High-Pressure/High-Temperature\nSynthesis of the Lanthanoid\nZinc Borates <i>Ln</i><sub>2</sub>Zn(B<sub>2</sub>O<sub>5</sub>)<sub>2</sub> (<i>Ln</i> = Eu, Tb, Yb, Lu) with\na Gad

Four new lanthanoid zinc borates with the composition <i>Ln</i><sub>2</sub>Zn(B<sub>2</sub>O<sub>5</sub>)<sub>2</sub> (<i>Ln</i> = Eu, Tb, Yb, Lu) were obtained by high-pressure/high-temperature\nsyntheses in a modified Walker-type multianvil device. The layered\ncrystal structures are built up of four- and eight-membered rings\nof corner-sharing [BO<sub>4</sub>] tetrahedra. Two adjacent layers\nare linked by the lanthanoid- and zinc-centered polyhedra. A detailed\ninvestigation of the luminescence properties and energy transfer in\nEu<sup>3+</sup>-activated Tb<sub>2</sub>Zn(B<sub>2</sub>O<sub>5</sub>)<sub>2</sub> and a thorough characterization of the title compounds\nby infrared spectroscopy and temperature-dependent powder X-ray diffraction\nare provided. It turns out that the energy transfer between Tb<sup>3+</sup> and Eu<sup>3+</sup> is extremely efficient (≥98%),\nwhich can be related to the unusually short interlanthanoid distance\nof around 3.45 Å in this structure. The potential use as a lifetime-based\nluminescent thermometer relying on a temperature-dependent back energy\ntransfer from Eu<sup>3+</sup> to Tb<sup>3+</sup> above room temperature\nwith appreciable relative sensitivity (S<sub>r</sub> = 0.92% K<sup>–1</sup> at 425 K) is indicated.