Synthesis-Controlled Polymorphism and Optical Properties of Phyllosilicate-Analogous Borosulfates M[B2 (SO4 )4 ] (M=Mg, Co)

Photoluminescence of Mn2+ in the Borosulfate Zn[B2 (SO4 )4 ] : Mn2+ -A Tool to Detect Weak Coordination Behavior of Ligands

The impact of the surrounding ligand field is successfully exploited in the case of Eu2+ to tune the emission characteristics of inorganic photoactive materials with potential application in, e.g., phosphor-converted white light-emitting diodes (pc-wLEDs). However, the photoluminescence of Mn2+ related to intraconfigurational 3d5 -3d5 transitions is also strongly dependent on local ligand field effects and has been underestimated in this regard so far. In this work, we want to revive the idea how to electronically tune the emission color of a transition metal ion in inorganic hosts by unusual electronic effects in the metal-ligand bond. The concept is explicitly demonstrated for the weakly coordinating layer-like borosulfate ligand in the Mn2+ -containing solid solutions Zn1-x Mnx [B2 (SO4 )4 ] (x = 0, 0.03, 0.04, 0.05, 0.10). Zn[B2 (SO4 )4 ]:Mn2+ shows orange narrow-band luminescence at 590 nm, which is an unusually short wavelength for octahedrally coordinated Mn2+ and indicates an uncommonly weak ligand field. On the other hand, the analysis of the interelectronic Racah repulsion parameters reveals ionic Mn-O bonds with values close to the Racah parameters of the free Mn2+ ion. Overall, this strategy demonstrates that electronic control of the metal-ligand bond can be a tool to make Mn2+ a potent alternative emitter to Eu2+ for inorganic phosphors.

Borate-Based Compounds as Mixed Polyanion Cathode Materials for Advanced Batteries

Rational design of new and cost-effective advanced batteries for the intended scale of application is concurrent with cathode materials development. Foundational knowledge of cathode materials’ processing−structure−properties−performance relationship is integral. In this review, we provide an overview of borate-based compounds as possible mixed polyanion cathode materials in organic electrolyte metal-ion batteries. A recapitulation of lithium-ion battery (LIB) cathode materials development provides that rationale. The combined method of data mining and high-throughput ab initio computing was briefly discussed to derive how carbonate-based compounds in sidorenkite structure were suggested. Borate-based compounds, albeit just close to stability (viz., <30 meV at−1), offer tunability and versatility and hence, potential effectivity as polyanion cathodes due to (1) diverse structures which can host alkali metal intercalation; (2) the low weight of borate relative to mature polyanion families which can translate to higher theoretical capacity; and a (3) rich chemistry which can alter the inductive effect on earth-abundant transition metals (e.g., Ni and Fe), potentially improving the open-circuit voltage (OCV) of the cell. This review paper provides a reference on the structures, properties, and synthesis routes of known borate-based compounds [viz., borophosphate (BPO), borosilicate (BSiO), and borosulfate (BSO)], as these borate-based compounds are untapped despite their potential for mixed polyanion cathode materials for advanced batteries.

Cd[B2(SO4)4] and H2[B2(SO4)4] - a phyllosilicate-analogous borosulfate and its homeotypic heteropolyacid

Borosulfates consist of heteropolyanionic networks of corner-shared (SO₄)- and (BO₄)-tetrahedra charge compensated by metal or non-metal cations. The anionic substructures differ significantly, depending on the different branching of the silicate-analogous borosulfate building blocks. However, only one acid has been characterized by single crystal X-ray diffraction so far. Herein, we present H₂[B₂(SO₄)₄] as the first phyllosilicate analogue representative, together with the homeotypic representative Cd[B₂(SO₄)₄]. The latter can be considered the cadmium salt of the former. Their crystal structures and crystallographic relationship are elucidated. For H₂[B₂(SO₄)₄], the bonding situation is examined using Hirshfeld-surface analysis. Further, the optical and thermal properties of Cd[B₂(SO₄)₄] are investigated by FTIR and UV-Vis spectroscopy, thermogravimetry, as well as temperature-programmed powder X-ray diffraction.

