University of Connecticut

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PhD Dissertation Defense

Monday, July 18, 2022
11:00am – 1:00pm

Storrs Campus

Graduate Student Amani Jayakody, Department of Physics, University of Connecticut

Perovskite SrCoOx - Surprising Structural and Magnetic Behavior

The concept of electronic phase separation explains several unusual magnetic and electronic properties of doped perovskite oxides such as cuprates, manganites, and cobaltites. The electronic phase separation observed in La1-ySryCoOx is an important example of the type of magnetic phases that can coexist, although there is an ongoing discussion as to the role of chemical segregation. Hole doping of this system can be achieved by substitution of Sr2+ for La3+ while keeping the full oxygen stoichiometry. An alternative way to do the same hole doping is to vary the oxygen concentration of the perovskite system. Strontium cobalt oxide with varying oxygen concentration (SrCoOx, 2.875 < x < 3) is one of the systems which shows magnetic phase separation for intermediate oxygen values. The end points are SrCoO2.875 which is ferromagnetic with Tc=220 K and SrCoO3 which is also ferromagnetic but with Tc=280K. Samples with intermediate values of oxygen concentration show two phase magnetic behavior between these values. Initial lab-based x-ray diffraction indicated that these compounds had only a single crystalline phase despite the two-part magnetism. We undertook a combined study using both high resolution x-ray powder diffraction measurements and pair distribution function measurements in order to understand possible differences between local and average structures. SrCoO3 remains in the simple cubic perovskite structure from 10–300 K. SrCoO2.875 has a much more complicated structure. At room temperature, it is tetragonal with an expanded unit cell. Upon cooling, the structure appears to undergo a second order transition to a cubic phase: i.e. a higher-symmetry phase at low temperature. This is extremely unusual, the opposite of typical structural transitions, and violates the usual assumption that entropy dominates the free energy changes with temperature in structural phase transitions. I present the structural studies of SrCoOx and discuss their implications.

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Prof. B. Wells

Physics Department (primary), College of Liberal Arts and Sciences, UConn Master Calendar

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