Thermal Stability, Optical and Electrical Properties of Substoichiometric Molybdenum Oxide

Substoichiometric molybdenum oxide ceramics have aroused widespread interest owing to their promising optical and electrical performance. In this work, the thermal stability and decomposition mechanism of Mo9O26 and Mo4O11 at 700–1000 °C and 700–1100 °C were investigated, respectively. Based on this information, MoOx (2 < x < 3) bulk ceramics were prepared by spark plasma sintering (SPS). The results show that Mo9O26 is stable up to 790 °C in an argon atmosphere. As the temperature rises, it decomposes into Mo4O11. Mo4O11 can exist stably at 830 °C, beyond which it will convert to MoO2. The MoOx ceramic bulks with four different components (MoO2.9, MoO2.8, MoO2.7 and MoO2.6) were successfully sintered by SPS, and their relative density was greater than 96.4% as measured by the Archimedes principle. The reflectivity of MoOx ceramic bulk is low and only 6.3% when the composition is MoO2.8. The resistivity increases from 10−3 to 10−1 Ωcm with the increase in the O/Mo atomic ratio x. In general, the thermal stability information provides a theoretical basis for the processing of MoOx materials, such as the sintering of the MoOx target. The optical and electrical properties show that MoOx is a low-reflective conductive oxide material with great photoelectric application value.

Although MoO x has excellent properties, little research has been conducted on its thermal stability. As weakly stable oxides, Gunnar Hägg and Arne Magnéli found that Mo 4 O 11 decomposes at 700-850 • C, and Mo 9 O 26 converts to Mo 8 O 23 at 765 • C; it has also been proposed that Mo 4 O 11 melts into a liquid phase and MoO 2 at 818 • C, and Mo 9 O 26 melts into Mo 4 O 11 and a liquid phase at 780 • C [24]. In recent years, studies on its stability have mainly focused on the usage scenarios of two-dimensional (2D) materials. Jinyoun Cho et al. [25] and Shuangying Cao et al. [26] found that the maximum heat treatment temperature of the MoO x layer in silicon heterojunction solar cells should not exceed 170-200 • C, otherwise a significant redox reaction and metal atom diffusion will occur. The study also found that annealing under argon can effectively improve the thermal stability of MoO x [27,28]. It can also be stacked with high work function metals, e.g., Ni, and subjected to certain heat treatments for higher stability [29]. Hennrik Schmidt et al. [11] found that the original amorphous MoO x films prepared by magnetron sputtering began to transform into crystalline MoO 2 films after thermal treatment at 350 • C. It can be observed that this substoichiometric molybdenum oxide exists stably only within a specific temperature range, and the thermal stability, decomposition mechanism and phase generated in the decomposition process have not been studied in detail. Therefore, it is necessary to explore its thermal stability and provide a theoretical basis for expanding its application range, such as preparing MoO x ceramic targets, thus producing films with a specific O/Mo ratio.
In this work, the Mo 9 O 26 and Mo 4 O 11 substoichiometric molybdenum oxide powders were heated in argon to explore the thermal stability and decomposition behavior of these two powders, and the sintering process was determined according to the thermal stability. Four ceramic bulks, MoO 2.9 , MoO 2.8 , MoO 2.7 and MoO 2.6 , were prepared by SPS, and the relative density, reflectivity and resistivity were characterized. This work provides useful information for the use of substoichiometric molybdenum oxide in optical and electrical fields.

Preparation of Substoichiometric Molybdenum Oxide Powders
Two powders with main phases of Mo 4 O 11 (7.47 µm) and Mo 9 O 26 (5.58 µm) are prepared. The MoO 3 (99.95%, 2.89 µm) powder is placed into a corundum crucible (80 × 40 × 20 mm 3 ) and then into a tubular furnace. The tube furnace is evacuated to vacuum (−0.1 MPa); then, argon is injected at a flow rate of 70 mL/min. The MoO 3 is heated to 500-600 • C at a heating rate of 5 • C/min in a protective atmosphere. When the preset temperature is reached, the argon gas is turned off, and hydrogen gas is introduced. After holding for a period of time (Mo 9 O 26 is reduced for 15-45 min, Mo 4 O 11 is reduced for 45-75 min), the MoO 3 is slightly reduced by hydrogen to obtain substoichiometric molybdenum oxide. Substoichiometric molybdenum oxide is used for the characterization of Section 2.2 and the mixing and sintering of Section 2.3.

