Controlling oxygen coordination and valence of network forming cations

Abstract

Understanding the structure-property relationship of glass material is still challenging due to a lack of periodicity in disordered materials. Here, we report the properties and atomic structure of vanadium phosphate glasses characterized by reverse Monte Carlo modelling based on neutron/synchrotron X-ray diffraction and EXAFS data, supplemented by Raman and NMR spectroscopy. In vanadium-rich glass, the water durability, thermal stability and hardness improve as the amount of P 2 o 5 increases, and the network former of the glass changes from VO x polyhedra to the interplay between VO x polyhedra and po 4 tetrahedra. We find for the first time that the coordination number of oxygen atoms around a V 4+ is four, which is an unusually small coordination number, and plays an important role for water durability, thermal stability and hardness. Furthermore, we show that the similarity between glass and crystal beyond the nearest neighbour distance is important for glass properties. These results demonstrate that controlling the oxygen coordination and valence of the network-forming cation is necessary for designing the properties of glass. Oxide glass components are basically classified into network formers, network modifiers and intermediates by Zachariasen 1 and Sun 2. Typical network formers satisfy Zachariasen's rules 1 , but V 2 O 5 is classified as a network former 1 or intermediate 3. This is because it is hard for this oxide to form glass on its own because the oxygen coordination number is five in a crystalline phase, which is larger than the coordination number of typical network formers of three and four. V 2 O 5 based glasses have been widely studied for decades because V 2 O 5 glass is a typical semiconducting glass originating from hopping conduction. Indeed, not only fundamental research on electrical properties 4-8 but also applied research on cathode materials for lithium, sodium and magnesium ion batteries have been reported 9-12. This glass also has a low glass transition temperature and relatively low thermal expansion 4. These are quite attractive properties for low-melting glass used for sealing. In this field, lead borate glass with a high percentage of PbO has been applied to sealing below 400 °C in electronic devices, such as IC ceramic packages, crystal oscillators and micro-electro-mechanical systems. Since lead components are a hazardous substance for human health and the environment, it is necessary to avoid the use of PbO, and hence, V 2 O 5 based glass is a promising material for overcoming this problem 13-19. However, there are two critical problems that face the practical application of V 2 O 5 based glass: poor water durability and low thermal stability.

Topological analyses of structure of glassy materials toward extraction of order hidden in disordered structure

“…The present findings regarding the 3D network formation of Zn x O y units indicate that interplay between the PO 4 tetrahedra and ZnO polyhedra is important not only for forming glass but also for tuning the physical properties of glass. Such interplays related to the formation of the glass network in oxide glass have been reported for CaOAl 2 O 3 , 33) V 2 O 5 P 2 O 5 35) glass systems. For instance, in V 2 O 5 P 2 O 5 glass, Aoyagi et al reported that the glass network formed by the interplay between PO 4 tetrahedra and VO x polyhedra brings about the improvement of water durability, thermal stability, and hardness.…”

“…For instance, in V 2 O 5 P 2 O 5 glass, Aoyagi et al reported that the glass network formed by the interplay between PO 4 tetrahedra and VO x polyhedra brings about the improvement of water durability, thermal stability, and hardness. 35) Furthermore, in zinc-rich glass in ZnOP 2 O 5 system, it was found that the thermal expansion coefficient sensitively reflects the substitution of the phosphate chain network by a network formed of Zn x O y units.…”

An EPR investigation of defect structure and electron transfer mechanism in mixed-conductive LiBO2–V2O5glasses

“…It is important to note that in the glassy materials, two distinctly different and observable V 4+ environments can be expected, with the vanadium acting as a network forming or network modifying phase. [30][31][32] At low contents, in the network modifying phase, V 4+ defects are formed via charge compensation for Li and B, and can be considered well isolated in the bulk LiBO 2 network compared to other substitutional vanadium sites. On the other hand, in network forming phases, the structure is akin to defective V 2 O 5 (to short range order), whereby V 4+ sites are formed via incorporation of Li, B and oxygen vacancy defects.…”

Structural changes in V2O5-P2O5 glasses: non-constant force field molecular dynamics and IR spectroscopy

“…Vanadium glasses have a complex structure because vanadium ions can exist in four-, five-and six-coordinated states (tetrahedron, square pyramid, trigonal bipyramid, and octahedron [9]). Moreover, vanadium oxide is a transition metal which possesses at least four-and five-valent states in glasses, and ions with different valence can form the same structural groups [10]. According to [11], amorphous vanadium oxide consists of VO 4 and VO 5 structural units sharing edges and corners, while VO 4 units dominate in the molted V 2 O 5 and VO 5 units are formed due to the transformation of VO 4 ones at quenching.…”