XRD patterns of the NbO-80K and NbO-120K sintered pellets (C, cubic NbO; T, orthorhombic T-Nb 2 O 5 ; B, monoclinic B-Nb 2 O 5 ; H, monoclinic H-Nb 2 O 5 ). 

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... d represents the sample thickness, A is the electrode area, ε 0 is the empty space permittivity (8.854 Â 10 - 12 F m), ω is the angular frequency, and C and R are the measured capacitance and resistance, respectively. The electrodes of the analyzed samples were made by painting the opposite faces of the pellets with silver conductive epoxy. The DTA spectra of the NbO-80K and NbO-120K powders are shown in Figure 1. Two exothermic phenomena, between RT and 1100 ° C for both NbO powders can be clearly identi fi ed. The fi rst, at 430 and 390 ° C, for NbO-80K and NbO-120K, respectively, is associated with the phase transition from NbO to Nb 2 O 5 , as would be concluded ahead. A large exothermal band between ∼ 550 and 1100 ° C was detected in both samples, as a result of a phase transition from the orthorhombic to monoclinic structure of Nb 2 O 5 . This kind of large exothermal band, without clear crystallization peaks, was previously reported in a similar work. 14 The thermal annealing temperatures were de fi ned on the basis of the DTA data. The XRD patterns of the sintered pellets indexed using the ICDD reference cards, Figure 2, show that the heat treatment at 300 ° C does not modify the initial cubic NbO crystal structure, though the color of the pellets change from gray to dark blue. In fact, although this color is associated with the NbO 2 phase, 33 di ff raction maxima corresponding to this niobium oxide crystalline structure were not identi fi ed. The heat treatment at temperatures between 450 and 900 ° C promote the formation of Nb 2 O 5 with a di ff raction pattern corresponding to the orthorhombic structure (T-Nb 2 O 5 ). 6,7,14,34 These pellets become completely white, as expected for a wide band gap semiconductor. In the pellets treated at 900 ° C, besides the orthorhombic phase, a monoclinic phase of Nb 2 O 5 was identi fi ed (B-Nb 2 O 5 ). For treatment temperatures above 900 ° C, the XRD patterns revealed a new monoclinic phase with larger lattice parameters, corresponding to H- Nb 2 O 5 , as also was reported. 6,7,14,34 The SEM micrographs of the pellets treated at 300 ° C, with NbO cubic structure, show 200 nm grains with spherical shape (Figure 3). This morphology remains the same for the orthorhombic T-Nb 2 O 5 crystalline phase, even at 900 ° C, where an intermediate monoclinic crystalline phase (B-Nb 2 O 5 ) is also present. The increase of the heat-treatment temperature promotes an increase of the grain size from 200 nm to 1 μ m, approximately. Also, an abrupt change in the grain morphology was observed in the pellets heat treated at 1100 ° C. The grains of the monoclinic H-Nb 2 O 5 phase exhibit a parallelepiped shape with some local preferential orientation. Figure 4 shows the RT Raman spectra of all NbO-80K and NbO-120K treated samples. Independently of the used excitation, 325 or 532 nm (not shown), the pellets treated at 300 ° C exhibit a large band centered at 641 cm , suggesting the existence of an amorphous Nb 2 O 5 phase, 27 not detectable by XRD. For pellets heat treated between 450 and 800 ° C, which display the orthorhombic T-Nb 2 O 5 phase, shows the expected vibrational modes at 684, 235, and 100 cm - 1 , approximately for this Nb 2 O 5 polymorph. 27,28 The phonons identi fi ed on the sample heat treated at 900 ° C are those for the B-Nb 2 O 5 and T-Nb 2 O 5 crystalline phases. 29,30 These phases are not homo- geneously distributed on the sample volume, as evidenced by the Raman spectra taken in two di ff erent sample regions. A distinct vibrational spectrum is taken for the pellets heat treated at 1100 ° C, as expected due to the presence of the H-Nb 2 O 5 phase. 27 - 30 Figure 5a shows the low temperature PL spectra for pellets sintered at di ff erent temperatures, with the T-Nb 2 O 5 and B-Nb 2 O 5 crystalline phases. With 3.8 eV optical excitation, all the samples evidence broad luminescence bands in the visible spectral region due to optical active defects. Yellow (maxima ∼ 2.25 eV) and blue (maxima ∼ 2.55 eV) luminescence are exhibited by the pellets heat treated at temperatures lower and higher than 900 ° C, respectively. Broad emission bands located in the middle of the bandgap energy of wide bandgap oxides (e.g., ZnO) are usually associated with the presence of native defects such as the anion and cation vacancies or interstitial defects. 35 The observation of similar broad luminescence bands in the niobium oxide based structures suggests that the native defects could assume an important role on the control of the electrical properties for the passive components. Figure 5b,c shows the temperature dependent PL spectra for the 800 and 900 ° C sintered pellets, the ones with the highest PL intensity. For the 800 ° C T-Nb 2 O 5 sample a nearly persistent intensity is detected from 14 K to ∼ 100 K, which is accompanied by a ∼ 300 meV high energy shift. Typically, for the same optical center, high energy shifts with increasing temperatures could be explained by a thermal population of the high energy states. However, such a high energy shift in the aforementioned temperature region is unlikely to be assigned to a thermal population. A probable explanation is that the broad emission band has an overlap of two emitting centers with maxima near 2.25 and 2.55 eV. A faster quenching of the intensity for the 2.25 eV band promotes the overall shift of the band maxima, with the high temperature PL spectrum dominated by the 2.55 eV band, as observed in Figure 5b. A further increase of the temperature leads to the overall luminescence intensity quenching due to additional nonradiative processes (an intensity ratio of 5 was observed between 100 and 200 K). The quenching is accompanied by a ∼ 80 meV low energy shift, suggesting that shallow energy levels are involved in the recombination process. This is also the expected behavior for a bandgap shrinkage in semiconductor materials, suggesting a decrease of ∼ 80 meV for the orthorhombic T-Nb 2 O 5 crystalline phase. A similar bandgap shrinkage of ∼ 100 meV was recently reported for the H-Nb 2 O 5 samples. 12 Figure 5b also shows the RT ...

