J O U R N A L O F C H E M I S T R Y Materials Communication Double perovskites containing hexavalent molybdenum and tungsten: synthesis, structural investigation and proposal of a fitness factor to discriminate the crystal symmetry Yasutake Teraoka,*a Ming-Deng Weib and Shuichi Kagawaa aDepartment of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852-8521, Japan. E-mail: yasu@net.nagasaki-u.ac.jp bDepartment of Marine Resources Research and Development, Graduate School of Marine Science and Engineering, Nagasaki University, Nagasaki 852-8521, Japan Received 17th August 1998, Accepted 16th September 1998 Double perovskites AII 2BIIBVIO6 (AII=Ba, Sr, Ca; BII= vigorous stirring.The obtained solid mixture was ground and Mg, Ni, Co, Cd, Ca; BVI=W, Mo, W0.5Mo0.5) have been heat-treated at 623 K for 1 h in order to decompose remaining synthesized and their crystal structures investigated by metal nitrates. After regrinding, the mixture was calcined in powder X-ray diffraction measurements. In order to dis- air for 5 or 10 h at temperatures varying between 1173 and criminate the crystal systems of the double perovskites, a 1523 K with an interval of 50 K, and the calcination was fitness factor is proposed which corresponds to the size repeated with regrinding at every interval.Crystal phases in matching between the A cation and the cubo-octahedral the products were identified by powder X-ray diVraction cavity formed by eight BO6 octahedrons. The fitness factor (XRD) using Cu-Ka radiation (Rigaku RINT-2200VL, 30 kV, can discriminate the crystal systems of obtained com- 16 mA).Lattice constants were calculated using the PIRUM pounds more exactly than the well known tolerance factor. program.3 In the syntheses of the double perovskites, AMoO4 and AWO4 (A=Ba, Sr) were the main and obstinate impurity It is known that hexavalent Mo and W are stabilized not in phases. Calcination for prolonged time or at higher tempera- the primitive ABO3 perovskites but in the ordered double tures was repeated until the XRD peak intensities of the perovskites, AII2BIIBVIO6,1,2 in which AII is an alkaline earth impurity phase disappeared or, if present, became as low as ion, BII a divalent metal ion such as Mg, Ca, Co, Ni and Cu, possible.The lowest temperature, though with longer calci- and BVI a hexavalent Mo or W ion.These compounds, in nation period, was adopted as the synthesis condition for each which the charge diVerence between BII and BVI is four, adopt oxide (Table 1). an ordered structure with the rock-salt arrangement of BII and Double perovskites containing Mo were not obtained for BVI cations as shown in Fig. 1.2 Systematic series of AII2BIIWO6 AII=Ca but were for AII=Ba and Sr.To the best of our compounds have been so far reported with AII=Ba, Sr and knowledge, the oxides in Table 1, except for Sr2BIIMoO6 (BII= Ca and BII=Mg, Ca, Co, Cu, Fe, Ni and Zn.1,2 In contrast, Co and Ni),4 are new compounds. Compounds with AII=Ba reports on Mo-containing double perovskites are limited. The crystallized in the cubic double perovskite structure with the first aim of the present communication is to report the synthesis lattice constant close to 2ap; ap is the lattice constant of cubic and structural investigation of Mo-containing double perovperovskite of the primitive ABO3 type (ap#4 A° ). XRD pat- skites of AII2BIIMoO6 and AII2BIIW0.5Mo0.5O6 (AII=Ba, Sr, terns of cubic Ba2CoMoO6 and Ba2CoW0.5Mo0.5O6 are Ca; BII=Mg, Ni, Co, Cd, Ca).The success in the synthesis of depicted in Fig. 2. The appearance of superlattice lines of 111, the Mo-containing double perovskites and the systematic 311 and 511 evidences the rock-salt ordering of Co2+ structural investigation of the Mo and W systems lead to the and Mo6+/W6+ ions.2 The fact that Ba2CoMoO6 and proposal of a new fitness factor which can discriminate the Ba2CoW0.5Mo0.5O6 gave almost the same XRD patterns crystal system of the AII2BIIBVIO6 double perovskites more implies that in the latter oxide W6+ and