The average and local structures of ∼22 nm β-Er:Yb:NaYF 4 upconverting nanocrystals were probed using a dual-space approach combining Rietveld and pair distribution function analysis of X-ray total scattering. Comparison of the fits provided by the structural models derived from P6̅ 2m, P6̅ , and P6 3 /m space groups demonstrates that the latter yields a crystallochemically meaningful description of the nanocrystals' average and local structures. This result is in line with those previously reported for bulk β-Na 3x RE 2−x F 6 (x ∼ 0.45; RE = Y, Er, Tm, Yb) using powder X-ray diffraction, 1 and for β-NaREF 4 (RE = Y, Lu) nanocrystals using solid-state NMR; 2 however, it differs from those reported in studies of β-NaREF 4 (RE = La, Gd, Er) single crystals, which favored the structural model derived from the P6̅ space group. 3 The proposed structural model features sodium cations distributed in four translation-equivalent chains, each shifted relative to the others along the c axis. Interestingly, the structural model derived for the mineral gagarinite (NaCaREF 6 , P6 3 /m), in which sodium cations are distributed in eight chains, also provides an adequate fit to the X-ray scattering data. The potential implications of this finding are discussed from the perspective of the size dependence of the crystal structure of β-NaREF 4 .

The average and local crystal structures of scheelite-type double molybdates and tungstates of formula NaRE­(MO4)2 (RE = rare-earth, Y; M = Mo, W) are probed using a dual-space approach that combines Rietveld and PDF analyses of X-ray scattering data. The evolution of metal–oxygen distances as a function of the ionic radius of the rare-earth cation reveals that the crystal-chemical pictures derived from Rietveld and PDF analyses differ in their description of (1) the rigidity of MVI–O bonds defining MO4 tetrahedra and (2) the extent of Na/RE–O bond distance distortions in (Na/RE)­O8 dodecahedra. Analysis of the thermal ellipsoids of the oxygen atoms reveals that these differences stem from the presence of rotational disorder of MO4 tetrahedra. Rietveld analysis averages random rotations of these tetrahedra yielding an abnormally broad range of Mo–O and W–O bond distances across the rare-earth series. By contrast, bond distances extracted from PDF analysis remain unchanged upon chemical substitution, in agreement with the well-known rigidity of MVI–O bonds. In addition, PDF analysis shows that the rotational disorder of MO4 tetrahedra translates into local distortions of Na/RE–O bond distances defining (Na/RE)­O8 dodecahedra. These results highlight the complementarity of Rietveld and PDF analyses and the significance of coupling both approaches to accurately probe local structural distortions through a single scattering measurement.

A systematic investigation of the thermometric response of dysprosium-activated scheelite-type oxides demonstrates their potential as optical sensors for intermediate temperatures.

In an effort to discover a high-mobility p-type oxide, recent computational studies have focused on Sn2+-based ternary oxides. Ta2SnO6 has been suggested as a potentially useful p-type material based on the prediction of simultaneously high hole mobility and a wide range of synthesis conditions over which it is the energetically favored phase. In this study, we synthesized this material epitaxially for the first time and evaluated its properties experimentally. We measured the band gap to be 2.4 eV and attempted to substitutionally dope titanium for tantalum (TiTa ’) and potassium for tin (KSn ’) but found that both doped and undoped films were insulating. Amorphous Ta2SnO6 films were also grown via thermal atomic layer deposition (ALD) at 175 °C. Electrical characterization of the ALD-fabricated amorphous films found them to be insulating with an optical band gap of 2.24 eV. Density functional theory calculations indicate that, under MBE growth conditions, oxygen vacancies have a negative energy of formation in crystalline Ta2SnO6 when the Fermi energy lies near the valence band edge. These oxygen vacancies would lead to compensation of holes generated by TiTa ’ or KSn ’ dopants, which is consistent with our observations. We conclude that the direct growth of epitaxial p-type Ta2SnO6 films using MBE-accessible growth conditions is thwarted by the spontaneous formation of oxygen vacancies. While our growth conditions do not yield p-type films, we calculate that there are conditions under which Ta2SnO6 is the thermodynamically stable phase and spontaneous formation of compensating defects does not occur, motivating further studies with different synthesis techniques.

The design, synthesis, and characterization of a novel Ni(II) chelator SG-20 is reported. SG-20 is selective in binding to Ni(II) versus other metal ions including Cu(II), Fe(II), Co(II), and Zn(II). At pH = 7.1, SG-20 binds Ni(II) with a K d = 7.0 ± 0.4 μM. Job analysis indicates that SG-20 binds to both Ni(II) and Cu(II) with a 1:1 stoichiometry. Affinity of SG-20 for Ni(II) is pH dependent and decreases upon lowering to pH 4.0. A green solid was isolated from the reaction of SG-20 with NiCl2·6H2O in MeOH and characterized by X-ray photoelectron spectroscopy (XPS), electronic absorption and infrared (IR) spectroscopies, and mass spectrometry. Collectively, XPS and IR analysis revealed Ni–N and Ni–O interactions and a shift in C–O asymmetric and symmetric stretches consistent with Ni binding. Attempts to crystalize a mononuclear complex were unsuccessful, likely due to the Ni-SG-20 complex being in equilibrium with higher order species in solution. However, reaction of SG-20 with NiCl2·6H2O in water followed by slow evaporation yielded green crystals that were characterized by electronic absorption spectroscopy (λmax = 260 nm) and X-ray crystallography. These analyses revealed that SG-20 supports formation of a complex cluster containing six SG-20 ligands, 15 Ni(II), and three Na(I) centers, with two distinct types of Ni atoms in its outer and inner core. The nine Ni atoms present in the inner core were bound by oxo and carbonate bridges, whereas the six Ni atoms present in its outer shell were bound to N, O, and S donor atoms derived from SG-20. Overall, X-ray crystallographic analysis revealed that two chelator arms of SG-20 bind to one Ni(II) ion with an axial aqua ligand, whereas the third arm is free to interact with Ni ions within the central cluster, supporting the goal of Ni capture.