Vibrational spectroscopy and microwave dielectric properties of Ca 5 − x Ba x Nb 2 Ti O12 and Ca 5 − x Ba x Ta 2 Ti O 12 ceramicsAnderson Dias, Pazhoor Varghese Bijumon, Mailadil Thomas Sebastian, and Roberto Luiz Moreira Citation: Journal of Applied Physics 98, 084105 (2005); doi: 10.1063/1.2112179 View online: http://dx.doi.org/10.1063/1.2112179 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/98/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in First-principle calculation and assignment for vibrational spectra of Ba(Mg1/3Nb2/3)O3 microwave dielectricceramic J. Appl. Phys. 115, 114103 (2014); 10.1063/1.4868226 Crystal structure and dielectric properties of ( 1 − x ) Ca 0.61 Nd 0.26 TiO 3 + x Nd ( Mg 1 / 2 Ti 1 / 2 ) O 3complex perovskite at microwave frequencies J. Appl. Phys. 104, 124104 (2008); 10.1063/1.3043574 Intrinsic dielectric and spectroscopic behavior of perovskite Ba ( Ni 1 ∕ 3 Nb 2 ∕ 3 ) O 3 – Ba ( Zn 1 ∕ 3 Nb 2 ∕ 3 ) O3 microwave dielectric ceramics J. Appl. Phys. 102, 044112 (2007); 10.1063/1.2770870 Experimental investigations and three-dimensional transmission line matrix simulation of Ca 5 − x A x B 2 Ti O12 ( A = Mg , Zn, Ni, and Co; B = Nb and Ta) ceramic resonators J. Appl. Phys. 98, 124105 (2005); 10.1063/1.2149187 Low-loss Ca 5 − x Sr x A 2 Ti O 12 [ A = Nb , Ta ] ceramics: Microwave dielectric properties and vibrationalspectroscopic analysis J. Appl. Phys. 97, 104108 (2005); 10.1063/1.1897065

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Vibrational spectroscopy and microwave dielectric propertiesof Ca5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 ceramics

Anderson Diasa�

Departamento de Engenharia Metalúrgica e de Materiais, Universidade Federal de Minas Gerais,Rua Espírito Santo 35, Sala 206, Belo Horizonte, Minas Gerais, 30160-030, Brazil

Pazhoor Varghese Bijumon and Mailadil Thomas SebastianCeramic Technology Division, Regional Research Laboratory, Trivandrum, 695 019, India

Roberto Luiz MoreiraDepartamento de Física, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais,C.P. 702, Belo Horizonte, Minas Gerais, 30123-970, Brazil

�Received 27 April 2005; accepted 6 September 2005; published online 21 October 2005�

Complex ceramics Ca5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 were prepared by conventionalsolid-state ceramic route. The crystalline structures were determined by x-ray diffraction, whichshowed that solid solutions were formed for x=4 �CaBa4Nb2TiO12 and CaBa4Ta2TiO12� andcrystalline multiphases appeared for 1�x�3. The dielectric properties of the sintered ceramicswere measured in the microwave frequency range. The dielectric constant and temperature variationof resonant frequency increased and the quality factor decreased with increasing x value. Ramananalyses showed different spectra depending upon the composition, with variations in both intensityand frequency, as well as in the number of bands strictly related to the phases present. For both Nb-and Ta-based systems, the results indicated an increase in the short-range ordering with bariumsubstitution, which explained the reduction in the unloaded quality factor �Qu�. The results offar-infrared spectroscopy agree well with the Raman data, with few variations when Nb replaces Ta,and an increase in the complexity of the vibrational modes for Ca/Ba mixed ceramics. © 2005American Institute of Physics. �DOI: 10.1063/1.2112179�

I. INTRODUCTION

Telecommunication systems are composed of differentkinds of components, including dielectric resonators and fil-ters to store and transfer microwave signals. In the last threedecades, an enormous progress in this area occurred as adirect result of successive development of ceramic materialssuitable to be used for these applications.1 However, fewceramics meet the rigorous requirements for use in deviceapplications, such as the operating frequency, power levels,and type of applications �base station or hand-held device�.These commercial ceramics are today titanium-based perovs-kites, such as ZrTiO4, Ba2Ti9O20, and CaTiO3–NdAlO3, to-gether with complex tantalates such as Ba3ZnTa2O9.1 Manysystems are being studied in order to attain minimum dielec-tric losses and also costs.2 In this respect, solid solutionsbetween different types of ceramic materials are of specialinterest, including complex titanium- and niobium/tantalum-based oxides with perovskite-like structure. These materialsare formed by traditional titanates �Ca and Ba� and complexniobates and tantalates.3,4 For applications in wireless com-munication devices, high relative permittivities �dielectricconstant �r�, near-zero temperature coefficients of resonantfrequency �� f�, and low dielectric loss tangents �high un-loaded quality factor Qu� are essential. Both CaTiO3 andBaTiO3 exhibit high permittivities; however, these ceramicspresent large positive temperature coefficients.5 On the other

side, Ca-based complex niobates and tantalates present op-posite signs of � f, which suggest the possibility of tuning thisparameter to zero.6

