  Temperature-dependent FAR-infrared reflectivity spectra of partially ordered
magnetodielectric La2CoMnO6 is presented, from room temperature up to 675 K. A
clear first-ordered structural phase transition (SPT) from a monoclinic
structure with P2_1/n symmetry to a rhombohedral phase with R-3 symmetry was
evidenced from the behaviour of polar phonon modes at TC~590 K. The temperature
dependences of the transversal and longitudinal phonon branches, dielectric
strengths, and damping of the strongest dielectric modes confirm the
significant contribution of the phonon modes on the SPT, and revealed an
important lattice anharmonicity, particularly for the low frequency modes. In
addition, these investigations showed that structural ordering does not inhibit
the SPT, and provided valuable information towards the polar phonons, their
implications on intrinsic dielectric constant in double perovskites and in
related compounds.
