Investigation of Polymorphism and Vibrational Properties of MnMoO4 Microcrystals Prepared by a Hydrothermal Process

MnMoO4 microcrystals were synthesized by hydrothermal methods, and their structural, morphological, and vibrational properties were investigated. Conventional reactors and microwave-heated hydrothermal vessels were used to synthesize micrometer-sized MnMoO4 polymorphs at different temperature and time conditions. MnMoO4 center dot H2O crystals were obtained in both reactors at temperatures below 200 degrees C, for times up to 24 h. These microcrystals belong to the tridinic P (1) over bar space group, and their 26 characteristic Raman bands were identified. A monoclinic MnMoO4 polymorph belonging to the P2/c structure was synthesized at temperatures higher than 200 degrees C only into conventional hydrothermal reactors. Polarized micro-Raman analyses have evidenced and allowed the assignment of 18 phonon modes predicted by group-theory calculations for this polymorph. Another MnMoO4 polymorph, within the monodinic C2/m space group, was obtained by two processing routes: (i) heating the P2/c microcrystals at 600 degrees C; or (ii) heating the MnMoO4 center dot H2O phase at 250 degrees C. For this monoclinic phase, 33 Raman-active bands were identified and assigned, in very good agreement with group-theoretical calculations, which predict 36 modes for the C2/m polymorph. Well-faceted, highly crystalline microcrystals were dearly observed by scanning and transmission electron microscopies, in perfect agreement with X-ray diffraction and Raman spectroscopic analysis. Finally, the appearance of characteristic phonon modes related to molybdenum in octahedral coordination suggests an incipient crystallization of a new, unreported alpha-MnMoO4 polymorph, at least in a short-range degree.