Thus, potentially useful ceramics with temperaturestable relative permittivities can be obtained by formingsolid solutions between Ca or Ba titanates and suitable cal-cium complex perovskites. This combination was proposedby Cava et al.7 in a study of a pseudobinary diagram wherethey presented the mixing of three cations on the perovskiteB-site, Ca�Ca1/4Nb1/2Ti1/4�O3 and Ca�Ca1/4Ta1/2Ti1/4�O3.Their results suggested that an order-disorder transitionamong the B-site ions plays a major role in the determinationof the temperature dependence of dielectric constant for bothcompounds. Concerning the ordering behavior, the same au-thors found that different types of ordering between �111�planes, as well as distortions by tilting of octahedra, are in-volved in the formation of a microstructural state at roomtemperature.8 Later, Bendersky et al.9 established a phasediagram for these ceramics, which includes five single-phasefields related to each other by different reactions. Recently,Bijumon et al.3,4 studied the synthesis, dielectric properties,and microstructures of the Ca5Nb2−xTaxTiO12 system. Theyobtained materials with high dielectric constants and lowlosses �Qu� f �33 000�, as well as small temperature varia-tions of resonant frequencies. More recently, these materialswere identified as potential candidates for the fabrication ofbroadband dielectric resonator antennas.10,11

The extreme chemical and structural complexities ofCa5Nb2−xTaxTiO12 systems indicate that additional studiesa�Electronic mail: anderson@demet.ufmg.br

JOURNAL OF APPLIED PHYSICS 98, 084105 �2005�

0021-8979/2005/98�8�/084105/8/$22.50 © 2005 American Institute of Physics98, 084105-1

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must be carried out to unambiguously associate the structuralchemistry with the dielectric properties. Also, the introduc-tion of other isovalent metal ions in the system, such asstrontium or barium, could modify the structure and dielec-tric properties of these materials. In a previous work, Biju-mon et al.12 studied the introduction of strontium inCa5−xSrxA2TiO12 �A=Nb,Ta� ceramics. The results showedsingle-phase materials presenting a structural evolution withincreasing x, followed by changes in microwave dielectricproperties. It was observed that the quality factor decreasedwith increasing Sr substitution with an unexpected increaseat x=4, where maximum distortions in the octahedral cagesoccurred. The present work was undertaken to understandthe effect of Ba substitution for Ca in the A sites on thestructural and microwave dielectric properties ofCa5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 ceramics. In thisrespect, Raman and far-infrared spectroscopies were em-ployed to investigate the vibrational properties originatedfrom the substitution of Ca by Ba on both A and B sites, aswell as the effects of Nb and Ta change on the metal-oxygenstretching/bending vibrations.

II. EXPERIMENT

Ca5−xBaxA2TiO12 �A=Nb,Ta� complex ceramics wereprepared by conventional solid-state ceramic route. Stoichio-metric amounts of high-purity ��99.9% � CaCO3, BaCO3,TiO2 �Aldrich Chemicals, Milwaukee WI, USA�, andNb2O5/Ta2O5 �Nuclear Fuel Complex, Hyderabad, India�were weighed and wet mixed in distilled water using yttria-stabilized zirconia balls in a plastic container for 24 h. Theslurry was dried and calcined in platinum crucibles in therange of 1200–1350 °C for 4 h. The calcined powders werewell ground into fine form and were pressed under a uniaxialpressure of 100 MPa into cylindrical disks with 14 mm di-ameter and 6–7 mm height. The compacts were sintered atdifferent temperatures in the range of 1470–1625 °C for 4 hin a high-temperature furnace �Nabertherm Ltd., Germany�.The sintered ceramic pucks were polished and their bulkdensities were measured by the Archimedes method. Thephase constitution and crystal structure were examined byx-ray diffraction �Philips x-ray diffractometer, Netherlands�using Cu K� radiation. Surface morphology and microstruc-tural evolution of the specimens were recorded from the ther-mally etched surface of sintered samples by using scanningelectron microscopy �SEM, Model S2400-Hitachi, Japan�.

The dielectric constant and unloaded quality factor of theceramics were measured in the microwave frequency range�2–5 GHz� using an HP 8510C vector network analyzercoupled with a sweep signal generator and reflection/transmission test unit. The �r was calculated from the TE011

resonance mode of the end-shorted samples kept betweentwo well-polished conducting plates using the method pro-posed by Hakki,13 later modified by Courtney.14 Qu of theceramic samples were measured from the excited TE01�

mode, when the samples were placed over a quartz spacerkept inside a well-polished copper cavity.15 The temperature

coefficient of the resonant frequency was measured by not-ing the frequency shift of the TE011 resonant mode over thetemperature range of 20–80 °C.

