Order-disorder vs Displacive structural phase transitions in (Ca_xSr_1-x)₃Rh₄Sn₁₃

A three-dimensional quasi-skutterudite compound (Ca_xSr_1-x)₃Rh₄Sn₁₃ undergoes a second-order structural phase transitions from a simple cubic I phase structure (Pm-3n) to a body-centered cubic I’ superstructure (I-43d)  but the nature of transition either displacive or order-disorder has not been categorized yet. To distinguish between these two characters, local order parameter fluctuations were estimated by transforming the x-ray diffuse scattering data of single crystal (Ca_xSr_1-x)₃Rh₄Sn₁₃ with x=0.0, 0.1, 0.6, 0.65, 0.7, 0.9 to a real space producing a three-dimensional pair distribution function using a new ‘Gaussian tiling’ approach rather than a conventional ‘punch and fill’ method. The 3D-ΔPDF analysis shows that the amplitudes of local atomic displacements are independent of both temperature and doping, a much clearer signature of order-disorder transitions than possible by other methods.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.