Materials Research Bulletin 47 (2012) 883–888

Microwave dielectric properties of new SrLa4�xNdxTi5O17 ceramics

Abdul Manan, Yaseen Iqbal *

Materials Research Laboratory, Institute of Physics & Electronics, University of Peshawar, Pakistan, Post Code 25120, Pakistan

A R T I C L E I N F O

Article history:

Received 13 June 2011

Received in revised form 21 October 2011

Accepted 10 November 2011

Available online 1 December 2011

Keywords:

A. Ceramics

B. Dielectric properties

A B S T R A C T

New microwave dielectric ceramics in the SrLa4�xNdxTi5O17 (0 � x � 4) composition series were

prepared through a solid state mixed oxide route. All the compositions formed highly dense (�95%)

single phase ceramics upon sintering at 1500–1580 8C. The molar volume and theoretical density

decreased due to the substitution of small (1.27 A) Nd ions for large (1.36 A) La ions. This decrease was

associated with a decrease in the dielectric constant (er), temperature coefficient of resonant frequency

(tf) and quality factor (Qufo). An analysis of properties achieved in the present study indicated that

ceramics exhibiting nearly zero tf corresponding to er � 54 could be fabricated in the SrLa4�xNdxTi5O17

composition series at x � 1.6; however, further work is required to improve Qufo (�6000 GHz) for possible

practical applications.

� 2011 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Recent technological developments in the wireless telecom-munication systems utilizing microwave dielectric ceramics asresonators, filters and other components have increased interest indesigning and engineering of new materials for better performanceand miniaturization of the microwave components. Ideal materialsfor commercial applications as dielectric resonators (DR) arerequired to have a high dielectric constant, er > 24, tf � 0, and ahigh quality factor at microwave frequencies, Qufo (�30,000). Forcertain applications, e.g. antennas, the values of tf and Qufo can becompromised to �10 ppm/8C and >10,000 GHz respectively; how-ever, er must be high enough to miniaturize the device forincorporation into a handset. Additionally, as given by Eq. (1), theuse of a high er material is important for reducing the size and weightand hence manufacturing and operational cost of the electronicequipment [1].

ld/1

ðerÞ1=2(1)

where ld is the wavelength in a dielectric given by ld = lo/(er)1/2.

Several materials have got their commercial applications as DRsin handsets and base stations but there is still a constant search formaterials with ultra low losses, tf � 0 and high dielectric constants,er > 50 [2,3].

Dielectric ceramics with the general formula AnBnO3n+2 havebeen investigated for practical applications as DRs exhibiting high

* Corresponding author. Tel.: +92 91 5704615; fax: +92 91 9216473;

mobile: +92 3009016451.

E-mail address: dryaseeniqbal@yahoo.co.uk (Y. Iqbal).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.11.060

er. Recently, Manan et al. [4] investigated the dielectric propertiesof Sr5�xCaxNb4TiO17 (x = 0–5) ceramics and Sr2Ca3Nb4TiO17 wasreported to exhibit optimum properties with er � 53 andtf � �6.5 ppm/8C but Qufo (�1166 GHz) was too low for microwaveapplications. On the other hand, Ca5Nb4TiO17 and Ca5Ta5TiO17

have also been reported to have high er (44.9 and 40.1), good Qufo

values (17,600 and 16,450 GHz) but high negative temperaturecoefficient of resonant frequencies (i.e. tf � �112.9 and�53.6 ppm/8C) respectively preclude their commercial use [5].Other compositions like La4NdCrTi4O17 and La4SmCrTi4O17 havebeen reported to have er = 53, Qufo = 8800 GHz and tf � �27.6 ppm/8C and er = 50, Qufo = 6900 GHz and tf � �39.5 ppm/8C respectively[6]. Similarly, Jawahar et al. [7] reported CaLa4Ti5O17 with er � 53tf � �20 ppm/8C and Qufo � 17,359 GHz. SrLa4Ti5O17 was alsoreported to have er � 39.1, tf � 58 ppm/8C and Qufo � 14,200 [8].In most of the above reported compositions, the high tf precludestheir use as DRs in telecommunication devices. Gao et al. [9]reported that Sm substitution decreased tf from 80.9 ppm/8C to�30.4 ppm/8C while investigating the effect of Sm substitution forLa in Ba4La9.33(Ti0.95Zr0.05)18O54. Similarly Solomon et al. [10]reported lower tf for composition containing Nd and Sm incomparison to La in BaRE2Ti4O12 and BaRE2Ti5O14 (where RE = rareearths). The decrease in tf for Nd and Sm substituted compositionsfor La originated from smaller ionic radii of Nd and Sm incomparison to La. Yue et al. [11] reported that Nd substitution forLa in CaLa4Ti4O15 improved the temperature stability of theresonant frequency, i.e. tf decreased from �17.1 ppm/8C to�5.77 ppm/8C as the Nd concentration was increased from 0 to 0.5.