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... oxide based structures suggests that the native defects could assume an important role on the control of the electrical properties for the passive components. Figure 5b,c shows the temperature dependent PL spectra for the 800 and 900 ° C sintered pellets, the ones with the highest PL intensity. For the 800 ° C T-Nb 2 O 5 sample a nearly persistent intensity is detected from 14 K to ∼ 100 K, which is accompanied by a ∼ 300 meV high energy shift. Typically, for the same optical center, high energy shifts with increasing temperatures could be explained by a thermal population of the high energy states. However, such a high energy shift in the aforementioned temperature region is unlikely to be assigned to a thermal population. A probable explanation is that the broad emission band has an overlap of two emitting centers with maxima near 2.25 and 2.55 eV. A faster quenching of the intensity for the 2.25 eV band promotes the overall shift of the band maxima, with the high temperature PL spectrum dominated by the 2.55 eV band, as observed in Figure 5b. A further increase of the temperature leads to the overall luminescence intensity quenching due to additional nonradiative processes (an intensity ratio of 5 was observed between 100 and 200 K). The quenching is accompanied by a ∼ 80 meV low energy shift, suggesting that shallow energy levels are involved in the recombination process. This is also the expected behavior for a bandgap shrinkage in semiconductor materials, suggesting a decrease of ∼ 80 meV for the orthorhombic T-Nb 2 O 5 crystalline phase. A similar bandgap shrinkage of ∼ 100 meV was recently reported for the H-Nb 2 O 5 samples. 12 Figure 5b also shows the RT absorption spectra of the sintered pellet. The peak position around 3.5 eV is consistent with the reported bandgap energy in fi lms produced by sol - gel with similar crystalline phase. 36 The additional absorption maximum at 4.7 eV was also observed in T-Nb 2 O 5 sintered by a di ff erent route, and from the observation of the same band in the photoconductivity experiments, it was suggested that the high energy intrinsic absorption could be related with transitions from di ff erent critical points in the conduction band for the T-Nb 2 O 5 band structure. 12 Di ff erent RT bandgap energies were recently reported for Nb 2 O 5 polymorphs. 37 In agreement with the reported values in the literature, 11,12 Viet et al. found bandgap energy values in the 3.4 - 5.3 eV energy range. 37 However, an expansion of the absorption spectra for both low and high energies indicates that for H-Nb 2 O 5 and T-Nb 2 O 5 onset absorptions at ∼ 3.1 eV and ∼ 3.5 eV (with an additional onset at 4.7 eV) can be found at RT, respectively. 12 Di ff erent oxygen stoichiometry on samples processed by the di ff erent routes could also explain the bandgap variations. 12,37 A distinct temperature dependent PL behavior was found for the sample with mixed T þ B Nb 2 O 5 crystalline phases sintered at 900 ° C. Here, with the same excitation conditions, the dominant recombination has a maximum at ∼ 2.55 eV at low temperatures rather than at 2.25 eV, as found for the T-Nb 2 O 5 sample. Increasing the temperature between 14 and 100 K leads to an increase in the PL intensity, meaning that the 2.55 eV center is thermally populated. For higher temperatures a faster decrease of the luminescence intensity was found ( I 100K / I 150K ∼ 10 and no signal was observed at 200 K), suggesting that the presence of the additional phase induces extra nonradiative processes that com- pete with the luminescence. In a way similar to the one found for the heat treated sample at 800 ° C (T-Nb 2 O 5 ), a low energy shift of the broad band due to the bandgap shrinkage was observed at higher temperatures. In both the analyzed systems besides the role of the native defects in the optical active defects, we cannot rule out the participation of nitrogen as a potential acceptor in these niobium - oxide systems, which could be responsible for the identi fi ed shallow levels. The addition of nitrogen to the NbO powders makes them more stable toward oxidation and improves their thermal stabi- lity and dielectric strength. 