Article pubs.acs.org/IC
Phase Evolution, Crystal Structure, and Microwave Dielectric Properties of Water-Insoluble (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.9) Ceramics Dan Guo,† Di Zhou,*,†,§ Wen-Bo Li,† Li-Xia Pang,‡,§ Yan-Zhu Dai,† and Ze-Ming Qi∥ †
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China ‡ Micro-optoelectronic Systems Laboratories, Xi’an Technological University, Xi’an 710032, Shaanxi, China § Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, U.K. ∥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Anhui 230029, Hefei, China ABSTRACT: In the present work, a series of low-temperature firing scheelite structured microwave dielectric in water-insoluble La2O3−Nb2O5− V2O5 system was prepared via the traditional solid-state reaction method. Backscattering electron diffraction, X-ray diffraction (XRD), energydispersive analysis, and Rietveld refinements were performed to study the phase evolution and crystal structure. In the full composition range of (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.9) ceramics, at least four typical phase regions including monoclinic fergusonite, tetragonal sheelite, B-site ordered sheelite, and composite of monoclinic LaVO4 and tetragonal sheelite phases can be detected according to XRD analysis. The variations of relative dielectric constant εr, quality factor Q × f, and resonant frequency τf could be attributed to Nb/V−O bond ionicity, lattice energy, and the coefficient of thermal expansion. Infrared reflectivity spectra analysis revealed that ion polarization contributed mainly to the permittivity in microwave frequencies ranges. Furthermore, the 0.7LaNbO4−0.3LaVO4 ceramic sintered at 1160 °C possessed excellent microwave dielectric properties with an εr of ∼17.78, a Q × f of ∼75 940 GHz, and a τf of ca. −36.8 ppm/°C. This series of materials might be good candidate for microwave devices.
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INTRODUCTION The traditional solid-state reaction method is used to prepare microwave dielectric ceramics by more and more researchers. One key point for the method is sintering. The sintering process can make grains more compact and let us obtain waterinsoluble characteristic ceramics. Therefore, the method has been widely accepted to prepare microwave dielectric ceramics. Nowadays, microwave dielectric ceramics are widely used as dielectric resonators, filters, substrates for radio frequency components, and waveguides.1−5 With the rapid development of these devices and technology, microwave dielectric ceramics with good performance are urgently needed and have been investigated for more than half a century. Generally, researchers are exploring microwave dielectric ceramics with high quality factor (Q × f),6,7 high relative dielectric constant (εr),8 and near-zero temperature coefficient of resonant frequency (τf) to improve these properties by two approaches, namely, addition and substitution. Compared with the classic perovskite ABO3 family, the adaptable ABO4 (A3+, B5+) families include scheelite, zircon, wolframite, rutile, and fergusonite materials, which have attracted much attention.9−11 Recently two new families of microwave dielectric ceramics, namely, rare-earth orthoniobates and orthovanadates, have attracted much attention due to their high quality factors. © XXXX American Chemical Society
Excellent microwave dielectric properties of the ReNbO4 (Re = La, Nd, Sm, Dy, Er, and Lu) have been reported by Kim et al.12,13 They revealed the facts that the structural distortion induced by phase transformation would lead to a relatively low εr value as well as an anomalous τf value. In this series of material, the LaNbO4 ceramic, along with sintering temperature of 1250 °C, εr of ∼19.3, τf of ca. +9 ppm/°C, and Q × f of ∼54 400 GHz, is a perfect material for the latest microwave devices. The reason for the high performance of the ReNbO4 is that this material possesses two characteristics including [NbO6] octahedral structures and rare-earth cations, which might play an important role in permittivity and Q × f.14,15 Therefore, there are some studies about the LaNbO4 ceramic to improve microwave dielectric properties or research relevant structure. Three approaches can be adopted by the substitution of rare-earth cations in the La sites, the substitution of other equivalent cations in the Nb sites, and low melting point oxide additives, respectively.16−18 Mei et al.16 have obtained the NdNbO4 samples with microwave dielectric properties of εr = 29.8, Q × f = 49 010 GHz, and τf = +53.8 ppm/°C. Fride Vullum et al.17 have studied the significant reduction in the Received: June 8, 2017