In this paper, the microwave dielectric properties of SrLa4�xNdx-

Ti5O17 have been investigated in an attempt to process tempera-ture stable microwave dielectric ceramics for possible commercialapplications.

A. Manan, Y. Iqbal / Materials Research Bulletin 47 (2012) 883–888884

2. Experimental procedure

A number of compositions in the SrLa4�xNdxTi5O17 series with0 � x � 4 were prepared using a solid state mixed oxide route.SrCO3 (Aldrich, 99+%) was dried at �185 8C while La2O3 (Aldrich,99.95%), Nd2O3 (Aldrich, 99.95%) and TiO2 (Aldrich, 99+%) weredried at 900 8C overnight to remove the moisture prior to weighingto ensure initial stoichiometery of compounds. The driedcarbonates and oxides were weighted in stoichiometric ratiosand wet ball milled for 24 h in disposable polyethylene jars usingY-toughened ZrO2 balls as grinding media and isopropanol as alubricant to make freely flowing slurry. The slurry was dried in anoven at �95 8C overnight. The resulting powders were sieved andcalcined at 1250 8C for x = 0 (i.e. SrLa4Ti5O17) and 1350 8C for 6 h forthe compositions with x = 1, 2, 3 and 4 at a heating/cooling rate of5 8C/min. The calcined powder was ground in a mortar and pestlefor about 45 min in isopropanol to make fine powder. A smallamount of distilled water was added to the powder to avoidcracking of the pellets during pressing. The powder was pressedinto 10 mm diameter pellets of �4–5 mm height at 80 MPa. Thepellets were placed on platinum foils and sintered at 1500–1580 8Cfor 4 h at a heating/cooling rates of 5 8C/min. Phase analysis ofcalcined and sintered crushed pellets was carried out using aPhilips X-ray diffractometer operating at 30 kV and 30 mA at 18/min from 2u = 10–708 with a step size of 28. A STOE PSD X-raydiffractometer with Cu Ka radiation (l = 1.540598 A) was used forthe measurement of lattice parameters. Dense sintered pelletswere cut and finely polished before thermal etching for 30 min attemperatures �10% less than their corresponding sinteringtemperatures at a heating/cooling rate of 5 8C/min for scanningelectron microscopy (SEM). The etched surfaces of the sampleswere gold-coated to avoid charging and a JEOL 6400 SEM operatingat 20 kV was used for microstructural examination. The bulkapparent densities of the sintered pellets were measured usingArchimedes method. The theoretical densities of the compoundswere calculated using Eq. (2):

rth ¼ZM

VAg(2)

where Z is formula unit, M is the molecular weight, V is the volumeof the unit cell and Ag is the Avogadro number(6.022 � 1023 atoms/mol).

Fig. 1. X-ray diffraction patterns recorded from crushed pellets of SrLa4�xNdxTi5O17 (0 �compositions with x = 1–3, showing single phase formation for each composition with

Microwave dielectric properties were measured using a networkanalyzer (Agilent R3767CH) by cavity method. Cylindrical sampleswere placed at the centre of the cavity made of brass on a low lossquartz single crystal to avoid conduction loss from the walls of thecavity. The dimensions of the test cavity were �3 times larger thanthe dimensions of the DR and the TE01d mode of the DR was thelowest resonance mode and could be easily identified. Also thesample was placed away from the walls of the cavity to ensure thatthe a of cavity does not disturb the evanescent field of the resonator.tf was calculated in the temperature range of 20–70 8C using Eq. (3).

t f ¼f 2 � f 1

f 1DT(3)

where f1 and f2 are the resonant frequencies at 20 8C and 70 8Crespectively and DT is the difference of initial temperature andfinal temperature.

