High-Pressure/High-Temperature Synthesis of the Lanthanoid Zinc Borates Ln2Zn(B2O5)2 (Ln = Eu, Tb, Yb, Lu) with a Gadolinite-Analogue Crystal Structure and Luminescence Properties of Tb2Zn(B2O5)2:Eu3+

Four new lanthanoid zinc borates with the composition Ln2Zn(B2O5)2 (Ln = Eu, Tb, Yb, Lu) were obtained by high-pressure/high-temperature syntheses in a modified Walker-type multianvil device. The layered crystal structures are built up of four- and eight-membered rings of corner-sharing [BO4] tetrahedra. Two adjacent layers are linked by the lanthanoid- and zinc-centered polyhedra. A detailed investigation of the luminescence properties and energy transfer in Eu3+-activated Tb2Zn(B2O5)2 and a thorough characterization of the title compounds by infrared spectroscopy and temperature-dependent powder X-ray diffraction are provided. It turns out that the energy transfer between Tb3+ and Eu3+ is extremely efficient (≥98%), which can be related to the unusually short interlanthanoid distance of around 3.45 Å in this structure. The potential use as a lifetime-based luminescent thermometer relying on a temperature-dependent back energy transfer from Eu3+ to Tb3+ above room temperature with appreciable relative sensitivity (Sr = 0.92% K–1 at 425 K) is indicated.