Structure and synergy performance of

Journal of Alloys and Compounds 763 (2018) 990e996

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Structure and synergy performance of (1-x)Sr0.25Ce0.5TiO3 -xLa(Mg0.5Ti0.5)O3 based microwave dielectric ceramics for 5G architecture Burhan Ullah a, b, c, Wen Lei a, b, **, Yi-Feng Yao a, b, Xiao-Chuan Wang a, b, Xiao-Hong Wang a, b, Muneeb Ur‑Rahman c, Wen-Zhong Lu a, b, * a b c

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, PR China Key Lab of Functional Materials for Electronic Information (B), Ministry of Education, Wuhan, 430074, PR China Department of Physics, Islamia College University Peshawar, K.P 25000, Pakistan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2018 Received in revised form 13 May 2018 Accepted 20 May 2018 Available online 22 May 2018

The crystal structure and microwave (MW) dielectric characteristics of the compound in the perovskite type series (1-x) (Sr0.25Ce0.5TiO3)-xLa(Mg0.5Ti0.5)O3, x ¼ 0.0 to 0.6 have been studied. The structural phase evolutions from orthorhombic (for x ¼ 0.0) to cubic (for x  0.15) as a function of La(Mg0.5Ti0.5)O3 have been investigated and are related to variation in the dielectric characteristics. For x ¼ 0.0, the electron diffraction (TEM) revealed that Sr0.25Ce0.5TiO3 compound were tilted in both anti-phase and anti-parallel displacement of A-site cation. The nonlinear trend of the temperature coefficient of resonant frequency (tf) vs dielectric constant resulted from the tilted (for x ¼ 0.0) to nontilted (for x  0.15) structure transition. The value of tf decreases markedly from þ194 ppm/ C (x ¼ 0.0) to þ 0.8 ppm/ C (x ¼ 0.6), but the Q  f value (Q-factor) increases from 19,238 GHz to 37,508 GHz. Low loss microwave (MW) dielectric properties: εr ¼ 47.0, Q  f ¼ 37508 GHz and tf ¼ þ15 ppm/ C and εr ¼ 39, Q  f ¼ 34098 GHz and tf ¼ þ0.8 ppm/ C were achieved for compositions with x ¼ 0.45 and 0.6. © 2018 Elsevier B.V. All rights reserved.

Keywords: Structure Microstructure Microwave dielectric properties

1. Introduction In recent year, the lack of microwave (MW) dielectric ceramic materials with dielectric constant (40  εr  80) draws considerable commercial and scientific attention [1,2]. Considering that both Sr/ CaTiO3-based perovskites display a high relative permittivity (SrTiO3, ST, εr ~ 300, and CaTiO3, CT, εr ~180) and a large tf value (ST, tf ~ þ1600 ppm/ C, and CT, tf ~ þ800 ppm/ C) [1,2], the ceramics with tf ~ 0 ppm/ C and dielectric constant of 40  εr  80 are possible by forming the solid solutions of ABO3 with either CT and/ 0 00 or ST [1,2]. On the one hand, Sr-based [Sr(B1=3 B2=3 )O3] complex perovskites often display stoichiometric cation ordering on the B site [3,4]. These ceramic materials are usually referred to as 1:2 ordered structure perovskites. It is well recognized that the MW

* Corresponding author. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China. ** Corresponding author. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China. E-mail addresses: [email protected] (W. Lei), [email protected] (W.-Z. Lu). https://doi.org/10.1016/j.jallcom.2018.05.231 0925-8388/© 2018 Elsevier B.V. All rights reserved.

