Ph.D. (Doctor of Philosophy)
Department of Physics
Classification of structural phase transitions as being of the displacive or order-disorder types is usually done based on spectroscopic measurements performed above the transition. Transitions of the displacive type typically occur when soft phonon modes condense upon approaching the phase transition, whereas those of the order-disorder type are characterized by structures in which the atoms are located randomly above the phase transition at T* at the minima of a multiwell potential. In the ordered state, i.e., when T
We have performed single crystal x-ray scattering experiments and three-dimensional pair distribution functions (3D-∆PDF) analyses to understand the nature of phase transitions and investigate various structural correlations in the quasiskutterudites (Sr1−xCax)3Rh4Sn13, and NaNO2. Sr3Rh4Sn13 is an important member of the ternary stannides family (namely R3T4Sn13 where R = La, Sr, Ca, T = Ir, Rh, Co) in which the coexistence of two contrasting and competing electronic phenomena, superconductivity and Charge Density Waves (CDW), is not well understood. With Ca substitution, the CDW transition temperature is gradually suppressed with simultaneous enhancement of the superconducting transition temperature. At the Ca content x∼0.9, the CDW order is fully suppressed giving way at a quantum phase transition. Our detailed 3D-∆PDF analyses demonstrate the independent character of the local atomic displacements of the Sn atoms at all temperatures below, at and well above the phase transitions for all the samples. This is a clear signature of phase transitions of the order-disorder type, the implications of which on the associated electronic transitions are discussed.
NaNO2 is another prototypical system exhibiting an order-disorder type structural transition. It is ferroelectric system with a simple crystal structure that undergoes two successive phase transitions: from a room temperature ferroelectric phase persisting up to 434 K when the material undergoes a first-order phase transition to a short-lived intermediate antiferroelectric phase existing only until 436 K where it gives away through a second-order phase transition to a higher temperature paraelectric phase. The ferroelectric to paraelectric transition involves the re-arrangement of triangular NO2 molecules and Na+ ions along the b-axis of the real space structure. Our 3D-∆PDF analyses successfully explained the phase transition and provided unambiguous evidence for the full ordering of the NO2 molecules in the ferroelectric phase, and for their random fluctuations over various symmetry-allowed sites in the paraelectric phase above the transition temperature.
Most importantly, through the extensive analysis and modeling of the complex systems I investigated, my dissertation brings to light the immense power and usefulness of the newly developed 3D-∆PDF real-space mapping techniques in the direct understanding and interpretation of interatomic structural correlations showing how local site occupancies differ from those of the average structure. Understanding and quantifying the behavior and probability of the site-occupancy between nearest and next-nearest neighbors allow for the determination with certainty of the mechanisms leading to short-range or long-range ordered structures from which the exact nature of the phase transition can be unequivocally identified.
Upreti, Puspa, "Diffuse Scattering and 3D-∆PDF Analyses: Order-Disorder Phase Transitions in (sr1−xcax)3rh4sn13 And Nano2" (2023). Graduate Research Theses & Dissertations. 7193.
Northern Illinois University
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