A
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
analyze the crystal structure of ceramics using Fullprof program. Diffraction pattern was obtained with 2θ in the range of 5°−120° at a step size of 0.02°. The room-temperature infrared reflectivity spectra were measured using a Bruker IFS 66v (Bruker Optics, Ettlingen, Germany) Fourier transform infrared (FT-IR) spectrometer on the infrared beamline station (U4) at the National Synchrotron Radiation Lab (NSRL), China. Microwave dielectric properties at microwave frequency were measured with the TE01δ dielectric resonator method with a network analyzer (8720ES, Agilent, Palo Alto, CA) and a temperature chamber (Delta 9023, Delta Design, Poway, CA) in the temperature range of 25−85 °C. The temperature coefficient of resonant frequency (τf value) was calculated with the following formula:
difference between the monoclinic and tetragonal thermal expansion coefficients (TEC) of the solid solution LaNb1−xTaxO4 using as electrolyte for thin-film solid-oxide fuel cells. Lee et al.18 investigated influence of CuO addition on the sintering behavior and microwave dielectric properties of the LaNbO4 ceramic; 3 wt % CuO−LaNbO4 can be welldensified at 950 °C with a Q × f of 49 000 GHz, εr of 19.5, and τf of 1 ppm/°C. The rare-earth orthovanadates ReVO4 (Re = La, Ce) have been reported by Wang and Zuo.19 The LaVO4 ceramics possesses excellent microwave dielectric properties with a εr = 14.20, a Q × f = 48 197 GHz, and a τf = −37.9 ppm/ °C. Besides, this series of the ReVO4 ceramics possess lower sintering temperature than the melting point of Ag (961 °C), especially LaVO4 ceramic sintered at 850 °C, which make it a good candidate for low-temperature cofired ceramics technology (LTCC).20−22 Recently, Randall et al.23 from Penn State found that some V2O5-based water-soluble dense ceramics (including pure V2O5 ceramic) can be obtained from a moistening powder at 120 °C and a pressure above 130 MPa due to the recrystallization during the volatilization of water, which can be understood as a combination of hydrothermal method and hot pressing sintering method. However, waterinsoluble ones are still dominating the main microwave devices, because those vapors exist anywhere, and electronic devices are required to work with the existence of vapors. Water-insoluble microwave dielectric ceramics are essential for industry, since they can be easily manufactured by using traditional solid-state reaction method and with device geometries optimized through tape-casting and screen printing. Our recent work on waterinsoluble Bi2O3−TiO2−V2O5 systems win intrinsic low sintering temperature has attracted much attention.24 More and more researchers have noticed the advantage of intrinsic low-temperature cofired microwave dielectric ceramics. To our knowledge, the microwave dielectric properties and structure of the LaNbO4−LaVO4 ceramics have not been reported. In this paper, we adjust LaNbO4 microwave dielectric properties by adding LaVO4 with opposite τf values to change [NbO6] octahedral structures. The preparation, characterization, and structure−property relation of the LaNbO4− LaVO4 ceramics were studied in detail.
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TCF =
fT − fT
0
fT × (T − T0) 0
× 1 × 106
(ppm/°C) (1)
where the f T and f T0 were the TE01δ resonant frequencies at temperature 85 and 25 °C, respectively.
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RESULTS AND DISCUSSION To understand the mechanism of these phase transitions, XRD patterns of the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.9) ceramic samples sintered at their optimal temperatures are presented in Figure 1. The lattice parameters of pure LaNbO4 are calculated
Figure 1. XRD patterns of the (1 − x)LaNbO4−xLaVO4 (0.00 ≤ x ≤ 0.9) ceramic samples sintered at their optimal temperatures. (heart shape) Monoclinic fergusonite; (⧫) tetragonal sheelite; (clover shape) B-site monoclinic ordered sheelite; (○) composite of monoclinic monazite LaVO4 and tetragonal sheelite phases.
