The structure and stoichiometry of the lanthanide(III) (Ln) complexes with the ligand 2-thenoyltrifluoroacetone (Htta) formed in a biphasic aqueous room-temperature ionic liquid system have been studied by complementary physicochemical methods. Equilibrium thermodynamics, optical absorption and luminescence spectroscopies, high-energy X-ray scattering, EXAFS, and molecular dynamics simulations all support the formation of anionic Nd(tta)4(-) or Eu(tta)4(-) complexes with no water coordinated to the metal center in 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (C4mim+Tf2N(-)), rather than the hydrated, neutral complexes, M(tta)(3)(H2O)n)(n = 2 or 3), that form in nonpolar molecular solvents, such as xylene or chloroform. The presence of anionic lanthanide complexes in C4mim+Tf2N(-) is made possible by the exchange of the ionic liquid anions into the aqueous phase for the lanthanide complex. The resulting complexes in the ionic liquid phase should be thought of as weak C4mim+Ln(tta)4(-) ion pairs which exert little influence on the structure of the ionic liquid phase.

Neutron-diffraction measurements have been carried out to investigate the crystal structure, magnetic structures, and magnetic phase transitions in RNi 2 B 2 C (Rϭ Y, Ce, Pr, Nd, Tb, Dy, Ho, Er, Tm, and Yb͒. The materials that order magnetically exhibit a wide variety of both commensurate and incommensurate magnetic structures, which argues strongly that the dominant exchange interactions are of the indirect Ruderman-Kittel-Kasuya-Yosida type. The Nd system exhibits a commensurate antiferromagnetic ordering at 4.8 K, with wave vector ␦ϭ(1/2,0,1/2) and moment direction along a ͑or equivalently with ␦ϭ(0,1/2,1/2) and moment direction along b in this tetragonal system͒. For Dy (T N ϭ10.6 K), Pr (T N ϭ4.0 K), and the low-temperature phase of Ho, the magnetic structure is also a commensurate antiferromagnet that consists of ferromagnetic sheets of rare-earth moments in the a-b plane, with the sheets coupled antiferromagnetically along the c axis ͓␦ϭ(0,0,1)͔. Pr is not superconducting, while for Dy (T c ϭ6 K) and Ho (T c ϭ8 K) this magnetic order coexists with superconductivity. For Ho, though, the magnetic state that initially forms at T N Ϸ8.5 K is an incommensurate spiral antiferromagnetic state along the c axis in which the direction of these ferromagnetic sheets are rotated in the a-b plane by ϳ17°from their low-temperature antiparallel configuration ͓␦ϭ(0,0,0.91)͔. The intensity for this spiral state reaches a maximum near the reentrant superconducting transition (ϳ5 K); the spiral state then collapses at lower temperature in favor of the commensurate antiferromagnetic state. An incommensurate a-axis modulation, with ␦ϭ(0.55,0,0), is also observed above the spiral-antiferromagnetic transition, but it exists over a narrower temperature range than the spiral state, and also collapses near the reentrant superconducting transition. The Er system forms an incommensurate, transversely polarized spin-density wave ͑SDW͒ state at T N ϭ6.8 K, with ␦ϭ(0.553,0,0) and moment direction along b ͑or with ␦ along b and the moment direction along a). The SDW squares up at low T, and coexists with the superconducting state (T c ϭ11 K) over the full temperature range where magnetic order is present. Tb, which does not superconduct, orders with a very similar wave vector, but the SDW is longitudinally polarized in this case and again squares up at low T. Tm orders at T N ϭ1.5 K in a transversely polarized SDW state, but with the moments along the c axis and ␦ϭ(0.093,0.093,0). This state is coexistent with superconductivity (T c ϭ11 K). No significant magnetic moment is found to develop on the Ni site in any of the materials, and there is no magnetic ordering of any kind in the Y, Yb, or Ce materials. Profile refinements have also been carried out on these same samples to investigate the systematics of the crystallography, and the crystal structure is I4/mmm over the full range of compositions and temperatures investigated. The area of the a-b plane and the volume of the unit cell both decrease smoothly with either decreasing lanthani...

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The coordination environment of the hydrated Cm3+ ion is probed both in the solid state and in solution. The analysis of single-crystal X-ray diffraction data from [Cm(H2O)9](CF3SO3)3 determines that the Cm species is surrounded by nine coordinating waters with a tricapped-trigonal-prismatic geometry involving six short Cm-O distances at 2.453(1) A and three longer Cm-O distances at 2.545(1) A. The Cm nona-aqua triflate is isostructural with the series of lanthanide and actinide [R(H2O)9](CF3SO3)3 (R=La-Lu, Pu) compounds. A similar nona-aqua geometry is seen for the coordination environment of Cm in aqueous solution, as probed by high-energy X-ray scattering and extended X-ray absorption fine structure spectroscopy, although the splitting in the first coordination shell is increased from 0.092(2) in the solid to 0.16(2) A in solution. This increase in splitting of the Cm-water distances in the first coordination sphere is discussed in terms of its potential relevance to the previously observed decrease in coordinating waters with decreasing ionic radius about the f-ion in solution.