Scientific paper Crystal Structures of CaLa8Ti9O31 and Ca2La4Ti6O20 Determined from Powder Diffraction Data Katarina Stare,1,2 Radovan Cerny,3 Sreco D. Skapin,4 Danilo Suvorov4 and Anton Meden1'2 1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia 2 EN-FIST Centre of Excelence, Dunajska 156, Ljubljana, Slovenia 3 University of Geneva, Geneva, Switzerland 4 Jozef Stefan Institute, Ljubljana, Slovenia * Corresponding author: E-mail: katarina.stare@fkkt.uni-lj.si Received: 28-02-2011 Dedicated to Professor Dusan Hadzi on the occasion of his 90th birthday Abstract The ceramic materials of CaLa8Ti9O31 and Ca^a^^O^ were synthesized using the solid-state reaction method. The crystal structures were determined from powder diffraction data using direct space methods. Like in similar compounds of the general formula AnBnO3n+2, the title compounds are composed of perovskite-like slabs, separated by oxygen-rich layers, where Ca or La occupy the A site. Keywords: ceramic materials, Ca-La-Ti-O system, crystal structures, powder diffraction. 1. Introduction 2. Experimental Procedure Perovskite structures have been extensively investigated due to the important properties exhibited by many members of this series of materials. With the rapid increase of demands in communication and broadcasting, the development of good dielectric resonator materials for application in microwave frequency range are needed. Compounds, based on La2O3-TiO2 system, were also examined as microwave frequency dielectric ceramics.1 The addition of CaO2 causes formation of perovskite structural type. In the ternary system La2O3-TiO2-CaO, a number of compounds were identified. Nanot et al.3 reported series of compounds of perovskite structural type with general formula AnBnO3n+2. Among those there were CaLa4 Ti5O17,4 CaLa8Ti9O31 and Ca2La4Ti6O20. The scope of this paper was to determine and analyze the last two structures by synchrotron and laboratory X-ray powder diffraction. The ceramic samples were prepared by the solid-state reaction technique using La2O3 (99.99% Alfa Aesar), TiO2 (99.8% Alfa Aesar), and CaCO3 (99.5% Alfa Aesar). Since La2O3 shows a strong tendency to form a hydroxide and a carbonate with the moisture and the CO2 in the air, the oxide was routinely checked prior to weighing with an ignition loss measurement at 1300 °C. Samples were weighed out according to the stoichiometry and homogenized in ethanol media using YTZ ball mill for 0.5 h. Dried powders were uni-axially pressed into pellets and calcined in a tube furnace in air at 1300 °C for 12 h. The heat treated samples were ground, well homogenized in a ball mill and sintered at 1400 °C for 20 h. The X-ray powder diffraction patterns, used for solving the structures and preliminary refinements, were collected on a PANalytical X'Pert PRO MPD diffractometer in reflection geometry using CuKa1 radiation. The data were collected in the 28 range of 3-150° in steps of 0.017° with the total collection time of 16 hours. Preliminary search-match analyses were carried out with Crystal-lographica Search-Match software using the PDF-4 database release 2010.5 Indexing was performed using Treor and Ito programmes incorporated in X'Pert High Score Plus.6 The Fox programme7 was used for structure solution and the structure refinement was performed using the Rietveld method incorporated in the programme package TOPASA-4.8 The measurements, used for the final Rietveld refinements, were carried out on