# Double epitaxy of tetragonal and hexagonal phases in the FeSe system

## TL;DR

This study demonstrates the controlled growth of double epitaxy in FeSe thin films, achieving simultaneous tetragonal and hexagonal phases with well-defined orientations, which could alter their physical properties.

## Contribution

It introduces a method to grow FeSe films with coexisting tetragonal and hexagonal phases exhibiting specific epitaxial relationships and controllable phase fractions.

## Key findings

- Both phases can be grown with specific orientations on MgO substrates.
- The relative phase fraction is tunable by growth conditions.
- Distinct in-plane orientations are observed for each phase.

## Abstract

Epitaxial films which contains more than one crystallographic orientation are of interest due to the possibility of altered magnetic, electrical, and optical properties. Thin films of FeSe have been grown on single-crystal MgO substrates under conditions that produces the simultaneous, epitaxial growth of tetragonal and hexagonal phases. We show that this double epitaxy is characterized by phase domains with a well-defined epitaxial relationship to each other and that the relative phase fraction can be controlled. For growth temperatures of 350-450 degrees C, the (001)-oriented tetragonal phase (${\beta}$-FeSe) grows with its unit cell aligned with the cubic substrate, while a (101)-oriented hexagonal phase (Fe$_7$Se$_8$) shows domains with two different in-plane orientations separated by 45 degrees. Additionally, the ${\beta}$-FeSe phase can be chosen to be (001)- or (101)-oriented with respect to the substrate, with the (101) orientation containing three rotated domains.