4320 Chem. Mater. 2010, 22, 4320–4327 DOI:10.1021/cm1012735
Polytypism in the BaMn0.85Ti0.15O3-δ System (0.07eδe0.34). Structural, Magnetic, and Electrical Characterization of the 9R-Polymorph Laura Miranda,†,‡ Derek C. Sinclair,‡ Marı´ a Hernando,† Aurea Varela,† Julio Ramı´ rez-Castellanos,† Khalid Boulahya,† Jose M. Gonzalez-Calbet,† and Marina Parras*,† †
Departamento de Quı´mica Inorg anica, Facultad de Quı´micas, Universidad Complutense de Madrid, E-28040-Madrid, Spain, and ‡Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD, U.K. Received May 6, 2010. Revised Manuscript Received June 15, 2010
Chemical doping of 15 atom % Mn by Ti in 2H-BaMnO3 leads to two new polytypes, 4H- and 9R-BaMn0.85Ti0.15O3-δ, depending on the oxygen content. The 4H polytype (a = 5.71355(4) and c = 9.41230(8) A˚, space group P63/mmc) was prepared in an inert atmosphere and forms face sharing dimers linked by corners based on a stacking sequence (hc)2 where h and c refer to hexagonal and cubic BaO3 layers, respectively. Selected area electron diffraction patterns and high resolution electron microscopy reveal the coexistence of two types of crystals; the majority corresponds to an ordered (hc)2-4H polytype whereas a minority fraction shows a disordered microstructure. Such differences may be associated with nonhomogeneous cationic distribution within the crystals. Oxidation in air at 1523 K for 48 days leads to conversion to the 9R polytype (a = 5.68007(1) and c = 20.98208(6) A˚, space group R3m) with a (hhc)3 stacking sequence; Mn4þ ions are exclusively located on the central B-site (M1) of the face sharing trimers linked by corners with the oxygen vacancies located on the h-BaO3 layers leading to the formula Ba(Mn0.33)M1(Mn0.52Ti0.15)M2O2.93(1). The 9R polytype is an antiferromagnetic semiconductor with a Neel temperature of 120 K and a relative permittivity of ∼22. Introduction The structure-composition-property relationships of cubic close-packed perovskites have been widely studied, and they find widespread applications in, for instance, piezoelectric Pb(Zr,Ti)O3 and ferroelectric BaTiO3. In contrast, the structure-property relationships of hexagonal-type perovskites have received much less attention, and their potential for industrial applications has not been explored. ABO3 perovskites can be described as a close-packed arrangement of AO3 layers with B cations in octahedral sites. A cubic close packed array (ccp) of AO3 layers leads to perovskites where all BO6 octahedra share corners (3C-polytype) and the stacking sequence of the ccp layers is ...ccc..., where c refers to cubic layers. Perovskites based only on hexagonal close packing (hcp) of AO3 layers have infinite chains of facesharing BO6 octahedra leading to a two-layer hexagonal cell (2H-polytype) where the stacking sequence is ...hh..., where h refers to hexagonal layers. Hexagonal polytypes are generally formed by intergrowths of c and h layers as opposed to being exclusively h; therefore, depending on the stacking sequence a variety of corner and face sharing BO3 units exist along the c axis. In these polytypes, the unit cell is given by the number (n) of AO3 layers per cell, the symbols H or R being used to represent the symmetry. In BaM1-xM0 xO3-δ (M, M0 =transition metals), one of the most studied perovskite systems, the large size of the *Corresponding author. E-mail:
[email protected]. Fax: (34) 91 394 43 52.
