Comparison of Aqueous and Non-aqueous Soft-Chemical Syntheses of Lithium Niobate and Lithium Tantalate Powders May Nyman,* Travis M. Anderson, and Paula P. Provencio Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 1036–1040
ReceiVed August 3, 2008; ReVised Manuscript ReceiVed NoVember 4, 2008
ABSTRACT: Lithium niobates and tantalates have a number of characteristics exploitable for optical and electrical devices that include ion conductivity, self-activating or dopant luminescence, piezoelectricity, pyroelectricity and ferroelectricity. To form these materials as nanometric powders or thin-film coatings, soft-chemical processing is required, and a limited number of procedures have been reported. We have explored two soft-chemical routes to lithium niobate and lithium tantalate materials: a non-aqueous procedure and an aqueous procedure. The non-aqueous procedure, which utilizes 1,4-butanediol and simple chemical precursors produces pure phase rhombohedral LiTaO3 and LiNbO3. For aqueous synthesis, we utilize the single-source polyoxoniobate salt: Li8[M6O19] · xH2O (M ) Nb, Ta) in a hydrothermal reaction. If LiOH is added, the product is rock-salt type Li3MO4. Without LiOH, other phases are observed. The products are characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, compositional analysis, and thermogravimetry, and the two soft-chemical processes are compared. Introduction “Soft-chemical” syntheses of functional metal oxide materials, in which the targeted phase is nucleated and grown directly from solution, are extensively pursued as alternatives to high temperature, solid-state processing. Soft-chemical syntheses offer numerous advantages for the formation of novel or advanced material forms including control over morphology (i.e., nanomaterials, coatings), phase, and composition. The controlled synthesis of nanomaterials and coatings, in particular, is imperative for the development of optical and electrical devices with ever-shrinking dimensions. Lithium niobates and tantalates, rhombohedral LiMO3 (M ) Nb, Ta) in particular, have found use in a number of modern technology applications that exploit their electro-optic, piezoelectric, pyroelectric, and ferroelectric characteristics. Furthermore, both LiMO3 and the trilithium compounds, Li3MO4 (M ) Nb, Ta), are noted for their ionconducting and luminescent behaviors both in their native and doped forms.1-7 Solution precipitation of nanometric powders of trigonal (R3c) LiNbO38 has been achieved solvothermally using a variety of coordinating solvents or mineralizers including poly(4-vinylpyridine),9 lithium hydroxide plus ammonia,10 lithium hydroxide plus oxalic acid,11,12 lithium hydroxide plus ethyleneglycol and surfactants,13 lithium hydroxide plus hydrogen peroxide,14 ammonia plus malic acid,15 lithium hydroxide plus amines,16 and benzyl alcohol.17 Li3NbO4, on the other hand, neither the disordered rock-salt phase18 nor the cubic (I43jm) ordered phase19 has ever been reported to be synthesized by methods alternative to high temperature, solid-state reactions. Soft-chemical processing of the lithium tantalate phases has been far-less studied. Amorphous lithium tantalate precursor powders have been precipitated, and LiTaO3 crystallizes from these powders upon annealing at 450-700 °C.20,21 LiTaO3 nanoparticles were formed from the amorphous powder if capping reagents were used to stabilize the precursor sol.22 Thin films of LiTaO3 were similarly made by annealing a sol-gel coating at 550 °C.23 Both nanoparticles and thin films of LiTaO3 were formed from a H2O2-modified sol-gel precursor, after * To whom correspondence should be addressed. E-mail: mdnyman@ sandia.gov.