Micro-Raman-scattering spectra were recorded using aJobin-Yvon LABRAM-HR spectrometer, equipped with adiffraction grating of 1800 grooves/mm, a liquid-N2-cooledcharge-coupled device �CCD� detector, and a confocal mi-croscope �100� objective�. The experimental resolution wasbetter than 1 cm−1. The measurements were carried out inbackscattering geometry, at room temperature, using the632.8 nm line of a helium neon ion laser �power of12.5 mW� as excitation source. A holographic notch filterwas used to stray-light rejection �Rayleigh scattered light�.The sample surfaces of the sintered materials were previ-ously polished to an optical grade in order to improve theratio of inelastic to elastic scattered light. Infrared reflectancespectra were recorded in a Bomem spectrometer �DA8-02�,equipped with a fixed-angle specular reflectance accessory�external incidence angle of 11.5°�. The far-infrared range�50–500 cm−1� was studied by using a globar lamp, a6-�m-coated Mylar® hypersplitter, and a liquid-He-cooled Sibolometer. In the mid-infrared region �500–4000 cm−1� weused a “globar” source �SiC�, a Ge-coated KBr beamsplitter,and a LN2-cooled HgCdTe detector. One of the ceramic facesreceived a thin gold coating and was used as a “rough” mir-ror for the reference spectra. This procedure allowed us toimprove the reflectivity spectra, since the mirror surfacemimics the sample one, which compensates the effects ofdiffuse reflection at the sample surface. The measurementswere performed under low vacuum �10−3 bar� with a typicalresolution of 2 cm−1.

III. RESULTS AND DISCUSSION

In previous works, Ca5Nb2TiO12 and Ca5Ta2TiO12

were studied and the structural analysis showed similarpatterns of orthorhombic symmetry, with a slight shiftin the peak positions.3,4 Recently, the ceramic systemsCa5−xSrxNb2TiO12 and Ca5−xSrxTa2TiO12 were investigatedand the results showed that these materials also form ortho-rhombic solid solutions from the binary phases �CaTiO3 andCa4Nb2O9, for example�, with incorporation of Ti on Bsites.12 Now, the ceramic systems Ca5−xBaxNb2TiO12 andCa5−xBaxTa2TiO12 are studied by substituting Ba for Ca. Thex-ray-diffraction patterns obtained for Ca5−xBaxNb2TiO12

and Ca5−xBaxTa2TiO12 �0�x�5� are shown in Figs. 1 and2, respectively. The patterns appear similar for both Nb- andTa-based ceramics. Ba2+ being a bigger ion16 compared toCa2+ occupies the perovskite A site of the ceramics.17 Incomplex perovskite form, Ca5A2TiO12 �A=Nb,Ta� ceramicscan be represented as Ca�Ca1/4Nb2/4Ti1/4�O3 andCa�Ca1/4Ta2/4Ti1/4�O3. The reported crystal structure of theseceramics is orthorhombic with four molecular formulas perunit cell �Figs. 1�a� and 2�a��.7 The substitution of one Ca2+

ion by Ba2+ �x=1� leads to a multiphase ceramic instead of asingle-phase Ca4BaA2TiO12 �A=Nb,Ta�, as evidenced by thediffraction profile shown in Figs. 1�b� and 2�b�. The phases

084105-2 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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were identified as Ca4Nb2O9/Ca4Ta2O9 �ICDD File 49-911and 31-308� and BaTiO3 �ICDD File 34-129� and were in-dexed accordingly.

For compositions with x=2 and x=3, the materials pre-sented a cubic double perovskite �AA��BB�B��O3� structure�Figs. 1�c�, 1�d�, 2�c�, and 2�d��. The diffraction peaks werelabeled by comparing with those of Ba�Zn1/3Nb2/3�O3 �ICDDFile 17-182� with cubic symmetry. For x=2, traces of sec-ondary phases corresponding to Ca4Nb2O9 and BaTiO3 inthe Nb-based system and Ca4Ta2O9 and BaTiO3 in the tan-talates were also detected by x-ray diffraction �the presenceof BaTiO3 cannot be neglected at x=3�. For x=4, pure phaseBa�Ca1/4Nb2/4Ti1/4�O3 and Ba�Ca1/4Ta2/4Ti1/4�O3 wereformed, as shown in Figs. 1�e� and 2�e�. In these cases, thediffraction patterns consist of strong peaks characteristic ofprimitive cubic perovskite unit cell as well as few lines aris-ing from the superstructure reflections indicating the possiblecubic-orthorhombic structural transformation. For x=5, amixture of Ba4Nb2O9–BaTiO3 and a few peaks correspond-ing to BaNb2O6 �ICDD File 32-77� can be seen in Nb-basedsystem �Fig. 1�f��. In the tantalum system, Ba4Ta2O9,BaTiO3, and BaTa2O6 �ICDD File 20-146� appeared in thediffraction pattern �Fig. 2�f��. Attempts for the complete sub-stitution of calcium with barium in Ca5A2TiO12 �A=Nb,Ta� ceramics led to extensive distortion in both sites

because of the large ionic radius of barium compared withcalcium and resulted in the formation of a mixture of phases�with poor sinterability�.

