Zhang Shuofeng, Xu Ben, Lin Yuanhua, Nan Cewen, Liu Wei
School of Materials Science and Engineering, Tsinghua University People's Republic of China.
Key Laboratory of Advanced Materials of Ministry of Education of China People's Republic of China
RSC Adv. 2019 Apr 26;9(23):12886-12894. doi: 10.1039/c9ra00247b. eCollection 2019 Apr 25.
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 μV K at 300 K. The value of the dimensionless figure of merit, , along the axis can be as high as 0.661 at 800 K, corresponding to a lattice thermal conductivity as low as 1.34 W (m K). The low of TiNBr is associated with a collectively low phonon group velocity (2.05 × 10 m s on average) and large phonon anharmonicity that can be quantified using the Grüneisen 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.
层状结构材料通常被认为是热电材料的理想候选者，因为由于原子层间相互作用，它们往往表现出固有的低热导率。层状结构材料的电学性质可以通过各种方法轻松调节，如能带改性和插层。我们报道了TiNBr，它是层状结构金属氮化物卤化物体系MNX（M = Ti、Zr、Hf；X = Cl、Br、I）的一员，在300 K时表现出2215 μV K的超高塞贝克系数。在800 K时，沿 轴的无量纲品质因数 值可高达0.661，对应于低至1.34 W（m K）的晶格热导率。TiNBr的低 与集体较低的声子群速度（平均2.05×10 m s）和大的声子非谐性有关，声子非谐性可以用格林艾森参数和三声子过程来量化。高非谐模式下原子运动的动画主要涉及N原子的运动，电荷密度差表明N原子随着非谐性的合并而极化。此外，能量-位移曲线的拟合过程验证了除三声子过程外，四阶非谐效应在TiNBr的整体非谐性中也很重要。我们的工作是对TiNBr热电性质的首次研究，可能有助于建立低晶格热导率与声子振动模式行为之间的联系。
