A - AND B - SITE ORDER IN (Na1/2La1/2)(Mg1/3Ta2/3)O3 PEROVSKITES. L. DUPONT, L. CHAI, P.K. DAVIES. Dept. of Materials Science & Engineering, ...
A - AND B - SITE ORDER IN (Na1/2La1/2)(Mg1/3Ta2/3)O 3 PEROVSKITES L. DUPONT, L. CHAI, P.K. DAVIES Dept. of Materials Science & Engineering, University of Pennsylvania, 3231 Walnut St., Philadelphia, PA 19104-6272
ABSTRACT The “1:2” family of A2+ (B2+ 1/3B5+ 2/3)O3 (e.g. B2+ = Mg, Ni, Zn; B 5+ = Nb, Ta) mixed metal perovskites typically adopt a 1:2 layered ordered structure when A = Ba and Sr, and a 1:1 ordered doubled perovskite structure for A = Pb. To understand the stability of the cation order in these systems and explore how the different cation correlations affect their dielectric and ferroelectric properties, the response of several members of this family of perovskites to a series of chemical substitutions and thermal treatments have been explored. In this paper we present results for a system where the divalent A cation is replaced by a 1:1 mixture of Na+ and La 3+ . At all temperatures this substitution stabilizes 1:1 B-site ordering. Below ~ 950°C an additional ordering reaction occurs on the A sub-lattice with Na and La occupying alternate layers along the c axis. The formation of different orientational variants of the A-site ordered phase produces a twinned nano-domain structure which in turn perturbs the length-scale, but not the symmetry, of the order on the B-site lattice. The presence of (Na1/2La1/2) on the A-site is apparently effective in destabilizing the 1:2 layering of the B-site cations observed in alkaline earth systems, possibly through unfavorable local charge imbalances. INTRODUCTION Several members of the “1:2” A2+ (B2+ 1/3B5+ 2/3)O3 (A 2+ = Pb, Ba, Sr, Ca; B2+ = Mg, Ni, Co, Zn, etc.; B5+ = Nb, Ta) family of mixed-metal perovskites have received widespread attention due to their importance as temperature compensated, low-loss dielectrics in wireless communication devices (e.g. Ba(Zn1/3Ta2/3)O3 - BZT) and their ferroelectric and piezoelectric properties (e.g. Pb(Mg1/3Nb2/3)O3 - PMN) [1,2, and references therein]. For all these systems the electronic performance is mediated by the degree and nature of the ordering of the cations on the B-site positions.[1,2] In some cases quite small alterations in the ordering can induce order of magnitude variations in the dielectric response.[3] Over the last two years we have focused on identifying how and why small alterations in the B-site and/or A-site chemistry affect the cation order [see, for example, 3,4,]. In this paper we briefly review the crystal chemistry of these systems and describe new results on the effect of coupled (Na1/2La1/2) A-site substitutions on the B sub-lattice order. The stable low temperature forms of all the alkaline earth 1:2 niobate and tantalate perovskites adopt an ordered structure (space group p3 m1), in which the B-site cations occupy positions in individual (111) planes that repeat with a stoichiometric “1:2” {B2+ B5+ B5+ } sequence, see figure 1a. The driving force for the ordering is derived from the valence and size difference of the B2+ and B5+ cations which are accommodated by correlated displacements of the oxygen anions between the cation layers. The tantalates retain their ordered cation distribution to very high temperatures, in some cases to the melting point, while several niobate systems undergo a transformation at elevated temperature (e.g. Ba(Zn1/3Nb2/3)O3, 1375°C) to a fully
disordered perovskite.[5] The reduced thermal stability of the order in the niobate systems can be attributed to the higher covalency of the Nb cation. In a series of recent studies we have examined the response of the cation ordering in the Ba and Sr based 1:2 systems to small levels of tetravalent cation B-site substitutions.