Novel scheelite-type [Ca 0.55 (Nd 1- x Bi x ) 0.3 ]MoO 4 (0.2 ≤ x ≤ 0.95) microwave dielectric ceramics with low sintering temperature

Novel scheelite-type [Ca 0.55 (Nd 1- x Bi x ) 0.3 ]MoO 4 (0.2 ≤ x ≤ 0.95) ceramics were prepared using the solid-state reaction method. According to the X-ray diffraction data, a solid solution was formed in 0.2 ≤ x ≤ 0.95 and all the samples belong to pure scheelite phase with the tetragonal structure. As revealed by Raman spectroscopy, the number of vibrational modes decreased with the increase in x value, which further indicated that Bi 3+ ions occupied A-site of scheelite structure. As the x value increased, the sintering temperature decreased from 740°C to 660°C; the permittivity increased from 12.6 to 20.3; the Qf value first decreased slightly and gradually remained stable. Based on the infrared reflectivity spectrum analysis, the calculated permittivity derived from the fitted data shared the same trend with the measured value. The [Ca 0.55 (Nd 0.05 Bi 0.95 ) 0.3 ]MoO 4 ceramic sintered at 660 °C attained a near-zero value temperature coefficient ~τ f (−7.1 ppm/°C) and showed excellent microwave dielectric properties with a ɛ r ~ 20.3 and a Qf ~ 33 860 GHz, making this system a promising candidate in the ultralow temperature cofired ceramic (ULTCC) technology.

the ultralow-temperature co-fired ceramic (ULTCC) technology, 2 provide technical support for the fabrications of the miniaturized and integrated microwave devices. Hence, there is an urgent need to develop microwave ceramics suitable for LTCC/ULTCC technology. Microwave dielectric ceramics used in LTCC/ULTCC technology must meet several basic requirements. First, the microwave ceramics should have lower sintering temperatures than the melting points (MP) of metal electrodes, such as Ag with a MP ~961°C and Al with a MP ~660°C. Second, the dielectric loss should be very low (high-quality factor Q) to reduce the power consumption of microwave devices. Third, the temperature coefficient of resonant frequency τ f values of the microwave ceramics should approach the zero to improve the temperature independence of microwave devices. Finally, the raw materials of the microwave ceramics should be nontoxic for the environment. [3][4][5][6][7][8][9][10] Recently, the A 2+ B 6+ O 4 (A 2+ = Mg, Mn, Ca, Zn, Cd, and B 6+ = Mo, W) oxides with scheelite structure have been widely investigated on their microwave dielectric properties. 11,12 Due to the low sintering temperature and the adaptive crystal structure providing the potential for the various substitutions at A-or B-site, these compositions have received extensive attentions in LTCC technology. In the previous report, Choi et al found that the CaMoO 4 possessed a high Qf value (~89 700 GHz) and a low sintering temperature (~1100°C). 13 Rama et al used the Cd 2+ ion to substitute for the Ca 2+ ion and gained good microwave dielectric properties (ɛ r = 8-10, Qf = 46 000-53 000 GHz, τ f = −22-−57 ppm/°C) in Ca 1-x Cd x MoO 4 ceramics at low temperature (900-1075°C). 14 Zhou et al changed the elements at A-site and gained the temperature stable [(Li 0.5 Ln 0.5 ) 0.2 Ca 0.8 ]MoO 4 ceramics with excellent microwave dielectric properties (ɛ r = 10.6-11, Qf = 18 400-24 500 GHz, τ f = −7.1~ + 6.9 ppm/°C) and low sintering temperatures (S.T = 800-900 °C). 15,16 According to the literatures' reports, 17,18 the CaMoO 4 ceramic has a negative τ f value, and the Bi-doping could adjust the τ f to near-zero and even positive value. The bismuth oxide was reported to have intrinsic low-melting temperature and might help reduce the sintering temperature of scheelite oxides. In addition, the rare-earth oxide, such as Nd 2 O 3 , etc, was found to improve the dielectric property in many microwave dielectric ceramics. 19 The microwave dielectric properties are closely related to the microstructures of ceramics. Choi et al ever used the Clausius-Mosotti relation to calculate the polarizability of Mo in AMoO 4 (A = Mg, Zn, Sr, Ca and Ba) compounds. 13 Kim et al used the packing fraction to correlate the quality factor of scheelite A 2+ B 6+ O 4 oxides. 20 Furthermore, many researchers used the Raman spectroscopy to obtain the information of the vibrations and rotations. 21,22 Some researchers used the infrared spectroscopy to reveal the complex dielectric response in ceramics. 23,24 Based on the fitting data from the infrared spectroscopy, the permittivity and intrinsic dielectric losses could be calculated. Hence, in this work, the (Nd 1-x Bi x ) 3+ complex ions were used at A-site of scheelite structure to adjust the microwave dielectric properties and the sintering temperature of CaMoO 4 ceramic. The microcharacteristics and microwave dielectric properties of [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 (0.2 ≤ x ≤ 0.95) ceramics were studied. The relationship between the properties and vibrational modes were investigated in detail.

