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Journal of Alloys and Compounds 719 (2017) 58e62

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Effect of Co-substitution on microwave dielectric properties ofLi3(Mg1�xCox)2NbO6 (0.00 � x � 0.10) ceramics

C.F. Xing, J.X. Bi, H.T. Wu*

School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

a r t i c l e i n f o

Article history:Received 14 January 2017Received in revised form3 May 2017Accepted 13 May 2017Available online 17 May 2017

Keywords:Microwave dielectric ceramicsLi3(Mg1�xCox)2NbO6

Temperature stableHigh Q

* Corresponding author.E-mail address: mse_wuht@ujn.edu.cn (H.T. Wu).

http://dx.doi.org/10.1016/j.jallcom.2017.05.1390925-8388/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

The Li3(Mg1�xCox)2NbO6 (0.00 � x � 0.10) ceramics were prepared by the solid-state reaction route inthis paper. The effects of Co-substitution on phase composition, microstructure and microwave dielectricproperties of the Li3Mg2NbO6 were investigated by a variety of characterization methods. XRD patternsand Raman spectrum revealed that the all the compositions possessed orthorhombic structure with thespace group of Fddd. The Q·f values of samples sintered at 1300 �C increased from 91,600 GHz at x ¼ 0 to127,600 GHz at x ¼ 0.02, which was basically attributed to the appropriate amount of Co2þ. With the xvalue increasing from 0 to 0.10, the tf value was gradually shifted toward zero, which indicated that theionic substitution could effectively adjust the crystal stability of the matrix. In addition, there were nosignificant changes in the dielectric constant and grain sizes in the Li3(Mg1�xCox)2NbO6 compositions.Out these compositions, the Li3(Mg0.98Co0.02)2NbO6 ceramic sintered at 1300 �C exhibited excellentmicrowave dielectric properties of 3r ¼ 15.22, Q�f ¼ 127,600 GHz and tf ¼ �3.64 ppm/�C, which madethe ceramics promising for future applications.

© 2017 Elsevier B.V. All rights reserved.

1. Introduction

With the operating frequency of mobile telecommunicationsexpanding to the microwave range, microwave dielectric materialshave attracted increasing interest as key components of the filter,oscillator and antenna in the applications ranging from cellularphones to the global positioning system [1,2]. In order to promoteindustrial development in these passive components, scientists aresearching for suitable materials with light weight, low productioncost and appropriate dielectric properties. Themicrowave dielectricceramic is a good candidate for practical application, which couldbe easier to satisfy three major criteria: an appropriate dielectricconstant ( 3r), a high quality factor (Q·f) and a near zero temperaturecoefficient of resonant frequency (tf) [3e6].

Recently, the orthorhombic structured Li3Mg2NbO6 system hasbeen extensively studied due to its satisfactory dielectric propertiesat high frequencies [7e10]. Yuan et al. firstly reported that theLi3Mg2NbO6 ceramics exhibited microwave dielectric properties of3r ¼ 16.8, Q�f ¼ 79,643 GHz and tf ¼ �27.2 ppm/�C [7]. The similarresults were repeated in our previous work ( 3r ¼ 14.94,

Q�f ¼ 100,965 GHz and tf ¼ �21.96 ppm/�C). The intrinsic pa-rameters of Li3Mg2NbO6 such as bond ionicity and lattice energywere also quantified by using the complex chemical bond theory[8]. Problems however, the large negative temperature coefficientof resonant frequency limited its practical application in wirelesscommunication system. In addition, it is also meaningful to obtainbalanced properties as well as to investigate the structure-propertyrelationship of the system. Zuo et al. reported that theLi3(Mg0.92Zn0.08)2NbO6 ceramics possessed a higher quality factorof 142,311 GHz at 1120 �C, while the tf value showed a downwardtrend with the increasing content of Zn2þ [9]. The Mg2þ ions in thematrix were also substituted by different divalent ions (A ¼ Ca2þ,Ni2þ, Zn2þ) and it could be concluded that the tf-adjustedLi3(Mg0.95Ca0.05)2NbO6 ceramic produce appropriate microwavedielectric properties of 3r ¼ 15.62, Q�f ¼ 96,160 GHz, andtf ¼ �18.49 ppm/�C. The substitution of Co2þ (<10 mol%) has beenshown to possess a significant effect on the microwave dielectricproperties in many ceramics [10]. For instance, Huang et al. inves-tigated the effects of Co2þ substitution on the microwave dielectricproperties in Li2MgTi3O8 samples, and they confirmed that a suit-able amount of Co2þ decreased the dielectric loss as well as slightlyadjusted the tf toward zero [11]. Li et al. reported the similar resultsthat the tf value was shifted from �55.4 ppm/�C at x ¼ 0to �53.5 ppm/�C at x ¼ 0.05 in the (Mg1�xCox)TiO3 system [12].