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Ba2þ ion favors the presence of h-stacked layers, and depending on both the chemical nature of the M/M0 cations and the oxygen content, several polytypes can be stabilized. The oxygen content and therefore polytypism can often be adjusted by varying the experimental conditions, for example, reaction temperature, oxygen partial pressure, and postannealing conditions employed during synthesis. Recently, we have shown 12R-BaMn0.5Ti0.5O31 to be a high permittivity (εr ∼ 44) material with modest microwave dielectric resonance properties (Q 3 f ∼ 12 000 GHz, where Q=1/tan δ is the quality factor). Several factors are known to influence Q 3 f. Some are intrinsic, such as cationic order on the B sublattice, whereas others are extrinsic, such as grain boundaries, pellet density, or chemical impurities.2 In the case of 12R-BaMn0.5Ti0.5O3, partial ordering of Mn-Ti ions at the B sublattice could be responsible for its modest Q 3 f value. On the basis of these results we have explored the electrical properties of a variety of hexagonal perovskite systems.3-7 (1) Keith, G. M.; Kirk, C. A.; Sarma, K.; Alford, N. Mc.; Cussen, E. J.; Rosseinsky, M. J.; Sinclair, D. C. Chem. Mater. 2004, 16, 2007. (2) Wersing, W. Curr. Opin. Solid State Mater. Sci. 1996, 1, 715. (3) Miranda, L.; Feteira, A.; Sinclair, D. C.; Boulahya.; Hernando, M.; Ramı´ rez, J.; Varela, A.; Gonzalez-Calbet, J. M.; Parras, M. Chem. Mater. 2009, 21, 1731. (4) Miranda, L.; Boulahya, J.; Varela, A.; Gonzalez-Calbet, J. M.; Parras, M.; Hernando, M.; Fernandez-Dı´ az, M. T.; Feteira, A.; Sinclair, D. C. Chem. Mater. 2007, 19, 3425. (5) Miranda, L.; Sinclair, D. C.; Hernando, M.; Varela, A.; Wattiaux, A.; Boulahya, K.; Gonzalez-Calbet, J. M.; Parras, M. Chem. Mater. 2009, 21, 5272. (6) Miranda, L.; Ramı´ rez, J.; Varela, A.; Gonzalez-Calbet, J. M.; Parras, M.; Hernando, M.; Fernandez-Dı´ az, M. T.; Garcı´ a Hernandez, M. Chem. Mater. 2007, 19, 1503.
Published on Web 06/29/2010
r 2010 American Chemical Society
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An in-depth study of the composition-structure-physical property relationships of 6H- and 12R-BaMn1-xTixO3-δ (x g 0.5) revealed all stabilized polytypes to be leaky dielectrics.3 The electrical behavior was attributed to mixed valency of the M0 -site cations, that is, Mn3þ/Mn4þ/Ti4þ. Stabilization of different hexagonal polymorphs depending on the anionic content has been reported for the BaMnO3-δ system.8-13 Mn4þ substitution by the larger Mn3þ ion produces a negative pressure effect giving rise to polytypes with higher c/h ratio. The same effect is obtained when Ti is introduced on the B sublattice. Substitution of 15% Mn by Ti in 2H-BaMnO3 leads to two new polytypes, 9R and 4H, depending on the oxygen content. The 4H polytype is formed by dimers of face sharing octahedra linked by corners. This structural configuration gives rise to a very stable polytype which is consistent with the large number of 4H polytypes previously described.14-17 As we have previously shown, oxygen-deficient polytypes are semiconducting and are therefore not suitable as dielectric materials. For this reason, 4H ceramics have been reoxidized in air after synthesis in N2 gas in an attempt to suppress the B-site mixed valency and therefore reduce the leakage conductivity. This air annealing process has resulted in the formation of a new 9R-BaMn0.85Ti0.15O2.93 material. This polytype is formed by trimers of face sharing octahedra linked by corners. It is worth emphasizing that 9R-BaMnO3 has not been isolated; this perovskite has only been isolated by doping the A sublattice, under high pressure conditions, with Bi or Sr.13,18 Ti doping leads to stabilization of this new 9R-BaMn0.85Ti0.15O2.93 polytype. Here we report the synthesis, structural characterization, and physical properties of these new hexagonal polymorphs: 4H and 9R-BaMn0.85Ti0.15O3-δ. Experimental Section 4H-BaMn0.85Ti0.15O3-δ was prepared by solid state reaction from a well ground mixture of BaCO3 (Aldrich, 99.98%), TiO2 (Merck, 99%), and MnCO3 (Aldrich, 99%). This mixture was heated in an Al2O3 crucible at 1273 K in air to decarbonate the reactives. The product was ground and processed using the following treatments: 1473 K for 14 days in air, 1373 K for 3 days in N2, 1473 K for 6 days in N2, and finally 1523 K for 7 days in N2. Intermittent regrinding of the powder took place every 24 h, and samples were cooled slowly at 4 °C/min, after the treatments under (7) Miranda, L.; Feteira, A.; Sinclair, D. C.; Garcı´ a Hernandez, M.; Boulahya, K.; Hernando, M.; Varela, A.; Gonzalez-Calbet, J. M.; Parras, M. Chem. Mater. 