Ph. D. THESIS COLLOQUIUM
Name: Mr. Rohit Kumar Rohj
Research Supervisor: Prof. D. D. Sarma
Title: Ferroelectricity, Magnetism, and Thermal Transport in Selected Transition Metal Oxides and Chalcogenides
Date &Time: Friday, 07th November 2025 at 4:00 pm
Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
The observable properties of materials are often determined not merely by individual atoms/spins, but by their cooperative behavior. This gives rise to collective phenomena, such as magnetism, plasmons, dipolar ordering, superconductivity, and complex thermal transport behavior. This thesis explores ferroelectric, magnetic, and thermal properties in selected transition metal oxides and chalcogenides. The motivation stems from a desire to understand how collective excitations such as spins, dipoles, and phonons can be harnessed to design new energy materials.
It is highly desirable to realize a ferroelectric material that not only retains robust polarization but also exhibits a band gap within the visible range, enabling efficient light absorption and intrinsic polarization-driven charge separation. Through combined experimental and theoretical approaches, we present strategies to engineer low-band-gap ferroelectric materials for their potential use in photovoltaic and photocatalytic applications. [1–5]
According to conventional wisdom, mixing two antiferromagnetic materials would not be expected to produce a large net magnetization. However, a solid solution of FeTiO3 and Fe2O3 does so owing to competing magnetic interactions. For nearly 70 years, the magnetic properties of this system have remained controversial, with conflicting reports regarding its magnetic phase diagram. Using detailed DC and AC magnetic measurements, we study the unusual magnetic properties and clarify the low-temperature magnetic phase diagram. [6–9]
Finally, we discuss phonons and their role in governing heat capacity and thermal transport in transition metal chalcogenides. Specifically, in the context of low thermally conducting materials, we study the role of low-energy optical phonons within the Debye-Einstein framework in reducing the thermal conductivity and demonstrate ultralow thermal conductivity in TlCuZrSe3, revealing how atomic “rattling” and phonon anharmonicity govern energy transport in complex solids. [10,11]
References:
[1] S. Y. Yang et al., Above-bandgap voltages from ferroelectric photovoltaic devices, Nat. Nanotechnol. 5, 143 (2010).
[2] I. Grinberg et al., Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials, Nature 503, 509 (2013).
[3] S. Das, S. Ghara, P. Mahadevan, A. Sundaresan, J. Gopalakrishnan, and D. D. Sarma. Designing a Lower Band Gap Bulk Ferroelectric Material with a Sizable Polarization at Room Temperature, ACS Energy Lett. 3, 1176 (2018).
[4] R. K. Rohj, A. Hossain, P. Mahadevan, and D. D. Sarma. Band Gap Reduction in Ferroelectric BaTiO3 Through Heterovalent Cu-Te Co-Doping for Visible-Light Photocatalysis, Front. Chem. 9, (2021).
[5] R. K. Rohj, P. Boyal, D. Sharma, P. Yanda, D. P. Panda, S. Ghosh, A. Sundaresan, R. Ranjan, P. Mahadevan, and D. D. Sarma. Tuning the band gap while retaining large polarization in Cu-W codoped BaTiO3 (Manuscript to be submitted).
[6] Y. Ishikawa. Magnetic Properties of Ilmenite-Hematite System at Low Temperature, J. Phys. Soc. Jpn. 17, 1835 (1962).
[7] Y. Ishikawa, N. Saito, M. Arai, Y. Watanabe, and H. Takei. A New Oxide Spin Glass System of (1-x) FeTiO3–xFe2O3. I. Magnetic Properties, J. Phys. Soc. Jpn. 54, 312 (1985).
[8] B. P. Burton, P. Robinson, S. A. McEnroe, K. Fabian, and T. B. Ballaran. A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3, Am. Min. 93, 1260 (2008).
[9] R. K. Rohj et al., Magnetic properties of x FeTiO3-(1-x) Fe2O3 solid solutions with 0 ≤ x ≤ 1 (Manuscript to be submitted).
[10] R. K. Rohj, A. Bhui, S. Sett, A. Ghosh, K. Biswas, and D. D. Sarma. Ultralow Thermal Conductivity Approaching the Disordered Limit in Crystalline TlCuZrSe3, Chem. Mater. 37, 520 (2025).
[11] R. K. Rohj, A. Das, K. Biswas, and D. D. Sarma. Rational Analyses of Heat Capacity for Thermoelectric Materials (Manuscript under review).