Synthesis, Structural and Magnetic Studies of the Double Perovskites

Jul 16, 2012 - Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, ... Peter E. R. Blanchard , Zixin Huang , Brendan J. Kennedy , Samu...
1 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/cm

Synthesis, Structural and Magnetic Studies of the Double Perovskites Ba2CeMO6 (M = Ta, Nb) Qingdi Zhou,† Peter Blanchard,† Brendan J. Kennedy,*,† Emily Reynolds,† Zhaoming Zhang,‡ Wojciech Miiller,†,§ Jade B. Aitken,†,∥,⊥ Maxim Avdeev,§ Ling-Yun Jang,⊗ and Justin A. Kimpton∥ †

School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Institute of Materials Engineering and §Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, 2234, Australia ∥ Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia ⊥ Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki 305-0801, Japan ⊗ Facility Utilization Group, Experiment Facility Division, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan ‡

ABSTRACT: Two Ce3+ containing double perovskites Ba2CeMO6 (M = Nb and Ta) have been prepared through the use of mildly reducing conditions, and the Ce valence state has been shown to be +3 through Ce L-edge X-ray absorption measurements. Both oxides adopt a monoclinic structure in I2/m at room temperature and undergo two phase transitions upon heating, a discontinuous I2/ m → R3̅ and a continuous R3̅ → Fm3̅m transition. Analysis of the first order I2/m → R3̅ transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron powder diffraction data because of domain wall effects.

KEYWORDS: perovskite, trivalent cerium, phase transition, XAS Ce L-edge → I2/m → R3̅ → Fm3̅m. With the notable exception of the Ce containing oxides, the preparation of single phase samples of each member of these three series has been described. Brixner reported the preparation of BaCe0.5Nb0.5O310 in 1960 but detailed studies of this were not reported. The Ba2LnBiO6 oxides have also been investigated,11,12 although the situation is somewhat more complex because of the ease of reduction of Bi5+ to Bi3+ such that when the lanthanoid can exist in the tetravalent state (Ln = Ce, Pr, Tb) this tends to occur and extensive cation disorder follows.11 The Ce containing oxides Ba2CeMO6 (M = Sb5+, Nb5+, or Ta5+) are notable absences from our earlier work5,13,14 reflecting the ease of oxidation of Ce3+ to Ce4+ and the high stability of Nb5+ and Ta5+. Indeed IJdo and Helmholdt15 used the stability of Ce4+ to extend the earlier work of Rauser and Kemmler-Sack,16 to prepare, and structurally characterize, the novel B-site deficient perovskite Ba2Ce0.75SbO6, where the cerium is tetravalent. There appears to be no reports of the analogous Nb and Ta oxides. The recent reports17,18 that double perovskites Ba2CeMO6 (M = Nb, Ta) could be prepared by solid state methods in air is at variance with earlier studies. As elegantly demonstrated by Hammink, Fu, and

T

he need for materials with specific chemical, electrical, or physical properties drives the search for new compounds. Double perovskites with the general formula A2BB′O6 have attracted great interest and have proved to be a fertile ground for such discoveries. Perovskite-type oxides provide essential components for numerous applications including ferroelectrics, ionic conductors, magnetic devices, and so forth. Despite the vast body of information available to researchers and engineers responsible for the development of new devices, there are numerous examples of poorly characterized or incorrectly described structures, the title oxides being a case at point. For a number of years we have been interested in the rich variety of structures exhibited by double perovskites and the nature of the transitions between these. The A2BB′O6 double perovskite structure has two cations, B and B′, that exhibit a rock-salt type ordering with the structure being further embellished by cooperative tilting of the corner sharing octahedra.1 The precise structure depends on the composition, temperature, and/or pressure.2−4 The majority of double perovskites adopt one of five particular tilt systems, namely: a0a0a0 (Fm3̅m), a0a0c− (I4/m), a−a−a− (R3̅), a0b−b (I2/m), and a+b−b− (P21/n), although other variants are possible.1 X-ray and, as appropriate, neutron powder diffraction studies have demonstrated5−9 that Ba2LnMO6 (Ln = a trivalent lanthanoid and M = Sb5+, Nb5+ or Ta5+) oxides adopt a sequence of structures as a consequence of the progressive loss of octahedral tilting P21/n → I2/m → I4/m → Fm3̅m, or P21/n © 2012 American Chemical Society

Received: April 25, 2012 Revised: July 8, 2012 Published: July 16, 2012 2978

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

IJdo19 in their studies of Ba2InTaO6, the formation of highly crystalline double perovskites can be very sensitive to the precise methods used to prepare them; for example, Ba2InTaO6 only develops the ordered double perovskite structures after prolonged annealing. Disorder has been shown to impact on the phase transition behavior of Ba2YTaO6.20,21 It is possible that the oxides Ba2CeMO6 (M = Nb, Ta) are equally sensitive to the precise synthetic protocol employed. Our aim in the present work was to verify the recent reports of the preparation of Ba2CeMO6 (M = Nb, Ta) and, using high resolution synchrotron X-ray and neutron diffraction methods, determine the structures of these novel oxides. Here we demonstrate that the method described by Bharti and Sinha17,18 does not yield the single phase double perovskites Ba2CeMO6 (M = Nb, Ta). Nevertheless we have, apparently for the first time, prepared and structurally characterized these two oxides. Given the possibility that the Ce may be either trivalent or tetravalent, X-ray absorption measurements at the Ce L-edge were also conducted.