2008, 20, 2818. (8) Parras, M.; Alonso, J.; Gonzalez- Calbet, J. M. Solid State Ionics 1993, 63-65, 614–619. (9) Gonz alez-Calbet, J. M.; Parras, M.; Alonso, J. M.; Vallet- Regı´ , M. J. Solid State Chem. 1993, 169, 99. (10) Gonz alez-Calbet, J. M.; Parras, M.; Alonso, J. M.; Vallet- Regı´ , M. J. Solid State Chem. 1994, 111, 202–207. (11) Parras, M.; Gonzalez-Calbet, J. M.; Alonso, J.; Vallet- Regı´ , M. J. Solid State Chem. 1994, 113, 78–87. (12) Parras, M.; Alonso, J.; Gonzalez-Calbet, J. M.; Vallet- Regı´ , M. J. Solid State Chem. 1995, 117, 21–29. (13) Adkin, J. J.; Hayward, M. A. Chem. Mater. 2007, 19, 755. (14) Jacobson, A. J.; Horrox, A. J. W. Acta Crystallogr. 1976, B32, 1003. (15) Adkin, J. J.; Hayward, M. A. J. Solid State Chem. 2005, 178, 3829. (16) Adkin, J. J.; Hayward, M. A. Inorg. Chem. 2008, 47, 10959. (17) Battle, P. D.; Gibb, T. C.; Jones, C. W. J. Solid State Chem. 1988, 74, 60. (18) Boullay, Ph.; Hervieu, M.; Labbe, Ph.; Raveau, B. Mater. Res. Bull. 1997, 32, 35.
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N2, and quenched in air after treatments in air. Once 4H-BaMn0.85Ti0.15O3-δ was stabilized, the sample was reoxidized in air at 1523 K for 48 h and quenched in air to form the 9R-BaMn0.85Ti0.15O3-δ polytype. The average cation composition was determined by inductively coupled plasma mass spectrometry (ICP-MS). The overall oxygen content was determined by thermogravimetric analysis on a thermobalance based on a CAHN D-200 electrobalance which allows determination of the oxygen content within (1 10-3 on a sample of about 100 mg working under 300 mbar H2/ 200 mbar He atmosphere. The analyzed composition of the 9R sample was BaMn0.85Ti0.15O2.93, and this was also confirmed by analysis of the neutron diffraction data. Powder X-ray diffraction (XRD) patterns were collected using Cu KR monochromatic radiation (λ = 1.54056 A˚) at room temperature on a Panalytical X’PERT PRO MPD diffractometer equipped with a germanium 111 primary beam monochromator and X’Celerator fast detector. Neutron powder diffraction (NPD) data for 9R-BaMn0.85Ti0.15O3-δ were collected at room temperature on the high resolution powder diffractometer D2B at the Institute Laue Langevin (ILL), Grenoble (France), with neutrons of wavelength 1.594 A˚. The angular range covered by the detectors extended from 0 to 160° with a step size of 0.05°. Neutron diffraction data were collected from samples at different temperatures on the high flux D1B instrument. Diffraction data were analyzed by the Rietveld method19 using the Fullprof program.20 Selected area electron diffraction (SAED) and high resolution electron microscopy (HREM) were performed using a JEOL 3000 FEG electron microscope, fitted with a double tilting goniometer stage ((22°, (22°). Simulated HREM images were calculated by the multislice method using the MacTempas software package. DC magnetization was measured using a SQUID magnetometer, in the range ∼2 to 300 K under an applied magnetic field of 1000 Oe. Powder was milled using a mortar and pestle and uniaxially pressed (Specac, Kent, U.K.) into cylindrical (diameter ∼ 5 mm, thickness ∼ 2 mm) pellets under an applied pressure of 50 MPa and then isostatically pressed (model CIP 32330, Flow Autoclave System Inc., Columbus, OH) at 200 MPa. Pellets were sintered in air at 1523 K for 4 h, and a ceramic density of 5.15 g 3 cm-3 (85% of the theoretical X-ray density) was achieved. Electrodes fabricated from gold paste (T-10112, Engelhard-CLAL, Cinderford, Gloucestershire, U.K.) were applied to both major faces of the pellets, which were sintered in air at 1073 K for 1 h to remove volatiles and harden the residue. The electrical properties of 9R-BaMn0.85Ti0.15O2.93 ceramics were investigated in the temperature range 10-300 K using a cryocooler coupled to an LCR bridge (model E4980A, Agilent, Palo Alto, CA) and to an impedance analyzer (model HP 4192A, Hewlett-Packard, Palo Alto, CA) for fixed frequency capacitance and impedance spectroscopy (IS) measurements, respectively. For the temperature range 300450 K, samples were measured using a ceramic jig in a high temperature furnace coupled to an impedance analyzer (model HP 4192A, Hewlett-Packard, Palo Alto, CA). All impedance data were corrected for sample geometry and analyzed using the commercial software package Z-view (Scribner Associates, Inc., Charlottesville, VA, version 2.1).