New alkaline-earth amidosulfates and their unexpected decomposition to S4N4

The amidosulphates Mg(NH₂SO₃)₂·4H₂O (P2₁/c), Mg(NH₂SO₃)₂·3H₂O (P1̄), Ca(NH₂SO₃)₂·4H₂O (C2/c), Ca(NH₂SO₃)₂·H₂O (P2₁2₁2₁), Sr(NH₂SO₃)₂·4H₂O (C2/c), Sr(NH₂SO₃)₂·H₂O (P2₁/c) and Ba(NH₂SO₃)₂ (Pna2₁) could be obtained as cm-sized crystals from aqueous solutions of the corresponding metal carbonates, hydroxides and amidosulphonic acid, respectively, by careful control of the crystallisation conditions. β-Sr(NH₂SO₃)₂ (Pc) and α-Sr(NH₂SO₃)₂ (P2₁) could be obtained by careful thermal dehydration of Sr(NH₂SO₃)₂·H₂O. Their crystal structures were determined by single-crystal XRD and revealed a rich structural diversity with a significant tendency to form non-centrosymmetric crystals. The compounds were characterised by powder XRD, FT-IR, Raman and UV/vis spectroscopy and thermogravimetry. Temperature programmed single-crystal XRD, powder XRD and Raman spectroscopy, as well as DFT calculations were employed to aid the interpretation of vibrational and thermal properties. For the first time, SHG measurements were performed on metal amidosulphates, revealing the SHG intensities of β-Sr(NH₂SO₃)₂ and Ba(NH₂SO₃)₂ that were comparable to quartz and KDP. Thermal decomposition was additionally studied by the preparation of reaction intermediates, serendipitously revealing the formation of S₄N₄ as the decomposition product. This unprecedented reaction represents the first sulphur nitride synthesis process that neither employs a sulphur halide nor elemental sulphur.

Polycations Stabilised by Borosulfates: [Au 3 Cl 4 ][B(S 2 O 7 ) 2 ] and the One‐Dimensional Metal [Au 2 Cl 4 ][B(S 2 O 7 ) 2 ](SO 3 )

(SO₄)‐rich silicate analogue borosulfates are able to stabilise cationic cluster‐like and chain‐like aggregates. Single crystals of [Au₃Cl₄][B(S₂O₇)₂] and [Au₂Cl₄][B(S₂O₇)₂](SO₃) were obtained by solvothermal reaction with SO₃, and the electronic properties were investigated by means of density functional theory–based calculations. [Au₃Cl₄][B(S₂O₇)₂] exhibits a cluster‐like cation, and the cationic gold‐chloride strands in [Au₂Cl₄][B(S₂O₇)₂](SO₃) are found to resemble one‐dimensional metallic wires. This is confirmed by polarisation microscopy.

The Role of the Bi3+ Lone Pair Effect in Bi(H3O)(SO4)2, Bi(HSO4)3, and Bi2(SO4)3

Three new members in the Bi₂O₃-SO₃-H₂O system are identified by single crystal X-ray diffraction and Rietveld refinement after a fundamental examination of this phase space. Bi(H₃O)(SO₄)₂ crystallizes in space group P2₁/c (no. 14, a = 1203.5(4), b = 682.9(2), c = 821.2(2) pm, β = 102.99(1)°, 861 independent reflections, 88 refined parameters, wR₂ = 0.14) homeotypic with Nd(H₃O)(SO₄)₂ featuring edge-sharing BiO₉ polyhedra. Bi(HSO₄)₃ crystallizes in a new structure type in space group P1 (no. 2, a = 492.04(7), b = 910.8(1), c = 1040.8(2) pm, α = 85.443(5)°, β = 86.897(5)°, γ = 74.542(4)°, 3227 independent reflections, 154 refined parameters, wR₂ = 0.05) comprising dimers of edge-sharing BiO₈ polyhedra. For Bi₂(SO₄)₃, a new modification crystallizing in space group P2₁/n (no. 14, a = 1308.03(7), b = 473.25(3), c = 1452.61(8) pm, β = 100.886(2)°, 3189 independent reflections, 155 refined parameters, wR₂ = 0.03) isotypic to Sb₂(SO₄)₃ with noncondensed BiO₇ polyhedra is presented. The role of the Bi³⁺ lone pair effect as elucidated by density functional theory (DFT) calculations is discussed for all three compounds with respect to their structural and optical properties. Additionally, the Bi³⁺ lone pair activity is compared to the recently reported borosulfates Bi(H₃O)[B(SO₄)₂]₄ and Bi₂[B₂(SO₄)₆]. Geometrical calculations based on structural data are correlated with electron localization function (ELF) calculations to establish the origin of the direction and strength of the lone pair stereoactivity of Bi³⁺ in oxidic compounds. Finally, the thermal properties of the three compounds are reported.

Polymorphism and optical, magnetic and thermal properties of the either phyllo- or inosilicate-analogous borosulfate Cu[B2(SO4)4]

