PAPER
www.rsc.org/materials | Journal of Materials Chemistry
New stabilized phases in the Sr/Ca–Mn–Co–O system: structural–magnetic properties relationship K. Boulahya,a M. Hernando,ab M. Parrasa and J. M. Gonza´lez-Calbet*a Received 22nd September 2006, Accepted 9th January 2007 First published as an Advance Article on the web 2nd February 2007 DOI: 10.1039/b613844f Polycrystalline Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33 have been synthesized and characterised by X-ray and electron diffraction, high resolution electron microscopy and magnetic measurements. These oxides constitute the a = 3, b = 1 and a = 4, b = 1 terms of the homologous series (A3B2O6)a(A3B3O9)b, respectively. Isolated rows of polyhedra sharing faces along the c-axis are made up of units of two and one face-sharing octahedra (occupied by Mn) linked by one trigonal prism (occupied by Co) in an ordered way. The chains are separated by columns of Sr/Ca atoms. Magnetic measurements suggest antiferromagnetic correlations in the two new commensurate monodimensional oxides. The influence of the size of alkaline-earth atoms on the long-range magnetic interactions is discussed.
Introduction Recently, there has been much interest in the structural and magnetic characterization of one-dimensional (1D) oxides structurally related to the hexagonal perovskite 2H-BaNiO31 and the K4CdCl62 structural types. Both structures can be regarded as a hexagonal array of infinite 1D chains of facesharing polyhedra running parallel to the c-axis. In the 2Htype, the polyhedra chains are formed by [NiO6] octahedra; in Sr4PtO6,3 isostructural to K4CdCl6, the chains are formed by the sequence of one [PtO6] octahedron (Oh) and one [SrO6]4 trigonal prism (TP). These chains are separated by columns of alkaline-earth atoms. The ability of both structures to intergrow in an ordered way has played a paramount role in the stabilisation of new one-dimensional oxides with different octahedra/trigonal prism ratios. All of them can be considered as members of the (A3A9BO6)a(A3B3O9)b5,6 homologous series in which B stands for octahedrally coordinated cations and A9 refers to cations in TP environment. Structural variety and compositional flexibility are the most remarkable characteristics of this family. At present, at least fifteen structural types7 have been stabilized with a large variety of metal cations occupying both oxygen environments, the Oh and the TP sites. Among them, various 1D compounds related to 2H-BaMnO3, that is, where Mn cations are located into the octahedral sites can be selected. In all these phases containing manganese, a metal of different chemical nature occupies the TP sites. Thus, Ca3MnA9O6 (A9 = Co, Cu, Ni, Zn)8–10 constitutes the a = 3, b = 0 member of the series; Sr4Mn2A9O9 (A9 = Mg, Zn, Cu, Ni, Co)11–15 corresponds to the a = 3, b = 1 member; the a = 3, b = 2 member is stabilized with the A5Mn3A9O12 chemical composition,16 where A = Sr, Ba and A9 = Zn, Ni; finally, a
Departamento de Quı´mica Inorga´nica, Facultad de Quı´micas, Universidad Complutense, E-28040-Madrid, Spain. E-mail:
[email protected]; Fax: +34 91 394 43 52; Tel: +34 91 394 43 42/58 b Institut Laue Langevin, BP 156X, F-38042 Grenoble, France
1620 | J. Mater. Chem., 2007, 17, 1620–1626