Results and Discussion An oxygen content of 2.93 was determined by thermogravimetric analysis for 9R-BaMn0.85Ti0.15O3-δ and (19) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (20) Rodrı´ guez-Carvajal, J. Physica B 1993, 192, 55.
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Figure 1. SAED patterns along [010] (a) and [110] (b) for 9R-BaMn0.85Ti0.15O2.93.
Figure 2. [010] HREM image corresponding to 9R-BaMn0.85Ti0.15O2.93. Inset shows the simulated image for Δf = -550 A˚, Δt = 60 A˚.
confirmed by NPD data as discussed below. Thermogravimetrical analysis performed on the 4H-BaMn0.85Ti0.15O3-δ sample gave an average δ value of 0.34. Unfortunately, this value could not be confirmed by NPD data due to some degree of microstructural disorder in the sample. XRD patterns corresponding to both polytypes are depicted in Figure 1a,b of the Supporting Information, for 9R and 4H, respectively. Both data sets can be indexed on a hexagonal lattice with cell parameters a = 5.68007(1) A˚ and c = 20.98208(6) A˚ for 9R and a = 5.71355(4) A˚ and c = 9.41230(8) A˚ for 4H, c values being consistent with 9 and 4 layer polytypes, respectively. Although 4H-BaMn0.85Ti0.15O2.66 appears as a single-phase, peak broadening is observed in the diffraction pattern which is indicative of some type of disorder in the sample. This fact is also reflected in the slightly raised fit parameters obtained from the refinement. Full details of the XRD refinement are given in Table 1 of the Supporting Information. Structural characterization of both samples by SAED and HREM has been undertaken to establish the layer sequence and symmetry of the as-obtained polytypes and to verify the presence or not of disorder in the crystals. Figure 1a,b shows the SAED patterns corresponding to 9RBaMn0.85Ti0.15O3-δ along [010] and [110] zone axes, respectively. Sharp diffraction spots are apparent with no streaking or diffuse intensity being observed. All reflections can be indexed on the basis of a rhombohedral unit cell with lattice parameters of a ∼ 5.7 A˚, c ∼ 20.9 A˚. The reflection conditions are compatible with an R3m space group. According to these results 11 stacking sequences are possible, although the only layer sequence known until now for a hexagonal polytype belonging to this space group is (hhc)3. As HREM is a very efficient tool to determine the layer sequence along the c-direction, samples were characterized in a 300 kV microscope with 1.6 point resolution. The (hhc)3 layer arrangement is confirmed from the Ba contrast along the [010] zone axis, Figure 2. The image simulation (inset of Figure 2) perfectly fits the experimental image for Δf = -550 A˚ and Δt=60 A˚. The sequence is extended over the
Figure 3. (a) SAED patterns along [010] and (b) corresponding HREM image along [010] zone axis for an ordered crystal of 4H-BaMn0.85Ti0.15O2.66, calculated image for Δt = 6 nm and Δf = -50 nm is inset on the image.
Figure 4. (a) Electron micrograph of a disordered crystal corresponding to the 4H polytype. Three different domains (A, B, and C) are marked, whose corresponding FFTs are shown. (b) Enlargement of one domain (marked with a square in part a) showing a dislocation edge (lines) due to the insertion of a cubic layer. (c) Corresponding high resolution image along [001] zone axis. Dislocation edges, marked with lines along the [010] direction, are responsible for the different observed domains (see circles).
whole crystal; therefore, the presence of any significant intergrowth can be discarded. Two kinds of crystals were found when 4H-BaMn0.85Ti0.15O2.66 was characterized by HREM. The majority fraction of the crystals has an ordered structure. Figure 3a corresponds to the SAED pattern along [010] zone axis, and Figure 3b depicts the HREM image along the same zone axis. All maxima can be assigned to a four layer polytype with hexagonal symmetry belonging to P63/mmc with stacking sequence (hc)2. Careful inspection of the samples reveals a small percentage of crystals (around 15%) showing the presence of extended defects. Figure 4a corresponds to the
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Figure 5. Observed, calculated, and difference profile of NPD data for 9R-BaMn0.85Ti0.15O2.93 at room temperature.