EXPERIMENTAL SECTION
Proportionate amounts of reagent-grade starting materials of La2O3 (>99.95%, Guo-Yao Co. Ltd.), Nb2O5 (>99.5%, Guo-Yao Co. Ltd.), and V2O5 (>99%, Fuchen Chemical Reagent, Tianjin, China) were measured according to the stoichiometric compositions of the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.9). La2O3 powder was calcined at 900 °C for 4 h before weighting. Powders were mixed and milled for 4 h using a planetary mill (QM-1F; Nanjing Machine Factory, Nanjing, China) by setting the running speed at 450 rpm with the zirconia balls (2 mm in diameter) as milling media. The powder mixture was then dried and calcined at 750−900 °C for 4 h. The calcined powders were remilled for 4 h at 450 rpm to increase reactivity and homogeneity. Then the dried powders were pressed into cylinders in a steel die (12 mm in diameter and 5 mm in height) with 5 wt % poly(vinyl alcohol) (PVA) under a uniaxial pressure of 150 MPa. PVA was burnt out at 550 °C for 4 h (2 °C/min). Samples were sintered in the temperature range from 1000 to 1200 °C for 2 h. The crystal structures of (1 − x)LaNbO4−LaVO4 (0 ≤ x ≤ 0.9) were investigated using room-temperature X-ray diffraction (XRD) with Cu Kα radiation (Rigaku D/MAX-2400 X-ray diffractometry, Tokyo,Japan). Microstructures and energy-dispersive spectroscopy (EDS) of ceramics sintered at their optimum temperature were observed with scanning electron microscopy (SEM; Quanta 250 F, FEI). The Rietveld profile refinement method was employed to
as a = 5.570(5) Å, b = 11.538(5) Å, c = 5.209(8) Å, β = 94.055(0)°, and Vunit = 334.020(0) Å3. With the increase of substitution content, four phase regions including monoclinic fergusonite, tetragonal sheelite, B-site ordered sheelite, and composite of monoclinic LaVO4 and tetragonal sheelite phases can be detected as seen from XRD patterns. A fergusonitestructured solid solution with a space group I2/a is observed in the range of x = 0−0.2 as seen from the XRD patterns. It is clearly seen from Figure 1, the reflections (−121) and (−130) of (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.2) ceramics move toward each other slowly with increasing the substitution content, which is attributed to decrease of monoclinic distortion caused by the smaller ionic radii from V5+ (0.54 Å). According to Shannon, R. D. et al.’s25,26 reports, Nb5+ (0.64 Å) ions occupy the center of the octahedron (CN6) sites and may be substituted by V5+ (0.54 Å) in the monoclinic structure of (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.2) ceramics. Finally at x = 0.225, a single reflection peak (112) is observed, which means that crystal structure changed from monoclinic B
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry fergusonite to tetragonal scheelite. Both of the Nb5+ (0.48 Å) and V5+ (0.355 Å) begin to occupy the center of the tetrahedron (CN4) sites. Variation in lattice parameters of the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) are shown in Figure 2. Along with increase of V content, lattice parameters b
Figure 3. SEM images of the (1 − x)LaNbO4−xLaVO4 ceramics (a) x = 0.05, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4 sintered at 1160 °C for 2 h; backscattering electron diffraction (f) and the associated EDS analysis of surfaces of the 0.4LaNbO4−0.6LaVO4 ceramics sintered at 1160 °C for 2 h.
Figure 2. Lattice parameters variation in the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) ceramics.
and c in the solid solution region (x ≤ 0.2) increase slowly. Meanwhile, lattice parameter a decreases. With further increasing the concentration of x, the lattice parameters a and c became equal to each other, whereas the b value increased to 11.707(8) Å in tetragonal structure. Similarly, the monoclinic angle β decreased from 94.055° to 90° gradually. All these changes in the lattice parameters indicate the phase transition from monoclinic fergusonite to tetragonal scheelite structure. In the tetragonal sheelite structure region (0.225 ≤ x ≤ 0.35), both Nb and V ions scattered randomly on the B site, which is four-coordinated. This result is similar to Brandão et al.’s.27 report on the similar composition Sr0.02La0.98Nb0.75xV0.25O4−δ. When x is increased to 0.4, tetragonal sheelite structure transformed