Rhombohedral-calcite and hexagonal-vaterite types of LuBO(3):Eu(3+) microparticles with various complex self-assembled 3D architectures have been prepared selectively by an efficient surfactant- and template-free hydrothermal process for the first time. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, photoluminescence, and cathodoluminescence spectra as well as kinetic decays were used to characterize the samples. The pH, temperature, concentration, solvent, and reaction time have a crucial influence on the phase formation, shape evolution, and microstructure. The reaction mechanism is considered as a dissolution/precipitation process; it is proposed that the self-assembly evolution occurs by homocentric layer-by-layer growth. Under UV excitation and low-voltage electron beam excitation, calcite-type LuBO(3):Eu(3+) particles show a strong orange emission corresponding to the (5)D(0)-->(7)F(1) transition of Eu(3+) whereas vaterite-type LuBO(3):Eu(3+) particles exhibit a strong red emission with much higher R/O values (that is, chromatically redder fluorescence than that of crystals grown from a direct solid-state reaction). The tunable luminescent properties have potential applications in fluorescent lamps and field emission displays.

Ratiometric luminescence thermometry with trivalent lanthanide ions and their 4fn energy levels is an emerging technique for non-invasive remote temperature sensing with high spatial and temporal resolution. Conventional ratiometric luminescence thermometry often relies on thermal coupling between two closely lying energy levels governed by Boltzmann’s law. Despite its simplicity, Boltzmann thermometry with two excited levels allows precise temperature sensing, but only within a limited temperature range. While low temperatures slow down the nonradiative transitions required to generate a measurable population in the higher excitation level, temperatures that are too high favour equalized populations of the two excited levels, at the expense of low relative thermal sensitivity. In this work, we extend the concept of Boltzmann thermometry to more than two excited levels and provide quantitative guidelines that link the choice of energy gaps between multiple excited states to the performance in different temperature windows. By this approach, it is possible to retain the high relative sensitivity and precision of the temperature measurement over a wide temperature range within the same system. We demonstrate this concept using YAl3(BO3)4 (YAB):Pr3+, Gd3+ with an excited 6PJ crystal field and spin-orbit split levels of Gd3+ in the UV range to avoid a thermal black body background even at the highest temperatures. This phosphor is easily excitable with inexpensive and powerful blue LEDs at 450 nm. Zero-background luminescence thermometry is realized by using blue-to-UV energy transfer upconversion with the Pr3+−Gd3+ couple upon excitation in the visible range. This method allows us to cover a temperature window between 30 and 800 K.