Effects of "stuffing" on the atomic and electronic structure of the pyrochlore Yb$_2$Ti$_2$O$_7$

TL;DR

This study uses ab initio calculations to explore how stuffing and oxygen vacancies in Yb$_2$Ti$_2$O$_7$ affect its atomic and electronic structure, revealing changes in hybridization and magnetic properties relevant to its ground state.

Contribution

It provides the first detailed ab initio analysis of the effects of stuffing and oxygen vacancies on Yb$_2$Ti$_2$O$_7$'s electronic and magnetic structure.

Findings

Oxygen vacancies are energetically favorable in stuffed Yb$_2$Ti$_2$O$_7$.

Stuffing and vacancies alter the hybridization between O 2p and metal orbitals.

Electronic structure and magnetic moments are significantly affected by stuffing.

Abstract

There are reasons to believe that the ground state of the magnetic rare earth pyrochlore Yb$_2$Ti$_2$O$_7$ is on the boundary between competing ground states. We have carried out $\mathrm{\textit{ab initio}}$ density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the "stuffed" system. We find that it is energetically favorable in the "stuffed" crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4$\mathrm{\textit{f}}$ orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit-coupling (SOC) on top of the GGA+U approach, we investigated the electronic structure and the magnetic moments of the most stable "stuffed" system. In our determined "stuffed" structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2$\mathrm{\textit{p}}$ orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.