■

cycle refrigerator. The wavelength of the incident neutrons, obtained using a Ge 335 monochromator, was 1.622 Å as determined using a NIST SRM676 Al2O3 standard. This instrument has a maximum dspacing resolution of Δd/d ∼ 1 × 10−3. The structures described here were refined by the Rietveld method as implemented in the program RIETICA.25 For both the neutron and synchrotron X-ray analysis the peak shapes were modeled using a pseudo Voigt function, and the background was estimated by interpolating between, up to, 40 selected points. X-ray absorption near-edge structure (XANES) analysis was carried out on beamline 16A1 at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan.26 The Ce L3-edge spectra were obtained from a number of samples of Ba2CeMO6 (M = Nb, Ta) and three Ce standards (CeO2, SrCeO3, and CeAlO3), in fluorescence mode using a Lytle detector from powder samples dispersed on Kapton tape. Energy steps as small as 0.2 eV were employed near the absorption edges with a counting time of 2 s per step. The spectra were normalized to the incident photon current. The energy scale was calibrated using the K-edge of a pure Cr foil at 5989.2 eV. Additional measurements at the Nb K-edge were undertaken from Ba2CeNbO6 on Beamline 20B at the Photon Factory in Japan.27 The X-ray beam was monochromated by diffraction from a channel cut Si (111) monochromator. Harmonic rejection was achieved by detuning the monochromator to 50%. These measurements were conducted in transmission mode with the samples diluted, as appropriate, in BN and held between Kapton tape in a 1 mm thick sample holder. The energy ranges used for X-ray absorption near edge structure (XANES) data collection were: pre-edge region 18756 to 18956 eV (3.3 eV steps); 18956 to 19036 eV (0.25 eV steps); and postedge region to 7 Å-1(0.05 Å-1 steps in k-space) with 2s/point. The energy scale was calibrated using a simultaneously collected Nb foil at 18.986 keV. Magnetic susceptibility and magnetization measurements were carried out using a Quantum Design PPMS9 device operating between 2 and 350 K and with magnetic fields of up to 9 T.

EXPERIMENTAL SECTION

The crystalline samples of Ba2CeMO6 (M = Nb, Ta) were synthesized using a conventional solid state method from stoichiometric quantities of BaCO3 (99.997%, Aithaca), CeO2 (99.9%, Aldrich), Nb2O5 (99.9%, Aldrich), and Ta2O5 (99.99%, Aldrich). The starting materials were preheated at 600 °C/12 h for BaCO3 and at 1000 °C/12 h for CeO2. The powders were finely ground and then pressed into 13 mm diameter pellets prior to being heated in a tube furnace initially at 1400 °C/10 h in 3.5% H2/N2 with a gas flow of about 20 cm3/min and then, after regrinding and repressing into pellets, at 1425 °C/20 h in 3.5% H2/N2. Before each heating step the reaction tube was purged by 3.5% H2/N2, at room temperature (RT), for at least 1 h. The phase purity of the samples was checked using laboratory powder X-ray diffraction (XRD) with Cu Kα radiation. Samples of Ba2CeMO6 (M = Nb, Ta) could be prepared from Ce2(CO3)3 (99.9%, Aldrich) as the source of Ce, by heating pellets of the starting materials under a H2/N2 atmosphere at 1350 °C for 25 h with intermittent regrinding and repressing. The Ce2(CO3)3 was used as received. Such products typically contain small amounts of Nb2O5 or Ta2O5 containments. Further heating (1400 °C/10 h and 1425 °C/ 20 h in 3.5% H2/N2 with intermitted regrinding and repressing) reduced the amounts of such impurities, but did not totally eliminate. This may be related to the complex decomposition chemistry of Ce2(CO3)3.22 Attempts to prepare Ba2CeNbO6 or Ba2CeTaO6 using methods described in the literature17,18 were unsuccessful. In these reactions stoichiometric quantities of BaCO3, Ce2(CO3)3, and Nb2O5 or Ta2O5 were well mixed with a small amount of acetone in an agate mortar and pestle for 30 min. After drying, the mixtures were heated at 1350 °C in air for 15 h and then cooled to RT at 1 °C per min. The mixture was reground and pressed into pellet and heated in air at 1370 °C for 10 h. The diffraction patterns of the materials appeared to be identical to those reported.17,18 The use of a mechanical ball-mill rather than hand grinding did not alter the product. Synchrotron X-ray powder diffraction data were collected from Ba2CeMO6 (M = Nb, Ta) using the powder diffractometer at BL-10 beamline of the Australian Synchrotron.23 The samples were finely ground, and these were housed in sealed 0.3 mm diameter capillaries that were rotated during the measurements. The wavelength was set at ∼0.825 Å, and the precise value of this was determined using a NIST LaB6 standard reference material. Data were collected from 100 to 500 K using an Oxford cryostream and from RT to 770 K using a Cyberstar hot-air blower. A 5 g sample of Ba2CeNbO6 was also sealed in a 9 mm diameter vanadium can for neutron powder diffraction measurements; such data were obtained using the high resolution powder diffractometer Echidna at ANSTO’s OPAL facility at Lucas Heights.24 Low-temperature data were collected at 4 K using a closed-

■

RESULTS AND DISCUSSION Sample Preparation. The powder XRD pattern recorded using Cu Kα radiation of the oxide of nominal composition Ba2CeTaO6, prepared as described by Bharti and Sinha,17 appears identical to that presented in Figure 1 of their work.17 Bharti and Sinha described this as having a monoclinic double perovskite structure although they neither identified the space

Figure 1. XRD pattern of “Ba2CeTaO6” prepared in air recorded using Cu Kα radiation. The symbols are the measured data, and the solid line is the best fit obtained using a LeBail type analysis in space group P21/n. The lower solid line is the difference between the observed and the calculated profiles. The short vertical lines illustrate the position of the space group allowed Bragg reflections. The inset highlights the inability to reproduce the observed splitting of certain reflections. 2979

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

Figure 2. Ce L3-edge XANES spectra of (a) Ce standards, (b) Ba2CeNbO6, and (c) Ba2CeTaO6. All spectra were collected in fluorescence mode.

