Anisotropic thermal expansion in Sr2RhO4 – A variable temperature Synchrotron X-ray diffraction study

• Thermal expansion of Sr2RhO4 was studied using Synchrotron-XRD.
• Unusual negative thermal expansion along c-axis observed.
• I41/acd → I4/mmm phase transition detected near 850 K.
• Tilting of the corner sharing RhO6 octahedra related to the observed NTE.

Synchrotron X-ray diffraction studies of Sr2RhO4, between 100 and 1273 K, demonstrate the presence of anisotropic thermal expansion that is significantly impacted by the progressive reduction in the size of the cooperative tilting of the corner sharing RhO6 octahedra. The tilting results in negative thermal expansion along the c-axis, demonstrating the importance of soft phonon modes. This anisotropy in thermal expansion is reflected in the changes in the individual Rh–O bond distances. Near 850 K Sr2RhO4 undergoes an apparently continuous transition from I41/acd to I4/mmm and above this temperature the material displays typical positive thermal expansion.

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