[3,5,6,7] From a technological perspective these substitutions (e.g. Zr4+ in the BZT-BaZrO3 system) are important as they permit the dielectric properties to be tailored to meet commercial demands. For the BZT system (and also Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3, and Sr(Mg1/3Ta2/3)O3) small levels of Zr and Ce (~ 3 mole %), introduced through solid solution formation with BaZrO 3 (BZ) and BaCeO3, promote a transformation to a cubic “1:1” ordered (Fm3 m) doubled perovskite structure, see figure 1b. The structure of these 1:1 ordered A(β I1/2β II 1/2)O3 phases, which had a range of homogeneity up to 25 mole % of the M4+ cation, was consistent with the so-called “random site” model for the cation order. In this model one of the ordered positions (β II ) is occupied by the B5+ cations and the second (β I) contains a random mixture of the B2+ , M 4+ , and remaining B5+ cations. For this model the compositions of the 1:1 ordered phases in the BZT-BZ system could be represented by Ba[(Zn(2-2x)/3 Ta(1-4x)/3 Zr 2x)1/2(Ta)1/2]O3. + + +
+
A
B 2+
B 5+
A=
+ +
+ + + + ++ + + + + ++ + + + ++ + + ++ +
'=
+ + + + + + ++ + + + ++ + + + ++ + + ++ + + + +
"=
O=
+
Figure 1. Schematic illustration of (a) 1:2 B-site order, oxygen omitted for clarity; (b) the 1:1 ordered A(β I1/2β II 1/2)O3 doubled perovskite structure. In contrast to their alkaline earth counterparts, none of the Pb-based 1:2 perovskites exhibit a stoichiometric 1:2 layered cation ordering scheme, instead they adopt a 1:1 ordered doubled perovskite, Pb(β I1/2β II 1/2)O3 -type structure (space group, Fm3 m). Although the cation order in the Pb systems has been extensively examined, controversies still exist regarding the nature and extent of the 1:1 cation order.[2] Several investigators have shown that in their assintered forms the PMN-type systems contain nano-sized 1:1 ordered domains surrounded by a disordered matrix [8, 9]. Until recently the ordering in these systems was interpreted using the “space charge” model. For this model the 1:1 ordered nano-domains were claimed to contain a 1:1 ratio of the divalent and pentavalent cations, with the resultant charge imbalance being compensated by an equal and oppositely charged, pentavalent-rich disordered matrix [8, 9]. Support for this “two-phase” model had come primarily from the apparent lack of coarsening of
the ordered domains upon annealing [9]. However, recent results from our group on the tantalate relaxor Pb(Mg1/3Ta2/3)O3 (PMT) reveal that significant domain growth can be induced in a narrow window of thermodynamic stability and kinetic activity.[4] This observation coupled with structure refinements of PMT and PMN based ordered relaxors has shown that the 1:1 order in these systems can also be interpreted using the “random site” model. In this case the ordered Pb(β I1/2β II 1/2)O3 forms of PMT (or PMN) can be represented as Pb(Mg2/3Ta1/3)1/2(Ta)1/2O3 with Ta in the β II site and a random distribution of (Mg2/3Ta1/3) on β I . Because the formation of the 1:1 ordered, random site structure has important ramifications for understanding and predicting the properties of the Ba and Pb based systems, we have attempted to explore other methods for stabilizing this type of order in the 1:2 perovskite family. In this paper we report on the effect of A-site substitutions on the B-site cation order. In particular we present preliminary results for a system in which the A2+ cations are completely replaced by a 1:1 mixture of A+ and A3+ . The example considered here is the (Na1/2La1/2)(Mg1/3Ta2/3)O3 system. EXPERIMENTAL METHODS Powders of (Na1/2La1/2)(Mg1/3Ta2/3)O3 were prepared by conventional mixed oxide methods using high purity (>99.9%) oxides (La2O3, MgO, Ta2O5) and carbonates (Na2CO3). Stoichiometric quantities of the reagents were milled in acetone for 6 hours with Y2O3 stabilized ZrO2 balls, dried and calcined at 700°C for 6 hours, and then re-fired at 900°C for 12 hours. After a second ball milling the powders were sintered either at 1200°C for 12 hours or at 1300°C for 6 hours in air. By monitoring the sample mass these conditions were found to avoid any significant loss of Na. Phase analysis was conducted using X-ray diffraction (XRD) and the microstructures of the ceramics were characterized using TEM with JEOL 2010FEG and JEOL 4000EX electron microscopes. RESULTS The x-ray patterns (fig 2a) collected after firings at either 1200 or 1300°C indicated the formation of a single phase perovskite. In addition to