Trivalent Cation-Controlled Phase Space of New U(IV) Fluorides, Na₃MU₆F₃₀ (M = Al³⁺, Ga³⁺, Ti³⁺, V³⁺, Cr³⁺, Fe³⁺): Mild Hydrothermal Synthesis Including an in Situ Reduction Step, Structures, Optical, and Magnetic Properties

Abstract

A series of new, complex U(IV) fluorides, namely, Na3MU6F30 (M = Al(3+), Ga(3+), Ti(3+), V(3+), Cr(3+), and Fe(3+)), containing trivalent transition- and main-group metal cations were synthesized via an in situ reduction step of U(VI) to U(IV). Single crystals of the series were grown in high yield under mild hydrothermal conditions and were characterized by single-crystal X-ray diffraction. The reported compounds crystallize in the trigonal space group P3̅c1 and exhibit complex crystal structures with a three-dimensional (3-D) framework composed of corner- and edge-shared UF9 polyhedra. The arrangement of U2F16 dimers forms two types of hexagonal channels, where MF6 polyhedra and sodium atoms are located. The most interesting structural feature is the presence of the 3-D framework that can accommodate various transition-metal ions in low oxidation states, indicating that the framework acts as an excellent host. Trivalent transition metal ions, even reduced Ti(3+) and V(3+), were stabilized by both the rigid framework and by our synthetic conditions. Utilizing ionic radii of transition metal ions, a phase boundary was investigated, suggesting that there exists a size limit for the M site in the crystal structure. The valence state of uranium was studied by U 4f X-ray photoelectron spectroscopy, which confirmed the presence of U(4+). Temperature-dependent magnetic susceptibility measurements yielded effective magnetic moments of 3.50 and 3.35 μB for Na3MU6F30 (M = Al(3+) and Ga(3+)), respectively. For the other compounds, combined effective magnetic moments of 8.93, 9.09, 9.18, and 10.39 μB were obtained for Ti, V, Cr, and Fe members, respectively. In all cases, large negative Weiss constants were observed, which are indicative of the existence of a spin gap in U(4+). Field-dependent magnetic property measurements at 2 K for Na3FeU6F30 demonstrated that U(4+) attains a nonmagnetic singlet ground state at low temperature. Optical and thermal properties were measured and are reported.

Expansion of the Na3MIII(Ln/An)6F30 Series: Incorporation of Plutonium into a Highly Robust and Stable Framework

“…The composition and the oxidation states (by charge balance and bond distances) of the elements in Na 3 MPu 6 F 30 (M=Al 3+ , Fe 3+ , Co 3+ ) and Na 2.4 Mn 1.6 Pu 6 F 30 compounds were determined using single crystal X‐ray diffraction. All compounds crystallize in the trigonal space group P c 1 and are isostructural to the Ce, Th, and U phases described in detail previously . All atoms in the structures of Na 3 MPu 6 F 30 (M=Al 3+ , Fe 3+ , Co 3+ ) are fully occupied and ordered.…”

“…We have extensively studied the Na n M(Ln/An) 6 F 30 series and have previously identified phase boundaries for the formation of Na 4 M II (Ln/An) 6 F 30 versus Na 3 M III (Ln/An) 6 F 30 on the basis of M 2+ /M 3+ and Ln 4+ /An 4+ sizes . It was determined that compositions containing the larger Th 4+ and U 4+ cations can accommodate a variety of di‐ and trivalent metal cations ranging in size from Al 3+ (Shannon radius of six‐coordinated Al 3+ , 0.675 Å) to Mn 2+ (0.97 Å), while compositions containing the smaller Ce 4+ cation are limited to the incorporation of only the smaller trivalent metal cations (Al 3+ , 0.675 Å; Ga 3+ , 0.760 Å; Fe 3+ , 0.785 Å; Cr 3+ , 0.755 Å).…”

“…We have extensively studied the Na n M(Ln/An) 6 F 30 series and have previously identified phaseb oundaries for the formation of Na 4 M II (Ln/An) 6 F 30 versus Na 3 M III (Ln/An) 6 F 30 on the basis of M 2 + /M 3 + and Ln 4 + /An 4 + sizes. [17][18][19][20] It was determined that compositions containing the larger Th 4 + and U 4 + cations can accommodate av arietyo fd i-and trivalent metal cationsr anging in size from Al 3 + (Shannon radius of six-coordinated Al 3 + , 0.675 )t oM n 2 + (0.97 ), [21] while compositions containing the smaller Ce 4 + cation are limited to the incorporation of only the smaller trivalent metal cations (Al 3 + ,0 .675 ;G a 3 + ,0 .760 ; Fe 3 + ,0 .785 ;C r 3 + ,0 .755 ). Given the similarityi ns ize between Ce 4 + and Pu 4 + we expectedt he plutoniums eries to exhibit at rend similart ot he one found for Ce 4 + ;h owever,d ifferences in their redox behavior,afactor which hadn ot seemed to play as ignificant role in the formation of the other members of the Na n M(Ln/An) 6 F 30 series, gave rise to notable differences between the cerium and plutonium systems.…”

“…All compounds crystallize in the trigonal space group P3 ¯c1a nd are isostructural to the Ce, Th, and Up hases described in detail previously. [17][18][19][20] All atoms in the structures of Na 3 MPu 6 F 30 (M = Al 3 + , Fe 3 + ,C o 3 + )a re fully occupied and ordered. As shown in Figure 2, the three-dimensional Pu 6 F 30 6À framework is made up of PuF 9 polyhedra connected to form channels containing charge-balancing M n + (Wyckoff site 2a)a nd Na + cations (site 4d); as econd set of crystallographically unique Na + cations (site 2b)a lso reside in the larger voids between MF 6 octahedra.…”

“…Am ild hydrothermals ynthesis methods imilar to that described in previousr eports [17][18][19][20] wase mployeda sastarting point for scaling down and optimizing the reactions to find the first members of the Na n MPu 6 F 30 family( see the Supporting Information for experimental details andT able S1 for specific reaction conditions). The target phases were prepared from mixtures of PuO 2 ,d ivalent or trivalent metal fluorides or nitrates,a nd NaF in hydrofluoric acid that were heatedi np olytetrafluoroeythylene (PTFE)-lined autoclaves.…”