Chemical composition tuning induced variable and enhanced dielectric properties of polycrystalline Ga2‐2xWxO3 ceramics

We report on the tunable and enhanced dielectric properties of tungsten (W) incorporated gallium oxide (Ga2O3) polycrystalline electroceramics for energy and power electronic device applications. The W‐incorporated Ga2O3 (Ga2−2xWxO3, 0.00 ≤ x ≤ 0.20; GWO) compounds were synthesized by the high‐temperature solid‐state chemical reaction method by varying the W‐content. The fundamental aspects of the dielectric properties in correlation with the crystal structure, phase, and microstructure of the GWO polycrystalline compounds has been investigated in detail. A detailed study performed ascertains the W‐induced changes in the dielectric constant, loss tangent (tan δ) and ac conductivity. It was found that the dielectric constant increases with addition of W in the system as a function of temperature (25°C‐500°C). Frequency dependence (102‐106 Hz) of the dielectric constant follows the modified Debye model with a relaxation time of ∼20 to 90 μs and a spreading factor of 0.39 to 0.65. The dielectric constant of GWO is temperature independent almost until ∼300°C, and then increases rapidly in the range of 300°C to 500°C. W‐induced enhancement in the dielectric constant of GWO is fully evident in the frequency and temperature dependent dielectric studies. The frequency and temperature dependent tan δ reveals the typical behavior of relaxation loses in GWO. Small polaron hopping mechanism is evident in the frequency dependent electrical transport properties of GWO. The remarkable effect of W‐incorporation on the dielectric and electrical transport properties of Ga2O3 is explained by a two‐layer heterogeneous model consisting of thick grains separated by very thin grain boundaries along with the formation of a Ga2O3‐WO3 composite was able to account for the observed temperature and frequency dependent electrical properties in GWO. The results demonstrate that the structure, electrical and dielectric properties can be tailored by tuning W‐content in the GWO compounds.


INTRODUCTION
effects of W into Ga 2 O 3 is not well understood at this time. Therefore, understanding of the W-mixed Ga 2 O 3 ceramics could be useful to predict the surface/interface diffusion and electrical properties of reaction compounds (if any) in such device applications involving W-Ga 2 O 3 contacts. 46,47 Also, the GWO bulk ceramic materials with controlled structure and properties may be useful to employ them as target materials for high-quality thin film deposition using physical vapor deposition. In view of all these considerations, the dielectric properties of GWO bulk ceramics were investigated as a function of W-concentration and under variable conditions. The present work may also contribute to the understanding of the effect of sintering behavior and chemical composition on the properties of mixed oxides or ceramic solid solutions based on Ga that is, Ga 2−x M x O 3 , where M represents a different cation. In fact, as widely reported in the literature, there have been significant efforts with a focus toward understanding various growth mechanisms and/or elucidating the effect of processing conditions on the synthesis of intrinsic and doped bulk Ga 2 O 3 ceramics for a range of optoelectronic applications. It was reported that the sintering temperature strongly influences the microstructure and final density of sintered Ga 2 O 3 ceramics. 48 The Ga 2 O 3 ceramic targets with high density were obtained by using Ga 2 O 3 micro-particles with uniform particle size, which shows promising applications for optoelectronic devices. 48 Similarly, the γ-Ga 2 O 3 -Al 2 O 3 solid solutions derived based on the reaction mixtures exhibit high catalytic activities for selective reduction of NO using methane as the reducing agent. 49 The annealing controlled growth optimization has been proved to grow La-doped a-GaOOH into nanostructures of -Ga 2 O 3 and -Ga 2 O 3 which has an influence on luminescence on La-doped -Ga 2 O 3 nano-spindles. A mixture of gallium oxide and silicon powders have been subjected to vapor-liquid-solid process or vapor-solid process to produce Si-doped Ga 2 O 3 which are sensitive to UV/ blue intensity ration and eventually causes a decline in it. 50 Spectacular cactus-like nanostructures of -Ga 2 O 3 were grown to investigate field emission properties which made a headway due to its first time reporting for possible applications. 51 Ga 2 O 3 transparent ceramics prepared by ceramic method with a controlled density and morphology were shown to exhibit excellent photoluminescence properties, which can mainly be attributed to the recombination between donors and acceptors. Ga 2 O 3 transparent ceramics have promising applications as transparent conductive materials and inorganic scintillators. 52 Similarly, even the growth mechanisms for synthesizing rod-shape like -Ga 2 O 3 structures by calcination pointed to a noticeable change in the porosity and pore distribution 53 which is essential to understand the inclusion of impurity materials for a given targeted application. Therefore, we believe that the present work on sintered W-mixed Ga 2 O 3 ceramics may also contribute to further advancements in the field, especially, in the light of existing efforts to the large class of mixed oxides and solid-solutions based on Ga 2 O 3 .

