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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Structural Evolution and Microwave Dielectric Properties of xZn0.5Ti0.5NbO4‑(1−x)Zn0.15Nb0.3Ti0.55O2 Ceramics Hongyu Yang,†,‡ Shuren Zhang,†,‡ Hongcheng Yang,†,‡ Xing Zhang,†,‡ and Enzhu Li*,†,‡ †
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National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu, 610054, China ‡ Key Laboratory of Multi-Spectral Absorbing Materials and Structures of Ministry of Education, University of Electronic Science and Technology of China, Chengdu, 610054, China S Supporting Information *
ABSTRACT: Structure and microwave properties of xZn0.5Ti0.5NbO4-(1 − x)Zn0.15Nb0.3Ti0.55O2 ceramics in the range of x = 0.0−1.0 were investigated. Rietveld refinement analysis and Raman spectra show that rutile- and orthorhombic-type solid solutions formed at 0−0.2 and 0.65−1, a composite at 0.2−0.64. In the solid solution regions, chemical bonds are enlarged. In this case, the Zn/Ti/Nb−O1 bond covalency and bond susceptibility are reduced, and lattice energy and thermal expansion coefficient increase along with x increases, which is mainly responsible for the development of microwave dielectric properties. Furthermore, far-infrared spectra and a classical oscillator model were used to discuss the intrinsic dielectric properties in detail. Temperature stable ceramic was obtained for x = 0.516: εr ∼ 46.11, Q × f ∼ 27 031 GHz, and τf ∼ −1.51 ppm/°C, which is promising for microwave applications.
1. INTRODUCTION Microwave dielectric ceramics (MWDCs) have played an important role in the electronic industry. Their practical use requires a temperature coefficient of resonant frequency (τf) with an appropriate dielectric constant (εr) and an excellent quality factor (Q × f).1−4 ZnO-TiO2-Nb2O5 materials are a new type of microwave dielectric ceramic systems. According to their phase diagram, ceramics with tunable dielectric constant and great microwave dielectric properties can be obtained by adjusting the contents of raw materials TiO2:5,6 columbite ZnNb2O6 (εr ∼ 25, Q × f = 83 700 GHz), ixiolite Zn0.5Ti0.5NbO4 (εr ∼ 34, Q × f = 42 500 GHz, abbreviated as O-ZTN) and rutile TiO2 (εr ∼ 104, Q × f = 40 000 GHz) ceramic. O-ZTN ceramic possess a moderate sintering temperature (1100−1200 °C) and excellent microwave dielectric properties (εr of ∼34, Q × f value of ∼42 500 GHz, and τf of ca. −52 ppm/°C).7 It is a solid solution of ZnNb2O6 first reported in 1992 which shares the same orthorhombic structure and space group of Pbcn(No. 60) as ZnNb2O6; their biggest difference is the disappearance of the (040) superlattice diffraction peak and atomic site occupation, © XXXX American Chemical Society
indicating a disordered structure. However, some applications require a near zero τf value.8,9 Designing a solid solution for improving the τf value is a classical method. For instance, Chen’s group studied MRAlO4, a tetragonal K2NiF4 structure. Solid solution was successfully formed in Sr1+xSm1−xAl1−xTixO4 (x = 0−0.15) and SrLaAl1−x(Zn0.5Ti0.5)xO4 (x = 0−0.9) systems.10,11 They explored the exact contributions of chemical bonds to lattice distortion, anomalous structure evolution, and microwave dielectric properties in depth and confirmed their results with Raman and far-infrared reflectivity spectra analysis and HRTEM observation, putting forward the abnormal variations come from Sr/(Sm/La)−O(2b) and Al/Ti(Zn,Ti)−O(2) bonds. Zhou et al. also quantify the contribution of chemical bonds in LaNbO4-LaVO4 and BiVO4-LaNbO4 systems with chemical bond theory and far-infrared reflectivity spectra analysis.12,13 In ixiolite-type O-ZTN ceramics, solid solution may be inapplicable due to its limited solid solubility; i.e., Ni2+ has a Received: April 1, 2018
A
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
States). The τf value was measured from the difference between the resonant frequency at 25 and 85 °C.
limited solid solubility about x ∼ 0.2 in (Ni1−xZnx)0.5Ti0.5NbO4.14 Mg2+ replaced for Zn2+ causes a formation of a columbite structure.15 Ca2+ and Sn4+ co-substituting induces the formation of rutile Zn0.15Nb0.3Ti0.55O2 phase (shortened as R-ZNT), turning the τf value to a positive value of 21.20 ppm/ °C.16 Co2+ replaces for Zn2+, forming solid solution less than x ∼ 0.2, where the R-ZNT phase also appears, and the ceramic shows great properties under two phase coexistences with a εr of 35.93, Q × f of 35 125 GHz, and zero τf value.17 Sb5+ for the Nb site also leads to the appearance of R-ZNT phase.18 Ta5+ and Co2+ codoped (Zn1−xCox)0.5Ti0.5(Nb1−yTay)NbO4 ceramic also improves the τf value when the R-ZNT phase is detected after x ∼ 0.1, y ∼ 0.3.19 It is worth noting that R-ZNT phase always appears in the process of ion modification. R-ZNT is a tetragonal structure with a space group of P42/ mnm(136), which is substituted by tetravalent (Zn1/3Nb2/3)4+ in TiO2 and reported as a solid solution of rutile phase with properties of: ST = 1200 °C, εr = 95, Q × f = 15 000 GHz, and τf = 237 ppm/°C.20 Although it has insufficient microwave dielectric properties than TiO2 (ST = 1400 °C, εr = 104, Q × f = 40 000 GHz, and τf = 400 ppm/°C),20 its lower sintering temperature and τf value may be more suitable than TiO2 for improving the properties of O-ZTN ceramics. Moreover, reports on the intrinsic dielectric properties of Zn0.5Ti0.5NbO4 and Zn0.15Nb0.3Ti0.55O2 ceramics are rare, which is important for discussing the intrinsic origin of dielectric constant and dielectric loss. Therefore, directly introducing R-ZNT phase into O-ZTN ceramics is designed. xZn0.5Ti0.5NbO4-(1 − x)Zn0.15Nb0.3Ti0.55O2 (Zn0.15+0.35xTi0.55−0.05xNb0.3+0.7xO2+2x, hereafter xZTN(1 − x)ZNT) (0 ≤ x ≤ 1), ceramics were synthesized via solid-state reaction. The relationships between crystal structure and microwave dielectric properties were discussed in detail.
