Doctoral Defense - Md Towhidur Rahman

About the Event

The Department of Mechanical Engineering

Michigan State University

Ph.D. Dissertation Defense

July 7, 2025 at 11:00am EST

3405 Engineering Building

ABSTRACT

ORDER-DISORDER TRANSITION IN SUPERIONIC ACrX2 COMPOUNDS AND THEIR ALLOYS:
IMPACT ON ELASTIC AND THERMAL TRANSPORT PROPERTIES (A = Ag, Cu; X = S, Se, Te)

By: Md Towhidur Rahman

Advisor: Dr. Alexandra Zevalkink

Interest in the layered ACrX2 (A = Ag, Cu; X = S, Se) compounds arises from the intriguing possibility that the same structural characteristics enabling fast ion transport — such as soft bonding, high anharmonicity, and intrinsic disorder — also contribute to exceptionally low lattice thermal conductivity. In this study, we systematically investigate the relationship between these behaviors by measuring the elastic and thermal properties of this family of layered chalcogenides. First, phase pure AgCrSe2, CuCrSe2, AgCrS2 and CuCrS2 compounds were synthesized, followed by temperature-dependent measurements of elastic moduli, thermal diffusivity, heat capacity, and electronic properties. The thermoelectric figure of merit was determined for each compound. To explore the effect of anion-site disorder, we synthesized a series of alloyed compounds with mixed anion occupancy, specifically CuCrSe2-ySy (x = 0.1, 0.25, 0.5, 0.75 and 1.0) and CuCrSe2-xTex system (x = 0.1, 0.15, 0.175, 0.2, 0.3, 0.4). We investigated three central hypothesis - (1) that the superionic conductors exhibit mixed occupancy at the anion site, showing evidence of forming solid solution; (2) that the mixing at anion site significantly affect thermal and elastic properties of the compounds; and (3) that the superionic transition temperature of the compounds changes as a function of composition of the alloy. The formation of solid solution was confirmed for the entire range of composition in CuCrSe2-ySy series and up to x = 0.15 in CuCrSe2-xTex series. Variable-temperature Xray diffraction and thermal diffusivity measurements were conducted to track the orderdisorder/superionic transition temperature (Tc) of the compounds. The transition temperature was found to be highly composition-dependent, exhibiting a decreasing trend with the incorporation of larger anions; CuCrSe1.85Te0.15 had the lowest Tc at 282 K, marking the first reported instance of Tc < 300 K for this crystal structure type. We also investigated the elastic properties and speed of sound in the CuCrSe2-xTex series as a function of composition and temperature. We show that the samples soften sharply as anion size increased. As a function of temperature, we see only a small inflection the temperature coefficient of elasticity, dCij/dT, at order-disorder phase transition, confirming prior findings that long-wavelength acoustic phonons are largely unaffected by the phase transition. These findings demonstrate that tuning interatomic distances and bond stiffness through anion-site alloying can effectively tailor the behavior of solid-state ionic conductors.


Persons with disabilities have the right to request and receive reasonable accommodation. Please call the Department of Mechanical Engineering at 355-5131 at least one day prior to the seminar; requests received after this date will be met when possible.