Materials and ingredients
After carrying out the stoichiometric calculations, Ga 2−2x W x O 3 (GWO) compounds were synthesized by conventional solid-state reaction method. The WO 3 concentration was varied from 0 ≤ x ≤ 0.20. The precursors Ga 2 O 3 (99.99%) and WO 3 (99.9% purity) were procured from Sigma-Aldrich. We adopted the previously established procedures and methods to synthesize all the GWO compounds. 42 Briefly, to prepare selected GWO composition, the precursors were weighed in stoichiometric proportions according to the calculations. An agate mortar is ideal for the quantity of powders which was used to pulverize the powder with acetone as wetting media. This method gives a homogeneous mixture of the GWO compounds.

Calcination and sintering
The homogeneous mixture of GWO powders were then transferred into crucibles and calcined at 1050 • C, 12 hours and 1150 • C, 12 hours in a muffle furnace. Each calcination was followed by intermediate grinding to ensure and assist the solid-state reaction. After the final calcination, the powders are thoroughly ground to enhance sinterability. The addition of polyvinyl alcohol (PVA) at this step is to give binding strength to the pulverized powder which is followed by pelletization into circular disc shape of 8 mm diameter and ∼1 mm thickness. A uniaxial hydraulic press was used to apply 1.5 ton of load for this process. These green pellets were then sintered at 1250 • C for 6 hours with a ramp rate of 5 • C/min, and binder burnout of pellets was ensured by holding at an intermediate temperature (500 • C) for 30 minutes. The crystal structure, surface chemistry and chemical composition of all the synthesized GWO compounds are thoroughly established. 42

Dielectric measurements
Dielectric measurements were performed using a HIOKI IM3536 LCR meter. Circuit corrections were made to ensure the accuracy of the readings taken on the LCR meter. The sample pellets were prepared by fine polishing and coating with silver paste on both sides prior to measurement. The capacitors fabricated using the GWO as dielectric while silver (Ag) serves as the metal electrodes of the capacitor. The silver coated GWO pellets were cured at 90 • C for 2 hours to ensure the proper functioning of the electrodes. The capacitance, dielectric dissipation (tan ) and inductance data were collected between 1 kHz and 1 MHz frequency (at 125 frequencies) and a temperature range of 30 • C to 500 • C (for every 10 • C). It is well known that the electrical energy stored in a capacitor is function of capacitance, which is determined by the capacitor geometry and the dielectric constant of the oxide. 47,[54][55][56] Thus, the dielectric constant of a material represents the charge storage capacity when a potential is applied to it. 47,[54][55][56] It is calculated by the following equation, where the capacitance is given 47,56 by: where, is the dielectric constant of the material under investigation, C represents the capacitance, A the area of the capacitor's plate, d the distance between the capacitor's parallel plates, and 0 the dielectric constant of free space.

