Luminescence of rare earth ions in strontium barium niobate around the phase transition: the case of Tm3+ ions

The photoluminescence of ferroelectric Sr0.6Ba0.4(NbO3)(2) congruent crystals doped with four different trivalent rare earth ions, Nd3+, Eu3+, Tm3+ and Yb3+, has been examined in the temperature range around the Curie temperature, when the system undergoes the transition to the paraelectric phase. While the luminescence of large ions is not sensitive (Nd3+) or weakly sensitive (Eu3+) to the phase transition, that of small ions (Tm3+ and Yb3+) is highly sensitive to temperature changes around the Curie temperature. Particular emphasis is given to the Sr0.6Ba0.4(NbO3)(2):Tm3+ system, for which the luminescence spectra around the phase transition and low temperature laser site selective spectra are systematically investigated. The different spectral sensitivity of the rare earth ions to the phase transition is interpreted in terms of the lattice sites occupied by the different rare earth ions in the Strontium Barium Niobate crystal host