Anna Llobet

Observation of local non-centrosymmetry in weakly biferroic YCrO3

Using high-resolution neutron powder diffraction we have determined the average and local structure of YCrO3 in order to explain the recently reported properties of YCrO3. Unlike conventional ferroelectric systems, YCrO3 is reported to possess a centrosymmetric crystal structure which is inconsistent with the development of weak electric polarization above the dielectric transition around 440 K because it requires atomic off-centring. We have characterized the different length scales existent in YCrO3 and found that, although the average crystallographic structure above and below the dielectric transition YCrO3 is orthorhombic and centrosymmetric (Pnma), in the ferroelectric-like state it is locally noncentrosymmetric and Cr is displaced about 0.01 ˚ A from its position along the z direction. We conclude that the local character of the Cr off-centring and the small value of the displacement observed could explain the weak ferroelectriclike polarization. This new concept of ‘local non-centrosymmetry’ might be of great importance for the understanding of unusual properties of other multifunctional materials as well.

Interstitial oxygen in oxygen-stoichiometric apatites

Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La9.33(Si1−xGexO4)6O2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La9.33(SiO4)6O2. Time-of-flight neutron data have been collected for La9.33(Si0.5Ge0.5O4)6O2, La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. The room-temperature structures have been derived from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data. High temperature structures have been obtained only from Rietveld refinements of neutron powder diffraction data. The refinements show that La9.33(SiO4)6O2 and La9.33(Si0.5Ge0.5O4)6O2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. Hence, the novelty of this work is to explain the high oxide conductivity of the lanthanum-deficient samples which it is due to the presence of interstitial oxygens. Lanthanum stoichiometric samples do not have interstitial oxygens and, so, their conductivities are much lower.

Magnetic structure of CeRhIn5 as a function of pressure and temperature

We report magnetic neutron-diffraction and electrical resistivity studies on single crystals of the heavy-fermion antiferromagnet CeRhIn

Average and Local Structural Origins of the Optical Properties of the Nitride Phosphor La3–xCexSi6N11 (0 < x ≤ 3)

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Phase relations in K xFe 2-ySe 2 and the structure of superconducting K xFe 2Se 2 via high-resolution synchrotron diffraction

at pressures up to 2.3 GPa. These experiments show that the staggered moment of Ce and the incommensurate magnetic structure change weakly with applied pressure up to 1.63 GPa, where resistivity, specific heat and NQR measurements confirm the presence of bulk superconductivity. This work places new constraints on an interpretation of the relationship between antiferromagnetism and unconventional superconductivity in CeRhIn

Iron displacements and magnetoelastic coupling in the antiferromagnetic spin-ladder compound BaFe2Se3

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Atomic displacements in the charge ice pyrochlore Bi_{2}Ti_{2}O_{6}O^{?} studied by neutron total scattering

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Magnetic transition broadening and local lattice distortion in the negative thermal expansion antiperovskite Cu1−xSnxNMn3

Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.

Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen

Superconductivity in iron selenides has experienced a rapid growth, but not without major inconsistencies in the reported properties. For alkali-intercalated iron selenides, even the structure of the superconducting phase is a subject of debate, in part because the onset of superconductivity is affected much more delicately by stoichiometry and preparation than in cuprate or pnictide superconductors. If high-quality, pure, superconducting intercalated iron selenides are ever to be made, the intertwined physics and chemistry must be explained by systematic studies of how these materials form and by and identifying the many coexisting phases. To that end, we prepared pure K

Orbital-selective magnetism in the spin-ladder iron selenides Ba 1 − x K x Fe 2 Se 3

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