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Microstructure of xZTN-(1 − x)ZNT. The X-ray diffraction (XRD) patterns of xZTN-(1 −
Figure 1. Representative X-ray diffraction patterns of xZTN-(1 − x)ZNT ceramics for x = 0−1.
x)ZNT ceramics sintered at optimum temperatures are shown in Figure 1. According to the evolutions of diffraction peaks, three phase regions can be divided. Rutile-structured Zn0.15Nb 0.3Ti0.55O 2 (R-ZNT) solid solution with P42/ mnm(136) space group was formed at x ≤ 0.2, which is well-indexed to JCPDS # 79-1186. The (111) peak existing at 30° indicates the formation of orthorhombic-type Zn0.5Ti0.5NbO4 (O-ZTN) phase (JCPDS # 48-0323). With x further increases, the (110) peak belonging to R-ZNT phase becomes weaker and weaker and then disappears at x = 0.65. No traces of R-ZNT phase were detected at 0.65 ≤ x ≤ 1, which suggests O-ZTN solid solution with a space group of Pbcn(60) was formed. The XRD patterns for all the compositions can be found in Figure 1S. Si powder was used as internal standard to obtain precise lattice parameters. The xZTN-(1 − x)ZNT ceramics were obtained by synthesizing at 1050−1175 °C for 4 h. The SEM micrographs of the ceramics sintered at optimum temperatures are shown in Figure 2. All of these compact microstructures indicate a high densification. At x = 0−0.516 (a−c) and 0.8−1 (e,f), the average grain size drops from 14.06 to 1.65 μm and 8.42 to 2.24 μm. However, in the region of 0.516−0.8, it has an increase tendency. A small grain size (approximately 1.8 μm) of pure Zn0.5Ti0.5NbO4 ceramic sintered at 1080 °C has also been reported by Tseng.23 3.2. Structural Evolution and Microwave Dielectric Properties of R-ZNT (0 ≤ x ≤ 0.2) and O-ZTN (0.65 ≤ x ≤ 1) Solid Solutions. 3.2.1. Rietveld Refinement and Normalized Bond Analysis. To further study the structure variations of R-ZNT phase and O-ZTN phase, Rietveld refinement was carried out. Prior to performing the refinement process, the occupancies of atoms are considered by following the chemical formula: Zn0.15+0.35xTi0.55−0.05xNb0.3+0.7xO2+2x. The initial models for Rietveld refinement are selected from ICSD #15208,5 reported by Baumgarte and Blachnik, and ICSD #40710,24 reported by Abrahams and Bruce et al. The variations of lattice parameters are given in Figure 3a,b. A
2. EXPERIMENTAL SECTION Reagent-grade starting materials of ZnO (ChengDu Kelong Chemical Co., Ltd., Chengdu, China, 99.7%), TiO2 (ChengDu Kelong Chemical Co., Ltd., Chengdu, China, 99.9%), and Nb2O5 (ChengDu Kelong Chemical Co., Ltd., Chengdu, China, 99.9%) were proportionately weighed in accordance with the xZTN-(1 − x)ZNT (0 ≤ x ≤ 1) chemical formula. Powders were mixed, and planetary ball-milled with deionized water in a Nylon jar for 6 h by using a zirconia ball and choosing the speed at 280 rad/s. After drying and sieving from a 60-mesh sieve, the mixtures were sintered to 950 °C for 3 h in air. Then, the mixtures were remilled for 4 h. After drying, the mixtures were adhered 3 wt % of a 10% solution of poly(vinyl alcohol) (PVA) to form pellets (15 mm in diameter and 7 mm in thickness) and sintered from 1050 to 1175 °C for 4 h. The bulk density of the sintered specimen is measured by the Archimedes method. X-ray diffraction (Philips X’Pert ProMPD, Amsterdam, The Netherlands) with Cu Kα radiation was applied to detect the crystal structure, and the scanning angle of 2θ varies from 10° to 120° with a step of 0.013°. Rietveld refinements were carried out by using GSAS-EXPGUI program.21,22 The microstructure of ceramic was observed by scanning electron microscopy (SEM, FEI Inspect F, the United Kingdom). Room temperature Raman spectra were performed using a Raman microscope (LabRAM HR Evolution); the 532 nm line of an Ar laser source with constant power acted as the exciting wavelength. Infrared reflectivity spectra were measured using a Bruker IFS 66v FT-IR spectrometer on the Infrared Spectroscopy and Microspectroscopy Endstation (BL01B) at the National Synchrotron Radiation Lab (NSRL) of China, Hefei, covering the middle-infrared region (600−4000 cm−1) and farinfrared region (50−600 cm−1). The microwave dielectric properties were measured by the Hakki−Coleman dielectric resonator method in the TE011 mode using a network analyzer (HP83752A, the United B
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. SEM micrographs and grain size distributions of the xZTN-(1 − x)ZNT ceramics for (a)−(f) x = 0, 0.4, 0.516, 0.65, 0.8, 1 (R, Co, and O refer to R-ZNT phase, two coexisting phases, and O-ZTN phase, respectively).
Figure 3. Variations of lattice parameters for (a) R-ZNT phase, (b) O-ZTN phase.
