Doctoral Defense - Sevan Chanakian

About the Event

The Department of Chemical Engineering and Materials Science

Michigan State University

Ph.D. Dissertation Defense

May 9, 2025 at 1:00pm (EST)

ECE Conference room 2219EB and Zoom

Contact Department or Advisor for Zoom Information

ATOMIC STRUCTURE TO TRANSPORT PROPERTY RELATIONS IN SELECT RARE

EARTH CONTAINING CaAl2Si2 AND ZrBeSi STRUCTURED ZINTL PNICTIDES

By: Sevan Chanakian

Advisor: Dr. Alexandra Zevalkink

Abstract

Thermoelectric materials convert heat into electricity when a temperature gradient is applied across them; conversely, they can generate a temperature gradient when an electric current is passed through. While thermoelectric devices have yet to impact the terrestrial energy economy, they have enabled deep space exploration since the 1970s.

Beyond their applications, thermoelectric materials represent a unique amalgam of thermal, electrical, and—though less commonly explored—optical properties in a unique subset of materials: small-bandgap semiconductors. This makes thermoelectrics a rich field for studying fundamental material physics across a wide temperature range, from near absolute zero to over 1000 K.

Understanding the origins of electronic carrier transport in semiconductors is fundamental for developing new technologies and guiding the future design of electronic and energy materials. Simplified physical models are often employed to interpret complex semiconductor trends; among these, the thermoelectrics community widely uses the single parabolic band (SPB) model. This model requires the selection of a dominant scattering mechanism that limits electron transport. Traditionally, the SPB model assumes acoustic deformation potential (ADP) scattering as the primary carrier scattering mechanism and has long been used to rationalize transport trends in thermoelectric materials. These explanations have shaped the community’s approach to material selection and development for thermoelectric applications.

The present work investigates the relationships between atomic structure, carrier scattering mechanisms, and transport properties in rare earth-containing Zintl pnictides with CaAl₂Si₂ and ZrBeSi structure types, focusing on the underlying factors that enable or limit thermoelectric performance. Through a combination of experimental measurements, advanced computational modeling (including DFT, AMSET, and CSLD), and synthesis of new compositions, this dissertation critically examines the conventional SPB model and its assumption of ADP scattering as the dominant carrier relaxation process. Notably, it is shown that for RECuZnP₂ (RE = Pr, Nd, Er) phosphides, polar optical phonon (POP) scattering—rather than ADP scattering—primarily limits carrier mobility. This insight challenges long-standing assumptions in thermoelectric materials design and underscores the need for more nuanced models to interpret experimental data.

The role of atomic disorder and vacancies is further explored in the ZrBeSi-type solid solution EuCu₁₋ₓZn₀.₅ₓSb, where increasing vacancy concentration leads to pronounced non-linear changes in lattice parameters, softening of elastic moduli, and a significant reduction in lattice thermal conductivity—from 3 to 0.5 W/m·K—while simultaneously impacting electronic transport through increased effective mass and band gap. This study demonstrates that vacancy-induced point-defect scattering effectively suppresses thermal conductivity without excessively degrading carrier mobility.

Subsequently, the related Zintl compound Eu₂ZnSb₂₋ₓBiₓ is investigated. Despite severe local structural disorder introduced by half-occupied Zn sites, this material exhibits exceptionally high electronic mobility (~100 cm²/V·s) alongside ultralow lattice thermal conductivity. The anomalous increase in mobility with rising point-defect concentration in Eu₂ZnSb₂₋ₓBiₓ highlights the need for experimental validation of effective mass and relaxation times. Substituting Sb with Bi in this series further reduces the speed of sound and lattice thermal conductivity, increases carrier concentration by two orders of magnitude, and decreases the band gap. These findings underscore the complex interplay between chemical composition, atomic-scale disorder, and macroscopic transport properties, enabling "phonon-glass, electron-crystal" behavior in thermoelectric Zintl phases.

The insights gained from these rare-earth Zintl pnictides highlight the necessity of revising established models of carrier transport for the discovery and design of next-generation thermoelectric materials.

Persons with disabilities have the right to request and receive reasonable accommodation. Please call the Department of Chemical Engineering and Materials Science at 355-5135 at least one day prior to the seminar; requests received after this date will be met when possible.