Oxidation and coordination states assumed by transition metal dopants in an invert ultrabasic silicate glass

An ultrabasic invert silicate glass (46SiO₂·11Na₂O·21CaO·22BaO, optical basicity index equal to 0.71) was synthesized (O₂ atmosphere) and used as host for various transition metal dopants. Optical absorption, emission and electron paramagnetic spectroscopies were used to characterize oxidation and coordination states. Some of the dopants displayed only their maximal oxidation state (Ti⁴⁺, V⁵⁺, Cr⁶⁺, Mo⁶⁺ and W⁶⁺). Others exhibited mixed valences: (i) Mn³⁺ was the dominant species, alongside Mn²⁺ and Mn⁵⁺; (ii) stable Fe³⁺ prevailed, although some Fe²⁺ was preliminarily suggested by the absorption spectrum; (iii) Co³⁺ probably accompanied the dominant Co²⁺ tetrahedral oxide complex; (iv) like in “conventional” silicate glasses, only Ni²⁺ was detected, though simultaneously located in tetrahedral and octahedral sites (somewhat distorted); (v) Cu⁺ was surprisingly identified alongside the expected 6-fold coordinated Cu²⁺. Drastic reduction of the oxygen content in the melting atmosphere led to conversion of Cr⁶⁺ to Cr³⁺, despite the extreme basicity of the host.