Zn0.15Nb0.3Ti0.55O2 is regarded as a Ti4+ cation partial substituted by (Zn1/3Nb2/3)4+, which has the same rutile structure with TiO2. In its structure, the Ti site is randomly occupied by Zn2+, Ti4+, and Nb5+ cations. The Zn/Ti/Nb atom is coordinated by six O atoms, which forms oxygen octahedrons. The octahedron has two Zn/Ti/Nb−O distances ranging from 1.9885 to 2.0062 Å (x = 0). They are arranged as chains by edge-sharing. In the structure of Zn0.5Ti0.5NbO4, similarly, Zn2+, Ti4+, and Nb5+ cations are distributed at the same position, forming octahedrons that interconnects by sharing edges or vertexes of octahedrons. According to Figure 4, it can be concluded that the two Zn−O1 bonds are in the state of “compression”, while all the Ti−O1 and Nb−O1 bonds are in “dilation” in the R-ZNT structure. More specifically, all the bonds are dilated at 0−0.2. Therefore, the oxygen octahedron will go through a series of change. For OZTN phase, Zn/Ti/Nb−O1(1) and Zn−O1(3) bonds are compressed, while others are elongated. All the Zn/Ti/Nb−
monotonous increase for lattice parameters of R-ZNT phase is observed at 0−0.2. For O-ZTN phase, lattice parameters of a-, b-axis length, and volume also show an increasing trend, while c-axis length has an abnormal change from 0.65 to 1. The XRD profiles after fitting are shown in Figure 2S. Exact structural parameters of the two phases can be found in Tables 1S and 2S. The atomic fractional coordinates (taking x = 0.2 and x = 0.8 as examples) are listed in Table 1. Normalized bond length and bond valence parameters are used to further investigate the structure variation.25 Normalized bond length was defined as the ratio of actual bond length in a crystal structure to the ideal bond length, while normalized bond valence was described as the ratio of actual bond valence to the formula valence.11,26 Both of the two parameters can represent the state of cations. Any bond with a valence shorter than 1 is dilated, otherwise, compressed. The normalized bond length and bond valence of R-ZNT and O-ZTN phases are shown in Figure 4. C
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Atomic Fractional Coordinates and Bond Types in Oxygen Octahedron for Zn0.15+0.35xTi0.55−0.05xNb0.3+0.7xO2+2x (x = 0.2 for R-ZNT and 0.8 for O-ZTN) structure
atom
position
x 0.00000 0.00000 0.00000 0.30375(5)
R-ZNT
Zn Ti Nb O bond type
2a 2a 2a 4f
O-ZTN
Zn Ti Nb O bond type
4c 4c 4c 8d
y
z
occ
Uiso
0.00000 0.00000 0.183 0.00516(7) 0.00000 0.00000 0.450 0.00727(9) 0.00000 0.00000 0.367 0.03141(5) 0.30375(3) 0.00000 1.000 0.02010(8) Zn−O1(1) × 2: 2.0126(2) Å, Zn−O1(2) × 4: 1.9959(3) Å Ti−O1(1) × 2: 2.0126(2) Å, Ti−O1(2) × 4: 1.9959(3) Å Nb−O1(1) × 2: 2.0126(2) Å, Nb−O1(2) × 4: 1.9959(3) Å 0.00000 0.32230(2) 0.25000 0.239 0.01302(6) 0.00000 0.32230(2) 0.25000 0.283 0.00687(9) 0.00000 0.32330(2) 0.25000 0.478 0.01235(7) 0.26870(18) 0.11220(9) 0.41820(16) 1.000 0.00584(7) Zn−O1(1) × 2: 1.9243(1) Å, Zn−O1(2) × 2: 2.1383(2) Å, Zn−O1(3) × 2: 2.0199(2) Å Ti−O1(1) × 2: 1.9243(1) Å, Ti−O1(2) × 2: 2.1383(2) Å, Ti−O1(3) × 2: 2.0199(2) Å Nb−O1(1) × 2: 1.9243(1) Å, Nb−O1(2) × 2: 2.1383(2) Å, Nb−O1(3) × 2: 2.0199(2) Å
their different crystal structures, the sum of binary crystals can be written as
O1(1) and Zn/Ti/Nb−O1(2) bonds enlarge, while a slight decrease in Zn/Ti/Nb−O1(3) bond length is detected at 0.65−0.7 and 0.8−0.9. 3.2.2. Chemical Bond Characteristics and Microwave Dielectric Properties Analysis. Microwave dielectric properties of xZTN-(1 − x)ZNT ceramics are shown in Table 2. The detailed density information can be found in the Figure 3S. Since all the samples have high densifications (>95%), the influences of density are not discussed here. Previous analysis shows an increase in bond length and cell volume. As seen from Table 2, the εr value decreases from 94.35 to 82.45 in RZNT solid solution regions, and the τf value also exhibits a dramatic decrease trend. Therefore, the correlations between crystal structure and the microwave dielectric properties should be discussed in detail. Later, complex chemical bond theory involving bond ionicity (f i), bond covalency (fc), lattice energy (U), and thermal expansion coefficient (αL) has been applied to discuss the influences of chemical bonds on crystal structure.27,28 It also has been developed for explaining the relationships between bond characteristics and microwave dielectric properties.29−32 Since the bond analysis provides structural variation of R-ZNT phase and O-ZTN phase, bond theory was applied to better verify the chemical bonds characteristics on the effects of dielectric properties. Generally speaking, the crystal can be regarded as the aggregation of chemical bonds between ions. The molecular formula of crystals is also the sum of chemical bonds corresponding to all kinds of bonds. Any chemical bond is binary, and its chemical formula can be expressed by binary compounds. The complex crystal should be decomposed into the sum of binary crystals according to its crystal structure. After that, the properties of multibonds can be converted into single bond, and then the chemical bond properties of a complex crystal can be solved. For a complex crystal (molecular formula is AaBbCcDd), A, B, C, and D represent the different elements or the different positions of the same element, while a, b, c, and d are the numbers of elements.27,28 In this work, ion occupancy changes along with x increases; therefore, the m and n values in the bonding subformula AmBn are changing based on Zn0.15+0.35xTi0.55−0.05xNb0.3+0.7xO2+2x correspondingly. After decomposing into bonding subformula AmBn, the bond parameters can be calculated. According to