Thermal Stability Tests
The prepared Mo 9 O 26 and Mo 4 O 11 powders are subjected to synchronous differential scanning calorimetry (DSC) analysis and thermogravimetric analysis (TGA) on a synchronous thermal analyzer to explore the thermal stability of the oxides. In order to further explore the decomposition behavior and phase evolution of the two oxides at different temperatures, Mo 9 O 26 and Mo 4 O 11 are heat-treated in the temperature range of 700-1000 • C and 700-1100 • C for 1 h, respectively, using a tube furnace and corundum crucible (50 × 40 × 20 mm 3 ). The isothermal stability test is carried out at an argon flow rate of 70 mL/min. The tubular furnace maintains the same heating rate (10 • C/min) as the synchronous thermal analysis. When the oxide melts at high temperatures and cools to room temperature to become bulky, the bulk is ground into powder using a mortar. Pure phase Mo 9 O 26 and MoO 3 (phase pure from XRD analysis; pure Mo 9 O 26 is prepared by optimizing the hydrogen reduction process of Section 2.1, and MoO 3 is the same as that used in Section 2.1) are used for heat treatment at 800-900 • C and 900 • C, respectively (conditions consistent with the isothermal thermal stability tests), as supplementary experiments to the isothermal thermal stability test experiments.

Sintered MoO x Ceramics
The substoichiometric molybdenum oxide powders prepared in Section 2.1 are mixed for sintering the ceramic bulks. By adjusting the ratio of substoichiometric molybdenum oxide, the O/Mo atomic ratio of the ceramic bulks can be controlled. In order to distinguish it from the unmixed powder, the mixed powder and ceramic bulk are expressed in the form of MoO x . For molybdenum oxide with an O/Mo atomic ratio of 2.9, there is no need to mix other substoichiometric molybdenum oxide to adjust the atomic ratio, so Mo 9 O 26 powder is directly used as the raw material for sintering MoO 2.9 . Using a v-type mixer, Mo 9 O 26 (62.3 wt.%), Mo 4 O 11 (28.3 wt.%) and MoO 2 (9.4 wt.%) are uniformly mixed to obtain MoO 2.8 powder. Mo 4 O 11 (80 wt.%, 70 wt.%) and MoO 2 (20 wt.%, 30 wt.%) are uniformly mixed to obtain MoO x powder with O/Mo of 2.6 and 2.5. The speed of the v-type mixer is 50 rad/min, and the mixing time is 6 h. Then, the oxide is placed into a graphite mold with an inner diameter of 20 mm. The sintering process is carried out in the LABOX-675F SPS furnace (SINTER LAND INC., Niigata, Japan) at a pressure of 60 MPa for 15 min. The sintering temperature is between 700 and 800 • C, the heating rate is 100 • C/min, and the temperature in the furnace is detected using infrared temperature measuring equipment.

Characterization
The phase evolution and composition after heat treatment were examined by an X-ray diffractometer (XRD, Empyrean Alpha 1, Malvern Panalytical, Alemlo, The Netherlands) equipped with a diffracted beam monochromator using a Cu Kα radiation source. The micromorphology of the powders was analyzed by scanning electron microscope (SEM, Quanta 200 FEG, FEI, Hillsboro, OR, USA). The powders were sprayed with gold for better electronic conductivity. The TG-DSC measurements of the powders were carried out on a synchronous thermal analyzer (STA 449 F3, NETZSCH, Selbu, Germany) with a heating rate of 10 • C/min in argon (gas flow rate 100 mL/min) using alumina crucibles. The reflectivity was tested with a UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan). The resistivity was measured with a four-point probe (RTS-8, Guangzhou Four Probe Technology Co., Ltd., Guangzhou, China).

