Investigation of Polymorphism and Vibrational Properties of MnMoO4Microcrystals Prepared by a Hydrothermal Process

MnMoO 4 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 MnMoO 4 polymorphs at different temperature and time conditions. MnMoO 4 ·H 2 O crystals were obtained in both reactors at temperatures below 200 °C, for times up to 24 h. These microcrystals belong to the triclinic P 1̅ space group, and their 26 characteristic Raman bands were identified. A monoclinic MnMoO 4 polymorph belonging to the P 2/ c structure was synthesized at temperatures higher than 200 °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 MnMoO 4 polymorph, within the monoclinic C 2/ m space group, was obtained by two processing routes: (i) heating the P 2/ c microcrystals at 600 °C; or (ii) heating the MnMoO 4 ·H 2 O phase at 250 °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 C 2/ m polymorph. Well-faceted, highly crystalline microcrystals were clearly 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 α-MnMoO 4 polymorph, at least in a short-range degree.