ARTICLE pubs.acs.org/IC
Suppression of Magnetic Order in Sr3Fe2O4Cl2 by Fe-Site Substitution by Cobalt Edward Dixon and Michael A. Hayward* Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom
bS Supporting Information ABSTRACT: The low-temperature topotactic reduction of Sr3Fe2xCoxO5Cl2 oxychloride phases with LiH allows the preparation of phases of composition Sr3Fe2xCoxO4Cl2 (0 e x e 1). The reduced phases adopt body-centered tetragonal structures which are isostructural with Sr3Fe2O4Cl2 and contain square-planar (Fe/Co)O4 centers connected into apex-linked sheets, analogous to the CuO2 sheets present in superconducting cuprate phases. As the cobalt concentration in Sr3Fe2xCoxO4Cl2 is increased the antiferromagnetic order of the Sr3Fe2O4Cl2 host phase is suppressed, ultimately leading to spin-glass behavior, at low temperature, in Sr3Fe2xCoxO4Cl2 phases with x g 0.8. The limited influence of cobalt substitution on the reactions which form the Sr3Fe2xCoxO4Cl2 phases is discussed and contrasted to that of the related SrFeO3δSrFeO2 system.
’ INTRODUCTION Since the discovery of high-temperature superconductivity in complex copper oxides, there has been great interest in preparing materials which contain square-planar MO4 transition-metal centers, particularly when these centers are arranged into corner-linked sheets, analogous to the CuO2 sheets present in the superconducting cuprate phases. However despite extensive investigations, very few thermodynamically stable phases have been discovered which exhibit this structural motif in the absence of copper. Given the apparent low thermodynamic stability of extended oxide phases containing square-planar transition metal centers, a logical synthetic approach is to exploit kinetically controlled chemical processes to prepare suitable metastable phases. Topotactic reduction reactions, which exploit the large difference between cation and anion mobilities in complex oxides to deintercalate oxide ions from extended solids, facilitate the preparation of kinetically stabilized phases which contain extended arrays of transition-metal centers in unusual coordination environments and oxidation states. This includes phases which contain apex-linked sheets of planar MO4 centers. Thus, for example, the reduction of the Ni(III) oxides LaNiO3 and LaSrNiO4 at low-temperature yields the Ni(I) phases LaNiO2 and LaSrNiO3, respectively,1,2 which contain extended arrays of corner-linked NiIO4 square-planes. Specifically LaNiO2 forms the so-called infinite-layer structure and is isostructural with Ca0.84Sr0.16CuO23 while LaSrNiO3 adopts a one-dimensional structure isostructural with Sr2CuO34 as shown in Figure 1. The formation of anion deficient phases such as LaNiO2 and LaSrNiO3, which contain Ni(I) centers in square-planar r 2011 American Chemical Society
coordination sites, is attributable to the favorable crystal-field stabilization energy associated with the d9 electron count of these ions in such a coordination environment. It is however possible to prepare phases with extended sheets of corner-linked MO4 centers in the absence of such favorable interactions, as demonstrated by the reduction of SrFeO3 and Sr3Fe2O6 to the respective Fe(II) phases SrFeO2 and Sr3Fe2O5,5,6 both of which contain arrays of apex-linked FeIIO4 planar centers as shown in Figure 1. In this instance, it is not clear what directs SrFeO2 and Sr3Fe2O5 to adopt anion deficient structures which contain Fe(II) centers in these highly unusual square-planar coordination sites. The high-spin d6 electronic configuration adopted by the Fe(II) centers in these phases is offered no obvious stabilization by square-planar coordination geometries. Likewise the large scale rearrangement of the anion lattice during the course of the reduction reactions that lead to the formation of both SrFeO2 and Sr3Fe2O5, make it hard to rationalize the structures adopted by these phases on the basis of the coordination preferences of the Sr2+ A-cations, in contrast to a number of related anion deintercalation reactions.7,8 To shed some light on the structural selectivity exhibited during the formation of SrFeO2, a recent study by Kageyama et al. investigated the effect of replacing some of the iron centers with cobalt or manganese.9 They observed that at low levels of substitution the reduction of SrFe1x(Mn/Co)xO3δ substrate phases led as expected to the formation of the corresponding Received: April 21, 2011 Published: June 29, 2011 7250
dx.doi.org/10.1021/ic200832n | Inorg. Chem. 2011, 50, 7250–7256
Inorganic Chemistry
ARTICLE
(0 e x e 1) to complement and contrast those of the analogous SrFe1xCoxO2 (0 e x e 0.3) phases.
’ EXPERIMENTAL SECTION
Figure 1. (a) Infinite layer structure adopted by LaNiO2, Ca0.84Sr0.16CuO2, and SrFeO2. (b) The structure of LaSrNiO3. (c) The structure of Sr3Fe2O5. (d) The structural transformation which occurs during topotactic reduction of Sr3Fe2O5Cl2 to Sr3Fe2O4Cl2.
infinite layer SrFe1x(Mn/Co)xO2 phases. However, at substitution levels of greater than 30%, reduction reactions led to alternative products which are yet to be identified. The sensitivity of the reduction reactions of SrFe1x(Mn/Co)xO3δ phases to cation substitution does suggest that the d6 electronic configuration of the Fe(II) centers plays some role in directing the formation of anion deficient structures with square-planar FeIIO4 coordination sites. However, a more immediate consequence of the limited range of substituted SrFe1x(Mn/Co)xO2 phases which can be prepared is the frustration of the second goal of the Kageyama investigation: to probe the electronic behavior of substituted SrFeO2 phases as a function of a wide range of d-electron count.9 Recently we reported the preparation of the Fe(II) phase Sr3Fe2O4Cl2 via the topotactic reduction of Sr3Fe2O5Cl2 with LiH.10 In this instance the formation of the anion deficient structure adopted by Sr3Fe2O4Cl2, which also contains sheets of apex-linked planar FeIIO4 centers (Figure 1), can be seen to be directed by the coordination preferences of the Sr2+ A-cations. As a result the reactions which form Sr3Fe2O4Cl2 are much more tolerant to cation substitution than those which form SrFeO2. Here we report the effects of cobalt substitution on the crystal structure and magnetic behavior of Sr3Fe2xCoxO4Cl2 phases
Preparation of Sr3Fe2xCoxO5Cl2 Phases. Samples of Sr3Fe2xCoxO5Cl2 (x = 0.2, 0.6, 0.8, 1) were prepared by the direct-combination route previously described by Weller et al. for the synthesis of Sr3FeCoO5Cl2.11 Suitable quantities of SrO (prepared by the decomposition of SrCO3 at 1100 °C under vacuum), SrCl2 (dried at 180 °C under vacuum), Fe2O3 (99.99%), and Sr2Co2O5 (prepared using the method described by Grenier et al.12) were thoroughly mixed using an agate pestle and mortar in an argon filled glovebox (O2 and H2O levels
