Can fresnoite (Ba2TiSi2O8) incorporate Ti3+ when crystallizing from highlyreduced melts?

The crystallization of fresnoite (Ba2TiSi2O8) in supercooled melts can be t riggered by the reduction of Ti4+ to Ti3+. We address the question of wheth er Ti3+ is incorporated into the fresnoite crystal structure during such el ectrochemically induced crystallization. Fresnoite crystals grown from a va riety of highly reduced melts were studied by electron energy-loss spectros copy in order to distinguish between Ti3+ and Ti4+ contents in the correspo nding phases. Although the presence of Ti3+ strongly promotes crystallizati on, Ti3+ is not incorporated into the fresnoite crystal lattice in a signif icant amount (less than 10 at.%) but, at sufficiently high Ti3+ concentrati ons, forms a previously unreported barium oxotitanate(III) containing triva lent titanium only. Since, in the fresnoite crystal structure, Ti4+ solely adopts an unusual fivefold coordination, we also discuss the energy-loss ne ar-edge structure (ELNES) of the Ti L-2,L-3 ionization edge and compare it with the Ti L-2,L-3 ELNES of tetrahedrally and octahedrally coordinated Ti4 +.