Scientific paper Rietveld Refinement and Dielectric Properties of CaLa4Ti5O17 and SrLa4Ti5O17 Ceramics Katarina Dem{ar,a Sre~o D. [kapin,b Anton Medena and Danilo Suvorovb a Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia b Jožef Stefan Institute, Ljubljana Slovenia * Corresponding author: E-mail: katarina.demsar@fkkt.uni-lj.si Received: 25-04-2008 Dedicated to the memory of Professor Ljubo Golic Abstract The ceramic materials of CaLa4TÌ5Oj7 and SrLa4TÌ5Oj7 were synthesized using the solid-state reaction method. Rietveld refinements of XRD data were carried out using different models obtained from the structural database and the orthor-hombic structure (Pmnn space group) proved to be most suitable for both compounds. Like in similar compounds of the general formula AnBnO3n+2, where perovskite-like slabs are separated by oxygen-rich layers, Ca or La and Sr or La occupy the A site. The refined occupancies indicate that Ca preferably occupies the A sites within the perovskite slabs, while Sr is preferably found within the oxygen-rich layers. The characterization of microwave dielectric properties revealed that ceramics based on these compounds exhibit permittivity in the range of 39-51, high quality factor (Qxf) in the range of 13140-14200 and negative value of temperature coefficient of resonant frequency Tf = -29 ppm/K for Ca-La4Ti5O17 and positive value of Tf = 58 ppm/K for the Sr-analog. Keywords: Powder diffraction, Rietveld refinement, Microwave ceramics 1. Introduction In the past two decades a lot of effort was put into investigations of new dielectric ceramic materials due to specific demands. The most important and constant interest has been focused on miniaturization, which imposes higher permittivity (e), high quality factor (Qxf) and temperature stable resonant frequencies (xf), of the new materials. Compounds in the La2O3-TiO2-based system were also examined as microwave frequency dielectric cera-mics.1 The addition of selected oxides, for example Al2O3,1 Ga2O3,2 CaO3 causes formation of an A-site deficient perovskite La2/3TiO3 compound. Solid solutions (1-x)La2/3TiO3(stab)-xLaAlO3 stable over the range 0.04< x <1 and (1-x)La2/3TiO3(stab)-xCaTiO3 stable for 0.1< x <0.96 were identified in the systems La2O3-TiO2-Al2O3 and La2O3-TiO2-CaO, respectively. Single phase ceramics based on these solid solutions exhibit excellent microwave dielectric properties, which can be simply tuned by changing the La2/3TiO3/(LaAlO3 or CaTiO3) ratio.3'4 In the ternary La2O3-TiO2-CaO system, a number of compounds were identified. Nanot et al. reported a perovskite-related series of compounds with a general formula AnBnO3n+2, where n = 4.5, 5 and 6 along the compositional line CaTiO3-La2Ti2O7. Thus, the stable compounds with n = 4.5, 5 and 6 are CaLa8Ti9O31, CaLa4Ti5O17 and Ca2La4Ti6O20, respectively.5 Furthermore, along the composition line CaTiO3 -La4Ti3O12 two compounds CaLa4Ti4O15 and Ca2La4 Ti5O18 were reported.6 CaTiO3 dissolves up to 40 mol % of La2O3. The end member of this solid solution is the componds Ca3La4Ti3O15.7 Some of these componds e.g. CaLa4Ti4O15, Ca2La4Ti5O18, CaLa8Ti9O31 and CaLa4 Ti5O17 have been reported to have promising microvawe dielectric properties with permitivity over 44, quality factor (Qxf) higher than 17350 and low temperature coefficient (Tf) in the range of -25 to +6 ppm/K.8 To date no detailed structure report has been published on Ca-La-Ti-O or Sr-La-Ti-O ceramic materials. Extensive crystallographic database search yielded no match for any compound of the CaLa4Ti5O17 or SrLa4Ti5O17 stoic-hiometry. However, it was found that the two structures are most probably isostructural with La5Ti4FeO17. This resulted in a motivation to explore structures of these materials by X-ray powder diffraction techniques. The scope of this paper is to analyze the structures of the title materials by X-ray powder diffraction, based on the Rietveld refinement. Additionally, the ceramics based on these compounds were characterized according to their microwave dielectric properties. 