dielectric characteristics can be improved by increasing the B-site degree of ordering and tailoring the ordering domain structures [5]. 0 00 On the other hand, A(B1=2 B1=2 )O3 stoichiometric perovskites are also important ceramic materials with a 1:1 B-site cation ratio [4,6], which may contain remarkable features on structure, microstructures and MW dielectric characteristics [7]. However, among 0 00 A(B1=2 B1=2 )O3 perovskites, La(Mg1=2 Ti1=2 )O3 with a cubic structure has a high value of εr, Q-factor and a negative tf value (εr ~ 27.6, Q  f ~11,4312 GHz, tf ~  81 ppm/◦C) (Fig. 1) [7]. 0 00 Recently, (1x)ABO3-xA(B1=2 B1=2 )O3 solid solution has been studied with promising dielectric characteristics (Fig. 1) [8,9]. For instance, Chen et al. have reported xLa(Mg0.5Ti0.5)O3-(1x)SrTiO3 ceramics with dielectric characteristics of εr ~ 46, Q  f ~34,000 GHz, and tf ~  0.12 ppm/ C, and (1x)SrTiO3  xNd(Mg0.5Ti0.5)O3 ceramics with εr ~ 45, Q  f ~ 44,000.0 GHz, and tf ~ 3 ppm/ C [10,11]. Meanwhile, a large εr value of 42, Q  f ~35,000.0 GHz, and tf ~ 10 ppm/ C is obtained in the xNd(Mg0.5Ti0.5)O3  (1x) CaTiO3 ceramics by Huang et al. [12]. In the meantime Fu et al. as well as Kim et al. focused on the structure chemistry and dielectric characteristics of xLnAlO3(1-x) (Sr0.4Na0.3Nd0.3)TiO3 (Ln ¼ Nd3þand Sm3þ) and xLnAlO3  (1-x)

B. Ullah et al. / Journal of Alloys and Compounds 763 (2018) 990e996

991

Fig. 1. Plot of the Q  f values and temperature coefficient of resonant frequency (TCF) as a function of dielectric constant for past and present work.

(Ca0.85Nd0.1)TiO3 (Ln ¼ Dy3þ, Sm3þ, and Er3þ) perovskite system; the solid solution displays εr ¼ 45.0e58.0, tf ¼ 1.2e12.8 ppm/ C and Q  f ¼ 38,000.0e14,600.0 GHz [13,14]. Nevertheless, replacement of A-site (Sr2þ/Ca2þ) ions by trivalent Nd3þ, La3þ and Na1þ has reverse effect on the εr and Q-factor (Q  f value) of the solid solution [15]. Therefore, for better understanding the structure chemistry and MW dielectric property correlation of doped SrTiO3based ceramics; it would be key problem to moved research in this possible path for its extrinsic and intrinsic mechanism. In our earlier reported work, stoichiometric Sr(11.5x)CexTiO3 (x ¼ 0.1 to 0.67, sintered in air and closed pipe) ceramics were studied systematically. The ceramic system is characterized by εr ¼ 40e61, Q  f ¼ 5699 22,682 GHz and tf ¼ þ0.9  þ5 ppm/ C respectively [16,17]. Besides this, the X-ray photoelectron spectroscopy (XPS) analyses revealed that all the samples displayed a cluster of Ce3þ  95%, rather than Ce4þ  5% [15,16]. Taking into account the XPS analysis on (1-x) (Sr0.5Ce0.5)TiO3þd ‒ xNdAlO3 [15], the existence of minor amount of Ce4þ (Ce4þ  2%, Ce3þ  98%) on the Sr2þ-sublattice should not change the defect chemistry mechanism to a significant extent. In addition, for stoichiometric Sr(11.5x)CexTiO3 system; the more Ce-ions content, the more cation defects (vacancies) and hence excessive formation of TiO2 (rutile) and CeO2 phases. This makes the perovskite structure more distorted, and consequently limited the solubility. Conversely, a high εr value (εr ¼ 72 and tf ¼ þ5 ppm/ C) with low Q  f value (high loss, 12052 GHz) have been obtained by forming solid solution between nonstoichiometric Sr0.5Ce0.5TiO3þd (tetragonal structure) and NdAlO3 (rhombohedral structure), such as (1x)Sr0.5Ce0.5TiO3þd -xNdAlO3 (0.0  x  0.4) [15]. On the one hand, the stoichiometric Sr(11.5x)CexTiO3 (for x ¼ 0.5) composition shows limited solubility (excessive formation of TiO2 (rutile) and CeO2 phases) for NdAlO3 instead of La(Mg0.5Ti0.5)O3 (cubic like structure), thus, the reduced and/or low dielectric loss (high Q-factor) is expected by Mg substitution for Ti in Sr0.25Ce0.5TiO3 (orthorhombic like structure) ceramics. On the other hand, the structure ordering is detected only in stoichiometric Sr(11.5x)CexTiO3 (x ¼ 0.5, see Fig. 2) ceramics while the ordered structure is disrupted (disappeared) in nonstoichiometric Sr(1x)CexTiO3þd (x ¼ 0.5) ceramics which affect the Q  f value [15e17]. Additionally, Al and Ti display different chemical and physical properties expect that Al has a tendency to densify at high sintering temperature [15,18], resultant in lattice defects in Sr(1x)CexTiO3þd compound, which improves the MW dielectric loss harshly.

