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This work provides new insights into the nature of the polaronic transport in xV2O5–(100−x)P2O5, 41 ≤ x ≤ 89 mol% glasses from analysis of their conductivity over a wide range of frequencies and temperatures, and correlation of the electrical parameters to the structural models of these materials. The results show that the linear increase in direct current (DC) conductivity with the increase in V2O5 could be related to the formation of a vanadate subnetwork, which is characterized by a length distribution of V–O bonds and variations in linkages between different vanadate units. Such a structurally complex network offers multiple conduction pathways which are favorable for polaron hopping. The Summerfield scaling of conductivity spectra reveals a temperature‐invariant mechanism of polaron transport for all glasses and the same local structural environment of polarons at higher V2O5 contents due to the same ratio of vanadate units present in the network. Also, this study highlights the similarities and differences in the nature of the polaronic transport of vanadate–phosphate glasses and other polaronically conducting phosphate glasses containing WO3, MoO3, and Fe2O3, and pins down a pivotal role of the glass structure in electrical processes in these materials. [ABSTRACT FROM AUTHOR]

Na-V-P-Nb-based materials have gained substantial recognition as cathode materials in high-rate sodium-ion batteries due to their unique properties and compositions, comprising both alkali and transition metal ions, which allow them to exhibit a mixed ionic–polaronic conduction mechanism. In this study, the impact of introducing two transition metal oxides, V2O5 and Nb2O5, on the thermal, (micro)structural, and electrical properties of the 35Na2O-25V2O5-(40 − x)P2O5 − xNb2O5 system is examined. The starting glass shows the highest values of DC conductivity, σDC, reaching 1.45 × 10−8 Ω−1 cm−1 at 303 K, along with a glass transition temperature, Tg, of 371 °C. The incorporation of Nb2O5 influences both σDC and Tg, resulting in non-linear trends, with the lowest values observed for the glass with x = 20 mol%. Electron paramagnetic resonance measurements and vibrational spectroscopy results suggest that the observed non-monotonic trend in σDC arises from a diminishing contribution of polaronic conductivity due to the decrease in the relative number of V4+ ions and the introduction of Nb2O5, which disrupts the predominantly mixed vanadate–phosphate network within the starting glasses, consequently impeding polaronic transport. The mechanism of electrical transport is investigated using the model-free Summerfield scaling procedure, revealing the presence of mixed ionic–polaronic conductivity in glasses where x < 10 mol%, whereas for x ≥ 10 mol%, the ionic conductivity mechanism becomes prominent. To assess the impact of the V2O5 content on the electrical transport mechanism, a comparative analysis of two analogue series with varying V2O5 content (10 and 25 mol%) is conducted to evaluate the extent of its polaronic contribution. [ABSTRACT FROM AUTHOR]

Sodium-phosphate-based glass-ceramics (GCs) are promising materials for a wide range of applications, including solid-state sodium-ion batteries, microelectronic packaging substrates, and humidity sensors. This study investigated the impact of 24 h heat-treatments (HT) at varying temperatures on Na-Ge-P glass, with a focus on (micro)structural, electrical, and dielectric properties of prepared GCs. Various techniques such as powder X-ray diffraction (PXRD), infrared spectroscopy-attenuated total reflection (IR-ATR), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) were employed. With the elevation of HT temperature, crystallinity progressively rose; at 450 °C, the microstructure retained amorphous traits featuring nanometric grains, whereas at 550 °C, HT resulted in fully crystallized structures characterized by square-shaped micron-scale grains of NaPO3. The insight into the evaluation of electrical and dielectric properties was provided by Solid-State Impedance Spectroscopy (SS-IS), revealing a strong correlation with the conditions of controlled crystallization and observed (micro)structure. Compared to the initial glass, which showed DC conductivity (σDC) on the order of magnitude 10−7 Ω−1 cm−1 at 393 K, the obtained GCs exhibited a lower σDC ranging from 10−8 to 10−10 Ω−1 cm−1. With the rise in HT temperature, σDC further decreased due to the crystallization of the NaPO3 phase, depleting the glass matrix of mobile Na+ ions. The prepared GCs showed improved dielectric parameters in comparison to the initial glass, with a noticeable increase in dielectric constant values (~20) followed by a decline in dielectric loss (~10−3) values as the HT temperatures rise. Particularly, the GC obtained at @450 stood out as the optimal sample, showcasing an elevated dielectric constant and low dielectric loss value, along with moderate ionic conductivity. This research uncovers the intricate relationship between heat-treatment conditions and material properties, emphasizing that controlled crystallization allows for precise modifications to microstructure and phase composition within the remaining glassy phase, ultimately facilitating the fine-tuning of material properties. [ABSTRACT FROM AUTHOR]

