[en] We investigate the effects of post-growth annealing on the structural and magnetic properties of BaFe_2As_2. Magnetic susceptibility measurements, which exhibit a signal corresponding to the magnetic phase transition, and high-resolution x-ray diffraction measurements, which directly probe the structural order parameter, show that annealing causes the ordering temperatures of both the phase transitions to increase, sharpen and converge. In the as grown sample, our measurements show two distinct transitions corresponding to structural and magnetic ordering, which are separated in temperature by approximately 1 K. After 46 days (d) of annealing at 700 °C, the two become concurrent in temperature. These measurements demonstrate that the structural phase transition is second-order like when the magnetic and structural phase transitions are separated in temperature, and first-order like when the two phase transition temperatures coincide. This observation indicates that annealing causes the system to cross a hitherto undiscovered tricritical point. In addition, x-ray diffraction measurements show that the c-axis lattice parameter increases with annealing up to 30 d, but remains constant for longer annealing times. Comparisons of BaFe_2As_2 to SrFe_2As_2 are made when possible. (paper)

[en] Soft x-ray resonant scattering (XRS) has been used to observe directly, for the first time, the ordering of localized electronic states on both the Mn and the Tb sites in multiferroic TbMnO₃. Large resonant enhancements of the x-ray scattering cross-section were observed when the incident photon energy was tuned to either the Mn L or Tb M edges which provide information on the Mn 3d and Tb 4f electronic states, respectively. The temperature dependence of the XRS signal establishes, in a model independent way, that in the high-temperature phase (28 K≤T≤42 K) the Mn 3d sublattice displays long-range order. The Tb 4f sublattices are found to order only on entering the combined ferroelectric/magnetic state below 28 K. Our results are discussed with respect to recent hard XRS experiments (sensitive to spatially extended orbitals) and neutron scattering. (fast track communication)

[en] Ga_2(Se_0_._3_3Te_0_._6_7)_3 belongs to a family of materials with large intrinsic vacancy concentrations that are being actively studied due to their potential for diverse applications that include thermoelectrics and phase-change memory. In this article, the Ga_2(Se_0_._3_3Te_0_._6_7)_3 structure is investigated via synchrotron x-ray diffraction, electron microscopy, and x-ray absorption experiments. Diffraction and microscopy measurements showed that the extent of vacancy ordering in Ga_2(Se_0_._3_3Te_0_._6_7)_3 is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga_2(Se_0_._3_3Te_0_._6_7)_3 (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga_2(Se_0_._3_3Te_0_._6_7)_3 material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga_2(Se_0_._3_3Te_0_._6_7)_3 band gap redshifts by ≈0.05 eV as the vacancies order and accompanied by gains in charge carrier mobility. The results serve as an encouraging example of altering material properties via intrinsic structural rearrangement as opposed to extrinsic means, such as doping