“…61–64 Other peaks, corresponding to the transitions from 3 P 0 , are also observed: 3 P 0 → 3 H 5 , 3 H 6 , 3 F 2 , 3 F 3 , 3 F 4 at 540, ∼618, 657, ∼720, and 739 nm, respectively. 61,62,65 The attribution of the strongest luminescence peaks and their corresponding electron transitions are related to the praseodymium concentration: both 1 D 2 → 3 H 4 (607 nm) and 3 P 0 → 3 H 5 (∼618 nm) transitions have a dependence on the praseodymium concentration in the crystal. However, the 1 D 2 energy level of Pr 3+ is more sensitive to Pr 3+ concentration than 3 P 0 one; 66–69 thus, for x < 0.1 the maximum intensity is associated with the 1 D 2 → 3 H 4 transition at 607 nm, for x = 0.1, the band corresponding to 3 P 0 → 3 H 6 transition has the maximum intensity, and for x ≥ 0.2, the peak at 657 nm, related to 3 P 0 → 3 F 2 transition, becomes the strongest luminescence peak.…”