First-principles study of the layered thermoelectric material TiNBr

Layer-structured materials are often considered to be good candidates for thermoelectric materials, because they tend to exhibit intrinsically low thermal conductivity as a result of atomic interlayer interactions. The electrical properties of layer-structured materials can be easily tuned using various methods, such as band modification and intercalation. We report TiNBr, as a member of the layer-structured metal nitride halide system MNX (M = Ti, Zr, Hf; X = Cl, Br, I), and it exhibits an ultrahigh Seebeck coefficient of 2215 $\mu V/K$ at 300K. The value of the dimensionless figure of merit, ZT, along A axis can be as high as 0.661 at 800K, corresponding to a lattice thermal conductivity as low as 1.34 W/(m K). The low ${\kappa_l}$ of TiNBr is associated with a collectively low phonon group velocity ($2.05\times 10^3 $ m/s on average) and large phonon anharmonicity that can be quantified using the Gr\"uneisen parameter and three-phonon processes. Animation of the atomic motion in highly anharmonic modes mainly involves the motion of N atoms, and the charge density difference reveals that the N atoms become polarized with the merging of anharmonicity. Moreover, the fitting procedure of the energy-displacement curve verifies that in addition to the three-phonon processes, the fourth-order anharmonic effect is also important in the integral anharmonicity of TiNBr. Our work is the first study of the thermoelectric properties of TiNBr and may help establish a connection between the low lattice thermal conductivity and the behavior of phonon vibrational modes.