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In this review we discuss general trends in the use of alkali bromide and iodide (ABI) fluxes for exploratory crystal growth. The ABI fluxes are ionic solution fluxes at moderate to high temperatures, 207 to ~1,300°C, which offer a good degree of flexibility in the selection of the temperature profile and solubility. Although their main use is to dissolve and recrystallize "soft" species such as chalcogenides, many compositions with "hard" anions, including oxides and nitrides, have been obtained from the ABI fluxes, highlighting their unique versatility. ABI fluxes can serve to provide a reaction and crystallization medium for different types of starting materials, mostly the elemental and binary compounds. As the use of alkali halide fluxes creates an excess of the alkali cations, these fluxes are often reactive, incorporating one of its components to the final compositions, although some examples of non-reactive ABI fluxes are known., (Copyright © 2020 Klepov, Juillerat, Pace, Morrison and zur Loye.)

The title salt, (PMIM) 2 [Ni(P 2 S 8 ) 2 ] (PMIM = 3-methyl-1-propyl-1 H -imidazol-3-ium, C 7 H 13 N 2 + ), consists of a nickel-thio-phosphate anion charge-balanced by a pair of crystallographically independent PMIM cations. It crystallizes in the monoclinic space group P 2 1 / n . The structure exhibits the known [Ni(P 2 S 8 ) 2 ] 2- anion with two unique imidazolium cations in the asymmetric unit. Whereas one PMIM cation is well ordered, the other is disordered over two orientations with refined occupancies of 0.798 (2) and 0.202 (2). The salt was prepared directly from the elements in the ionic liquid [PMIM]CF 3 SO 3 . Whereas one of the PMIM cations is well behaved (it does not exhibit disorder even in the propyl side chain), the other is found in two overlapping positions. The refined occupancies for the two orientations are roughly 80:20. Here, too, there appears to be little disorder in the propyl arm., (© Dalecky et al. 2020.)

Topochemical reactions between CaCrO3 and polyvinylidene difluoride yield the new fluorinated phase CaCrO2.5F0.5, which was characterized by powder synchrotron X-ray diffraction, X-ray photoemission spectroscopy, and magnetic susceptibility measurements. The reaction proceeds via reduced oxide intermediates, CaCrO2.67 and CaCrO2.5, in which CrO6 octahedral and CrO4 tetrahedral layers are stacked in a different manner along the c axis of CaCrO3. These two intermediate phases can be selectively synthesized by the carbothermal reduction with g-C3N4. Both CaCrO3 and CaCrO2.5F0.5 adopt the same orthorhombic space group, Pbnm; however, the fluorinated phase has decreased Cr-O-Cr bond angles as compared to the parent compound in both the ab plane and along the c-direction, which indicates an increased orthorhombic distortion due to the fluorination. While the oxygen vacancies are ordered in both intermediate phases, CaCrO2.67 and CaCrO2.5, a site preference for fluorine in the oxyfluoride phase cannot be confirmed. CaCrO3 and CaCrO2.5F0.5 undergo antiferromagnetic phase transitions involving spin canting, where the fluorination causes the transition temperature to increase from 90 K to 110 K, as a result of the competition between the increased octahedral tilting and the enhancement of superexchange interactions involving Cr3+ ions in the CaCrO2.5F0.5 structure.

