Cation vacancy order in the K0.8+xFe1.6-ySe2 system: five-fold cell expansion accommodates 20% tetrahedral vacancies

TL;DR

This study reveals how vacancy ordering in K-Fe-Se compounds causes a fivefold expansion of the crystal lattice, accommodating significant tetrahedral vacancies and maintaining iron's charge state, with implications for doping mechanisms.

Contribution

It introduces a novel fivefold cell expansion model that explains vacancy ordering and charge balance in K-Fe-Se systems, highlighting a new doping mechanism.

Findings

Fivefold expansion accommodates 20% vacancies.

Vacancy ordering maintains Fe charge state near +2.

Coupling of alkali and iron vacancies influences doping.

Abstract

Ordering of the tetrahedral site vacancies in two crystals of refined compositions K0.93(1)Fe1.52(1)Se2 and K0.862(3)Fe1.563(4)Se2 produces a fivefold expansion of the parent ThCr2Si2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels.