AGH - Paweł Stoch

Raman imaging as a useful tool to describe crystallization of aluminum/iron-containing polyphosphate glasses

Polyphosphate glasses are materials of a wide spectrum of applications in many fields. The subject of the work is polyphosphate glasses containing aluminum and iron. Three compositions of the glasses were obtained and the materials have been characterized in terms of their crystallization. The differences in crystallization behavior between powder and bulk materials were compared. The crystallized materials were analyzed by Raman scattering spectroscopy and X-ray diffraction method. It was evidenced that depending on the glass composition the main crystalline phases were Al(PO₃)₃, AlPO₄, FePO₄, Fe₃(P₂O₇), Fe₄(P₂O₇)₃, FePO₄. The glass crystallization leads to enrichment of the residual glassy phase in P₂O₅ and increases its polymerization. Thus, it was observed the glass inhomogeneities are being increased due to crystallization. The two dimensions spectral maps of the bulk crystallized samples were executed to describe the mechanism and type of crystallization. The depth profiling proves the differences between surface and interior phase composition.

The most important conclusions are:

The pristine glass materials are built of chains which are the longest for iron-free glass and with the Fe₂O₃ content are being gradually depolymerized.
The glass network becomes less rigid what is observed as the decrease of T_g and ∆C_p. The effect is consistent with the observed depolymerization of the phosphate network and increases the mean bond ionicity.
In the case of the aluminum-phosphate material, there is evidenced the formation of AlPO₄ and Al(PO₃)₃ compounds in which formation and quantity were confirmed by XRD and Raman.
It was evidenced that the glass crystallization proceeds from the surface. On which it was observed AlPO₄ layer which was evidenced by Raman imaging and not visible in XRD.
The crystallization confirms the inhomogeneous character of the glass with the possible evaporation of P2O₅ out of the surface.
The residual vitreous phase is mostly pure P₂O₅. In the iron-containing glasses crystallize Fe₃(P₂O₇)₂, Fe₄(P₂O₇)₃, and FePO₄. The first compound is the mixed iron valence of the Fe³⁺/Fe²⁺ similar to in the pristine glass. Its quantity on the surface is lower what indicates their oxidation with the formation of Fe₄(P₂O₇)₃ and FePO₄ phases. Thus, during the devitrification Fe²⁺ is transformed to Fe³⁺ and the mixed iron valence compound is transformed to the pure Fe³⁺ compounds.
The oxidization proceeds from the surface to the bulk what is related to the diffusion of atmospheric oxygen from the surface.
Partial substitution of Al/Fe slows down the glass crystallization and promotes the formation of berlinite like the FePO₄ phase.
The partial substitution may prevent the iron-phosphate glass crystallization. The opposite effect may be detected in the case of the aluminum-phosphate glasses where iron inhibits aluminum-phosphates crystallization.

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Phosphate glasses from ab initio molecular dynamics perspective

The ab initio simulation methods are nowadays very precise in prediction of glass network structural properties. On the other hand, they are very computationally demanding methods and the size of the simulated system is limited to several hundreds of atoms, and extremely short timescales. What is important they can give you information not only of atoms arrangement but also about bonding properties. The aim of our latest studies is to test influence of aluminium on structural and bonding properties of phosphate glasses by Car-Parrinello molecular dynamics.

The most important conclusions are: Mayer’s bond order analysis can be a useful tool in the glass network description. Non-network oxygen atoms, which do not take part in a glass network depolymerization, is an important parameter which needs to be considered in the prediction of the network dimensionality based on the O/P ratio. On the other side, the difference between the dimensionality obtained from an experiment and the ratio gives the possibility to estimate the part of the non-network oxygen atoms. This is a valuable parameter describing an inhomogeneity of a glass network. In the case of the studied glasses, the number of oxygen atoms increases with Al₂O₃ what increases the Al-O-Al joining’s. The second parameter which leads to a glass network inhomogeneity is the disproportionation reaction of the structural units. The consideration of the reaction and the non-network oxygens leads to very good convergence between the dimensionality predicted from the O/P ratio and the simulations. The simulations showed the theoretical possibility of occurrence of Q⁴ structural units, which are joined to Q² what lead to their charges compensation. The glass network charge neutrality is a predominant parameter which needs to be fulfilled by the network as a whole. There is a strong willingness of the atoms to obtain the specific total bond orders. The condition is at first fulfilled for oxygen, later phosphorus, and finally aluminium. In effect, the Al atoms at first need to compensate the charge of O_t oxygen atoms which leads to its modifier role in the network. Due to the lowering the number of O_t oxygens the Al atoms can change their position to the glass network formers. Nevertheless, their negative charge needs to be compensated by the network due to its depolymerization or charge compensating role of the Al atoms in the modifier positions. The more details in article:

Paweł Stoch, Paweł Goj, Aleksandra Wajda, Agata Stoch

Alternative insight into aluminium-phosphate glass network from ab initio molecular dynamics simulations

Ceramics International, Volume 47, Issue 2, 15 January 2021, Pages 1891-1902.

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Raman imaging as a useful tool to describe crystallization of aluminum/iron-containing polyphosphate glasses

Phosphate glasses from ab initio molecular dynamics perspective