Dielectric constant -frequency dependence
The sintered GWO pellets were subjected to a frequency sweep ranging from 1 kHz to 1 MHz and the data obtained are shown in Figure 1. As it can be seen from Figure 1, the dielectric constant ( ′ ) at lower frequency range shows comparatively higher values. However, as the frequency increases, the ′ then decreases. It decreases until it plateaus out with the increase in frequency. This behavior of ′ with respect to frequency is typical for all the GWO compounds. The dielectric dispersion of any dielectric material is a complicated function of the frequency of applied electric field. It also depends on the microstructure (eg, grain size) of the material system under investigation. As reported elsewhere, the incorporation of W into Ga 2 O 3 results in microstructural changes. 42 Initially, the pristine Ga 2 O 3 exhibits a rod-shaped structure. The addition of small quantities of W into the system changes the morphology to spherical and as the W concentration increases the grains become faceted with square or hexagonal features. The facets even exhibit twin lamellae which are unique to the GWO system. Overall the grain growth is abnormal and is attributed to the unreacted phase that is accumulated at the grain boundaries. Also, the presence of twin lamellae is due to the WO 3 which enhances the diffusion of vacancies. The fact that this abnormal grain growth with twin lamellae is contributing to the increase in grain boundary area which is assisted with the unreacted WO 3 aggregated at the grain boundaries. This overall phenomenon is contributing to the increased resistance at grain boundaries which impairs conductivity. It is evident from Figure 1 that the real part of the dielectric constant, ′ , shows usual behavior with alternating frequency. Which can be explanied by addressing the polarization source in the system. 57,58 At the lower end of the frequency range, ′ assumes higher values throughout the sweep profile. This is due to the fact that ionic, space charge and grain boundary polarization contribute to the higher values of ′ at lower frequencies. 58,59 Perhaps, the presence of space charge polarization at the grain boundaries, may generate a potential barrier. Then, an accumulation of charge at the grain boundary occurs leading to higher values of the dielectric constant. 59,60 However, it may be noted that as the frequency increases, the ′ values deccreases rapidly. This can be attributed to the species that are contributing to the polarization phenomenon, lag behind the applied voltage in the high frequency domain. This typical dielectric behvaior can be elaborated by the dispersion due to Maxwell-Wagner polarization, 61 Figure 2 shows the dielectric dispersion behavior for GWO materials using the modified Debye function as shown in the following equation 59,60,62,64 : where ′ ( ) gives the complex permittivity, ( ′ 0 − ′ ∞ ) gives the dielectric relaxation strength, ′ 0 represents the low frequency permittivity (static) while ′ ∞ represents high frequency permittivity. ω is the angular frequency which is derived from the linear frequency (f ) of the applied electric field as = 2 f . represents the Debye average relaxation time while denotes the spreading factor of actual relxation time about the mean value. The intrinsic paramters of the GWO compunds were determined using Cole -Cole plots 65 as can be seen in Figure 3. The values for spreading factor " " and relaxation time " " were obtained by plotting ln [( ] as a function of ln ω with only the real part of the dielectric dispersion in context. In other words, the pioneering work of Cole-Cole with the standard procedure 65 were adopted to fit the experimental data, based on the real part of the dielectric constant instead of the complex part of the dielectric constant, and to obtain information on the dielectric relaxation behavior. 60 As reported F I G U R E 2 Real part of dielectric constant fitted to modified Debye Function. The plots are for different compositons of GWO elsewhere, such analyses and procedures were found to be quite useful to understand the dielectric relaxation behavior in complex ceramics or chemical compounds with multivalent cations present. 54,55,60 The values obtained for and were used to fit the data by computing the Debye function mentioned in Equation (1) and fitting it with the experimentally measured values for ′ at room temperature. As it can be seen from Figure 2, the experimental and calculated values show a good agreement which further corroborates the validity of the modified Debye's function in claiming the multiple ion contribution to the relaxation process. The and values determined are tabulated in Table 1. It can be noted that these values are in reasonable agreement with doped semiconductors and complex ceramic compounds. 54,55,60

