Ordering of Interstitial Iron Atoms and Local Structural Distortion Induced by Iron Polycomplex in Fe1+yTe1-xSex as Seen via Transmission Electron Microscopy

TL;DR

This study uses advanced electron microscopy to analyze how interstitial iron atoms order and induce local structural distortions in Fe1+yTe1-xSex, revealing how Se substitution suppresses Fe interstitials and causes nanoscale phase separation.

Contribution

It provides detailed atomic-level insights into Fe interstitial ordering and structural distortions in Fe1+yTe1-xSex, highlighting the effects of Se substitution on Fe interstitials and phase heterogeneity.

Findings

Interstitial Fe atoms predominantly occupy the 2c site.

A superstructure phase from Fe interstitial ordering is observed in Fe1+yTe.

Se substitution reduces Fe interstitial concentration and ordering.

Abstract

Employing aberration-corrected scanning transmission electron microscopy (STEM), we meticulously investigated the intrinsic chemical heterogeneity of Fe1+yTe, Fe1+yTe0.8Se0.2, and Fe1+yTe0.5Se0.5. Comprehensive analysis reveals the presence of interstitial iron atoms (Feint) across all samples, pre-dominantly occupying the 2c site of the P4/nmm space group. Moreover, a superstructure phase characterized by a wave vector q = 2/5a + 1/2c, originating from the ordering of Feint, is distinctly observable in the parent compound Fe1+yTe. In this scenario, the Feint atoms interact with adjacent Fe atoms, forming iron polycomplex and leading to an evident distortion of the FeTe4 tetrahedral. Experimental results further demonstrate effective suppression of Feint concentration and ordering through appropriate Se substitution; notably, Fe1+yTe0.5Se0.5 manifests the lowest concentration of Feint atoms. Our findings additionally indicate that Se substitution is random, and nanoscale phase separation induced by Te/Se chemical heterogeneity is commonly observed within Fe1+yTe1-xSex crystals.