Zn 0.15Nb0.3Ti 0.55O2 = Zn 0.05O1(1)0.1 + Zn 0.1O1(2)0.2 + Ti 0.55/3O1(1)1.1/3 + Ti1.1/3O1(2)2.2/3 + Nb0.1O1 (1)0.2 + Nb0.2O1(2)0.4 Zn 0.5Ti 0.5NbO4 = Zn1/3O1(1)2/3 + Zn1/3O1(2)2/3 + Zn1/3O1(3)2/3 + Ti1/3O1(1)2/3 + Ti1/3O1(2)2/3 + Ti1/3O1(3)2/3 + Nb2/3O1(1)4/3 + Nb2/3O1(2)4/3 + Nb2/3O1(3)4/3
As known, the dielectric constant is a reflection of dielectric polarizability, which is usually determined by constituent atoms of a crystal. Zhang’s32 work indicated the variation of Nd/La−O(1)2 bond covalency in (Nd1−xLax)NbO4 ceramics is closely related to the polarization, which mainly influences the variation of εr value. Therefore, to evaluate the polarizability, bond covalency fc is calculated. The bond covalency (fc) parameter of bond μ can be estimated by the series of equations fc =
(Eh μ)2 μ 2
(Eg )
Eh μ =
Egμ,
=
(C μ)2 (Eh ) + (C μ)2 μ 2
39.74 (d μ)2.48 μ
(1)
(2) μ
where Eh , and C are the average energy gap, homopolar part, and heteropolar part. dμ is the bond length obtained from Rietveld refinement results. Moreover, the heteropolar part Cμ is defined as follows Ä ÑÉÑ i d μ y−1 i d μ y ÅÅ n C μ = 14.4·b μ ·expjjjj−ks μ zzzz·ÅÅÅÅ(ZA μ)* − (ZB μ)*ÑÑÑÑ·jjjj zzzz ÑÖ k 2 { 2 { ÅÇ m k (3)
where bμ is a correction factor that is determined by the coordination number (NCAμ and NCBμ) of cation A and anion B. In our work, bμ is taken as 1.424. (ZAμ)* and (ZBμ)* represent the effective number of valence electrons. exp(−ksμroμ) is the Thomas−Fermi screening factor D
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Normalized bond length (a) and bond valence (b) of R-ZNT phase and normalized bond length (c) and bond valence (d) of O-ZTN phase. μ
ks =
jij 4kF zyz jj z j πaB zz k {
vb
(4)
(5)
nv μ vb μ
(6)
Ne μ =
where nvμ and vbμ refer to the number of valence electrons per bond and bond volume nv μ =
(ZB μ)* (ZA μ)* μ + NCA NCB μ
ÄÅ ÉÑ−1 ÅÅ ÑÑ ÅÅ v 3 vÑ = (d ) ·ÅÅ∑ (d ) Nb ÑÑÑ ÅÅ ÑÑ ÑÖ ÅÇ v μ 3
(8)
where Nbv is determined by the bond length and crystal structure, which can be obtained from crystal structure information. In our work, the average bond covalencies (Afc) of the Zn/ Ti/Nb−O1 bond in R-ZNT (0−0.2) and O-ZTN (0.65−1) solid solution regions are calculated and given in Figure 5. In R-ZNT and O-ZTN phases, decreasing trends for all the bonds are also observed, suggesting a drop trend in εr value. Among the three types of bond covalencies, Afc(Zn−O) possesses the largest value both in R-ZNT and in O-ZTN solid solutions, as one would have expected, from the electronegativity difference between Zn2+ and O2−. However, those narrow variations of fc value (maximum 0.06%) are not sufficient to support the large decrease of εr value at x = 0−0.2 (94.35 to 82.45) and 0.65−1
where kF, aB are Fermi wave vector, Bohr radius (0.5292 Å) kF μ = (3π 2Ne μ)1/3
μ
(7) E
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 2. Microwave Dielectric Properties of xZTN-(1 − x)ZNT Ceramics Sintered at Optimum Temperature for 4 h x (mol)
ST (°C)
εr
Q × f (GHz)
τf (ppm/°C)
phase
0 0.05 0.1 0.15 0.2 0.4 0.45 0.516 0.64 0.65 0.7 0.8 0.9 1
1050 1100 1100 1100 1125 1150 1100 1100 1125 1125 1125 1125 1125 1100
94.35 92.50 90.34 88.37 82.45 58.41 50.70 46.11 40.92 40.63 40.33 40.18 39.49 37.21
10889 11712 12042 12238 13017 18519 22384 27031 41851 42181 45214 47829 44128 41231
353.43 344.11 330.96 322.80 302.15 141.13 50.57 −1.51 −64.40 −73.78 −74.59 −76.52 −77.21 −77.53
R-ZNT
coexist
O-ZTN
Figure 6. Total lattice energy Utotal and Q × f value as a function of x value.
crystal, which can be explained by bond susceptibility χμ.33 Therefore, the χμ is calculated by using the obtained bond parameters, shown in Table 3. According to Table 3, the χμ(Zn−O1) > χμ(Nb−O1) > χμ(Ti−O1), the decreasing rates of χμ(Zn−O1) and χμ(Nb−O1) are larger than the increment of the Ti−O1 bond, and the total χμ in the crystal drops apparently, which well-explains the decline of εr value in RZNT and O-ZTN solid solutions. On the other hand, in high-εr dielectric materials, such as rutile phase, the εr value also can be characterized by the ratting of the octahedron that was predicted by the ratio of dZn/Ti/Nb−O1(1)/dZn/Ti/Nb−O1(2). Still, a decreasing trend of 1.0089 to 1.0083 can be obtained, suggesting that the variations of εr value could be estimated through the complex chemical bond theory. The Q × f value is closely related to the dielectric loss, which can be separated into intrinsic loss and extrinsic loss. In a crystal structure, intrinsic loss can be regarded as loss caused by the anharmonic lattice vibration in a perfect crystal. As reported before, lattice energy (U) represents the binding ability between anions and cations. The stronger the ion binding, the more stable the crystal would stay. In other words, the intrinsic loss is reduced.31 Extrinsic loss, such as densification, grain boundary, grain size, phase composition. Therefore, total lattice energy of one crystal is used by obtaining the fc value, and the U value of any bond can be calculated
Figure 5. Average bond covalency as a function of x value in R-ZNT phase and O-ZTN phase.