Transmission mode cylindrical cavity method proposed byKrupka et al. [12] was used for the measurement of er and Qu. Firstthe resonant frequency (fo) was measured from the resonant peakusing a vector network analyzer (Agilent R3767CH) by cavitymethod and er was calculated using the formula ‘‘er = (c/Dfo)2’’,where c is the speed of light in vacuum and D is the diameter of thecylindrical DR. Qu is calculated using the relation, Qu = QL/(1 � 10�IL/20), where QL = fo/Df is the loaded quality factor, Df is3 dB bandwidth and IL is the insertion loss. In this study, Qu and er

were calculated by the software ‘Dielectric Resonator Calculator’using resonant frequency, and the dimensions of the sample, cavityand quartz spacer as input parameters.

3. Results and discussion

X-ray diffraction patterns recorded from SrLa4�xNdxTi5O17

(0 � x � 4) compositions sintered at their optimum sinteringtemperatures for 4 h are shown in Fig. 1. The reflections from thecompositions with x = 0, 1 and 2 were identical and could beindexed according to the orthorhombic (Pnnm) CaLa4Ti5O17 unitcell (PDF# 27-1059) and the reflections from the compositionswith x = 3 and 4 matched PDF# 49-256 for SrPr4Ti5O17 which alsocrystallize into an orthorhombic crystal structure. The unit cellparameters for each composition were refined by the least squaresmethod (Fig. 2) to account for the small variations due to

x � 4) sintered at 1550 8C for the compositions with x = 0 and 4, and 1580 8C for the

in the XRD detection limits.

Fig. 2. Variation in lattice parameter (a) ‘a’ and ‘c’ and (b) ‘b’ with increase in Nd content (x) for SrLa4�xNdxTi5O17 (0 � x � 4) series, showing a decrease in ‘a’ and ‘c’, with

increase in Nd-content, and an initial increase and then decrease in ‘b’ with increase in x.

Table 1X-ray diffraction data of SrNd4Ti5O17 refined by least squares method.

d (A) I/Io h k l d (A) I/Io h k l d (A) I/Io h k l

7.827488 22.54 0 4 0 2.584981 26.72 0 9 1 1.825687 20.64 1 14 1

5.2192 21 1 2 0 2.527792 21.69 1 11 0 1.748624 21.11 1 5 2

4.483925 18.99 1 4 0 2.458066 16.29 1 8 1 1.664209 22.48 1 16 1

4.127070 49.00 1 5 0 2.356266 17.42 1 12 0 1.643909 24.10 0 10 2

3.833420 26.24 0 1 1 2.292107 19.16 0 11 1 1.631854 15.32 3 3 1

3.780391 19.04 1 6 0 2.229989 33.12 2 1 1 1.616579 20.91 3 4 1

3.468379 18.46 1 7 0 2.150254 16.70 2 4 1 1.578345 28.64 1 19 0

3.290892 17.81 0 5 1 2.115155 17.43 1 11 1 1.575398 30.87 3 6 1

3.189223 18.78 1 8 0 2.062811 34.37 2 10 0 1.534480 15.95 1 11 2

3.133255 51.83 0 10 0 2.056087 22.66 2 6 1 1.524173 19.40 1 18 1

3.093782 36.97 1 2 1 2.012717 20.66 1 12 1 1.422694 15.68 2 17 1

3.017072 19.52 1 3 1 2.000502 18.50 2 7 1 1.409895 22.41 2 10 2

2.928164 100.0 1 4 1 1.950888 18.37 1 15 0 1.371408 16.22 0 16 2

2.741714 45.75 2 0 0 1.942546 20.10 2 8 1 1.350488 18.02 2 10 2

2.701525 70.58 2 2 0 1.930087 42.13 0 0 2

2.611266 17.54 0 12 0 1.891101 24.25 2 12 0

A. Manan, Y. Iqbal / Materials Research Bulletin 47 (2012) 883–888 885

substitution of Nd for La and the patterns were indexedaccordingly (Table 1). There was no evidence of second phaseformation within the XRD detection limits which demonstratedthe phase purity of all the sintered ceramics.

