Optical spectroscopy of the Sr4Al14O25:Mn4+,Cr3+ phosphor: pressure and temperature dependences

We present a spectroscopic study of the doubly Mn4+ and Cr3+-doped Sr4Al14O25, synthesised via solid state reaction, as a function of pressure and temperature to check its potential as a red emitting phosphor. The strong crystal field acting on Cr3+ yields a ruby-like narrow R-line emission that is used as an intrinsic temperature sensor and, in comparison to ruby, provides a fair estimate of the bulk modulus K = 172 GPa for Sr4Al14O25. The photoluminescence (PL) and associated excitation spectra of Sr4Al14O25:Mn4+,Cr3+ unravel a strong-field 3d(3) configuration with R-1 and R-2 emission lines largely separated by 10.7 meV, which characterizes the non-centrosymmetric distorted Al4 site in the Sr4Al14O25 structure. Eighteen Raman modes have been detected at ambient pressure. They correspond to the stretching modes of the Al-O bonds and show frequency shifts with pressure consistent with Gruneisen parameters (gamma = 0.65), similar to those measured for other aluminates. The variations of PL intensity and lifetime with temperature exhibit a similar concomitant behaviour indicating an efficient pumping to the T-4(2) level and hence down to the E-2 emitting level in both Mn4+ and Cr3+ ions at low and moderate temperatures. Multiphonon non-radiative processes yield a quenching temperature of 400 K for Mn4+ with an effective activation energy of 0.57 eV. Unexpectedly, this energy decreases with pressure enhancing the non-radiative processes and thus PL intensity reduction. A thoroughly coordinate configurational energy diagram is presented for explaining the main spectroscopic features and excited-state dynamics.