reflections from the perovskite sub-cell, the patterns contained a series of strong peaks located at positions associated with the formation of a cubic (Fm3 m) 1:1 ordered doubled perovskite structure. Least squares refinement of these reflections yielded a cell with a = 7.922Å. Examination by TEM found no evidence for 1:2 B-site order and the microstructure was comprised of large 1:1 ordered domains. The TEM studies and simulations of the x-ray patterns were consistent with a structure where the supercell formation arises from a 1:1 “random-site” ordering of the B-site cations (figure 1b). For this model the structure of the phase can be represented as (Na1/2La1/2)(Mg2/3Ta1/3)1/2(Ta)1/2O3. Although the x-ray and electron diffraction patterns of the phases annealed at 1200 or 1300°C showed no evidence for additional order, very weak and broad extra reflections could be discerned in the patterns of samples that were slowly cooled to room temperature. These diffuse peaks were located at positions [e.g. (00l/2)] close to those reported for mixed A-site titanates such as (La1/2Li1/2)TiO 3 where a (001) layered ordering of the A-site cations has been observed.[see, for example, 10] A series of annealing experiments were conducted on the B-site ordered samples of (Na1/2La1/2)(Mg1/3Ta2/3)O3 to identify any possible ordering transformations
at lower temperatures. While extended treatments at 1000°C induced no significant changes to the x-ray pattern (figure 2b), the additional reflections were clearly strengthened in the patterns of samples annealed at 900 or 935°C for three days (figure 2c and 2d). The kinetics of this ordering reaction were slow; however, the transformation was readily reversed at higher temperature. For example, the pattern in figure 2e was collected after heating sample 2d to 1000°C for six hours and quenching to room temperature.
Figure 2. X-ray patterns of (Na1/2La1/2)(Mg1/3Ta2/3)O3 after: (a) 1300°C calcine; (b) 3 day heat at 1000°C; (c) 3 day heat at 900°C; (d) 3 day heat at 935°C; (e) re-heating sample (d) to 1000°C and quenching. A = peaks from A-site order, 1:1 from B-site order, P from the sub-cell. Direct observations of the samples containing the extra set of reflections were made using transmission electron microscopy (these results will be described in detail elsewhere). The images confirmed that the low temperature phase transition arises from a (001) layered ordering of the Na and La cations on the A-site positions. The correlation length of the order was limited by the formation of a micro-twinned domain structure. Each grain was comprised of nano-sized (5-20 nm) domains of three possible orientational variants of a layered A-site and 1:1 ordered Bsite structure (possible space group P4) with a = √2asub, and c = 2a sub (see figure 3). Least squares refinement of the reflections in the x-ray patterns yielded a cell with a = 5.603Å, c = 7.915Å. The features of the A-site order and the formation of a nano-domain structure are essentially identical to those reported for the corresponding titanate perovskites.[10] However,
while the titanates contain a single B-site cation, (Na1/2La1/2)(Mg1/3Ta2/3)O3 also contains positional order on the B-site sub-lattice. We note that simultaneous A and B - site order has been observed previously in the (Na1/2La1/2)(Mg1/2W1/2)O3 system where the B - site positions have a 1:1 Mg:W distribution.[11] The transformation to a long-range ordered arrangement of Na and La on the A-sites does not seem to change the symmetry of the order on the B-sites. However, the formation of the Asite ordered nano-domain structure does increase the width of the reflections associated with the B-site ordering. The decrease in the range of the 1: 1 B-site order associated with the onset of long-range order on the A sub-lattice is still under investigation but appears to result from the disruption of the structure at the twin and anti-phase boundaries.
βII βI La Na
c = 2asub
a = asub√2 Figure 3. Schematic illustration of A and B-site ordered structure of (Na 1/2La1/2)(Mg1/3Ta2/3)O3.
AII AI
B5+ B2+ O
Figure 4. Different A-site positions in the 1:2 ordered structure.