Ce2O3, and Nb2O5 at 1200 °C in argon gettered with Zr−Ti alloy.10 We next explored the use of a reducing environment. Since Nb5+ can be reduced by H2 at high temperatures (see for example ref 29) we initially focused on the Ta oxide by heating Ta2O5, BaCO3, and CeO2 under a H2/N2 atmosphere. For safety reasons we elected to use a commercially supplied nonexplosive 3.5% H2/N2 mixture. The influence of gas fugacities was not investigated. Once we had established that a single phase sample of Ba2CeTaO6 could be obtained we prepared the analogous Nb oxide employing Nb2O5 as starting material. Attempts to prepare the analogous Sb containing oxide, Ba2CeSbO6, were unsuccessful irrespective of the environment, reducing conditions resulting in the reduction of both the Ce and the Sb cations. Heating a sample of Ba2CeTaO6, prepared under reducing conditions, at 1350 °C in air overnight results in decomposition of the material, and the diffraction pattern of the resulting material is very similar to that illustrated in Figure 1. In situ diffraction measurements of Ba2CeTaO6 showed that heating this to ∼600 K resulted in transition to the cubic Fm3̅m structure. Further heating of the sample, either in air or under a moderate vacuum (∼ 10−2 Torr) resulted in a noticeable broadening of the various diffraction peaks. Peak broadening is often associated with the formation of nanocrystalline materials. In the present case we believe that this is a consequence of the introduction of strains into the lattice associated with the oxidation of Ce3+ to Ce4+, while retaining a perovskite type structure. Of the two possible mechanisms to charge balance this oxidation, by reduction of Ta or by the introduction of excess oxygen, we favor the latter. Oxygen excess nonstoichiometry in perovskite oxides is relatively rare since introduction of interstitial oxygen in perovskite structure is thermodynamically unfavorable. Nevertheless a number of systems containing oxygen excess, including LaMnO3+x30 and Sr1−xLaxTiO3+x/231 have been identified, although it should be noted that such oxides are better described as cation deficient, rather than oxygen excess. The complexity that can arise where the cations can exist in multiple oxidation states is evident from studies of “Ba2CeBiO6” that contains a mixture of Ce4+, Bi3+, and Bi5+, with the three cations disordered on a common site.11 Cation disorder has been observed in other systems containing Tb or Pr including the related oxides Ba2LnSnxBb1−xO6−δ and Ba2LnSnxSb1−xO6−δ (Ln = Pr and Tb),13,14 in the extreme case leading to phase separation.32 X-ray Absorption Near-Edge Spectroscopy. Since a reducing atmosphere was necessary to prepare samples of Ba2CeMO6 (M = Nb or Ta) the Ce atoms are expected to be trivalent. To confirm this, the Ce L3-edge XANES spectra were analyzed. The Ce L-edge corresponds to a dipole-allowed 2p-

group nor did they refine the structure.17 The work of Howard, Kennedy, and Woodward1 shows two monoclinic structures developing as a consequence of tilting of the rock-salt like ordered double perovskite structure, in space groups P21/n or I2/m. These differ in the nature of the cooperative tilting of the BO6 octahedra. The likelihood of tilting in Ba2CeTaO6 can be estimated by considering the perovskite tolerance factor (t), defined as t = (rA + rO)/21/2(rB + rO) where rB is the average ionic radius of the B-site cations.28 On the basis of studies of other A2LnBO6 systems, it appears that where t is above about 0.98 a cubic structure in Fm3̅m is expected. A tolerance factor in the range 0.98−0.96 typically results in either the rhombohedral R3̅ or the tetragonal I4/m space groups. A monoclinic structure in either I2/m or P21/n is typically encountered for smaller values of t. Assuming Ce present as Ce3+ the tolerance factor in Ba2CeTaO6 is 0.96. The XRD pattern shown in Figure 1 could be indexed to the cell described previously by Bharti and Sinha17 with a ∼ 9.75 Å, b ∼ 9.02 Å, and c ∼ 4.27 Å and β ∼ 93.8°, although we note that this cell does not coincide with any known double perovskite structure, both the above-mentioned monoclinic tilt systems are on a (2)1/2ap × (2)1/2ap × 2ap superstructure where ap is the unit cell length of the cubic aristotype ∼3.9 Å. Attempts to fit the observed diffraction data using a LeBail type analysis, in either space group P21/n or I2/m, to the reported cell17 were unsuccessful. Both space groups predicted a large number of unobserved reflections, and a number of reflections were not indexed. A LeBail analysis is more sensitive to errors in the estimated lattice parameters than simple index programs since it involves fitting the entire profile using appropriate peak shapes. Since a LeBail analysis does not consider the content of the unit cell, a Rietveld type analysis is required if the structure is to be refined. At this point it was concluded that although the method described by Bharti and Sinha yielded a crystalline product this was not a double perovskite of composition Ba2CeTaO6. Likewise LeBail analysis of an X-ray pattern for a sample of Ba2CeNbO6 prepared as described in reference 1717 was unsuccessful. The most probable reason for this inability to prepare the two Ba2CeMO6 oxides is that heating in air results in the formation of tetravalent Ce. X-ray absorption measurements of the samples showed them to contain Ce4+ (see below); oxidation of the Ce3+ present in Ce2(CO3)3 occurs when this is heated in air. We have not yet successfully indexed the diffraction patterns of these materials. Consequently we then explored alternate synthetic methods. Direct reaction of BaCO3, Ta2O5, and Ce2(CO3)3 either under vacuum or in an inert atmosphere (N2 or Ar) produced materials that were clearly nonhomogeneous, although we note that Brixner reported the preparation of cubic BaCe0.5Nb0.5O3 from BaO, 2980

