On the photoluminescence of Pr(III) activated Ca2P2O7 polymorphs

In this work the optical properties of two distinct, praseodymium activated, UV-C emitting pyrophosphate polymorphs are presented. The materials were obtained as single-phase samples using a facile solid state method with annealing at target phase-dependent temperatures. The activator concentration dependent luminescence quenching was investigated revealing significant differences between the [Xe]4f² → [Xe]4f² and the [Xe]4f¹5d¹ → [Xe]4f² transitions. The highest emission intensities were observed at Pr³⁺ concentration of 0.5 and 2.0 % for the [Xe]4f² → [Xe]4f² line and the [Xe]4f¹5d¹ → [Xe]4f² band emission, respectively. Furthermore, temperature dependent fluorescence spectroscopy and VUV spectroscopy were employed to investigate the thermal quenching behaviour as well as the excitation and emission properties in the deep UV range. Fitting of the temperature dependent emission integrals showed thermal quenching temperatures exceeding 700 K. It was revealed that the samples show two types of emission that can be traced back to the inter- and intraconfigurational transition of trivalent praseodymium. Praseodymium emission bands around 235 and 265 nm correspond to the various [Xe]4f¹5d¹ → [Xe]4f² interconfigurational transitions. Meanwhile, narrow emission lines throughout the visible and NIR range are caused by the [Xe]4f² → [Xe]4f² intraconfigurational transitions of Pr³⁺. Qualitative and quantitative comparisons between the emission properties of the two polymorphs revealed significant differences arising from the different coordination environments. α-Ca₂P₂O₇:Pr phosphors exhibited around 60 % of the emission intensities of the β-Ca₂P₂O₇ materials in the red spectral range while a reversed trend was observed for the UV emission caused by [Xe]4f¹5d¹ → [Xe]4f² transitions.