2. Experimental Details Synthesis. The ceramic samples were prepared by the solid-state reaction technique using La2O3 (99.99% Alfa Aesar), TiO2 (99.8% Alfa Aesar), CaCO3 (99.5% Alfa Aesar) and SrCO3 (99.9% 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. The dried powders were uni-axially pressed into pellets and fired in a tube furnace in air on a Pt foil. Heating rate up to 600 °C was 5 °C/min, followed by 0.9 °C/min up to the different final temperatures between 1420 °C and 1580 °C and soaked there for 10 to 20 h. The final sintering temperatures were optimized for maximum density of the pellets. After the heat treatment, the samples were furnace-cooled to room temperature. The bulk densities were measured using the Archimedes method. Dielectric measurements. The microwave dielectric properties were measured using a network analyzer (HP, Model HP 8719 C). We used the closed resonant cavity method using TE01g mode. X-ray diffraction measurements. The X-ray powder patterns were collected on a PANalytical X'Pert PRO MPD diffractometer in Bragg-Brentano geometry using CuKa1 radiation. The data was collected in the 20 range of 3-150° in steps of 0.017° with the total collection time of 64 hours for the Sr-compound and 16 hours for the Ca-compound. Preliminary search-match analyses were carried out with Crystallographica Search-Match software using the PDF data base release 20079. The structural data was obtained from the ICSD10 data base and the structure refinement was performed using the Rietveld method incorporated in the program package TOPASR2-111. 3. Results and Discussion 3. 1. Rietveld Refinement and Structure Consideration The diffraction pattern of the CaLa4Ti5O17 compound did not correspond to any known structure of Ca-La-Ti-O compound. However, we found that the position of the diffraction peaks in the XRD-pattern coincided with the ones of La5Ti5O1712'9 (281282-ICSD10), which is monoclinic with the space group P21/c. For that reason the structure of La5Ti5O17 was used as an initial model for CaLa4Ti5O17 crystal structure refinement. On the basis of the reduced cell search, another suitable model was found in the ICSD database - La5FeTi4O17,13 which is orthorhombic with the space group Pmnn. The following transformation matrix between the unit cells was determined: 1/2 0 0, 0 1 0, 1/2 0 1. The volume of the orthorhombic unit cell was two times smaller than the monoclinic one and explained all observed peaks. A very careful investigation of, whether there is anything in the pattern that would justify the use of a larger monoclinic unit cell (peak splitting or broadening, weak reflections, indexed only by the larger cell) gave no support to the large monoclinic unit cell. To verify the model, we also performed the Le-Bail fit with both unit cells and the results were insignificantly different (the Rwp value was 3.115 for the larger monoclinic P21/c cell and 3.483 for the smaller Pmnn orthorhombic cell). Therefore we decided to use the more symmetric La5FeTi4O17 structural model in further studies. The CaLa4Ti5O17 diffraction pattern was then fitted using Rietveld refinement to adjust the unit cell and structural parameters. The atomic positions and occupancies were refined, taking into account the fact that the lanthanum sites can be occupied by calcium, while the sum of the occupancies at all sites was constrained to be unity and the net stoichometric ratio between calcium and lanthanum was preserved. Isotropic displacement