the Swiss-Norwegian beam line at the European Synchrotron Radiation Facility, Grenoble, France. The sample was mounted in a glass capillary (o. d. 0.5 mm). The data were collected using the Debye-Scherrer geometry and MAR345 image plate detector at a sample to detector distance 150 mm, and radiation with selected wavelength of X = 0.701378 À. The capillary was oscillated by 60° during exposure to the X-ray beam for 60 s, followed by a readout for ~83 s. Obtained raw images were transformed to 2D-powder diffraction patterns in 28 range of 1-48° in steps of 0.03580° using the FIT2D programme.9 3. Results and Discussion The two studied compounds were previously identified by Nanot et al.3 Here presented diffraction patterns correspond well with those published. The unit cells obtained by the indexing procedure seems to be similar to the proposed ones, but the unit cell for the compound Ca-La8Ti9O31 was found to be doubled in ^-direction regarding the prediction. On the basis of extinction conditions the most probable space groups were determined, being P2,2,2, and Pbn2, for CaLa5TiQO2, and Ca2La4Ti6O20, res- pectively. According to the cell volume, most probable symmetry and expected material density for the Ca2La4 Ti6O20 three independent atoms for La/Ca positions and three independent TiO6 octahedra were put into the Fox program. Since we expected calcium and lanthanum to share the same positions in atomic ratio 1:2 we substituted them by rhodium whose number of electrons roughly corresponds to the occupancy - weighted average number of electrons of lanthanum and calcium. For CaLa8Ti9O31 we applied the same strategy, where nine free atom positions were occupied by tellurium and nine octahedra were used. The resulting structural models were treated manually in order to exclude all the redundant oxygen atoms in the model due to the shared corners of the octahedra. For Ca-La8Ti9O31 higher symmetry was observed in the solved structure so we increased the symmetry to Pnma and removed the obsolete atoms. Table 1. Crystal data and refinement parameters. Compound CaLa8Ti9O31 Ca2La4Ti6O20 Crystal system, Space group Orthorombic, Orthorombic, Pnma Pbn21 a (A) 5.528(3) 3.88104(19) b (A) 7.798(4) 36.771(3) c (A) 57.01(3) 5.5031(3) v (A3) 2457.4(2) 785.35(8) Temperature (K) 293 293 No of reflections 2246 740 No of independent parameters 138 88 rp Rp-dash 4.657 4.645 6.748 8.063 Rwp R wp-dash 6.049 6.047 8.292 9.341 Rexp R exp-dash 1.519 2.103 2.082 3.248 Excluded regions None None No of Anti-Bump restraints 4 4 The interatomic distances Ti-O, O-O, Ca-O, La-O, anti-bump values used were 1.96 A, 2.77 A, 2.40 A, 2 .40 A respectively.10 IlJIL ■ .1" ' 'I ............................1*11 l.nW^HH J I. I fc:ll H> «*- 18 20 22 2A 26 23 30 32 2Th Degrees Figure 1. Final Rietveld plot of Ca2La4Ti6O20-bottom mark reflection positions. solid black line - measured, circles - calculated, gray line below - difference. Vertical bars at the Figure 2. Final Rietveld plot of CaLagTi9O31- solid black line - measured, circles - calculated, gray line below - difference. Vertical bars at the bottom mark reflection positions. Both structure models were refined by Rietveld method to adjust the unit cell and structural parameters. All the atomic positions were refined, while inter-atomic bond distances were restrained against becoming unreasonably short by using anti-bump restraints, as implemented in TOPASA-4. The site occupancies were fitted only for