The synthesizing conditions, cell volume, density, toler-ance factor, and microwave dielectric properties ofCa5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 �0�x�5� aregiven in Tables I and II, respectively. It is evident from TableI that, in the Nb system, the sintering temperature decreasesfrom 1550 to 1470 °C for different Ba substitutions,whereas in the Ta-based system the variation in sinteringtemperature for the same values of x was from1625 to 1500 °C. At x=5, the ceramics were not able to sin-ter to a compact form since they crumbled to powder onfiring. Efforts made to sinter these materials by adding dif-ferent amounts of low melting CuO were not successful. Theenhancement in cell volume observed in both systems withcompositional variation is due to the substitution of biggerbarium ion16 for the comparatively smaller calcium ion. Thestandard deviation in calculating cell volume was 0.0008 Å3.In Ca5−xBaxNb2TiO12 ceramics, the theoretical density in-creased from 4.19 to 5.84 g/cm3 �the experimental densityvaried from 96.4% to 98.3%�, when x shifted from 0 to 4,with orthorhombic-to-cubic transformation. A similar varia-tion was observed in Ta-based ceramics, where the densityincreased from 5.41 to 6.83 g/cm3 �experimental density be-

FIG. 1. X-ray-diffraction patterns of �a� Ca5Nb5TiO12, �b�Ca4Nb2O9–BaTiO3, �c� Ca3Ba2Nb2TiO12, �d� Ca2Ba3Nb2TiO12, �e�CaBa4Nb2TiO12, and �f� Ba4Nb2O9-BaTiO3 ��=Ca4Nb2O9, =BaTiO3, =Ba4Nb2O9, and �=BaNb2O6�.

FIG. 2. X-ray-diffraction pattern of �a� Ca5Ta2TiO12, �b�Ca4Ta2O9–BaTiO3, �c� Ca3Ba2Ta2TiO12, �d� Ca2Ba3Ta2TiO12, �e�CaBa4Ta2TiO12, and �f� Ba4Ta2O9–BaTiO3 ��=Ca4Ta2O9, =BaTiO3, =Ba4Ta2O9, and �=BaTa2O6�.

084105-3 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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tween 97.2% and 98.0%�. In both ceramic systems, the tol-erance factor increased and approached unity, which can beattributed to the increase in average A-site ionic radius due tothe substitution of Ba for Ca ions. This is in agreement withthe fact that as the tolerance factor tends to unity, the sym-metry transforms to a cubic perovskite form. The microwavedielectric properties of Ca5−xBaxNb2TiO12 �0�x�5� ceram-ics are also presented in Table I. Qu� f shows an abruptdecrease when x is varied from 0 to 1 and, thereafter, de-creases gradually. �r and � f show a linear increase with in-creasing x. The same trend in the variation of microwavedielectric properties was observed in Ca5−xBaxTa2TiO12 �0�x�5� ceramics �Table II�. Qu� f varies from 33 000 �at4.5 GHz� to 1400 �at 2 GHz�, �r changes from 38 to 64, and� f increases from +10 to +24 ppm/ °C, when x increasesfrom 0 to 4, in the Ta-based ceramics. The microwave dielec-tric properties of the Ca5−xBaxA2TiO12 �A=Nb,Ta� ceramicsappear to be influenced by the presence of the secondaryphase BaTiO3 which possess high �r and positive � f.

5

The microstructures of few typical Ca5−xBaxA2TiO12

�A=Nb,Ta� ceramics are shown in Fig. 3. The surface mor-phology of Ca5Nb2TiO12 and Ca5Ta2TiO12 has been reportedearlier as constituted by uniform distribution of large grainsof about 10 �m in size.4 Figure 3�a� shows the surface mor-

phology of Ca4Nb2O9–BaTiO3 mixture phases formed aftersintering the Ca5−xBaxNb2TiO12 system with x=1. Two dif-ferent types of grains are clearly visible with an additionalindication of liquid phase formation. The secondary electronimage from the surface of Ca3Ba2Ta2TiO12 sample is shownin Fig. 3�b�. Comparing with Fig. 3�a�, the grain distributionbecomes more uniform but still presents some stray phasescorresponding to BaTiO3. The SEM picture of aCa2Ba3Nb2TiO12 ceramic is shown in Fig. 3�c�. The graindistribution becomes more uniform in this specimencompared with the precedent material. A gradual changein grain shape is visible, indicating a structural transforma-tion with barium introduction. Figure 3�d� shows the micro-structure of a CaBa4Ta2TiO12 ceramic. The SEM pictureresembles that of parent Ca5Ta2TiO12 ceramics4 except forthe slight change in the grain size. With x=4, the ceramicshave grain with 1–2 �m sizes. No secondary phases wereobserved for this composition indicating pure phase forma-tion of CaBa4Ta2TiO12 material. The grain size of Ba-substituted single-phase compositions �CaBa4Nb2TiO12 andCaBa4Ta2TiO12� is smaller than that of Ca5Nb2TiO12 andCa5Ta2TiO12,

4 which implies that the substitution of Ca2+ byBa2+ improved the sinterability and enhanced the densifica-tion, as observed from Tables I and II.