parameters were set to be equal for all the atoms of the same type. No Table 1. Crystal data and refinement parameters of CaLa4Ti5Oj7 and SrLa4Ti5Oj7. Compound Space group a b c V Profile function No. of profile points No. of reflections No. of structural parameters No. of profile parameters* Rp R R,, "p-dash wp-dash R 'exp-dash CaLa4Ti5Oj7 SrLa4Ti5Oj7 Pmnn Pmnn 3.89534(3) À 3.91122(4) À 31.2774(3) À 31.3193(4) À 5.51568(48) À 5.53561(6) À 672.008 (8) À3 678.094(1) À3 Fundamental Fundamental Parameters Parameters 8058 8058 747 747 33 33 14 14 4.43 4.67 16.73 15.95 6.14 7.13 16.75 18.29 3.60 1.70 9.82 4.37 *Profile parameters: 10 background - Chebyshev 10*^ order, 1 zero correction, 1crystal size L (nm), 1 strain L, 1 scale factor. Figure 1. Final Rietveld plot of CaLa4TÌ5Oj7 (top) and SrLa4TÌ5Oj7 ceramics (bottom): solid black line - measured, circles - calculated, black line below - difference. further constraints or restraints were used. After such refinement of all atomic parameters the calculated pattern corresponded well to the experimental one (Rwp= 6.14, further details are presented in Tables 1 and 2 and in Fig. 1). As the structure is rather constrained by symmetry in the space group Pmnn (all the atoms lie on the mirror plane m perpendicular to x at x = 0 or x = 1/2), an attempt to relax these symmetry restraints and improve the structural model has been made by decreasing the symmetry. By removing certain symmetry elements, structural models based on two orthorhombic (P2nn and Pm21n) and two monoclinic subgroups (P21/c and Pc) of the Pmnn space group were constructed. Structural parameters of these four models were subjected to Rietveld refinement. However, since the resulting Rwp factors obtained with these space subgroups were not significantly better (e.g., 6.033, 6.100, 5.950 and 6.063 Figure 2. Crystal structure of CaLa4TÌ5Oj7 viewed along the a axis. Gray spheres are lanthanum or calcium atoms, of which shaded positions are more preferably occupied by lanthanum and thus less preferably by calcium. The white octahedra have oxygen atoms in the corners and titanium atoms in the centre. Figure 3. Crystal structure of SrLa4Ti5Oj7 viewed along the c axis. Gray spheres are lanthanum or strontium atoms, of which shaded positions are more preferably occupied by lanthanum but less so by strontium. The white octahedra have oxygen atoms in the corners and titanium atoms in the centre. for the P2nn, Pmn21, P21/c and Pc space group, respectively) and atomic positions did not change notably with respect to the original model (maximum difference in the respective atom positions were 0.25 À for oxygen, 0.18 À for lanthanum and calcium, and 0.07 À for titanium), none of the lower symmetry models was recognized as a better so- lution and the model that explains all the available data with the lowest number of parameters was accepted. The structural model of SrLa4Ti5O17 was treated in the same manner as for the calcium analog; the same space groups were considered and the Pmnn group was again determined to provide the most suitable model. The final Table 2. Fractional atomic coordinates and site occupancies of CaLa4Ti5O17 ceramics. x .y z Biso Occ. (<1) La1 0.5 0.08991(5) 0.5010(3) 0.393(18) 0.7747(31) Ca1 0.5 0.08991(5) 0.5010(3) 0.393(18) 0.2253(31) La2 0.5 0 0 0.393(18) 0.7712(82) Ca2 0.5 0 0 0.393(18) 0.228(82) La3 0.5 0.20982(5) 0.0721(3) 0.393(18) 0.8397(27) Ca3 0.5 0.20982(5) 0.0721(3) 0.393(18) 0.1603(27) Til 0 0.09337(12) -0.0001(8) 0.30(4) Ti2 0 0.17790(12) 0.5398(8) 0.30(4) Ti3 0 0 0.5 0.30(4) Ol 0.5 0.0823(4) -0.010(3) 0.67(9) O2 0 0.9637(5) 0.212(3) 0.67(9) O3 0.5 0 0.5 0.67(9) O4 0 0.1356(5) 0.215(3) 0.67(9) O5 0 0.2253(5) 0.347(3) 0.67(9) O6 0.5 0.1699(4) 0.550(3) 0.67(9) O7 0 0.1211(5) 0.697(3) 