ISSN 00201685, Inorganic Materials, 2015, Vol. 51, No. 9, pp. 923–927. © Pleiades Publishing, Ltd., 2015. Original Russian Text © E.A. Bovina, D.V. Tarasova, F.Kh. Chibirova, 2015, published in Neorganicheskie Materialy, 2015, Vol. 51, No. 9, pp. 1003–1007.
Ion Exchange Synthesis of Lanthanum Zirconate E. A. Bovina, D. V. Tarasova, and F. Kh. Chibirova Karpov Institute of Physical Chemistry (Russian State Scientific Center), per. Obukha 31/12, str. 6, Moscow, 105064 Russia email:
[email protected] Received December 4, 2014
Abstract—We have studied the formation of lanthanum zirconate through ion exchange in aqueous solutions of lanthanum and zirconyl chlorides and the AV178 anion exchanger. The results demonstrate that the ion exchange leads to the formation of hydrosol of an amorphous lanthanum zirconium compound in the form of spherical particles about 4 nm in diameter. Heat treatment of the hydrosol at 700°C leads to crystallization of lanthanum zirconate with the fluorite structure, which transforms into the pyrochlore structure at higher temperatures. DOI: 10.1134/S0020168515090022
INTRODUCTION
EXPERIMENTAL
Lanthanum zirconate, La2Zr2O7 (pyrochlore structure, lattice parameter а = 1.079 nm [1]) pos sesses high thermal stability (tm = 2280°С) and excel lent thermophysical [2], catalytic, and optical [3] properties, which makes it an attractive material for hightemperature coatings [4, 5] and buffer layers of currentcarrying ribbons for secondgeneration superconductors [6]. Lanthanum zirconate can be synthesized by a variety of methods: solidstate reac tion [7], coprecipitation [8], citrate route [9], sol–gel processing [10], and others. However, the synthesis product typically contains additional phases. The best known method that enables a pure pyrochlore phase to be obtained is that proposed by Kido et al. [11]: mixing of lanthanum and zirconium acetylacetonates in acetylacetone at a temperature of 135°С, followed by the addition of water, stirring for 12 h at a temperature of 120°С, water evaporation, washing of the resultant precipitate with ethanol, drying, and heat treatment at temperatures of up to 1000°С. Knoth et al. [12] reported a simpler process for lanthanum zirconate synthesis, by boiling down a solution of La(III) and Zr(IV) 2,4pentadionates in propionic acid, but a pure pyrochlore phase, with no lanthanum or zirconium oxide impurities, was only obtained at a temperature of 1200°С.
The starting chemicals used were lanthanum chlo ride hexahydrate, LaCl3 · 6H2O; zirconyl chloride octahydrate, ZrOCl2 · 8H2O; and the AV178 anion exchanger (RF State Standard GOST 2030174). Ion exchange was conducted by mechanically stirring a suspension of the anion exchanger in an aqueous solu tion of a mixture of the chlorides (La : Zr atomic ratio of 1 : 1) at room temperature for a predetermined time. In addition, we carried out ion exchange between indi vidual LaCl3 and ZrOCl2 solutions. The initial con centration of the chloride solutions was 0.05 mol/L. The solutions were first passed through a Millipore 0.2 µm filter. The anion exchanger was treated twice with an aqueous 20% NaOH solution and each time it was washed with distilled water until neutral pH was reached. The amount of the anion exchanger was cal culated from the predetermined ratio (1.5) of the mg equiv of OH– to the sum mgequiv of Cl– in the chlo rides. After the ion exchange process, the anion exchanger was separated from the liquid phase by fil tering though nylon6 cloth. Note that there was no precipitation in the suspension stirring or anion exchanger separation steps. The filtrates were boiled down in a water bath, dried in a drying oven at 100°С for 2 h, and then calcined in a muffle furnace at a pre set temperature for 2 h. The filtrates were characterized by pH measure ments, and we determined the solids concentration, optical density, chloride ion content, and the particle size and shape. The pH of the medium was determined using a PP20 Sartorius Professional Meter pH meter equipped with a pH/ATC combination glass elec trode. The solids concentration in the filtrates was determined gravimetrically, by boiling down an ali quot of the filtrate, followed by heat treatment in a muffle furnace at 600°С for 2 h. The optical density
The purpose of this work was to investigate the for mation of lanthanum zirconate via an ion exchange process that has not been described previously, with the use of an aqueous solution of lanthanum chloride, zirconyl chloride, and the highly basic anion exchanger AV178.
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RESULTS AND DISCUSSION
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Fig. 1. pH of the medium as a function of the stirring time of the anion exchanger with (1) a solution of lanthanum and zirconyl chlorides, (2) lanthanum chloride solution, and (3) zirconyl chloride solution.