The lattice parameters a and c (Fig. 2a) decreased linearly withan increase in the Nd content due to the relatively small ionicradius (1.27 A) of Nd+3 substituted for La+3 with large ionic radius(1.36 A) in the orthorhombic unit cell [13]. As evident from Fig. 2b,the lattice parameter ‘b’ first increased with an increase in the Ndcontent from 0 to 1 and then decreased upon further increase in theNd content to 4; however, as a whole the parameter ‘b’ increasedonly by �0.01 A with increase in x from 0 to 4. It is noticeable thatthe observed variation in ‘b’ was too small to affect the decreasingtrend in the molar volume of the unit cell. The calculated unit celldata using least squares method is given in Table 2. The observeddecrease in lattice parameters due to the substitution of small ionsfor large ones in perovskite related structures is consistent withprevious studies [14,15]. Consistent with these observations, theposition of XRD peaks (Fig. 1) also shifted towards relatively higherdiffraction angles (smaller d-values) with increase in Nd+3 content,suggesting shrinkage of unit cell.

Table 2Structural data of SrLa4�xNdxTi5O17 (0 � x � 4) ceramics refined by least squares metho

x a (A) b (A) c (A)

0 5.549(5) 31.299(9) 3.9104(14)

1 5.530(3) 31.355(12) 3.9052(11)

2 5.5164(13) 31.348(5) 3.8892(7)

3 5.494 (3) 31.318(13) 3.8748(13)

4 5.4841(16) 31.313(7) 3.8613(10)

Consistent with the observed decrease in lattice parameters, themolar volume (Vm = Vcell/Z) of the SrLa4�xNdxTi5O17 (0 � x � 4) unitcell also decreased from 339.5 A3 to 331.6 A3 with increase in Nd+3

content from 0 to 4 (Fig. 3). In a similar study the molar volume ofCa (La1�xNdx)4Ti4O15 was reported to decrease from 285.32 to280.13 A3 with an increase in Nd content from 0 to 0.5 [11]. Thetheoretical density (rth) of SrLa4�xNdxTi5O17 (0 � x � 4) ceramicsincreased from 5.66 g/cm3 to 5.89 g/cm3 with increase in the Nd+3

concentration from 0 to 4 as shown in Fig. 3. The increase in thetheoretical density was due to the increase in the Mw/Vcell ratiowith increase in Nd+3 content. The plot of the apparent densities‘rap’ of SrLa4�xNdxTi5O17 (0 � x � 4) versus sintering temperatureis shown in Fig. 4. As evident from Fig. 4, the bulk density of Lacontaining ceramics decreased due to its lower theoretical densitybecause of its lower Mw/Vcell (molecular weight/molar volume)ratio in comparison to Nd-containing ceramics.

The apparent density increased with an increase in sinteringtemperature from 1500 8C to 1580 8C for SrLa3NdTi5O17, SrLa2Nd2-

Ti5O17, and SrLaNd3Ti5O17 ceramics (Table 3). For SrNd4Ti5O17, rap

increased from �5.11 g/cm3 to �5.64 g/cm3 with increase insintering temperature from 1500 8C to 1550 8C but decreased to

d.

Z Structure Vunit (A3) Vm (A3)

2 Orthorhombic 679.1(8) 339.5

2 Orthorhombic 677.1(6) 338.5

2 Orthorhombic 672.5(16) 336.3

2 Orthorhombic 666.6(6) 333.3

2 Orthorhombic 663.2(8) 331.6

Fig. 4. The plot of apparent densities ‘rap’ versus sintering temperatures for

different compounds in the SrLa4�xNdxTi5O17 (0 � x � 4) composition series,

showing an increase in rap with sintering temperature and Nd-content.

Fig. 3. Variation in the molar volume ‘Vm’ and theoretical density ‘rth’ of the unit cell

of SrLa4�xNdxTi5O17 (0 � x � 4) ceramics as a function of Nd+3 content substituted

for La+3, showing a decrease in ‘Vm’ and an increase in ‘rth’ with increase in the Nd+3

concentration.