Dielectric constant -temperature dependence
The variation of ′ with temperature is shown in Figure 4. The data shown are for GWO compounds with variable W-content and measured at different frequencies. At lower frequency (1 kHz-10 kHz), polycrystalline -Ga 2 O 3 shows a single relaxation peak at ∼400 • C. The peak intensities fade at higher frequencies with no relaxation peaks at f = 100 kHz to 1 MHz. almost all the GWO composites show a relaxation peak and the dielectric constant follows an increasing monotonic function. The high intensity relaxation peaks observed in -Ga 2 O 3 are due to the conduction between the grains and grain boundaries. As the W concentration in -Ga 2 O 3 increases, more and more W 6+ ions are introduced into Ga 2 O 3 altering the grain morphology from rod shaped (un-doped Ga 2 O 3 ) to spherical shape which provides a larger area for conduction and hence the reduced intensity of relaxation peaks. Note that the dielectric properties of ceramics are highly dependent upon the microstructure, defect structure, type of ionic dopants, and temperature. 4,55,60,65,66 In fact, realizing that the energy storage properties mainly depends on the defect chemistry of the dielectric, several research groups have paid attention recently to tailor the multilayered materials for significant enhancement of energy storage performances by regulating the dielectric contrast between adjacent layers. 4,66 In the present case, as the W 6+ substitution increases in -Ga 2 O 3 , the microstructural properties are altered as evident in our previous work as well as the TEM analyses discussed in subsequent sections. Owing to the smaller ionic radius of W 6+ , when it substitutes for Ga 3+ (larger ionic radius), there is a shrinkage of the unit cell volume. This shrinkage results in enhanced charge carrying capability which in turn reduces the hopping distance. At x = 0.10, the dielectric relaxation is unnoticed but as the concentration increases, the low intensity relaxation peaks resurface. This phenomenon is attributed to the undissolved WO 3 agglomerated at the grain boundaries and increasing the resistance at interface. The abnormal grain growth arising due to vacancy assisted enhanced mass F I G U R E 5 Temperature dependence of ′′ in GWO compounds as a function of frequency at different W concentration transport is also contributing to the twin lamellae which explains the resurfacing relaxation peaks as x > 0.10. Thermal energy enhanced charge carrier mobility and improved hopping is generally observed at higher temperature. Thus, an increase in dielectric polarization contributes to the increased ′ values. The temperature dependence of the corresponding imaginary part of dielectric constant for GWO compounds is presented in Figure 5. The data presented are for GWO compounds with variable W-content and measured at various frequencies. It may be noted that as WO 3 concentration increases in the system, the value for ′′ also increases. It can be seen that there is a shift in the dielectric values due to the change in temperature. This is due to the increase in charge carrier mobility and the enhanced hopping rate due to increased temperature. The lower temperature does not support this phenomenon and hence the lower values of ′′ .
The frequency dependence of the corresponding imaginary part of dielectric constant for GWO compounds is presented in Figure 6. The data presented are for GWO compounds with variable W-content and measured at various temperatures. It is evident that the ′′ values tend to be generally higher at lower frequencies but decreases rapidly with increasing frequency. On the other hand, ′′ is seen to increase with increasing temperature. Also, the data clearly indicates that the temperature dependence of ′′ is strongly dependent on the frequency of measurements. These variations can be understood if we consider the microstructure variation and interfacial contributions in the W-doped Ga 2 O 3 materials. The contribution of the interfacial losses and the loss from electrical conductivity (as discussed in the subsequent F I G U R E 6 The dependence of ′′ in GWO compounds as a function of W concentration at a particular frequency sections) is generally dominant at lower frequencies; however, these factors becomes negligible at higher frequencies. [55][56][57][58][59] Thus, the observed decrease in the imaginary part of the dielectric constant observed at higher frequencies can be attributed to the rapidly fading off the contributions from the interfacial as well as grain conductivity mechanisms. However, the large values -observed at lower frequency is mainly due to the W-doping induced complex chemistry of Ga 2 O 3 in terms of multiple valence cations, vacancies, and grain boundary defects. 55,57-59

Dielectric loss
The commonly referred dielectric loss factor is defined as the ratio of imaginary part of dielectric constant to the real part and it is given by tan = ′′ / ′ , where δ is the phase difference between current and voltage of the applied electric field. 57,63 As shown in Figure 7, the loss factor with reference to change in temperature indicates that -Ga 2 O 3 assumes a lower value for lower frequencies until temperatures of 350 • C to 420 • C. Then the loss tan rises exponentially toward the higher range of measuring temperatures. This is true for higher frequency range as well, indicating a dormant behavior of the participating species. -Ga 2 O 3 is known to have intrinsic oxygen defects leading to space charge polarization which exists during the entire temperature sweep. As the temperature increase, this effect becomes predominant and the loss tan δ rises exponentially as seen in Figure 7.  Figure 8 shows the variation of tan with temperature at different frequencies, it may be noted as the concentration of WO 3 in the system increase, tan increases. As reported elsewhere, a mixture of W 4+ and W 6+ valence states is present at lower W concentrations. 67 This gives rise to a charge imbalance which explains the substitution of Ga 3+ ions and with free W ions for conduction reducing the dielectric loss at lower concentrations. With the increment in WO 3 , the aggregation of mixed phase may give an improved mass transportation assisted Ga 3+ substitution. This may finally result in a certain amount of dopant infusion and additional amount to agglomerate. Figure 9 shows the ac conductivity of GWO samples as a function of ln . It is evident from the figure that the conductivity increases with the increase in frequency. It can also be seen that the conductivity increases with the increase in W content in the GWO system. This is attributed due to the electron hopping between various cations. This is also influenced by the increase in frequency which increases the hopping rate and hence the improved ac conductivity. To further expand the understanding of transport mechanism in GWO dielectric, a plot of acdc in log scale has been plotted against the log 2 as shown in Figure 10. The plots obtained validate the polaron hopping conduction mechanism involved in all the GWO dielectric materials. The polaron is often described in context of a deformable polar medium as a self-stablilized electronic charge. The lattice distortion induced slow motion in the polarons is known as polaron hopping. 55,68,69 As it can be seen in Figure 10, the W-doped Ga 2 O 3 ceramics exhibit a linear growth before being achieiving the saturation state at higher frequency. Intrinsic Ga 2 O 3 has the lowest magnitude as compared to other doped GWO samples which forms a cluster as the dopant level increases. The underlying mechanism behind this can be explained the following equation 68,69 :

