Evidence and Practical Applications of Site Occupancy Theory (SOT) of Eu3+ in Scheelite Compounds

The determination of the site occupancy of activators in phosphors is essential for precise synthesis, understanding the relationship between their luminescence properties and crystal structure, and tailoring their properties by modifying the host composition. Herein, one simple method was proposed to help determine the sites at which the doping of rare earth ions or transition metal ions occupies in the host lattice through site occupancy theory (SOT) for ions doped into the matrix lattice. SOT was established based on the fact that doping ions preferentially occupy the sites with the lowest bonding energy deviations. In order to provide detailed experimental evidence to prove the feasibility of SOT, several scheelite-type compounds were successfully synthesized using a high-temperature solid-phase method. When Eu3+ ions occupy a similar surrounding environment site, the photoluminescence spectra of the activators Eu3+ are similar. Therefore, by comparing the intensity ratio of photoluminescence spectra and the mechanism of all transitions of KEu(WO4)2, KY(WO4)2:Eu3+, Na5Eu(WO4)4, and Na5Y(WO4)4:Eu3+, it was proved that SOT can successfully confirm the site occupation when doped ions enter the matrix lattice. SOT was further applied to the sites occupied by Eu3+ ion-doped LiAl(MoO4)2 and LiLu(MoO4)2.

Ba2[WO3F(IO3)][WO3F2]: the first polar fluorinated tungsten iodate featuring a direct W-O-I bond

The introduction of the transition metal cations with d⁰ electron configurations and F in the iodate systems generates a new polar compound, Ba₂[WO₃F(IO₃)][WO₃F₂], which features the first example of a direct W-O-I bond in the structure. It exhibits excellent properties, including a large second harmonic generation response (∼3.5 × KH₂PO₄), a wide visible and mid-infrared transparency region (0.28-10.74 μm), and a moderate birefringence of 0.061@532 nm.

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.

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.