annealing at 700 °C.24 Finally, the ordered monoclinic Li3TaO43 has only been synthesized by solid-state methods or annealing precursor powders.3,25 However, hydrothermal synthesis of disordered rock-salt type Li3-xHxTaO4 has been reported,26 with a Li/Ta ratio of 1.2. While the literature review above describes a multitude of solution chemistries from which lithium niobate or tantalate phases may be obtained, rarely are both compositions investigated using a single synthetic procedure. We have learned from niobate and tantalate cluster chemistry that despite the apparent structural similarities of these Group V oxides, they behave in surprisingly different ways, especially at the aqueous interface.27-29 Thus we are interested in developing soft-chemical syntheses of tantalate and niobate materials where a single synthetic procedure from which both tantalate and niobate end-members of a solid-solution can be formed. This offers the opportunity to vary and optimize material properties based on phase and composition.3,30-32 Here we present two general procedures: (1) An aqueous synthesis that takes advantage of lithium salts of [M6O19]8- (M ) Nb, Ta) polyoxoanion, and (2) a very simple non-aqueous, solvothermal synthesis. We have learned in this study that like niobate/tantalate aqueous cluster chemistry, there are differences in the niobate/tantalate materials obtained from aqueous solution. However, the lithium niobate and tantalate materials obtained from non-aqueous solution behave in an analogous fashion. While pure and highly crystalline forms are obtained from non-aqueous solution, only one phase is obtainable. Aqueous solutions on the other hand offer different lithium niobate and tantalate phases as a function of processing conditions, they are nanocrystalline, and it is more difficult to obtain a pure phase, as a reflection of the multitude of phases that are obtainable from aqueous solution. This report produces several firsts for soft-chemical syntheses: the first report of solution formation of rock-salt type Li3NbO4, and the first report of direct solution precipitation of crystalline LiTaO3. Experimental Section General Reagents and Procedures. LiOH · H2O was obtained from Fisher. Niobium ethoxide, tantalum ethoxide, 1,4-butanediol, and lithium acetate hydrate were obtained from Aldrich. Hydrous amorphous niobium pentaoxide for hexaniobate synthesis33 was a gift from CBMM (Companhia Brasileira de Metalurgia e Minerac¸a˜o), Brazil. X-ray
10.1021/cg800849y CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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Table 1. Summary of Aqueous Lithium Niobate/Lithium Tantalate Syntheses
1 2 3 4 5 6 7
solution composition
reaction time/temperature
no LiOH/Lithium hexaniobate 0.5 M LiOH/Lithium hexaniobate 1-4 M LiOH/Lithium hexaniobate 4 M LiOH/Lithium hexatantalate no LiOH/Lithium hexatantalate 0.0025-0.025 M LiOH/Lithium hexatantalate no LiOH/Lithium hexatantalate
1 day to 30 days/220 °C 1 day/220 °C 1-5 day/220 °C 3 day/220 °C 1 day/220 °C 4 h/220 °C 3 day/220 °C
powder diffraction was performed with a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Ni-filtered Cu KR radiation. Chemical analyses (Li, Nb, Ta) were performed by Galbraith, Inc., Knoxville, TN. Samples were examined with a JEOL JSM-6300V scanning electron microscope (SEM) equipped with a Link GEM Oxford detector and IRIDIUM IXRF Systems software for EDAX analysis. Samples for transmission electron microscopy (TEM) were prepared by very light grinding in a mortar/pestle, then covered with water. A TEM grid was placed and the bottom of the mortar and the sample was swirled resulting in a TEM grid with abundant small pieces of sample. High Resolution TEM (HR-TEM) was done on a JEOL 2010 F with Gatan energy filtered imaging, in the High Resolution Microscope User Facility at UNM in the Earth and Planetary Science Department. Synthesis of Li8[Nb6O19] · 22H2O. Lithium hexaniobate, Li8[Nb6O19] · 22H2O (FW ) 1311), was synthesized by a method