Evolution of the full energy structure of Mn 4+ in fluoride phosphors under high pressure conditions.

This paper analyzes the photoluminescence excitation and emission spectra of fluoride phosphors doped with Mn 4+ : KNaSiF 6 :Mn 4+ , Rb 2 GeF 6 :Mn 4+ , and Na 3 HTiF 8 :Mn 4+ under high pressure conditions. From the optical spectra, the pressure-dependent energies of optically active 4 T 2 , 4 T 1 , and 2 E crystal field subterms of Mn 4+ have been determined in the 0-30 GPa pressure range. A strong blueshift of the 4 T 2 and 4 T 1 subterms was found, as expected from the Tanabe-Sugano diagram for Mn 4+ (d 3 ). At the same time, the 2 E emitting state exhibited a redshift under pressure - an effect opposite to the prediction of the Tanabe-Sugano diagram. This is a manifestation of the pressure-driven nephelauxetic effect, governed by pressure induced changes of Racah parameters, which demonstrates the necessity of taking into account the Racah parameters for a correct description of Mn 4+ emission under pressure. The high pressure experimental data allowed to determine the pressure dependence of crystal field strength parameter Dq and Racah parameters B and C . Finally, obtaining the pressure dependence of Dq and Racah parameters allowed to calculate the full energy structure of the d 3 configuration of Mn 4+ in KNaSiF 6 , Rb 2 GeF 6 , and Na 3 HTiF 8 in the pressure range of 0-30 GPa. The calculations reproduced the redshift of the 2 E emitting state under pressure, as well as gave the pressure shift direction and magnitude for all crystal field subterms of Mn 4+ up to 50 000 cm -1 ( i.e. the equivalent of the Tanabe-Sugano diagram for high-pressure experiments). The approach presented in this paper can be easily extended for calculating the energy structure of materials doped with isoelectronic Cr 3+ as well as other transition metal ions.