Dedicated to Professor Josef Breu on the Occasion of his 60 th BirthdayTo unambiguously clarify the long-debated crystal structure of the low-temperature orthoborate π-YBO 3 , single-crystals of the europium-doped π-YBO 3 :Eu 3 + were prepared via a highpressure/high-temperature route at 7.5 GPa and 1400 °C. The structure was solved and refined in the monoclinic space group C2/c (no. 15) in the pseudowollastonite structure type, with the lattice parameters a = 11.3407( 5), b = 6.5501(3), c = 9.5798(4) Å, and β = 113.08(2)°. This structure determination was corrobo-rated by powder X-ray diffraction and vibrational spectroscopy. 11 B as well as 89 Y MAS NMR spectroscopy was used to verify that our results are also accurate for the undoped π-YBO 3 and samples with varying europium content prepared under ambient pressure conditions. Additionally, the luminescence properties of the Eu 3 + doped π-orthoborate were determined in comparison with these ambient-pressure samples.

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This work concerns the concentration dependence of the optical properties of microscale samples according to the solid solution Li2Ca1–2xPrxNaxSiO4 with x = 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, 0.07, and 0.1. Phase purity was inspected by using X-ray powder diffraction. In order to figure out concentration dependent similarities and differences of their properties with respect to down- and up-conversion, luminescence spectroscopy for vacuum UV (VUV), X-ray, and blue light excitation were performed. Furthermore, diffuse reflection spectroscopy as well as time dependent luminescence measurements were conducted. It turns out that the concentration quenching of the up-conversion lags the down-conversion. In addition, a rise time can be observed in the time dependent measurements of the up-conversion. From these observations, it is concluded that the up-conversion process takes place via an energy transfer process with a very high probability.

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