White light generation from tin-doped condensed borates

Introduction: Condensed borates are compounds constructed of three-dimensional lattices of borate groups BO₃ and BO₄. They can form amorphous materials (glasses), ordered matrices (crystals), or a mixture of the two (glass–ceramics). They are characterized by high stiffness, which yields a high emission efficiency and stability of the luminescence against host modifications. Divalent tin (Sn²⁺) is a ns²-type luminescent center, which has found use in the fabrication of phosphors and scintillating materials. Sn²⁺ can be excited in UV and exhibits a very wide emission band, spanning the whole visible range. The position of the excitation and emission maxima depend on the host material to some degree.

Methods: This study comprises an extended investigation into the luminescence of the divalent tin (Sn²⁺) in condensed borates. The studied condensed borate systems include 25 MgO–12.5 La₂O₃– 62.5 B₂O₃ (LaMgB₅O₁₀), labeled as LaMBO, which can be obtained in both a crystalline and an amorphous form. It can also form a glass–ceramic through the process of isochemical crystallization. The amorphous and crystalline samples of condensed borates were investigated and their structure, optical properties and luminescence were evaluated. Emission and excitation spectra, as well as decay times and quantum yield, were recorded.

Results: The results indicate that Sn²⁺ can exhibit a range of luminescence in the studied hosts, ranging from a blueish, cool white emission to red-tinted, warm white emission. The correlated color temperature (CCT) of the emission can be tuned using an excitation light of different wavelengths in the UV spectral range or by using two-wavelength excitation. The samples exhibit a high color rendering index (CRI) of >95 in the relevant CCT range and a high quantum yield >80%.

Conclusions: The condensed borate materials, doped with divalent tin, are promising materials for novel, efficient, dual-excitation phosphors for high CRI wLED with tunable CCT. Further insight into the structure of condense borate glass–ceramics can provide interesting fields of applications.