Glasses containing transition metal (TM) oxides exhibit polaronic conductivity which, according to a well-accepted view, depends on the overall amount of TMO and the fraction of reduced TM ions. In this study we show that in phosphate glasses containing WO3 and MoO3, polaronic conductivity is entirely governed by the structural features of the glass network and cannot be correlated with these parameters. Tungsten glasses are found to exhibit six orders of magnitude higher conductivity than their molybdenum counterparts despite a significantly lower fraction of W5+ ions and the compositional change of DC conductivity is non-monotonic in both glass systems. We relate the former effect to the tendency of WO6 units to aggregate by forming W–O–W–O–W bonds which facilitate polaronic transport, in contrast to MoO6 and MoO4 units which are prone to incorporate into a phosphate network uniformly thus failing to provide such fast conduction pathways. More interestingly, we show that the latter effect originates from the manner in which tungstate or molybdate units are mutually linked within the phosphate network: if they form chains with a sufficient number of flexible terminal oxygen bonds (lower TMO content) polaronic transport is facilitated whereas if they connect three-dimensionally, causing a reduction in the number of terminal oxygens (higher TMO content), conductivity is hindered. A detailed analysis of frequency-dependent conductivity and permittivity using the MIGRATION concept helps us understand the structural features that influence conductivity in these glasses and points out the importance of network connectivity in the propagation of polarons on long-range and localized scales. [ABSTRACT FROM AUTHOR]

Bulk glasses of the series (100−x)[0.4Na2O-0.2Nb2O5-0.4P2O5]-xB2O3 with x = 0–48 mol% B2O3 were prepared by slow cooling in air. Their glass transition temperature increases within the range of 0–16 mol% B2O3, but further additions of B2O3 result in its decrease. Their structure was investigated by Raman, 11B, and 31P MAS NMR spectroscopy. The relative number of BO4 units decreases with increasing B2O3 content, while the number of BO3 units increases up to 59 % at x = 48. The upfield shift of a broad resonance peak in the 31P MAS NMR spectra is ascribed to an increasing connectedness of the structural network with increasing B2O3 content. Strong Raman band at 916–929 cm−1 shows on the presence of NbO6 octahedra in the structural network of these glasses. With the B2O3 addition, a decrease in DC conductivity is observed, which is attributed to the decrease in the concentration of Na+ ions. [ABSTRACT FROM AUTHOR]

Ion conducting oxide glasses are attractive materials for application in various electrochemical devices and an understanding of the structure–transport properties relationship is crucial for their development. An interesting effect of glass structure on the dynamics of mobile ions is the mixed glass-former effect which causes a non-linear change of ionic conductivity when glass-forming oxides get gradually substituted. Here, we report a strong, positive effect of structural changes on the conductivity of sodium ions in two glass systems 40Na2O–xMoO3–(60−x)P2O5 and 40Na2O–xWO3–(60−x)P2O5; x = 0–50 mol% where a conventional glass-forming oxide (P2O5) is gradually replaced by WO3/MoO3 which are conditional ones. In both glass systems, the compositional change in DC conductivity is non-linear, with the maximal increase of four orders of magnitude in the case of WO3 and three orders of magnitude in the case of MoO3. This significant enhancement of ionic conductivity is related to the formation of mixed phosphate–tungstate and phosphate–molybdate units in the glass network. The facilitating effect of these structural units on sodium ion dynamics is also observed in the changes of the shape of frequency-dependent conductivity and in the values of typical spatial extent of diffusion of sodium ions known as the Sidebottom length. [ABSTRACT FROM AUTHOR]

