Christian A. Juillerat

Single crystals of CsGa ₇ O ₁₁ , RbGa ₇ O ₁₁ , and RbGa ₄ In ₅ O ₁₄ were grown from alkali halide melts and their structures were characterized by single crystal and powder X‐ray diffraction. CsGa ₇ O ₁₁ and RbGa ₇ O ₁₁ adopt the same structure type, reminiscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge‐sharing GaO ₆ octahedra, and two corner‐sharing octahedron/tetrahedron pairs. The structure of RbGa ₄ In ₅ O ₁₄ is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetrahedra, and contains similar sized tunnels as CsGa ₇ O ₁₁ and RbGa ₇ O ₁₁ . CsGa ₇ O ₁₁ and RbGa ₄ In ₅ O ₁₄ were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa ₇ O ₁₁ decomposes to GaO(OH) xH ₂ O when heated in warm aqueous solutions. CsGa ₇ O ₁₁ undergoes ion exchange in both an aqueous solution of RbCl and a RbNO ₃ melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO ₃ melt.

The flux synthesis of a uranium gallophosphate and a uranium gallate, Cs₄[UO₂Ga₂(PO₄)₄] and Cs₂UO₂Ga₂O₅, and 4 uranium phosphates, [Rb₂Rb_3.93Cl_0.93][(UO₂)₅(PO₄)₅], Rb₁₁[(UO₂)₈(PO₄)₉], Rb_7.6[(UO₂)₈O_8.6F_0.4(PO₄)₂], and Rb₆[(UO₂)₅O₂(PO₄)₄], is reported.

CaCrO₃ synthesized by high pressure methods is topochemically treated with polyvinylidene difluoride and undergoes stepwise reduction followed by fluorine incorporation.

The novel phases Rb₄NaU₃O_12−xF_x (1), K₄NaU₃O_12−xF_x (2), and Rb_2.1K_1.9KU₃O_12−xF_x (3) were synthesized by molten flux methods using mixed alkali fluoride melts. The oxyfluorides crystallize in the cubic space group Imm with a lattice parameters of 8.7472(2) Å, 8.6264(2) Å, and 8.8390(3) Å, respectively. All three structures crystallize in a cubic perovskite structure, ABO₃ (A₄BB′₃O₁₂), where the A site is fully occupied by an alkali cation, and the B site is shared by the remaining smaller alkali cation and uranium in an ordered fashion such that the alkali cation on the B site is surrounded by square uranyl bipyramids. The structures were characterized by single crystal X-ray diffraction, energy dispersive spectroscopy, X-ray absorption near edge structure spectroscopy, X-ray photoelectron spectroscopy, magnetic susceptibility measurements, DFT calculations, thermogravimetric analysis, and UV-vis spectroscopy, all of which support the presence of U(V) in the three new materials.

The flux synthesis, solid state synthesis, and characterization of a new aluminate, Cs₂(UO₂)Al₂O₅, are reported. Cs₂(UO₂)Al₂O₅ crystallizes in the tetragonal space group I4₁/amd with lattice parameters a = 7.3254(2) and c = 30.9849(7) and is constructed from edge-sharing chains of UO₇ pentagonal bipyramids that are connected to [Al₂O₅]^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, [Cs₁₃Cl₅][(UO₂)₃Al₂O(PO₄)₆], Rb₇[Al₂O(PO₄)₃][(UO₂)₆O₄(PO₄)₂], Cs₃[Al₂O(PO₄)₃][(UO₂)₃O₂], and Rb₃[Al₂O(PO₄)₃][(UO₂)₃O₂], the first uranyl phosphate salt-inclusion material [Cs₄Cs₄Cl][(UO₂)₄(PO₄)₅], and a related structure Cs₄[UO₂Al₂(PO₄)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, [Al₂O(PO₄)₆]^14–, that can be easily obtained and employed for the construction of new porous structures.