Fig. 1. Apparent density of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at1100e1300 �C, and the inset showed the relative densities of samples sintered at1250 �C.

C.F. Xing et al. / Journal of Alloys and Compounds 719 (2017) 58e62 59

Furthermore, the radius of Co2þ (0.65 Å, CN ¼ 6) was similar toMg2þ (0.72Å, CN¼ 6) and a small amount of the substitutionwouldnot apparently effect the phase compositions. However, to the bestof our knowledge, there is no report in substitutingMg2þ by Co2þ inthe Li3Mg2NbO6 ceramics. Thus, the effects of Co2þ substitution onmicrowave dielectric properties, phase composition and micro-structure of the Li3Mg2NbO6 system will be investigated in ourpresent work.

2. Experimental procedure

The Li3(Mg1�xCox)2NbO6 (x ¼ 0.00, 0.02, 0.04, 0.06, 0.08 and0.10) ceramics were prepared by a solid-state reaction route using

Fig. 2. SEM micrographs of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sinte

high purity raw materials (99.9%) of MgO, Li2CO3, CoO and Nb2O5.The starting powders were weighed as the formula of Li3(Mg1�x-

Cox)2NbO6 and milled in a nylon jar with zirconia balls in ethanolfor 4 h. The resulting mixtures were dried and calcined at 1000 �Cfor 4 h. Subsequently, the calcined powders were reground for 5 h,dried, mixed with polyvinyl alcohol as a binder and granulated. Theobtained powder was axially pressed into cylindrical disks with athickness of 6 mm and a diameter of 10 mm under a pressure of200 MPa. The pellets of Li3(Mg1�xCox)2NbO6 were sintered at1100 �C, 1150 �C, 1200 �C, 1250 �C and 1300 �C for 4 h in air with atemperature-ramp rate of 5 �C/min.

After sintering, the apparent densities of the samples weremeasured using the Archimedes method (Mettler ToledoXS64). Thephase analysis was examined by an X-ray diffractometer (XRD)using CuKa radiation (l ¼ 0.1542 nm) at 40 KV to 40 mA settings(Model D/MAX-B, Rigaku Co., Japan). The morphology and grainsizes were examined using a scanning electron microscope (ModelJEOL JEM-2010, FEI Co., Japan). Raman spectra of the samples werecollected using the high resolution Raman spectrometer (LobRAMHR Evolution, HORIBA Jobin Yvon S.A.S.). A network analyzer(N5234A, Agilent Co., America) was used for measurement of mi-crowave dielectric properties. Dielectric constants were measuredusing Hakki-Coleman post-resonator method by exciting the TE011resonant mode of dielectric resonator by using an electric probe assuggested by Hakki and Coleman [13]. Unloaded quality factorswere measured using TE01d mode by the cavity method [14]. Theabove results were measured at room temperature and in the fre-quency of 8e12 GHz. The temperature coefficients of resonantfrequency were calculated from data collected at 25 �C and 85 �Caccording to tf ¼△f/(f0△T), where f0 was the resonant frequenciesat 25 �C.