TABLE I. Synthesizing conditions, structure, cell volume, density, tolerance factor, and microwave dielectricproperties of Ca5−xBaxNb2TiO12 �0�x�5� ceramics.

x CTa STb Structure CVc Dd teQu� f�GHz� �r

� f

�ppm/°C�

0 1350 1550 Orthorhombic 247.8585 96.4 0.9189 26 000 48 +401 1300 1490 Mixture #f # # 4 000 58 +442 1275 1475 Cubic 278.8493 97.0 0.9585 2 600 66 +483 1240 1500 Cubic 283.4104 97.4 0.9810 1 600 75 +534 1210 1470 Cubic 288.5216 98.3 1.0035 1 200 83 +575 1200 �

g Mixture # # # � � �

aCT=calcination temperature �°C�.bST=sintering temperature �°C�.cCV=cell volume �Å3� with standard deviation of ±0.0008.dD=density �%�.et=tolerance factor.f#=form as a mixture.g�=cannot be sintered �powder sample�.

TABLE II. Synthesizing conditions, structure, cell volume, density, tolerance factor, and microwave dielectricproperties of Ca5−xBaxTa2TiO12 �0�x�5� ceramics.

x CTa STb Structure CVc Dd teQu� f�GHz� �r

� f

�ppm/°C�

0 1350 1625 Orthorhombic 246.1796 97.2 0.9189 33 000 38 +101 1300 1575 Mixture #f # # 5 000 43 +142 1275 1540 Cubic 278.8493 96.9 0.9585 3 000 48 +183 1240 1525 Cubic 283.4104 97.2 0.9810 1 800 61 +214 1210 1500 Cubic 288.5216 98.0 1.0035 1 400 64 +245 1200 �

g Mixture # # # � � �

aCT=calcination temperature �°C�.bST=sintering temperature �°C�.cCV=cell volume �Å3� with standard deviation of ±0.0008.dD=density �%�.et=tolerance factor.f#=form as a mixture.g�=cannot be sintered �powder sample�.

084105-4 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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Figures 4 and 5 show the Raman spectra forCa5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 �0�x�5� micro-wave ceramics, which can be interpreted as a combination oftypical Raman spectra of the end members BaTiO3 orCaTiO3 and Ca4Nb2O9 or Ca4Ta2O9. First, the effects of thesubstitution of Nb atom by Ta �B sites� in Ca5Nb2TiO12 andCa5Ta2TiO12 are discussed �x=0 in Figs. 4 and 5�.Ca5Nb2TiO12 and Ca5Ta2TiO12 have characteristic bands ofA sites occupied by Ca �120 cm−1� together with a set ofbands related to B sites �average occupancy ofCa1/4Nb1/2Ti1/4 and Ca1/4Ta1/2Ti1/4�. The broad bands ob-served at 640 cm−1 is related to the B–O symmetric stretch-ing vibration.18,19 The bands at 450–455 and 475–485 cm−1

are assigned to B–O3 torsional �bending or internal vibrationof oxygen cage� modes, although the frequencies verified by

Hirata et al.18 and Zheng et al.19 in similar perovskites werehigher. This behavior can be understood as an effect causedby the more distorted coordination environment of B sitesdue to the presence of both Ca and Nb/Ta in the solid solu-tions. The bands in the region of 235–370 cm−1 are relatedto the modes associated with rotations of oxygen cage andB-site ordering,19 while the bands at 530–600 and750–800 cm−1 correspond to oxygen motion �A1g mode�.

Concerning the B-site ordering, it is clear that the bandsin the regions of 300–400 and 720–800 cm−1 are relatedto 1:1 ordering, similar to the results of Raman spectroscopicanalysis carried out by Zheng et al.19 and Levin et al.20

in Ca�Ca1/3Nb2/3�O3- and CaTiO3-based ceramics. Accord-ing to group theory analysis for 1:1 superstructureswith Fm3m symmetry and for 1:2 ordered perovskites with

P3̄m1 space group, A1g and F2g modes become Raman activedue to B-site ordering.21,22 The substitution of Nb by Ta pro-duced a faint shift to lower frequencies �redshift�, as ex-pected. However, an exception have occurred at798–803 cm−1, corresponding to the A1g�O� mode, whichblueshifted for the ceramic with tantalum. This behavior canbe explained by the increasing of force constant or stiffnessof the oxygen octahedra cage, as also verified by Siny et al.in Ba�Mg1/3Ta2/3�O3 ceramics.22 Also, the intensification ofthe bands at 375 and 750 cm−1 for compounds with Nb areprobably related to a higher short-range ordering. Similarresults were observed by Zheng et al.19 for theCaTiO3–Sr�Mg1/3Nb2/3�O3 system.