Barium phosphate glasses of the composition (100 − x)Ba(PO3)2–xWO3 with 0–60 mol% WO3 were prepared and studied. Thermal studies of the glasses were carried out with DTA, dilatometry and hot-stage microscopy. Glass transition temperature in this glass series increases with increasing WO3 content from 461 °C (x = 0) to 623 °C (x = 60). Most glasses containing WO3 crystallize on heating within the range of 650–750 °C with the exception of glasses with 20–30 mol% WO3, which are thermally stable. Very high chemical durability of these glasses with dissolution rate of 1.4 × 10−8–6.2 × 10−9 g cm−2 min−1 was also revealed. Crystallization of glasses revealed the existence of a new compound Ba(WO2)2(PO4)2 in the studied compositional series. The compound Ba(WO2)2(PO4)2 possess hexagonal unit cell of dimensions a = 11.5191(1) Å and c = 6.3437(1) Å, and its melting point is 936 °C. Raman spectra of crystallized glasses confirmed the formation of crystalline compound in this glass series and revealed the presence of glass phase in some thermal-treated samples. 31P MAS NMR showed on only one coordination of phosphorus atoms in the Ba(WO2)2(PO4)2 compound. [ABSTRACT FROM AUTHOR]

Mixed ionic-polaronic transport has been investigated in two series of glasses containing Na2O and WO3/MoO3, namely xWO3-(30-0·5x)Na2O-(30-0·5x)ZnO-40P2O5 and xMoO3-(30-0·5x)Na2O-(30-0·5x)ZnO-40P2O5, 0=x=60 (mol%). The changes in the conduction mechanism with the systematic alternation of the glass composition have been analysed in correlation to the structural modifications and variations of molybdenum and tungsten in different oxidation states. Raman spectra revealed the clustering of WO6 units by the formation of W-O-W bonds in glasses with increasing WO3 content while the co-existence of MoO4 and MoO6 units is evidenced in glasses containing MoO3. In particular, the dominant molybdenum coordination in glasses with highest MoO3 content is four suggesting the absence of clustering of molybdate units. The DC conductivity of glasses with =20 mol% of WO3 and =30 mol% of MoO3 is almost identical due to the dominance of ionic conductivity. In this compositional region, the introduction of tungstate and molybdate units in the glass structure increases the mobility of sodium ions and compensates the decrease in sodium number density. The significant difference in conductivity is observed for glasses with higher WO3 and MoO3 content. While for glasses containing MoO3 the conductivity remains almost constant up to 50 mol% of MoO3, it constantly increases for almost six orders of magnitude for glasses with up to 60 mol% of WO3. The behaviour of DC conductivity for glasses containing MoO3 suggests the transition from ionic to polaronic conduction mechanisms which transport pathways are independent of each other. On the other hand, significantly higher conductivity of glasses with higher amounts of WO3 originates from a huge polaronic contribution, much larger than in a case of MoO3. Such a high conductivity is a consequence of the clustering of tungstate units which forms continuous W-O-W-O-W bridges which significantly facilitate polaronic transport. [ABSTRACT FROM AUTHOR]

Glasses from the Na2O-MoO3-P2O5 and Na2O-WO3-P2O5 systems were studied for the compositional series 40Na2O- xMoO3-(60-x)P2O5 and 40Na2O-yWO3-(60-y)P2O5, with x=0-50 mol% MoO3 and y=0-50 mol% WO3. The density of glasses increases more steeply for WO3-glasses than for MoO3-glasses with the transition metal oxides concentration, but the molar volume of glasses in both series decreases by similar amounts due to the similar radii of both transition metal ions. EPR measurements of the Mo5+ and W5+ species in the glasses showed that both ion concentrations decrease with increasing MoO3 and WO3 contents, respectively, consistent with the increasing basicity in both glass series, as described by the theory of J. A. Duffy. Glass transition temperature increases both with MoO3 and WO3 content revealing a maximum at 30 mol% MoO3 and 40 mol% WO3. We assume that these values correspond to an optimum connectivity of the glass network. The index of refraction for glasses from both series also increases with increasing MoO3 (WO3) concentrations, from 1·49 to 1·75. The evolution of 31P MAS NMR spectra is similar for glasses over the range of 0-30 mol% MoO3 (WO3), where the number of Q2 units sharply decreases. At 30 mol% MoO3 (WO3), Q1 units dominate the NMR spectra, whereas within the range of 30-40 mol% Q1, units are replaced by Q0 units. Raman spectra reveal a strong doublet of vibrational bands in the range of 850-1050 cm-1 for both glass series, ascribed to Mo-O and W-O vibrations, respectively. The observed shift of the maximum of the dominant vibrational band from 951 to 92 cm-1 for MoO3-doped glasses and from 956 to 934 cm-1 for WO3-doped glasses is ascribed to changes in the coordination number of both molybdenum and tungsten. Crystallization experiments showed that the glasses with the highest MoO3 (WO3) content crystallize with the formation of crystalline compounds Na2Mo2O7 and Na2W2O7, respectively, which contain both octahedral MO6 and tetrahedral MO4 units. [ABSTRACT FROM AUTHOR]