Fig. 2. XRD patterns for Sr0.25Ce0.5TiO3 ceramics. The inside figure displays, the peak triplet associated with the higher angle reflection.

In pursuit of intermediate εr (30e60), high Q-factor (Q  f ¼ 20,000.0e60,000.0 GHz) and nearby zero tf ~ 0 ppm/ C for practical usage, stoichiometric (1-x)Sr0.25Ce0.5TiO3exLa(Mg0.5Ti0.5) O3 ceramics have been proposed. Besides this, special consideration was paid to the suppression of tf ~0 ppm/ C in the (1-x) Sr0.25Ce0.5TiO3 e xLa(Mg0.5Ti0.5)O3 ceramics while maintaining values of the Q  f and εr, suitable for the use in 5G applications. 2. Experimental procedure Commercial high-purity (99%) raw powders of La2O3 (SCR, Shanghai, China), SrCO3 (SCR, Shanghai, China), MgO (Huizhou Si Ruier, Guangdong, China), CeO2 (SCR, Shanghai, China), and TiO2 (Xian-Tao, Hubei, China) were weighed according to the stoichiometric formulation (1-x)Sr0.25Ce0.5TiO3exLa(Mg0.5Ti0.5)O3 (x ¼ 0.0e0.6), abbreviated SCTLMT. The stoichiometric SCT and LMT raw powders were ball-milled overnight separately with distilled water. After the ceramics powder were dried at 100  C, the SCT and LMT ceramics were calcined separately at 1100  C 1200  C for 3 h in air. The calcined powders were mixed together, sieved, remilled overnight and pressed into cylindrical samples (6 mm  12 mm in height and diameter) at 150 MPa. The cylindrical samples were then preheated at 550  C for 1 h before sintering at 1450e1550  C for 4 h in air. The lattice structure were analyzed through X-ray diffractometer (10 e80 2q angle, XRD) with Cu Ka radiation (Shimadzu, Kyoto, Japan, XRD-7000) and electron diffraction (TEM, FEI, Oregon, Tecnai G2 20, USA). The surface of the sintered samples were detected by scanning electron microscope (SEM, FEI, Sirion 200; Eindhoven, Netherland). The XRD results were further examined by the Rietveld refinement technique, using powder cell and GSAS program [19]. The relative densities (Table 2) were measured from ratio of bulk density and theoretical density. The theoretical densities were calculated using the following relations [16]:

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B. Ullah et al. / Journal of Alloys and Compounds 763 (2018) 990e996

3. Results and discussion

ZM ¼ NA  Vunit

(1)

u1 þ u2 u1 =r1 þ u2 =r2

(2)

rtheo

r¼

where Z ¼ no of atoms/Unit cell, M is the molar weight (g/mol); NA ¼ 6.023  1023 atoms/mol is the Avogadro's number; r1 and r2 are the theoretical densities (r1 ¼ major phase and r2 ¼ minor phase); and u1 and u2 are the weight fractions (u1 ¼ major phase and u2 ¼ minor phase), respectively. The MW dielectric properties (εr, tf and Q  f value) over 2 GHze6 GHz were measured by TE011 mode method using network analyzer (Agilent E8362B, Agilent Technologies, USA) [20]. The tf was measured using the following formula:

tf ¼

1 ½f ðT1 Þ  f ðT0 Þ f ðT0 Þ T1  T0

(3)

where f (T1) and f (T0) represent the frequencies (resonant) at 80  C and 25  C, respectively.