38 Figure 6a presents the dielectric characteristics, measured at room temperature, of the NbO-80K pellets treated at 800, 900, and 1100 ° C. The results obtained with the NbO-120K pellets, treated at the same temperatures, are shown in Figure 7a. It was observed that the sample with orthorhombic (T-Nb 2 O 5 ) structure (treated at 800 and 900 ° C) shows a dielectric constant value of ∼ 25 at 100 kHz, increasing to ∼ 55 for the sample heat-treated at 1100 ° C with a monoclinic structure (H-Nb 2 O 5 ). This increment can be related to the formation of the parallelepipedic morphology, which gives rise to an evident preferential grain orientation (Figure 3). These results suggest that the development of a unit cell distorted structure, which is the case of the formation of polymorphic H-Nb 2 O 5 (Figure 2), facilitates the formation of a microstructure with grains oriented in a preferred direction. Thus, the presence of a preferential orientation promotes an increase of the dipole moment, 39 experimentally observed by the increase of the dielectric constant value. The presence of the B-Nb 2 O 5 monoclinic phase in the sample NbO-80K treated at 900 ° C, does not in uence signi cantly the dielectric results due, probably, to its low content. For the sample NbO-120K, treated at 900 ° C, the amount of the B-Nb 2 O 5 phase is, according to the XRD patterns, higher than in sample NbO-80K. The SEM micrographs (Figure 3) revealed that the grain size of the 120K samples is always lower than that of the 80K samples. More- over, in pellets treated at 900 ° C parallelepiped grain morphology was not observed , related to the B-Nb 2 O 5 structure, which indicates that these particles have a much smaller size. A possible justi fi cation for the decrease of ε 0 , on the 120K sample, with the increase of the heat-treatment temperature from 800 to 900 ° C, is the presence of a su ffi cient quantity of B-Nb O particles that can promote their interaction with the T-Nb 2 O 5 particles, a dielectric noncooperative e ff ect 40,41 leading to a decrease of ε 0 . The high value of ε 0 at low frequencies, for all samples, is due to the polarization of sample - electrode interfacial dipoles, which number increases with the rise of the heat-treatment temperature (Figures 6a and 7a). The dielectric loss (tan δ , Figure 6b) presents its higher value for the 1100 ° C pellets, which can be ascribed to the increase of the percolation path between the grains. This percolation path increase is justi fi ed by the preferential grain growth, observed through SEM, on the 1100 ° C treated pellets. The graph of the imaginary part of the dielectric modulus ( M 00 ) 42 as a function of the frequency revealed the presence of a dielectric relaxation phenomenon in all pellets heat-treated at 900 and 1100 ° C (Figure 8). The increase of the heat-treatment temperature promotes a shift of the M 00 peak to higher frequencies, indicating a decrease of the relaxation time. This result shows that for the pellets treated at high temperatures, their dielectric dipoles can follow the applied external ac fi eld, more easily promoting a high polarizability, already observed through the ε 0 values discussion. The XRD analysis performed on the sintered pellets showed an evolution from cubic NbO to orthorhombic T-Nb 2 O 5 and then to monoclinic H-Nb 2 O 5 , as the sintering temperature rises. An intermediate monoclinic phase (B-Nb 2 O 5 ) with smaller lattice parameters than the H-polymorph was also identi fi ed for pellets treated at 900 ° C. The XRD measurements were fully corroborated by the Raman data. All the sintered pellets, with the exception of those heat treated at 900 ° C, evidence a broad emission band peaked at ∼ 2.25 eV. For the 900 ° C sample the emission is blue-shifted. The temperature dependent PL analysis suggests that an overlap of emitting centers is responsible for the broad emission and that thermal population occurs between the two emitting defects. The analysis put forward that shallow energy levels must be involved in the recombination processes, and besides considering the native defects as responsible for the broad bands, we cannot rule out that nitrogen can assume an important role on the luminescence. Furthermore, for higher temperatures a bandgap shrinkage of ∼ 80 meV was found in samples with the T-Nb 2 O 5 crystalline phase is dominant. The dielectric constant of ∼ 55, for the samples with monoclinic structures (H-Nb 2 O 5 ), is approximately the double of the orthorhombic samples. These results show that the treatment temperature is a very important parameter for the production of solid electrolytic capacitors where the Nb 2 O 5 is the dielectric ...