| EXPERIMENTAL SECTION
High-purity oxide powders of Bi 2 O 3 (>99%), MoO 3 (>99.95%), Nd 2 O 3 (>99%) and the carbonate powder of CaCO 3 (>99%) were mixed proportionally for the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ] MoO 4 (0.2 ≤ x ≤ 0.95) ceramics. First, the raw materials were put into the nylon jars with zirconia balls and prepared by the method of high-energy milling. Second, the milled powders were calcined at 600°C for 4 hours. Then, the calcined powders were ground for the second time. After the milled powders were dried, the PVA binder can be used to mix the powders and make the powders granulated. Finally, the mixed powders were pressed into pillars under a pressure of 160 MPa using the uniaxial hydraulic press. All the green bodies were put into the muffle and sintered at the temperature from 660°C to 740°C, the holding time is 2 hours.
Phase identifications of the samples were obtained by the powder diffraction data using the X-ray diffractometer with Cu Kα source, operated at 40 kV and 30 mA, all the data were gathered in the range of 10-70° with a scan step of 0.02. The topography of the crystal grain of the ceramics was surveyed using the scanning electron microscopy (SEM). The IR spectra measurement of the ceramics used a Bruker Optik IFS 66v FTIR spectrometer on an infrared source base station (U4) at the National Synchrotron Radiation Lab (NSRL), China. The measurement of the Raman vibration modes of the ceramics used a Raman spectrometer, which was stimulated using an Ar + laser (532 nm). The quality factor was measured with a microwave network analyzer and the dielectric constant was calculated with the TE 01δ shielded cavity method, meanwhile, the τ f value was measured in a thermal cycling chamber, the calculated value could be obtained using the following formula: where the h and l were the values of high temperature and low temperature, respectively. The f h was the TE 01δ resonant frequencies at high temperature, and the f l was the TE 01δ resonant frequencies at low temperature.