Figs. (2)e(3) display X-ray and electron diffraction (XRD, TEM) patterns obtained from stoichiometric Sr0.25Ce0.5TiO3 compound. The compound was detected with the formation of two phases: Sr0.25Ce0.5TiO3 as the main phase with orthorhombic structure (Structure Database, JCPDS # 70e3939, space group Pmmm (47)), and minor impurity phase representing CeO2 (JCPDS # 81e0792) [16e18]. Based on the TEM/XRD analysis together with the earlier reported data [16e18], the evidence to the orthorhombic phase mainly arises from the existence of an extra super lattice and the splitting of the high angle reflection, both of which cannot be accounted for cubic/tetragonal phase structure [21]. In fact, the existence of the structure ordering peaks consistent with antiparallel displacement of the A-site cation (b) and antiphase tilting reflection (a) may be resulted from unit cell doubling (a ¼ 3.8852 Å, b ¼ 3.8493 Å, c ¼ 7.8030 Å, and V ¼ 116.7 Å3) due oxygen vacancy ordering and/or to octahedral tilting [16,21]. The surface morphology and/or microstructure of the Sr0.25Ce0.5TiO3 compound are detected by SEM as shown in Fig. 3(e). It can be seen that the samples exhibit a dense microstructure.

Fig. 3. 〈111〉 zone axis electron diffraction patterns (ZADP's) from Sr0.25Ce0.5TiO3 ceramics: (aec) for powder sample, (def) for pellet sample and (e) SEM image for pellet sample. Superlattice are specified as follows: antiparallel displacement of the A-site cation (b) and antiphase tilt reflection (a).

Fig. 4. (a) XRD patterns for SCT-LMT (0.15  x  0.6) composite ceramics. (bec) Refinement profile of calculated and observed XRD patterns for SCT-LMT (x ¼ 0.4) solid solution.

U

1a 1b 3c 1a 1a 1b 1b 0.025 0.024 1.000 0.004 0.400 0.200 0.002 0.150 0.800 1.000 0.285 0.400 0.200 0.015

occ. z

0.00000 0.50000 0.50000 0.00000 0.00000 0.50000 0.50000 0.00000 0.50000 0.50000 0.00000 0.00000 0.50000 0.50000

y x site

1a 1b 3c 1a 1a 1b 1a

occ.

0.023 0.032 0.004 0.002 0.002 0.009 0.002 0.00000 0.50000 0.50000 0.00000 0.00000 0.50000 0.00000

z y

0.00000 0.50000 0.50000 0.00000 0.00000 0.50000 0.00000 0.00000 0.50000 0.00000 0.00000 0.00000 0.50000 0.00000 Sr Ti O Ce(1) La Mg Ce(2)

x

0.150 0.800 1.000 0.285 0.400 0.200 0.015

U

Sr Ti O Ce(1) La Mg Ce(2)

0.00000 0.50000 0.00000 0.00000 0.00000 0.50000 0.50000

4þ Ce3þ/4þ mixed substitution (Ce3þ A þCeB ) 4þ Ce3þ/4þ in Sr sublattice (Ce3þ A þCeA )

Table 1 Structure parameters of SCT-LMT, (x ¼ 0.4) ceramics after refinement based upon, Pm-3m space group.

