Journal Article

Interpretation of experimental findings on the structure of glass in the CaO–MoO3–P2O5 system using a thermodynamic model including oxidation–reduction equilibria

Macháček, Jan, Liška, Marek, Hruška, Branislav, Gedeon, Ondrej

2024

Overview

This work extends the thermodynamic model of associated solutions used in the past to describe the structure and properties of glasses to the area of complex multicomponent glasses with polyvalent elements, where it has not been applied until now either due to the absence of Gibbs energies of formation of the necessary compounds or due to oxidation–reduction equilibrium in the presence of a gas phase containing oxygen. While the fitting of unknown Gibbs energies based on experimental data has already been applied to some extent in our previous work, the implementation of redox is, to the best of our knowledge, new. Four concentration series were taken from the published data from the glass-forming ternary system CaO–MoO 3 −P 2 O 5 : A) xMoO 3 −(0.5–0.75x)CaO−(0.5–0.25x)P 2 O 5 ; B) xMoO 3 −(0.5–0.875x)CaO−(0.5–0.125x)P 2 O 5 ; C) xMoO 3 −(0.5−x)CaO−0.5P 2 O 5 ; M) xMoO 3 −(1−x)P 2 O, for which the distributions of Q n units were also published (Q denotes the PO 4 tetrahedral unit with n bridging oxygens) by the 31 P MAS NMR method and the Mo [V] /ΣMo fraction by the ESR method [Černošek et al. (J Solid State Chem 303:122522, 2021); Holubová et al., (J Non-Cryst Solids 607:122222, 2023)]. The following compounds were considered in the TD model: P 2 O 5 , CaO, Mo [VI] O 3 , Ca(PO 3 ) 2 , Ca 2 P 2 O 7 , (Mo [VI] O 2 )(PO 3 ) 2 , (Mo [VI] O 2 ) 2 (P 2 O 7 ), (Mo [VI] O 2 ) 3 (PO 4 ) 2 , (Mo [V] O) 2 (PO 3 ) 2 (P 2 O 7 ), (Mo [V] O)PO 4 . All except the hypothetical compound (Mo [VI] O 2 ) 3 (PO 4 ) 2 exist, and their structure is known. Binary phosphate compounds with molybdenum lack Gibbs energies of formation. Therefore, one of the series, namely A, was used to determine these energies by nonlinear regression with the help of a genetic algorithm, without/with redox, and then the distribution of Q n units and the fraction of Mo [V] /ΣMo was predicted for the remaining series. It was found that the distribution of Q n units can be described by the TD model with redox only. During the reduction of molybdenum, the distribution of Q n unit’s changes, and thus also the connectivity of the phosphate network, for example, according to the reactions: (MoO 2 ) 2 (P 2 O 7 )—> 2(MoO)PO 4  + 1/2O 2 , in which Q 1 —> Q 0 and 2(MoO 2 )(PO 3 ) 2 —> (MoO) 2 (PO 3 ) 2 (P 2 O 7 ) + 1/2O 2 in which Q 2 —> Q 1 . Despite the fact that the TD model with redox gives excellent agreement in the case of the Q n distribution, the agreement with the ESR measurements of the Mo [V] /ΣMo ratio is not good. The TD model predicts significantly more pentavalent molybdenum in the glass.

• Publisher: Springer International Publishing,Springer Nature B.V
• Subject: Analytical Chemistry / Calcium oxide / Chemistry / Chemistry and Materials Science / Energy / Equilibrium / Genetic algorithms / Glass / Inorganic Chemistry / Measurement Science and Instrumentation / Molybdenum / Molybdenum oxides / Molybdenum trioxide / NMR spectroscopy / Oxidation / Phosphorus pentoxide / Physical Chemistry / Polymer Sciences / Ternary systems / Thermodynamic models / Thermodynamics / Vapor phases