Hydrothermal synthesis and characterization of the layered titanates MLaTiO4 (M = Li, Na, K) powders

Materials Research Bulletin, Vol. 34, No. 5, pp. 685– 691, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00075-6

HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION OF THE LAYERED TITANATES MLaTiO4 (M ⴝ Li, Na, K) POWDERS Dairong Chen1*, Xiuling Jiao1, and Ruren Xu2 Department of Chemistry, Shandong University, Jinan 250100, P.R. China 2 Department of Chemistry, Jilin University, Changchun 130023, P.R. China 1

(Refereed) (Received March 19, 1998; Accepted July 28, 1998)

ABSTRACT Layered titanates MLaTiO4 (M ⫽ Li, Na, K) powders were hydrothermally synthesized, derived from double hydrous oxides La2O3䡠2TiO2䡠nH2O in a basic medium. The formation conditions of these compounds with a pure phase were investigated. Details such as the structural and particulate properties of these powders were obtained from X-ray diffraction (XRD), IR spectroscopy, high-resolution electron microscopy (HREM), and other techniques. © 1999 Elsevier Science Ltd KEYWORDS: A. electronic materials, A. layered compounds, A. oxides, B. chemical synthesis INTRODUCTION Numerous layered titanates have been discovered. Particular interest has been in the perovskite AA⬘BO4 compounds, because of their ionic conductivity, luminescence, and two-dimensional physical properties, which are attributed to the A-site cation. These layered compounds are usually synthesized by the solid-state reaction, such as in the synthesis of NaLaTiO4, derived from a mixture of sodium carbonate, rare-earth oxide, and titanium oxide [1,2]. It is common knowledge that the layered compound, one metastable phase, can either transform into the stable phase at high temperature or decompose below the synthetic temperature. However, high temperature is necessary for

*To whom correspondence should be addressed. 685

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the solid-state reaction. Thus, the product is usually contaminated due to its decomposition at high temperature, and it is difficult to synthesize some of these compounds by solid-state reaction [3]. In a previous study [4], the ion exchange reaction was considered to be an effective process for the syntheses of certain layered compounds. Using this method, Toda et al. [4] prepared LiLaTiO4 powders, derived from NaLaTiO4, and Byeon et al. [5–7] synthesized a family of protonated oxides HLnTiO4 and hydrated HLnTiO4, derived from NaLnTiO4 (Ln ⫽ lanthanides). However, the difficulty in using this synthesis method for some layered compounds is that the ion exchange reaction depends on the properties of the parent-layered compound and exchanged ions. Hydrothermal synthesis is regarded as being superior to other methods for synthesizing metastable materials. One reason for this is that many molecular sieves and layered compounds can be prepared by this method [8]. In this study, we present the hydrothermal synthesis of isomorphous crystalline LiLaTiO4, NaLaTiO4, and KLaTiO4 powders. The new KLaTiO4 compound, which was synthesized for the first time, is also discussed. The purpose of this research was to find a new process for the synthesis of these perovskite-layered compounds and determine the borderline reaction conditions. Moreover, the structural properties of the products are characterized in detail to compare with those of products prepared by other methods.

EXPERIMENTAL Synthesis. All the reagents were of analytical grade and were not further purified before utilization. Into the mixed solution of lanthanum chloride (⬎99.9%) and titanium chloride (ⱖ99.5%), in which the molar ratio of La to Ti was 1:1, extensively dilute ammonia was added under magnetic stirring. The formed coprecipitate La2O3䡠2TiO2䡠 nH2O was filtered under reduced pressure and thoroughly washed with pure water. Some La2O3䡠2TiO2䡠nH2O was added into 0.4 mol䡠dm⫺3 of MOH (M ⫽ Li, Na, K, Rb) solution, to form a suspension solution. 16 cm3 of the suspension solution, in which the La concentration was 0.1 mol䡠dm⫺3, was poured into a 20 cm3 Teflon-lined stainless steel autoclave. The autoclave was heated at a predetermined temperature and held for a set time. After the autoclave had cooled to room temperature, the precipitate was filtered and thoroughly washed with pure water, then dried at ambient temperature overnight. Characterization. Powder X-ray diffraction (XRD) patterns were measured on a Rigaku D/Max-IIIA diffractometer equipped with a graphite monochrometer using Cu K␣ radiation (␭ ⫽ 0.15418 nm). Chemical analyses of the metal content in the products were made by inductively coupled plasma atomic emission spectroscopy (Leemen Labs, ICP-1000). A transmission electron microscope (Hitachi, H-8100IV) equipped with an energy dispersive X-ray (EDX) microanalyzer was used to observe the particulate morphologies and determine the components of the product. IR spectra were recorded on a Nicolet FT-IR spectrometer, and differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed with a Perkin-Elmer DTA-1700 and TGA-7 analyzer system at a heating rate of 5.0°C䡠min⫺1 in air.