Ba-based compounds with nominal compositionsBa5Nb2TiO12 and Ba5Ta2TiO12 were studied by Raman spec-troscopy and the results are displayed in Figs. 4 and 5 �x=5�. These materials represent, as a first approximation,solid solutions between BaTiO3 and Ba4Nb2O9 or Ba4Ta2O9,

FIG. 3. SEM images of �a� Ca4Nb2O9–BaTiO3-, �b� Ca3Ba2Ta2TiO12-, �c�Ca2Ba2Nb2TiO12-, and �d� CaBa4Ta2TiO12-sintered ceramics.

FIG. 4. Raman results of Nb-based complex perovskites with isovalent sub-stitution of Ca and Ba.

FIG. 5. Raman results of Ta-based complex perovskites with isovalent mix-ing of Ca and Ba on A and B sites.

084105-5 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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with titanium incorporation on B sites of the perovskitelikestructure. However, as verified by x-ray diffraction �Figs.1�f� and 2�f��, these ceramics presented multiple phases�Ba4Nb2O9, BaTiO3, and BaNb2O6 for the Nb-based materi-als� and could not be sintered. Although the diffraction pro-files of Nb- and Ta-based ceramics are quite similar, theirRaman spectra look different because of the presence of thesecondary phases BaNb2O6 and BaTa2O6. At least 22 bandscan be seen in both spectra as a result of the superposition ofdifferent combination modes of all the crystalline phases ob-served. The appearance of different phases leads to the de-velopment of multiple bands within the broad Raman peaks,which are difficult to assign in these materials. Figures 4 and5 also present Raman spectra for Nb- and Ta-based micro-wave ceramics with isovalent cation substitution on A and/orB sites: Ca5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12, respec-tively. With Ba substitution, we note that the spectra for x=0 and x=1 show absolutely the same number of features.This is compatible with the interpretation that, although thestructure evolves to multiple phases for higher values of x, itremains orthorhombic at x=1. The only noticeable change inthe Raman spectra is the shifting to lower frequencies as Bais introduced in the system. On the other hand, microwavedielectric properties varied significantly by changing Ca byBa, as it can be seen in Tables I and II: the introduction ofonly one Ba atom in the structure led to the sudden decreasein Qu� f , from 26 000 to 4000 in Nb-based ceramics andfrom 33 000 to 5000 in tantalum-based materials.

The increase in Ba contents led to the reduction of theamount of both Ca4Nb2O9 and Ca4Ta2O9 phases at x=2 andx=3 �see Figs. 1�c� and 2�c��. This result produced moresignificant changes in the Raman spectra. Bands around 260and 360 cm−1 appeared, increasing their intensity and be-coming narrower for increasing x. Also, the bands at 120 and800 cm−1 shifted to 100 cm−1 �redshift� and to 820 cm−1

�A1g�O� mode, which blueshifted�. The strongest bandaround 800 cm−1 is related to the “breathing” mode of the�NbO6� / �TaO6� octahedra. Shoulders are visible on bothsides of this band and become more pronounced for ceramicswith tantalum. The A1g�O� mode shifted to higher frequen-cies for increasing Ba content as a result of the volumechange �rather than mass�. This result was also observed in�Ca,Sr�A2TiO12 �A=Nb,Ta� materials and has been attrib-uted to short-range ordering.12 Levin et al.20 observed similarshoulders in the Raman spectra of Ca4Nb2O9 and attributedthem to the presence of metastable superstructures.

The observed changes in the Raman bands around 100and 820 cm−1 for increasing x denote the presence of asingle-phase material for x=4. The first band of the Ramanspectra reflects the A-site environment, as well as probablerotations of the oxygen cage.12 For this composition, the re-maining peaks of BaTiO3 present in the x-ray-diffractiondata for x=3 �Figs. 1�d� and 2�d�� disappeared completelyand solid solutions could be obtained with the nominal com-positions Ba�Ca1/4Nb2/4Ti1/4�O3 and Ba�Ca1/4Ta2/4Ti1/4�O3.Here, all Ba ions occupy now the perovskite A sites and thedistortions observed for lower values of x �Ca ion in the Asite� are no longer detected. The broad Raman bands for x=4 �Figs. 4 and 5� show that the B-site long-range ordering is