Single crystals of four new layered uranyl phosphates, including three oxyfluoride-phosphates, were synthesized by molten flux methods using alkali chloride melts, and their structures were determined by single-crystal X-ray diffraction. Cs 11 [(UO 2 ) 12 (PO 4 ) 3 O 13 ] ( 1 ) and Rb 11 [UO 2 ) 12 (PO 4 ) 3 O 12 F 2 ] ( 2 ) contain uranyl phosphate layers exhibiting a new sheet topology that can be related to that of β-U 3 O 8 , while Cs 4.4 K 0.6 [(UO 2 ) 6 O 4 F(PO 4 ) 4 (UO 2 )] ( 3 ) and Rb 4.4 K 0.6 [(UO 2 ) 6 O 4 F(PO 4 ) 4 (UO 2 )] ( 4 ) contain layers of a known isomer of the prominent phosphuranylite topology. The location of the fluorine in structures 2 - 4 is discussed using bond valence sums. First principles calculations were used to explore why a pure oxide structure is obtained for the Cs containing phase ( 1 ) and in contrast an oxyfluoride phase for the Rb containing phase ( 2 ). Ion exchange experiments were performed on 1 and 2 and demonstrate the ability of these structures to exchange approximately half of the parent alkali cation with a target alkali cation in an aqueous concentrated salt solution. Optical measurements were performed on 1 and 2 and the UV-vis and fluorescence spectra show features characteristic of the UO 2 2 + uranyl group.

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs 2 (UO 2 )Al 2 O 5 , are reported. Cs 2 (UO 2 )Al 2 O 5 crystallizes in the tetragonal space group I4 1 / amd with lattice parameters a = 7.3254(2) and c = 30.9849(7) and is constructed from edge-sharing chains of UO 7 pentagonal bipyramids that are connected to [Al 2 O 5 ] 4- two-dimensional sheets. The cesium cations, which are heavily disordered, occupy small channels in the a and b directions in the framework structure. The optical properties and ion exchange behaviors are reported along with DFT calculations that support the observed results of the ion exchange experiments.

This frontier article focuses on the use of flux crystal growth for the preparation of new actinide containing materials, reviews the history of flux crystal growth of uranium containing phases, and highlights the recent advances in the field. Specifically, we discuss how recent developments in f-element materials, fueled by accelerated materials discovery via crystal growth, have led to the synthesis and characterization of new families of complex uranium containing oxides, namely alkali/alkaline uranates, oxychlorides, oxychalcogenides, tellurites, molybdates, tungstates, chromates, phosphates, arsenates, vanadates, niobates, silicates, germanates, and borates. An overview of flux crystal growth is presented and specific crystal growth approaches are described with an emphasis on how and why they - versus some other method - are used and how they enable the preparation of specific classes of new materials.

The synthesis of four non-Löwenstein uranyl aluminophosphates, [Cs 13 Cl 5 ][(UO 2 ) 3 Al 2 O(PO 4 ) 6 ], Rb 7 [Al 2 O(PO 4 ) 3 ][(UO 2 ) 6 O 4 (PO 4 ) 2 ], Cs 3 [Al 2 O(PO 4 ) 3 ][(UO 2 ) 3 O 2 ], and Rb 3 [Al 2 O(PO 4 ) 3 ][(UO 2 ) 3 O 2 ], the first uranyl phosphate salt-inclusion material [Cs 4 Cs 4 Cl][(UO 2 ) 4 (PO 4 ) 5 ], and a related structure Cs 4 [UO 2 Al 2 (PO 4 ) 4 ], all prepared by molten flux methods, is reported. All compounds are discussed from the point of view of their structural features favoring, in some cases, ion-exchange properties. Löwenstein's rule, well known in the realm of zeolites, aluminosilicate, and aluminophosphate minerals, describes the tendency of tetrahedra (Al, P, Si, and Ge) linked by an oxygen bridge to be of two different elements resulting in the avoidance of Al-O-Al bonds. Zeolites and related aluminosilicate/aluminophosphate minerals are traditionally formed under relatively mild temperatures, where zeolites are synthesized using the hydrothermal synthetic technique. Few exceptions to Löwenstein's rule are known among aluminophosphates, and four of the five exceptions are synthesized under either high temperature or high pressure methods. For that reason, the high-temperature flux synthesis of four new non-Löwenstein uranyl aluminophosphates realizes a unique synthetic approach to forming the new pyroaluminate-based building block, [Al 2 O(PO 4 ) 6 ] 14- , that can be easily obtained and employed for the construction of new porous structures.

Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.

