Spin glass and semiconducting behavior of the flux grown BaFe$_{2-\delta }$Se$_{3}$ crystals

In this talk, physical properties and crystal and electronic structures of BaFe$_{2-\delta }$Se$_{3}$ crystals, synthesized using tellurium flux, will be discussed. This phase is an iron-deficient derivative of the ThCr$_{2}$Si$_{2}$-type and its structure is made of double chains formed from edge-sharing FeSe$_{4}$ tetrahedra. The semiconducting BaFe$_{2-\delta }$Se$_{3}$ with \textit{$\delta $} $\approx $ 0.2 does not order magnetically, however, there is evidence for short-range magnetic correlations of spin glass type below 50 K in magnetization, heat capacity and neutron diffraction results. The semiconducting behavior of BaFe$_{2-\delta }$Se$_{3}$ is in line with the detrimental influence of iron deficiency to the superconductivity in $A_{x}$Fe$_{1.8}$Se$_{2}$ ($A$ = alkali metal) superconductors. The electronic structure calculations suggest that this compound can be considered as a low-dimensional (1D) ladder structure with a weak interchain coupling. Based on the survey of available data on BaFe$_{2}$Se$_{3}$ so far, lower concentrations of iron vacancies may lead to a long range antiferromagnetic order, whereas higher concentrations of iron vacancies may suppress long range order and then lead to a spin glass behavior.