Point defects play a critical role in the transport properties of Mg 3 Sb 2based thermoelectric materials. However, the determination of point defects is rather challenging. Herein, Mg 3 Sb 2 single crystals were prepared by the self-flux method with the Sb flux (p-type sample) and Mg flux (n-type sample). The crystal structure as well as the atomic occupancy parameters of these samples were determined by single-crystal and synchrotron powder X-ray diffractions. Comparison between the experimental and calculated Fourier electron density maps generated by single-crystal data indicates the existence of Mg interstitials [1/3, 2/3, 0.38(3)] and vacancies at the Mg2 sites [2/3, 1/3, 0.8653(3)] in the n-type sample and Mg vacancies at both the Mg1 and Mg2 sites in the p-type sample. In addition, differences in the peak positions and intensities of the two types of Mg−Sb vibration modes in Raman spectra were observed between different Mg 3 Sb 2 crystals, which suggests that point defects can impact the vibration modes.

Herein we report two reduced rare-earth metal-based superconductors, La 26 Ge 19 M 5 O 5 (M = Ag, Cu), that feature an unprecedented [La 18 O 5 ] cluster composed of five oxygen-centered [La 6 O] octahedra condensed through shared faces and capped with [Ge 4 ] butterfly rings. The structure, determined by single-crystal X-ray diffraction, crystallizes in a tetragonal space group (P4/ nmm), with a = 15.508(2) Å and c = 11.238(2) Å. Resistivity and magnetic susceptibility measurements show onsets of superconductivity at T c = 5.4 and 6.4 K for the Ag and Cu compounds, respectively. Applying high pressures, up to 1.3 GPa, results in increased superconducting transition temperatures (T c = 6.8 K for Ag and 7.2 K for Cu compounds), with no sign of saturation.