Ba7Mn5PdO1817 constitutes the a = 3, b = 4 and Ba6Mn4A9O15 (A9 = Cu, Zn)18 constitutes the a = 3, b = 3 member. It is worth mentioning that for Ba6Mn4A9O15 (A9 = Cu, Zn) some disorder in the Oh and TP sites has been found since both polyhedra can be occupied by either Mn or Cu(Zn). Keeping manganese in the octahedral sites, the A–(Co/Mn)– O system nicely reflects the structural diversity of these 1D phases. Actually, stoichiometric Ca3MnCoO6 (a = 3, b = 0),8 Sr4Mn2CoO9 (a = 3, b = 1),15 and Sr9Mn5Co2O21 (a = 2, b = 1)19 have been stabilized and fully characterized. Besides these phases, the stabilization of non-stoichiometric compounds structurally close to the idealized compositions Sr14Mn8Co3O33, a = 9, b = 5, and Sr9Mn5Co2O21, a = 2, b = 1, has been recently reported.20 The first one is isostructural to Sr14Co11O335,21 which shows an ordered intergrowth of two structural blocks corresponding to the a = 3, b = 2 member, –1TP–3Oh– polyhedra sequence, and one block of the a = 3, b = 1 member, –1TP–2Oh–. On the other hand, Sr9Mn5Co2O21 has a rhombohedral structure, where one structural block of the a = 3, b = 2 member, constituted by 3Oh–1TP, alternates with one a = 3, b = 1 block, –2Oh–1TP–, along the c-axis. The stoichiometry and structure of these oxides control their electronic properties. Thus, the magnetic properties of Mn/Co oxides depend on both the A9 chemical nature and the Oh/TP sequence along the chains. For instance, Ca3MnCoO6,8 formed by one octahedron [MnO6] sharing faces with one [CoO6] trigonal prism, shows long magnetic ordering of an antiferromagnetic type.22 The magnetic properties of Sr4Mn2CoO9 (2Oh–1TP) also evidence strong antiferromagnetic correlation along the chain although no three-dimensional magnetic long range order is observed. The global magnetic behavior corresponds to a partially ordered system close to a magnetic spin glass. Following the series, the increase of the number of octahedra between prisms gives rise to more complex magnetic systems. For instance, the magnetic behavior of Sr9Mn5Co2O21 is rather complex. In this case, the polyhedra chains are composed by a sequence of three facesharing octahedra (Mn4+), followed by one trigonal prism This journal is ß The Royal Society of Chemistry 2007
(Co2+) and finally, two face-sharing octahedra (Mn4+). Therefore, the magnetic chain consists of one Mn trimeric unit separated by a Co prism of one dimer of Mn ions. Obviously, the distances and angles inside the dimers and trimers are different, and these structural features increase the magnetic disorder compared to Sr4Mn2CoO9. As a consequence, long range magnetic order is not present, and the short range magnetic order is rather small. The structural and magnetic properties which occur as the Mn/Co ratio and the chemical nature of the A cation vary incited us to explore the (Sr/Ca)–Mn/Co–O system. We report in this paper the study of two new commensurate compounds with Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33 compositions. To fully determine the structural details and physical properties, X-ray powder diffraction (XRPD), selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) studies were performed and magnetic properties were investigated.
Fig. 1 Experimental, calculated and difference X-ray diffraction patterns corresponding to Sr3CaMn2CoO9.
Experimental Polycrystalline Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33 were synthesised by heating stoichiometric amounts of SrCO3 (Aldrich 99.98%), CaCO3 (Aldrich 99.98%), Co3O4 (Aldrich 99+%) and MnO2 (Aldrich 99+%) in air at 1300 uC for 7 days and 1450 uC for 5 days, respectively, and then quenching to room temperature. XRPD patterns were collected with CuKa radiation at room temperature on a PHILIPS X9PERT diffractometer equipped with a graphite monochromator. The diffraction data were analysed by the Rietveld method23 using the Fullprof program.24 The sample was characterised by SAED and HREM in a JEOL 3000FEG electron microscope, fitted with a double tilting goniometer stage (¡22u, ¡22u). Local composition was analysed with an INCA analyser system attached to the above microscope. Simulated HREM images were calculated by the multislice method using the MacTempas software package. Magnetic properties were measured in a SQUID magnetometer, in the temperature range from 2 to 300 K, and in magnetic fields up to 5 T.