3. Results and discussion

The apparent densities of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10)ceramics sintered at different temperatures were plotted in Fig. 1.

red at 1300 �C (aef corresponding to x ¼ 0.00, 0.02, 0.04, 0.06, 0.08, 0.10).

Fig. 3. XRD patterns of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at1300 �C, and the inset showed the variation of (111) and (004) reflection peaks.

C.F. Xing et al. / Journal of Alloys and Compounds 719 (2017) 58e6260

With the sintering temperature increasing from 1100 to 1250 �C,the apparent densities of all the samples witnessed a upward trend,reaching around 3.6 g/cm3 at 1250 �C. Specifically, the apparentdensities of the low doped samples (x ¼ 0e0.06) obtained themaximum values at 1250 �C, while those of higher doped samples(x ¼ 0.08e0.10) peaked around 3.7 g/cm3 at 1200 �C. Thus, it couldbe considered that the substitution of Co2þ indirectly influencedthe best sintering temperature of the system. The inset of Fig. 1listed the relative densities of Li3(Mg1�xCox)2NbO6 sintered at1250 �C. The values fluctuated between 92.15% and 96.60% with theincrease of Co2þ content, implying the substitution played littleeffect on the density of the matrix. However, the apparent densitiesof all the samples significantly decreased by 0.1e0.4 g/cm3 at1300 �C, whichmight be caused by the evaporation of lithium oxidein air atmosphere. Fig. 2(aef) demonstrated the typical surface SEMmicrographs of the Li3(Mg1�xCox)2NbO6 ceramics with different xvalues sintered at 1300 �C. Closely-packed grains as well asdiscernable grain boundaries could be observed in all the samples,which indicated the substitution of Co2þ played little effect on thecrystal growth in Li3Mg2NbO6 ceramics. In addition, although theapparent densities slightly decreased at 1300 �C, there were nosignificant pores on the surface of the samples.

The XRD patterns of the Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10)ceramics sintered at 1300 �C were shown in Fig. 3. At x ¼ 0.00, thepeaks of Li3Mg2NbO6 were indexed as (111), (11 3), (0 0 4), (0 2 6)and so on. It was obvious that all the observed reflection peakscould be indexed in terms of Li3Mg2NbO6 (JCPDS-PDF 36-1018)with orthorhombic structure. With the x value increasing from 0.00to 0.10, there were no significant peaks of second phase in all theXRD patterns, implying continuous solid solutions were formed inthe whole composition range. As shown in the inset of Fig. 3, thediffraction peaks of crystal face (1 1 1) and (4 0 0) were shiftedtoward higher angle with the increasing x, suggesting the lowerionic size of Co2þ (0.65 Å) in comparisonwith that of Mg2þ (0.72 Å).In addition, the small amount of evaporation might indirectly affectthe unit cell volume of Li3(Mg1�xCox)2NbO6, which could beobserved in the inset of Fig. 3.

Fig. 4(a) illustrated the deconvoluted Raman spectrum ofLi3(Mg0.98Co0.02)2NbO6 ceramic in the frequency range of30e1000 cm�1. According to the group theory calculation, theLi3Mg2NbO6 crystal with Fddd space group and D2h (mmm) pointgroup was expected to possess 51 fundamental Raman activemodes.

G ¼ 8Agþ12B1gþ15B2gþ16B3g (1)

However, only 10 vibration modes could be observed from theRaman spectra, which was mainly caused by the overlapped orinteracted modes. As shown in Fig. 4(a), four typical Raman peakswere observed at 347, 423, 495 and 744 cm�1, which wereconfirmed as the signature of Li3Mg2NbO6 [15]. For instance, thepeak at 744 cm�1 was correspond to Ag(O) mode and was assignedto stretching vibrations of the NbO6 octahedra, while the othermodes were considered as the vibration of Li/MgeO bonds. In orderto investigate the crystal structure of the orthorhombic-structuredcompositions, the Raman spectra of the Li3(Mg1�xCox)2NbO6(0.02 � x � 0.10) ceramics were carried out in Fig. 4(b). With the xvalue increasing from 0.02 to 0.10, there were no significantchanges in both the peak position and full width at half maximums(FWHMs) of the intense Ramanmodes, implying the substitution ofCo2þ played a little effect on the crystal structure of the matrix.

