P-01 Behavior of molybdenum in calcium phosphate glasses

Topic: Advanced glasses, ceramics and polymeric materials and nano materials

M. Chládková^*, J. Holubová, Z. Černošek

Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic

^*Email: monika.chladkova@student.upce.cz

 Keywords: phosphate glasses; molybdenum; structure; ³¹P MAS NMR; ESR

Abstract: Transition metals are often used as an electropositive part of phosphate glasses, but their chemistry in high temperature melts is often not given enough attention. While for the s-and p-metals their interaction with the phosphate glassy network can be predicted with a high probability, in the case of transition metals (d-elements) the situation is more complicated, as they can occur in several oxidation states and form various complex ions, which can come in to the formation and strengthening of the glass-forming network by means of coordination bonds. For these reasons, it is not possible to predict unambiguously the chemical behavior of transition metals in the phosphate melt. It is therefore appropriate to pay more detailed attention to the chemistry of transition metals in phosphate melts, as the literature data are often contradictory and not entirely sufficient. One of the elements that can occur in different oxidation states and form different complex cations is molybdenum. In glasses, this should normally occur in two oxidation states, Mo^VI (d⁰ system) and Mo^V (d^l) and can form molybdenyls (Mo^XO₂)^(X-4)+ and (Mo^XO)^(X-2)+, where X is 6 or 5.

A series of glasses in which the composition changes from calcium metaphosphate to molybdenum pyrophosphate (Mo^VO)₄(P₂O₇)₃, i.e. xMoO₃-(50-0.875x)CaO-(50-0.125x)P₂O₅, x = 0, 1, 10, 20, 30, 40, 45, 50 and 57.14, was used for this work. Homogeneous glasses were prepared over the whole compositional line. The composition of glasses was checked by XRF. The Mo free glass (x=0) was colorless and transparent and the addition of Mo produces a gradual coloration of glasses changing from light to dark green indicating by that the reduction of Mo(VI) to Mo(V) during synthesis (d⁰ to d¹ system). The degree of reduction of molybdenum and the symmetry of its coordination polyhedron were derived from electron paramagnetic resonance measurements. The amount of reduced Mo did not exceed 3%, only in the case of the two highest concentrations of MoO₃ the reduction was higher ~ 7%, which is still a very low degree of reduction.

The phosphate base of glassy network was characterized by ³¹P MAS NMR and correlated with Raman spectra. It has been found that Ca²⁺ is gradually being replaced by molybdenyl (MoO₂)²⁺ and thus starting metaphosphate network of Ca(PO₃)₂ remains maintained as much as possible and only a significantly increasing O/P ratio leads to the partial formation of pyrophosphates. It has been found that like in aqueous solution, reduction of Mo^VI is almost negligible in the glass-forming phosphate melts and that composition of phosphate glassy network is primarily controlled by the presence of divalent cations. Unlike calcium, molybdenyl is incorporated into the glass network structure by four covalent bonds which, in addition to forming a cation-anion network, also allows the involvement of small pyrophosphate anions, i.e. (O₃P-O-PO₃)^4- units in the glass network and thus improves the glass forming ability of the studied system even with complete replacement of calcium by molybdenum.