Raman spectroscopy of Na_3Co_2SbO_6

Raman spectroscopy together with density functional calculations were used to study lattice dynamics in a layered honeycomb cobaltite Na_3Co_2SbO_6, which can host a field-induced phase related with the Kitaev physics. We show that there develops an additional mode well above Neel temperature (at ≈ 200K) at 525 cm^-1, which origin can be related to an electronic excitation to one of j_3/2 doublets. Moreover, our theoretical calculations demonstrate that the highest frequency intensive mode related to the oxygen vibrations is very sensitive to type of the magnetic order. Thus, we propose to use the softening of this mode as a hallmark of the transition to a fully polarized regime, which is stabilized in Kitaev materials in strong magnetic fields.