We investigate the effects of post-growth annealing on the structural and magnetic properties of BaFe2As2. 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 BaFe2As2 to SrFe2As2 are made when possible.

Ga2(Se0.33Te0.67)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 Ga2(Se0.33Te0.67)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 Ga2(Se0.33Te0.67)3 is highly dependent on thermal annealing. It is posited that stoichiometric vacancies play a role in local atomic distortions in Ga2(Se0.33Te0.67)3 (based on the fine structure signals in the collected x-ray absorption spectra). The effect of vacancy ordering on Ga2(Se0.33Te0.67)3 material properties is also examined through band gap and Hall effect measurements, which reveal that the Ga2(Se0.33Te0.67)3 band gap redshifts by 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.