Results and discussion I. Sr3CaMn2CoO9 The cationic composition, as analysed by energy-dispersive X-ray analysis, is in agreement with the nominal composition, Sr3.01(2)Ca1.04(2)Mn1.95(9)Co1.02(5). The corresponding XRPD pattern (Fig. 1) can be fully indexed on the basis of a threedimensional trigonal unit cell with lattice parameters a = ˚ , c = 7.7612 (18) A ˚ , no extra reflections being 9.4846 (7) A detected. SAED has been used to fully reconstruct the reciprocal space confirming the above unit cell. The most relevant zone axes, corresponding to the [12¯10] and [0001] ones, are shown in Fig. 2a and b. All maxima can be indexed in the above trigonal unit cell, the reflection conditions being compatible with the P321 space group, previously proposed for Sr4Mn2CoO9.15 However, when this reciprocal plane is compared with that This journal is ß The Royal Society of Chemistry 2007
Fig. 2 SAED patterns corresponding to Sr3CaMn2CoO9 along (a) [12¯10] and (b) [0001].
corresponding to isostructural Sr4Mn2CoO9, some differences can be appreciated. In fact, although both SAED patterns present the same structural characteristics, the intensity distribution of the superstructure spots is clearly different. In fact, this feature may originate from the presence of twinning in the crystals, as previously reported for Sr4Mn2ZnO9.12 The multitwinned microstructure is confirmed by HREM. Actually, the image taken along the [12¯10] zone, is shown in Fig. 3a. Several domains, tilted by 90u with respect to each other around the c-axis, are clearly evident. In each domain, d-spacings of 0.83 nm and 0.77 nm, along the a and c axes, respectively, are observed in agreement with the d100 and d001 interplanar distances corresponding to the a = 3, b = 1 1D oxide. Besides, the contrast variation in each domain is also characteristic of this member of the series. This can be clearly appreciated in the enlargement of one of them (Fig. 3b), where it can be observed that, following the c-axis, one bright dot alternates with two less intense ones. We have previously reported that, in all these 1D oxides, the metal columns in the trigonal interstices and the Sr/Ca atoms are imaged as bright dots. Therefore, the contrast sequence in this experimental image is that expected for the Oh–Oh–TP polyhedra sequence in the 1D chains. Moreover, the calculated images show a good fit with the experimental one for Df = 245 nm and Dt = 5 nm (see Fig. 3b). It is worth mentioning that the domains are quite narrow, and the twins are coherent since no relative displacement is observed at the twin boundaries. This is J. Mater. Chem., 2007, 17, 1620–1626 | 1621
T a b l e 1 Fi n a l s t r u c t u r a l p a r a me t er s co r r e s p o nd i n g t o Sr3CaMn2CoO9 Atom
x/a
y/b
Sr/Ca 1 Sr/Ca 2 Sr/Ca 3 Mn1 Mn2 Mn3 Co1 Co2 O1 O2 O3 O4 O5
0.0245(9) 0.358(1) 0.328(1) 0.33333 0.33333 0 0 0.33333 0.499(4) 0.680(5) 0.864(4) 0.671(5) 20.012(4)
0.688(8) 0 0 0.66666 0.66666 0 0 0.66666 0.650(4) 0.198(5) 0 0.191(1) 0.161(3)
z/c
˚2 B/A
Occ.
0.252(9) 0.56(3) 1 0.5 0.56(3) 1 0 0.56(3) 1 0.400(2) 1.53(4) 1 0.070(2) 1.53(4) 1 0.166(2) 1.53(4) 1 0.5 1.84(6) 1 0.744(4) 1.84(6) 1 0.244(5) 0.37(20) 1 0.441(4) 0.37(20) 1 0 0.37(20) 1 0.101(4) 0.37(20) 1 0.293(3) 0.37(20) 1 a ˚ , c = 7.76710(25) A ˚, Space group P321 (no. 150), a = 9.48677(25) A ˚ 3, RB = 0.064, Rexp = 0.076, Rwp = 0.148, x2 = 3.71. V = 605.377(30) A
Fig. 3 (a) Sr3CaMn2CoO9 HREM image along [12¯10]; a twinned microstructure is clearly seen; (b) enlargement of one of the domains; the calculated image is shown in the inset; (c) enlargement of three domains showing coherent twin boundaries; a schematic model illustrating the twin boundaries is shown in the inset. Optical Fourier transforms (FT) corresponding to domains marked with A (d) and B (e) in (a); (f) FT corresponding to the juxtaposition of both A and B domains.