Fig. 5 showed the changes in the dielectric constant ofLi3(Mg1�xCox)2NbO6 ceramics as a function of Co2þ content andsintering temperature. In general, the dielectric constant is affectedby both the extrinsic parameters (such as porosity and the sec-ondary phase) and intrinsic parameters (such as polarizability andcrystal structure) of the materials [16]. Fig. 3 indicated that therewas no second phase existed in the entire composition range.Therefore, the dielectric constant of Li3(Mg1�xCox)2NbO6 ceramicswas dependent on the variation of apparent density and dielectricpolarizability in this work. As shown in Fig. 5, when the sinteringtemperature increased from1050 to 1300 �C, the dielectric constantof Co2þ substituted ceramics initially increased, reaching thesaturated values (~16) at 1250 �C, and then decreased as the sin-tering temperature increased further. These changes were consid-ered to correlate with the compactness of the samples. For thesamples sintered at 1300 �C, the 3r values slightly fluctuated from15.06 to 16.17 with the x value increasing from 0 to 0.1, implyingthat the dielectric constant showed limited dependence on theCo2þ content. Based on the above results, the theoretical dielectricpolarizabilities (atheo.) were calculated according to the Shannon'sadditive rule given in Eq. (2) [17], while the observed dielectricpolarizabilities (aobs.) were obtained by Clausius-Mossotti equationgiven in Eq. (3) [18].

atheo�Li3ðMg1�xCoxÞ2NbO6

� ¼ 3aD�Liþ

�þ 2ð1� xÞaD

�Mg2þ

�

þ 2xaD�Co2þ

�þ aD

�Nb5þ

�

þ 6aD�O2�

�

(2)

aobs: ¼1bVm

3r � 13r þ 2

(3)

where a(Liþ), a(Mg2þ), a(Co2þ), a(Nb5þ), a(O2�) represented oxidespolarizability abilities reported by Shannon [18], Vm, 3and b indi-cated the molar volume of samples, dielectric constant and con-stant value (4p/3), respectively. As shown in the results listed inTable 1, the aobs. Values were slightly lower than atheo. Values whenthe samples were sintered at 1300 �C, which might be result fromthe evaporation of lithium oxide.

The Q�f values of Li3(Mg1�xCox)2NbO6 ceramics sintered atdifferent temperatures were shown in Fig. 6. In general, thedielectric loss in the range of microwave frequency consist ofintrinsic contributions such as structure characteristics, andextrinsic contributions such as impurities, density, secondaryphases and grain sizes [19]. With regard to the extrinsic factors, the

Fig. 4. (a) Deconvoluted Raman spectrum of Li3(Mg0.98Co0.02)2NbO6 ceramics sintered at 1300 �C (b) Raman spectra of Li3(Mg1�xCox)2NbO6 (x ¼ 0.02e0.10) ceramics sintered at1300 �C.

Fig. 5. Dielectric constant of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at1100e1300 �C.

Table 1Theoretical dielectric polarizability (atheo.), observed dielectric polarizability (aobs.),dielectric constant ( 3r), quality factor (Q�f) and temperature coefficient of resonantfrequency (tf) of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at 1300 �C.

Li3(Mg1�xCox)2NbO6 atheo. aobs. 3r Q�f (104 GHz) tf (ppm/�C)

X ¼ 0 22.34 22.01 15.51 9.16 �19.62X ¼ 0.02 22.28 21.95 15.22 12.76 �3.64X ¼ 0.04 22.23 22.21 16.17 9.88 �0.92X ¼ 0.06 22.18 21.73 15.06 9.70 �7.33X ¼ 0.08 22.12 21.97 15.59 9.83 �7.31X ¼ 0.10 22.07 21.95 15.46 9.80 �7.63

Fig. 6. Quality factor of Li3(Mg1�xCox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at1100e1300 �C.

