Transparent oxyfluoride glass ceramics are interesting composites
that combine the good chemical and mechanical stability of
oxide glasses with the excellent optical properties of fluoride
crystals [1e3]. These materials are ideal hosts for rare earth (RE)
ions e the low phonon energy of fluorides reduces the nonradiative
relaxations.
Among other RE ions, erbium is a widely investigated candidate
for infrared to visible upconversion luminescence (UCL) processes
due to favorable energy levels and long decay times of its excited
states [4]. When introduced to different hosts, the UCL efficiency of
erbium depends on its local environment. Significant efforts have
been devoted to the investigation of erbium doped b-NaREF4
crystals because they have low phonon energy and low local
symmetry of RE ions. Both of these factors contribute to a highly
efficient UCL in the materials [5]. Several hexagonal NaREF4 compounds,
such as NaLaF4 [6], b-NaGdF4 [7,8] and b-NaYF4 [9] have
been successfully precipitated also in aluminosilicate and borosilicate
glasses.