AC conductivity
F I G U R E 10 The log( ac − dc ) vs log 2 plots for GWO dielectric materials. The data shown are for GWO dielectrics with variable W-concentration. The initial linear plots are evident of small polaron hopping mechanism (see text) operative in the electrical transport properties of GWO dielectrics. The elevated set of data with a different linear behavior and slope can be also noted for W-doped vs intrinsic Ga 2 O 3 where, is angular frequency and represents the average relaxation time. It is worth noting that for conduction occuring in a localized neighborhood with a small polaron hopping, 2 2 < 1, the log ( ac − dc ) vs log 2 will always represent a linear behavior which is clearly evident in Figure 10. While this observation clearly supports the fact that the electrical transport mechanism in GWO materials is based on the polaron hopping among the localized sites, the W-induced distortion of the lattice is the source of such localization of charge carriers leading to the polaron formation. Furthermore, metal-doping induced lattice distortion leading to such localization of charge carriers leading to polaron formation and polaron-hopping in the localized states facilitated electrical conduction mechanism was noted and reported in some of the doped ferrite semiconductors. 55,70-72

Proposed mechanism and model
Finally, based on the observations made from previously reported structural details 42,67 and the present work on frequency and temperature dependent dielectric constant, dielectric loss, and ac electrical transport analysis, the effect of W-doping on the electrical conduction mechanism and dielectric properties of Ga 2 O 3 can be modelled simply from the microstructure and heterogeneity perspective. First of all, it must be emphasized that crystal structure, phase and microstructure analyses using X-ray diffraction (XRD), scanning electron microscopy (SEM) indicate that the W-doping induced changes are significant. 42,67 As reported previously, XRD analyses of GWO reveal the formation of a solid solution at lower concentrations W (x ≤ 0.10) while unreacted WO 3 secondary phase formation occurs at higher concentrations (x > 0.10). Insolubility of W at higher concentrations (x ≥ 0.15) leading to a Ga 2 O 3 -WO 3 composite formation is attributed to the difference in formation enthalpies of respective oxides that is, Ga 2 O 3 and WO 3 . Furthermore, the surface chemistry and electronic structure analyses using X-ray photoelectron spectroscopy (XPS) analyses also supported the formation of Ga 2 O 3 -WO 3 composite formation. 67 However, XPS studies reveal the lower valence state (W 4+ ) formation for GWO compounds with lower concentration of W, Thus, the structural and chemical analyses strongly support the idea of an heterogeneous and electronically differently characterized bilayer system for W-doped Ga 2 O 3 . Such simple two-layer or heterogeneous model can be formulated to account for the frequency and temperature dependent dielectric properties and electrical conduction mechanism in GWO materials. Formation of grain-interior and grain-boundary in GWO dielectric can be treated as a heterogeneous system as schematically presented in Figure 11. The proposed model contains grains of Ga 2 O 3 and the secondary phase of WO 3 , which may be nucleating and located more at the grain boundaries. We hypothesized the variation in dielectric constant is a result of the formation of heterogeneous system based on the Ga 2 O 3 -WO 3 composite with the gradual increase in W content. Although the electrical response of the grain and grain boundaries is entirely different, the grains are essentially separated by a thin layer of grain-boundaries. It is evident from Figure 11B,C that the effect of W-content on the dielectric constant of GWO samples is remarkable. The dielectric constant increases with increasing W-content and is attributed to the lattice distortion of the intrinsic Ga-oxide which is in-turn the result of enhanced atomic polarizability. Additionally, formation of a small amount of WO 3 leads to the heterogeneous system which presents itself as WO 3 phase at the grain boundaries leading to the accumulation of charges at the grain boundaries. The resulting interfacial polarization further contributes to an increase in dielectric constant. This would also explain the ac conductivity of the GWO dielectrics. At lower frequencies, the grain boundaries are highly F I G U R E 11 A, Proposed heterogeneous, two-layer model of the GWO dielectric. The chemical identification of the grain and grain boundaries are as indicated. B, Variation of the real part of the dielectric constant with W-concentration. The data shown is at room temperature. C, Variation of the real part of the dielectric constant with W-concentration. The data shown is at the highest temperature of the measurement (500 • C). It is evident from the data shown in B and C, that the effect of W-doping on the dielectric constant is significant active and the frequency of electron hopping frequency between the metal ions of variable valence states is observed to be at a lower level. This explains the lower conductivity of materials at lower frequency. However, as the frequency of the applied field increases, the GWO grains (interior) become more active. These highly active grains facilitate electron hopping between the same metal ions of variable valence state and, thereby, increases the hopping frequency. As a result, the electrical conductivity increases gradually with increasing frequency. This is clearly seen in frequency dependent electrical characteristics (Figure 10) of GWO dielectric materials. Having understood about the dielectric properties of GWO materials, it is imperative to shed light on their potential benefits, at least in the context of utilizing Ga 2 O 3 based mixed oxides. Ga 2 O 3 doping or mixed oxides are commonly used to design novel dielectric materials, particularly those ceramic compositions without any volatile elements such as Li or Na, for modern wireless communication devices, such as cellular phones, resonators, filters and oscillators in microwave integrated circuits. [73][74][75] Therefore, in addition to the traditional properties and applications, the present work on the dielectric properties of Ga 2 O 3 and Ga 2−2x W x O 3 ceramics may be useful and provide a road map when considering the mixed oxide ceramics based on Ga 2 O 3 for designing such materials.