[010] projection of a disordered crystal where the presence of various crystalline regions (labeled A, B and C in the figure) can be observed. FFT performed in the different areas shows the features of the (hc)2-4H polytype, indicating that all of them are structurally identical. The formation of such domains is due to edge dislocations clearly visible in the enlarged image, Figure 4b. Moreover, careful inspection of the compression area of the dislocation reveals the insertion of an extra cubic semiplane of atoms parallel to the c-axis (marked with lines in the inset). This defect occurs over the whole crystal. This structural peculiarity is also observed along the [001] projection (Figure 4c), where edge dislocations are visible between different domains. This disordered microstructure is consistent with the broad maxima observed in the XRD data. Stacking faults are a common occurrence in polytype materials. An intergrowth of different polytypes is the more usual way in which the variations of the content are accommodated. However, the presence of extended defects such as those observed in the disordered 4H phase is rather unusual in hexagonal perovskites. By considering the strong preference of Ti4þ to occupy corner-sharing octahedra, that is, a cubic layer, the insertion of such cubic semiplanes can be understood as Ti-rich areas suggesting that a nonhomogeneous cationic distribution can occur in the crystals. Moreover, since only a minor fraction of the crystals shows this disordered microstructure, it can be considered that in these, the average cationic Ti/Mn ratio could be slightly higher than that corresponding to the ordered ones. Unfortunately, EDS cannot be used to accurately analyze the cationic content in the different type of crystals because KR1 and 2 and Kβ1 Ti lines overlap with LR1 and Lβ1 Ba lines. The high degree of disorder in the 4H-BaMn0.85Ti0.15O2.66 microstructure prevents a deeper understanding of this material; therefore, further structural and
physical characterization has been carried out only for the 9R-BaMn0.85Ti0.15O3-δ polytype. Structural refinement of this oxide has been performed on neutron diffraction data collected at room temperature. The atomic positions corresponding to 9R-BaMnO3 described by Chamberland et al.21 were used as the starting model in the R3m space group. Mn and Ti were randomly distributed over the two distinct metal sites according to the Mn:Ti nominal ratio since neutron scattering lengths for these atoms are very similar (Ti = -3.438 fm and Mn=-3.73 fm). Atomic coordinates, isotropic displacement parameters for each atom, and oxygen occupancies were refined. Figure 5 shows the observed, calculated, and difference plots from this structural refinement. Refined structural parameters and selected bond lengths are listed in Tables 1 and 2, respectively. The structure, depicted in Figure 6, is formed by trimers of face-sharing octahedra linked by corners. M1 and M2 sites define each trimer. The M1 site corresponds to the central octahedron of the trimer, and it is regular with six identical M1-O2 distances, whereas M2 has three short and three long M2-O distances; see Table 2. To reduce electrostatic repulsions between metal ions in the trimers, M2 cations are displaced from their central position toward the adjacent cubic layer. Refinement of the oxygen occupancies shows that O1 is fully occupied, and therefore the oxygen vacancies are randomly located in the hexagonal layer (Ba2O2) see Table 1. The composition from the refinement was BaMn0.85Ti0.15O2.93(1), in good agreement with the TG analysis. Although the Mn/Ti distribution cannot be obtained from the neutron diffraction data, a cationic distribution can be proposed. Bond valence sum calculations (21) Chamberland, B. L.; Sleight, A. W.; Weiher, J. F. J. Solid State Chem. 1970, 1, 506.
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Table 1. Final Structural Parameters for 9R-BaMn0.85Ti0.15O2.93a,b Ba1 (0, 0, 0) Biso (A˚2) 0.65(7) Occ 1 Ba2 (0, 0, z) z 0.21627(14) 2 0.63(5) Biso (A˚ ) Occ 1 0.77(7) M1 (0, 0, 1/2) Biso (A˚2) Occ 0.85/0.15c M2 (0, 0, z) z 0.38101(17) 0.63(5) Biso(A˚2) Occ 0.85/0.15c O1 (1/2, 0, 0) Biso (A˚2) 0.90(3) Occ 1 O(2) (x, -x, z) x 0.14864(4) z 0.55818(7) 0.91(3) Biso (A˚2) Occ 0.964(9) a = b (A˚) 5.679543(28) c (A˚) 20.98079(13) a Space Group R3m. b Fit parameter; Rp =2.77, Rwp = 4.10, RB = 4.73, χ2=1.54. c Mn/Ti ratio. Occupancy of metal sites cannot be distinguished from the scattering lengths.