Differential scanning calorimetry (DSC), dilatometry and X-ray diffraction have been used to study thermal behavior and stability of glasses prepared simultaneously in Al2O3 or Pt crucibles in compositional series (40 − x)MgO-xFe2O3-60P2O5 within the concentration range of x = 0-40 mol% Fe2O3. Mössbauer spectra showed that all glasses with Fe2O3 contained both Fe(II) and Fe(III) ions. The contamination glasses from alumina crucibles ranged from 0.3 to 0.9 mol% Al2O3. The replacement of MgO by Fe2O3 resulted in a gradual decrease in glass transition temperature and dilatometric softening temperature, whereas thermal expansion coefficient did not change significantly. The DSC curves showed that all glasses crystallize on heating in the broad temperature range ≈ 560-900 °C. Crystallization temperature decreases with increasing Fe2O3 content. The highest tendency toward crystallization was found for the glass containing 40 mol% Fe2O3. X-ray diffraction measurements showed that the major compounds formed by glass crystallization were Mg(PO3)2, Fe(PO3)3, Fe3(P2O7)2 and FePO4. All thermoanalytical studies showed differences in thermal behavior of glasses prepared in Al2O3 or Pt crucibles. [ABSTRACT FROM AUTHOR]

The thermal behavior and crystallization of barium molybdate-phosphate glasses were studied in two compositional series, namely A: (100 − x)Ba(PO3)2–xMoO3 (with x = 0–70 mol%) and B: 50BaO–yMoO3–(50 − y)P2O5 (with y = 0–15 mol% MoO3). Thermal properties were studied with differential thermal analysis and dilatometry. Glass transition temperature increases in series A in the range of 0–40 mol% MoO3 and then decreases with a further increase in MoO3 content, whereas the thermal expansion coefficient in series A reveals a minimum at 50 mol% MoO3. These trends are not observed in series B, where the glass transition temperature increases in the range of 0–15 mol% MoO3 and the thermal expansion coefficient almost does not change. Crystallization of these glasses was studied by X-ray diffraction analysis, Raman and 31P MAS NMR spectroscopy. Large thermal stability of glass with a composition 35BaO–30MoO3–35P2O5 toward crystallization was demonstrated. Crystallization of glasses resulted in the formation of the compound Ba(MoO2)2(PO4)2 in this ternary system. Raman spectroscopy also provided information on the formation of glass-crystalline samples in some studied compositions. [ABSTRACT FROM AUTHOR]

Transformation of electrical transport from ionic to polaronic in glasses, which are a potential class of new cathode materials, has been investigated in four series containing WO3/MoO3 and Li+/Na+ ions, namely: xWO3-(30−0.5 x)Li2O-(30−0.5 x)ZnO-40P2O5, xWO3-(30−0.5 x)Na2O-(30.5 x)ZnO-40P2O5, xMoO3-(30−0.5 x)Li2O-(30−0.5 x)ZnO-40P2O5, and xMoO3-(30−0.5 x)Na2O-(30−0.5 x)ZnO-40P2O5, 0 ≤ x ≤ 60, (mol%). This study reports a detailed analysis of the role of structural modifications and its implications on the origin of electrical transport in these mixed ionic-polaron glasses. Raman spectra show the clustering of WO6 units by the formation of W-O-W bonds in glasses with high WO3 content while the coexistence of MoO4 and MoO6 units is evidenced in glasses containing MoO3 with no clustering of MoO6 octahedra. Consequently, DC conductivity of tungstate glasses with either Li+ or Na+ exhibits a transition from ionic to polaronic showing a minimum at about 20-30 mol% of WO3 as a result of ion-polaron interactions followed by a sharp increase for six orders of magnitude as WO3 content increases. The formation of WO6 clusters involved in W-O-W linkages for tungsten glasses plays a key role in significant increase in DC conductivity. On the other hand, DC conductivity is almost constant for glasses containing MoO3 suggesting an independent ionic and polaronic transport pathways for glasses containing 10-50 mol% of MoO3. [ABSTRACT FROM AUTHOR]