Phase Evolution of Substoichiometric Molybdenum Oxide during Heat Treatment
The XRD patterns of the Mo 9 O 26 and Mo 4 O 11 powders prepared by reducing molybdenum trioxide are shown in Figure 1. Figure 1a shows that the powder is mainly Simultaneous TG-DSC curves for the Mo9O26 and Mo4O11 powders are shown in Figure 2. Figure 2a shows that the DSC curve contains three endothermic peaks at 774.1 °C, 811.3 °C and 1012.6 °C. There are three exothermic peaks at 805.1 °C, 885.6 °C and 930.1 °C, indicating that new phase crystallization may occur at these three temperatures. According to the TG curve, the initial mass reduction occurred at 766.1 °C. At approximately 1000 °C, the TG curve has an obvious anomaly, which corresponds to the endo- Simultaneous TG-DSC curves for the Mo 9 O 26 and Mo 4 O 11 powders are shown in Figure 2. Figure 2a shows that the DSC curve contains three endothermic peaks at 774.1 • C, 811.3 • C and 1012.6 • C. There are three exothermic peaks at 805.1 • C, 885.6 • C and 930.1 • C, indicating that new phase crystallization may occur at these three temperatures. According to the TG curve, the initial mass reduction occurred at 766.1 • C. At approximately 1000 • C, the TG curve has an obvious anomaly, which corresponds to the endothermic peak at 1012.6 • C (Figure 2a). The final residual mass detected at 1200 • C is 27.7%. The Figure 2b DSC curve contains two endothermic peaks at 810.7 • C and 1037.9 • C. Exothermic peaks appear at 845 • C and 927.9 • C. According to the TG curve, the initial mass reduction occurred at 793.0 • C. At approximately 1030 • C, the TG curve, similar to Mo 9 O 26 , also has an obvious anomaly. This corresponds to the endothermic peak at 1037.9 • C (Figure 2b), which may be due to the violent sublimation [30]. The final residual mass detected at 1200 • C is 40.8%. According to the area method shown in Figure 2, the reaction enthalpy during the heat treatment is indicated. Simultaneous TG-DSC curves for the Mo9O26 and Mo4O11 powders are shown in Figure 2. Figure 2a shows that the DSC curve contains three endothermic peaks at 774.1 °C, 811.3 °C and 1012.6 °C. There are three exothermic peaks at 805.1 °C, 885.6 °C and 930.1 °C, indicating that new phase crystallization may occur at these three temperatures. According to the TG curve, the initial mass reduction occurred at 766.1 °C. At approximately 1000 °C, the TG curve has an obvious anomaly, which corresponds to the endothermic peak at 1012.6 °C (Figure 2a). The final residual mass detected at 1200 °C is 27.7%. The Figure 2b DSC curve contains two endothermic peaks at 810.7 °C and 1037.9 °C. Exothermic peaks appear at 845 °C and 927.9 °C. According to the TG curve, the initial mass reduction occurred at 793.0 °C. At approximately 1030 °C, the TG curve, similar to Mo9O26, also has an obvious anomaly. This corresponds to the endothermic peak at 1037.9 °C (Figure 2b), which may be due to the violent sublimation [30]. The final residual mass detected at 1200 °C is 40.8%. According to the area method shown in Figure 2, the reaction enthalpy during the heat treatment is indicated. Mo9O26 and Mo4O11 were subjected to isothermal heat treatment with reference to the temperature points obtained by the synchronous thermal analysis. As shown in Figure 3, the mass residual curve of substoichiometric molybdenum oxide after heat treatment is consistent with the TG curve. This same trend shows that the thermal decomposition behavior in the tubular furnace is consistent with that in the synchronous thermal analysis. Notably, as the heat treatment temperature increases, the powders become Mo 9 O 26 and Mo 4 O 11 were subjected to isothermal heat treatment with reference to the temperature points obtained by the synchronous thermal analysis. As shown in Figure 3, the mass residual curve of substoichiometric molybdenum oxide after heat treatment is consistent with the TG curve. This same trend shows that the thermal decomposition behavior in the tubular furnace is consistent with that in the synchronous thermal analysis. Notably, as the heat treatment temperature increases, the powders become bulky when cooled to room temperature. The Mo 9 O 26 powders form a bulk at 800 • C, and the Mo 4 O 11 powders form a bulk at 840 • C ( Figure 3). This is consistent with the judgment in the literature [24] that, above 800 • C, the Mo-O system will be in a liquid state.
Materials 2023, 16, x FOR PEER REVIEW bulky when cooled to room temperature. The Mo9O26 powders form a bulk at 800 the Mo4O11 powders form a bulk at 840 °C ( Figure 3). This is consistent with the jud in the literature [24] that, above 800 °C, the Mo-O system will be in a liquid state.  Figure 4 shows the XRD spectrums of Mo9O26 and Mo4O11 after heat treatm 750-950 °C and 800-1000 °C, respectively. Tables 1 and 2 list the phase composition on Figure 4. It can be observed from Table 1 that Mo9O26 still exists stably at 790 °C         Figure 4) and thermodynamically stable α-MoO 3 (PDF#05-0508) during the decomposition process of substoichiometric molybdenum oxide. There is a trend of β-MoO 3 to α-MoO 3 with the increase in temperature. This transition from the thermodynamically unstable phase to the thermodynamically stable phase is accompanied by heat release, which is consistent with the exothermic peak present at approximately 930 • C in Figure 2. From the above results, we can reasonably obtain the thermal stability of Mo 9 O 26 and Mo 4 O 11 . The former exists stably from room temperature to 790 • C, and the latter exists stably from room temperature to 830 • C.