to a B-site ordered monoclinic sheelite, similar to that of the (K0.5Sm0.5)MoO4 and (K0.5Nd0.5)MoO4 materials.28 With further increase of substitution content to 0.6 ≤ x ≤ 0.9, a composite phase region of monoclinic LaVO4 and tetragonal sheelite phases was observed in the (1 − x)LaNbO4−xLaVO4 ceramics. These results are in accordance with Bastide et al.’s29 and ManjÓ n et al.’s30 study on the phase transition of ABX4 compounds with various cation−anion radii ratio coordinates. SEM, backscattered electron (BSE) images, and related EDS analysis of the (1 − x)LaNbO4−xLaVO4 (0.05 ≤ x ≤ 0.6) ceramics sintered at 1160 °C for 2 h are shown in Figure 3. As shown in Figure 3a−c, there are more and more pores with increase of the substitution content of V5+. Dense microstructure is revealed at x = 0.3 as shown in Figure 3d. BSED image of the 0.4LaNbO4−0.6LaVO4 ceramics is shown in Figure 3f, and there are two different kinds of grains with gray and white contrast. According to EDS analysis on spot A and spot B, the regions of gray and white contrast are rich in vanadium and niobium elements, respectively. Conbined with the XRD analysis above, the gray grains belong to the LaVO4type solid solution, while the white ones belong to the (K0.5Sm0.5)MoO4-type B-site ordered monoclinic sheelite solid solution. To study the detailed influence of V5+ substitution on crystal structure, Rietveld refinements using Fullprof software were chosen to analyze the crystal structure of the (1 − x)LaNbO4− xLaVO4 ceramics. Rietveld refinement was continuously performed on all the XRD angles of the (1 − x)LaNbO4−
xLaVO4 (0.00 ≤ x ≤ 0.225) collected at different x values, and the plots are shown in Figure 4. The crystallographic information, such as space group, atomic positions, and occupancies of different vanadium content (0.00 ≤ x ≤ 0.225) from Rietveld refinements are given in Table 1, and bond lengths d are given in Table 2. There are two structures including monoclinic fergusonite with I2/a space group and tetragonal sheelite structure with I41/a (No. 88) space group when x are in regions of 0.00 ≤ x ≤ 0.2 and x = 0.225, respectively. The results of the phase structures are satisfactory to refinement parameters (Rwp, Rp). Figure 5 has shown schematic representation of monoclinic fergusonite LaNbO4 (a), tetragonal sheelite 0.775LaNbO4− 0.225LaVO4 ceramics (b), and pure LaVO4 (c), respectively. There are three typical space structures along with the Nb/V ion at the center of oxygen octahedon and tetrahedron. Properties of the (1 − x)LaNbO4−xLaVO4 ceramics have close connection to the complex crystals theories, which are related to the bond length affected by the increase of the substitution of V5+ ions. Nowadays, the complex crystal theories including chemical bond theory, lattice energy, and coefficient of thermal expansion have been reported by Zhang et al.31,32 This theory can decompose complex crystal into sum of binary crystals with the crystallographic data, and then we can more easily understand crystal structure. Table 3 has shown the calculated results of bond iconicity of the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) ceramics. The average of the bond iconicity (0 ≤ x ≤ 0.1) has a decreasing trend with the increase of V content, and the values rise at x = 0.225. This change is mainly attributed to structure transition from monoclinic fergusonite to tetragonal sheelite. As seen from Table 3, we can know that ΔAf i(La−O) = 0.1488 and ΔAf i(Nb/V−O) = 0.3660 (%), which means that substitution of V5+ has effect on the B-site bond ionicity and that the bond ionicity of the Nb/V−O type has a dominant effect on the microwave dielectric properties of the (1 − x)LaNbO4−xLaVO4 ceramics. The concept of lattice energy is defined as the heat of dissociation of one mole of solid into its structural components. Lattice energy can be applied to evaluate the phase stability of a crystal structure.33,34 The C
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Observed XRD (red points) and calculated pattern (black solid line) along with difference plot (at the bottome) of the (1 − x)LaNbO4− xLaVO4 at x = (a) 0, (b) 0.05, (c) 0.1, (d) 0.225. Allowed Bragg reflections are indicated by vertical bars and goodness-of-fit shown in inset.