Table 3. Bond Susceptibility χμ in R-ZNT Phase and O-ZTN Phase bond susceptibility χμ x (mol)
Zn−O1
0 0.05 0.1 0.15 0.2
2.1831 1.9571 1.7735 1.6222 1.4943
0.65 0.7 0.8 0.9 1
1.4194 1.3568 1.2473 1.1536 1.0745
Ti−O1 R-ZNT 0.6276 0.6314 0.6351 0.6391 0.6428 O-ZTN 1.1087 1.1146 1.1270 1.1389 1.1527
Nb−O1
total χμ
1.2333 1.1056 1.0017 0.9162 0.8439
4.0441 3.6941 3.4103 3.1775 2.9810
Utotal = U=
∑U
(9)
∑ (Ubc μ + Ubi μ) (10)
μ
Ubc μ = 2100m
(Z+ μ)1.64 (d μ)0.75
f μc
(m + n)Z+ Z − ij 0.4 y jj1 − μ zzzf μi d { (d μ)0.75 k μ
0.8124 0.7765 0.7137 0.6600 0.6147
Ubi μ = 1270
3.3404 3.2478 3.0880 2.9525 2.8418
μ
(11)
μ
(12)
μ
where Z+ , Z− are the valence states of cation and anion, respectively. The total lattice energies (Utotal) of R-ZNT and O-ZTN solid solutions are displayed in Figure 6. In R-ZNT phase, the Q × f value increases from 10 889 to 13 017 GHz, and Utotal shows a similar monotonous growth. In O-ZTN phase, the Q
(40.63 to 37.21) simply according to Figure 5. As mentioned before, dielectric constant is caused by polarizability in a F
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 4. Thermal Expansion Coefficient (αL) of R-ZNT Phase and O-ZTN Phase x (mol)
αZn−O1 (10−6 K−1)
0 0.05 0.1 0.15 0.2
1.4556 1.5497 1.6350 1.7139 1.7864
0.65 0.7 0.8 0.9 1
4.5013 4.5582 4.7054 4.8228 4.9333
αTi−O1 (10−6 K−1)
αNb−O1 (10−6 K−1)
αtotal (10−6 K−1)
0.2900 0.3244 0.3587 0.3934 0.4279
3.8985 3.9857 4.0666 4.1456 4.2193
3.3591 3.5166 3.8337 4.1492 4.4719
10.3679 10.4977 10.8075 11.1012 11.4114
R-ZNT 2.1529 2.1116 2.0730 2.0383 2.0050 O-ZTN 2.5074 2.4228 2.2684 2.1292 2.0062
grain size of x = 0.8 is about 4 times larger than that of x = 1. The influence of grain size is nonnegligible. Because there is correlation between τf value, temperature coefficient of the dielectric constant (τε), and thermal expansion coefficient (αL) iτ y τf = −jjj ε + αLzzz k2 {
(13)
The τf value is inversely proportional to the τε and αL values. The τε is usually considered as influenced by dielectric polarizability and it increases with the decline of εr value.34 Therefore, in our work, the large decreasing εr value enhances the τε value to a great extent. The αL is caused by the anharmonicity in the crystal structure.35 It can be estimated by using the calculated lattice energy U
Figure 7. Raman spectra of xZTN-(1 − x)ZNT ceramics for x = 0−1 (inset rutile TiO2 Raman spectrum is reproduced (in part) with permission from ref 37. Copyright 2017 with permission of The Royal Society of Chemistry).
α=
× f value goes through an increase at 0.65−0.8 and the highest Q × f value of 47 829 GHz is obtained at x = 0.8, while a continuous increase of Utotal appears. As suggested above, since all the samples are densified and phase composition remains unchanged at x = 0−0.2 and x = 0.65−1, the grain size effects should be taken into consideration. Generally speaking, larger grain size reduces the amounts of grain boundary, and improves the Q × f value. As shown in Figure 1, the average
∑ Fmnαmn μ
(14)
αmn μ = −3.1685 + 0.8376γmn
(15)
γmn =
kZANCA μ β U (A mBn)ΔA mn
(16)
βmn =
m(m + n) 2n
(17)
where Fmn is the proportion of this kind of chemical bond in the crystal. k and ΔA are the Boltzmann constant and correction parameters. The results are listed in Table 4.
Figure 8. (a) Enlargement of Ag(1) mode at x = 0.65−1 and (b) the Ag(1) position and bond lengths of Zn/Ti/Nb−O1(1) bonds and Zn/Ti/Nb− O1(2) bonds in O-ZTN phase as a function of x value. G
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Figure 9. Measured and fitted infrared spectra of samples for (a) x = 0−1, (b) especially for x = 0 and x = 1, and the real part (c) and imaginary part (d) of complex dielectric response (the blue circles are the measured values in microwave region).
Table 5. Phonon Parameters Obtained from the Fitting of the Infrared Reflectivity Spectra of xZTN-(1 − x)ZNT (x = 0, 1) Ceramics x=0
mode 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ∑
x=1
ωj
γj
Δεj
Δtan δ × 104
ωj
γj
85.33 120.95 152.3 176.87 203.31 251.58 418.37 472.26 588.44 800.2
44.42 40.14 34.13 32.79 40.55 53.76 57.77 106.19 187.85 75.04
36.6 18.6 13.3 13.1 8.69 1.43 0.562 1.63 0.803 0.013
2.2158 0.5065 0.1942 0.1363 0.0846 0.0121 0.0018 0.0077 0.0043 0.0000
88.30 123.15 172.89 204.21 242.31 304.63 344.85 413.36 475.41 523.09 614.56 679.60 770.86 834.77
23.72 51.16 42.61 46.98 44.53 50.00 53.11 83.58 90.22 108.68 105.19 76.86 91.41 37.48
93.17 ε∞ = 4.46, ε0 = 97.63
0.97 3.31 6.48 7.55 4.07 1.24 0.82 1.24 0.83 0.77 0.49 0.16 0.10 0.01 28.02 ε∞ = 4.14, ε0 = 32.16
3.16
As indicated before, the bonding subformula AmBn changes with x increases; that is to say, in the solid solution regions, despite the effects of lattice energy, the proportion of chemical bond Fmn also varies. Therefore, from Table 4, the total α value of the crystal slightly increases with x value. This phenomenon also illustrated the contributions of lattice thermal expansion to the decreasing τf value. On the other hand, since the variations of bond length lead to distortion of the oxygen octahedron, the distortion (Δ) also
Δεj
Δtan δ × 104 0.1552 0.5849 0.4843 0.4455 0.1617 0.0349 0.0191 0.0318 0.0173 0.0160 0.0071 0.0014 0.0008 0.0000 1.96
can reflect the stability of crystal structure. A structure with a less distorted oxygen octahedron has a smaller |τf| value. According to the equation ΔR ‐ZNT = ΔO‐ZTN = H
1 6
i R i − RA yz zz zz k RA {
∑ jjjjj
2
(18)
R largest − R smallest Raverage