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

to-5d transition. In general, Ce3+ and Ce4+ have very distinct lineshapes, as shown in Figure 2a. A single broad peak (labeled A′) is observed in Ce3+ species (such as CeAlO3). Four peaks (labeled A, B, C, and D) are commonly observed at the L3-edge of Ce4+ species. While peaks A and B are assigned to the 4f0 and 4f1L configurations (L denotes a ligand hole),33,34 peak C is believed to originate from either the 4f2L2 configuration33 or crystal splitting of the Ce 5d states.34 It has also been postulated that peak C may be the result of Ce3+ impurities.35,36 Peak D is attributed to final states with delocalized d character at the bottom of the conduction band.34 There are noticeable differences in the line shape of the Ce L3-edge for the two Ce4+ standards measured in this work, namely, CeO2 (8-coordindate Ce) and BaCeO3 (6-coordindate Ce), particularly with respect to peak C. Magnetic susceptibility measurements for the BaCeO3 sample (not shown) demonstrated the material to be diamagnetic suggesting that no Ce3+ was present in the sample. This demonstrates that Ce3+ impurities are not responsible for the difference in the line shape, rather the differences in the line shape are likely due to the more covalent character found in BaCeO337 as well as the difference in the coordination number. The line shape of the Ce L3-edge spectra for the Ba2CeNbO6 and Ba2CeTaO6 samples prepared under reducing conditions (Figures 2b and 2c) is similar to that found for the Ce3+ standard CeAlO3, the small differences possibly reflecting the difference in coordination environment between the A- and Bsite of perovskites. However, shoulders observed at higher photon energy from Ba2CeNbO6 and Ba2CeTaO6 occur at similar absorption energies to the peaks observed in BaCeO3, suggesting that a small amount of Ce4+ might be present in both samples (a greater amount of Ce4+ is observed in the sample prepared from Ce2(CO3)3 than CeO2), which is consistent with the presence of a small amount of Ce4+ (most likely CeO2) impurities observed in the diffraction studies (see below). The Ce L3 spectrum from a sample of “Ba2CeNbO6” prepared in open air using Ce2(CO3)3 as starting material is nearly identical to that obtained for BaCeO3, confirming that Ce has been completely oxidized to +4. To gain insight into the oxidation state of Nb for the Ba2CeNbO6 samples, the Nb K-edge was analyzed. The Nb Kedge corresponds to a dipole allowed transition from 1s electrons into 5p states. The Nb K-edge absorption edge energy was 19003.7 eV for the three Ba2CeNbO6 samples examined (estimated from the first derivative), showing the valence state of Nb to be identical (Figure 3). The first derivatives are shown

in supplementary data. The absorption energy is within error of that measured for Nd1/3NbO3 (19003.8 eV), confirming that Nb is pentavalent. Further structural information can be obtained from the pre-edge, which is attributed to transition to the p component in d-p hybridized orbitals and, to a lesser extent, to a 1s-to-4d quadruple transition.38,39 Under a perfect octahedral symmetry, the d-p hybridization cannot occur, and the pre-edge is very weak. However, distortions within the NbO6 octahedra allow for the d-p hybridization, leading to a more intense pre-edge feature. The RT crystal structure of Nd1/3NbO3 is orthorhombic in space group Cmmm with a relatively large octahedral distortion.40 Compared to the Nd1/3NbO3 standard, the pre-edge is less intense in the two Ba2CeNbO6 samples prepared under reducing conductions, consistent with a less distorted NbO6 octahedra. In contrast the sample prepared in open air shows a higher pre-edge intensity than Nd1/3NbO3, indicating the local environment surrounding the Nb ions is different from that in the Ba2CeNbO6 samples prepared under reducing conductions. On the basis of our XANES results, the oxidation state of Ce and Nb in the “Ba2CeNbO6” samples prepared in open air is +4 and +5 respectively. Charge balancing the molecular formula would result in Ba2CeNbO6.5, which should not be a perovskite. Magnetic Properties. The temperature dependence of the reciprocal magnetic susceptibilities for Ba2CeNbO6 and Ba2CeTaO6 are shown in Figure 4. No anomalies, indicative

Figure 4. Temperature dependence of the inverse magnetic susceptibility for Ba2CeNbO6 and Ba2CeTaO6 recorded with an external field of 1 T. The solid line corresponds to the values for an isolated f1 ion. The inset shows the field dependence of the magnetization at 2 K.

of a magnetic phase transition, were observed for either oxide. Since neither Nb5+ nor Ta5+ contains any unpaired d-electrons the Ce3+ is expected to dominate the magnetic susceptibility and consequently the observed magnetic susceptibilities for Ba2CeTaO6 and Ba2CeNbO6 are essentially identical. The strong nonlinear character of χ−1(T) is characteristic of a strong crystal field acting on the Ce3+ ion in 6-fold environment. Only at high temperatures, above 250−300 K, does χ−1(T) approach linearity, and the effective magnetic moment, μeff, is comparable to the theoretical value of 2.54 μB/Ce (solid line), predicted for the f1 electronic configuration appropriate for Ce3+. This is confirmed in the magnetization, m, measurements collected as a function of applied magnetic field, B, at 2 K (shown in the inset of Figure 4). The observed values of the magnetic moment induced at field of 9 T, m(2K,9T) = 0.28 μB/Ce, are much lower than the value expected for free Ce3+ ion, which may be estimated as m = gJJμM = 2.14 μB (for gJ = 6/7 and J = 5/2). The magnetic behavior of the two samples can be used to obtain qualitative information on the Ce3+ crystal field. The lowest-energy states for the f1 configuration belong to the 2F5/2

Figure 3. Nb K-edge XANES spectra of several Ba2CeNbO6 samples and Nd1/3NbO3 (Nb5+ standard). All spectra were collected in transmission mode. 2981

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

Figure 5. Synchrotron XRD pattern of Ba2CeNbO6 prepared from CeO2 under reducing conditions. The symbols are the measured data, and the solid line is the best fit obtained using a Rietveld type analysis in space group I2/m. The lower solid line is the difference between the observed and the calculated profiles. The lower set of short vertical lines illustrate the position of the space group allowed Bragg reflections. The upper set of reflections is from the small amount (∼ 0.7 wt %) of CeO2. Note the change in intensity scale at 2θ = 45° highlights both quality of the data and the fit.