(D) was measured using an EKOTEST 2020 instru ment (λ = 595 nm, 1.0cmthick cuvette). The chlo ride ion concentration (Cmeas [mg/mL]) was deter mined nephelometrically with the EKOTEST 2020 (λ = 470 nm) using silver nitrate. Next, the degree of exchange α was calculated:
α = 1 − (C mean C0 ), where C0 [mg/mL] is the initial chloride ion concen tration in solution. The shape and size of the particles in the filtrates were analyzed by transmission electron microscopy (TEM) on a JEOL JEM 2100F/URH (Japan) at an accelerating voltage of 200 kV. The filtrate drying and heat treatment products were characterized by Xray diffraction on a Rigaku D/MAX 2500 diffractometer (CuKα radiation, graph ite monochromator).
(b) (а)
We “assessed” the effect of the stirring time of the anion exchanger suspensions in the solution of lantha num and zirconyl chlorides on the pH of the medium. Curve 1 in Fig. 1 demonstrates that increasing the stir ring time from 0 to 3 min sharply increases the pH of the medium, from 2.3 to 11.0, which then remains unchanged as the stirring time increases to 30 min. When the anion exchanger suspension is stirred with the lanthanum chloride solution for 6 min, the pH of the medium rises from 6.0 to 10.9, remaining unchanged over the next 30 min, as above (curve 2). In contrast, during stirring of the suspension of the zirco nyl chloride solution, pH varies stepwise: it gradually increases from 2.3 to 6.8 during the first 10 min of stir ring, reaches 9.9 after 14 min, and then remains unchanged (curve 3). We investigated the filtrates after stirring for a time that ensured a constant pH of the medium. The results demonstrated that the pH of the filtrates was essen tially identical to the pH of the suspensions. The degree of ion exchange (α) of the chloride ions in solu tion for the hydroxyl groups of the anion exchanger was 0.97–0.98. The filtrates were colorless and trans parent, and their optical density was about 0.01. TEM examination of the filtrates (Fig. 2) showed that the fil trate obtained using the solution of lanthanum and zir conyl chlorides contained rounded amorphous parti cles about 4 nm in diameter (Fig. 2a). The filtrate obtained using the lanthanum chloride solution con tained needlelike crystals 70–100 nm in length and 2–4 nm in thickness (Fig. 2b). The filtrate obtained using the zirconyl chloride solution contained amor phous spherical particles about 4 nm in size (Fig. 2c). Therefore, ion exchange of the solution of lanthanum and zirconyl chlorides and individual salt solutions leads to the formation of nanoparticlecontaining hydrosols. The Xray diffraction pattern of the dried hydrosol prepared using the solution of lanthanum and zirconyl
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Fig. 2. TEM images of the filtrates obtained from (a) a solution of lanthanum and zirconyl chlorides, (b) lanthanum chloride solu tion, and (c) zirconyl chloride solution. INORGANIC MATERIALS
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Fig. 3. Xray diffraction patterns of dried hydrosols prepared using (1) a solution of lanthanum and zirconyl chlorides, (2) lan thanum chloride solution, and (3) zirconyl chloride solution.
chlorides (Fig. 3, scan 1) shows no reflections. The Xray diffraction pattern of the dried hydrosol pre pared using the lanthanum chloride solution (Fig. 3, scan 2) shows sharp 100, 101, 200, 111, 201, 211, 112, 131, 302, and 321 reflections. According to JCPDS PDF data (card no. 361481), these reflections char acterize wellcrystallized hexagonal lanthanum hydroxide, which has the form of needlelike particles (Fig. 2b). Note that Music and ŠipaloZ uljevic [13] reported the preparation of lanthanum hydroxide in the form of wellcrystallized needlelike crystals by coprecipitation. The Xray diffraction pattern of the dried hydrosol prepared using the zirconyl chloride solution (Fig. 3, scan 3) shows broad reflections in the range 2θ = 30° and 50°. According to JCPDS PDF data (card no. 712363), this characterizes poorly crystallized cubic zirconia. Figure 4 shows Xray diffraction patterns of the materials obtained by drying the La–Zr hydrosol cal cined in the temperature range from 300 to 1100°С. It is seen that the Xray amorphous state persists up to 500°С, with almost no changes. After heat treatment at 700°C, the Xray diffraction pattern shows the 220, 400, 440, and 622 reflections. According to Kido et al. [11], these reflections indicate the formation of crys talline lanthanum zirconate with the fluorite structure, FLa2Zr2O7. As the temperature is raised to 900°C, the ˆ
