“…If the energy separation between the ground (or lowest) J state and higher lying J states is large, the above relationship is still approximately valid because of the small J mixing by the crystal field. This point has been supported by lots of theoretical and experimental studies for f n ions in crystals [7,8,[19][20][21][22][23][24][25][26]. Pa 4 + ions in crystals belong to this case because the energy separation between the ground J ( ¼5/2) state and higher lying J ( ¼7/2) state is large ( $ 5000 cm À 1 [6,17,18]).…”