outlined in Fig. 3c and illustrated in the inset, where a schematic model of three domains is depicted. The optical Fourier transforms corresponding to domains A and B, see Fig. 3, are depicted in Fig. 3d and e, respectively. In domain A, the superstructure direction follows the [448¯6]*2H reciprocal direction, while in domain B, tilted 90u with respect to the former, the satellite reflections are located along the [4¯4¯86]*2H direction. In both domains, the intensity distribution of the spots is that expected in a modulated superstructure. The juxtaposition of two patterns corresponds to the experimental SAED (see Fig. 3f). In this one-dimensional series, a multitwinned microstructure has been associated with the presence of anti-site disorder between the two cations located into the different polyhedra constituting the chains. For instance, in Sr4Mn2ZnO9, Mn and Zn should be located at the octahedral and TP sites, respectively. However, there was evidence of occupational disorder between the Oh and TP sites, a fraction of 0.08 mol Zn per formula unit being located at the octahedral sites. This disordered cationic distribution seems to be at the origin of the twinned microstructure. According to that, the presence of twinning in Sr3CaMn2CoO9 could be also indicative, at least, of an irregular cationic distribution of Mn and Co atoms in a small proportion. However, the similar scattering factors of both cations make them indistinguishable by SAED and XRPD. On the basis of the above results, an X-ray profile refinement of Sr3CaMn2CoO9 was performed. The structure was solved in the P321 space group taking as the starting point 1622 | J. Mater. Chem., 2007, 17, 1620–1626
the Sr4Mn2CoO9 crystallographic data;15 Sr and Ca are randomly distributed over three different crystallographic positions and Co and Mn atoms are arranged in the TP and octahedral sites, respectively. Peak shapes were described by pseudo-Voigt functions. Fig. 1 shows the graphic result of the fitting of the experimental XRPD pattern and the difference between the observed and calculated data. The refinement was stable and it was possible to refine the positions of oxygen atoms, provided a temperature factor for each type of atoms was used. The final structural parameters are collected in Table 1, whereas Table 2 shows some selected inter-atomic distances. The structure refinement confirms isotypism with Sr4Mn2CoO9. The essential feature of the structure is the presence of 1D chains of face-sharing polyhedra, stacked in the –2Oh–TP– sequence, and folded along the [0001] direction. The chains are separated by columns of Ca and Sr atoms. The refined structural model is shown in Fig. 4. Within the polynuclear (CoMn2O9) group, the inter-cation ˚ ) and Mn–Co (2.50(4)– distances Mn–Mn (2.49(1)–2.57(3) A ˚ ) agree well with those of similar oxides also 2.70(4) A containing face-sharing polyhedra. For instance, the Mn–Mn ˚ 14 distances in Sr4Mn2NiO9 range from 2.556 to 2.572 A whereas in Sr4Mn2CoO9, the corresponding values range from Table 2
˚ ) in Sr3CaMn2CoO9 Selected inter-atomic distances (A
Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca
1–O1: 1–O1: 1–O2: 1–O2: 1–O3: 1–O4: 1–O5: 1–O5:
2.67(4) 2.59(4) 2.52(4) 2.91(4) 2.65(2) 2.32(3) 2.69(4) 2.69(3)
Co1–O5: 2.24(4) 6 6 Co2–O2: 2.01(5) 6 3 Co2–O4: 1.81(4) 6 3
Sr/Ca Sr/Ca Sr/Ca Sr/Ca
2–O1: 2–O2: 2–O2: 2–O5:
2.43(4) 2.71(5) 2.66(4) 2.42(4)
6 6 6 6
2 2 2 2
Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca
3–O1: 3–O3: 3–O4: 3–O4: 3–O5:
2.47(4) 2.70(3) 2.93(5) 2.83(3) 2.75(3)
6 6 6 6 6
2 2 2 2 2
Mn1–O1: Mn1–O3: Mn2–O1: Mn2–O4: Mn3–O3: Mn3–O5:
2.02(4) 1.83(4) 2.13(4) 1.91(4) 1.83(2) 1.88(5)
6 6 6 6 6 6
3 3 3 3 3 3
Co1–Mn2: 2.51(3) Co2–Mn1: 2.70(4) Co2–Mn2: 2.50(4) Mn1–Mn2: 2.57(3) Mn2–Mn3: 2.49(1)
This journal is ß The Royal Society of Chemistry 2007
Fig. 5 Experimental XRPD (Sr0.5Ca0.5)15Mn7Co4O33. Fig. 4
pattern
corresponding
to
Structural model corresponding to Sr3CaMn2CoO9.
