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

Abstract

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·H2O crystals were obtained in both reactors at temperatures below 200 °C, for times up to 24 h. These microcrystals belong to the triclinic P1̅ 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 °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 monoclinic C2/m space group, was obtained by two processing routes: (i) heating the P2/c microcrystals at 600 °C; or (ii) heating the MnMoO4­·H2O 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 C2/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-MnMoO4 polymorph, at least in a short-range degree.

Unraveling the MnMoO4 polymorphism: a comprehensive DFT investigation of α, β, and ω phases

“…The results for lattice parameters of MnMoO 4 polymorphs obtained by our simulations agree with experimental data at room conditions [ 24 , 27 , 28 , 48 , 49 ] and other theoretical calculations [ 2 ]. Meanwhile, there were not reported data for bond lengths and distortion degree in the literature for evaluated polymorphs.…”

“…In particular, this polymorph is obtained from MoO 3 and MnO (or MnCO 3 ) under 900 °C and 6 GPa; further, the β polymorph can be transformed into α phase when heated for more than 48 h at 60 °C [ 23 , 30 ]. This phase transition was also reported under 600 °C [ 28 , 32 ]. The third polymorph of MnMoO 4 , known as ω-phase (Fig.…”

“…[ 2 ] 10.64 9.70 7.25 90 105.77 90 720.30 β (P2/c) This work 4.868 5.829 5.035 90 89.938 90 142.88 Exp. [ 28 ] 4.818 5.759 4.965 90 90.82 90 137.75 Exp. [ 49 ] 4.821 5.740 4.999 90 90.861 90 – ω (C2/m distorted) This work 10.105 9.250 7.787 90 113.527 90 667.36 Exp.…”

“…Another polymorph often reported is the β phase (Fig. 1 b), which presents a P2/c space group (existing in a wolframite structure) and Mn and Mo octahedral sites [ 28 – 30 ]. In this polymorph, both Mn and Mo cations occupy one-quarter of the interstitial position of the distorted polyhedral close packing [ 30 , 31 ].…”

Effects of tungsten source and tartrate additive on the microstructure and photoluminescence of hydrothermally crystallized ZnWO4

“…Rational control over reaction parameters enables to tailor material properties for a specific application [2,3]. Among various synthesis strategies developed for inorganic compounds, hydrothermal reaction, which utilizes mildly high temperature and pressure to enhance the reactivity of chemical species [4,5], is widely used, since it can crystallize the targeted compound within a reasonably short period of reaction and allows to tailor characteristics of the product via facilely manipulating reactant source/concentration, solution pH, reaction temperature/ duration, and surfactant/chelate species [6]. The key to morphology control of a hydrothermal product is to manipulate the growth habit of the crystallite, where the crystallite may grow via Ostwald ripening [7] and/or oriented attachment [8].…”