“…The broadband absorption of BaCuSi 4 O 10 in the range 500–800 nm includes three peaks at 534, 625 and 828 nm, which can be assigned to d – d transitions 2 B 1g → 2 A 1g , 2 B 1g → 2 E g , and 2 B 1g → 2 B 2g , respectively, of the square-planar Cu 2+ with a symmetry of D 4h . 16,48 Before the first phase transition, the main absorption of transition 2 B 1g → 2 E g gradually redshifts and decreases in intensity as the pressure increases, corresponding to the color lightening. Combining the red-shifted NIR emission peak associated with the transition from 2 B 2g to 2 B 1g , the branch-off tendency of the absorption peaks provides direct proof of the enlarged crystal-field splitting, which supports the conclusion of structural evolution where there is only the shortening of bond distances without any deformation in planarized configuration of the Cu 2+ coordination.…”