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FIG. 1 XRD patterns of the layered compounds (a) LiLaTiO4, (b) hydrate NaLaTiO4, (c) hydrate KLaTiO4, (d) NaLaTiO4, and (e) KLaTiO4. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the hydrothermal products. It is seen that the distances between two neighboring layers in the NaLaTiO4 and KLaTiO4 compounds are longer than previously reported values [2,3]. Considering the possible influence of water molecules, especially of interlayered hydration water, on the crystal structures of the compounds, the products were heated to 400°C and maintained at that temperature for 2 h before being examined by XRD. XRD patterns of these powders after dehydration were essentially identical with those of the NaLaTiO4 and KLaTiO4 (Fig. 1(d) and (e)) structurally characterized in previous works [2– 4]. The ICP results as well as EDX analyses are in good agreement with the formula MLaTiO4. This indicates that the desired powders were obtained. To determine suitable reaction conditions for the formation of these powders with a pure phase, the reactions were carried out at 160, 180, 200, 220, and 240°C for 48, 72, 148, and

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TABLE 1 The Reaction Conditions for the Formation of Crystalline MLaTiO4 Powders with a Pure Phase Sample LiLaTiO4 NaLaTiO4 KLaTiO4

Temperature (°C)

Time (h)

Alkali concentration (mol䡠dm⫺3)

180–200 180–220 180–200

ⱖ148 ⱖ72 ⱖ48

0.4–0.6a 0.3–0.6 0.4–0.6

The precursor concentration (in La) was 0.1 mol䡠dm⫺3 for all reactions. KOH concentration, but the LiOH concentration was 0.1 mol䡠dm⫺3.

a

160 h, respectively, in various basic media. The pyrochlore La2Ti2O7 or rutile TiO2 was produced when the alkaline concentration exceeded 1.0 mol䡠dm⫺3, but slightly basic media gave the mixed precipitate of the La(OH)3 and TiO2. To prepare crystalline MLaTiO4 powders, a suitable concentrated alkaline solution was necessary. Further experiments showed that a reaction temperature higher than 180°C was necessary, although when the reaction was carried out at 160°C, the desired powders were produced. Pure phase powders in which La2Ti2O7 was present were not obtained by increasing the reaction temperature to over 240°C. In order to synthesize LiLaTiO4, KOH must be used to catalyze the reaction. After some trial and error, the borderline reaction conditions for the formation of the layered compounds with a pure phase were determined and are summarized in Table 1. Unfortunately, we failed to synthesize the RbLaTiO4 compound by the hydrothermal reaction. Strong absorption bands of IR spectra centered at about 3340 cm⫺1 and 1680 cm⫺1 for the Na and K compounds, respectively, before being dried at 400°C indicated that these powders contained water molecules. The IR spectrum for the Li compound showed that it did not contain any water molecules. All of the IR spectra showed a wide and flat, strong absorption peak below 700 cm⫺1. The perovskite structure is usually characterized by octahedral TiO6 groups. According to calculations, the wavenumbers for the absorption bands of TiO6 are lower than 700 cm⫺1. The bands above 405 cm⫺1 are assigned to Ti–O stretching vibration, and those below 405 cm⫺1 are attributed to Ti–O bending vibration [9]. However, the strong absorption band centered at about 900 cm⫺1 is considered to be due to Ti–O stretching vibration because of the effect of the superstructure in the compound [10]. After these products were dried, the absorption bands assigned to the water molecules disappeared, and the characteristic bands of the Na and K compounds corresponding to the Ti–O stretching vibration shifted to higher wavenumber. According to the DTA and TGA curves of the hydrothermal products, the K compound showed a weight loss of 0.8% of adsorbed surface water molecules from room temperature to 180°C and a weight loss of 6.0% of interlayer hydration water molecules from 180 to 380°C. The Na compound showed a 3.2% weight loss of interlayer hydration water in addition to a 1.2% weight loss of adsorbed surface water. However, the DTA and TGA curves showed that the Li compound did not exist in any hydrous form, because the weight remained constant when the temperature was below 400°C. The weight losses at high temperature for all compounds were due to the evaporation of the alkali metals. The larger interlayer space for KLaTiO4 compound allows more water molecules to interlay and the small interlayer space for LiLaTiO4 does not allow insertion of water molecules. Thus, the