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reflections in the Xray diffraction pattern become narrower and additional reflections emerge: 444, 800, and a very weak 331 reflection from lanthanum zir conate with the pyrochlore structure, РLа2Zr2O7. As the temperature is raised further, to 1000°C, the inten sity of the 331 reflection increases and a weak 511 reflection emerges, which also arises from the structure of РLа2Zr2O7. Above 1100°C, a very weak 111 reflection from the structure of РLа2Zr2O7 emerges (JCPDS card no. 73444). It should be emphasized that, in the temperature range examined, no reflections from lanthanum oxide or zirconium oxide were detected in Xray diffraction patterns, sug gesting that the synthesized lanthanum zirconate had the stoichiometric composition. Thus, in ion exchange synthesis, crystalline lanthanum zirconate forms in the heat treatment step and has first the FLa2Zr2O7 struc ture (700°С), which transforms into the РLа2Zr2O7 structure with increasing temperature. The formation of lanthanum zirconate without the starting oxides is obviously due to the formation of an amorphous lanthanum zirconium compound in the form of spherical particles about 4 nm in diameter in the ion exchange step. Since the ion exchange of the lanthanum chloride solution leads to the formation of hexagonal La(OH)3 hydrosol in the form of long, nee dlelike crystals, whereas the ion exchange of the zir
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2θ, deg Fig. 4. Xray diffraction patterns of a dried hydrosol prepared using a solution of lanthanum and zirconyl chlorides and calcined at temperatures of (1) 300, (2) 500, (3) 700, (4) 900, (5) 1000, and (6) 1100°C.
conyl chloride solution leads to the formation of dis ordered zirconia in the form of spherical particles, the ion exchange of the solution of lanthanum and zirco nyl chlorides without interaction between the chlo rides would yield hydrosol containing both spherical ZrO2 · xH2O particles and long lanthanum hydroxide needles. The absence of needles in the hydrosol obtained from the solution of lanthanum and zirconyl chlorides suggests that there is interaction between the lanthanum and zirconium ions. It should be empha sized that, when the starting solution contains even a slight excess of lanthanum with respect to the stoichi ometric composition, the resultant hydrosol contains not only spherical particles of a lanthanum zirconium compound but also needlelike crystals. The ion exchange of dissolved lanthanum and zir conyl chlorides involves exchange of chloride ions for the hydroxyl groups of the anion exchanger, which leads to an increase in the pH of the suspension. In aqueous solutions, lanthanum ions are in the form of [La(H2O)n]3+ aqua complexes [14]. In the case of ion exchange between the lanthanum chloride solution and the anion exchanger, the plot of pH against stirring time (Fig. 1, curve 2) is characteristic of ion exchange of cat ions in the oxidation state 3+ [15, 16]. In aqueous zir conyl chloride solutions, the zirconium 4+ ions are present in the form of the [Zr4(OH)8(OH2)16]8+ tet ramer [17]. In the course of ion exchange, an increase
in pH may be accompanied by decomposition of the tetramer and, for example, the formation of the [Zr(OH)5(OH2)]1– monomeric aqua complex [18], which may be responsible for the stepwise variation in pH with suspension stirring time (Fig. 1, curve 3). The fact that the plot of pH against stirring time for the solution of lanthanum and zirconyl chlorides is not a combination of the corresponding curves for ion exchange in the individual solutions and the shorter time (3 min) needed to reach the maximum pH value in comparison with the individual solutions (6 and 14 min) can be interpreted as evidence of interaction between the lanthanum and zirconyl aqua complexes in the starting chloride solution, which ensures the formation of an amorphous lanthanum zirconium compound in the exchange step. Comparison of the present results with previous data [11, 12] demonstrates that, yielding pure lantha num zirconate, ion exchange synthesis is simpler and requires no organic compounds or solvents. CONCLUSIONS Ion exchange between an aqueous solution of lan thanum and zirconyl chlorides (La : Zr atomic ratio of 1 : 1) and the AV178 anion exchanger ensures the formation of a nanoparticulate hydrosol of an amor phous lanthanum zirconium compound. After drying, INORGANIC MATERIALS
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ACKNOWLEDGMENTS This work was supported by the RF Ministry of Edu cation and Science, grant agreement no. 14.576.21.0025, June 27, 2014. REFERENCES 1. Von Deiseroth, H.J. and MüllerBuschbaum, Hk., Ein Beitrag zur Pyrochlorstruktur an L2Zr2O7, Z. Anorg. Allg. Chem., 1970, vol. 375, pp. 152–156. 2. Mazilin, I.V., Baldaev, L.Kh., Drobot, D.V., Akhmet gareeva, A.M., Zhukov, A.O., and Khismatullin, A.G., Thermal and thermophysical properties of lanthanum zirconatebased heatresistant coatings, Perspekt. Mater., 2013, no. 7, pp. 21–30. 3. Sevast’yanov, V.G., Simonenko, E.P., Ignatov, N.A., Pavelko, R.G., and Kuznetsov, N.T., Synthesis and thermal stability of fineparticle refractory lanthanum and neodymium zirconates and hafnates for thermal barrier coatings, Kompoz. Nanostrukt., 2009, no. 1, pp. 50–58. 4. Xu, Z., He, L., Zhong, X., Zhang, J., Chen, X., Ma, H., and Cao, X., Effects of Y2O3 addition on the phase evo lution and thermophysical properties of lanthanum zir conate, J. Alloys Compd., 2009, vol. 480, no. 2, pp. 220– 224. 5. Seo, J.W., Interface formation and defect structures in epitaxial La2Zr2O7 thin films on (111) Si, Appl. Phys. Lett., 2003, vol. 83, no. 25, pp. 5211–5213. 6. Molina, L., Tan, H., Biermans, E.J., Batenburg, K., Verbeeck, J., Bals, S., and Van Tendeloo, G., Barrier efficiency of spongelike La2Zr2O7 buffer layers for YBCOcoated conductors, Supercond. Sci. Technol., 2011, vol. 24, pp. 1–7.