multiplet. As Ce3+ is a Kramer’s ion, the crystal-field potential is expected to split this 6-fold-degenerate multiplet into three doublets. At high temperatures, above ∼250−300 K, most of the levels are almost completely populated, resulting in the linear temperature dependence of χ−1, and leads to the effective moments of ∼2.25 μB/C close to the theoretical value of 2.54 μB/Ce. As the temperature is lowered the two excited doublets are progressively depopulated resulting in nonlinear χ−1(T). At very low temperature, T < 40 K, the χ−1 vs T plot is essentially linear intercepting the origin demonstrating the absence of any cooperative magnetic effects in either oxide. The low values of the induced magnetic moment observed at very low temperatures results from the splitting of the ground state singlet in the magnetic field. Room Temperature Structure. Having prepared single phase samples of Ba2CeNbO6 and Ba2CeTaO6, we then sought to establish their precise structures using high resolution diffraction methods. The synchrotron X-ray powder diffraction pattern of Ba2CeNbO6 measured at RT, shown in Figure 5, contained a weak (111) type reflection near 2θ = 9.5° indicative of ordering of the Ce and Nb cations.1,41 The pattern was indexed to a double perovskite structure with a ∼ 2ap = 8.58 Å; however, scrutiny of the pattern demonstrated the symmetry to be lower than cubic. The patterns were measured to 2θ = 4.2°; however, no reflections in addition to those shown in Figure 5 were observed. The absence of Bragg reflections at low angles rules out larger unit cells and subsequent Rietveld refinements used only the data in the range 7.5 ≤ 2θ ≤ 84°. Examination of the XRD pattern showed the structure to be monoclinic, where the (222)P reflection appeared as a triplet and the (400)p reflection was a doublet. This, together with the absence of any (ooe) M-point reflections indicative of the presence of in-phase tilting of the corner sharing octahedra around the primitive [001]p axis, demonstrates I2/m to be the appropriate space group.1 No (ooe) type reflections were evident in the neutron diffraction pattern recorded at RT either, Figure 6. Cooling the sample to 4 K did not result in the appearance of any additional reflections in the neutron profile, indicating the absence of any structural or magnetic phase transitions at low temperatures.

Figure 6. Observed, calculated, and difference powder neutron diffraction profiles for the Rietveld refinement of Ba2CeNbO6 at RT, using a structural model in space group I2/m. The short vertical lines illustrate the position of the space group allowed Bragg reflections.

The absence of in-phase tilting, corresponding to a transition to a structure in P21/n upon cooling to 4 K Ba2CeNbO6 whereas such a transition was observed in Ba2LaTaO65 is a further example of the sensitivity of the structures of these oxides. The parameters for the refinement in I2/m against the neutron data are tabulated in Table 1. No evidence was found from the refinements for any anti-site disorder of the Ce and M-type cations. The structure of Ba2CeTaO6 prepared under reducing conditions using CeO2 as reagent was also investigated using synchrotron XRD data. The diffraction pattern measured at RT was well fitted to a model in I2/m, although as discussed in more detail below the data showed the effects of domain walls. Space group I2/m allows for rock-salt like ordering of the corner sharing MO6 and CeO6 octahedra in the Ba2CeMO6 oxides with the larger Ba2+ cations located in the resulting cavities, see Figure 7. The average Ba−O distance of 3.048 Å 2982

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

Table 1. . Refined Crystallographic and Associated Parameters for Ba2CeMO6 (M = Nb and Ta) at RTa

Table 2. Selected Bond Distances in Ba2CeMO6 (M = Nb, Ta) Estimated from Rietveld Refinements

M = Nb Rp

Rwp (%)

7.11%

9.22%

Rexp (%) 3.41% M = Nb

bond χ2

RB (%)

7.33

4.96%

ATOM

x

y

z

Biso

Ba Ce Nb O1 O2

0.5039(10) 0 0 0.0535(8) 0.2750(5)

0 0 0 0 0.2630(8) M = Ta

0.2520(10) 0 0.5 0.2734(6) −0.0263(3)

1.00(4) 0.51(7) 0.82(6) 1.35(8) 1.58(5)

Rp

Rwp (%)

Rexp (%)

χ2

RB (%)

3.26

4.46

2.44 M = Ta

3.33

6.93

ATOM

x

y

z

Biso

Ba Ce Ta O1 O2

0.5077(4)) 0 0 0.060(3) 0.277(2)

0 0 0 0 0.267(3)

0.2846(5) 0 0.5 0.284(2) −0.024(2)

1.05(2) 0.36(4) 0.58(5) 2.4(3) 1.58(5)

Ce−O1 Ce−O2 M−O1 M−O2 Ba−O1 × Ba−O1 × Ba−O1 × Ba−O2 × Ba−O2 × Ba−O2 × Ba−O2 ×

1 2 1 2 2 2 2

Ba2CeNbO6

Ba2CeTaO6

2.364(5) 2.326(5) 1.969(5) 1.999(5) 2.755(7) 3.059(1) 3.359(7) 2.849(7) 2.897(8) 3.187(8) 3.242(7)

2.46(2) 2.35(2) 1.89(2) 1.98(2) 2.75(2) 3.075(5) 3.39(2) 2.83(2) 2.92(1) 3.16(2) 3.27(2)

large difference in the size of the two B-type cations (1.01 vs 0.64 Å)42 coupled with the charge difference drives the ordering of these two cations. The representation of the refined structure in Figure 7 illustrates both the out-of-phase tilting of the octahedra and the relative sizes of the CeO6 and NbO6 octahedra. The precision of the structural refinement for Ba2CeTaO6 is somewhat lower since only X-ray methods were employed (Table 1) and will not be discussed here. Temperature-Induced Phase Transitions. The thermal evolution of the structure of Ba2CeNbO6 between 80 and 670 K was studied using synchrotron XRD measurements. These measurements are summarized by Figure 8 and showed that

a

The refined lattice parameters are, for M = Nb, a = 6.1031(3), M = 6.0617(2), c = 8.5668(3) Å, β = 90.181(4)°; and for M = Ta, a = 6.10944(7), M = 6.07119(7), c = 8.57961(9) Å, β = 90.1986(8)°. The refinement for Ba2CeNbO6 utilised neutron diffraction data and that for Ba2CeTaO6 synchrotron XRD data.