calcium and lanthanum, which are expected to share the positions in the structure; the sum of the occupancies at all sites was constrained to unity and the net stoichiometric ratio between calcium and lanthanum was preserved; the exact atomic positions for both were Table 2. Fractional atomic coordinates, displacement parameters and site occupancies of Ca2La4Ti6O20 ceramics. x J" z Biso Occ. (<1) La1 0.002(4) 0.54086(15) 0.38037 0.58(7) 0.513(5) Ca1 1.016(9) 0.5297(5) 0.376(5) 0.58(7) 0.487(5) La2 0.996(3) 0.38447(11) 0.3755(14) 0.58(7) 0.630 (6) Ca2 0.005(15) 0.3964(7) 0.399(5) 0.58(7) 0.370(6) La3 0.514(2) 0.78304(9) 0.9527(12) 0.58(7) 0.857(8) Ca3 0.51(4) 0.8050(14) 0.906(7) 0.58(7) 0.143(8) Til 0.492 (5) 0.46252(18) 0.3772(16) 0.31(8) O21 0.485(17) 0.5654(4) 0.175(3) 1.13(16) O31 0.989(15) 0.4635(4) 0.374(4) 1.13(16) O41 0.484(17) 0.4924(4) 0.083(3) 1.13(16) Ti2 0.502(6) 0.68740(15) 0.9095(17) 0.31(8) O22 0.469(12) 0.3452(4) 0.094(2) 1.13(16) O42 0.517(17) 0.7174(4) 0.192(3) 1.13(16) O52 0.51(3) 0.7285(4) 0.702(3) 1.13(16) O62 0.988(16) 0.3161(3) 0.415(3) 1.13(16) Ti3 0.488(4) 0.38412(17) 0.8863(16) 0.31(8) O13 0.982(12) 0.3902(3) 0.891(3) 1.13(16) O23 0.516(19) 0.6381(4) 0.082(2) 1.13(16) O43 0.00(3) 0.0809(4) 0.684(3) 1.13(16) fitted independently. Isotropic displacement parameters were set to be equal for all the atoms of the same type, including calcium and lanthanum. Applying all those conditions in the refinement, we obtained good agreement between calculated and observed powder diffraction patterns (Figs. 1 and 2). All the details are given in Table 1, final atomic parameters are given in Tables 2 and 3. The crystal structures of both compounds belong to the group of compounds with general formula AnBnO3n+2 (where site A can be occupied eider with Ca or La, and site B is Ti). These are oxygen rich perovskite type structures that consist of slabs of the perovskite structure separated by layers of additional oxygen atoms, where the thickness of the slabs depends on the n-value. The consecutive blocks are shifted with respect to each other by one half of the unit cell edge. In the CaLa8Ti9O31 (n = 4.5) compound the four and five octahedra thick perovskite blocks are alternating, while in the Ca2La4Ti6O20 (n = 6) structure the building blocks have the thickness of six octahedra (Figs 3 and 4). The TiO6 octahedra in both structures are rather regular, also due to anti-bump restraints that had to be used to prevent unrealistically short interatomic distances. The ranges and average values of distances and angles are given Table 4. In the Ca2La4Ti6O20 compound, the positions of calcium and lanthanum, occupying the same crystallographic sites, do not coincide exactly. The differences between their positions are moderate [0.41-0.85 A], and it can be seen that they are larger in-between the slabs than within them. Within the perovskite structure region the calcium and lanthanum atoms are surrounded by twelve oxygen atoms; the cation - anion distances ranges are presented in Table 5. The interatomic distances between these two atoms and ti- Table 3. Fractional atomic coordinates, displacement parameters and site occupancies of CaLa8Ti9O31 ceramics. x 3 z B iso Occ. (<1) Lai 0.4260(l3) 0.0008(l6) 0.6l590(l0) 0.63(7) 0.923(9) Cal 0.39(2) 0.02(3) 0.6209(l6) 0.63(7) 0.077(9) La3 0.5 0 0.5 0.63(7) 0.883(l4) Ca3 0.5 0 0.5 0.63(7) 0.ll7(l4) La4 0.496(2) 0.25 0.2740(2) 0.63(7) 0.94(3) Ca4 0.5l(6) 0.25 0.269(4) 0.63(7) 