0.67(9) O8 0 0.2086(5) 0.793(3) 0.67(9) O9 0 0.0515(5) 0.288(3) 0.67(9) Table 3. Fractional atomic coordinates and site occupancies of SrLa4Ti5O17 ceramics. x J Z ^iso Occ. (<1) La1 0.5 0.09003(5) 0.5037(4) 0.66(2) 0.8427(84) Sr1 0.5 0.09003(5) 0.5037(4) 0.66(2) 0.1573(84) La2 0.5 0 0 0.66(2) 0.768(22) Sr2 0.5 0 0 0.66(2) 0.232(22) La3 0.5 0.20986(7) 0.0733(4) 0.66(2) 0.7731(70) Sr3 0.5 0.20986(7) 0.0733(4) 0.66(2) 0.2269(70) Ti1 0 0.09415(16) 0.0033(11) 0.30(4) Ti2 0 0.17822(16) 0.5433(11) 0.30(4) Ti3 0 0 0.5 0.30(4) O1 0.5 0.0859(6) -0.016(4) 0.78(11) O2 0 0.9627(7) 0.226(4) 0.78(11) O3 0.5 0 0.5 0.78(11) O4 0 0.1317(6) 0.212(4) 0.78(11) O5 0 0.2245(6) 0.349(4) 0.78(11) O6 0.5 0.1690(6) 0.531(4) 0.78(11) O7 0 0.1219(7) 0.718(4) 0.78(11) O8 0 0.2083(7) 0.800(4) 0.78(11) O9 0 0.0541(7) 0.281(4) 0.78(11) crystallographic data for both compounds are presented in Table 1. The fitted diffraction patterns are shown in Fig. 1, the crystal structures are presented in Figs. 2 and 3 and atomic fractional coordinates with site occupancies and isotropic atomic displacement parameters are collected in Tables 2 and 3. The crystal structures of both compounds are related to the group of compounds with the chemical formula AnBnO3n+2 (where A is Sr, La or Ca and B is Nb or Ti)12 in which part of the A or B site is occupied by two different atom types. Structures of this type consist of slabs of the pe-rovskite-like structure, separated by layers of additional oxygen atoms and do not differ much from the structure of La5Ti5O17.12 Compared to the La5Ti5O17 analogue, the unit cell of CaLa4Ti5O17 is two times smaller, and the structure can be described as orthorhombic while the organization of the polyhedra remains the same. Worthy to note is that our choice of the smaller orthorhombic unit cell was based on the fact that we observed no particular peak in the diffraction pattern that could have been explained only by a larger unit cell. The coordination polyhedra of Ti are tilted octahe-dra. The Ti-O distances are in the range of 1.7-2.2 À and the O-Ti-O angles from 79 to 104 degrees. Similar distortions of Ti-O octahedra have been observed in some related systems.10 The La atoms are coordinated by 12 oxygen atoms, the distances are in the range from 2.4 to 3.1 À (see Table 4). Exception is the La3 site, located between two individual slabs of the perovskite structure, where two oxygen atoms are on a notably larger distance of 3.9 À. The second neighbors of the lanthanum sites within the perovski-te slabs are eight Ti atoms which form a distorted cube, while the La3 site has seven Ti atoms as second neighbors. Additional insight into the structure of La/Ca and La/Sr polyhedra was obtained by performing a Bond Valence calculation on both systems.14 The bond valence sums together with the corresponding ranges of atomic di- stances are listed in Table 4. For the Ca compound the bond valence sums of La1 and La2 are close to the nominal valence of 3, but the bond valence sum of La3 is notably smaller. This is consistent with the fact that the La3 site lies between the perovskite slabs so that it is less tightly coordinated than the La1 and La2 sites which are located within the slabs. As expected, the corresponding bond valences of calcium at all sites is considerably smaller than 2.0, mainly due to the notably smaller size of the Ca2+ ion. Similar features can be found in the Sr compound; however the calculated bond valence sum of La2 is much lower than in the Ca compound, apparently due to longer La-O distances. This possibly originates form the fact that the Sr occupancy at this site is relatively high, inducing strain to the polyhedron and resulting in its expansion. Although Sr2+ is still slightly smaller than La3+, it Tabele 4. Bond