1. bookVolume 34 (2016): Issue 1 (March 2016)
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2083-134X
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16 Apr 2011
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Raman spectroscopy and microwave dielectric properties of Sn substituted SrLa4Ti5O17 ceramics

Published Online: 27 Apr 2016
Volume & Issue: Volume 34 (2016) - Issue 1 (March 2016)
Page range: 1 - 5
Received: 24 Aug 2014
Accepted: 11 Nov 2015
Journal Details
License
Format
Journal
eISSN
2083-134X
First Published
16 Apr 2011
Publication timeframe
4 times per year
Languages
English
Abstract

SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics were fabricated through solid state ceramic route and their microwave dielectric properties were investigated in an attempt to tune their temperature coefficient of resonant frequency (τf) to zero. The compositions were sintered to single phase SrLa4Ti5O17 and SrLa4Ti4.5Sn0.5O17 ceramics at x = 0 and x = 0.5, and SrLa4Ti4−xSnxO17 along with a small amount of La2Ti2O7 at x = 1. The major phase observed at x = 2 was La2Ti2O7 but along with SrLa4Ti4SnO17 and SrLa4Ti4O15 as the secondary phases. τf decreased from 117 to 23.0 ppm/°C but at the cost of dielectric constant (εr) and quality factor multiplied by resonant frequency (Qufo) which decreased from 65 to 33.6 and 11150 to 4191 GHz, respectively. The optimum microwave dielectric properties, i.e. τf = 38.6 ppm/°C, εr = 45.5 and Qufo = 7919 GHz, correspond to the SrLa4Ti5−xSnxO17 composition with x = 1.

Keywords

Introduction

Lead free dielectric oxide ceramics with high electric permittivity (εr), high unloaded quality factor (Qu) and a near zero temperature coefficient of resonant frequency (τf) are critical elements in the components, such as resonators, oscillators and filters for wireless communication. For commercial applications, any material used as a dielectric resonator must have εr > 24, Qufo > 30,000 GHz and |τf| ≤ 3 ppm/°C [1]. For certain applications, such as antennas, the requirements for low τf and high Qufo are flexible but εr is generally required to be as high as possible to miniaturize the device so that it might be incorporated into a handset [2]. Recently, layered perovskites with a general formula AnBnO3n+2 (where A and B are cations) have received much attention due to their high dielectric performance and applications in patch antennas. Jawahar et al. [2] reported CaLa4Ti5O17 with εr = 53, τf = −20 ppm/°C and Qufo = 17359 GHz that was sintered at 1625 °C. The microwave dielectric properties of CaLa4Ti5O17 ceramics were improved by substituting Ca2+ ions with Zn2+ ions [4]. εr = 57, Qufo = 15,000 GHz, and τf = −8.16 ppm/°C were obtained for Ca0.99Zn0.01La4Ti5O17 ceramics with 0.5 wt.% CuO additive that were sintered at 1450 °C for 4 h [5]. On the other hand, SrLa4Ti5O17 was reported to have εr = 61, Qufo = 9969 GHz and τf = 117 ppm/°C [3]. The high positive τf precluded its use as dielectrically loaded antenna. In previous studies, Sm and Nd substitution for La in SrLa4Ti5O17 resulted in τf ~ 0 ppm/°C but at a cost of a decrease in Qufo to 3000 GHz and 6000 GHz, respectively [6, 7]. The substitution of Sn4+ for Ti4+ has been reported to lower τf of some compounds, for example, Ba4LaMNb3O15 (M = Ti, Sn) [8, 9] without any significant effect on the Qufo; therefore, in the present study, the effects of Sn4+ substitution for Ti4+ on the phase, microstructure and microwave dielectric properties of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics were investigated in an attempt to lower τf of the resulting compounds.