Decomposition Process of Substoichiometric Molybdenum Oxide during Heat Treatment
In order to exclude the interference of Mo 4 O 11 generated by Equation (1)   As shown in Table 2, Mo4O11 is still stable at 830 °C. After heat treatment at 840 °C, the content of Mo4O11 decreases, and a trace of MoO3 appears, indicating that Mo4O11 begins to fuse at this temperature, and the enthalpy of fusion is 1.26 J/g. With the increasing temperature, Mo4O11 continues to decompose, MoO3 sustains to sublimate, and the content of MoO2 gradually increases. Finally, only MoO2 exists at 1000 °C. It is worth noting that there are thermodynamically unstable β-MoO3 (PDF#47-1320, not marked in Figure  4) and thermodynamically stable α-MoO3 (PDF#05-0508) during the decomposition process of substoichiometric molybdenum oxide. There is a trend of β-MoO3 to α-MoO3 with the increase in temperature. This transition from the thermodynamically unstable phase to the thermodynamically stable phase is accompanied by heat release, which is consistent with the exothermic peak present at approximately 930 °C in Figure 2. From the above results, we can reasonably obtain the thermal stability of Mo9O26 and Mo4O11. The former exists stably from room temperature to 790 °C, and the latter exists stably from room temperature to 830 °C.