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TABLE 2 Lattice Parameters for the Compounds Studied Sample

a (nm)

c (nm)

V (nm3)

c/a

LiLaTiO4 NaLaTiO4 KLaTiO4 NaLaTiO4䡠0.5H2O KLaTiO4䡠H2O

0.3773 0.3776 0.3863 0.3892 0.3921

1.2080 1.3020 1.3606 1.3231 1.3626

0.1720 0.1856 0.2030 0.2004 0.2095

3.20 3.45 3.52 3.40 3.48

hydrated Na and K compounds are defined by the formula NaLaTiO4䡠0.5H2O and KLaTiO4䡠 H2O, respectively. An exothermic peak centering at 792, 1110, and 808°C on the DTA curve, corresponding to the Li, Na, and K compounds, respectively, was assigned to the decomposition of the compound. The decomposition was confirmed by XRD patterns of the compounds heated at different temperatures. LiLaTiO4 decomposes into the stable phase perovskite LiLaTi2O6 when the heating temperature is greater than 800°C [11], and the Na and K compounds transform into another type of layered M2La2Ti3O10 compounds [12]. The transformation temperature for the NaLaTiO4 compound is 1120°C and that of KLaTiO4 is 815°C. This is evidence that LiLaTiO4 and KLaTiO4 cannot be prepared by the solid-state reaction due to their decomposition at high temperature. The lattice parameters were determined from the XRD patterns of the crystalline powders (scanning rate 0.1°䡠min⫺1, using KCl as internal standard) and are summarized in Table 2. The lattice parameters a, c, and c/a of the compounds increased with the increasing M⫹ radii. The MLaTiO4 construction can be regarded as being derived from the K2NiF4 structure by ordering of the M⫹ and La3⫹ ions among the layer cation sites in which the double layers of La3⫹ and M⫹ are perpendicular to the c axis. In general, the c/a ratio for the structure type of K2NiF4 phase was found to be 3.3 ⫾ 0.1. The high c/a ratio observed for KLaTiO4 is caused by the small value of the cell parameter a; it is presumed that there is considerable pressure on the Ti–O equatorial distances, which results in the deformation of TiO6 octahedra. Na–O in NaLaTiO4 has a rock-salt-type coordination, whereas LiLaTiO4 consists of an intergrowth of perovskite sheets with layers of Li–O in a tetrahedral coordination and those of La–O in a distorted rock-salt coordination [4]. Thus, LiLaTiO4 and KLaTiO4 are less stable, compared to NaLaTiO4. Only in considering c/a might the insertion of water molecules be favorable to the stability of the layered structure. After being dried at 400°C for 2 h, these powders were dispersed in acetone. TEM observations revealed that the particulate morphology of NaLaTiO4 (length: 500 – 650 nm, width: 40 –50 nm) appeared to be lathe-like, and that of LiLaTiO4 (length: 230 –340 nm, width: 140 –210 nm) and KLaTiO4 (length: 180 –220 nm, width: 120 –160 nm), plate-like. Electron diffraction was used to correlate the powder data with the additional information available from single crystal diffraction. Using the first higher-order Laus zone, an approximate spacing for the (001) plane was obtained for LiLaTiO4 (1.209 nm), NaLaTiO4 (1.306 nm), and KLaTiO4 (1.341 nm). The long c axes are consistent with the X-ray diffraction data. This result is also supported by the HREM investigation. The patterns of parallel lines along the (001) plane in Figure 2 reveal the layered structures of the LiLaTiO4, NaLaTiO4, and

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FIG. 2 HREM of the layered compounds (a) LiLaTiO4, (b) NaLaTiO4, and (c) KLaTiO4. KLaTiO4 compounds. The distances between the two neighboring layers in the layered compounds are 1.20, 1.30, and 1.35 nm, respectively. CONCLUSIONS The layered perovskite MLaTiO4 (M ⫽ Li, Na, K) powders with a pure phase were hydrothermally prepared and characterized by XRD, IR, HREM, and other techniques. NaLaTiO4 is more stable than the corresponding Li and K compounds, which for the first time have been obtained by direct synthesis. The lattice parameters of the compounds increase with increasing M⫹ radii. More hydration water molecules enter into KLaTiO4, due to its larger interlayer space; the small interlayer space in LiLaTiO4 compound does not allow

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water molecules to be inserted. HREM observations confirmed the two-dimensional layered structures of these compounds. ACKNOWLEDGMENT The authors sincerely thank the Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, for financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

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