valence calculations for La/Ca and La/Sr structures CaLa4Ti5O17 BVS* Distance range [A] N** La1 3.139 2.432-3.018 12 Ca1 1.804 12 La2 3.009 2.540-2.985 12 Ca2 1.730 12 La3 2.753 2.379-3.139 10 Ca3 1.582 10 SrLa4Ti5O17 DVS* Distance range [À] N** La1 3.089 2.478-2.880 12 Sr1 2.670 12 La2 2.559 2.599-3.017 12 Sr2 2.212 12 La3 2.523 2.402-2.854 7 Sr3 2.180 7 Bond valence sums ** No of neighboring atoms included probably cannot accommodate the same amount of oxygen counterions because of the lower valence. Note that this effect is not observed in the Ca compound, because the much smaller size of Ca2+ ion compensates for the lack in its valence capability. In contrast to the Ca compound in which the valences of Ca2+ are notably lower than 2.0, the bond valence sums of strontium are considerably higher than 2.0 (about 2.6 at sites 1 and 2), because strontium ion is much larger than calcium ion and the resulting computed valences of Sr2+ are higher. On the whole, part of the bond valence calculations fairly reproduce the assumed valences of the atoms considered (mainly for lanthanum ion), while the agreement is poorer for calcium ion and strontium ion. The discrepancies can readily be assigned to the fact that calcium and strontium ion share their sites with lanthanum, hence the resulting structure represents an average weighted by relative occupancies of the respective atoms. While the net occupancies of calcium and strontium are clearly the same, a very slight difference has been observed in their preferential location in the crystal structure. In the calcium compound the calcium atoms are less preferably located at the sites between the perovskite slabs, while in the strontium analogue the strontium atoms are altogether less likely to be located within the slabs (see Tables 2 and 3, Figs. 1 and 2). This observation is consistent with the fact that the ionic radius of calcium is slightly smaller than that of lanthanum; hence the former can be more easily incorporated into the perovskite slab than the strontium ion, whose radius is slightly larger than that of lanthanum.15 3. 2. Microwave Dielectric Properties The ceramics based on MLa4Ti5O17 (M=Ca, Sr) compounds exhibit interesting microwave dielectric characteristics. The TE01g modes of the samples were obtained in the range of 5.5-6GHz. Dielectric properties of the ceramics are given in Table 5. The CaLa4Ti5O17 shows relative permittivity at 20 °C e = 51.1 and negative temperature coefficient of resonant frequencies xf = -29 ppm/K, however the SrLa4Ti5O17 has e = 39.1 and positive value of Tf = 58 ppm/K. Both compounds show high quality factor (Qxf) = 13 140 for CaLa4Ti5O17 and 14 200 for SrLa4Ti5O17. The compounds show very promising microwave dielectric properties. Their opposite value of temperature coefficient of resonant frequencies is not understood satisf-yingly but applying this phenomenon a to solid solution of both compounds which would show Tf near zero value, required for the use in electronic devices, may be prepared. 4. Conclusions In this work we have characterized two microwave dielectric crystalline ceramic materials, namely Ca-La4Ti5O17 and SrLa4Ti5O17, found as single phases in the ternary phase diagrams systems of the corresponding metal oxides. They are isostructural and crystallize in a pe-rovskite-like structure type with a general formula AnBnO3n+2. The structures of the two compounds were determined by powder diffraction X-ray crystallography using the pattern matching, followed by Rietveld refinement method. The final structures are of the orthorhombic symmetry (Pmnn space group). We have presented the dielectric properties in microwave frequency range of two isostructural compounds. Their high dielectric constants e in the range 39 to 