An impurity phase (second phase) appeared on the surface of polish sample which further supports the XRD result (Fig. 2). The Sr0.25Ce0.5TiO3 compound possess a good microwave dielectric properties with εr ¼ 104, Q  f ¼ 19, 238 GHz, and tf ¼ þ194 ppm/  C. Fig. 4 displays the X-ray diffraction patterns obtained from SCTLMT, where x ¼ 0.15e0.6, composite ceramics. All the peaks (reflections) were indexed based on the perovskite type structure (SrTiO3 cubic like structure, space group Pm-3m, JCPDS # 05e0634). There were no any other extra peaks corresponding to Mg/Ti structure ordering, or anti-parallel (Sr, La, Ce) displacement. Besides this, taking into account the structure stability factor for perovskite-type- structure it is expected that [15], Ti-site (B-site, 2þ 2þ r4þ Ti ¼ 0.605 Å) is occupied by the small ions (Mg , rMg ¼ 0.72 Å) [11], Sr-site (A-site, r2þ ¼ 1.44 Å) is occupied by the large ions Sr 3þ (r3þ Ce ¼ 1.34 Å, and rLa ¼ 1.36 Å) and the cations of intermediate sizes occupy both sites with different partitioning (r4þ Ce ¼ 0.87 Å) [11,12]. Further, for the sample with x ¼ 0.15, a shift to higher angles (2q angle) was detected in the main (110) peak, which shows the shrinkage of the lattice due to the substitution by Ce3þ/4þ 4þ 3þ (r3þ (r3þ Ce ¼ 1.34 Å, rCe ¼ 0.87 Å) and La La ¼ 1.36 Å) of Sr-site 2þ (rSr ¼ 1.44 Å) cation [15,22]. However, at 0.3  x  0.6 the main peak shift towards lower angle, which could be due to the substi2þ tution by Ce4þ (r4þ (r2þ Ce ¼ 0.87 Å) and Mg Mg ¼ 0.72 Å) cations in place of Ti4þ (r4þ ¼ 0.605 Å) at B-site. Ti Further, the XRD spectra were refined based on the cubic space group Pm-3m (Fig. 4). In order to obtain a reliable phase structure information, two possible paths of substitution were assumed: 4þ 3þ/4þ Ce3þ/4þ in Sr-site (Ce3þ mixed substitution A þCeA ) and Ce 4þ (Ce3þ þCe ). B A 4þ The first possibility (Sresite substitutions by Ce3þ A and CeA ) is based on computer simulation study [23], while the second pos3þ sibility is based on the ionic radii of Sr2þ (r2þ Sr ¼ 1.44 Å), Ce 3þ 4þ 4þ 4þ 4þ (rCe ¼ 1.34), Ti (rTi ¼ 0.605 Å) and Ce (rCe ¼ 0.87 Å) [22] respectively. Based on this finding, the final agreement factors and the structure parameters suggested that SCT- LMT crystallized into the cubic Pm-3m structure (Table 1). The grain surface morphology and/or microstructure of the SCTLMT (0.15  x  0.6) composite ceramics are detected by SEM as shown in Fig. 5(aed). It can be seen that the all samples exhibit a non-uniform grain size with dense microstructures. The average grain sizes increase with increasing LMT content together with low porosity. In summary, the ceramics sample exhibits a non-homogenous grain structure (microstructure) with large type of grains surrounded by small ones, suggesting that the amount of LMT (x) increases in the composite ceramics. However, increasing the content of LMT (x), the small grains are restricted within the grain boundary. Further, it can be seen that the minor amount of impurity phase (like CeO2 phase) indeed exists within the grain boundary in the (1-x)SCTxLMT with x ¼ 0.6 sample, and it is according with the XRD results. The variation in the dielectric properties as a function of LMT content of (1-x) SCTex LMT (0.0  x  0.6) composite ceramics are shown in Fig. 6(a)-(c) and Table 3. The composition dependent tendency of εr and tf displays the same variation behaviors, whereas the Q-factor (Q  f value) exhibits an opposite variation, that is with the addition of LMT content the εr and tf decreases while the Q  f values increase nonlinearly. The interpretations of the εr and relative density are comparative straightforward in that the properties display a strong dependence on composition [Fig. 6(a)]. At 0.15  x  0.45, a second phase was not detected; however εr decreased from 83.0 to 47.0, which is believed to be dominated by the relative density and smaller εr of La(Mg0.5Ti0.5)O3 (LMT, εr ¼ 27, Q  f ¼ 11,4312 GHz tf ¼ 81 ppm/ C) than Sr0.25Ce0.5TiO3 (SCT, εr ¼ 104, Q  f ¼ 19, 238 GHz, and

993

site

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Table 2 Structure parameters of SCT, (x ¼ 0.4) ceramics after refinement based upon, Pm-3m space group. Nominal composition

Structure parameters

SCT-LMT x ¼ 0.4

Symmetry Space group

x ¼ 0.4

Cubic, Pm-3m

x ¼ 0.4

Cubic, Pm-3m

Reliability factors (Overall GOF)