A. Manan, Y. Iqbal / Materials Research Bulletin 47 (2012) 883–888886

5.62 g/cm3 upon further increase in the sintering temperature to1580 8C. The observed increase in apparent density might be due tothe substitution of Nd3+ with smaller ionic radius (1.27 A) incomparison to La3+ (1.36 A) [13]. Hu et al. [16] also reported anincrease in the optimum sintering temperature due to thesubstitution of Sr2+ with smaller radius for larger radius Ba2+.Additionally, consistent with the author’s previous work [17], themaximum apparent density (�5.36 g/cm3) for SrLa4Ti5O17 (con-taining no Nd3+), was obtained at 1500 8C.

The secondary electron images from thermally etched gold-coated surfaces of SrLa4�xNdxTi5O17 (0 � x � 4) ceramics sinteredat their optimum sintering temperatures are shown in Fig. 5a–e. Ingeneral, the microstructures of all the investigated samplescomprised densely packed (rrel > 94%) elongated and plate-likegrains, typical of layered perovskites with orthorhombic crystalstructure [18], although non-uniform grain growth could be seenin all the samples which made the determination of exact grain sizedifficult. For example, elongated grains >15 mm in length could beseen in the micrograph from the sample with x = 0. The averagegrain size in the microstructure of the examined samples variedfrom �1 mm � 1 mm to 5 mm � 10 mm (e.g. in x = 0) and from0.5 mm � 0.5 mm to �5 mm � 7 mm (e.g. in x = 4). An analysis ofSEM micrographs indicated that the number of grains withrelatively smaller size increased with increase in the Nd contentwhich may be one of the reasons for the observed increase inapparent density with increasing x.

The relative permittivity ‘er’ and ionic dielectric polarizability‘aD’ of SrLa4�xNdxTi5O17 (0 � x � 4) decreased almost linearly withincrease in the Nd content (Fig. 6 and Table 3). The observeddecrease in er could be explained on the basis of the higherdielectric polarizability of La+3 (6.07 A3) than that of Nd+3 (5.01 A3)reported in 1993 [19]; however, if the revised dielectricpolarizability of La+3 (i.e. 4.82 A3) is considered [20], er ofSrLa4�xNdxTi5O17 should increase with an increase in the Ndcontent. More recently, the dielectric polarizability of La+3 was

Table 3Synthesizing conditions, apparent, theoretical and relative densities, and microwave d

x CT (8C) ST (8C) rap (g/cm�3) rth (g/cm

0 1250/6 h 1500/4 h 5.36 5.66

1 1350/6 h 15,804 h 5.42 5.69

2 1350/6 h 1580/4 h 5.40 5.75

3 1350/6 h 1580/4 h 5.57 5.83

4 1350/6 h 1550/4 h 5.64 5.89

ST, sintering temperature, CT, calcination temperature, rap, apparent density, rth, theo

recalculated as 5.71 A3 by plotting the linear fit for the dielectricpolarizabilities versus r3 (r being the ionic radius) of lanthanideions for the coordination no.8 [21] which is higher than that ofNd+3. This suggests that the observed decrease in er is consistentwith the variation in aD of SrLa4�xNdxTi5O17 with Nd concentrationas shown in Fig. 6.

An almost linear decrease from +117 to �142.3 ppm/8C wasobserved in tf for SrLa4�xNdxTi5O17 (0 � x � 4) with increase in x

due to the substitution of smaller ionic radius (1.27 A) of Nd+3 forLa+3 (1.36 A) for coordination no. of 12 [19]. The substitution ofNd+3 for La+3 causes an antiphase tilting of octahedra [3] resultingin the decrease in tf. It can be seen from Fig. 7 that near zero tf

corresponds to a dielectric constant of about 54.The quality factor ‘Qufo’ for SrLa4�xNdxTi5O17 was observed to

