Various borate and borosilicate glasses doped with Tb3+ in a broad concentration range were prepared and characterized by different methods. Static fluorescence excitation and emission spectra of Tb3+ in the ultraviolet-visible range were recorded. The fluorescence decay rates of the green emission were measured and fitted. Different lifetimes dependent on different local sites of Tb3+ ions were detected. To identify these different local sites crystalline terbium borate, TbBO3, was prepared and investigated. XRF and electron microscopy measurements were used. Very strong clustering effects of Tb3+ ions were detected as preliminary stage of phase separation by formation of a very short decay rate in optically clear borate containing glasses with low basicity. In glass samples with phase separation, the Tb3+ ions were accumulated in the phase with higher optical basicity.
