Study of energy transfer processes among 'Yb POT. 3+' and 'Tm POT. 3+' ions in metaphosphate glasses - Terra, Idelma Aparecida Alves; Terrile, Maria Cristina; de Camargo, Andrea Simone Stucchi; Nunes, Luiz Antônio de Oliveira (International Symposium on Advanced Materials and Nanostructures - ISAMN, 2007)