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between R-ZNT phase and TiO2 phase. Since there are no relevant Raman reports about R-ZNT phase, its Raman-active modes can be inferred from the rutile TiO2 phase. From Figure 7, four Raman-active modes of rutile TiO2 were detected by Yang,37 which appear at 143, 239, 445, and 613 cm−1 assigned to B 1g , two phonon scattering, E g and A 1g modes, respectively.38 The broadened band at 239 cm−1 is usually considered as a characteristic of rutile TiO2. Similarly, a near position peak at 250 cm−1 detected in pure R-ZNT phase (x = 0). It has been reported that the difference of Ti−O bond length and bond angle makes this peak blue-shifted.39 Therefore, the peak at 250 cm−1 is inferred also representative for rutile ZNT phase, and its shifts are caused by the variation of Ti−O bond length and angle. Ramarao also reported that electronegativity discrepancies of Nb and W at the same site are responsible for the peak splitting behavior of Ag mode in solid solutions.40 So, it is considered that the broadening peaks at 424 cm−1 and two peaks at 612 and 678 cm−1 for x = 0 are generated by the peak at 445 and 613 cm−1 of TiO2 due to the electronegativity difference of Zn2+ (1.65), Nb5+ (1.6), and Ti4+ (1.54) at the same 2a Wyckoff position. Besides, the peak at 143 cm−1 of the B1g mode of TiO2 disappears and an additional peak at 868 cm−1 shows compared with pure TiO2, which is unclear for now and will need further deep investigations. Ceramics of x = 0.2 have the similar Raman-active peaks with x = 0, which confirms the formation of R-ZNT solid solution. After x = 0.2, new peaks corresponding to other vibrational modes are distinguished. For instance, some peaks at 135, 295, 316, and 895 cm−1 are clearly observed, which are assigned to B3g(4), B1g(3), B2g(1), and Ag(2) of O-ZTN phase, respectively. Also, the XRD results prove that O-ZTN phase is formed at x ≥ 0.4; the Raman intensities of O-ZTN phase modes increase along with the increase of x value, especially for B1g(3), B2g(1), and Ag(2) modes, which represents the bending behaviors of O−cation−O bonds and stretching mode of the oxygen octahedron. For Raman-active modes of R-ZNT phase, they become broadened (250 cm−1) and some bands even overlapped (612 cm−1, 678 cm−1) by new modes of O-ZTN phase. This phenomenon illustrates the contributions of OZTN phase to the vibration rise, which is in accordance with the Rietveld refinement analysis. For x = 1, pure O-ZTN phase was formed, and all the detected vibrational modes are assigned according to the previous work:41 Peaks with wavenumber larger than 450 cm−1 present the stretching vibration of Zn/Ti/Nb−O bonds, less than 250 cm−1 are caused by cation vibrations, and in the range of 250−450 cm−1 are the bending vibrations of Zn/Ti/Nb−O bonds. It is worth noting that Ag(1) mode (after 450 cm−1) symbolizes the stretching vibration of Zn/Ti/Nb−O bonds. The Raman spectra after deconvolution of x = 0.65−1 are
Figure 10. Comparisons between theoretical and measured dielectric constant.
where Ri and RA are the bond length of each bond and average value in R-ZNT phase. Rlargest, Rsmallest, and Raverage are the bond lengths in O-ZTN phase. There is a decreasing trend of ΔR‑ZNT value from 8.67 × 10−5 to 7.70 × 10−5 at x = 0−0.2 in R-ZNT phase, corresponding to the increase in structural stability. Similarly, the ΔO‑ZTN value of 10.552% at 0.65 is smaller than that of 10.561% at 0.9, which also indicates a lower |τf| value at x = 0.65. 3.2.3. Raman Spectra and Far-Infrared Reflectivity Spectra Analysis. As known, Raman spectra analysis is very sensitive to the lattice vibrations and provides evidence for the structural variation. For better verifying structure evolution of xZTN-(1 − x)ZNT ceramics, Raman analysis is applied, and the spectra are shown in Figure 7. Due to the differences of RZNT and O-ZTN phases in Wyckoff position, symmetry, and crystal structure, the Raman-active modes can be assumed by group theory. For R-ZNT phase with a space group of P42/ mnm(136) and point group of D4h (4/mmm), its Zn, Ti, and Nb atoms occupy the same 2a, and O occupies the 4f Wyckoff position. Therefore, its optic modes can be predicted by the Bilbao Crystallographic Server,36 which is represented as Γoptic = A1g + A2g + 3A2u + B1g + 4B1u + B2g + 7Eu + Eg, where subscript “g” stands for Raman-active vibrations. Four Ramanactive modes (A1g + B1g + B2g + Eg) and 10 infrared-active modes of 3A2u + 7Eu (acoustic modes not included) can be expected. Similarly, group theory can be applied for O-ZTN phase. In its structure, Zn, Ti, and Nb atoms are in the 4c position while O is in the 8d position, and 57 vibrations are detected, where there are 30 Raman-active modes (6Ag + 9B1g + 6B2g + 9B3g) and 21 infrared-active modes (8B1u + 5B2u + 8B3u). R-ZNT phase was reported as a type of solid solution of TiO2.20 However, there is a large difference of Raman spectra
Table 6. Reported Microwave Dielectric Materials with an εr ∼ 46 and Near Zero τf Value materials
ST (°C)
εr
Q × f (GHz)
τf (ppm/°C)
ref
(Ba1−xSrx)La4Ti4O15 (x = 0.8) Ba3La2Ti2Nb2−xTaxO15 (x = 1) 0.3SrTiO3-0.7Ca(Mg1/3Nb2/3)O3 0.55La(Mg1/2Ti1/2)O3-0.45SrTiO3 0.42ZnNb2O6-0.58TiO2 0.65LiNb3O8-0.35TiO2 0.516ZTN-0.484ZNT
1600 1500 1475 1475 1250 1100 1100
46.1 46.5 46.0 46.3 45.0 46.2 46.11
52800 27140 29300 34000 29000 5800 27031
−3 −4 2 0 0 0 −1.51
49 50 51 52 53 54 this work
I