relatively poor. The A1g�O� mode represents a qualitative in-dication of the degree of B-site ordering and, obviously, dif-ferences in charge or ionic sizes will influence the distribu-tion of ions on any given site. Ca and Ba atoms presentsignificant differences in their Raman spectra because of thedifference in their ionic radius and the larger this differenceis, the greater is its influence on the vibrational modes, par-ticularly on those related to ordering. Zheng et al.21,23 con-sidered the possibility that the width and frequency of theA1g�O� mode band may be affected by the size distribution ofthe A-site cations. Their conclusions are that the A-site dis-tribution influences the degree of short-range order on the Bsite or, also, the size differences in the A site could constrictin some manner the octahedra breathing mode to occur in agiven range of frequencies. In our case, it is believed that theA1g�O� mode is a function of the volume cage, which pre-sents the maximum variation for x=4 �one Ca atom in Bsite�. Finally, correlating this result to the microwave prop-erties, the decrease in Qu with Ba introduction �see Tables Iand II� could be an expected result, since its presence altersthe A-site distribution and probably increases the degree ofshort-range order. This increase is detrimental to Qu becauseit induces anharmonicity and increases phonon damping. Asverified by Zheng et al.21,23 and Webb et al.,24 only when theorder changes from short to long range is that the values ofQu increase.

Figures 6 and 7 show the infrared-reflectivity spectrabetween 50 and 700 cm−1 for Ca5−xBaxNb2TiO12 andCa5−xBaxTa2TiO12 microwave ceramics, respectively. As ageneral trend, the results of far infrared agree with thoseobtained from the Raman analyses. As it can be seen, thereare no significant differences in the spectra of Ba-pure com-pounds �Figs. 6 and 7, x=5�, which consist of four sets ofbroad bands because of the multiple-phase nature of thesesamples. The first of these bands is located below 180 cm−1,

FIG. 6. Far-infrared spectra of Nb-based complex perovskites with isovalentsubstitution of Ca and Ba.

084105-6 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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the second one at 180–260 cm−1, the third one at260–440 cm−1, and the fourth one between 440 and700 cm−1. Other features, abundantly observed in the spec-tra, must be due to the additional polar modes activated bythe multiple phases present. On the other hand, Ca-puresamples showed important differences �Figs. 6 and 7, x=0�,including a more defined set of bands at 220, 265, 318, 370,396, 470, 540, 570, and 670 cm−1. The frequency region be-low 150 cm−1 can be attributed to the A–BO3 external mode,while the O–B–O bending modes appear between 170 and500 cm−1.12,25 The highest wavenumber range,500–700 cm−1, is due to the oxygen octahedral-elongationmode, that is, BO6 stretching. Tantalum substitution leads toshifting of modes to lower frequencies in Ca-based materials,as expected, as well as to the enhancement of the bands at370, 470, and 540 cm−1. This behavior can be associatedwith the stiffness of the oxygen octahedra with Ta, comparedto Nb ions.

Figures 6 and 7 also present the results of the far-infrared analyses for the Ca- and Ba-mixed microwave ce-ramics. The modes below 100 cm−1 and above 600 cm−1 aredifficult to be assigned due to instabilities in the measure-ments with consequent poor quality. Both spectra of niobatesand tantalates presented similar behavior. Barium substitu-tion leads to a better definition of the bands at around130 cm−1 with a maximum at x=4. Also, a continuous shiftoccurred, especially in the dip centered at 490–460 cm−1

with the addition of one Ba atom. It is interesting to note theincreasing of the band at 520 cm−1, which is related to oxy-gen octahedral-elongation modes.12 Stronger bands at 130and 180–260 cm−1 are a result of perturbations in theO-metal-O bending modes �inner mode vibrations� on Bsites, as well as in the band around 320 cm−1. The maximumbroadening occurred at x=4 in the spectral range of150–370 cm−1 and coincided with the formation of solid so-

lutions of nominal compositions Ba�Ca1/4Nb2/4Ti1/4�O3 andBa�Ca1/4Ta2/4Ti1/4�O3. This frequency range can be consid-ered as specific for ordered structures and their strength canbe used as a relative measure of this ordering.12,25 The fre-quency shifts observed from 440 to 420 cm−1 and from550 to 520 cm−1, for increasing x, could be related to BO3

torsional ��2� and BO stretching ��1� modes, and confirm ourassumptions that Ba ions share the A sites with Ca ions, forx=1–3, or fully occupy this site, for x=4.