Figure 8. Temperature evolution of, suitably scaled, lattice parameters for Ba2CeNbO6 as obtained from synchrotron XRD measurements. The rhombohedral and monoclinic phases coexist in the region shown by the hatched background. Figure 7. Polyhedral view of the Ba2CeNbO6 structure (space group I2/M) showing the ordering of the corner sharing CeO6 and NbO6 octahedra. The Nb atoms are located at the center of the smaller octahedra, and the barium cations are represented by the solid spheres.

Ba2CeNbO6 undergoes two successive temperature induced phase transitions near 400 and 640 K. The first transition from monoclinic I2/m to rhombohedral R3̅ is required to be first order,1 and our data shows these two phases coexist over a ∼ 120 K temperature range. The second phase transition is the continuous rhombohedral to cubic transition. These three phases can be distinguished through examination of the (hhh) and (h00) type reflections. The (222)p reflection is a triplet with an approximately 1:1:2 intensity ratio in I2/m, a doublet with a ∼3:1 intensity ratio in R3,̅ and a singlet in Fm3m ̅ . The (400)p is a 2:1 doublet in I2/m and a singlet in both R3̅ and Fm3̅m. That a tetragonal (I4/m) intermediate phase is not formed is evident from the lack of splitting of the (400)p

found for Ba2CeNbO6 is typical for Ba2+ cations in a 12coordinate environment (Table 2). Neither of the MO6 octahedra showed evidence for appreciable distortion, and the average Ce−O bond distance 2.339 Å is in good agreement with the value predicted from the ionic radii of 6-coordinate Ce3+ (1.01 Å)42 although bond valence calculations43 suggest the Ce is somewhat overbonded, with the bond valence sum (BVS) being 3.61. The much shorter average Nb−O distance of 1.989 Å is unexceptional, as is the derived BVS of 4.86. The 2983

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

reflection in the high temperature phases. The I4/m tetragonal phase is present in the closely related oxide Ba2HoTaO6.6 Figure 9 illustrates the changes in the (222)p and (400)p reflections at selected temperatures.

of tetragonal ferroelectric perovskites such as Pb(Zr1−xTix)O3 “PZT”44 and in perovskites containing Jahn−Teller active cations such as SrLaCuTaO645 and Sr2CuUO6.6 Where the twins are associated with a first order phase transition it is possible that domains of the lower symmetry phases can have ferroelastic domain walls that, because of crystal chemistry constraints present during the development of the domains (i.e., the I2/m to R3̅ phase transition), are not aligned at the same angle φ. This introduces strain fields in the vicinity of the domain walls that result in diffuse scattering close to certain Bragg peaks. As illustrated by Locherer et al.,46 this can result in a ridge of diffuse scattering that connects the two Bragg peaks. This diffuse scattering is anisotropic and in addition to the extra diffuse intensity appearing between the peaks can also result in asymmetry of the Bragg reflections. Contemporary Rietveld programs allow anisotropic broadening parameters to be included in the refinements; however, modeling of the asymmetry due to domain walls remains problematic. In simple cases the addition of a second phase to mimic the domain wall diffraction can lead to successful structural refinements. In the present case we are confronted with a temperature range where the I2/m and R3̅ phases coexist, and both of these exhibit anisotropic peak broadening and asymmetry from the domain walls. This combination is currently beyond Rietveld analysis. An unwelcome consequence of this combination is that establishing precise lattice parameters requires a nontrivial correction of the hkl dependent peak shifts. Accordingly we have not been able to obtain reliable estimates of the lattice parameters in the two phase region. Nevertheless the essential details of the phase transitions are apparent. An unexpected feature of the evolution in the lattice parameters is the large discontinuous changes associated with the monoclinic to rhombohedral transition. The dramatic increase in a, and to a lesser extent b, that occurs upon cooling below 350 K results in a large discontinuous increase in the volume, Figure 11. A similar but somewhat smaller discontinuity was evident for Ba2CeTaO6. Tilting of the BO6 octahedra in perovskites arises when the cation at the A-site is too small for the cuboctahedral site. The tilts introduce

Figure 9. Evolution of the (400)p and (222)p and reflections in Ba2CeNbO6. The coexistence of the I2/m and R3̅ phases at 323 and 373 K is responsible for the appearance of three reflections near 2θ = 22°.

The high resolution synchrotron data show the presence of anisotropic peak broadening and peak shifts associated with the I2/m to R3̅ phase transition, see for example Figure 10.

Figure 10. Portion of the synchrotron diffraction pattern for Ba2CeNbO6 illustrating the coexistence of the rhombohedral and monoclinic phases. The failure to adequately fit the observed profile is a consequence of diffuse scattering from the domain walls.