0.06(3) La5 0.956(2) 0.25 0.66283(l7) 0.63(7) 0.95(3) Ca5 l.00(6) 0.25 0.675(3) 0.63(7) 0.05(3) La8 l.00l9(l5) 0.5008(l8) 0.45056(ll) 0.63(7) 0.845(9) Ca8 -0.038(l0) 0.50(2) 0.4469(ll) 0.63(7) 0.l55(9) La7 0.092(2) 0.25 0.34304(l9) 0.63(7) 0.9l(2) Ca7 0.0l(3) 0.25 0.32l(2) 0.63(7) 0.09(2) La6 0.990(3) 0.25 0.7736(3) 0.63(7) 0.79(6) Ca6 0.030(l4) 0.25 0.7850(ll) 0.63(7) 0.2l(6) Til 0.458(4) 0.75 0.9044(3) 0.26(9) Ti2 -0.00l(2) 0.997(3) 0.7240l(l8) 0.26(9) Ti3 0.47l(3) l.006(3) 0.676l(2) 0.26(9) Ti4 0.978(4) 0.25 0.9489(4) 0.26(9) Ti6 0.5l9(4) 0.75 0.5493(3) 0.26(9) Ti8 0.492(5) 0.25 -0.0006(4) 0.26(9) TilO 0.459(4) 0.25 0.8987(4) 0.26(9) Ol 0.549(7) 0.25 0.7737(6) 0.46(l9) O2 0.782(4) 0.965(5) 0.792l(4) 0.46(l9) O3 0.303(4) 0.025(5) 0.8029(4) 0.46(l9) O4 0.560(8) 0.25 0.2276(6) 0.46(l9) O5 0.286(5) 0.527(5) 0.2565(4) 0.46(l9) O9 0.9ll(8) 0.25 0.8230(7) 0.46(l9) OlO 0.045(8) 0.75 0.8l9l(6) 0.46(l9) Oil 0.5 0.5 0 0.46(l9) Ol2 0.257(8) 0.25 0.0242(7) 0.46(l9) Ol3 0.772(8) 0.25 0.0l83(7) 0.46(l9) Ol4 0.306(8) 0.75 0.0286(7) 0.46(l9) Ol5 0.230(9) 0.25 0.9795(7) 0.46(l9) Ol6 0.990(5) 0.488(6) 0.9534(3) 0.46(l9) Ol8 0.77l(7) 0.25 0.9234(7) 0.46(l9) O20 0.966(5) 0.484(6) 0.5947(3) 0.46(l9) O2l 0.287(7) 0.25 0.9344(7) 0.46(l9) O22 0.300(7) 0.75 0.9347(7) 0.46(l9) O23 0.293 (8) 0.75 0.57l9(7) 0.46(l9) O24 0.649(8) l.25 0.6l23(8) 0.46(l9) O25 0.l33(8) 0.25 0.6273(7) 0.46(l9) O26 0.207(8) 0.75 0.8842(8) 0.46(l9) O27 0.690(7) 0.75 0.8805(7) 0.46(l9) O28 0.3l4(4) 0.987(6) 0.3498(4) 0.46(l9) O30 0.800(4) 0.034(4) 0.34l4(4) 0.46(l9) tanium are slightly shorter for calcium atoms (3.09 A and 3.08 A as opposed to 3.34 A and 3.28 A for La). In between the slabs ten neighboring oxygen atoms are found around calcium or lanthanum. The distances to titanium are approximately the same for both atoms -3.34 A. For the CaLa8Ti9O31 the Ca-La position differences are more notable [0.30-1.33 A], but the same trend can be found. Within the slabs the Ca,La coordination by oxygen is 12 and the distances ranges are in Table 5; distances to titanium are shorter for calcium atoms. In-between the slabs the coordination is ten and also the Ca-Ti distance is shorter than La-Ti. The average distances Ca-O and La-O are expec-tedly very similar and close to the values for similar structures. As the two cations occupy the same voids, the differences to the surrounding oxygen atoms average out and the differences in positions are best viewed and understood on Figs 3 and 4. Table 4. Interatomic distances between Ti and oxygen atoms. Figure 3. Projection of the structure of Ca2La4Ti6O20 along the a-axis. Gray spheres are calcium atoms, black spheres are lanthanum. The octahedra have oxygen atoms in the corners and titanium atom in the centre. The unit cell is outlined. Figure 4. Projection of the structure of CaLa8Ti9O31 along the b-axis. Gray spheres are calcium atoms, black spheres are lanthanum. The octahedra have oxygen atoms in the corners and titanium atom in the centre. The unit cell is outlined. Distance range Angle range Average distance [A] [°] [A] CaLa8Ti9O31 Til 1.80-2.29 80.7-102.9 2.00 Ti2 1.83-2.21 76.5-96.0 1.97 Ti3 1.82-2.20 80.0-95.6 2.01 Ti4 1.85-2.23 82.5-102.3 1.96 Ti6 1.80-2.11 83.8-103.5 1.99 Ti8 1.84-1.95 85.6-91.7 1.92 Ti10 1.76-2.24 75.9-103.1 1.97 Ca2La4Ti6O20 Til 1.92-2.01 86.7-93.6 1.95 Ti2 1.89-2.11 82.9-97.6 1.97 Ti3 1.83-2.19 82.0-102.7 1.98 Table 5. Interatomic distances between Ca/La and oxygen atoms. Distance range N* Average