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7. Tong, Y., Zhu, J., Lu, L., Wang, X., and Yung, X., Prep aration and characterization of Ln2Zr2O7 (Ln = La and Nd) nanocrystals photocatalytic properties, J. Alloys Compd., 2008, vol. 465, nos. 1–2, pp. 280–284. 8. Chen, D. and Xu, R., Hydrothermal synthesis and characterization of La2M2O7 (M = Ti, Zr), Mater. Res. Bull., 1998, vol. 33, no. 3, pp. 409–417. 9. Nair, J., Nair, P., Doesburg, E.M.B., van Ommen, J.G., Ross, J.R.H., Burggraaf, A.J., and Mizukami, F., Prep aration and characterization of lanthanum zirconate, J. Mater. Sci., 1998, vol. 33, pp. 4517–4523. 10. Rao, K.K., Banu, T., Vithal, M., Swamy, G.Y.S.K., and Ravi Kumar, K., Preparation and characterization of bulk and nano particles of La2Zr2O7 and Nd2Zr2O7 by sol–gel method, J. Mater. Lett, 2002, vol. 54, nos. 2–3, pp. 205–210. 11. Kido, H., Komarneni, S., and Roy, R., Preparation of La2Zr2O7 by sol–gel route, J. Am. Ceram. Soc., 1991, vol. 74, no. 2, pp. 422–424. 12. Knoth, K., Hühne, R., Oswald, S., Schultz, L., and Holzapfel, B., Detailed investigations on La2Zr2O7 buffer layers for YBCOcoated conductors prepared by chemical solution, Acta Mater., 2007, vol. 55, pp. 517– 529. 13. Music, S. and ŠipaloZuljevic, J., Preparation of La(OH)3 colloids and tactoid structures, Colloid Polym. Sci., 1978, vol. 256, pp. 970–972. 14. Buz’ko, V.Yu., Sukhno, I.V., Buz’ko, M.V., Polushin, A.A., and Panyushkin, V.T., Study of the structure and stabil (n = 8, 9) by ab initio ity of aqua ions La(H2O)3+ n methods, Russ. J. Inorg. Chem., 2008, vol. 53, no. 8, pp. 1249–1255. 15. Tarasova, D.V., Bovina, E.A., Sergeev, A.M., Soderzhi nova, M.M., Dulina, R.S., and Chibirova, F.Kh., Syn thesis of cerium dioxide sols by an ion exchange method, Colloid. J., 2007, vol. 69, no. 2, pp. 227–231. 16. Bovina, E.A., Tarasova, D.V., Soderzhinova, M.M., Dulina, R.S., and Chibirova, F.Kh., Synthesis of yttrium hydroxide hydrosols, Russ. J. Inorg. Chem., 2011, vol. 56, no. 1, pp. 1–5. 17. Solovkin, A.S. and Tsvetkova, Z.N., The chemistry of aqueous zirconium salt solutions, Usp. Khim., 1962, vol. 31, no. 11, pp. 1394–1416. 18. Jolivet, J.P., Metal Oxide Chemistry and Synthesis from Solution to Solid State, Chichester: Wiley, 1988. ˆ
heat treatment of the hydrosol leads to the formation of La2Zr2O7 with the fluorite structure, which trans forms into the pyrochlore structure with increasing temperature. This approach can be used to prepare lanthanum zirconate powders. However, because of the low con centration of starting solutions, it is more reasonable to use the hydrosol as a precursor for the preparation of various types of coating.
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Translated by O. Tsarev