Without accurately modeling the peak profiles, Rietveld analysis will not produce useful results. Peak broadening often occurs when the effective crystallite (domain) size reduces the coherence of the diffraction or where, micro or macrostrain effects, stacking faults, microtwins, stresses, dislocations, and so forth become important. Anisotropic (hkl) dependent peak broadening can occur as a consequence of twinning faults. The lattice orientations change through the twin planes preventing coherent diffraction from the different crystal twins. This has been observed in a number

Figure 11. Temperature dependence of the, appropriate scaled, cell volumes for Ba2CeNbO6 highlighting the unusual variation at the monoclinic to rhombohedral transition. The two-phase region is marked. 2984

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

rhombohedral with the cubic phase forming only above 663 K. Evidently the precise structure and phase transition behavior for both systems are sample dependent, and we postulate that this may reflect the impact of defects at the domain walls.

anisotropic change in the cell dimensions, giving rise to spontaneous strains. These strains can be quantified from the small increase in the cell volume that invariably accompanies the octahedral tilting.47,48 Where the only distortion present is the tilting of the octahedra, the magnitude of the spontaneous volume strains is typically small. Although the I2/m to R3̅ transition must be first order, and therefore allows the possibility for a discontinuous change in cell volume, recent studies demonstrate that this is generally not the case.48 The absence of a significant volume change in most perovskites can be rationalized by noting that although the I2/m to R3̅ transition involves a reorientation of the octahedral tilting, the magnitude of the tilt does not change. The amplitude of the symmetry mode responsible for the breaking of the symmetry is associated with anion displacement, and therefore does not change abruptly at the transition. At this stage we have no explanation for the observed volume change in Ba2CeMO6. Finally we would like to comment on the apparent sensitivity of the structure of Ba2CeNbO6 to the precise method of preparation. During this work we prepared a number of samples, using a H2 atmosphere, where either the starting materials or the precise heating regime was altered. The synchrotron diffraction pattern of one such sample (prepared from Ce2(CO3)3 starting material, sintered at 1350 °C for 35 h), see Figure 12, was well fitted to a model in R3̅ with a =

■

CONCLUSIONS We have successfully prepared samples of the two Ce3+ containing double perovskites Ba2CeMO6 (M = Nb and Ta) through the use of mildly reducing conditions. The valence state of the Ce was established through both X-ray absorption measurements at the Ce L3-edge and magnetic susceptibility measurements. These are a rare example of oxides containing trivalent Ce in an octahedral environment. The structure of the two oxides was refined using synchrotron XRD, and in the case of M = Nb neutron diffraction as well. Both oxides adopt a monoclinic structure in I2/m at RT, and neutron diffraction measurements show that this structure persists to 4 K in Ba2CeNbO6. There is no evidence from either the magnetic susceptibility or the neutron measurements for magnetic ordering. The evolution of the structure Ba2CeNbO6 at high temperatures is complex, with two phase transitions observed, a discontinuous I2/m → R3̅ near 350 K and a continuous R3 → Fm3m ̅ transition at 580 K. Analysis of the first order I2/m → R3̅ transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron data because of domain wall effects.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was, in part, performed at the powder diffraction beamline at the Australian Synchrotron. B.J.K. acknowledges the support of the Australian Research Council for this work. The work performed at the NSRRC was supported by the Australian Synchrotron International Access Program. Part of this research was undertaken at the Australian National Beamline Facility at the Photon Factory in Japan, operated by the Australian Synchrotron. We acknowledge the Australian Research Council for financial support and the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Japan, for operations support.

Figure 12. RT synchrotron X-ray powder diffraction pattern of a Ba2CeNbO6 sample prepared from Ce2(CO3)3 sintered in H2/N2. The (111) reflection indicative of rock-salt type ordering of the Ce and Nb cations is marked. The inset shows the splitting of selected reflections, consistent with rhombohedral symmetry. The reflections indicated by * in the inset are indicative of the octahedral tilting. The lower set of markers indicate the position of the peaks due to the “cubic” phase included to model the effects of domain wall broadening.

■

REFERENCES

(1) Howard, C. J.; Kennedy, B. J.; Woodward, P. M. Acta Crystallogr., Sect. B: Struct. Sci. 2003, 59, 463−471. (2) Zhou, Q.; Kennedy, B. J.; Elcombe, M. M. J. Solid State Chem. 2007, 180, 541−548. (3) Zhou, Q.; Kennedy, B. J. J. Solid State Chem. 2005, 178, 3589− 3594. (4) Zhou, Q.; Kennedy, B. J.; Howard, C. J.; Elcombe, M. M.; Studer, A. J. Chem. Mater. 2005, 17, 5357−5365. (5) Saines, P. J.; Spencer, J. R.; Kennedy, B. J.; Avdeev, M. J. Solid State Chem. 2007, 180, 2991−3000. (6) Saines, P. J.; Spencer, J. R.; Kennedy, B. J.; Kubota, Y.; Minakata, C.; Hano, H.; Kato, K.; Takata, M. J. Solid State Chem. 2007, 180, 3001−3007. (7) Saines, P. J.; Kennedy, B. J.; Elcombe, M. M. J. Solid State Chem. 2007, 180, 401−409. (8) Fu, W. T.; Ijdo, D. J. W. J. Solid State Chem. 2006, 179, 1022− 1028.

6.07241(2) Å, α = 60.1918(4)°, although it should be noted that we included a small amount of a cubic phase in Pm3m ̅ to reproduce the additional diffuse scatter introduced by the domain walls, doing this resulted in a reduction in χ2 from 15.54 to 4.76%. It is possible that this reflects the presence of some disordered material in the sample. Heating this sample resulted in a transition to a cubic structure, although the transition temperature was somewhat lower than that found for the sample prepared from CeO2, ∼ 540 vs 580 K. The behavior of Ba2CeNbO6 mimics that of Ba2LaIrO6 where Zhou et al.49 showed the sample to be monoclinic in I2/m at RT, transforming to a cubic (Fm3̅m) structure near 473 K, whereas Fu et al. 50 described the structure of Ba 2 LaIrO 6 as 2985

