F. Rodriguez, D. Hernandez and H. U. Güdel
Double-excitation transitions in the perovskite NH₄MnCl₃ studied by pressure techniques
Phys. Rev. B 60, 10598-10601 (1999)                 

Abstract: This work investigates double-excitation (DE) transitions in the cubic chloroperovskite NH₄MnCl₃ through high-pressure absorption spectroscopy. This procedure has proved to be a powerful tool for revealing DE from the peak energy variations induced by pressure. Particularly, this is important for manganese chlorides where DE assignment is a difficult task to accomplish given that the associated bands appear in the absorption threshold and overlap with several Mn^{2 +} single excitation (SE) bands. Apart from Mn^{2 +} SE, the optical-absorption spectrum of NH₄MnCl₃ microcrystals shows a new intense band at 39 200 cm^–1 that is assigned to the ⁴T_1g+⁴T_1g DE transition. The proposed assignment is based on the transition energy as a sum of the corresponding SE energies, E(⁴T_1g+⁴T_1g) = E(⁴T_1g) + E(⁴T_1g), and is clearly confirmed by the variation of the band position with pressure. The measured pressure shift of E/P = -76 cm^–1/kbar is twice the pressure shift experienced by the first ⁴T_1g(G) SE located at 19590 cm^–1, E/P = -37 cm^–1/kbar. From this result, the possibility of SE transitions ⁴A_2g(F), ⁴T_1g(F), and ⁴T_2g(F) located around 40 000 cm^–1 as responsible for this band is definitively ruled out due to their different pressure shift.