Vanado-phosphate glasses

Vanado-phosphate glasses (VPGs) are well known for their semi-conductive behaviour [72]. In the last decades they were widely studied in order to understand the mechanism underneath their electronic properties. The electronic conductivity was correlated to small polaron hopping between vanadium ions with different valence state [86], favoured by their electronic configuration [87]. In VPGs, vanadium atoms are mainly characterized by two oxidation states, 4+ and 5+, and the and based interconnected polyhedra become the preferential path for the polaron hopping [88]. Therefore, the relative amount of these two species (V + and V +), which is usually expressed as V +A tot ratio, influences the final electrical properties of VP systems [86, 89]. 

The analysis of structural rearrangement could be essential to explain the properties that characterize VPG systems. The main features that could affect the electronic behaviour are the number of V +-0-V + linkages, the vanadium coordination number and the V-V distance. In our knowledge, a unique description of VPGs structure has not been already reached. For instance, Jordan et al. [36, 91] supposed that a 

Several vanadium configurations and geometries could be found in VPG systems, opening to a wide range of final properties for these systems. Coordination is comprised between 4 and 6, and tetrahedral, square pyramidal, trigonal pyramidal, octahedral units are found in VPGs [47, 78, 91]. The structural units are further characterized by distorted structure that could produce intermediate geometries, such as 5 H- 1 or 4 + 1 [47]. 

In the glass network, vanadium plays an intermediate role [59, 78] depending by its content. Generally, for low V2O5 content, vanadium ions tend to have higher coordination number and a stronger modifier character. Increasing the content, the coordination decreases and a more former behavior could be associated to vanadium ions. 

Alkaline ions have strong effects on the electrical properties of VPGs. Increasing the network modifier content the total conductivity could decrease even several orders of magnitude [21]. Various hypotheses are proposed to explain this behaviour. Firstly, high percentages of alkali result in the increase of the number of NBO that could break the paths of the electronic conductivity [21, 96]. Furthermore, modifier ions could interact with polarons, creating neutral entities that inhibited the electronic contribution [21, 96, 97]. 

---

As previously reported, the peculiarity of alkali vanado-phosphate glasses is ascribed to the mixed electronic-ionic conductivity. The electronic conductivity is due to the small polaron hopping between the TM ions with different valence state and it could be related to the presence of V +-0-y + species in the glass network. For NaVP systems, TableS.lO reports the various contributions that are included in the V-O-V linkages. It could be highlighted that the increase of V content results in a major number of V -O-V " linkages, which might be correlated to an increase of the electronic conductivity. 

In the present work, MD simulations were used to study the structure of alkali vanado-phosphate glasses providing insight into the short and medium range order of the glass network and to the role of the vanadium ions and their environment in the glass matrices. 

G. Tricot, H. Vezin, Description of the intermediate length scale structural motifs in sodium vanado-phosphate glasses by magnetic resonance spectroscopies. J. Phys. Chem. C 117(3), 1421-1427(2013)... 

G. Ori, C. Mugoni, G. BrogUa, C. Siligardi, M. Montorsi, Short- and medium-range order structure of alkali vanado-phosphate glasses a Molecular Dynamics study (Submitted) (2014)... 

In 2013, the D-HMQC technique has been involved in the structural characterisation of vanado-phosphate-based system within the 50Na2O-3cV2O5-(50 — 3c)P20s composition line [53]. These glasses have already been investigated by Raman and ID P and NMR spectroscopies [54,55]. The results suggest that the network was built by interconnected vanadate and phosphate polyhedra but no direct evidence about the presence of P—O—V bond was available. D-HMQC experiment was applied... 

---