Decomposition Process of Substoichiometric Molybdenum Oxide during Heat Treatment
In order to exclude the interference of Mo4O11 generated by Equation (1) during the heat treatment, high-purity Mo9O26 was heat treated at 800 °C and 900 °C. As can be observed from Figure 5a, Mo4O11 appeared after heat treatment. As shown in Figure 5b, MoO3 forms Mo4O11 when heat treated at 900 °C, that is, oxygen escape occurs in molybdenum oxide during heat treatment. The unassigned peaks in Figure 5b are Al2(MoO4)3 and Al2O3 impurities introduced by the alumina crucible.  According to the phase evolution ( Figure 4) and the decrease in the O/Mo atomic ratio (Figure 5b) Thermodynamic analysis software HSC6.0 was used to calculate the thermodynamics of the equations. Figure 6 shows the Gibbs free energies as a function of temperature for the four decomposition reactions possible for the two substoichiometric molybdenum oxides in the standard state (101.325 kPa), and it can be observed that all four reactions reach the thermodynamic condition at high temperature. Thermodynamic analysis software HSC6.0 was used to calculate the the namics of the equations. Figure 6 shows the Gibbs free energies as a function of ature for the four decomposition reactions possible for the two substoichiome lybdenum oxides in the standard state (101.325 kPa), and it can be observed that reactions reach the thermodynamic condition at high temperature. According to the research results in 3.1, only MoO2 exists above 1000 °C, curve has significant jitter, and the DSC curve has an endothermic peak; it is reaso speculate that Mo4O11 will sublime rapidly above 1000 °C (Figure 2a,b sublimatio and 8.9%, respectively). For Mo4O11, it decomposes below 1000 °C according to E (4) or (5) (excluding the sublimated part above 1000 °C), and the final theoretic residue is 28.7% or 47.4%, respectively. The actual mass residue is 40.8% (Fig  which is between Equations (4) and (5). This indicates that, during the Mo4O11 de sition, both Equations (4) and (5) (3), the theoretical mass residues are 16.4% or 29.7%, resp Therefore, during the decomposition of Mo9O26, Equations (2) and (3) occur sim ously with the former accounting for 15.1 wt.%, and the latter accounting for 84 In summary, most of the mass loss of the two substoichiometric molybdenum come from MoO3, and a few are due to the sublimation of Mo4O11 above 1000 °C. Figures 7 and 8 show the morphology of the Mo9O26 and Mo4O11 powders a treatment at 790-950 °C and 820-1100 °C, respectively. From Figure 7a,b, it can b that the layered features of Mo9O26 disappear after heat treatment. As shown in 7c,d, Mo9O26 begins to melt between 800-820 °C. The melting phenomenon can According to the research results in Section 3.1, only MoO 2 exists above 1000 • C, the TG curve has significant jitter, and the DSC curve has an endothermic peak; it is reasonable to speculate that Mo 4 O 11 will sublime rapidly above 1000 • C (Figure 2a,b sublimation 10.1% and 8.9%, respectively). For Mo 4 O 11 , it decomposes below 1000 • C according to Equation (4) or (5) (excluding the sublimated part above 1000 • C), and the final theoretical mass residue is 28.7% or 47.4%, respectively. The actual mass residue is 40.8% (Figure 2b), which is between Equations (4) and (5). This indicates that, during the Mo 4 O 11 decomposition, both Equations (4) and (5)  . There are obvious traces of droplets on the powders, and there is no particle adhesion on the surface of the droplets. The droplets are the residues of liquid MoO 3 that are not volatilized completely. As the temperature continues to increase to 860 • C (Figure 7e), subgrains are found on the powder surface, and the powder again shows distinct layered features (Figure 7d,e). The presence of layered features in Figure 7d,e may be due to the Mo 9 O 26 of the layered structure [32] with high energy at the interlaminar interface, such that the decomposition reaction occurs here first, and the powder is turned into thinner particles. This transformation is very similar to the crackling core model (CCM). During the temperature increase from 860 • C (Figure 7e) to 950 • C (Figure 7f), the subgrains on the surface of the powder begin to grow and increase significantly, and the platelike particles are completely covered at 950 • C. From the perspective of phase transition between 860-950 • C, MoO 2 continues to increase, and Mo 4 O 11 gradually decreases, so it can be judged that the small particles added on the surface are newly formed MoO 2 phases. This indicates that the transition from Mo 4 O 11 to MoO 2 during heat treatment is an external-to-internal reaction.  As shown in Figure 8a,b, Mo4O11 grows from an irregular polygonal structure to a regular geometry in which small disk-shaped particles are embedded. This adhesion of different particles may be caused by the low melting point eutectic [33]. At 830 °C (Figure  8c), the Mo4O11 powder is nearly spherical. Decomposition begins at 840 °C (Figure 8d), and small particles begin to appear on the surface. At 900 °C (Figure 8e), small particles increase and grow significantly and coat on the powder surfaces. From the phase change, it can be observed that, between 840-900 °C, the MoO2 content continues to increase, so these gradually increasing small particles are MoO2. At 1100 °C (Figure 8f), the powder is completely transformed into polygonal MoO2 (the XRD pattern of heat treatment at 1100 °C is consistent with that at 1000 °C, so it is not shown in Figure 4). It can be found that the two kinds of Mo4O11 are (1) obtained by thermal decomposition of Mo9O26 at 860 °C and above, and (2) obtained by hydrogen reduction and have the same transformation behavior, which fits the chemical vapour transport (CVT) model from outside to inside; the former is shown in Figure 7e,f, and the latter is shown in Figure 8e,f. The transformation of molybdenum oxide undergoes CCM and CVT, which is different from the view of Werner V. Schulmeyer et al. [34], who think that MoO2 is obtained by hydrogen reduction only through the CVT model, which may be because they have not found that Mo9O26 or the conditions of the two reactions (heat treatment or hydrogen reduction) are different. The stability and decomposition mechanism of substoichiometric molybdenum oxide are shown in Figure 9. Below 800 °C, Mo9O26 can exist stably. Then, Mo9O26 cracks through the CCM at 800 °C and begins to convert to Mo4O11. Until 840 °C, Mo9O26 almost decomposes completely. Mo4O11 is stable below 840 °C and forms MoO2 subgrains on the surface by CVT at 840 °C; then, the subgrains increase and grow, and finally replace As shown in Figure 8a,b, Mo 4 O 11 grows from an irregular polygonal structure to a regular geometry in which small disk-shaped particles are embedded. This adhesion of different particles may be caused by the low melting point eutectic [33]. At 830 • C (Figure 8c), the Mo 4 O 11 powder is nearly spherical. Decomposition begins at 840 • C (Figure 8d), and small particles begin to appear on the surface. At 900 • C (Figure 8e), small particles increase and grow significantly and coat on the powder surfaces. From the phase change, it can be observed that, between 840-900 • C, the MoO 2 content continues to increase, so these gradually increasing small particles are MoO 2 . At 1100 • C (Figure 8f), the powder is completely transformed into polygonal MoO 2 (the XRD pattern of heat treatment at 1100 • C is consistent with that at 1000 • C, so it is not shown in Figure 4). It can be found that the two kinds of Mo 4 O 11 are (1) obtained by thermal decomposition of Mo 9 O 26 at 860 • C and above, and (2) obtained by hydrogen reduction and have the same transformation behavior, which fits the chemical vapour transport (CVT) model from outside to inside; the former is shown in Figure 7e,f, and the latter is shown in Figure 8e,f. The transformation of molybdenum oxide undergoes CCM and CVT, which is different from the view of Werner V. Schulmeyer et al. [34], who think that MoO 2 is obtained by hydrogen reduction only through the CVT model, which may be because they have not found that Mo 9 O 26 or the conditions of the two reactions (heat treatment or hydrogen reduction) are different.
The stability and decomposition mechanism of substoichiometric molybdenum oxide are shown in Figure 9. The stability and decomposition mechanism of substoichiometric molybdenum o ide are shown in Figure 9. Below 800 °C, Mo9O26 can exist stably. Then, Mo9O26 crack through the CCM at 800 °C and begins to convert to Mo4O11. Until 840 °C, Mo9O26 almo decomposes completely. Mo4O11 is stable below 840 °C and forms MoO2 subgrains on th surface by CVT at 840 °C; then, the subgrains increase and grow, and finally repla Mo4O11 completely from the surface to the interior.