distance [A] [A] CaLa8Ti9O31 Lal 2.31-3.27 10 2.74 Cal 2.34-2.79 7 2.76 3.19 1 3.51-3.52 2 La3 2.54-3.05 12 2.74 Ca3 La4 2.32-3.21 12 2.79 Ca4 2.35-3.06 10 2.80 3.38 2 La5 2.25-3.35 10 2.71 Ca5 2.31-3.23 10 2.72 La8 2.41-3.05 12 2.74 Ca8 2.41-3.05 10 2.76 3.20-3.25 2 La7 2.34-3.19 9 2.74 3.78 1 Ca7 2.35-3.11 11 2.74 La6 2.34-3.09 12 2.75 Ca6 2.26-2.94 9 2.80 3.35-3.25 3 Ca2La4Ti6O20 Lal 2.37-3.15 12 2.75 Cal 2.39-2.96 10 2.75 3.14-3.25 2 La2 2.37-3.19 12 2.74 Ca2 2.41-2.95 10 2.74 3.10-3.26 2 La3 2.40-3.20 10 2.70 Ca3 2.38-3.14 10 2.75 * number of neighboring oxygen atoms 4. Conclusions In this work we have characterized two microwave dielectric crystalline ceramic materials, namely Ca-La8Ti9O31 and Ca^a^^O^, found as single phases in the ternary phase diagram of the corresponding metal oxides. They crystallize in a perovskite-like structure type with a general formula AnBnO3n+2, n being 4.5 and 6, respectively. The structures of the two compounds were determined by X-ray powder diffraction using laboratory data and direct space methods. Sinchrotron radiation measurements were used for final Rietveld refinement. The structural analysis reveals rather well ordered perovskite slabs of 6 and alternating 4 and 5 octahedral layers for Ca2La4Ti6O20 and CaLa8Ti9O31 respectively, separated by oxygen-rich layers. It was found that the positions of Ca and La, which occupy the same crystallographic sites (A), do not coincide exactly. The voids among the oxygen atoms in the structure allows for a moderate differences in the positions of Ca and La, thus allowing somewhat different distribution of individual coordination distances of the two atoms according to their different charge and slightly different ionic radii. 5. Acknowledgements This work was supported by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia (Grants P2-0091-0106, P1-0175 and MR-28339) and AdFutura scholarship programme. We thank to Yaro-slav Filinchuk (SNBL Grenoble) for the help with the synchrotron data collection. 6. References 1. S. D. Skapin, D. Kolar, D. Suvorov, D, J. Am. Ceram. Soc. 1993, 76, 2359-2362. 2. I-S. Kim, W-H. Jung, Y. Inaguma, T. Nakamura, M. Itoh, Mat. Res. Bull. 1995, 30, 307-316. 3. M. Nanot, F. Queyroux, J-C. Gilles, A. Carpy, J. Galy, J. Solid State Chem. 1974, 11, 272-284. 4. K. Demsar, S. D. Skapin, A. Meden, D.Suvorov, Acta Chim. Slov. 2008, 55, 966-972. 5. PDF-2_ Release 2006; McClune, W. F., Ed.; International Centre for Diffraction Data: Newtown Square, PA, 2006. 6. X'Pert High Score Plus 2007; P. A. Nalytical B.V., Almelo, The Netherlands. 7. V. Favre-Nicolin, R. Cerny, J. Appl. Cryst. 2002, 35, 734743. http://vincefn.net/Fox/FoxWiki 8. Coelho, A. A. TOPAS-Academic, Version 4.1. 2007; Coelho Software, Brisbane, Australia. 9. Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Häusermann, D. High Pressure Res. 1996, 14, 235-248. 10. ICSD (2010). Inorganic Crystal Structure Database. FIZ-Karlsruhe, Germany, and the National Institute of Standards and Technology (NIST), USA. Povzetek Enofazna keramična materiala na osnovi spojin CaLagTi9O31in Ca2La4Ti6O20 sta bila pripravljena z reakcijo v trdnem. Kristalno strukturo smo jima določili na osnovi rentgenske praškovne difrakcije z uporabo metod v direktnem prostoru. Kot v podobnih strukturah s splošno formulo AnBnO3n+2 smo opazili plasti perovskitnega strukturnega tipa, ki so med seboj ločeni s plastmi bogatimi s kisikom, atomi La in Ca pa si delijo ista kristalografska mesta.