Experimental

SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) batch compositions were prepared by weighing the required amounts of SrCO3 (Aldrich, 99+ %), La2O3 (Aldrich, 99.95 %), TiO2 (Aldrich, anatase, 99+ %) and SnO2 (Aldrich, 99.95 %). The mixed batches were ball milled for 24 h in polyethylene disposable mill-jars using Y-toughened ZrO2 balls as grinding media and isopropanol as lubricant to make freely flowing slurries. The resulting slurries were dried in and oven kept at 95 °C for about 24 h and then calcined at 1250 °C to 1300 °C for 6 h at a heating/cooling rate of 5 °C/min. The calcined powders were finely ground in an agate pestle and mortar for 45 min and then pressed into 4 to 5 mm high and 10 mm diameter pellets in a steel die at 80 MPa. The pellets were sintered at 1450 to 1650 °C for 4 h at a heating/cooling rate of 5 °C/min. Densities of the sintered pellets were measured using Archimedes method. Phase analysis of the sintered samples was carried out using a Philips X-ray diffractometer (XRD) with CuKα radiation (λ = 1.5406 Å) operating at 30 kV and 40 mA at 1°/min in 2θ = 10 to 70° with a step size of 0.02°. A Renishaw’s inVia Raman microscope was used for Raman measurements. The microwave dielectric properties were measured using a R3767CH Agilent network analyzer by placing the cylindrical pellets on a low loss quartz single crystal at the center of an Au-coated brass cavity. τf was measured by measuring the temperature variation of TE01δ resonance mode in the temperature range of 20 to 80 °C.

Results and discussion

Fig. 1 shows the XRD patterns recorded at room temperature for optimally sintered SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics. The patterns of the compositions with x = 0, 0.5 and 1 were identical and matched with the one reported for SrLa4Ti5O17 (PDF# 57-00940) but with an appropriate shift in the peaks positions towards lower angles for the samples with x = 0.5 and 1 due to the presence of relatively larger Sn ions at the B-site of the perovskite unit cell. This indicated the formation of some SrLa4Ti4.5Sn0.5O17 and SrLa4Ti4SnO17 compounds containing isostructural phases with SrLa4Ti5O17 at x = 0.5 and 1. The presence of a few low intensity XRD peaks indicated the beginning of the formation of a secondary La2Ti2O7 (PDF# 70-1690) phase at x = 1. At x = 2, La2Ti2O7 (PDF# 70-1690) was observed as the major phase but along with SrLa4Ti4SnO17 and SrLa4Ti4O15 (PDF# 49-0254) as minor phases. The observed increase in the intensity of the XRD peaks due to La2Ti2O7 indicated an increase in their amount with increasing x.

Fig. 1

XRD patterns of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics, showing the formation of SrLa4Ti5O17, SrLa4Ti4.5Sn0.5O17 at x = 0 and 0.5, SrLa4Ti4Sn1O17 along with La2Ti2O7 at x = 1 and La2Ti2O7 and SrLa4Ti4O15 along with a few diminishing peaks due to SrLa4Ti4SnO17 at x = 2.

The secondary electron images (SEIs) of thermally etched and gold-coated surfaces of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics sintered at their optimum sintering temperatures are shown in Fig. 2. The microstructure of the compositions with x = 0, and 0.5 comprised of elongated grains of the size ranging from 1 × 1 μm2 to 5 × 7 μm2 and 1 × 3 μm2 to 5 × 20 μm2 (Fig. 2a and Fig. 2b). Semi-quantitative SEM EDS (Table 1) of the grains labeled as “A” and “B” indicated that the composition of these grains was close to SrLa4Ti5O17 and SrLa4Ti4.5Sn0.5O17, respectively. Generally, the microstructure of the composition with x = 1 also comprised of elongated rod and plate-shaped grains labeled as “c” (Fig. 2c), where the molar elemental composition of these grains was close to SrLa4Ti4SnO17 (Table 1). The microstructure of the composition with x = 2 comprised of micro-regions including cubical grains labeled as “D” (Fig. 2d). Semi-quantitative EDS (Table 1) indicated that the composition of these grains was close to La2Ti2O7 phase as the EDS detected little Sr or Sn in these grains. This suggested the presence of grains due to La2Ti2O7 phase as observed by XRD of the same composition. The observed variation in the composition of the grains (Table 1) combined with the morphological change from rod-shaped grains to cubical grains with an increase in Sn content indicated the formation of secondary phases.

