Attributions of rich Raman modes and their temperature dependences in Mn4Nb2O9 single crystals

It is an important task of single crystal Raman spectroscopy to identify the lattice vibration modes in terms of symmetry and further study the temperature-dependent behavior of various Raman modes. A4B2O9 (A = Co, Mn, Fe; B = Nb, Ta) type magnetodielectric or magnetoelectric materials represented by Mn4Nb2O9 not only have high structural anisotropy, but also have strong electron-phonon or spin-phonon coupling, which is very meaningful for the Raman scattering study. Here, we conduct the first investigation of the angle and temperature-dependent Raman scattering behaviors in highly crystalline Mn4Nb2O9 crystals with perfect a-cut. Polarization experiments show that most of the Raman peaks can be clearly identified as A1g and Eg vibration modes, showing a clear anisotropic structure. As the temperature increases from 83 K to 283 K, the Raman shifts of most Raman peaks of Mn4Nb2O9 exhibit a linear softening behavior with the temperature coefficient in the range of −0.010 cm−1 K−1 to −0.015 cm−1 K−1. No remarkable change of Raman shifts occurs at antiferromagnetic (AFM) transition nearby, indicating that the change of the ion position in the AFM transition has little effect on its Raman scattering behavior.