To whom correspondence should be addressed. Journal of Solid State Chemistry 163, 513 } 518 (2002) doi:10.1006/jssc.2001.9439 available online at http://www.idealibrary.com on Magnetic Susceptibility and Spin Exchange Interactions of the Hexagonal Perovskite-Type Oxides Sr 4/3 (Mn 2/3 Ni 1/3 )O 3 A. El Abed,*- E. Gaudin,* J. Darriet,* and M.-H. Whangbo? *Institut de Chimie de la Matie ` re Condense H e de Bordeaux (ICMCB UPR 9048 CNRS), 87 Avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France; -Laboratoire de Physique du Solide, Faculte H des Sciences, Universite H Mohamed 1er, Oujda, Morocco; and ?Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received July 24, 2001; in revised form October 8, 2001; accepted October 26, 2001 Magnetic susceptibility measurements were carried out for two hexagonal perovskite-type oxides Sr 1x (Mn 1x Ni x )O 3 with slightlydi4erentcompositions(i.e., x  1 3 and0.324).Asigni5cant di4erence in the susceptibilities of the two phases demonstrates the need to control phase compositions accurately. Sr 4/3 (Mn 2/3 Ni 1/3 )O 3 consistsoftwospinsublattices,i.e.,theMn 4 andtheNi 2 ionsublattices.Spindimeranalysiswascarriedout to examine the relative strengths in the spin exchange interac- tionsoftheMn 4 ionsublattice.Thetemperaturedependenceof themagneticsusceptibilityofSr 4/3 (Mn 2/3 Ni 1/3 )O 3 wasfoundcon- sistentwithapictureinwhichtheMn 4 ionsublatticehasweakly interacting antiferromagnetically coupled (Mn 4 ) 2 dimers, the Ni 2 ionsublatticeactsasaparamagneticsystem,andthetwo sublattices are nearly independent.  2002 Elsevier Science (USA) 1. INTRODUCTION In recent years the structures and magnetic properties of hexagonal perovskite-type oxides A  MO  (x"  ,     ; A"Sr, Ba; M"Co, Ni) have received much attention (1}6). The structures of these oxides can be viewed in terms of in"nite (MO  )  and A  chains (Fig. 1), where the (MO  )  chains are made up of face-sharing MO  octahedra and trigonal prisms. In the (MO  )  chains, single MO  trigonal prisms may alternate with single MO  octahedra (1) or with M  O  octahedral oligomers made up of n face-sharing MO  octahedra (2}6) (Fig. 2). In the in"nite (MO  )  chains of the incommensurate phase Sr  NiO  (7), which is approximated by the commensurate structure Sr  NiO  , the MO  trigonal prisms alternate randomly with two types of octahedral oligomers M  O  and M  O  . Analysis of the M}O bond lengths in Sr  CoO  (5) and Sr  NiO  (7) indicates that the oxidation states of the transition metal atoms in the octahedral (Oh) and trigonal prism (TP) MO  units are close to #4 and #2, respective- ly. Thus, in hexagonal perovskite-type oxides containing two di!erent transition metals, the more electronegative ones should occupy the TP sites. Indeed, the recent study of Sr  (Mn  Ni  )O  (6), whose (MO  )  chains have Mn  O  octahedral dimers alternate with single NiO  trig- onal prisms (Fig. 2b), shows that the Oh and TP sites are occupied exclusively by Mn (d) and Ni (d) ions, respectively. As found for Sr  NiO  (7), the Ni ions of Sr  (Mn  Ni  )O  occupy several di!erent coordination sites of the NiO  trigonal prisms. According to the X-ray powder di!raction and XANES studies (6), approximately 80% of the Ni ions are located at the three square-planar (SP) coordination sites, and approximately 20% of the Ni ions at the TP centers. (A unit cell of Sr  (Mn  Ni  )O  has one (MO  )  chain consisting of Mn(1) and Ni(1) atoms and two (MO  )  chains consisting of Mn(2), Mn(3), Ni(2), and Ni(3) atoms. The occupancies of the SP and TP centers are 0.751(4) and 0.249 (4), respectively, in the (MO  )  chain containing Ni(1) atoms, and 0.829(4) and 0.171(4), respec- tively, in the (MO  )  chain containing Ni(2) and Ni(3) atoms.) The Ni ions at the SP centers prefer the S"0 spin state, but those at the TP centers the S"1 spin state (8). The Mn K-edge XANES study (6) of Sr  (Mn  Ni  )O  showed that the Mn ions are in a high-spin (S"  ) state. Each Mn ion can have spin-exchange interactions with adjacent Mn ions and also with adjacent Ni ions if the latter are located at the TP centers. Thus, a number of di!erent local spin arrangements are possible in Sr  (Mn  Ni  )O  , and one would expect the description of its magnetic properties to be rather complicated. Never- theless, as will be shown below, the essential features of the magnetic susceptibility of this compound are well described using the simple approximation that the spin exchange interactions between the Mn and Ni ions are negli- gible. In the present work, we report the magnetic suscep- tibilities of two hexagonal perovskite-type oxides Sr  513 0022-4596/02 $35.00  2002 Elsevier Science (USA) All rights reserved.