C.F. Xing et al. / Journal of Alloys and Compounds 719 (2017) 58e62 61

XRD results indicated that all the samples were classified as singlephase, and thus the effect of the second phase could be neglected.As shown in Fig. 6, the Q$f values of all the samples initially showedan increasing tendency with the sintering temperature increasingfrom 1100 to 1200 �C, which was explained by the reduction ofporosity shown in Fig. 1. These changes were considered to corre-late with the compactness of the samples: a higher Q�f value wasusually related to a denser microstructure [20]. Thereafter, almostall the values remained stable in the temperature region of1150e1200 �C, and the intrinsic factors played an important role in

affecting dielectric losses when the ceramics densified.Table 1 also illustrated the Q�f, tf and 3r values of Li3(Mg1�x-

Cox)2NbO6 (x ¼ 0.00e0.10) ceramics sintered at 1300 �C and dis-played comprehensive properties as a function of the x values. TheQ�f value gradually increased from 91,600 GHz to 127,600 GHz andthen decreased to a stable value with the x value increased from0.00 to 0.10. Out of all doped samples, the x ¼ 0.02 sample sinteredat 1300 �C shown the highestQ� f values (127,600 GHz), whichwasmuch higher than that of the sample Li3Mg2NbO6 (100,965 GHz)reported in our previous and present work [8]. However, furtherincreasing x value resulted in the decrease of Q�f value, which wasbasically attributed to the appropriate amount of Co2þ. The tf valuewas well-known to be influenced by the composition, the additiveand second phase of the materials [21e23], while the variation of tfin Li3(Mg1�xCox)2NbO6 ceramics was closely related to theincreasing content of Co2þ in this work. As shown in Table 1, the tfvalues increased from �19.62 to �3.64 ppm/�C and then decreasedto around�7.5 ppm/�Cwith the increasing x value. Especially, near-zero tf values could be observed in Li3(Mg0.98Co0.02)2NbO6(�3.64 ppm/�C) and Li3(Mg0.96Co0.04)2NbO6 (�0.92 ppm/�C) ce-ramics, implying the substitution of Co2þ effectively stabilized thestructure of thematrix. Although the 3r values kept relatively steady

C.F. Xing et al. / Journal of Alloys and Compounds 719 (2017) 58e6262

in the range of 0.00 � x � 010, the substitution of Co2þ could beconsidered as an effective method to adjust tf and Q�f values.Typically, Li3(Mg0.98Co0.02)2NbO6 ceramics sintered at 1300 �Cpossessed excellent microwave dielectric properties with a 3r of15.22, a high Q�f of 127,600 GHz and a near-zero tf of �3.64 ppm/�C.

4. Conclusion

The Li3(Mg1�xCox)2NbO6 ceramics were successfully preparedby the conventional solid state method. The effect of Co2þ substi-tution on the crystal structure and microwave dielectric propertiesof Li3Mg2NbO6 was systematically discussed. All the ceramicsdensified and possessed a space group of Fddd above 1250 �C,implying that the ionic substitution did not affect the phasecomposition of the matrix. Although the theoretical dielectric po-larizabilities matched well with the observed values, there were noobvious changes in 3r values with the increasing x values. Thevariation of Q�f values was closely related to the increasing sin-tering temperature below 1300 �C, while the appropriate amountof Co2þ (x ¼ 0.02) effectively reduced the dielectric losses ofLi3Mg2NbO6 at 1300 �C. In addition, the tf value was graduallyshifted toward zero with the increasing amount of Co2þ, whichindicated the ionic substitution could stabilize the structure of thematrix. The Li3(Mg0.98Co0.02)2NbO6 ceramics sintered at 1300 �Cpossessed best combination of microwave dielectric properties of3r ¼ 15.22, Q�f ¼ 127,600 GHz, tf ¼ �3.64 ppm/�C, which made theceramics promising for further researching.

Acknowledgments

This work was supported by the National Natural ScienceFoundation (No. 51472108) and Project funded by China Post-doctoral Science Foundation.

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