similar to that which we reported earlier.34 One liter of a 1 molar LiOH solution is prepared (42 g LiOH · H2O). Eight grams of cesium hexaniobate, Cs8[Nb6O19] · 14H2O (synthesis reported earlier33), is dissolved in 15 mL of DI water. The cesium hexaniobate solution is added dropwise to the LiOH solution while stirring. Crystals of Li8[Nb6O19] · 22H2O start forming almost immediately and are left at room temperature for 4 days to ensure maximum yield. A white crystalline product is collected by pressure filtration and rinsing with DI water. Thermogravimetric analysis (TGA): 30 wt % volatile (H2O) (yield ) 4.16 g, 86% based on [Nb6O19]). Synthesis of Li8[Ta6O19] · 24H2O. Lithium hexatantalate, Li8[Ta6O19] · 24H2O (FW ) 1871). Hexagonal sodium hexatantalate27(1 g) was dissolved in 30 mL of hot (∼90 °C) DI water with stirring. This solution is added to 150 mL of 1 M LiOH solution and left to crystallize at room temperature for 4 days. The crystalline solid (0.8 g; yield ∼ 80% based on [Ta6O19]) was collected by pressure filtration. TGA: 23 wt % volatile (H2O). Synthesis of Li3MO4 (disordered rock salt; M ) Nb, Ta). Ten milliliters of a 4 molar LiOH solution (1.68 g LiOH · H2O) is placed in a 23 mL Teflon vessel for a Parr reactor. Li8[Nb6O19] · 22H2O or Li8[Ta6O19] · 24H2O, 0.2 g, is ground as a methanol slurry with a mortar and pestle and added to the LiOH solution. The Parr vessel is sealed and placed in a 225 °C oven for 5 days. Yield ) 0.15-0.19 g of rocksalt type Li3NbO4 or Li3TaO4 is collected. Chemical analyses for Li3NbO4, calcd: 52.2% Nb, 11.8% Li. Found: 49.7% Nb, 11.4% Li. Chemical analyses for Li3TaO4, calcd: 69.0% Ta, 7.7% Li. Found: 67.8 Ta, 7.3% Li. Non-aqueous synthesis of LiMO3 (M ) Nb, Ta). Lithium acetate hydrate (0.25 g, 2.45 mmol) and tantalum(V) ethoxide or niobium(V) ethoxide (2.45 mmol; 1.00 or 0.784 g, respectively) are combined in 10 g of 1,4-butanediol in a 23 mL Teflon vessel for a Parr reactor. The Parr vessel is sealed and placed in a 225 °C oven for 3-5 days. Freeflowing, fine white powders of LiMO3 are collected by centrifuging at 3500 RPM and washing with methanol. Additional Aqueous Syntheses of Lithium Niobate/Lithium Tantalate Phases. Phase formation was investigated in aqueous LiOH-lithium hexametalate solutions; predominantly with variation in LiOH concentration. In general, lithium hydroxide was dissolved in 10 mL of DI water, and combined with 0.1 to 0.2 g of lithium hexametalate in a 23 mL Teflon Parr reaction vessel. Solutions were reacted at 225 C for 1 to 14 days. Pertinent experiments are summarized in Table 1.
Results and Discussion Non-Aqueous Synthesis of Lithium Niobates and Tantalates. Non-aqueous precipitation of oxide materials is sometimes preferred over aqueous synthesis because of the
products 6.5 Å hydrous phase 6.5 Å hydrous phase rock-salt Li3NbO4 rock-salt Li3TaO4 6.5 Å hydrous phase 6.5 Å hydrous phase 6.5 Å hydrous phase
plus rock-salt Li3NbO4 plus rock-salt Li3NbO4 plus rock-salt Li3TaO4 plus rock-salt Li3TaO4 plus LiTaO3
relative simplicity of the solution chemistry. In aqueous synthesis pH, ionic strength, and spectator ions produce complex and unpredictable effects that sometimes render control difficult. On the other hand, non-aqueous solvents can undergo reactions at elevated temperature that result in viscous products that are dark in color. This process seems to be accelerated in the presence of redox-active metals such as vanadium. While this does not always affect the phase that forms, it can affect morphology, surface characteristics, and especially mask or alter optical properties. We and others35-38 have found 1,4-butanediol a very useful solvent for glycothermal synthesis of nanometric oxides. A number of oxide phases have been produced in this medium, and the solvent appears unaltered upon heating for extended time. The X-ray diffraction pattern of LiTaO3 formed in 1,4-butanediol is shown in Figure 1a; the diffraction pattern for LiNbO3 looks quite similar. The powders are single-phase and highly crystalline. The morphology can be observed in the SEM image in Figure 1b. The grains are very regular, ∼1 µm in size and disk-shaped. The synthetic procedure is exceedingly simple, and the precursors (alkoxide plus acetate) are not particularly hazardous or air sensitive. The reaction likely proceeds by ester elimination39 between the alkoxide and the acetate ligand to form the oxo-ligands:
