  First-principles calculations are employed to investigate the phonon
transport of BiCuOSe. Our calculations reproduce the lattice thermal
conductivity of BiCuOSe. The calculated gruneisen parameter is 2.4~2.6 at room
temperature, a fairly large value indicating a strong anharmonicity in BiCuOSe,
which leads to its ultralow lattice thermal conductivity. The contribution to
total thermal conductivity from high-frequency optical phonons, which are
mostly contributed by the vibrations of O atoms, is larger than 1/3, remarkably
different from the usual picture with very little contribution from
high-frequency optical phonons. Our calculations show that both the high group
velocities and low scattering processes involved make the high-frequency
optical modes contribute considerably to the total lattice thermal
conductivity. In addition, we show that the sound velocity and bulk modulus
along $a$ and $c$ axes exhibit strong anisotropy, which results in the
anisotropic thermal conductivity in BiCuOSe.