IV. CONCLUSIONS

Ca5−xBaxNb2TiO12 and Ca5−xBaxTa2TiO12 were synthe-sized and their structural characteristics and microwave di-electric properties were studied aiming to their application atmicrowave frequencies. Raman and far-infrared spec-troscopies were employed to study the influence of isovalentsubstitution of Ca/Ba and Nb/Ta on the vibrational modesof these materials. Structural characterization was carried outby x-ray diffraction, which showed different crystallinephases as a function of cation substitution. The dielectricproperties were measured at microwave frequencies and theresults showed a decreasing tendency of Qu for both Nb- andTa-based ceramics with Ba introduction. Raman spectrashowed bands related to A-site as well as B-site vibrations.From the spectroscopic point of view, materials with Ta ionsdiffer from those with Nb ones only by shifts to lower fre-quencies �redshift�. However, the A1g�O� mode blueshiftedfor the ceramics with Ta atom as a result of a higher stiffnessof the oxygen octahedral cage. Isovalent Ba substitution forCa produced significant changes on the Raman spectra. Asingle-phase solid solution was obtained only for the compo-sition CaBa4Nb2�Ta2�TiO12, which represents an orderedstructure, where all Ba ions lie on A sites and all Ca ions onB sites. Far-infrared results agree well with the Raman data,with few variations when Ta replaced Nb, except by bandshifts towards lower frequencies. The introduction of cal-cium in the structure of Ba-based ceramics produced pertur-bations on the polar vibrational modes, especially on thebending modes �B sites�, which increased their intensities.

ACKNOWLEDGMENTS

The Brazilian authors �A.D. and R.L.M.� acknowledgethe financial support from CNPq/MCT �Profix and The Mil-lennium Institute: Water, a mineral approach�. The Indianauthors �P.V.B. and M.T.S.� thank Council of Scientific andIndustrial Research �CSIR� New Delhi, India.

1T. A. Vanderah, Science 298, 1182 �2002�.2R. J. Cava, J. Mater. Chem. 11, 54 �2001�.3P. V. Bijumon, P. Mohanan, and M. T. Sebastian, Jpn. J. Appl. Phys., Part1 41, 3834 �2002�.

4P. V. Bijumon, P. Mohanan, and M. T. Sebastian, Mater. Lett. 57, 1380�2003�.

5A. Feteira, K. Sarma, N. McN Alford, I. M. Reaney, and D. C. Sinclair, J.Am. Ceram. Soc. 86, 511 �2003�.

6H. Kagata and J. Kato, Jpn. J. Appl. Phys., Part 1 33, 5463 �1994�.7R. J. Cava, J. J. Krajewski, and R. S. Roth, Mater. Res. Bull. 34, 355�1999�.

8L. A. Bendersky, J. J. Krajewski, and R. J. Cava, J. Eur. Ceram. Soc. 21,2653 �2001�.

9L. A. Bendersky, I. Levin, R. S. Roth, and A. J. Shapiro, J. Solid State

FIG. 7. Far-infrared results of Ta-based complex perovskites with mixing ofCa and Ba in their composition.

084105-7 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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Chem. 160, 257 �2001�.10P. V. Bijumon, S. K. Menon, M. N. Suma, M. T. Sebastian, and P. Moha-

nan, Electron. Lett. 41, 7 �2005�.11S. Mridula, S. K. Menon, P. Mohanan, P. V. Bijumon, and M. T. Sebastian,

Microwave Opt. Technol. Lett. 40, 316 �2004�.12P. V. Bijumon, P. Mohanan, M. T. Sebastian, R. L. Moreira, and A. Dias,

J. Appl. Phys. 97, 104108 �2005�.13B. W. Hakki, IEEE Trans. Microwave Theory Tech. MTT-8, 402 �1960�.14W. E. Courtney, IEEE Trans. Microwave Theory Tech. MTT-18, 476

�1970�.15J. Krupka, K. Derzakowski, B. Riddle, and J. B. Jarvis, Meas. Sci.

Technol. 9, 1751 �1998�.16R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen.

Crystallogr. A32, 751 �1976�.17A. M. Glazer, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem.

B28, 3384 �1972�.

18T. Hirata, K. Ishioka, and M. Kitasima, J. Solid State Chem. 124, 353�1996�.

19H. Zheng, G. D. C. Csete de Gyorgyfalva, R. Quimby, H. Bagshaw, R.Ubic, I. M. Reaney, and J. Yarwood, J. Eur. Ceram. Soc. 23, 2653 �2003�.

20I. Levin, J. Y. Chan, R. G. Geyer, J. E. Maslar, and T. A. Vanderah, J.Solid State Chem. 156, 122 �2001�.

21H. Zheng, I. M. Reaney, G. D. C. Csete de Gyorgyfalva, R. Ubic, J.Yarwood, M. P. Seabra, and V. M. Ferreira, J. Mater. Res. 19, 488 �2004�.

22I. G. Siny, R. Tao, R. S. Katiyar, R. Guo, and A. S. Bhalla, J. Phys. Chem.Solids 59, 181 �1998�.

23H. Zheng, H. Bagshaw, G. D. C. Csete de Gyorgyfalva, I. M. Reaney, R.Ubic, and J. Yarwood, J. Appl. Phys. 94, 2948 �2003�.

24S. J. Webb, J. Breeze, R. I. Scott, D. S. Cannell, D. M. Iddles, and N. McNAlford, J. Am. Ceram. Soc. 85, 1753 �2002�.

25A. Dias and R. L. Moreira, J. Appl. Phys. 94, 3414 �2003�.

084105-8 Dias et al. J. Appl. Phys. 98, 084105 �2005�

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