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Pyrochlore Formation, Phase Relations, and Properties in the Cao–Tio2–(Nb,Ta)2O5 Systems R.S

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Pyrochlore Formation, Phase Relations, and Properties in the Cao–Tio2–(Nb,Ta)2O5 Systems R.S ARTICLE IN PRESS Journal of Solid State Chemistry 181 (2008) 406–414 www.elsevier.com/locate/jssc Pyrochlore formation, phase relations, and properties in the CaO–TiO2–(Nb,Ta)2O5 systems R.S. Rotha, T.A. Vanderaha,Ã, P. Bordetb, I.E. Greyc, W.G. Mummec, L. Caid, J.C. Ninod aMaterials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA bCNRS Laboratoire de Cristallographie, BP 166, 38042 Grenoble, France cCSIRO Minerals, Box 312, Clayton South, Vic. 3169, Australia dDepartment of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6400, USA Received 16 August 2007; received in revised form 29 November 2007; accepted 1 December 2007 Available online 15 December 2007 Abstract 0 Phase equilibria studies of the CaO:TiO2:Nb2O5 system confirmed the formation of six ternary phases: pyrochlore (A2B2O6O ), and five members of the (110) perovskite-slab series Can(Ti,Nb)nO3n+2, with n ¼ 4.5, 5, 6, 7, and 8. Relations in the quasibinary Ca2Nb2O7ÀCaTiO3 system, which contains the Can(Ti,Nb)nO3n+2 phases, were determined in detail. CaTiO3 forms solid solutions with Ca2Nb2O7 as well as CaNb2O6, resulting in a triangular single-phase perovskite region with corners CaTiO3–70Ca2Ti2O6:30Ca2 Nb2O7–80CaTiO3:20CaNb2O6. A pyrochlore solid solution forms approximately along a line from 42.7:42.7:14.6 to 42.2:40.8:17.0 CaO:TiO2:Nb2O5, suggesting formulas ranging from Ca1.48Ti1.48Nb1.02O7 to Ca1.41Ti1.37Nb1.14O7 (assuming filled oxygen sites), respectively. Several compositions in the CaO:TiO2:Ta2O5 system were equilibrated to check its similarity to the niobia system in the pyrochlore region, which was confirmed. Structural refinements of the pyrochlores Ca1.46Ti1.38Nb1.11O7 and Ca1.51Ti1.32V0.04Ta1.10O7 using single-crystal X-ray diffraction data are reported (Fd3m (#227), a ¼ 10.2301(2) A˚(Nb), a ¼ 10.2383(2) A˚(Ta)), with Ti mixing on the A-type Ca sites as well as the octahedral B-type sites. Identical displacive disorder was found for the niobate and tantalate pyrochlores: Ca occupies the ideal 16d position, but Ti is displaced 0.7 A˚to partially occupy a ring of six 96g sites, thereby reducing its coordination number from eight to five (distorted trigonal bipyramidal). The O0 oxygens in both pyrochlores were displaced 0.48 A˚from the ideal 8b position to a tetrahedral cluster of 32e sites. The refinement results also suggested that some of the Ti in the A-type positions may occupy distorted tetrahedra, as observed in some zirconolite-type phases. The Ca–Ti–(Nb,Ta)–O pyrochlores both exhibited dielectric relaxation similar to that observed for some Bi-containing pyrochlores, which also exhibit displacively disordered crystal structures. Observation of dielectric relaxation in the Ca–Ti–(Nb,Ta)–O pyrochlores suggests that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi3+. r 2007 Elsevier Inc. All rights reserved. Keywords: Ca–Ti–Nb–O; CaO:TiO2:Nb2O5; Phase diagram; Pyrochlore; Crystal structure; Dielectric relaxation 1. Introduction especially those involving ternary compounds. Studies of the CaO:TiO2:Ta2O5 system have not been reported, Ceramic dielectric materials based on complex titanates, to the best of our knowledge. Available information on the niobates, and tantalates are important for a variety of CaO:TiO2:Nb2O5 system is limited to the study of Jongejan components in communication systems [1–6]. In addition, and Wilkins [8], which included work on phase relations these systems have received considerable attention for both above and below the subsolidus. The formation of ‘‘at photocatalytic materials capable of splitting water [7]. The least two ternary compounds’’ was reported, a pyrochlore- present study of the CaO–TiO2–(Nb,Ta)2O5 systems was type phase at 3CaO:3TiO2:Nb2O5 ( ¼ Ca1.5Ti1.5NbO7), undertaken to clarify the equilibrium phase relationships, and other phases near 1:1:1 and 8:7:6 CaO:TiO2:Nb2O5 which were associated with liquids. The 8:7:6 composition ÃCorresponding author. Fax: +1 301 975 5334. was reported to melt congruently at 1472 1C and exhibit a E-mail address: [email protected] (T.A. Vanderah). distinct X-ray powder diffraction pattern. A CaTiO3 solid 0022-4596/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2007.12.005 ARTICLE IN PRESS R.S. Roth et al. / Journal of Solid State Chemistry 181 (2008) 406–414 407 solution formed along the CaTiO3–CaNb2O6 join, and prepared by melting the respective mixture of CaCO3 and changes in the X-ray powder diffraction pattern of V2O5 at 950 1C for 2 h, and then pouring the liquid onto a Ca2Nb2O7 were observed when CaTiO3 was added along chill plate. The pre-heated (@ 1300 1C for 94 h) composi- the Ca2Nb2O7–CaTiO3 composition line. The