Elemental composition (in moles) calculated from semi-quantitative EDS data of the grains labelled in Fig. 2.

GrainSrLaTiSn
A14.124.260
B14.104.200.6
C2.9510.179.361
D1.3513.9315.091

Fig. 2.

SEIs recorded from thermally etched, gold-coated sintered SrLa4Ti5−xSnxO17 compositions showing (a) elongated grains at x = 0, (b) elongated grains at x = 0.5, (c) elongated grains at x = 1, (d) cuboidal shaped grains at x = 2.

The change from one compound to another with compositions can also be proved by Raman spectra analysis.

Raman spectroscopy is considered to be an ideal tool for probing the degree of cation ordering, also suitable to study dynamic changes in a structure. Fig. 3 illustrates the Raman spectra of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics. The spectra are very similar to those of the related compounds in the Ca1−xZnxLa4Ti5O17 series [4]. Generally, corner shared and edge-shared octahedra are predominant in (Nb, Ti)–O polyhedra. In the corner-shared octahedra, the symmetric stretching vibrations are observed in the 750 to 850 cm−1 region [10].

Fig 3.

Raman spectra recorded at room temperature for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions.

In the present study, the highest frequency A1g mode at 793.08 cm−1 for x = 0 corresponds to the symmetric metal-oxygen stretching vibrations of the BO6 octahedra, but it is shifted towards lower frequency of 781 cm−1 as the value of x increased from 0 to 2, along with a decrease in its intensity. This is due to the incorporation of bigger cation of Sn+4 (0.69 Å) in place of smaller cation of Ti4+ (0.605 Å) [11] which induced a decrease in the force constant or the stiffness of the oxygen octahedral cage as a result of the covalent character of the central TiO6 octahedra [4]. The vibrational modes in the 750 to 850 cm−1 region, supported the existence of the corner shared octahedra in the SrLa4Ti5−xSnxO17 series of compounds. The XRD revealed La2Ti2O7 as the major phase and SrLa4Ti4O15 as the secondary phase with the increase in peak intensities. Since La2Ti2O7 has only one cation at the B site, no A1g Raman mode was active. The new Raman active mode band with increased intensity that appeared at 734 cm−1, suppressing the A1g mode at 781 cm−1 for x ≥ 1, could be attributed to the A1g mode of BO6 octahedra in SrLa4Ti4O15 ceramics that developed as a secondary phase along with La2Ti2O7 as a major phase. The weak band observed at 605 to 620 cm−1 range can be assigned to the B–O symmetric stretching vibration [12, 13]. The broad bands at 450 to 570 cm−1 can be represented as symmetric breathing of the BO6 octahedra [14]. The Eg modes in the range of 200 to 400 cm−1 have been assigned to O–B–O bending mode [12]. The modes in the range of 470 to 490 cm−1 were described as B–O torsional modes [15]. The modes at 314 and 464 cm−1 can be attributed to the rotating and tilting of the BO6 octahedron [15]. The intensity of the bands around 250 to 350 cm−1 increased with an increase in the x value and also shifted towards higher frequencies.

The modes below 250 cm−1 can be assigned to lattice vibrations of the A-site cations.

The microwave dielectric properties of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics sintered at their optimum sintering temperatures are shown in Fig. 4 and Fig. 5 and are also compared in Table 2. εr, τf and Qufo were observed to decrease from 65.0 to 33.8, 118 ppm/°C to 21.0 ppm/°C and 11150 GHz to 4191 GHz with an increase in Sn4+ content from 0 to 2, respectively. It has been reported that the substitution of Sn4+ for Ti4+ caused a decrease in εr, hence, τf in other compounds [9]. Therefore, the observed decrease in εr (Fig. 4) could be attributed to the less ionic dielectric polarizability (2.83 Å3) of Sn4+ in comparison to (2.93 Å3) of Ti4+[16]. However, for 1 ≤ x ≤ 2, the decrease in εr could be attributed to the formation of the secondary phase of La2Ti2O7 with εr = 22 [4]. The observed decrease in Qufo (Fig. 5) upon increasing Sn content from 1 to 2 may be due to the formation of secondary phases, whose interface causes additional dielectric loss [10]. τf decreased from 117 to 23.0 ppm/°C (Fig. 4) with the increase in x from 0 to 2 but at the cost of εr and Qufo which decreased from 65 to 33.6 and 11150 to 4339 GHz, respectively, due to the formation of secondary phases which made higher concentrations of Sn unfavorable. Thus, the optimum microwave dielectric properties (τf = 39 ppm/°C, εr = 46 and Qufo = 7900 GHz), corresponded to the SrLa4Ti5−xSnxO17 composition with x = 1 with minimum Sn content, hence, the secondary phases.

