First-principles study of the layered thermoelectric material TiNBr

Area of Science:

• Materials Science
• Solid State Physics
• Thermoelectrics

Background:

• Layer-structured materials possess intrinsically low thermal conductivity due to atomic interlayer interactions.
• Their electrical properties are tunable via methods like band modification and intercalation.
• Thermoelectric materials convert heat energy into electrical energy and vice versa.

Purpose of the Study:

• To investigate the thermoelectric properties of TiNBr, a novel compound within the MNX (M = Ti, Zr, Hf; X = Cl, Br, I) system.
• To understand the origins of its low thermal conductivity and high thermoelectric performance.

Main Methods:

• Experimental synthesis and characterization of TiNBr.
• Measurement of Seebeck coefficient and thermal conductivity along the A axis.
• Phonon group velocity and anharmonicity analysis using Grüneisen parameter and three-phonon processes.
• Analysis of atomic motion and charge density differences.

Main Results:

• TiNBr exhibits an ultrahigh Seebeck coefficient of 2215 μV K^-1 at 300 K.
• A dimensionless figure of merit (ZT) of 0.661 at 800 K was achieved along the A axis.
• Lattice thermal conductivity was as low as 1.34 W (m K)^-1, attributed to low phonon group velocity and high phonon anharmonicity.
• Both three-phonon and fourth-order anharmonic effects contribute to the overall anharmonicity.

Conclusions:

• TiNBr is a promising thermoelectric material with exceptional performance.
• Low lattice thermal conductivity is linked to collective low phonon group velocity and significant phonon anharmonicity.
• This study provides insights into the relationship between phonon behavior and thermoelectric properties in layer-structured materials.