˚ .15 The Mn–Co distance corresponds to 2.41(6) to 2.58(5) A ˚ in Ca3MnCoO68 and ranges from 2.62(3) to 2.73(6) A ˚ 2.645 A ˚, in Sr4Mn2CoO9.15 The Mn–O distances, 1.83(2)–2.13(3) A are typical for Mn4+ octahedra in oxides. For instance, in ˚ ,25 Sr4Mn3O10 the Mn–O distances are between 1.77–2.10 A 26 ˚ ˚ 1.83–2.05 A in Sr7Mn4O15, and 1.85–2.04 A in Sr4Mn2CoO9.15 Finally, the Ca/Sr–O distances (2.32(3)– ˚ ) are close to those observed in other Sr and/or Ca 2.93(5) A ˚ in Sr7Mn4O15 and 2.33–2.93 in oxides, d(Sr–O) = 2.36–3.02 A Sr4Mn2CoO9.15 The ensemble of the XRPD, SAED and HREM results shows that Sr3CaMn2CoO9 constitutes a new example of a commensurate 1D oxide. The structural study shows that the partial substitution of Sr by Ca in Sr4Mn2CoO9 modifies the structural features, since a multitwinned microstructure is observed. In each domain, the polyhedra rows are formed by dimers of octahedra sharing faces linked to trigonal prisms. According to the structure refinement from XRPD data, Co and Mn occupy the TP and Oh sites, respectively.
Fig. 6 SAED patterns corresponding to (Sr0.5Ca0.5)15Mn7Co4O33 along (a) [12¯10] and (b) [11¯00].
II. (Sr0.5Ca0.5)15Mn7Co4O33 Fig. 5 shows the XRPD pattern corresponding to the sample with nominal composition (Sr0.5Ca0.5)15Mn7Co4O33. In Fig. 6a and b the SAED patterns corresponding to the most relevant zone axes, [12¯10] and [11¯00], of the (Sr0.5Ca0.5)15Mn7Co4O33 sample are depicted. All reflections can be indexed on the basis of a rhombohedral symmetry, with possible space group R32 ˚ and c = 57.1183(9) A ˚. and unit cell parameters a = 9.3871(1) A In order to confirm the proposed unit cell, an X-ray profile refinement was performed (Fig. 5), no atomic parameters being refined. Fig. 7 shows the HREM image along [1–210] and the corresponding Fourier transform. An apparently well-ordered material with d-spacings of 0.81 nm and 0.95 nm, along the a and c axes, respectively, are observed in agreement with the d100 and d006 interplanar distances. As previously observed in other related 1D oxides, bright dots can be associated to TP sites, whereas less bright dots along the c-axis should This journal is ß The Royal Society of Chemistry 2007
Fig. 7 (Sr0.5Ca0.5)15Mn7Co4O33 HREM image along [12¯10]; calculated images are shown as insets (A (Dt = 3 nm, Df = 260 nm), B (Dt = 5nm, Df = 250 nm)), with the corresponding FT.