| RESULTS AND DISCUSSIONS
In Figure 1A, the room temperature XRD patterns and the crystal structure ceramics are clearly observed. All the diffraction peaks are indexed as a tetragonal scheelite structure without any secondary phase. 25 Therefore, the complete solid solution is obtained in the whole compositions. With the increase in x value, the corresponding peaks moved to lower 2θ angles direction, which indicates an increase in the unit cell volume.
According to the ionic radius reported by Shannon, 26 the ionic radius of Bi 3+ (1.17Å) is larger than that of Nd 3+ (1.109Å) and introduction of Bi 3+ expanded the volume of the unit cell. Meanwhile, the intensity of (101) peak gradually decreases, indicating that the increase in content of Bi 3+ ions enhanced the disordering at A-site. 27 The lattice parameters calculated from the measured XRD data are shown in Figure 1B. When the x value increases, the lattice parameters a, c and the unit cell volume V all increase, which are consistent with the peaks shifts in XRD patterns. Meanwhile, a slight decrease in a/c could be observed as shown in Figure  In the scheelite primitive cell, the A g , B g , and E g vibrations are Raman active vibrations, the A u and E u belong to the infrared active vibrations and the B u is the inactive vibrations. Furthermore, as the literature reported, 28 the lattice vibrations are divided into two parts in the tetragonal scheelite structured materials. The first one is external mode, which corresponds to the movements of A-site cations and rigid Mo-O tetrahedra, the measured data below 350 cm −1 all belong to the external modes. The second one is internal mode, which derives from the vibrations inside the Mo-O tetrahedra, the measured data above 350 cm −1 all belong to the internal modes.
In this study, Raman spectra of [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramics are demonstrated in Figure 3 in range 100 cm −1 -1100 cm −1 . The Raman spectra were fitted using the Lorenz and Gaussian functions. The measured and fitted values match well with each other. At x value ~0.2, 13 vibrational modes are clearly observed in the ceramic. With the increase in x, the number of the vibrational modes decreases and it should be attributed to the distortions of MoO 4 tetrahedra. As shown in Figure 3, the A peak located at 141 cm −1 corresponds to the translational movement of cation at A-site, and the B peak located at 191 cm −1 belongs to the rotation mode between the ion bonds. These spectrum data are in good agreement with that of Porto's report. 21  (2) Γ = 3A g + 5B g + 5E g + 5A u + 5E u + 3B u are shifted by a small amount. These phenomena indicate the increasing disordering in the MoO 4 tetrahedral with the introduction of (Nd 1-x Bi x ) 3+ . When the x value increases to 0.95, there are two more vibrational modes (1, 4) observed.
The infrared spectra can be analyzed using the Fresnel formula (3) and the Lorentz formula (4) in the typical one-dimensional harmonic oscillator model. Furthermore, these methods are effective to analyze the complex dielectric response of [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramics.
where the IR reflectance denoted as R(ω); the dielectric function of R(ω) denoted as ɛ'(ω); the high-frequency dielectric constant attributed to the electronic polarization denoted as ɛ ∞ ; the plasma and transverse frequency denoted as ω pj and ω oj , respectively, γ j and n represent the damping factor and the phonon mode number, respectively.
In Figure 5, the fitted results of the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ] MoO 4 ceramics can be observed. According to the experimental measurement data (sign as red circles in picture), all the measured permittivities are slightly larger than that of the calculated ones. These two results show the same trend with each other. When the x value is 0.   where n is the number of per unit cell in the compounds. M and V are molecular weight; the cell volume of the ceramics, respectively, N represents the Avogadro's number (6.022 × 10 23 mol −1 ). Using the measured densities and the theoretical ones, the relative densities of [Ca 0.55 (Nd 1-x Bi x ) 0.3 ] MoO 4 ceramics could be calculated with the following equation.
Densities and relative densities of [Ca 0.55 (Nd 1-x Bi x ) 0.3 ] MoO 4 ceramics sintered at different temperature are shown in Figure 6A-F. It can be observed that all the values of density and relative density have the same variation trend. In Figure 6A, the maximal density of the [Ca 0.55 (Nd 0.8 Bi 0.2 ) 0.3 ]MoO 4 ceramic is 4.51 g/cm 3 , the sintering temperature is 740°C. As we know, the bismuth oxide has a low-melting point and addition of Bi 2 O 3 help reduce the sintering temperature. Therefore, with the x value increasing, the optimal sintering temperature decreases from 740°C to 660°C, and the relative density reduces from 95% to 92%. All the samples are well densified and these results are consistent with the SEM results. Figure 7A-F shows the permittivities and Qf values of the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramics as a function of sintering temperature. As presented in Figure 7A, the permittivity of [Ca 0.55 (Nd 0.8 Bi 0.2 ) 0.3 ]MoO 4 ceramic increase gradually and then reaches a maximum value at 740 °C as the sintering temperature increases. Meanwhile, the Qf value has a quite similar trend vs sintering temperature. When the x value rises, the sintering temperature of the ceramic with best microwave dielectric properties shifts to the low field, these results are similar to the change in the density and relative density. Then, the optimal sintering temperature of the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ] MoO 4 ceramics could be obtained.
The measured microwave dielectric properties of the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramics with optimal sintering temperature are shown in Figure 8. It is clearly observed that the ɛ r value increases from 12.6 to 20.3. According to the Clausius-Mosotti equation, the dielectric constant depends on the polarizability. When the ionic substitution happens, the total ionic polarizability will increase, resulting in an increase in ɛ r value. the polarizability of Bi 3+ and Nd 3+ ions are 6.12 Å 3 and 5.01 Å 3 , respectively. The molecular polarizability increases from 14.63 Å 3 to 14.88 Å 3 as the x value increases, indicating that the increasing ɛ r value mainly derives from the enhanced cationic polarizability. In this work, the change in relative permittivity is consistent with the Shannon's theory. Generally, the Qf value is inversely proportional to the intrinsic dielectric losses. When the x value increases, the expanding unit cell volume and the enhancing disorder lead to the increase in dielectric loss, then the Qf value has a decreasing trend range from 68 380 to 33 860 GHz. The near-zero τ f value is a significant factor for the microwave ceramic, when x value increases, the  Table 2. As shown in Table 2, the ceramics (eg, x = 0.8, 0.9) sintered at low temperature with excellent microwave dielectric properties can be potentially applied in LTCC technology.

| CONCLUSION
A series of scheelite-type [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 (0.2 ≤ x ≤ 0.95) ceramics are successfully prepared by solidstate reaction method. According to the XRD data, the solid solution was formed and the pure tetragonal phases are clearly observed in the [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramics. When the x value increases, the sintering temperature of the sample gradually decreases from 740°C to 660°C, and the average grain size of the sample also decreases from 3.16 μm to 2.78 μm. These results indicate that the Bi 3+ ion derived from Bi 2 O 3 could effectively lower the sintering temperature and refine grains. Meanwhile, the permittivity is found to increase from 12.6 to 20.3 with increasing x, which is due to the enhancement of polarizability derived from Bi 3+ . The Qf value decreased and gradually remained stable, which is closely related to the rare earth element Nd 3+ . The Raman spectroscopy showed that the internal modes gradually disappear with increasing x value, this phenomenon is due to the distorting MoO 4 tetrahedra, which is also related to the introduction of the (Nd 1-x Bi x ) 3+ ions. The far infrared frequency is close to the microwave frequency, the measured dielectric response from IR spectra can extrapolate to the microwave region. With the fitting data of the IR spectra, the calculated permittivity has the same trend with the measured one, this result indicates that the A-site cations mainly dominate the variation of dielectric response at microwave frequency. The [Ca 0.55 (Nd 0.05 Bi 0.95 ) 0.3 ] MoO 4 ceramic sintered at low temperature (660°C) possesses optimal microwave dielectric properties with a relative permittivity of 20.3, a Qf value of 33 860 GHz and a approach zero τ f value (−7.1 ppm/°C). The [Ca 0.55 (Nd 1-x Bi x ) 0.3 ]MoO 4 ceramic system with excellent microwave dielectric properties can be potentially used in LTCC or ULTCC technologies.