Origin of the (2)E <->(4)T(2) Fano resonance in Cr(3+)-doped LiCaAlF(6): Pressure-induced excited-state crossover.

Abstract

This work investigates pressure-induced phase transition (PT) and excited-state-crossover effects on the photoluminescence (PL) properties of LiCaAlF(6):Cr(3+). We report a PL study by means of time-resolved emission, excitation, and lifetime measurements as a function of pressure. We focus on Cr(3+) PL variations around pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF6 at 7 GPa. Moreover, the structural requirements for changing Cr(3+) PL from a broadband emission at 1.59 eV (781 nm) at ambient conditions, to a rubylike narrow-line emission at 1.87 eV (663 nm) are analyzed in the 0-35 GPa range. We show how pressure progressively transforms Cr(3+) broadband PL into a rubylike emission that becomes the dominant feature of the room-temperature emission spectrum at 28 GPa. This behavior, together with the pressure dependences of the (2)E and (4)T(2) excited-states energy and PL lifetime, are explained on the basis of the electron-phonon coupling associated with the (4)T(2) and (2)E states. We demonstrate that both excited states interact through spin-orbit coupling yielding Fano resonance rather than antiresonance as is frequently assumed.