LisO*OCCH3+NbsOC2H5 f LisO∗sNb + CO(CH3)(OC2H5)
(1)
where the “*” indicates the oxygen that is converted to an oxoligand. To our knowledge this is the first report to date of a soft-chemical synthesis of LiTaO3. Varying the Li/Nb or Li/Ta ratio (i.e., 3:1 Li/M) does not result in formation of other phases, such as Li3MO4. Therefore this synthesis is a very simple and reliable method to form LiMO3 but does not afford the flexibility to form related lithium niobates or tantalates that we obtain from the aqueous syntheses, discussed further below. Aqueous Synthesis of Lithium Niobates and Tantalates. The polyoxometalate Lindqvist ion (illustrated in Figure 2), [HxM6O19]8-x (x ) 1-3; M ) Nb, Ta) is charge-balanced by Li, Na, K, Rb, and Cs, and these salts are highly soluble in water.27,33,34 Thus, they have been shown to be ideal singlesource precursors or intermediates for aqueous synthesis of the related binary oxides such as perovskite NaNbO3 and KNbO3.40-42 We carried out a series of experiments where the hexametalate salts, Li8[Nb6O19] · 22H2O or Li8[Ta6O19] · 24H2O lithium, were hydrothermally treated in an aqueous solution with LiOH concentration ranging from 0-4 molar, see Table 1. The hexatantalate and hexaniobate behaved similarly in high LiOH concentration (i.e., > 0.5 M), forming the disordered, rock-salt type Li3MO4. A typical X-ray diffraction pattern is shown in Figure 3. With decreasing LiOH concentration, we observe a poorly crystalline hydrous phase, always mixed with rock-salt Li3MO4. A diffraction pattern of this phase is shown in Figure 4, its largest d-spacing is 6.5 Å, so we will refer to it as the 6.5 Å hydrous phase. This phase loses water weight in a thermogravimetric analysis (Figure 5), but since it always co-
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Figure 1. Reaction of lithium acetate with niobium or tantalum ethoxide in 1,4-butanediol produces crystalline, pure LiMO3 (M ) Nb, Ta). Left (1a) is an X-ray diffraction pattern of LiTaO3. Right (1b) is an SEM image of LiNbO3.
Figure 2. Ball-and-stick view of the Lindqvist ion, [M6O19]8- (M ) Nb, Ta), precursor for lithium niobate and lithium tantalate materials. Black spheres are Nb5+/Ta5+ and gray spheres are O2-.
Figure 4. Powder X-ray diffraction pattern of the unidentified 6.5 Å hydrous phase, formed by hydrothermal treatment of Li8[M6O19] · xH2O (M ) Nb, Ta) at with minimal LiOH added (see Table 1).
Figure 3. Powder X-ray diffraction pattern of disordered, rock-salt type Li3MO4 synthesized by hydrothermal treatment of Li8[M6O19] · xH2O (M ) Nb, Ta) with high concentration of LiOH (see Table 1).
Figure 5. TGA of the 6.5 Å lithium niobate layered phase (with some Li3NbO4 impurity) showing conversion to LiNbO3 at 600 °C. The weight loss (5%) is attributable to the layered phase.
precipitates with Li3MO4, the exact amount of water cannot be readily determined. In the TGA analysis, this phase undergoes an exothermic crystallization to LiMO3 at 600 °C. Thus we presume the Li/M ratio in the 6.5 Å hydrous phase is close to 1:1. Extensive hydrothermal treatment does not result in better crystallization nor does the product mixture convert entirely to
one of the anhydrous phases, Li3MO4 or LiMO3. This is unusual; hydrous phases tend to convert to condensed phases with increased hydrothermal treatment time. We have noted this earlier in the synthesis of niobate materials.43,44 Indexation of this phase was not possible because the diffraction peaks are very broad.
Syntheses of Lithium Niobate and Tantalate Powders
Figure 6. (left) SEM image and (right) TEM image of rock-salt type Li3NbO4.