Two polymorphs of the borosulfate Cu[B₂(SO₄)₄] can be selectively prepared by solvothermal syntheses. The crystal structures of inosilicate-analogous α-Cu[B₂(SO₄)₄] (P1̄, no. 2, a = 5.2636(2), b = 7.1449(2), c = 7.9352(2) Å, α = 73.698(2)°, β = 70.737(2)°, γ = 86.677(2)°, 65 parameters, R_Bragg = 0.0052) and the new phyllosilicate-analogous polymorph β-Cu[B₂(SO₄)₄] (P2₁/n, no. 14, a = 7.712(3), b = 8.149(3), c = 9.092(3) Å, β = 111.22(1)°, 3829 independent reflections, 106 parameters, wR₂ = 0.054) are discussed. Further, the optical, magnetic and thermal properties of both polymorphs are investigated with focus on the role of the Cu²⁺ cation and its Jahn-Teller effect. The findings are confirmed by DFT calculations yielding insights in the stability of the synthesised polymorphs as well as a predicted γ-modification. Additionally, the crystal structures of two polymorphs of copper hydrogensulfate Cu(HSO₄)₂-I (P2₁/n, no. 14, a = 4.7530(2), b = 8.5325(4), c = 7.3719(3) Å, β = 100.063(1)°, 1063 independent reflections, 55 parameters, wR₂ = 0.052) and Cu(HSO₄)₂-II (P1̄, no. 2, a = 4.79.88(8), b = 7.857(1), c = 8.057(1) Å, α = 77.86(1)°, β = 87.02(1)°, γ = 89.82(1)°, 1044 independent reflections, 109 parameters, wR₂ = 0.132) as well as that of Cu[S₂O₇] (C2/c, no. 15, a = 6.6341(4), b = 8.7302(5), c = 9.0555(8) Å, β = 104.763(3)°, 1117 independent reflections, 48 parameters, wR₂ = 0.049) are presented and the cyclosilicate-analogous borosulfate Cu[B(SO₄)₂(HSO₄)] is fully characterised with respect to its optical and thermal properties.

Ba2B5O8(OH)2(NO3)·3H2O: the design of an alkaline earth metal borate-nitrate optimized from a hydroxylic borate

The first alkaline earth metal borate-nitrate, namely Ba₂B₅O₈(OH)₂(NO₃)·3H₂O (BBNOH), has been synthesized by the hydrothermal method. BBNOH crystallizes in the space group of P2₁/c and shows two-dimensional (2D) ²_∞[B₅O₈(OH)₂]^3- borate anion layers, and the hydrated barium cations and the [NO₃]^- anions are located between the layers. The process of optimizing the structure of Ba₂B₅O₈(OH)₂OH to BBNOH has been discussed. The first principles calculation has been used to calculate the birefringence of Ba₂B₅O₈(OH)₂(NO₃)·3H₂O, and the value is 0.033@1064 nm, which is mainly originated from the borate anions and the π conjugated [NO₃]^- anions.

Triple-Vertex Linkage of (BO4 )-Tetrahedra in a Borosulfate: Synthesis, Crystal Structure, and Quantum-Chemical Investigation of Sr[B3 O(SO4 )4 (SO4 H)]

Borosulfates are classified as silicate analogue materials. The number of crystallographically characterized compounds is still limited, whereas the structural diversity is already impressive. The anionic substructures of borosulfates exhibit vertex-connected (BO₄ )- and (SO₄ )-tetrahedra, whereas bridging between two (SO₄ )- or even between two (BO₄ )-tetrahedra is scarce. The herein presented compound Sr[B₃ O(SO₄ )₄ (SO₄ H)] is the first borosulfate with a triple-vertex linkage of three (BO₄ ) tetrahedra via one common oxygen atom. DFT calculations complement the experimental studies. Bader charges (calculated for all atoms) as well as charge-density calculations give hint to the electron distribution within the anionic substructure and density-of-states calculations support the interpretation of the bonding situation.

Strong Lewis and Brønsted Acidic Sites in the Borosulfate Mg3 [H2 O→B(SO4 )3 ]2

Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg3 [H2 O→B(SO4 )3 ]2 is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid-base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF3 and B(C6 F5 )3 revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Brønsted acidity than comparable silicates or phosphates. Additionally, Mg3 [H2 O→B(SO4 )3 ]2 was characterised by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations.

The First Bismuth Borosulfates Comprising Oxonium and a Tectosilicate-Analogous Anion

The first bismuth borosulfate (H3 O)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 H2 O 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 4 ‾ (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 H3 O+ 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.

Synthesis-Controlled Polymorphism and Optical Properties of Phyllosilicate-Analogous Borosulfates M[B2 (SO4 )4 ] (M=Mg, Co)

Increased synthetic control in borosulfate chemistry leads to the access of various new compounds. Herein, the polymorphism of phyllosilicate-analogous borosulfates is unraveled by adjusting the oleum (65 % SO3 ) content. The new polymorphs β-Mg[B2 (SO4 )4 ] and α-Co[B2 (SO4 )4 ] both consist of similar layers of alternating borate and sulfate tetrahedra, but differ in the position of octahedrally coordinated cations. The α-modification comprises cations between the layers, whereas in the β-modification cations are embedded within the layers. With this new synthetic approach, phase-pure compounds of the respective polymorphs α-Mg[B2 (SO4 )4 ] and β-Co[B2 (SO4 )4 ] were also achieved. Tanabe-Sugano analysis of the Co2+ polymorphs reveal weak ligand field splitting and give insights into the coordination behavior of the two-dimensional borosulfate anions for the first time. DFT calculations confirmed previous in silico experiments and enabled an assignment of the polymorphs by comparing the total electronic energies. The compounds are characterized by single-crystal XRD, PXRD, FTIR, and UV/Vis/NIR spectroscopy, thermogravimetric analysis (TGA), and density functional theory (DFT) calculations.