Diversity of Microstructural Phenomena in Superconducting and Non-superconducting Rb_xFe_2–ySe₂: A Transmission Electron Microscopy Study at the Atomic Scale

Abstract

Superconducting (SC) and non-superconducting (non-SC)Rb(x)Fe(2-y)Se2 crystals were grown using the "self-flux" technique in order to assign the microstructural changes to the onset of superconductivity in complex iron selenides. The crystals were thoroughly characterized by magnetic susceptibility and transport measurements as well as powder X-ray diffraction. Special attention was paid to the comparison of the microstructure of the crystals with and without the superconducting transition by means of transmission electron microscopy (TEM). It is shown that the alternation of ordered and disordered regions on the sample surface and along the c-axis is characteristic for both SC and non-SC materials and therefore does not necessarily represent a trigger of superconductivity. Three types of electron diffraction patterns were found for the superconducting Rb(x)Fe(2-y)Se2 sample, of which one is observed for the first time and originates from alkali metal ordering. Moreover, for the superconducting Rb(x)Fe(2-y)Se2 material a monoclinic distortion with β ∼ 87° was observed, leading to the space group I2/m. This monoclinic distortion seems to be an attribute of the superconducting material only, whereas in the non-superconducting sample the orthogonality of the crystallographic axes is preserved.

Cesium vacancy ordering in phase-separatedCsxFe2−ySe2Taddei1, Sturza2, Chung3 et al. 2015Phys. Rev. B9360View full textAdd to dashboardCite

“…[14,15,16,17] Phase separation in the intercalated iron selenides is well-established and this induction of microscopic domains of minority 122-type structure in a host matrix with robust magnetic properties is unique and interesting. [17,18,19,20,21,22] Though its formation is widely reported, the exact nature of this minority phase remains debated, being described as isostructural with the higher temperature paramagnetic 122 phase from x-ray diffraction on polycrystalline K x Fe 2-y Se 2 or as being distorted to lower orthorhombic or monoclinic symmetries as found in x-ray diffraction on Rb x Fe 2-y Se 2 and Cs x Fe 2-y Se 2 single crystals, respectively. [18,19,23] Other studies of K x Fe 2-y Se 2 have reported the formation of a new '234' phase, which may be unique to the K intercalated family or perhaps simply related to the growth conditions of that particular crystal.…”

“…Superstructure formation due to long range ordering of the Cs vacancies is observed as sharp superlattice reflections in contrast to the commonly seen out-of-plane intensity rods. [18,20,21,26] Modeling based on group theoretical representation analysis shows that three-dimensional Cs vacancy ordering takes place in the ab plane and at alternating layers along the c-axis in both the majority and minority phases.…”

High-temperature superconductivity: From macro- to nanoscale structures

“…Along with this, research was conducted on superconductivity of non-cuprate compounds, in particular, of Mg-B, Ba-Na-Ge, Na-WO 3 systems [31,[92][93][94][95][96], as well as iron compounds, ferronickel and iron selenides [97][98][99][100][101][102][103][104][105][106][107][108][109][110]. To date, there is a wide class of such materials which give new opportunities to further increase the transition temperature of the superconductivity with the simultaneous increase in the critical current value.…”

“…Subsequently, superconducting Fe compounds were synthesized with Ce, As, F, As, K, Se, Te, Rb, K, Na and other elements [102][103][104][105][106][107][108].…”

“…were obtained, among which there are superconducting compounds [104][105][106][107][108]123]. Superconducting properties are found in the β-phase of iron monoselenide FeSe 1−δ , with a significant non-stoichiometry of the chalcogen δ ∼ 0.18.…”