Origin of the E-2 <-> T-4(2) Fano resonance in Cr3+-doped LiCaAlF6: Pressure-induced excited-state crossover

This work investigates pressure-induced phase transition (PT) and excited-state-crossover effects on the photoluminescence (PL) properties of LiCaAlF6:Cr3+. We report a PL study by means of time-resolved emission, excitation, and lifetime measurements as a function of pressure. We focus on Cr3+ PL variations around pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF6 at 7 GPa. Moreover, the structural requirements for changing Cr3+ 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 Cr3+ 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 E-2 and T-4(2) excited-states energy and PL lifetime, are explained on the basis of the electron-phonon coupling associated with the T-4(2) and E-2 states. We demonstrate that both excited states interact through spin-orbit coupling yielding Fano resonance rather than antiresonance as is frequently assumed.