Selective Spin-State Switch and Metal-Insulator Transition in \boldmath $\rm GdBaCo_2O_{5.5}$

TL;DR

This study uses synchrotron diffraction to investigate the metal-insulator transition in GdBaCo2O5.5, revealing a spin-state switch in octahedral cobalt ions linked to electron excitation and structural changes.

Contribution

It provides detailed structural insights into the spin-state transition and electron excitation mechanisms underlying the metal-insulator transition in GdBaCo2O5.5.

Findings

Anomalous expansion of CoO6 octahedra at transition

Electron excitation from t2g^6 to eg^2 states in octahedra

No change in pyramidal Co states or orbital ordering at T_MI

Abstract

Ultra-high resolution synchrotron diffraction data for $\rm GdBaCo_2O_{5.5}$ throw new light on the metal-insulator transition of Co$^{3+}$ Ba-cobaltites. An anomalous expansion of CoO$_6$ octahedra is observed at the phase transition on heating, while CoO$_5$ pyramids show the normal shrinking at the closing of the gap. The insulator-to-metal transition is attributed to a sudden excitation of some electrons in the octahedra ($t_{2g}^6$ state) into the Co $e_g$ band (final $t_{2g}^4e_g^2$ state). The $t_{2g}^5e_g^1$ state in the pyramids does not change and the structural study also rules out a $d_{3x^2-r^2}/d_{3y^2-r^2}$ orbital ordering at $T_{MI}$.