Phase transitions in the 5 at. % Nd3+-doped Ba2NaNb5O15 self-doubling laser crystal -: art. no. 104110

Structural and dynamical properties of 5 at. % Nd3+-doped Ba2NaNb5O15 (BNN) self-doubling laser fibers are studied over the 15-300 K temperature range. Both Raman scattering and x-ray diffraction (including complete refinement of the structure) are carried out for this purpose. On decreasing the temperature, the 5% doped compound is found to follow the same tetragonal A-orthorhombic B-tetragonal C phase transition sequence as in pure BNN. The critical temperature of the B-C phase transition is only slightly influenced by doping (113 K in pure BNN-->90 K in 5 at. % Nd3+ BNN). In contrast that of the A-B phase transition is strongly shifted (573 K in pure BNN-->200 K in 5 at. % Nd3+ BNN), which indicates a marked stabilization of the tetragonal A phase versus the B and C phases on doping. A phenomenological discussion of these doping-induced shifts involving both impurity ions and intrinsic cationic vacancies is sketched out in the framework of Ginzburg-Landau theory. It is suggested how charge compensation can be locally restored in the region of a Na+ or Ba2+ vacancy by substituting an adequate neighboring cation with a Nd3+ impurity. This mechanism is supported by the experimentally observed high quality of 3% doped crystals. The practical advantages of having thus stabilized the laser fiber in the tetragonal phase at room temperature owing to a suitable doping rate are pointed out and briefly discussed.