Table 1. Crystallographic Parameters of Different x Values of the (1 − x)LaNbO4−xLaVO4 (0.00 ≤ x ≤ 0.225) Ceramics x 0
0.05
0.10
0.2
0.225
atom La Nb O1 O2 La Nb V O1 O2 La Nb V O1 O2 La1 Nb1 V1 O1 La1 Nb1 V1 O1
x
y
z
occupancy
at x = 0, 0.05, 0.10, 0.20 space group: I2/a 0.250 00 0.119 98 0.000 00 0.250 00 0.645 63 0.000 00 −0.001 65 0.706 13 0.188 05 0.907 69 0.456 50 0.241 22 0.250 00 0.120 76 0.000 00 0.250 00 0.641 90 0.000 00 0.250 00 0.641 90 0.000 00 0.002 48 0.709 20 0.177 37 0.913 44 0.459 24 0.236 78 0.250 00 0.121 56 0.000 00 0.250 00 0.640 87 0.000 00 0.250 00 0.640 87 0.000 00 0.014 21 0.712 03 0.173 76 0.906 19 0.455 33 0.234 09 0.000 00 0.250 00 0.625 00 0.000 00 0.250 00 0.125 00 0.000 00 0.250 00 0.125 00 0.142 97 0.003 80 0.201 78 at x = 0.225 space group: I41/a 0.000 00 0.250 00 0.625 00 0.000 00 0.250 00 0.125 00 0.000 00 0.250 00 0.125 00 0.148 88 0.009 71 0.214 13
Table 2. Bond Length d from Rietveld Refinement for the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) Ceramics
bond type
0.500 0.500 1.000 1.000 0.500 0.475 0.025 1.000 1.000 0.500 0.450 0.050 1.000 1.000 0.250 0.200 0.050 1.000
4 4 8 8 4 4 4 8 8 4 4 4 8 8 4 4 4 16
0.250 0.194 0.056 1.000
4 4 4 16
bond length d (Å)
d (Å)
mult 1
La−O(1) ×2 La−O(1)2 ×2 La−O(2)1 ×2 La−O(2)2 ×2 Nb/V− O(1)1 ×2 Nb/V− O(1)2 ×2 Nb/V− O(2) × 2 Vcell (Å3)
x=0
x = 0.05
x = 0.1
2.421
2.477
2.528
2.590
2.556
2.557
2.423
2.432
2.460
2.513
2.522
2.554
1.899
1.872
1.821
2.685
2.722
2.701
1.893
1.893
1.839
334.0182
334.5279
334.2744
bond type 1
x = 0.225
La−O(1) ×2 La−O(1)2 ×2 La−O(1)3 ×2 La−O(1)4 ×2 Nb/V− O(1)1 ×2 Nb/V− O(1)2 ×2
1.96
Vcell (Å3)
335.9034
2.507 2.562 2.849 1.839
1.913
of them possess a rapid rising trend at x = 0.225, which can become a signal of structure transition. Therefore, there is a close relationship between phase transition and bond ionicity, lattice energy, and coefficient of thermal expansion. εr and Q × f of the (1 − x)LaNbO4−xLaVO4 ceramics as a function of composition (0 ≤ x ≤ 1) are shown in Table 6. εr decreases as V content increases basically, and it is ∼13 to 20. The relationship between the constant and bond ionicity35 was found as follows:
calculated results of lattice energy and coefficient of thermal expansion are show in Table 4 and Table 5, respectively. Both D
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Schematic of crystal structure of (a) monoclinic fergusonite structured LaNbO4, (b) tetragonal sheelite 0.775LaNbO4−0.225LaVO4 ceramics, and (c) monoclinic monazite LaVO4.
Table 3. Bond Ionicity f i for the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) Ceramics f i (%)
f i (%) x = 0.225 (88)
bond type
x=0
x = 0.05
x = 0.1
bond type
La−O(1)1 × 2
90.1041
90.0883
90.0932
La−O(1)1 ×2 La−O(1)2 ×2 La−O(1)3 ×2 La−O(1)4 ×2 Nb/V−O(1)1 ×2 Nb/V−O(1)2 ×2
La−O(1) × 2
90.2908
90.1726
90.1221
La−O(2)1 × 2
80.3207
80.1908
80.1622
La−O(2)2 × 2
80.5116
80.3764
80.3428
Nb/V−O(1)1 ×2 Nb/V−O(1)2 ×2 Nb/V−O(2) ×2 Af i(La−O)a Af i(Nb/V−O)a ΔAf i(La−O)a (%) ΔAf i(Nb/V−O)a (%)
88.3826
88.2345
88.0727
88.6813
88.5402
88.4862
78.5641
78.4022
78.1370
85.3068 85.2093 0.1488
85.2070 85.0590
85.1801 84.8986
2
Table 4. Lattice Energy U for the (1 − x)LaNbO4-xLaVO4 (0 ≤ x ≤ 0.225) Ceramics
bond type
96.3688
La−O(1)1 ×2 La−O(1)2 ×2 La−O(2)1 ×2 La−O(2)2 ×2 Nb/V− O(1)1 × 2 Nb/V− O(1)2 × 2 Nb/V−O(2) ×2 ULa−O UNb/V−O total
96.8195 96.8456 96.9442 86.4664 86.5861
96.7445 86.5263
0.3660
x=0
x = 0.05
x = 0.1
bond type La−O(1)1 ×2 La−O(1)2 ×2 La−O(1)3 ×2 La−O(1)4 ×2 Nb/V− O(1)1 × 2 Nb/V− O(1)2 × 2
1494
1467
1443
1414
1430
1429
1409
1406
1393
1369
1366
1352
7184
7258
7403
5487
5424
5459
6498
6498
6635
5686 19 169 24 855
5669 19 180 24 849
5617 19 497 25 114
ULa−O UNb/V−O total
x = 0.225 (88) 2027 1674 1645 1507 15 862 15 420
6853 31 282 38 135
Table 5. Coefficient of Thermal Expansion α for the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) Ceramics