Crystal Growth & Design, Vol. 9, No. 2, 2009 1039
phases other than LiNbO3 are favored. This was unexpected because an alkaline solution of niobium oxide, regardless of the precursor source, is dominated by the Lindqvist ion. That being said, it is also true that aqueous hydrothermal syntheses are extremely sensitive and the outcome likely altered by any parameter, including the precursor source. We found this to be the case in polyoxoniobate chemistry; when we used Nb2O5 · H2O as a precursor, we obtain isolated Keggin ion clusters, and when we use the [Nb6O19]8- Lindqvist ion as a precursor source under otherwise identical conditions, our Keggin ion clusters are linked together by [Nb2O2] bridges into a one-dimensional material.45,46 With the exception of the work reported by Yan et al.11 in 2008, other mineralizers, complexing or templating reagents were added to the alkaline aqueous mixture, which may have prevented the formation of the Lindqvist ion intermediate. Yan et al., however, did not provide enough detail on either the experiment or the product to determine which phase of lithium niobate was formed. Conclusions
Figure 7. Two views of nanotubes of the 6.5 Å lithium niobate hydrous phase.
The aqueous lithium tantalate system behaves slightly differently at low LiOH content. With less than 0.025 M LiOH and heating for less than a day, we observe the mixture of the 6.5 Å hydrous phase and Li3TaO4. However, hydrothermal treatment for 3 days results in LiTaO3 plus the 6.5 Å hydrous phase. We never observe the formation of LiNbO3 in our aqueous syntheses. The only other hydrothermal or solvothermal synthesis of the trilithiate phase, Li3MO4, was reported by Kumada et al. for the hydrothermal synthesis of lithium tantalates.26 They used what might be described as tantalic acid or a hydrous precipitate of tantalum oxide, along with LiOH for reactants. They also observed the rock-salt phase at higher LiOH concentration but with a Li/Ta ratio of only 0.22, very broad diffraction peaks, and a formula described as Li3-xHxTaO4. We obtained exactly the expected 3:1 ratio of Li/M (see Experimental Section) for both rock-salt Li3MO4 phases and reasonably sharp diffraction peaks. However, we utilized both higher LiOH concentration and a more soluble form of niobate/ tantalate. At lower LiOH concentration, they also observed an unknown phase that they indexed as triclinic, which does not seem to be related to our 6.5 Å hydrous phase. The typical morphology of the disordered rock-salt Li3MO4 precipitated from aqueous solution is shown in the SEM image in Figure 6a. These agglomerated, globular particles are around 200-500 nm in diameter. Lattice fringes are only weakly observed by HR-TEM (see Figure 6b), probably because the only high-Z element (Nb) is disordered with the low-Z lithium. Two views of the 6.5 Å hydrous phase are shown in the HRTEM images in Figure 7. It appears as if this phase forms nanotubes; otherwise the crystallinity is too poor to obtain any useful structural information. As outlined in the introduction, many soft-chemical syntheses of lithium niobate have been reported, and most of them are aqueous. All of the aqueous syntheses are alkaline, which favors solubility of niobium oxide. All of these syntheses report the formation of trigonal LiNbO3. It is somewhat surprising that by utilizing the [Nb6O19]8- Lindqvist ion precursor instead of hydrous niobium oxide, niobium alkoxide, or niobium halide
Two new soft-chemical synthetic routes were developed for solution precipitation of lithium niobate and lithium tantalate materials. Non-aqueous solvothermal synthesis in 1,4-butanediol using lithium acetate and niobium or tantalum ethoxide produced pure-phase, highly crystalline LiNbO3 or LiTaO3. The synthetic procedure is extremely simple to execute and safe; and this is the first report describing the formation of LiTaO3 without solidstate annealing or processing. We utilized the single-source Lindqvist ion precursor, Li8[M6O19] · xH2O (M ) Nb, Ta), for the aqueous synthesis of lithium niobate and lithium tantalate materials. While the Na and K salts of the niobate Lindqvist ion have been utilized as precursors for hydrothermal synthesis of NaNbO3 and KNbO3 perovskite, conversion of the [Ta6O19]8to oxide materials has not been explored, likely because of the difficulty in synthesizing these precursor salts. Likewise, synthesis of the lithium salts of [Nb6O19]8-34 and [Ta6O19]8has not been reported prior to our work, and therefore not available for further studies. This is the first report of direct precipitation of crystalline, rock-salt type Li3MO4 from solution, which was obtained via hydrothermal processing of Li8[M6O19] · xH2O. Future work on lithium niobate and lithium tantalate materials will include the introduction of heterometals to improve functionality, such as ion conductivity, and crystallinity for structural information on the 6.5 Å hydrous phase. Acknowledgment. This work was funded by Sandia National Laboratories’ LDRD program. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.
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CG800849Y