Table 2. Selected Interatomic Distances (A˚) for 9R-BaMn0.85Ti0.15O2.93 Ba1-O1 6 Ba1-O2 6 Ba2-O1 3 Ba2-O2 6 Ba2-O2 3 M1-O2 6 M2-O1 3 M2-O2 3 M1-M2 M2-M2
2.839(1) 2.9124(11) 2.953(2) 2.8510(2) 2.900(2) 1.9048(9) 1.9206(19) 1.940(2) 2.739(8) 3.841(6)
were performed for Mn and Ti in the different M sites by using bond lengths from Table 2. The M1-O distance of 1.9048(9) A˚ is the same as the Mn-O distance reported for this environment in 9R-Ba0.875Sr0.125MnO313 (1.8956(3) A˚) or in BaMnO3 (1.9044(4) A˚).22 This confirms the M1 site to be completely occupied by Mn. Besides, the larger distance M2-O (∼1.93 A˚) is in agreement with the presence of Ti cations on this site. The obtained values, Table 3, reflect the strong preference of Mn4þ ions to occupy the central site of the face-sharing trimers (M1 site), whereas both Mn and Ti must be distributed over the terminal octahedra of the trimer. On this basis, the cationic distribution is proposed to be Ba(Mn0.33)M1(Mn0.52Ti0.15)M2O2.93(1). Magnetic Properties. Magnetic susceptibility versus temperature measured under a field of 1000 Oe is shown in Figure 7 and shows the irreversibility between zero field cooled (ZFC) and field cooled (FC) data below ∼250 K. In addition, at ∼50 K there is a significant increase in the field-cooled magnetization. A noteworthy feature is the presence of an anomaly in the magnetization data at ∼45 K. This has been observed in other Mn-perovskites, for example, 12R- and 6H-Ba(MnTi)O3-δ, and was ascribed to the magnetic phase transition of Mn3O4. We therefore attribute this signal to be due to a small amount of Mn3O423 present in the sample that was not detectable by either XRD or NPD techniques. NPD data collected on D1B diffractometer from 300 to 5 K showed additional diffraction features assigned to the (22) Cussen, E. J.; Battle, P. D. Chem. Mater. 2000, 12, 831. (23) Jensen, G. B.; Nielsen, O. V. J. Phys. C: Solid State Phys. 1974, 7, 409.
Figure 6. Structural model for the 9R polytype. Table 3. Bond Valence Sums for Ti and Mn in 9R-BaMn0.85Ti0.15O2.93 P PVMn-O VTi-O
M1 site
M2 site
3.838 4.538
3.649 4.315
Figure 7. DC magnetic susceptibility versus temperature under an applied magnetic field of 1 kOe for 9R-BaMn0.85Ti0.15O2.93.
presence of long-range magnetic order. These additional scattering peaks were indexed on the basis of the same hexagonal crystallographic cell (R3m) with a propagation vector k = (0, 0, 1/2). Observed, calculated, and difference plots from D1B data at 5 K are shown in Figure 2 (Supporting Information). The refinement was performed using a magnetic model in which the interactions between manganese cations were antiferromagnetic. Figure 8 shows a representation of the magnetic structure formed by ferromagnetic sheets with the magnetic moments aligned along the a-axis and stacked antiferromagnetically along the c-axis. This kind of magnetic structure has been found in other hexagonal Mn-related structures, for example, the BaMnO3-δ system13 and BaMn1-xFexO3-δ.6 The saturated magnetic moment at 5 K was 2.43 μB, and the thermal variation of the magnetic moment per
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Figure 9. Thermal variation of the magnetic moments per Mn obtained from the refinement corresponding to 9R-BaMn0.85Ti0.15O2.93.
Figure 10. Impedance complex plane (Z*) plots at 300 K with InGa and Au electrodes for 9R-BaMn0.85Ti0.15O2.93.
