Structural Diversity of Molybdate Iodate and Fluoromolybdate: Syntheses, Structures, and Calculations on Na3(MoO4)(IO3) and Na3Cs(MoO2F4)2

Chromium(VI) Oxyfluoride Dianions, [Cr2 O4 F6 ]2- and [CrO2 F4 ]2- ; Syntheses and Structures of [XeF5 ]2 [Cr2 O4 F6 ], [XeF5 ]2 [Cr2 O4 F6 ] ⋅ nX (X=HF, n=4; X=XeOF4 , n=2), and [XeF5 ][Xe2 F11 ][CrO2 F4 ]

The fluorobasic character of the strong oxidative fluorinator, XeF6 , and the oxidative resistance of the [XeF5 ]+ and [Xe2 F11 ]+ cations have been exploited for the syntheses of several novel Cr(VI) dianion salts. The reactions of XeF6 and CrO2 F2 in anhydrous HF and by direct fusion of the reactants in melts have yielded the first dinuclear Cr(VI) oxyfluoro-dianion salts, [XeF5 ]2 [Cr2 O4 F6 ], [XeF5 ]2 [Cr2 O4 F6 ] ⋅ 4HF, [XeF5 ]2 [Cr2 O4 F6 ] ⋅ 2XeOF4 , and mononuclear Cr(VI) oxyfluoro-dianion salt, [XeF5 ][Xe2 F11 ][CrO2 F4 ]. The salts were structurally characterized by low-temperature (LT) single-crystal X-ray diffraction (SCXRD) and LT Raman spectroscopy. The [CrO2 F4 ]2- and [Cr2 O4 F6 ]2- dianions have distorted octahedral cis-dioxo Cr(VI) coordination spheres in which two F-atoms are trans to one another and two F-atoms are trans to O-atoms, where the [Cr2 O4 F6 ]2- dianion is the fluorine-bridged dimer of the [CrO2 F3 ]- anion. Quantum-chemical calculations were used to obtain the energy-minimized, gas-phase geometries, and the calculated vibrational spectra of the gas-phase dianions and their ion-pairs, which were used to aid in the vibrational frequency assignments of the crystalline salts. NBO and MEPS analyses and SCXRD show these salts are comprised of intimate ion-pairs in which their cations and anions interact through primarily electrostatic Xe- - -F σ-hole bonds.

A6Mo2P4O19 (A = Rb, Cs) and Rb2MoP2O9: new molybdophosphates with distinct polyanionic configurations

The exploration of A-Mo-P-O (A = Rb, Cs) systems has allowed several new Mo(V) phosphates, Rb₂MoP₂O₉, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉, to be synthesized through the spontaneous nucleation method. Single-crystal X-ray diffraction analysis reveals that the identical stoichiometry compounds Rb₂MoP₂O₉ and Cs₂MoP₂O₉ belong to different space groups C2/c and Pbca, respectively. Both compounds consist of dissimilar 1D [Mo-O-P]_∞ chains with different repeated building units, while monovalent cations fill in spaces to form 3D structures. However, Rb₆Mo₂P₄O₁₉ and Cs₆Mo₂P₄O₁₉ are isostructural and crystallize in the same space group of P2₁/c. They exhibit a 3D framework structure with 0D Mo₂O₅O₆P₄O₈ groups, which are separated by Rb/Cs atoms. Interestingly, structural relationships between the different monophosphates of the A-Mo-P-O (A = Rb, Cs) systems are presented in which distinct polyanionic configurations appear owing to the A/P ratios, as well as the size of the univalent cations. Further, detailed structural comparisons, optical properties and theoretical calculations are also discussed.

BaI3O9H: a new alkaline-earth metal hydroxy iodate with two groups

A new alkali metal hydroxyl iodate BaI3O9H with a three-dimensional network structure and a moderate birefringence of 0.073 at 1064 nm.

Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

A set of new triple molybdates, Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45, was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 with variation of the Li_xNa_1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho³⁺ = 0.05 and Yb³⁺ = 0.45 were obtained, and the effect of Li⁺ on their spectroscopic features was investigated. The crystal structures of Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I4₁/a by Rietveld analysis. Pure NaCaGd_0.5Ho_0.05Yb_0.45(MoO₄)₃ has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) ų, Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the ⁵S₂/⁵F₄ → ⁵I₈ green emission and the ⁵F₅ → ⁵I₈ red emission. The incorporated Li⁺ ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho³⁺ and Yb³⁺ ions, and they further affected the UC transition probabilities in triple molybdates Li_xNa_1-xCaGd_0.5(MoO₄)₃:Ho³⁺_0.05/Yb³⁺_0.45. The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of Li_xNa_1-xCaGd_0.5(MoO₄)₃ doped with Ho³⁺ and Yb³⁺ ions were totally superimposed with the luminescence signal of Ho³⁺ ions in the range of Mo–O stretching vibrations, and increasing the Li⁺ content resulted in a change in the Ho³⁺ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO₄)₃:Ho³⁺,Yb³⁺ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.

Intriguing Dimensional Transition Inducing Variable Birefringence in K2Na2Sn3S8 and Rb3NaSn3Se8

The birefringent crystals capable of modulating the polarization of lights are of the current research interests. Although many oxide crystals have been discovered and widely used in UV and visible regions, the birefringent crystals in the infrared (IR) region are still rare. Herein, two new chalcogenides, K₂Na₂Sn₃S₈ and Rb₃NaSn₃Se₈, have been synthesized by the solid-state method. We have used the single crystal X-ray diffraction to determine their structures. K₂Na₂Sn₃S₈ crystallizes in the monoclinic space group C2/c and exhibits a three-dimensional framework constructed by the corner-sharing SnS₄ and SnS₅ units, whereas Rb₃NaSn₃Se₈ crystallizes in the tetragonal space group P4/nbm and features a zero-dimensional [Sn₃Se₈]^4- trimer built by the three edge-sharing SnSe₄ tetrahedra. The physical property measurements indicate that Rb₃NaSn₃Se₈ has a wide IR transparent window up to 20 μm and large birefringence, ∼0.196, suggesting its potential application as a birefringent crystal in the IR region. However, compared with Rb₃NaSn₃Se₈, the birefringence of K₂Na₂Sn₃S₈ is relatively small, ∼0.070. The study of their structure-property relationship indicates that the different connection modes of SnQ_n (Q = S, Se; n = 4, 5) polyhedra are the main reason for the large difference of birefringence between the two compounds. These studies will provide a new insight for the origin of birefringence and will facilitate the exploration of new IR birefringent crystals.

Influence of structure-directing polyhedra and heterocyclic ligands on the chain structures and O/F ordering in a series of zinc vanadium oxyfluorides

Transition metal oxyfluorides composed of distinctive structure-directing vanadium oxyfluoride polyhedra and highly polarizable Zn²⁺ cations were synthesized by mild hydrothermal reactions.