Preparation conditions, apparent densities and microwave dielectric properties of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2).

xCT[°C]ST[°C]ρexp[g/cm3]εrQufo [GHz]τf [ppm/°C]
01250/6h1500/4h5.4165.011150+118
0.51300/6h1600/4h5.6356.29450+54.5
11300/6h1625/4h5.6048.28278+35.6
21300/6h1625/4h5.7533.84345+21.0

CT = Calcination temperature, ST = Sintering temperature, ρap = Apparent density

Fig. 4

Variation in εr and τf versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.

Fig. 5

Variation in Qufo versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.

Conclusions

SrLa4Ti5−xSnxO17 compositions crystallize into single phase ceramics at x = 0, and x = 0.5, while the formation of a small amount of second phase begins at x = 1. The substitution of Sn for Ti causes a substantial decrease in τf but at a cost of εr and Qufo due to the second phase formation, which makes higher concentration of Sn unfavorable. The optimum microwave dielectric properties, i.e. τf = 35.6 ppm/°C, εr = 48.6 and Qufo = 8278 GHz, correspond to the SrLa4Ti5−xSnxO17 composition with x = 1.

Fig. 1

XRD patterns of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics, showing the formation of SrLa4Ti5O17, SrLa4Ti4.5Sn0.5O17 at x = 0 and 0.5, SrLa4Ti4Sn1O17 along with La2Ti2O7 at x = 1 and La2Ti2O7 and SrLa4Ti4O15 along with a few diminishing peaks due to SrLa4Ti4SnO17 at x = 2.
XRD patterns of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) ceramics, showing the formation of SrLa4Ti5O17, SrLa4Ti4.5Sn0.5O17 at x = 0 and 0.5, SrLa4Ti4Sn1O17 along with La2Ti2O7 at x = 1 and La2Ti2O7 and SrLa4Ti4O15 along with a few diminishing peaks due to SrLa4Ti4SnO17 at x = 2.

Fig. 2.

SEIs recorded from thermally etched, gold-coated sintered SrLa4Ti5−xSnxO17 compositions showing (a) elongated grains at x = 0, (b) elongated grains at x = 0.5, (c) elongated grains at x = 1, (d) cuboidal shaped grains at x = 2.
SEIs recorded from thermally etched, gold-coated sintered SrLa4Ti5−xSnxO17 compositions showing (a) elongated grains at x = 0, (b) elongated grains at x = 0.5, (c) elongated grains at x = 1, (d) cuboidal shaped grains at x = 2.

Fig 3.

Raman spectra recorded at room temperature for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions.
Raman spectra recorded at room temperature for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions.

Fig. 4

Variation in εr and τf versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.
Variation in εr and τf versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.

Fig. 5

Variation in Qufo versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.
Variation in Qufo versus Sn4+ content (x) for SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2) compositions sintered at their optimum sintering temperatures.

Preparation conditions, apparent densities and microwave dielectric properties of SrLa4Ti5−xSnxO17 (0 ≤ x ≤ 2).

xCT[°C]ST[°C]ρexp[g/cm3]εrQufo [GHz]τf [ppm/°C]
01250/6h1500/4h5.4165.011150+118
0.51300/6h1600/4h5.6356.29450+54.5
11300/6h1625/4h5.6048.28278+35.6
21300/6h1625/4h5.7533.84345+21.0

Elemental composition (in moles) calculated from semi-quantitative EDS data of the grains labelled in Fig. 2.

GrainSrLaTiSn
A14.124.260
B14.104.200.6
C2.9510.179.361
D1.3513.9315.091

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