Low-Temperature Synthesis of Magnetic Pyrochlores (R2Mn2O7, R = Y, Ho-Lu) at Ambient Pressure and Potential for High-Entropy Oxide Synthesis.

Rare-earth manganese pyrochlores (R2Mn2O7) are frustrated magnetic materials, which previously have only been accessed using expensive high-pressure and high-temperature synthesis. In the present work, we demonstrate a convenient synthetic approach to synthesize R2Mn2O7 pyrochlores at ambient pressure. A series of pyrochlores (R = Y, Ho-Lu) were prepared by a simple and cost-effective molten salt method using NaCl and KCl as the flux. Moreover, phase-selectivity was demonstrated for yttrium manganese oxides (YMnO3 and Y2Mn2O7) by a simple variation of synthesis temperature and precursors-to-chlorides ratio. The synthetic procedure does not require high pressures or temperatures nor oxygen flow. All synthesized pyrochlores demonstrated ferromagnetic behavior at low temperature, and the magnetic properties were in good agreement with those of high-pressure-synthesized materials. The versatility of the method was confirmed by the preparation of a mixed-rare earth Y0.4Er0.4Tm0.4Yb0.4Lu0.4Mn2O7 solid solution─a compositionally complex high-entropy oxide.

Synthesis of Mixed-Valent Lanthanide Sulfide Nanoparticles.

In targeting reduced valent lanthanide chalcogenides, we report the first nanoparticle synthesis of the mixed-valent ferromagnets Eu3 S4 and EuSm2 S4 . Using divalent lanthanide halides with bis(trimethylsilyl)sulfide and oleylamine, we prepared nanoparticles of EuS, Eu3 S4 , EuSm2 S4 , SmS1.9 , and Sm3 S4 . All nanoparticle phases were identified using powder X-ray diffraction, transmission electron microscopy was used to confirm morphology and nanoparticle size, and magnetic susceptibility measurements for determining the ordering temperatures and valence. The UV/Vis, Raman and X-ray photoelectron spectroscopies for each phase were compared. Surprisingly, the phase is influenced by the halide and the reaction temperature, where EuCl2 formed EuS while EuI2 formed Eu3 S4 , highlighting the role of kinetics in phase stabilization. Interestingly, at lower temperatures EuI2 initially forms EuS, and converts over time to Eu3 S4 .

Solution Synthesis of NdTe3 Magnetic Nanosheets.

Neodymium tritelluride is a layered van der Waals material, with correlated electronic properties including high electronic mobility, charge density waves, and antiferromagnetism. We developed a solution synthesis method to form free-standing nanosheets of NdTe3, with nanosheet lateral dimensions of 200-400 nm. The morphology of the nanosheet was influenced by the neodymium precursor. When Nd[(N(SiMe3)2]3 was used as the metal source the nanosheet thickness average was 12 ± 2.5 nm, alternatively the combination of NdCl3 and Li(N(SiMe3)2) led to thicker nanosheets, approximately 19 ± 2.4 nm. We believe that the difference in thickness and changes in surface chemistry point to the role of chloride in accelerating nanocrystal growth for the synthesis with NdCl3 (and Li(N(SiMe3)2). Both types of nanosheets exhibit charge density wave (CDW) distortions as measured using electron diffraction and investigated using variable temperature Raman scattering. Interestingly, the magnetic studies suggest a distinct change in properties between 12 and 19 nm thickness in antiferromagnetic NdTe3.

Molecular Parameters Promoting High Relaxivity in Cluster-Nanocarrier Magnetic Resonance Imaging Contrast Agents.

We have investigated the mechanism of relaxivity for two magnetic resonance imaging contrast agents that both employ a cluster-nanocarrier design. The first system termed Mn8Fe4-coPS comprises the cluster Mn8Fe4O12(L)16(H2O)4 or Mn8Fe4 (1) (L = carboxylate) co-polymerized with polystyrene to form ∼75 nm nanobeads. The second system termed Mn3Bpy-PAm used the cluster Mn3(O2CCH3)6(Bpy)2 or Mn3Bpy (2) where Bpy = 2,2'-bipyridine, entrapped in ∼180 nm polyacrylamide nanobeads. Here, we investigate the rate of water exchange of the two clusters, and corresponding cluster-nanocarriers, in order to elucidate the mechanism of relaxivity in the cluster-nanocarrier. Swift-Connick analysis of O-17 NMR was used to determine the water exchange rates of the clusters and cluster-nanocarriers. We found distinct differences in the water exchange rate between Mn8Fe4 and Mn8Fe4-coPS, and we utilized these differences to elucidate the nanobead structure. Using the transverse relaxivity from O-17 NMR line widths, we were able to determine the hydration state of the Mn3Bpy (2) cluster as well as Mn3Bpy-PAm. Using these hydration states in the Swift-Connick analysis of O-17 NMR, we found the water exchange rate to be extremely close in value for the cluster Mn3Bpy and cluster-nanocarrier Mn3Bpy-PAm.