Understanding the Efficiency of Mn⁴⁺ Phosphors: Study of the Spinel Mg₂Ti_1–xMn_xO₄

Abstract

We present a spectroscopic study of Mn-doped Mg 2 TiO 4 as a function of pressure and temperature to check its viability as a red-emitting phosphor. The synthesis following a solid-state reaction route yields not only the formation of Mn 4+ but also small traces of Mn 3+ . Although we show that Mn 4+ photoluminescence is not appreciably affected by the presence of Mn 3+ , its local structure at the substituted Ti 4+ host site causes a reduction of the Mn 4+ pumping efficiency yielding a drastic quantum-yield reduction at room temperature. By combining Raman and time-resolved emission and excitation spectroscopies, we propose a model for explaining the puzzling nonradiative and inefficient pumping processes attained in this material. In addition, we unveil a structural phase transition above 14 GPa that worsens their photoluminescence capabilities. The decrease of emission intensity and lifetime with increasing temperature following different thermally activated de-excitation pathways is mostly related to relatively small activation energies and the electric−dipole transition mechanism associated with coupling to odd-parity vibrational modes. A thorough model based on the configurational energy level diagram to the A 1g normal mode fairly accounts for the observed excitation and emissionthe quantum yieldof this material.

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

“…8b )). At temperatures below 300 K, the intensity I PL (T) decreases slowly while t(T) increases up to 110 K and then decreases, both I PL (T) and t(T) follow a similar temperature dependence above 300 K. In a previous study on Mg 2 TiO 4 :Mn 4+ , 28 the introduction of a term associated with the pumping efficiency in the PL intensity (eqn (10)) was crucial to account for both I PL (T) and t(T) variations and their delay in the temperature range where the non-radiative processes take place. Although this term doesn't affect the PL lifetime, it has, however, a significant influence on the PL intensity.…”

“…In particular, the PL 2 E → 4 A 2 transition at 1.908 eV (649.8 nm) with a salient vibronic component at 1.863 eV (665.5 nm) is shifted by about 0.2 eV with respect to Cr 3+ , and the crystal-field strength given by the first 4 A 2 → 4 T 2 PLE transition at 2.78 eV (446 nm) is 0.55 eV higher. The fitted Racah and crystal-field splitting parameters from the Mn 4+ transition energies following the procedure given elsewhere 28 are B = 0.103 eV, C = 0.412 eV ( C / B = 3.97), and Δ = 2.78 eV at 20 K. These values yield a reduction parameter β 1 = 1.053 using B 0 = 1160 cm −1 and C 0 = 4303 cm −1 for the free Mn 4+ ion. 25 This β 1 value is in the range of Mn 4+ in oxides and reflects the higher covalency of the Mn–O bond compared to the Cr–O bond in the same lattice.…”

“…Such a coincidence in the thermal decays of τ ( T ) and I PL ( T ) yielding PL quenching unravels a very efficient pumping into 4 T 2 , which subsequently transfers the excitation to the emitting 2 E. This behaviour is at variance with findings in spinel Mg 2 TiO 4 :Mn 4+ that show a less efficient pumping. 28 The analysis of the PL lifetime and intensity is essential for appropriately understanding this highly efficient phosphor. There are two main mechanisms that account for the observed PL behaviour: (1) the relaxation dynamics of the 2 E emitting state, and (2) the non-radiative process previously mentioned arising mainly from the 4 T 2 excited state.…”

“…Here, B 0 = 918 cm À1 and C 0 = 3850 cm À1 are the Racah parameters of the free Cr 3+ ion. 25 Using the analytical expressions for B and C parameters as a function of the transition energies derived from the optical spectra, 28 we obtain B = 0.078 and 0.081 eV (629 and 653 cm À1 ) for Cr 3+ in Sr 4 Al 14 O 25 and Al 2 O 3 , respectively, with C = 0.42 eV (3370 cm À1 ) and D = 2.23 eV (17 990 cm À1 ) in both compounds. This means that b 1 = 1.12 and 1.13, respectively, similar to those found for Cr 3+ in other oxide hosts, 25,42 thus supporting the slightly higher covalency of Cr 3+ in the investigated crystal.…”

“…In addition, charge compensation effects may lead to local noncentrosymmetric crystal-field perturbations at Mn 4+ that are interesting and effectively increase the transition oscillatorstrength of excitations (parity forbidden transitions in ideal octahedral sites), and hence, the pumping capability. 21,28 The orthorhombic Sr 4 Al 14 O 25 structure belongs to the Pmma space group and in the structure there are three tetrahedral and three octahedral sites for Al 3+ coordinated by oxygen ions. The octahedra of Al4 and Al6 sites are connected via common edges to form chains parallel to the b axis.…”