A-site Occupancy 3þ

La Ce3þ Ce4þ La3þ Ce3þ

B-site Occupancy

a (ű0.01)

b (ű0.01)

c (ű0.01)

Vunit (Å3)

RWP (%)

RP (%)

c2 (%)

a¼b¼g (Degree)

2þ

3.86433

3.86433

3.86433

57.70621

10.30

6.86

4.176

90

Mg2þ Ce4þ

3.86497

3.86497

3.86497

57.73489

10.15

7.02

4.041

90

Mg

Fig. 5. SEM images of (1-x)SCT-x LMT (0.15  x  0.6) composite ceramics with: (a) x ¼ 0.15, (b) x ¼ 0.3, (c) x ¼ 0.45, (d) x ¼ 0.6. Fig. 5[(e)e(f)] indicates the EDX analysis for x ¼ 0.6 compound.

Fig. 6. (a) Plot of the εr and relative density, (b) tf and porosity corrected εr value, and (c) Q-factor (Q  f) of the (1-x) SCTex LMT (0.0  x  0.6) ceramics vs x. The inside figure shows plot of tf versus εr.

tf ¼ þ194 ppm/ C) [7]. Whereas, the decrease in the value of relative density could be attributed to the rapid grain growth and/ or inhomogeneous grain morphology [Fig. 6(a)] [10,24].

Nevertheless, the shift in the value of εr for x ¼ 0.6 was very large, which was probably due to the formation of CeO2 phase (εr ¼ 24, tf ¼ 56 ppm/ C, Q  f ¼ 60,000 GHz) which exhibits a low

B. Ullah et al. / Journal of Alloys and Compounds 763 (2018) 990e996

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Table 3 Relative Density and Microwave Dielectric Parameters of SCT-LMT (0.0  x  0.6) composite ceramics. (1-x) (Sr0.25Ce0.5)TiO3 e x La(Mg0.5Ti0.5)O3

Microwave Dielectric Parameters

0.0  x  0.6

εr (Observed, ± 2%)

Q  f (GHz,± 3%)

tf (ppm/ºC, ±2%)

Relative density (observed,± 0.05)%

εr (Porosity corrected)

La(Mg0.5Ti0.5)O3, LMT Sr0.25Ce0.5TiO3, x ¼ 0.0 (Sr,Ce)T‒LMT, x ¼ 0.15 (Sr,Ce)T‒LMT, x ¼ 0.3 (Sr,Ce)T‒LMT, x ¼ 0.45 (Sr,Ce)T‒LMT, x ¼ 0.6

27.6 104.0 83.0 68.0 47.0 39.0

11,4312 19238 24640 31624 37508 34098

81.0 þ194.0 þ112.0 þ81.0 þ15.0 þ0.8

97.0 99.3 98.8 98.2 97.9 97.4

28.8 105.1 84.5 69.8 48.5 40.5

dielectric constant [16]. Generally, these variations are initiated by fluctuation between the A-site cation and TiO6 octahedron, subsequently affecting the effective ion polarizability [13,15,25]. Fig. 6(b) illustrates the tf and porosity corrected εr of (1-x)SCTex LMT (0.0  x  0.6) ceramics at constant sintering temperature. The porosity correct εr values were calculated by using Bosman and Having's relation [26];

εcorrected ¼ εr ð1 þ 1:5pÞ

(4)

where p is the fractional porosity, εcorrected and εr are the corrected and exprimental (measured) values of dielectric constant. The porosity corrected εr values for the composite ceramics varied from 105.1 (x ¼ 0.15) to 40.5 (x ¼ 0.6), slightly greater than the actual measured (experimentally) values [Fig. 6(b) and Table 3]. Meanwhile, the variations in tf of the sintered ceramics are not linear with LMT (x), but are significantly more pronounced for low LMT (x) content. The initial increase to x ¼ 0.15 results in the decrease of tf from þ194.0 to þ112.0 ppm/ C, whereas with the increase in LMT (x) from x ¼ 0.3 to 0.6 the tf deceases from þ81.0 to þ0.8 ppm/ C. This indicates that the initial increase in the LMT (x) content has a stronger effect on the tf than subsequent additions. Since, the tf is a composite parameter related to the linear thermal expansion coefficient (aL) and temperature coefficient of dielectric constant εr (tε) by Ref. [27]:



tf ¼ 

1 tε þ aL 2

 (5)