Phase Composition, Optical and Electrical Properties of MoOx Ceramic Bulks
The Archimedes drainage method was used to determine the relative density different x ceramic materials prepared by SPS sintering, which were higher than The phase composition of the ceramic bulk is shown in Figure 11. The phase of (Sample A) and MoO2.8 (Sample B) did not change before and after sintering. A tering with MoO2.6 (Sample C) and MoO2.5 (Sample D), Mo4O11 increased by 10 wt MoO2 decreased by 10 wt.%. C and D rose to MoO2.7 and MoO2.6, respectively should be due to the increase in MoO2 content that promoted its reaction with th tected high x (O/Mo atomic ratio, x > 2.75) molybdenum oxide in the raw materi producing Mo4O11.
The optical and electrical properties of the MoOx bulks are characterized by tivity and resistivity tests. The reflectivity and resistivity are shown in Figure 12 most sensitive area of the human eye (550 nm), the reflectivity is between 6.3 t among which the reflectivity of MoO2.8 is the lowest at 6.3%. With the decrease i resistivity decreases from 10 −1 to 10 −3 Ωcm, and the measured value is close to th recorded in the literature [36]. From the change in resistivity, MoO2 with low re significantly affects the conductivity of the MoOx ceramics. MoO2.9 contains only which exhibits the highest resistivity. The resistivity of the other three MoOx c decreases significantly with the addition of MoO2. Through the analysis of opti electrical properties, it can be determined that MoOx material is a low reflective c tive oxide material with great application prospects.

