Anisotropic thermal expansion of AEFe₂As₂ (AE = Ba, Sr, Ca) single crystals

Abstract

We report anisotropic thermal expansion of the parent, AEFe₂As₂ (AE = Ba, Sr, and Ca), compounds. Above the structural/antiferromagnetic phase transition anisotropy of the thermal expansion coefficients is observed, with the coefficient along the a-axis being significantly smaller than the coefficient for the c-axis. The high temperature (200 K ≤ T ≤ 300 K) coefficients themselves have similar values for the compounds studied. The sharp anomalies associated with the structural/antiferromagnetic phase transitions are clearly seen in the thermal expansion measurements. For all three pure compounds, the ‘average’ a-value increases and the c-lattice parameter decreases on warming through the transition, with the smallest change in the lattice parameters observed for SrFe₂As₂. The data are in general agreement with the literature data from X-ray and neutron diffraction experiments.

Keywords:

• iron-arsenides
• thermal expansion
• anisotropy
• structural phase transition

Acknowledgements

Work at the Ames Laboratory was supported by the US Department of Energy – Basic Energy Sciences under Contract No. DE-AC02-07CH11358. We are indebted to George M. Schmiedeshoff for his help in establishing the dilatometry technique in the Ames Laboratory Novel Materials and Ground States Group and for much propitious advice. We thank Andreas Kreyssig and Shibabrata Nandi for useful discussions and Jiaqiang Yan for help in synthesis.