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Especially, the fitting spectra for x = 0 and x = 1 are shown in Figure 9b and the real and imaginary parts of the complex dielectric response are given in Figure 9c,d. The phonon parameters obtained from the fitting of the infrared reflectivity spectra of xZTN-(1 − x)ZNT (x = 0, 1) ceramics are shown in Table 5. As seen from Figure 9 and Table 5, the measured dielectric constant and dielectric loss are close to the fitted values, which illustrates that the majority dielectric contributions at microwave frequency should be associated with the absorptions of structural phonon oscillation.13 For R-ZNT phase, the vibration modes below 250 cm−1 (especially for modes 1 and 2) provide the primary contributions to the dielectric constant (91.70%) and dielectric loss (99.41%). For O-ZTN phase, similarly, modes between 100−250 cm−1: the infraredactive frequencies46 of 123.15 cm−1 assigned to B1u mode and B3u mode for 172.89 cm−1, 204.21 and 242.31 cm−1 give 10.29% and 56.28% of the total polarizability contributions and contributes 29.84% and 55.69% of the dielectric loss, respectively. 3.3. Phase Compositions (0.2 ≤ x ≤ 0.64) and Microwave Dielectric Properties. As suggested by Figure 1, R-ZNT phase and O-ZTN phase coexist at the region of x = 0.2−0.64. It is widely accepted that extrinsic loss, such as phase compositions, plays an important role in determining the development of microwave dielectric properties. In a multiphase system, some mixture rules can give predictions of the εr value (see Figure 10), i.e., Serial mixing rule, Parallel mixing rule, Lichtenecker rule, and Bruggeman rule.47,48 The consistent experimental results with theoretical value confirm that the decreasing εr value is possibly affected by the phase composition. Due to the difference in measuring frequency of ceramics, the Q value is extracted from their Q × f values. According to the theoretical calculation of Q value (see Table 3S), phase composition surely plays a vital role in determining the actual Q value. Additionally, it could be considered that the negative τf value of O-ZTN phase adjusts the positive value of R-ZNT phase to zero in x = 0.4−0.6. Theoretical calculation indicates that VR‑ZNT:VO‑ZTN = 0.180:0.820 can be achieved at a zero τf value composition, which is very close to our refinement results at x = 0.516. Comparisons between this work and other ceramic systems are listed in Table 6. The 0.516ZTN-0.484ZNT ceramic has a lower sintering temperature and combination of great microwave dielectric properties, which is promising for electronic devices.
shown in Figure 4S, and the peak position of Ag(1) mode is marked as well. A distorted octahedral NbO6 structure presents major Raman frequencies at 500−700 cm.42 The variation of Raman position of Ag(1) mode is shown in Figure 8. Hardcastle43 indicates that stretching frequency of a bond is inversely proportional to its bond length. In the experiment results, the peak position of Ag(1) shifts to lower frequency along with x increases. Therefore, stretching behaviors of Zn/ Ti/Nb−O1(1) and Zn/Ti/Nb−O1(2) chemical bonds are testified by the variation of Ag(1) vibrational mode. The infrared reflectivity spectra are also measured for the samples to investigate the intrinsic dielectric properties, shown in Figure 9a. The measured infrared reflectivity spectra are fitted with a classical oscillator model. Before fitting, the spectra are transformed into a complex dielectric function with44 ε*(ω) − 1
R(ω) =
2
ε*(ω) + 1
(20)
The complex dielectric function can be expressed as ε*(ω) = ε′(ω) − iε″(ω) Sj(ωj 2 − ω 2)
n
∑
= ε∞ +
j=1
(ωj 2 − ω 2)2 + ω 2γ 2
n
− i∑ j=1
Sjωγj 2
(ωj − ω 2)2 + ω 2γ 2
(21)
where ε∞ is the dielectric constant caused by electronic polarization at the optical frequency. n, Sj, ωj, and γj represent the number of transverse polar-phonon modes, strength, eigen frequency, and damping constant, respectively. The spectra are transformed into complex dielectric spectra by using the K-K relationship to obtain the real ε′(ω) and imaginary parts ε″(ω). After making these considerations, the microwave dielectric properties of εr and tan δ can be achieved under the condition of ω ≪ ωj: n
ε′(ω) ≈ ε∞ +
∑ j=1
n
ε″(ω) ≈
∑ j=1
tan δ =
Sj ωj 2
(22)
Sjωγj ωj 4
ε″(ω) = ε′(ω)
(23) n
∑ j=1
Sjωγj ε′(ω)ωj 4
4. CONCLUSION The present work focuses on the xZn0.5Ti0.5NbO4-(1 − x)Zn0.15Nb0.3Ti0.55O2 ceramics and mainly discusses the correlation between structural evolution of rutile-type Zn0.15Nb0.3Ti0.55O2 (R-ZNT) and orthorhombic-structured Zn0.5Ti0.5NbO4 (O-ZTN) phase and microwave dielectric properties via Rietveld refinement, vibration spectra, and complex chemical bond theory analysis. The results reveal that, in the RZNT and O-ZTN solid solution regions (x = 0−0.2 and x = 0.65−1), chemical bonds are stretched along with x increases, which results in the expansion of bond volume and cell volume. In this case, bond covalency ( fc) and bond susceptibility (χμ) are reduced, indicating the continuous decrease of dielectric constant. Lattice energy (U) increases monotonously, suggesting an increasing structural stability and
(24)
From Figure 9a, the measured infrared spectra are well-fitted with the classical oscillator model. Specially, 10 bands are observable for R-ZNT phase (x = 0, 0.2) after fitting, which confirms the R-ZNT solid solution formed. As reported before,45 rutile TiO2 gives 3 visible bands (183, 408, and 596 cm−1) and the band at 183 cm−1 is the strongest. However, our results show some differences that the new band at approximately 85 cm−1 (circle) provides 37% and 46% contribution of dielectric constant at x = 0 and 0.2, respectively. After x = 0.2, additional bands are clearly seen (dotted line) and some bands belonging to R-ZNT phase become broadened. For x = 0.4−1, the band at 190−210 cm−1 (circle) contributes the majority of dielectric constant. J
DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry a higher Q × f value. Far-infrared spectra and a classical oscillator model were used to study the dielectric constant and dielectric loss. A combination of great properties is obtained at x = 0.516 when sintered at 1100 °C: εr = 46.11, Q × f = 27 031 GHz, and τf = −1.51 ppm/°C.
