D. Sutar b , S.V. Subramanyam b , G.N. Subbanna c , J. Gopalakrishnan a, * a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India b Department of Physics, Indian Institute of Science, Bangalore 560 012, India c Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India (Refereed) Received 19 April 1999; accepted 1 June 1999 Abstract We report the synthesis and investigation of electrical and magnetic properties of double perovskites of the formula ALaMnBO 6 for A = Ca, Sr, Ba and B = Fe, Ru. Powder X-ray diffraction (XRD) shows formation of cubic/pseudocubic perovskite structure for all the phases, with no obvious long-range ordering of B-site cations. Electron diffraction, however, reveals an ordering of Mn and Ru in ALaMnRuO 6 , showing a doubling of the primitive cubic perovskite cell. While the B = Fe phases are paramagnetic insulators, the B = Ru phases are ferrimagnetic semiconductors. It is likely that in the B = Ru phases, a valence equilibrium exists between Mn III /Ru IV and Mn II /Ru V states that is biased toward the latter. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Electrical properties; D. Magnetic properties 1. Introduction Double perovskite oxides, A 2 BB'O 6 and AA'BB'O 6 , where A and A' are alkaline-earth and/or rare-earth metals and B and B' are transition metals, have been widely investigated * Corresponding author. Tel.: +91-80-309-2537; +91-80-331-1310. E-mail address: gopal@sccu.iisc.ernet.in (J. Gopalakrishnan). Pergamon Materials Research Bulletin 35 (2000) 559 –565 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0025-5408(00)00248-8