Raman spectroscopy and microwave dielectric properties of Sn substituted SrLa₄Ti₅O₁₇ ceramics

Lead free dielectric oxide ceramics with high electric permittivity (ε_r), high unloaded quality factor (Q_u) and a near zero temperature coefficient of resonant frequency (τ_f) 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 ε_r > 24, Q_uf_o > 30,000 GHz and |τ_f| ≤ 3 ppm/°C [1]. For certain applications, such as antennas, the requirements for low τ_f and high Q_uf_o are flexible but ε_r 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 A_nB_nO_3n+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 ε_r = 53, τ_f = −20 ppm/°C and Q_uf_o = 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]. ε_r = 57, Q_uf_o = 15,000 GHz, and τ_f = −8.16 ppm/°C were obtained for Ca_0.99Zn_0.01La₄Ti₅O₁₇ 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 ε_r = 61, Q_uf_o = 9969 GHz and τ_f = 117 ppm/°C [3]. The high positive τ_f precluded its use as dielectrically loaded antenna. In previous studies, Sm and Nd substitution for La in SrLa₄Ti₅O₁₇ resulted in τ_f ~ 0 ppm/°C but at a cost of a decrease in Q_uf_o to 3000 GHz and 6000 GHz, respectively [6, 7]. The substitution of Sn⁴⁺ for Ti⁴⁺ has been reported to lower τ_f of some compounds, for example, Ba₄LaMNb₃O₁₅ (M = Ti, Sn) [8, 9] without any significant effect on the Q_uf_o; therefore, in the present study, the effects of Sn⁴⁺ substitution for Ti⁴⁺ on the phase, microstructure and microwave dielectric properties of SrLa₄Ti_5−xSn_xO₁₇ (0 ≤ x ≤ 2) ceramics were investigated in an attempt to lower τ_f of the resulting compounds.

SrLa₄Ti_5−xSn_xO₁₇ (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. τ_f was measured by measuring the temperature variation of TE_01δ resonance mode in the temperature range of 20 to 80 °C.

Fig. 1 shows the XRD patterns recorded at room temperature for optimally sintered SrLa₄Ti_5−xSn_xO₁₇ (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 SrLa₄Ti_4.5Sn_0.5O₁₇ 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

The secondary electron images (SEIs) of thermally etched and gold-coated surfaces of SrLa₄Ti_5−xSn_xO₁₇ (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 SrLa₄Ti_4.5Sn_0.5O₁₇, 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.

Fig. 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 SrLa₄Ti_5−xSn_xO₁₇ (0 ≤ x ≤ 2) ceramics. The spectra are very similar to those of the related compounds in the Ca_1−xZn_xLa₄Ti₅O₁₇ 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.

In the present study, the highest frequency A_1g 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 SrLa₄Ti_5−xSn_xO₁₇ 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 A_1g Raman mode was active. The new Raman active mode band with increased intensity that appeared at 734 cm⁻¹, suppressing the A_1g mode at 781 cm⁻¹ for x ≥ 1, could be attributed to the A_1g 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 E_g 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 SrLa₄Ti_5−xSn_xO₁₇ (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. ε_r, τ_f and Q_uf_o 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 ε_r, hence, τ_f in other compounds [9]. Therefore, the observed decrease in ε_r (Fig. 4) could be attributed to the less ionic dielectric polarizability (2.83 ų) of Sn⁴⁺ in comparison to (2.93 ų) of Ti⁴⁺[16]. However, for 1 ≤ x ≤ 2, the decrease in ε_r could be attributed to the formation of the secondary phase of La₂Ti₂O₇ with ε_r = 22 [4]. The observed decrease in Q_uf_o (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]. τ_f decreased from 117 to 23.0 ppm/°C (Fig. 4) with the increase in x from 0 to 2 but at the cost of ε_r and Q_uf_o 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 (τ_f = 39 ppm/°C, ε_r = 46 and Q_uf_o = 7900 GHz), corresponded to the SrLa₄Ti_5−xSn_xO₁₇ composition with x = 1 with minimum Sn content, hence, the secondary phases.

Fig. 4

Fig. 5

SrLa₄Ti_5−xSn_xO₁₇ 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 τ_f but at a cost of ε_r and Q_uf_o due to the second phase formation, which makes higher concentration of Sn unfavorable. The optimum microwave dielectric properties, i.e. τ_f = 35.6 ppm/°C, ε_r = 48.6 and Q_uf_o = 8278 GHz, correspond to the SrLa₄Ti_5−xSn_xO₁₇ composition with x = 1.