manganese ion obtained from the refinement is shown in Figure 9. According to these data, a Neel temperature of 120 K is obtained. The value of the refined magnetic moment at 5 K for Mn is similar to that found for 9RBa0.875Sr0.125MnO3 and close to the predicted value for Mn4þ allowing for a degree of covalency,24 approximately 2.6 μB. This phase has a higher Neel temperature (∼250 K) than our 9R-phase (120 K). The presence of non-magnetic TiIV cations dilutes the antiferromagnetic interactions, and, as a result the magnetic ordering temperature decreases. Electrical Properties. 9R-BaMn0.85Ti0.15O2.93 is semiconducting at room temperature (σ ∼ 10-4 S 3 cm-1); therefore, impedance spectroscopy (IS) measurements were performed at 10-300 K with different types of electrodes. The impedance complex plane (Z*) plots at 300 K for a sample with InGa and Au electrodes are shown in Figure 10. In both cases, three arcs are observed. The two high frequency arcs are independent of the electrode type; however, the magnitude of the low frequency arc is dependent on the electrode type. For Au electrodes, the low frequency arc has an associated resistance of ∼200 kΩ 3 cm, whereas for InGa electrodes it has
a value of ∼100 kΩ 3 cm. In both cases the associated capacitance is ∼3-5 nF 3 cm-1. This low frequency arc is therefore attributed to a nonohmic electrode contact between the semiconducting ceramic and the metal electrode as observed in other systems. The work function of InGa is lower (∼4.1 eV) than that of Au (∼5.1 eV), and the lower resistance of the nonohmic contact with the InGa electrodes suggests that BaMn0.85Ti0.15O2.93 is n-type; however, other measurements such as thermoelectric power are required to confirm this hypothesis. The associated capacitance values of the intermediate and high frequency arcs were ∼2 10-10 and ∼5 10-12 F 3 cm-1, respectively, and, based on the brickwork layer model for electroceramics, are attributed to grain boundary and bulk (grain) responses, respectively.25 This assignment was confirmed by the IS response obtained at 200 K, Figure 11. The Z* plot shows the arcs associated with the grain boundary and bulk responses at intermediate and high frequencies, respectively; the electrode response is observed as an incline in the data at low frequency, Figure 11a. The spectroscopic plot of the real component of capacitance shows plateaus at high and intermediate frequencies with values of ∼10-12 (bulk) and 10-10 F 3 cm-1 (grain boundary), respectively, with a high capacitance incline at low frequencies associated with the electrode response, Figure 11b. Finally, a combined spectroscopic plot of the imaginary components of impedance
(24) Tofield, B. C.; Fender, B. E. F. J. Phys. Chem. Solids 1970, 31, 2741.
(25) Irvine, J. T. S.; Sinclair, D. C.; West, A. R. Adv. Mater. 1990, 2, 132.
Figure 8. Antiferromagnetic structure of 9R-BaMn0.85Ti0.15O2.93.
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Figure 12. Arrhenius plot of the bulk and grain boundary conductivity (σ = 1/R) for 9R-BaMn0.85Ti0.15O2.93.
Figure 11. Impedance complex plane (Z*) plot (a) and spectroscopic capacitance (C0 ) (b) and M00 /Z00 (c) plots at 200 K for 9R-BaMn0.85Ti0.15O2.93.
(Z00 ) and electric modulus (M00 ) reveals a Debye-like peak in the M00 spectrum that is coincident with the high frequency Z00 peak which confirms this to be the bulk response, Figure 11c. On this basis, the IS data were modeled on an equivalent circuit of three parallel resistor-capacitor (RC) elements connected in series; ReCe represents the low frequency electrode response; RgbCgb represents the grain boundary regions; and RbGb, represents the semiconducting grains. An Arrhenius plot of bulk and grain boundary conductivity (where σ=1/R) data extracted from the intercepts of the respective arcs on the real axis of the Z* plots is shown in Figure 12. Both follow an Arrhenius-type law in the measured temperature range; however, the bulk conductivity deviates from linearity at low temperature. This variation in slope may be due to a change in the conduction mechanism as has been observed in related materials such as 5H-BaMn0.2Co0.8O2.80,7 12R-BaMn0.67Ti0.33O3,3 and 6HBaTiO3.26 Our previous results on the BaMn1-xTixO3-δ system showed that an increase in Mn content as well as in oxygen vacancy concentration gives rise to an increase in (26) Sinclair, D. C.; Skakle, J. M. S.; Morrison, F. D.; Smith, R. I.; Beales, T. P. J. Mater. Chem. 1999, 9, 1327.
