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In the important widely-studied class of thermoelectric materials,it has been long standing problem to improve the energy conversion efficiencies.The talk provides one solution to this problem by looking for perfect crystals with intrinsically low lattice thermal conductivity.The lattice thermal conductivity evaluation method is based on the combination of first-principle calculations of the vibrational properties within the quasi-harmonic approximation and the Debye-Cailaway model.The method is validated by studying two experimentally well measured Cu3SbSe3 and Cu3SbSe4,and the theoretical thermal conductivity is good agreement with the experimental measurements.Additionally,based on the quasi-harmonic approximation and band structure calculations,the theoretically predicted Cu12Sb4Se13 compound is a promising thermoelectric material due to a low thermal conductivity and a good Seebeck coefficient.I also report high thermoelectric figure of merit ZT(> 2.6)for SnSe,a simple layered binary compound free of Pb and expensive Te.The highest ZT value is due the ultralow lattice thermal conductivity.The proposed rationale for low thermal conductivity in SnSe opens additional opportunities to explore new sets of layered materials that exhibit such bonding anharmonicity.