Also, Harrop related the temperature coefficient of capacitance (tc) and εr by Ref. [28]:

tc ¼ aL εr

(6)

Besides this, tε and tc is related by the following relation [29]:

tε ¼ tc  aL

(7)

Thus the relation amog εr,tf and aL is consider to be;

1 2

tf ¼ aL ð1  εr Þ

(8)

Thus the relationship between tf and εr is likely to be almost linear whentf value is larger than aL, which practically happen in the higher εr range [30]. Fig. 6(b) indicates that the value of tf decreased approximately linearly because of the compensation completely analogous to that of εr value. Nevertheless, the nonlinear trend of the tf seem to be abnormal [insert Fig. 6(c)]. Further, for the (1-x)SCTex LMT ceramics without octahedron titling,tf is primarily turned by tε. By considering the ClausiusMossotti differential equation, tε value is always negative [5,26], and supposed to decline with increasing εr within the range 10.0  εr  200 [5,26]. In the current ceramics compound, as εr decreases, tε become more positive, therefore the tf value

[7]

decreases. On the one hand, the tf value altered from large positive (SrTiO3, tf ¼ þ1650 ppm/ C, and εr ¼ 300) to a less positive value (Sr0.25Ce0.5TiO3, tf ¼ þ196 ppm/ C, and εr ¼ 104) at the onset of the cubic-to-orthorhombic structure phase evolution which is described by both antiparallel displacement of the A-site cation and anti-phase super lattice [31]. On the other hand, the tf changed from a less positive (Sr0.25Ce0.5TiO3, tf ¼ þ196 ppm/ C, and εr ¼ 104) to near zero (0.4Sr0.25Ce0.5TiO3 e 0.6 La(Mg0.5Ti0.5)O3, tf ¼ þ 0.8 ppm/ C, and εr ¼ 39.0) at the onset of the orthorhombic to cubic phase transition. As there is loss of structure symmetry at the structure phase transition there will be an increase in the structure distortion, and this will tend to encourage reduction intf [15,25]. Therefore,tf in Ti-based perovskite-type-structure ceramics is fundamentally associated with onset of structure phase transition relating the tilting of the octahedral [12,15,31]. The variation in Q  f values (Q-factor) as a function of LMT (x) content. The Q  f value of (1-x)SCTex LMT (0.0  x  0.6) ceramics increases nonlinearly with addition of La(Mg0.5Ti0.5)O3 content as shown in Fig. 6(c). Although, the Q-factor were markedly improved, however, still the observed enhancement deteriorated (limited) with addition of LMT content Fig. 6(c). This is attributed to phase structure evolution (ST cubic to Sr0.25Ce0.5TiO3 orthorhombic and turned to (1x)SCTxLMT cubic), where the atoms are in a state of re-orientation to form new type structure [15]. Meanwhile the increase in Q  f value may be originate from the variations of cation distribution in A-site and B-site respectively [5,29]. Thus, the reduced intrinsic dielectric loss is expected by La(Mg0.5Ti0.5)O3 substitution for Sr0.25Ce0.5TiO3, and the Q  f value increases with increasing x in (1-x)SCT-xLMT ceramics. Further, most perovskite ceramics, with large dielectric constant, have smaller Q-factor (Q  f value) due to the contribution of increasing anharmonic terms [16]. Besides this, many parameters can affect the dielectric loss (Q-factor) of dielectric resonator such as the pores, the secondary phases (CeO2, for x ¼ 0.6) and the lattice vibration modes. In general, a smaller grain boundary (a large grain size), shows a decrease in lattice imperfection and the MW dielectric loss was accordingly reduced. 4. Conclusion Orthorhombic phase structure with both antiparallel displacement of the A-site cation and anti-phase tilt reflection is identified for Sr0.25Ce0.5TiO3, where x ¼ 0.0 ceramics. The (1-x)SCTex LMT (0.0  x  0.6) composite ceramics exhibited a structure phase evolution from an orthorhombic (for x ¼ 0.0) to cubic (0.15  x  0.6) phase. Both εr and tf value decreased while Q  f value increased non-linearly with addition of LMT(x) content. Meanwhile, the εr remains nearly 47 at x ¼ 0.45, the tf value which is affected by both εr and octahedral tilting could be adjusted to near zero (tf ¼ 0.8 ppm/ C at x ¼ 0.6). Low loss microwave (MW) dielectric properties: εr ¼ 47, Q  f ¼ 37508 GHz and tf ¼ þ15 ppm/  C and ε ¼ 39, Q  f ¼ 34098 GHz and t ¼ þ0.8 ppm/ C were r f

