How can the same manganese activator produce different colours in different host crystals?

The luminescent properties of phosphor centres are strongly dependent on the chemical nature of the host crystal. When manganese ions (${\text{Mn}}^{2+}$) act as activators, they produce different emission colours and decay times depending on the specific host material in which they are embedded.1

This phenomenon occurs because the host crystal exerts an electrical field—often described by ligand field theory—that influences the electronic structure of the activator ion. In a crystal lattice, the activator's energy levels are altered by the surrounding ions, which act as ligands. The geometric arrangement and the strength of this field cause the activator's orbitals to split into different energy states.12

Because the energy of the emitted photon corresponds to the difference between these split energy levels, changing the host crystal changes the magnitude of this splitting, thereby shifting the wavelength (and thus the colour) of the emitted light. For example, manganese(II) produces orange light in tetragonal zinc fluoride but green light in rhombohedral zinc orthosilicate.12