We present measurements of the thermal expansion coefficient $\alpha$ of polycrystalline ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ $\left(x\le 0.1\right)$. The magnetic and structural transitions of the samples with $x\le 0.04$ give rise to large anomalies in $\alpha \left(T\right)$ while the onset of superconductivity in the crystals with $x\ge 0.05$ is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for $x=0.05$ is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping.

• Received 6 March 2009

DOI:https://doi.org/10.1103/PhysRevB.80.094512