Mo6+ ions randomly exactly than the well known tolerance factor.occupy the smaller octahedra (BVIO6 in Fig. 1) and they form Polycrystalline powders of double perovskites were synthethe rock-salt sublattice with larger BIIO6 octahedra.As exem- sized from starting materials of MoO3, WO3 and nitrates of plified by the XRD pattern of Sr2CoMoO6 (Fig. 2), splitting other elements. Molybdenum and/or tungsten trioxide was of some diVraction peaks was observed for all the Sr com- added into a mixed aqueous solution of metal nitrates, and pounds, and their powder XRD patterns could be satisfactorily the suspended solution was evaporated to dryness under indexed with an orthorhombic (BII=Ca) or tetragonal (others) unit cell having a size close to Ó2ap×Ó2ap×2ap.It was reported2 that only cubic (2ap) and monoclinic (Ó2ap×Ó2ap×2ap) unit cells occur in double perovskites with the rock-salt sublattice. Accordingly, it is speculated that the monoclinic distortion of the Sr compounds synthesized in this study, if present, is too small to distinguish the monoclinic system from the orthorhombic and tetragonal systems by powder XRD measurements alone.It is noted here that, in accordance with a previous report,5 we could discern a monoclinic Ó2ap×Ó2ap×2ap unit cell of Ca2CaWO6 with a slight distortion (b=90.18°). Fig. 3 shows the relation between the primitive perovskite parameter (ap) and the ionic radius6 of divalent BII cations. Fig. 1 Crystal structure of AII2BIIBVIO6 double perovskites with the rock-salt ordering of larger BII and smaller BVI cations. Values of ap were calculated from (VUC/8)1/3 for cubic oxides J. Mater. Chem., 1998, 8(11), 2323–2325 2323Table 1 Lattice parameters and synthesis conditions of Mo-containing double perovskites Compound CSa Lattice parameter/A° Synthesis conditions Ba2BIIMoO6 1 BII=Ni C a=8.035(1) 1373 K, 30 h 2 BII=Co C a=8.076(1) 1273 K, 20 h 3 BII=Cd C a=8.3242(9) 1173 K, 20 h Sr2BIIMoO6 4 BII=Ni T a=5.5464(7), c=7.892(1) 1373 K, 40 h 5 BII=Mg T a=5.598(2), c=7.875(2) 1373 K, 60 h 6 BII=Co T a=5.562(2), c=7.941(5) 1423 K, 25 h 7 BII=Ca O a=5.753(2), b=5.841(1), c=8.186(3) 1373 K, 5 h Ba2BIIW0.5Mo0.5O6 8 BII=Ni C a=8.053(1) 1273 K, 70 h 9 BII=Co C a=8.0928(6) 1273 K, 60 h 10 BII=Cd C a=8.3360(8) 1173 K, 30 h Sr2BIIW0.5Mo0.5O6 11 BII=Ni T a=5.587(2), c=7.852(2) 1423 K, 40 h 12 BII=Mg T a=5.603(2), c=7.882(2) 1373 K, 20 h 13 BII=Co T a=5.611(3), c=7.872(4) 1473 K, 20 h 14 BII=Ca O a=5.766(1), b=5.847(1), c=8.183(3) 1323 K, 60 h aCrystal system: C; cubic, T; tetragonal, O; orthorhombic.and (VUC/4)1/3 for tetragonal and orthorhombic oxides, where VUC is the unit cell volume of the original double perovskite. As expected, the cell size increases with increasing the radius of BII cations in each series of oxides. The cell size diVerences of Ba2BIIBVIO6>Sr2BIIBVIO6 and A2BIIW0.5Mo0.5O6> A2BIIMoO6 are also consistent with ionic size diVerences6 of Ba>Sr and W>Mo.In parallel with the investigation of the Mo-containing double perovskites, the synthesis and structural investigation of the W analogues have been carried out. The following compounds were obtained, and their crystal systems were in accordance with the literature. (2ap)-type cubic phase; Ba2BIIWO6 (BII=Ni,7 Co,7 Cd,8 Ca7) (Ó2ap×Ó2ap×2ap)-type tetragonal phase; Sr2BIIWO6 (BII=Ni,7 Mg,8 Co,7 Cd) (Ó2ap×Ó2ap×2ap)-type orthorhombic phase; Sr2CaWO6,9 Ca2BIIWO6 (BII=Ni, Co)1 (Ó2ap×Ó2ap×2ap)-type monoclinic phase; Ca2CaWO65 The Goldschmidt tolerance factor (t)10 is often used to predict the formation of perovskites and the crystal symmetry for a given pair of A- and B-site cations.The tolerance factor is given by t=(rA+rO)/Ó2(rB+rO) (1) where rX is the ionic radius6 of X ion, and rB=(rBII+rBVI)/2 for A2BIIBVIO6 double perovskites.The relation between the t Fig. 2 Powder XRD patterns of AII2CoMoO6 (A=Ba, Sr) and value and the crystal symmetry is depicted in Fig. 4A. All of Ba2CoW0.5Mo0.5O6. V: BaWO4. the Ba compounds with t values >0.96 are cubic. In each series of