Single crystals of 13 new uranyl germanate salt-inclusion materials were grown from alkali halide fluxes: [Cs 2 Cs 5 F][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (1), [Cs 6 Ag 2 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (2), [Cs 6 Ag 0.3 Na 1.7 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (3), [Cs 6 Ag 0.4 Na 1.6 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (4), [Cs 6 K 2 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (5), [Cs 6 K 1.9 Ag 0.1 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (6), [KK 6 Cl][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (7), [KK 6 Br 0.6 F 0.4 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (8), [Na 0.9 Rb 6.1 F][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (9), [K 0.6 Na 0.4 K 5 CsCl 0.5 F 0.5 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (10), [K 0.8 Na 0.2 K 4.8 Cs 1.2 Cl 0.5 F 0.5 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (11), [KK 1.8 Cs 4.2 F][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (12), and [Cs 6 Cs 0.71 Cl 0.71 ][(UO 2 ) 3 O 3 (Ge 2 O 7 )] (13). Structures 1-12 contain the same [(UO 2 ) 3 (Ge 2 O 7 ) 2 ] 6- framework whose pores are filled with varied salt species selected by the choice of the specific alkali halide flux used for crystal growth. The size and identity of the salt species also influence whether the [(UO 2 ) 3 (Ge 2 O 7 ) 2 ] 6- framework adopts a monoclinic or orthorhombic symmetry. The 13th composition, [Cs 6 Cs 0.71 Cl 0.71 ][(UO 2 ) 3 O 3 (Ge 2 O 7 )] (13), crystallizes in a new structure type in the hexagonal crystal system and contains large channels. Optical characterization was performed on [Cs 6 K 1.9 Ag 0.1 Cl 2 ][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (6) and [KK 1.8 Cs 4.2 F][(UO 2 ) 3 (Ge 2 O 7 ) 2 ] (12), and both exhibit UV-vis absorption and luminescence typical of the uranyl group. The fluorine-containing composition luminesces 10 times as intensely as does the chlorine-containing composition.

Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Cs 0.7 K 3.3 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], K 4 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], K 2.9 Na 0.9 Rb 0.2 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], K 2.1 Na 0.7 Rb 1.2 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Cs 1.7 K 4.3 [(UO 2 ) 5 O 5 (PO 4 ) 2 ], and Rb 1.6 K 4.4 [(UO 2 ) 5 O 5 (PO 4 ) 2 ]. All structures crystallize in the monoclinic space group, P2 1 / c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on Cs 0.7 K 3.3 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], and K 4 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] demonstrated that Cs and Rb cations cannot be exchanged for K cations; however, K cations can be readily exchanged for Na, Rb, and Cs. Enthalpies of formation were calculated from density functional theory (DFT) and volume-based thermodynamics (VBT) for all six structures. A value for the enthalpy of formation of the phosphuranylite sheet, [(UO 2 ) 3 O 2 (PO 4 ) 2 ] 4- , was derived using single-ion additive methods coupled with VBT. DFT and VBT calculations were used to justify results of the ion exchange experiments. Cs 0.7 K 3.3 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], and K 4 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] exhibit typical luminescence of the uranyl group.

Single crystals of two new uranyl phosphates, A 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] (A = Cs, Rb), featuring cation-cation interactions (CCIs) rarely observed in uranium(VI) compounds were synthesized by molten flux methods. This structure crystallizes in the triclinic space group P1̅ with lattice parameters, a = 9.2092(4) Å, b = 9.8405(4) Å, c = 10.1856(5) Å, α = 92.876(2)°, β = 95.675(2)°, and γ = 93.139(2)° for A = Cs and a = 9.2166(9) Å, b = 9.3771(10) Å, c = 10.1210(11) Å, α = 89.981(4)°, β = 96.136(4)°, and γ = 92.790(4)° for A = Rb. The optical properties are reported for both compounds and compared to a layered uranyl phosphate, K 4 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], having a similar phosphuranylite-based structure but no CCIs. Partial ion exchange of Cs and Rb cations into the Rb 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] and Cs 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] structures, respectively, was achieved.