decrease from 9969 GHz to 6030 GHz (at 5.40 GHz) with theincrease in Nd content from 0 to 3 and then increased to 8678 GHz(at 5.53 GHz) with further increase in x to 4 (Fig. 8). The variation inthe Qufo value might be due to various factors including grain size,porosity, dislocations, and lattice defects that vary from sample tosample [22]. Additionally, the difference in the ion size between Srand La is less in comparison to that of Nd and Sr and thus asincrease in Nd content may cause an increase in lattice strainwhich may adversely affect the quality factor [6,23]. Themicrowave dielectric properties measured for the samples sinteredat the optimum sintering temperatures are given in Table 3. Acareful analysis of the microwave dielectric properties of theceramics designed and processed in the present study indicatedthat near zero tf corresponds to er � 54 and Qufo � 6000 GHz atx � 1.6. The substantial increase in er from 39.1 [8] to 60.8 (presentstudy) for SrLa4TiO17 may be due to the �3% increase in the relativedensity of the final ceramics. The relatively higher quality factor(14,200 GHz) and lower tf (58 ppm/K) reported in Ref. [8] forSrLa4TiO17 and the observed increase in er and decrease in tf due toNd substitution demonstrated the possible application potential ofthe compositions in the SrLa4�xNdxTiO17 series. Nd3+ and Ca2+ have

ielectric properties of SrLa4�xNdxTi5O17 (0 � x � 4) ceramics.

�3) rrel er Qufo (GHz) tr (ppm/8C)

94.7 60.8 9969 +117.0

95.2 56.0 6243 40.9

93.9 51.8 6030 �32.3

95.5 48.0 6701 �97.0

95.7 44.1 8678 �142.3

retical density, rrel, relative density.

Fig. 5. SEI from thermally etched surfaces of optimally sintered SrLa4�xNdxTi5O17 (0 � x � 4) ceramics showing the microstructure comprising elongated and plate-like grains.

Fig. 6. The plot of dielectric constant ‘er’ and ionic dielectric polarizability ‘aD’ for

SrLa4�xNdxTi5O17 (0 � x � 4) compounds, showing an almost linear decrease in er

and aD with increase in Nd+3 concentration.

Fig. 7. The plot of tf versus er for SrLa4�xNdxTi5O17 (0 � x � 4) compounds, showing

near zero tf corresponding to er � 53 at x � 1.6.

A. Manan, Y. Iqbal / Materials Research Bulletin 47 (2012) 883–888 887

Fig. 8. Variation in the Qufo value with an increase in Nd+3 content (x) for

SrLa4�xNdxTi5O17 (0 � x � 4) compounds.

A. Manan, Y. Iqbal / Materials Research Bulletin 47 (2012) 883–888888

lower ionic radii and polarizabilities than La3+ and Sr2+ respec-tively. The substitution of Ca2+ for Sr2+ decreased the ionic sizedifference at the A-site resulting in a decrease in the average bondlength [24] and hence an increase in the quality factor, aspreviously reported by the authors [25]. Nd3+ substitution for La3+

increases the ionic size difference at the A-sit of the perovskitestructure leading to an increase in average bond length and hence adecrease in the quality factor [24]. On the other hand, Nd3+

substitution for La3+ decreased while Ca2+ substitution for Sr2+

increased the packing fraction of the unit cell which may be areason for the observed decrease in the quality factor for Nd-containing compounds and an increase in quality factor for the Ca-containing compounds [24]. This indicates that the large sizespread is bad, small size spread is good.

4. Conclusions

Single phase, 95% dense ceramics were prepared for eachcomposition in the SrLa4�xNdxTiO17 (0 � x � 4) series sintered at1500–1580 8C. The unit cell parameters and volume decreasedwith increase in the Nd content due to the smaller ionic radius ofNd than La. The substitution of Nd for La increased the theoreticaldensity from 5.66 g/cm3 (for x = 0) to 5.89 g/cm3 (for x = 4).Ceramics with nearly zero tf and er � 54 can be prepared in theSrLa4�xNdxTiO17 series with x � 1.6; however, further work is

required to improve Qufo (�6000 GHz) for possible commercialapplications.

Acknowledgements

The authors acknowledge the financial support of the HigherEducation Commission of Pakistan for the Development ofMaterials Connection Centre (Pak-US Project ID 131) and NRPU20-569, IRSIP and electro-ceramics group in facilitating the authorsat the Electroceramics Laboratory, Department of Material Scienceand Engineering Materials, University of Sheffield (UK).

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