The very first thorough investigation of the rare earth borosulfatesRE₂[B₂(SO₄)₆] withRE= Y, La–Nd, Sm, Eu and Tb–Lu focusses on synthetic aspects and especially optical properties.

The pentasodium rare-earth tungstates Na₅M(WO₄)₄ are closely related to the sodium rare-earth double tungstates Na₅M(WO₄)₂ both adopting the scheelite structure type (space group I4₁/a, no. 88). After the preparation of polycrystalline powders via flux syntheses improving the phase purity significantly, the crystal structures of Na₅M(WO₄)₄ (M = Y, La–Nd, Sm–Lu, Bi) were determined by single crystal XRD and Rietveld analysis. Na₅M(WO₄)₄ is a promising phosphor material both as a host and as a 100% phosphor due to the possible charge transfer of the tungstate group and the absence of any concentration quenching. Na₅M(WO₄)₄ incongruently melts to Na₅M(WO₄)₂ and Na₂WO₄. After the clarification of the crystallographic relationship of Na₅M(WO₄)₄ and Na₅M(WO₄)₂ based on a rare isomorphic transition of index 5 (i5) the non-linear trend of the decomposition temperature within the row of rare earth ions is explained systematically taking into account the existence of domains within the crystal structure predetermining the posterior decomposition. A miscibility gap for solid solutions of Na₅Y(WO₄)₄ and Na₅Eu(WO₄)₄ or Na₅Tb(WO₄)₄ is identified and its temperature dependence is investigated. Furthermore, the investigation of the fluorescent properties of Na₅M(WO₄)₄ (M = Pr, Sm, Eu, Tb, Tm, Bi), Na₅Y_1−xEu_x(WO₄)₄ and Na₅Y_1−yTb_y(WO₄)₄ provided insights into the weak ligand field and the energy transfer from WO₄ ²⁻ to M³⁺ governed by the emission of the sensitiser within Na₅M(WO₄)₄. Additionally, the compounds were characterised by magnetic measurements and vibrational, UV/Vis and ¹⁵¹Eu Mössbauer spectroscopy.

The first bismuth borosulfate (H₃O)Bi[B(SO₄)₂]₄ is only the second featuring a three‐dimensional anion, the first tectosilicate‐analogous borosulfate synthesised solvothermally without a precursor (from Bi(NO₃)₃⋅5 H₂O and B(OH)₃ in oleum); moreover, it is the first comprising two differently charged cations and crystallises in a new structure type in space group I (no. 82) (a=11.857(1), c=8.149(1) Å, 1947 refl., 111 param., wR2=0.037), confirmed by a second harmonic generation (SHG) measurement. The B(SO₄)₄ supertetrahedra are connected via all four sulfate tetrahedra resulting in a three‐dimensional anion with both H₃O⁺ and Bi³⁺ cations in channels. Additionally, the crystal structure of a further bismuth borosulfate, Bi₂[B₂(SO₄)₆], is elucidated crystallising isotypically to the rare‐earth borosulfates R₂[B₂(SO₄)₆] in space group C2/c (No. 15) (a=13.568(2), b=11.490(2), c=11.106(2) Å, 3127 refl., 155 param., wR2=0.035). Moreover, the optical and thermal properties of both compounds are discussed.

Apart from the energy gap law, control parameters over nonradiative transitions are so far only scarcely regarded. In this work, the impact of both covalence of the lanthanoid–ligand bond and varying bond distance on the magnitude of the intrinsic nonradiative decay rate between the excited ⁶P_5/2 and ⁶P_7/2 spin–orbit levels of Gd³⁺ is investigated in the chemically related compounds Y₂[B₂(SO₄)₆] and LaBO₃. Analysis of the temperature‐dependent luminescence spectra reveals that the intrinsic nonradiative transition rates between the excited ⁶P_J ( J = 5/2, 7/2) levels are of the order of only 10 ms⁻¹ (Y₂[B₂(SO₄)₆]:Gd³⁺: 8.9 ms⁻¹; LaBO₃:Gd³⁺: 10.5 ms⁻¹) and differ due to the different degree of covalence of the GdO bonds in the two compounds. Comparison to the established luminescent Boltzmann thermometer Er³⁺ reveals, however, that the nonradiative transition rates between the excited levels of Gd³⁺ are over three orders of magnitude slower despite a similar energy gap and the presence of a single resonant phonon mode. This hints to a fundamental magnetic dipolar character of the nonradiative coupling in Gd³⁺. These findings can pave a way to control nonradiative transition rates and how to tune the dynamic range of luminescent Boltzmann thermometers.

Crystalline orthorhombic rare earth carbonate hydroxides Ln[CO₃][OH] (Ln=La, Pr, Nd, Sm, Eu, Gd) were synthesised as phase pure powders via a simple hydrothermal reaction. CO₂ from air acted as natural carbonate source and cetyltrimethylammonium bromide was added as templating agent to an aqueous rare earth nitrate solution. Single-crystal X-ray structure determination was performed on La[CO₃][OH] (Pnma, a=7.4106(5), b=5.0502(3), c=8.5901(6) Å, 563 independent reflections, 38 refined parameters, wR2=0.037), Pr[CO₃][OH] (Pnma, a=7.2755(4), b=4.9918(3), c=8.5207(5) Å, 744 independent reflections, 38 refined parameters, wR2=0.04), Eu[CO₃][OH] (Pnma, a=7.1040(4), b=4.8940(3), c=8.4577(5) Å, 1649 independent reflections, 38 refined parameters, wR2=0.05) and Gd[CO₃][OH] (Pnma, a=7.069(7), b=4.874(5), c=8.464(9) Å, 431 independent reflections, 38 refined parameters, wR2=0.051). These findings are supported by powder XRD, infrared spectroscopy, UV/Vis spectroscopy and, for Pr[CO₃][OH] and Eu[CO₃][OH], by measurements of the non-linear optical properties. Thermal analysis could demonstrate the possible use of the Ln[CO₃][OH] phases as precursors for rare earth carbonate dioxides Ln ₂[CO₃]O₂ and rare earth oxides Ln ₂O₃. The decomposition products inherit the precursor’s morphology. The lattice parameters of Pr₂[CO₃]O₂ were refined from high-temperature powder XRD data.

The phyllosilicate-analoguous borosulfate H ₂ [B ₂ (SO ₄ ) ₄ ] and its cadmium salt, Cd[B ₂ (SO ₄ ) ₄ ], have been synthesized via various paths and analyzed thoroughly. The heteropolyacid is the second structurally determined acid within borosulfate chemistry.