Phonon and magnetic properties of nanocrystalline MnWO4 prepared by hydrothermal method

Well-crystallized MnWO4 nanoparticles with 20–100 nm sizes were synthesized by a mild hydrothermal crystallization process. X-ray diffraction and TEM results show that particles have tendency to grow in a shape of nanorods elongated in the c direction. They also show that the particles synthesized from MnCl2 have smaller concentration of defects than those synthesized from MnSO4·H2O. Raman spectra revealed weak changes with decreasing particle size due to phonon confinement effect and defects. In contrast to this behavior, IR spectra showed much more pronounced changes. Origin of this behavior is discussed. Raman and IR spectra showed also presence of additional bands at 923 and 914 cm−1, respectively. These bands have been attributed to presence of terminal WO bonds at the surface of MnWO4 nanocrystallites. Specific heat and magnetization studies revealed broad anomalies at 11.0–11.4 K, which correspond to the two magnetic transitions found for bulk MnWO4 at 12.5 and 13.5 K.

► Nanoparticles of MnWO4 were synthesized with size 20–100 nm.
► Particles synthesized from MnCl2 have smaller concentration of defects than those synthesized from MnSO4·H2O.
► IR spectra of MnWO4 nanoparticles differ significantly from IR spectrum of bulk due to polar character of the observed modes.
► Nanoparticles exhibit clear magnetic phase transition at 11.0–11.4 K.