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achieved for compositions with x ¼ 0.45 and 0.6. In conclusion, these ceramics system occupy a unique place in a number of microwave dielectric ceramics and have potential for commercial application as 5G resonator architecture. 5. Prime novelty statement  Orthorhombic phase structure with both antiparallel displacement of the A-site cation and anti-phase tilt reflection is identified for Sr0.25Ce0.5TiO3.  The nonlinear trend of tf vs εr resulted from the tilted (for x ¼ 0.0) to nontilted (for x  0.15) structure transition.  Low loss dielectric properties: εr ¼ 47.0e39, Q  f ¼ 37508 34098 GHz and tf ¼ þ15e0.8 ppm/ C were achieved for compositions with x ¼ 0.45 and 0.6.  These ceramics system have potential for commercial application as 5G resonator architecture. Acknowledgements This work was supported by National natural science foundation of China (NSFC-51772107, 51572093). The author wishes to thank the China Government for a scholarship. The authors are grateful to the analytical testing centre, Huazhong University of Science and Technology for XPS and TEM analysis. References [1] I.M. Reaney, D. Iddles, Microwave dielectric ceramics for resonators and filters in mobile phone networks, J. Am. Ceram. Soc. 89 (7) (2006) 2063e2072. [2] R. Freer, F. Azough, Microstructural engineering of microwave dielectric ceramics, J. Eur. Ceram. Soc. 28 (7) (2008) 1433e1441. [3] S. Kawashima, M. Nishida, I. Ueda, H. Ouchi, Ba(Zn1/3Ta2/3)O3 ceramics with low dielectric loss at microwave frequencies, J. Am. Ceram. Soc. 66 (1983) 421e423. [4] C.A. Randall, D.J. Barber, R.W. Whatmore, P. Groves, A TEM study of ordering in the perovskite, Pb(Sc1/2Ta1/2)O3, J. Mater. Sci. 21 (1986) 4456e4462. [5] P.P. Ma, X.Q. Liu, F.Q. Zhang, J.J. Xing, X.M. Chen, Sr(Ga0.5Nb0.5)1-xTixO3 Lowloss microwave dielectric ceramics with medium dielectric constant, J. Am. Ceram. Soc. 98 (2015) 2534e2540. [6] H. Wu, P.K. Davies, Influence of Non-stoichiometry on the structure and properties of Ba(Zn1/3Nb2/3)O3 microwave dielectrics, J. Am. Ceram. Soc. 89 (2006) 2271e2278. [7] M.P. Seabra, M. Avdeev, V.M. Ferreira, R.C. Pullar, N.M. Alford, I.M. Reaney, Structureeproperty relations in xBaTiO3e(1- x)La(Mg1/2Ti1/2)O3 solid solutions, J. Am. Ceram. Soc. 87 (2004) 584e590. [8] N. Santha, I.N. Jawahar, P. Mohanan, M.T. Sebastian, Microwave dielectric properties of (1-x)CaTiO3-xSm(Mg1/2Ti1/2)O3 [0.1 x 1] ceramics, Mater. Lett. 54 (2002) 318e322. [9] S. Kucheiko, J.W. Choi, H.J. Kim, H.J. Jung, Microwave dielectric properties of CaTiO3-Ca(Al1/2Ta1/2)O3 ceramics, J. Am. Ceram. Soc. 79 (1996) 2739e2743. [10] Y.B. Chen, New dielectric material system of xLa(Mg1/2Ti1/2)O3- (1-x)SrTiO3 in

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