The Af i was the average of the bond iconicity, and Δ was the variation in Af i, ΔAf i = (Af imax − Af imin)/Af imin. a
n2 − 1 ε= +1 1 − fi
U (kJ mol−1)
U (kJ mol−1)
α (1 × 10−6 K−1)
(2)
where n is the refractive index. It is indicated that the εr decreased with the decrease of bond ionicity. The variations of εr and B-site bond ionicity Ib of (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) along with x are shown in Figure 6a. Both of the εr and Ib have decreasing trends in (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.1) ceramics. When x increases to 0.225, the Ib has an abnormal change, which could be attributed to phase transition from monoclinic fergusonite to tetragonal sheelite structure. There is a reciprocal relationship between quality factor and loss in theory. And the dielectric loss of ceramics can be divided into two parts, namely, intrinsic loss and extrinsic loss. The intrinsic loss can be regarded as loss of the perfect crystal itself caused by lattice vibration, which is decided by structure and composed elements. The extrinsic loss arose from all defects, including point defects, line defects, face defects, and body defects.36 To a great extent, Q × f can also be affected by complex phase composition. Therefore, there are not simple
bond type
bond type
x=0
x = 0.05
x = 0.1
La−O(1)1 ×2 La−O(1)2 ×2 La−O(2)1 ×2 La−O(2)2 ×2 Nb/V− O(1)1 ×2 Nb/V− O(1)2 ×2 Nb/V− O(2) × 2 α
11.2861
11.5521
11.797
12.1039
11.933
11.9436
10.8809
10.9109
11.0423
11.2914
11.3231
11.4732
2.3982
2.3414
2.2335
4.1198
4.2045
4.1572
1.9799
1.9799
1.8736
7.7229
7.7493
7.7886
α (1 × 10−6 K−1) x = 0.225(88)
La−O(1)1 ×2 La−O(1)2 ×2 La−O(1)3 ×2 La−O(1)4 ×2 Nb/V− O(1)1 ×2 Nb/V− O(1)2 ×2
total
9.2609 11.8819 12.1472 13.5497 0.6133
0.7217
8.0291
change rules of quality factor with x increasing. In this paper, quality factor could reach to maximum of 75 940 GHz at x = 0.3 with tetragonal sheelite phase and minimum of 6770 GHz E
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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microwave properties and structure aspects. Every pure phase in the (1 − x)LaNbO4−xLaVO4 has certain linear laws. In pure monoclinic fergusonite at x = 0−0.2, tetragonal sheelite at x = 0.225−0.375, B-site ordered sheelite at x = 0.4, and composite of monoclinic LaVO4 and tetragonal sheelite phases structure at x = 0.6−0.9, εr and Q × f possess rising or falling trends. These changing trends can further suggest the correlation of structures and properties. Q × f values and the B-site lattice energy Ubc for (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) are shown in Figure 6b. In pure monoclinic fergusonite structure, both Q × f and Ubc increased with increase of x. The trends changed when x increased to 0.225, which may indicate that Q × f was also affected by phase transition besides lattice energy Ubc. The relationship between τf, the temperature coefficient of the dielectric constant (τε), and the linear thermal expansion coefficient (αL) is shown in the following formula:38 τ τf = − ε − αL (3) 2
Table 6. Microwave Dielectric Properties of Different x Values of the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.9) Microwave Dielectric Ceramics x 0 0.05 0.1 0.2 0.225 0.25 0.3 0.35 0.375 0.4 0.5 0.6 0.8 0.9 119
S.T. (°C)
εr
Q × f (GHz)
TCF (ppm/°C)
1140 1160 1160 1180 1160 1180 1160 1160 1160 1180 1160 1160 1160 1160 900
19.29 18.21 18.18 14.39 16.82 17.07 17.78 16.86 16.89 17.12 16.51 16.13 14.49 13.64 14.5
6770 29 130 62 370 34 930 27 360 7180 75 940 31 640 31 810 7290 24 130 21 970 13 120 17 030 19 000
27.3 77.7 106.7 106.5 −25.5 −24.0 −36.8 −35.9 −43.5 −34.4 −43.4 −44.4 −56.0 −59.0 −41.0