correspond to Oh sites.27,28 Therefore, a careful analysis of the contrast variation along the c-axis leads to the polyhedra sequence along the polyhedra rows; according to that, the J. Mater. Chem., 2007, 17, 1620–1626 | 1623
structure of these materials is formed by an ordered intergrowth between 3 unit cells of (Sr/Ca)4Mn2CoO9 (–2Oh–TP– 2Oh–TP–2Oh–TP–) and half a unit cell of (Sr/Ca)3MnCoO6 (–Oh–TP–). Fig. 8 shows the schematic representation of the resulting structural model of (Sr0.5Ca0.5)15Mn7Co4O33, the corresponding ideal atomic positions being gathered in Table 3. On the basis of this model, an image simulation was performed in the R32 space group. A good fit with the experimental micrograph is obtained for different zones of the crystal (see Fig. 7). The ensemble of the XRPD, SAED and HREM results shows that (Sr0.5Ca0.5)15Mn7Co4O33 constitutes a new example of a commensurate 1D oxide. The structural study shows that the polyhedra rows are formed by three blocks of octahedra dimers sharing faces with trigonal prisms alternating with one
Table 3 Ideal atomic positions corresponding (Sr0.5Ca0.5)15Mn7Co4O33
to
Atom
x/a
y/b
z/c
Occ
Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca Sr/Ca Co Co Co Co Mn Mn Mn Mn Mn Mn Mn O O O O O O O O O O O O
0.6666 0.3333 0.3333 0.6666 0.6666 0.3333 0 0 0 0 0 0 0 0 0 0 0 0.1666 0.5 0.5 0.3333 0.3333 0.5 0.6666 0.8333 0.6666 0.8333 0.5 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.1666 0.3333 0.3333 0.5 0.5 0.3333 0.5 0.8333 0.5 0.8333 0.1666 0.1666
0.1 0.1333 0.2663 0.3666 1/2 0 0.0666 0.2 0.3 0.4333 0.0166 0.1166 0.1500 0.25 0.35 0.3833 0.4833 0 0 0.0666 0.0666 0.2 0.2 0.3666 0.3666 0.1 0.1 0.5 0.5
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
octahedron along the c-axis. It is worth mentioning that this phase constitutes the first example of the a = 4, b = 1 member of the (A3A9BO6)a(A3B3O9)b5,6 homologous series. Physical properties
Fig. 8 Structural model corresponding to (Sr0.5Ca0.5)15Mn7Co4O33.
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Fig. 9a and b show the temperature dependence of the magnetic susceptibility corresponding to Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33, respectively, measured in a field of 1000 Oe. The two monodimensional oxides present very similar magnetic behavior. The zero field-cooled (ZFC) and field-cooled (FC) data overlie and reach a maximum at 13.5 and 16.5 K for Sr3CaMn2CoO9 and Sr7.5Ca7.5Mn7Co4O33, respectively. Below this temperature, hysteresis appears between the ZFC and FC data and the latter passes through a local minimum and increases with decreasing temperature. The inverse of the magnetic susceptibility follows a Curie– Weiss law at temperatures above 200 K, with two effective paramagnetic moments calculated from the slope of 6.99 mB f.u.21 (f.u. = formula unit) for Sr3CaMn2CoO9 and 13.8 mB f.u.21 for (Sr0.5Ca0.5)15Mn7Co4O33. These values agree rather well (7.2 mB f.u.21 and 13.9 mB f.u.21for Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33, respectively) with the assumption of Mn4+ in octahedral coordination (3.8 mB Mn21) and Co2+ in a trigonal prismatic environment (4.8 mB Co21). The extrapolated Weiss constant (H = 296.3 K for Sr3CaMn2CoO9 and H = 260.8 K for (Sr0.5Ca0.5)15Mn7Co4O33) clearly indicates that antiferromagnetic interactions dominate in these oxides although competition between two different interactions (ferro- and antiferromagnetic) between magnetic cations inside the chain cannot be disregarded. This journal is ß The Royal Society of Chemistry 2007
Fig. 9 Temperature dependence of the magnetic susceptibility corresponding to (a) Sr3CaMn2CoO9 and (b) (Sr0.5Ca0.5)15Mn7Co4O33.