Phase Composition, Optical and Electrical Properties of MoO x Ceramic Bulks
The Archimedes drainage method was used to determine the relative density of four different x ceramic materials prepared by SPS sintering, which were higher than 96.4%. The phase composition of the ceramic bulk is shown in Figure 11. The phase of MoO 2.9 (Sample A) and MoO 2.8 (Sample B) did not change before and after sintering. After sintering with MoO 2.6 (Sample C) and  The optical and electrical properties of the MoO x bulks are characterized by reflectivity and resistivity tests. The reflectivity and resistivity are shown in Figure 12. At the most sensitive area of the human eye (550 nm), the reflectivity is between 6.3 to 7.8%, among which the reflectivity of MoO 2.8 is the lowest at 6.3%. With the decrease in x, the resistivity decreases from 10 −1 to 10 −3 Ωcm, and the measured value is close to the value recorded in the literature [36]. From the change in resistivity, MoO 2 with low resistivity significantly affects the conductivity of the MoO x ceramics. MoO 2.9 contains only Mo 9 O 26 , which exhibits the highest resistivity. The resistivity of the other three MoO x ceramics decreases significantly with the addition of MoO 2 . Through the analysis of optical and electrical properties, it can be determined that MoO x material is a low reflective conductive oxide material with great application prospects.

Conclusions
The thermal stability and decomposition mechanism of Mo9O26 and Mo4O11 are systematically studied. Under an argon atmosphere, Mo9O26 has good thermal stability below 790 °C, gradually decomposes into Mo4O11 and MoO3 between 800-840 °C, and the transformation form is similar to the CCM. At 860-950 °C, Mo4O11 generated by thermal decomposition decomposes into MoO2 and MoO3. Mo4O11 has good thermal stability below 830 °C, is decomposed into MoO2 and MoO3 from 840 °C and completely transforms into MoO2 at 1000 °C. This transition is consistent with the CVT model. The decomposition of molybdenum oxide is the process of oxygen loss with the increase in temperature. In a word, during the heat treatment, the transformation of molybdenum oxide undergoes CCM and CVT successively. The relative density of ceramic bulks obtained by SPS sintering can reach 96.4%. All four MoOx have low reflectivity that ranges from 6.3-7.8%,

Conclusions
The thermal stability and decomposition mechanism of Mo 9 O 26 and Mo 4 O 11 are systematically studied. Under an argon atmosphere, Mo 9 O 26 has good thermal stability below 790 • C, gradually decomposes into Mo 4 O 11 and MoO 3 between 800-840 • C, and the transformation form is similar to the CCM. At 860-950 • C, Mo 4 O 11 generated by thermal decomposition decomposes into MoO 2 and MoO 3 . Mo 4 O 11 has good thermal stability below 830 • C, is decomposed into MoO 2 and MoO 3 from 840 • C and completely transforms into MoO 2 at 1000 • C. This transition is consistent with the CVT model. The decomposition of molybdenum oxide is the process of oxygen loss with the increase in temperature. In a word, during the heat treatment, the transformation of molybdenum oxide undergoes CCM and CVT successively. The relative density of ceramic bulks obtained by SPS sintering can reach 96.4%. All four MoO x have low reflectivity that ranges from 6.3-7.8%, especially MoO 2.8 at 6.3%. The resistivity of MoO x decreases from 10 −1 to 10 −3 Ωcm with the decrease in x.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.