In (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.1) ceramics, αL and τf increase with increase of V5+, which indicates that αL plays an important role in τf. The different trends at x = 0.225 could be attributed to phase transition, and αL is no longer the only factor. τf as a function of composition in the (1 − x)LaNbO4− xLaVO4 ceramics is shown in Figure 7a. τf increased with concentration of x (x ≤ 0.2) within the monoclinic LaNbO4 structure. Then τf rapidly falls to negative value at x = 0.225 along with phase transition to tetragonal structure. The phenomenon indicated that change of tendency of τf was dominated by phase transition. In general, the ReNbO4 ceramics could be affected by two factors, namely, A-site chemical element and oxygen polyhedron. This series of
at x = 0 with monoclinic fergusonite phase. The integral irregular performances of permittivity and quality factor is in connection with structural deformation caused by vanadium ion doping. The structure of LaNbO4 ceramics is basically stable at room temperature with monoclinic (space group I2/a) and also can turn into a tetragonal structure (space group I41/a) above the critical temperature (Tc).37 The stress and the temperature can, respectively, induce ferroelasticity and phase transformation, which means that the LaNbO4 ceramics are likely to show distortion of the unit cell and changes of dielectric properties. Therefore, the microwave dielectric ceramics of the (1 − x)LaNbO4−xLaVO4 have much more instabilities in
Figure 6. (a) The εr value and the average of B-site bond ionicity Ib, (b) the Q × f value and B-site lattice energy Ubc, (c) the TCF value and coefficient of thermal expansion α for the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.225) microwave dielectric ceramics. F
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 7. Variation of (a) τf with x values in the (1 − x)LaNbO4−xLaVO4 and (b) their temperature dependence of microwave εr and Q × f.
Figure 8. Measured and calculated infrared reflectivity spectra (solid line for fitting values and circle for measured values) and fitted complex dielectric spectra of (a) the 0.8LaNbO4−0.2LaVO4 and (b) the 0.7LaNbO4−0.3LaVO4 ceramics (circles are experimental in microwave region).
Table 7. Phonon Parameters Obtained from the Fitting of the Infrared Reflectivity Spectra of the (1 − x)LaNbO4−xLaVO4 Ceramics (x = 0.2 and 0.3) x = 0.2
x = 0.3
mode
ωoj
ωpj
γj
Δεj
ωoj
ωpj
γj
Δεj
1 2 3 4 5 6 7 8 9 10 11 12
56.49 88.38 105.95 132.08 184.30 225.86 262.11 325.40 425.26 529.84 619.96 746.38
28.65 155.21 140.77 190.45 202.64 72.63 49.77 210.95 157.03 300.44 322.50 159.42
8.02 58.45 31.60 51.50 44.89 41.19 18.89 55.78 53.67 141.78 96.68 99.95
0.257 3.084 1.765 2.079 1.209 0.103 0.036 0.420 0.136 0.322 0.271 0.046
59.77 116.28 178.04 221.97 327.45 424.85 537.81 616.15 719.66 754.66 814.15
51.68 311.68 252.87 156.40 236.33 218.75 378.10 423.18 104.87 58.41 23.08
25.13 52.12 47.41 72.70 46.50 56.88 118.43 88.64 42.13 38.17 14.92
0.748 7.185 2.017 0.496 0.521 0.265 0.494 0.472 0.021 0.006 0.001
ε∞ = 1.47
εο = 11.20
ε∞ = 1.70
εο = 13.93
(1 − x)LaNbO4−xLaVO4 ceramics. Figure 7b shows the external temperature dependence of εr and Q × f of the (1 − x)LaNbO4−xLaVO4 (x = 0.2, 0.225, and 0.3) ceramics. The 0.8LaNbO4−0.2LaVO4 ceramics possessed permittivity of 14, and Q × f could rise from 40 000 to 85 000 GHz with increase of temperature. εr and Q × f of the 0.775LaNbO4−0.225LaVO4 and 0.7LaNbO4−0.3LaVO4 ceramics basically keep constant when temperature rises to 85 °C, which implies that the ceramics are promising materials from temperature. The best microwave dielectric properties, εr of ∼17.50 and Q × f of
material is known to undergo a reversible, pure ferroelastic phase transformation from a high-temperature, scheelite-type structure (tetragonal, I41/a) to a low-temperature, fergusonitetype structure (monoclinic, C2/c), with the transition temperature depending on the rare-earth ions.39 In (1 − x)LaNbO4− xLaVO4 ceramics, the lanthanide element (La at A-site) is fixed, which means that transition temperature is basically determined. Then, the only factor of change in τf values trend might be attributed to oxygen polyhedron variation. The theoretical explanation is needed to further research. In this paper, we pay more attention to research microwave dielectric properties of G
DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry ∼70 000 GHz, were obtained in the 0.7LaNbO4−0.3LaVO4 ceramic sintered at 1160 °C. Infrared reflectivity spectra of the (1 − x)LaNbO4−xLaVO4 (x = 0.2, 0.3) ceramics can be used to analyze the intrinsic microwave dielectric properties by using a classical harmonic oscillator model:
∑ j=1
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (U1632146), the Young Star Project of Science and Technology of Shaanxi Province (2016KJXX-34, 2015KJXX-39), the Fundamental Research Funds for the Central University, and the 111 Project of China (B14040). The authors would like to thank the administrators in IR beamline workstation of National Synchrotron Radiation Laboratory (NSRL) for their help. The SEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong Univ., Xi’an, China, and the authors thank Ms. Y.-Z. Dai for her help in using SEM.
ωpj 2
n
ε × (ω) = ε∞ +
Notes
ωoj 2 − ω 2 − jγjω
(4)
where ε × (ω) is complex dielectric function, ε∞ is the highfrequency dielectric constant that represents the contribution of all oscillators at very high frequencies, γj, ωoj, and ωpj are the damping factor, the transverse frequency, and plasma frequency of the jth Lorentz oscillator, respectively. The complex reflectivity R(ω) can be written as R(ω) =
1−
ε × (ω)
1+
ε × (ω)
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2
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(5)
Fitted infrared reflectivity values, complex permittivities, and phonon parameters are shown in Figure 8 and Table 7. In microwave frequencies ranges of the (1 − x)LaNbO4−xLaVO4 (x = 0.2, 0.3) ceramics, we can obtain the permittivity value with 11.20 and 13.93, respectively. The measurement values of permittivity are 14.39 and 17.78, respectively. Generally, the fitted infrared values, especially permittivity and quality factors, are lower than measurement values because of the extrinsic loss affected by all kinds of defects. The optical dielectric constants from the infrared spectra of the 0.8LaNbO4 −0.2LaVO4 ceramics and the 0.7LaNbO4−0.3LaVO4 ceramics are ∼1.47 and 1.70, which is only 13.1 and 12.2% of the total polarizability contribution at microwave frequencies, respectively, which means that the main polarization contributions to microwave permittivity of high εr materials come from ionic polarization rather than electronic, because only the ionic and electronic polarization can follow the microwave frequencies changes.
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CONCLUSIONS In the present work, microwave dielectric ceramics were explored in the (1 − x)LaNbO4−xLaVO4 (0 ≤ x ≤ 0.90) ceramics in the whole composition range. Excellent microwave dielectric properties with a εr = 17.78, a Q × f = 75 940 GHz, and τf = −36.8 ppm/°C were obtained in the 0.7LaNbO4− 0.3LaVO4 ceramic sintered at 1160 °C. The XRD patterns could demonstrate that four phase regions were detected as increase of V2O5, including monoclinic fergusonite, tetragonal sheelite, B-site ordered sheelite, and composite of monoclinic LaVO4 and tetragonal sheelite phases, respectively. Far-infrared reflectivity fitting shows the main polarization stems from ionic polarization rather than electronic. These materials have great potential for equipment and devices with high quality factors.
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REFERENCES
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Di Zhou: 0000-0001-7411-4658 Author Contributions
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DOI: 10.1021/acs.inorgchem.7b01462 Inorg. Chem. XXXX, XXX, XXX−XXX
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