51, high quality factor (13 140 for CaLa4Ti5O17 and 14 200 for SrLa4Ti5O17) and temperature coefficient of resonant frequencies Tf = -29 - +58 ppm/K make them interesting for use in electronic devices, especially where a narrow bandwidth is necessary. The final structural data (including coordinates, displacement and geometrical parameters) have also been deposited with FIZ Karlsruhe Crystal Structure Deposition (CSD) Center as supplementary material with the deposition numbers 419425 and 419426 for compounds Ca-La5Ti5O17 and SrLa5Ti5O17, respectively. Copies of the data can be obtained, free of charge, contacting crysdata @fiz-karlsruhe.de 5. Acknowledgements This work was suported by the Ministry of Higher Education, Science and Technology of the Republic of Slovenia (Grants P2-0091-0106, P1-0175 and MR-28339). 6. References 1. S. D. Skapin, D. Kolar, D. Suvorov, D, J. Am. Ceram. Soc. 1993, 76, 2359-2362. 2. D. Kolar, S. D. Skapin, D. Suvorov , M. Valant, Phase equilibria and dielectric properties in the La5O3 - Ga2O3 - TiO2 system, in: High Temperature Materials Chemistry IX, ed. K. E. Spear. The Electrochemical Soc. Inc. Pennington, New Jersey, 1997, pp. 109-115. Table 5. Microwave dielectric properties of the MLa4Ti5Oj7 ceramics (M=Ca, Sr) Compound Theor. density Rel. density Tsint. e20°C Qxf Tf f [g/cm3] [%] [°C] [GHz] [ppm/K] [GHz] CaLa4Ti5O17 5.471 94 1550 51.1 13 140 29 5.56 SrLa4Ti5O17 5.655 92 1580 39.1 14 200 58 5.96 3. I-S. Kim, W-H. Jung, Y. Inaguma, T. Nakamura, M. Itoh, Mat. Res. Bull. 1995, 30, 307-316. 4. D. Suvorov, M. Valant, S. D. Skapin, D. Kolar, J. Mat. Sci. 1998, 33, 85-89. 5. M. Nanot, F. Queyroux, J. C. Gilles, Mat. Res. Bull. 1975, 10, 315-318. 6. M. German, L. M. Kovba, Russ. J. Inorg. Chem. 1983, 28, 1340-1350. 7. V. A. Saltykova, O. V. Mel'nikova, N. F. Fedorov, I. A. Pole-vaya, Russ. J. Inorg. Chem. 1989, 34, 1520-1522. 8. I. N. Jawahar, N. I. Santha, M. T. Sebastian, J. Mater. Res. 2002, 17, 3084-3089. 9. PDF-4_ Release 2006; McClune, W. F., Ed.; International Centre for Diffraction Data: Newtown Square, PA, 2006. 10. ICSD (2007). Inorganic Crystal Structure Database. FIZ-Karlsruhe, Germany, and the National Institute of Standards and Technology (NIST), USA. 11. Bruker AXS GmbH. Topas: General profile and structure analysis software for powder diffraction data. Karlsruhe; 2003. 12. P. Daniels, F. Lichtenberg, S. Smaalen, Acta Cryst. 2003, C59, i15-i17. 13. Yu. O. Titov, N. M. Belyavina, V. Ya Markiv, M. S. Slobod-yanik, V. V. Chumak, V. P. Yashchuk, Dopov. Nats. Akad. Nauk. Ukr. 2005, 12, 149-154. 14. I. D. Brown, D. Altermatt, Acta Cryst. 1985, B41, 244-247. 15. R. D. Shannon, Acta Cryst. 1976, A32, 751-767. Povzetek Keramična materiala CaLa4TijO17 in SrLa4TijO17 smo pripravili z reakcijo v trdnem. Strukture smo določili na osnovi že poznanih struktur iz baze podatkov, ki smo jih uporabili kot začetne modele za Rietveldovo prilagajanje na izmerjenih difraktogramih. Za obe spojini se je izkazalo, da je najbolj ustrezen ortorombski strukturni model s prostorsko skupino Pmnn. Tako kot v podobnih strukturah s splošno formulo AnBnO3n+2 smo opazili perovskitne plasti, ločene z dodatnimi plastmi kisikov, v katerih Ca/La oziroma Sr/La naključno zasedata mesto A. Prilagajanje zasedenosti na mestu A je pokazalo, da Ca preferenčno zaseda mesta znotraj perovskitne plasti, Sr pa mesta znotraj plasti, bogatih s kisikom. Meritve mikrovalovnih dielektričnih lastnosti keramik na osnovi teh spojin pokašejo, da le-te izkazujejo vrednost die-lektrične konstante od 39 do 51, visok faktor kvalitete (Qxf) od 13100 do 14200 in negativno vrednost temperaturnega koeficienta resonančne frekvence Tf = -29 ppm/K v primeru spojine CaLa5Ti5O17, oziroma pozitivno vrednost Tf = 58 ppm/K v primeru spojine SrLa4Ti5O17.