CONCLUSIONS
A wide range of capacitor devices were fabricated using the tungsten doped Ga 2 O 3 (GWO) as dielectric and silver (Ag) as the metal electrodes of the capacitor. The electrical performance of the capacitor devices based on W-doped Ga 2 O 3 (Ga 2−2x W x O 3 , 0.00 ≤ x ≤ 0.20; GWO) dielectric materials were evaluated by comprehensively studying the dielectric properties and electrical transport mechanisms. The W-concentration strongly influences the dielectric properties of GWO. The significant effect of W-doping is, however, dominant for higher concentration of W, where there is a secondary phase formation leading to the Ga 2 O 3 -WO 3 composite. At f = 1 kHz, at room temperature, varying W-content (x) from 0.0 to 0.2 increases the GWO dielectric constant from ∼9 to ∼90. Further enhancement in the dielectric constant due to W-doping was evident in the dielectric studies as a function of temperature, 25 • C to 500 • C. While the dielectric constant is temperature independent until ∼300 • C, rapid increase in dielectric constant beyond 300 • C indicates the contribution from interfacial polarization to the enhanced dielectric constant. Modified Debye model accounts for the frequency (10 3 -10 6 Hz) dependent variation in the dielectric constant of GWO dielectrics. The fitting of the experimental data to the modified Debye model yield a relaxation time of ∼20 to 90 μs and a spreading factor of 0.39 to 0.65. We propose a two-layer heterogeneous model consisting of thick grains separated by very thin grain boundaries along with the formation of a Ga 2 O 3 -WO 3 composite to explain the remarkable effect of W-incorporation on the dielectric and electrical transport properties of Ga 2 O 3 . The results demonstrate that the structure, electrical and dielectric properties can be tailored by tuning W-content in the GWO compounds. We believe that this fundamental study of dielectric properties as a function of frequency, temperature and W concentration could be useful to initiate further studies on the doping effects of wide-band gap semiconductors and could be useful for designing dielectric materials for electronic, optoelectronic and high power electronic device applications. In addition to the traditional electronic and optoelectronic device, the present work on the dielectric properties of Ga 2 O 3 and Ga 2-2x W x O 3 ceramics may be useful and provide a road map when considering the mixed oxide ceramics based on Ga 2 O 3 for application in advanced communications.