the Sr and Ca compounds, oxides with the smallest t values (BII=Ca) have unit cells with lower symmetry than the others; orthorhombic vs. tetragonal for the Sr compounds, and monoclinic vs.orthorhombic for the Ca compounds. In this way, the t value can discriminate the crystal symmetry in the series of each AII cation. When all the compounds are taken into account, however, overlap regions exist.At t values between 0.95 and 1.0, both the cubic (Ba compounds) and tetragonal (Sr compounds) systems occur, and the t value of tetragonal Sr2CdWO6 is smaller than those of orthorhombic Ca2BIIWO6 (BII=Ni, Co). This suggests that a new parameter, which reflects more pronouncedly the size diVerence of A site cations, is necessary to exactly discriminate the crystal system of double perovskites.For this purpose, we propose a new fitness factor (W) defined by eqn. (2). Fig. 3 Primitive perovskite parameter (ap) of Mo-containing double perovskites as a function of ionic radius of divalent BII cation. See W=Ó2rA/(rB+rO) (2) text for the calculation of ap and Table 1 for the listing of compounds. For the ideal cubic unit cell of the primitive ABO3 type (t= 2: Ba2 BIIMoO6, %: Ba2 BIIW0.5Mo0.5O6, +: Sr2 BIIMoO6, #: Sr2BIIW0.5Mo0.5O6. 1.0, rA=rO), the B–O interionic distance, rB+rO, is equivalent 2324 J.Mater. Chem., 1998, 8(11), 2323–2325fitness factor, the borderline between the cubic Ba compounds and the tetragonal Sr compounds becomes clear. In addition, all of the orthorhombic Sr and Ca compounds have similar W values, giving a clear borderline at W=0.93 to distinguish from tetragonal systems. The cubic system occurs at 1.00<W, the tetragonal system at 0.93<W<1.00, the orthorhombic system at 0.90<W<0.93, and the monoclinic system at W<0.90; the borderline value of W=0.90 between orthorhombic and monoclinic systems is not definitive owing to lack of data.The reduction of crystal symmetry of perovskites usually results from the tilting of BO6 octahedrons.2,11 When an A cation is closely packed in the cubo-octahedral cavity (W1), tilting would be suppressed and the cubic system results.At W<1 with the loose packing of the A cation, tilting of BO6 octahedrons or distortion would be expected, and would become larger with decreasing W. It is well known that B site vacancies in perovskite-type oxides are rare, while those at A sites are commonly found in, for example, ReO3, NaxWO312 and La2/3TiO3.13 This indicates that the framework made of BO6 octahedra is of primary importance for the construction of the perovskite structure and that A site cations stabilize the structure by sitting in the cubo-octahedral cavities.The concept of the proposed fitness factor is in line with this.References 1 G. Blasse, J. Inorg. Nucl. Chem., 1965, 27, 993. 2 M. T. Anderson, K. B. Greenwood, G. A. Taylor and K. P. Poeppelmeier, Prog. Solid State Chem., 1993, 22, 197. 3 P-E. Werner, Ark. Kemi, 1969, 1, 513. 4 JCPDS 15-556 (B¾=Co) and 15-601 (B¾=Ni). 5 JCPDS 22-541. 6 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751. 7 E. J. Fresia, L. Katz and R. Ward, J. Am. Chem. Soc., 1959, 81, 4783. 8 G. Blasse and A. F. Corsmit, J. Solid State Chem., 1973, 6, 513. Fig. 4 Tolerance factor (A) and fitness factor (B) for discriminating 9 E. G. Steward and H. P. Rooksby, Acta Crystallogr., 1951, 4, 503. unit cell systems of AII2BIIBVIO6 double perovskites. 10 V. M. Goldschmidt, Skrifter Nordske Videnskaps-Akad. Oslo I, Mat-Naturvidensk Kl., 1926, 8, 2. to half of the cell edge and rA/(rB+rO) is equal to 1/Ó2 (W= 11 A. M. Glazer, Acta Crystallogr., Sect. B, 1972, 28, 3348. 1.0). This parameter corresponds to size matching between 12 A. Magneli, Acta Chem. Scand., 1953, 7, 315. the A cation and the cubo-octahedral cavity formed by eight 13 M. Abe and K. Uchino, Mater. Res. Bull., 1974, 9, 147. BO6 octahedrons. As shown in Fig. 4B, the W factor can exactly discriminate the crystal system. By introducing the Communication 8/06442C J. Mater. Chem., 1998, 8(11), 2323–2325 2325