Fig. 10a shows the hysteresis loop corresponding to Sr3CaMn2CoO9 at 2 K. In this curve, the magnetization increases at low magnetic fields, followed by hysteretic behavior without reaching saturation. This abrupt increase of the magnetization curve can be associated with a spin canting behavior. When the temperature increases (see inset), the magnetization curves are progressively smeared out and the hysteresis disappears. The magnetization curves versus applied magnetic field corresponding to (Sr0.5Ca0.5)15Mn7Co4O33 are shown in Fig. 10b. At low temperatures (T = 2 K) the slope of the curve increases with the field. The tendency to linearity increases with increasing temperature, corresponding to a paramagnetic system. In both oxides, the departure from a straight line may involve the existence of a ferromagnetic component at low temperature (T = 2 K), below the maximum temperature. The magnetic behavior of these new (Sr,Ca)–Mn–Co oxides is different to that shown by the three commensurate Mn–Co oxides previously reported, i.e., Ca3MnCoO6,8 Sr4Mn2CoO915 and Sr9Mn5Co2O2119 and the two incommensurate Mn–Co oxides Sr14Mn8Co3O33,20 Sr9Mn5Co2O21.20 In fact these 1D oxides present similar magnetic behavior. All of these compounds are constituted by chains of octahedra of Mn4+ connected by a trigonal prism of Co2+ separated by columns of This journal is ß The Royal Society of Chemistry 2007
Fig. 10 Magnetization curves versus applied field at different temperatures corresponding to (a) Sr3CaMn2CoO9 and (b) (Sr0.5Ca0.5)15Mn7Co4O33.
alkaline-earth cations (Ca/Sr). The susceptibility curves versus temperature present a maximum at low temperature. This maximum corresponds to the Ne´el temperature in Ca3MnCoO6 and, therefore, to long range 3D antiferromagnetic order. By contrast, in the other phases this signal is related to short range magnetic interaction, probably within the chain, no long-range magnetic order coupling between chains being found. This fact can be related to the bigger size of Sr that avoids the 3D magnetic ordering and, therefore, they exhibit magnetically disordered behavior. Although Sr3CaMn2CoO9 is isostructural to Sr4Mn2CoO9,15 the replacement of 25% Sr by Ca introduces a significant change in the magnetic behavior. The smaller size of the Ca cations respect to the Sr ones decreases the distance between polyhedra chains and, therefore, the magnetic coupling between magnetic cations can be modified. A careful comparison of the magnetic susceptibility curves reveals that the Sr3CaMn2CoO9 magnetic behavior is closer to that observed in Ca3MnCoO6 than that of the isostructural Sr4Mn2CoO9. Actually, in Sr4Mn2CoO9 and Sr9Mn5Co2O21 the ZFC and FC data diverge at maximum temperature, as J. Mater. Chem., 2007, 17, 1620–1626 | 1625
corresponds to magnetic cluster (or spin glass) behavior. This divergence between the ZFC and FC data collected from (Ca/Sr)–Mn–Co–O oxides (Ca3MnCoO6, Sr3CaMn2CoO9 and (Sr0.5Ca0.5)15Mn7Co4O33) is observed below the maximum temperature, characteristic of an antiferromagnetic system. These magnetic features seem to indicate a 3D antiferromagnetic order in the two new monodimensional oxides. This fact evidences the great influence of the size of alkaline-earth atoms on the long-range magnetic interactions. To confirm this longrange antiferromagnetic order, the nature of the magnetic phase as well as the features of the interaction between magnetic cations in the complex structure, neutron diffraction studies of these new oxides at low temperature are in progress.
Acknowledgements Financial support through research projects MAT2004-01248, MAT/0627/2004 and CAM/S-0505/PPQ0316 is acknowledged. K.B. thanks the MCYT (Spain) for financial support under the ‘‘Ramo´n y Cajal’’ Program.
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