Figure 13. Temperature dependence of the bulk permittivity for 9R-BaMn0.85Ti0.15O2.93.
conductivity. 9R-BaMn0.85Ti0.15O2.93 is more conductive than all the samples studied previously and therefore confirms this trend to be maintained in this system. The difference in grain and grain boundary conductivity is attributed to oxygen concentration gradients between the grain and grain boundary regions in the ceramics. The temperature dependence of the bulk relative permittivity, εr, for 9R-BaMn0.85Ti0.15O2.93 was obtained from the Debye-like M00 peak using the relationship Cb =1/2M00 and is shown in Figure 13. εr is temperature dependent and has a maximum value at ∼160 K. The origin of this peak is currently unknown as there is no evidence of a structural phase change from the low temperature NPD data and it occurs at ∼40 K above the Neel temperature. It may be associated with the change in conduction mechanism as observed by the curvature of the bulk conductivity data at low temperatures, Figure 12. Unfortunately the sample was too resistive to obtain meaningful M00 data below ∼110 K so it was not possible to investigate εr below the magnetic phase transition at ∼120 K using IS. Instead, fixed frequency capacitance measurements were performed to investigate the permittivity behavior at low
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frequency and near room temperature are attributed to a nonohmic electrode effect.27-29 The semiconducting nature of the ceramic results in large values of tan δ, and the heterogeneity of the electrical microstructure is revealed by the presence of two peaks in the tan δ data (for a fixed frequency), Figure 14c. These peaks are associated with a changeover from measuring the bulk to grain boundary response at low temperatures (first peak in tan δ) and subsequently from measuring the grain boundary to electrode response at higher temperatures (second peak in tan δ). Many perovskite type materials such as CaCu3Ti4O1230 and 5H-BaMn0.2Co0.8O2.807 have this electrical microstructure of semiconducting grains and resistive grain boundaries and give rise to similar so-called “giant” permittivity responses with values >10 000 near room temperature. It is therefore difficult to obtain bulk permittivity data for 9R-BaMn0.85Ti0.15O2.93 near the magnetic phase transition temperature (∼120 K) as the fixed frequency measurements are heavily dominated by the extrinsic responses associated with the grain boundaries and nonohmic electrode contacts, Figure 14b. We have previously reported that the bulk permittivity decreases with increasing Mn content in the BaMn1-xTixO3-δ system.3 The results obtained here for 9R-BaMn0.85Ti0.15O2.93 are consistent with our previous studies, and the bulk permittivity value of ∼22 at ∼10 K is similar to that reported for the BaMn1-xFexO3-δ system.4,5 Conclusions The experimental conditions required to stabilize the 4H- and 9R-polytypes of BaMn0.85Ti0.15O3-δ have been established, and their crystal structures have been elucidated by a combination of XRD, NPD, ED, and HREM. The structural analysis reveals the stacking sequences to be (hc)2 and (hhc)3 for 4H and 9R, respectively. The 4Hphase is formed by two kinds of crystals, the major part corresponding to an ordered 4H-structure, although a minority fraction shows the presence of extended defects that are rather unusual in hexagonal perovskites. The 9R polytype is a fully ordered phase with a composition of Ba(Mn0.33)M1(Mn0.52Ti0.15)M2O2.93(1) and is an antiferromagnetic semiconductor with a bulk permittivity of ∼22 and a Neel temperature of ∼120 K .
Figure 14. Temperature dependence of the permittivity (ε) (a and b) and dielectric loss (c) of 9R-BaMn0.85Ti0.15O2.93.
temperatures, Figure 14. The data indicate a bulk permittivity value of ∼22, Figure 14a,b, but the data rise significantly above 100 K, especially for the low frequency data, Figure 14a. This large increase in apparent permittivity and the peaks observed in the tan δ data, Figure 14c, are consistent with the equivalent circuit used to model the IS data. The apparent permittivity values of >10000 at low (27) Li, M.; Feteira, A.; Sinclair, D. C. J. Appl. Phys. 2009, 105, 114109. (28) Ferrarelli, M. C.; Sinclair, D. C.; West, A. R.; Dabkowska, H. A.; Dabkowski, A.; Luke, G. M. J. Mater. Chem. 2009, 19, 5916–5919.
Acknowledgment. Financial support through research projects MAT2007-61954 (Madrid), Consolider CSD200900013, and EPSRC (Sheffield) is acknowledged. We wish also thank Dr. Antonio Feteira for valuable help on preliminary synthesis. We thank Dra. M. T. Fern andez-Dı´ az for assistance in collecting the neutron power diffraction data. Supporting Information Available: XRD patterns corresponding to both polytypes, full details of the XRD refinement, and observed, calculated, and difference plots from D1B data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. (29) Li, M.; Feteira, A.; Sinclair, D. C.; West, A. R.; Shen, Z.; Nygren, M. J. Appl. Phys. 2009, 106, 104106. (30) Sinclair, D. C.; Adams, T. B.; Morrison, F. D.; West, A. R. Appl. Phys. Lett. 2002, 80, 2153.