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986

Chemistry of Materials

Article

(9) Fu, W. T.; Ijdo, D. J. W. J. Solid State Chem. 2005, 178, 2363− 2367. (10) Brixner, L. J. Inorg. Nucl. Chem. 1960, 15, 352−355. (11) Harrison, W. T. A.; Reis, K. P.; Jacobson, A. J.; Schneemeyer, L. F.; Waszczak, J. V. Chem. Mater. 1995, 7, 2161−2167. (12) Macquart, R. B.; Kennedy, B. J. Chem. Mater. 2005, 17, 1905− 1909. (13) Saines, P. J.; Kennedy, B. J.; Elcombe, M. M.; Harris, H. H.; Jang, L. Y.; Zhang, Z. M. J. Solid State Chem. 2008, 181, 2941−2952. (14) Saines, P. J.; Kennedy, B. J.; Johannessen, B.; Poulton, S. J. Solid State Chem. 2008, 181, 2994−3004. (15) Ijdo, D. J. W.; Helmholdt, R. B. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1993, 49, 652−654. (16) Rauser, G.; Kemmler-Sack, S. J. Solid State Chem. 1980, 33, 135−140. (17) Bharti, C.; Sinha, T. P. Mater. Res. Bull. 2011, 46, 1431−1436. (18) Bharti, C.; Sinha, T. P. Phys. B 2011, 406, 1827−1832. (19) Hammink, T. S.; Fu, W. T.; Ijdo, D. J. W. J. Solid State Chem. 2011, 184, 848−851. (20) Zhou, Q.; Kennedy, B. J.; Kimpton, J. A. J. Solid State Chem. 2011, 184, 729−734. (21) Zhou, Q.; Kennedy, B. J.; Avdeev, M. J. Solid State Chem. 2010, 183, 1741−1746. (22) Padeste, C.; Cant, N. W.; Trimm, D. L. Catal. Lett. 1994, 24, 95−105. (23) Wallwork, K. S.; Kennedy, B. J.; Wang, D. AIP Conf. Proc. 2007, 879, 879−882. (24) Liss, K. D.; Hunter, B.; Hagen, M.; Noakes, T.; Kennedy, S. Phys. B 2006, 385−86, 1010−1012. (25) Hunter, B. A.; Howard, C. J. RIETICA A Computer Program for Rietveld Analysis of X-Ray and Neutron Powder Diffraction Patterns, 1998 (26) Dann, T. E.; Chung, S. C.; Huang, L. J.; Juang, J. M.; Chen, C. I.; Tsang, K. L. J. Synchrotron Radiat. 1998, 5, 664−666. (27) Sabine, T. M.; Kennedy, B. J.; Garrett, R. F.; Foran, G. J.; Cookson, D. J. J. Appl. Crystallogr. 1995, 28, 513−517. (28) Glazer, A. M. Acta Crystallogr., Sect. B: Struct. Sci. 1972, B 28, 3384−3392. (29) Bolzan, A. A.; Fong, C.; Kennedy, B. J.; Howard, C. J. J. Solid State Chem. 1994, 113, 9−14. (30) Tofield, B. C.; Scott, W. R. J. Solid State Chem. 1974, 10, 183− 194. (31) Ruiz-Morales, J. C.; Canales-Vázquez, J.; Savaniu, C.; MarreroLópez, D.; Zhou, W.; Irvine, J. T. S. Nature 2006, 439, 568−571. (32) Saines, P. J.; Kennedy, B. J. J. Solid State Chem. 2008, 181, 298− 305. (33) Kotani, A.; Kvashnina, K. O.; Butorin, S. M.; Glatzel, P. J. Electron Spectrosc. Relat. Phenom. 2011, 184, 210−215. (34) Soldatov, A. V.; Ivanchenko, T. S.; Dellalonga, S.; Kotani, A.; Iwamoto, Y.; Bianconi, A. Phys. Rev. B 1994, 50, 5074−5080. (35) Nachimuthu, P.; Shih, W. C.; Liu, R. S.; Jang, L. Y.; Chen, J. M. J. Solid State Chem. 2000, 149, 408−413. (36) Purans, J.; Azens, A.; Granqvist, C. G. Electrochim. Acta 2001, 46, 2055−2058. (37) Finkelstein, L. D.; Postnikov, A. V.; Efremova, N. N.; Kurmaev, E. Z. Mater. Lett. 1992, 14, 115−118. (38) Yamamoto, T. X-Ray Spectrom. 2008, 37, 572−584. (39) Piilonen, P. C.; Farges, F.; Linnen, R. L.; Brown, G. E.; Pawlak, M.; Pratt, A. Can. Mineral. 2006, 44, 775−794. (40) Zhang, Z. M.; Howard, C. J.; Kennedy, B. J.; Knight, K. S.; Zhou, Q. D. J. Solid State Chem. 2007, 180, 1846−1851. (41) Anderson, M. T.; Greenwood, K. B.; Taylor, G. A.; Poeppelmeier, K. R. Prog. Solid State Chem. 1993, 22, 197−233. (42) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751−767. (43) Brown, I. D. Chem. Rev. 2009, 109, 6858−6919. (44) Daniels, J. E.; Jones, J. L.; Finlayson, T. R. J. Phys. D: Appl. Phys. 2006, 39, 5294−5299. (45) West, D. V.; Davies, P. K. J. Appl. Crystallogr. 2011, 44, 595− 602.

(46) Locherer, K. R.; Chrosch, J.; Salje, E. K. H. Phase Transitions 1998, 67, 51−63. (47) Carpenter, M. A.; Howard, C. J.; Kennedy, B. J.; Knight, K. S. Phys. Rev. B 2005, 72, 024118. (48) Kennedy, B. J.; Howard, C. J.; Knight, K. S.; Zhang, Z. M.; Zhou, Q. D. Acta Crystallogr., Sect. B: Struct. Sci. 2006, 62, 537−546. (49) Zhou, Q.; Kennedy, B. J.; Avdeev, M.; Giachini, L.; Kimpton, J. A. J. Solid State Chem. 2009, 182, 3195−3200. (50) Fu, W. T.; Gotz, R. J.; Ijdo, D. J. W. J. Solid State Chem. 2010, 183, 419−424.

2986

dx.doi.org/10.1021/cm301278a | Chem. Mater. 2012, 24, 2978−2986