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(9) Xiong, Z.; Tang, B.; Yang, C.; Zhang, S. Correlation Between Structures and Microwave Dielectric Properties of Ba3.75Nd9.5‑xSmxTi17.5(Cr1/2Nb1/2)0.5O54 Ceramics. J. Alloys Compd. 2018, 740, 492− 499. (10) Liu, B.; Li, L.; Liu, X. Q.; Chen, X. M. Structural Evolution of SrLaAl1‑x(Zn0.5Ti0.5)xO4 Ceramics and Effects on Their Microwave Dielectric Properties. J. Mater. Chem. C 2016, 4, 4684−4691. (11) Mao, M. M.; Liu, X. Q.; Chen, X. M. Structural Evolution and Its Effects on Dielectric Loss in Sr1+xSm1‑xAl1‑xTixO4 Microwave Dielectric Ceramics. J. Am. Ceram. Soc. 2011, 94, 2506−2511. (12) Guo, D.; Zhou, D.; Li, W. B.; Pang, L. X.; Dai, Y. Z.; Qi, Z. M. Phase Evolution, Crystal Structure, and Microwave Dielectric Properties of Water-Insoluble (1-x)LaNbO4-xLaVO4 (0 ≤ x ≤ 0.9) Ceramics. Inorg. Chem. 2017, 56, 9321−9329. (13) Pang, L. X.; Zhou, D.; Qi, Z. M.; Liu, W. G.; Yue, Z. X.; Reaney, I. M. Structure-property Relationships of Low Sintering Temperature Scheelite-structured (1-x)BiVO4-xLaNbO4 Microwave Dielectric Ceramics. J. Mater. Chem. C 2017, 5, 2695−2701. (14) Chen, T.; Ma, W.; Sun, Q.; Tang, C.; Huan, Z.; Niu, B. The Microwave Dielectric Properties of (Ni, Zn)0.5Ti0.5NbO4 Solid Solution. Mater. Lett. 2013, 113, 111−113. (15) Liao, Q.; Li, L.; Ding, X.; Ren, X. A New Temperature Stable Microwave Dielectric Material Mg0.5Zn0.5TiNb2O8. J. Am. Ceram. Soc. 2012, 95, 1501−1503. (16) Li, L.; Cai, H.; Yu, X.; Liao, Q.; Gao, Z. Structure Analysis and Microwave Dielectric Oroperties of CaxZn1‑xSn0.08Ti1.92Nb2O10 Ceramics. J. Alloys Compd. 2014, 584, 315−321. (17) Huan, Z.; Sun, Q.; Ma, W.; Wang, L.; Xiao, F.; Chen, T. Crystal Structure and Microwave Dielectric Properties of (Zn1‑xCox)TiNb2O8 Ceramics. J. Alloys Compd. 2013, 551, 630−635. (18) Li, L.; Cai, H.; Ren, Q.; Sun, H.; Gao, Z. Microstructure and Microwave Dielectric Characteristics of ZnTi(Nb1−xSbx)2O8 Ceramics. Ceram. Int. 2014, 40, 12213−12217. (19) Yang, H.; Li, E.; Duan, S.; He, H.; Zhang, S. Structure, Microwave Properties and Low Temperature Sintering of Ta2O5 and Co2O3 Codoped Zn0.5Ti0.5NbO4 Ceramics. Mater. Chem. Phys. 2017, 199, 43−53. (20) Kim, E. S.; Kang, D. H. Relationships Between Crystal Structure and Microwave Dielectric Properties of (Zn1/3B2/35+)xTi1‑xO2 (B5+=Nb, Ta) Ceramics. Ceram. Int. 2008, 34, 883−888. (21) Toby, B. EXPGUI, A Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (22) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86748; U.S. Department of Energy: Washington, DC, 2004. (23) Tseng, C. F. Microwave Dielectric Properties of Low Loss Microwave Dielectric Ceramics: A0.5Ti0.5NbO4 (A = Zn, Co). J. Eur. Ceram. Soc. 2014, 34, 3641−3648. (24) Abrahams, I.; Bruce, P. G.; David, W. I. F.; West, A. R. Structure Determination of Substituted Rutiles by Time-of-flight Neutron Diffraction. Chem. Mater. 1989, 1, 237−240. (25) Fan, X. C.; Chen, X. M.; Liu, X. Q. Structural Dependence of Microwave Dielectric Properties of SrRAlO4 (R = Sm, Nd, La) Ceramics: Crystal Structure Refinement and Infrared Reflectivity Study. Chem. Mater. 2008, 20, 4092−4098. (26) Magrez, A.; Cochet, M.; Joubert, O.; Louarn, G.; Ganne, M.; Chauvet, O. High Internal Stresses in Sr1‑xLa1+xAl1‑xMgxO4 Solid Solution (0 ≤ x ≤ 0.7) Characterized by Infrared and Raman Spectroscopies Coupled with Crystal Structure Refinement. Chem. Mater. 2001, 13, 3893−3898. (27) Levine, B. F. Bond Susceptibilities and Ionicities in Complex Crystal Structures. J. Chem. Phys. 1973, 59, 1463−1486. (28) Xue, D. F.; Zhang, S. Y. Calculation of the Nonlinear Optical Coefficient of the NdAl3(BO3)4 Crystal. J. Phys.: Condens. Matter 1996, 8, 1949−1956. (29) Xia, W. S.; Li, L. X.; Ning, P. F.; Liao, Q. W. Relationship Between Bond Ionicity, Lattice Energy, and Microwave Dielectric Properties of Zn(Ta1‑xNbx)2O6 Ceramics. J. Am. Ceram. Soc. 2012, 95, 2587−2592.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00873. X-ray diffraction patterns; X-ray diffraction profiles after fitting; deconvoluted Raman spectra; density information; structural parameters of R-ZNT and O-ZTN solid solutions after Rietveld refinement; measured and calculated Q values (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: 86-28-8320-6695. E-mail:
[email protected]. ORCID
Hongyu Yang: 0000-0002-5894-1693 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the managers in the Infrared Spectroscopy and Microspectroscopy Endstation (BL01B) of the National Synchrotron Radiation Laboratory (NSRL), especially Prof. Chuan-Sheng Hu and Prof. Ze-Ming Qi for their assistance in the IR measurement and fitting. The authors also thank Prof. Shu Zhang in the School of Energy Science and Engineering of University of Electronic Science and Technology of China for her helpful discussions on the complex chemical bond theory.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b00873 Inorg. Chem. XXXX, XXX, XXX−XXX
