A series of new U(IV) and Th(IV) fluorides, NaUF (1), NaUF (2), NaUF (3), KThF (4), NaThF (5), (HO)ThF (6), and (HO)UF (7), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded 1, 7, and the monoclinic polymorph of NaUF, whereas direct reactions between UF and NaF led to the formation of 2 and orthorhombic NaUF (3). This highlights an unexpected difference in reaction products when different starting uranium sources are used. All seven compounds were characterized by single-crystal X-ray diffraction, and their structures are compared on the basis of cation topology, revealing a close topological resemblance between fluorides on the basis of the layers observed in NaUF(HO). Phase-pure samples of 1, 2, and both polymorphs of NaUF were obtained, and their spectroscopic and magnetic properties were measured. The UV-vis data are dominated by the presence of U cations and agree well with the electronic transitions. Effective magnetic moments of the studied compounds were found to range from 3.08 to 3.59 μ.
Geometrically frustrated systems
play an important role in studying
new physical phenomena and unconventional thermodynamics. Charge ordered
defect pyrochlores AM
2+
M
3+F6 offer a convenient platform for probing
the interplay between electron distribution over M
2+ and M
3+ sites and structural
distortions; however, they are limited to compounds with M
2+/3+ = V, Fe, Ni, and Cu due to difficulties in the simultaneous
stabilization of other 3d elements in the +2 and +3 oxidation states.
Herein, we employ Cl– anions under hydrothermal
conditions for the mild reduction of Mn2O3 in
concentrated HF to obtain the CsMn2+Mn3+F6 composition as a phase pure sample and study its properties.
The magnetism of CsMn2F6 was characterized by
measuring the magnetic susceptibility and isothermal magnetization
data, and a magnetic transition to a canted antiferromagnet state
was found at 24.1 K. We determined the magnetic structure of CsMn2F6 using powder neutron diffraction, which revealed
successive long-range ordering of the Mn2+ and Mn3+ sites that is accompanied by a second transition. The role and strength
of magnetic exchange interactions were characterized using DFT calculations.
Low-dimensional
solids are highly anisotropic by nature and show
promise as new quantum materials, leading to exotic physical properties
not realized in three-dimensional materials. To discover correlations
in low-dimensional systems, studying robust crystal structures that allow for chemical tuning is critical
for optimizing materials properties. In our search for novel quantum
intermetallic materials, we discovered a new homologous series, A
n+1B
n
X3n+1 (A = rare earth; B = transition metal; X = tetrels; n = 1–5) which crystallizes in orthorhombic space
groups Cmmm (for odd “n”)
and Cmcm (for even “n”).
This series, best characterized by the stacking of structural subunits
of AlB2, AuCu3, and BaNiSn3, represents
a bulk architecture of highly correlated quantum materials. Though
not a conventional “low dimensional” material with a
van der Waals gap, the lattice parameters of the members of this series
have a high aspect ratio (b/a) and
can systematically be “tuned” as a function of dimensionality.
This new homologous series can serve as a robust intermetallic system
to study collective phenomena in quantum materials.
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