A Brief Comment on the Low-Temperature Structure of LaOFeAs

TL;DR

This paper clarifies the low-temperature structure of LaOFeAs, demonstrating that x-ray and neutron diffraction results are consistent and supporting the Cmma symmetry through first-principles calculations.

Contribution

It resolves previous discrepancies by showing the structural equivalence of different space groups and supports the Cmma symmetry with computational evidence.

Findings

X-ray and neutron diffraction structures are essentially identical.

Oxygen and iron atoms prefer to lie on specific planes, supporting Cmma symmetry.

The structural distortion can be conveniently described in the P2/c cell.

Abstract

In a recent paper [arXiv:0804.3569], Takatoshi Nomura {\it et al.} reported a structural phase transition near 150 K in LaOFeAs and used space group "Cmma" to describe their X-ray diffraction data. However, they did not discuss how their proposed structure compares with the early neutron study by Cruz {\it et al.}[arXiv:0804.0795] where the low temperature structure of LaOFeAs was described by space group "P112/n". This caused some confusion, suggesting that there may be some disagreement on the low temperature structure of LaOFeAs as evidenced by several inquiries that we received. Here we show that the proposed structures from x-ray and neutron diffraction are basically identical. The P2/c (i.e., P112/n) cell becomes the primitive cell of the Cmma cell when the z-coordinate of the oxygen and iron are assumed to be exactly 0 and 0.5 (these numbers were reported to be -0.0057 and 0.5006 in neutron study). Our first-principles total-energy calculations suggest that the oxygen and iron atoms prefer to lie on the z=0 and 1/2 plane, respectively, supporting Cmma symmetry. However it is more convenient to describe the structural distortion in the primitive P2/c cell which makes it easier to see the connection between the high (i.e., P4/nmm) and low temperature structures.