Structural transitions in a doped lanthanum cuprate

TL;DR

This study uses 139La NMR to investigate structural phase transitions in Eu-doped La{2-x}Sr{x}CuO{4}, revealing non-vanishing tilt angles and indicating order-disorder transitions influenced by Eu doping.

Contribution

It demonstrates that Eu doping changes the nature of structural transitions from displacive to order-disorder in La{2-x}Sr{x}CuO{4}.

Findings

Non-zero tilt angles in HTT phase contrary to undoped samples.

Evidence of critical fluctuations at structural transitions.

Eu doping induces LTO to LTT transition absent in undoped material.

Abstract

139La NMR and relaxation measurements have been performed on La{1.8-x}Eu{0.2}Sr{x}CuO{4} (x = 0.13 and 0.2) single crystals. The temperature dependence of the 139La NMR spectra in all the structural phases (HTT -> LTO -> LTT) reveals the non-vanishing tilt angle of the CuO6 octahedra in the HTT phase, opposed to the case of La{2-x}Sr{x}CuO{4} where the tilt angle disappears immediately above the transition. Since 139La relaxation data provide evidence of the thermodynamic critical fluctuations associated with the structural phase transitions, HTT -> LTO and LTO -> LTT, we conclude that the structural transitions in Eu-doped La{2-x}Sr{x}CuO{4} should be of the order-disorder type rather than of the displacive type observed in La{2-x}Sr{x}CuO{4}. The change of the nature of the structural transitions caused by doping Eu appears to be consistent with the LTO -> LTT transition that is absent in La{2-x}Sr{x}CuO{4}.