Abstract: The classification of structural phase transitions as displacive or order-disorder in character is usually based on spectroscopic data above the transition. Displacive type of transition results from the condensation of soft phonon modes at low-temperature phase while for the order-disorder transition the distorted atoms resides randomly on the minima of multiwell potential for T>T* and for ordered state, T<T*, one of the well is preferentially populated. However, often times some of the system exhibits the mixed type of structural phase transition i.e. the system possess the soft phonon modes behaviour of displacive nature while displaying the central peak of the order-disorder transition at the same time. The spectroscopic based measurements above phase transition cannot be reliable particularly when measuring at the temperature comparable to the potential barriers because thermally activated disorder blurs the distinction between two categories.

We have performed the single crystal x-ray diffraction to investigate the structural correlation in the quasiskutterudites, (Ca_xSr_(1-x)Rh₄Sn₁₃) and typical ferroelectric, NaNO₂. Sr₃Rh₄Sn₁₃ shows the interesting coexistence of two opposing electronic phenomena, superconductivity and charge density wave modulation (CDW). With the Ca substitution the CDW transition is suppressed while T_c is increased and at x ≈ 0.9 it is believed to have the quantum phase transition. NaNO₂ is the prototypical order-disorder system that undergoes two successive phase transitions- first order type transition from room temperature ferroelectric to intermediate antiferroelectric phase at T≈ 434K and subsequent second order type transition to high temperature paralectric phase at T≈ 436K. The ferroelectric to paraelectric transition is modulated by the rearrangement of triangular NO₂- ions along the crystal b-axis. The x-ray diffuse scattering measurements were transformed into real space producing three-dimensional pair distribution functions (3D-ΔPDF). The 3D-ΔPDF analysis shows that the amplitude of local atomic displacements are temperature independent below the transition and persist to well above the transition, a signature of order-disorder behaviour.