stoichiome- tion 42.50:42.50:15.00 CaO–TiO2–Ta2O5 was used as the try of Ca1.5Ti1.5NbO7 reported for the pyrochlore charge. Greenish pyrochlore crystals were grown from both VIII VI 0 [ A2 B2O6O ] phase drew our interest, as this would 4:1 and 1:1 (by mass) charge:flux mixtures, which were require 0.5 mol of the smaller B-type octahedral cations to heated in Pt capsules (crimped but not welded shut) at mix on the (6+2)-coordinated sites of the larger A-type 1425 1C for 1 h, cooled at 3 1C/h to 960 1C, and then air- cations. This behavior has been observed in a number of quenched. After removing the flux with dilute (3 M) pyrochlore systems [9] which exhibit static displacive hydrochloric acid, the 4:1 mixture yielded pyrochlore 0 disorder in the A2O sub-network that facilitates reduced crystals as well as black rutile crystals, as did the 1:1 coordination numbers for resident B-type cations; how- growth mixture, which in addition contained yellow ever, for these systems the A-type majority cations have all CaTa2O6 crystals. Small crystals from the 4:1 product been lone-pair or d10 species such as Bi3+,Tl+,Sn2+, were selected for the structural study. Pb2+or Cd2+. The presence of these large, highly polariz- Crystals of both pyrochlores were analyzed using a able cations has been associated with the occurrence of the JEOL electron probe microanalyzer model JXA 8900R, static displacive disorder [10], which in turn has been linked operated at 20 kV and 20 nA with 40 s counts on peaks and to unusual dielectric properties, including high relative 20 s counts on backgrounds on either side. Standards used permittivities [11] and relaxation behavior [12–15]. A key were TiO2, V metal, Ta2O5 and Ca3Nb2O8. The averages objective of the present work was to characterize the from 11 analyses on two crystals of the niobate, expressed stoichiometry and dielectric properties of pyrochlores in as oxides, are 24.1 wt% CaO, 32.5 wt% TiO2 and 43.3 wt% the CaO–TiO2–(Nb,Ta)2O5 systems. Nb2O5. The corresponding formula scaled to seven oxygen atoms is Ca1.46Ti1.38Nb1.11O7. For the tantalate, the 2. Experimental methods averages from 24 analyses on four crystals are 19.5 wt% CaO, 24.4 wt% TiO2, 55.9 wt% Ta2O5 and 0.84 wt% V2O5. Approximately 30 polycrystalline specimens (3–4g each) The small amount of vanadium is due to incorporation were prepared in air by solid-state reactions of mixtures from the calcium vanadate flux. The formula scaled to of CaCO3 (99.99%), TiO2 (phosphate-free), and Nb2O5 seven oxygen atoms is Ca1.51Ti1.32V0.04Ta1.10O7. (99.9985%) or Ta2O5 (99.993%). Prior to each heating, Phase assemblages were ascertained using the disappear- each sample was mixed by grinding with an agate mortar ing phase method [16,17] and X-ray powder diffraction and pestle for 15 min, pelletized, and placed on a bed data obtained with a Philips [18] diffractometer equipped of same-composition sacrificial powder supported on with incident Soller slits, a theta-compensating slit and platinum foil placed on alumina ceramic. After an initial graphite monochromator, and a scintillation detector. overnight calcination at 950 1C, multiple 1–7d heatings Samples were mounted in welled glass slides. Diffraction (with intermediate grinding and re-pelletizing) were carried patterns were collected at ambient temperature using Cu out at temperatures ranging from 1300 to 1650 1C. Samples Ka radiation over the range 3–701 2y with a 0.021 2y step were either furnace-cooled to E700 1C and air-quenched size and a 2 s count time. Intensity data measured as on the bench-top, or quenched from high temperature into relative peak heights above background were obtained liquid nitrogen. Equilibrium was presumed when no using the DATASCAN software package, and processed further changes could be detected in the weakest peaks using JADE [19]. Single pyrochlore-type crystals were observed in the X-ray powder diffraction patterns. characterized by the precession camera method (Zr-filtered Pyrochlore-type crystals in the CaO–TiO2–Nb2O5 sys- Mo Ka radiation) to assess quality, cell parameter, and tem were obtained from partial melts of the respective space group. Single-crystal intensity data were collected (equilibrated) molar compositions 34.00:45.00:21.00 (reported using Nonius Kappa CCD diffractometers (at CNRS to occur in the pyrochlore’s primary phase field [8]) and Grenoble for the Ca–Ti–Ta–O pyrochlore and at Monash 41.67:41.67:16.67 in welded Pt capsules. Heating the first University, Clayton for the Ca–Ti–Nb–O pyrochlore). composition at 1375 1C for 143 h (followed by cooling at Dielectric properties of the Ca–Ti–Nb–O pyrochlore 2001/h to 700 1C) produced a sample that was approxi- were measured using a disk (E6 mm in diameter, 1.5 mm mately half melted and contained large crystals, some of thick) of pressed equilibrated powder, composition which were euhedral octahedra.
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