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

Abstract

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 3d³ configuration with R₁ and R₂ 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 Grüneisen parameters (γ = 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 ⁴T₂ level and hence down to the ²E 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.