DISCUSSION AND CONCLUSIONS In (Na1/2La1/2)(Mg1/3Ta2/3)O3 the 1:2 mixture of Mg and Ta cations are ordered on a 1:1 doubled perovskite B-site sub-lattice. All of the features of this structure are consistent with a random site arrangement for the octahedral cations with one position occupied by Ta and the other by a random distribution of 2/3Mg and 1/3Ta. Above ~950°C the A-site sub-lattice is disordered, below this temperature Na and La adopt a [001] ordered layered arrangement. The ordering of Na and La doubles the periodicity of the A-site sub-cell along one of the cube axes. The existence of three equivalent orientational variants of the A-site ordering induces the formation of a nano-domain structure in which the cation correlations are limited to ~ 5-20 nm. The onset of the long-range A-site ordering does not affect the geometry of the B-site order, but does seem to reduce the length scale of the B-site correlations. The combined A and B-site ordered low temperature phase can be described by a tetragonal cell (possible space group P4) with a = √2asub and c = 2asub (figure 3). Because the geometry of the B-site ordering in (Na1/2La1/2)(Mg1/3Ta2/3)O3 is independent of the degree of order on the A-site sub-lattice, the destabilization of the “stoichiometric” 1:2 B-
site order observed in all the alkaline earth A2+ (Mg1/3Ta2/3)O3 systems must originate from chemical differences between the Na and La cations rather than the detailes of their periodicity. It seems unlikely that size effects are important in stabilizing the 1:1 B-site order; the radii of Na and La are essentially identical and similar to that of Sr2+ . A more likely explanation is that 1:2 B-site order in (Na1/2La1/2)(Mg1/3Ta2/3)O3 is incompatible with the different valences of the Asite cations. In the 1:2 structure the {..B2+ B5+ B5+ ..} layering creates two different environments for the A-site cations (see figure 4). Two A-sites (AI) are located between a B2+ and a B 5+ layer, these have 5 B5+ and 3 B2+ nearest neighbors and the formal charge, q, on the octahedral framework in those sub-cells = {[nB5+(5) + nB2+(2)]/8 - 6} = 2.125-. The second A-site position (AII ) is located between two B5+ layers and has 6 B5+ and 2 B2+ nearest neighbors. In this sub-cell the formal framework charge is 1.75-. When all of the A-sites are occupied by a divalent cation the excess formal charge per sub-cell (∆q) is - 0.125 and + 0.25 for the A' and A" positions respectively. This type of small formal charge imbalance is not unusual and can be accommodated by small distortions from the ideal geometries. However, a 1:1 mixture of Na+ and La3+ on the A sub-lattice, where each cation must occupy both types of A-site, would produce much larger excess charges in the sub-cells (Na+ , ∆q = -1.125 and -0.75; La, ∆q = +0.875 and + 1.25 for A' and A" respectively). These high local charge imbalances may be partially responsible for de-stabilizing a 1:2 layered B-site sub-lattice. In the 1:1 B-site ordered random site structure all of the A positions are equivalent and have 4 B5+ and 4 (B 2+ 2/3B5+ 1/3) neighbors. The average formal framework charge is -2 in each sub-cell and the local charge imbalance is reduced to -1 and +1 for Na and La respectively. While this is still higher than for a divalent A cation, the reduction may be an important contributing factor in promoting the formation of the 1:1 B-site order. It would also be expected that concerted ionic displacements, particularly in the lower temperature A-site ordered structure, could further alleviate the local imbalance. This expectation is supported by our investigations of the tetragonal ordered phase using high resolution TEM where simulations of a structure with small displacements of the Asite atoms give the best fit to the experimental images. ACKNOWLEDGMENTS This work was supported by the NSF under grants DMR 94-21184 and DMR 98-09035. The electron microscopy facility was funded by NSF/MRSEC program (DMR 96-32598). REFERENCES 1. L.E. Cross, Ferroelectrics, 76, 305 (1994) 2. P.K. Davies, in Materials for Wireless Communication, edited by T. Negas and H. Ling, (Am. Ceram. Soc., Westerville, OH, 1994), p. 137-152. 3. P.K. Davies, J. Tong, and T. Negas, J. Am. Ceram. Soc ., 32(9), 1261 (1997) 4. M.A. Akbas and P.K. Davies, J. Am. Ceram. Soc., 80(11), 2933 (1997) 5. M.A. Akbas and P.K. Davies, J. Am. Ceram. Soc., 81(3), 670 (1998) 6. L. Chai, M.A. Akbas, P.K. Davies and J. Parise, Mater. Res. Bull., 32, 1261 (1997) 7. L. Chai and P.K. Davies, Mater. Res. Bull.,33, 1283 (1998) 8. M.P. Harmer, J. Chen, P. Peng, H.M. Chen and D.M. Symth, Ferroelectrics, 97, 263 (1989). 9. A.D. Hilton, D.J. Barber, A. Randall and T.R. Shrout, J. Mater. Sci., 25, 3461 (1990).
10. A. Varez, F. Garcia-Alvarado, E. Moran, and M. Alario-Franco, J. Solid State Chem., 118, 78 (1995) 11. T. Sekiya et al., Bull. Chem. Soc. Jpn., 57, 1859 (1984)
