Solvothermal Synthesis and Crystal Structures of Alkali Molybdates

Abstract

Within the frame of systematic morphological studies concerning the solvothermal formation of nanoscale and microscale molybdenum oxides from the interaction of a molybdenum-based precursor such as MoO 3 ¥ 2 H 2 O with ionic additives such as alkali and earth alkali halides, we studied ± with the aim to elaborate preparative guidelines ± the influence of the precursor structure and the alkali halide upon the crystal structure of the emerging alkali polymolybdates in terms of solvothermal fields and high-throughput solvothermal techniques. The discussion of the resulting crystal structures revealed a structure-directing potential of the alkali cations that was explored for the synthesis of new mixed alkali polymolybdates.

Crystal chemistry and topology of Rb–MIII molybdates (M = Fe, Sc, In) and triclinic Rb2Mo4O13: novel building blocks, decorated chains and layers

“…Crystal symmetries and unit-cell dimensions of these compounds are listed in Table 10, and are briefly discussed in the next section. Recently, Michailovski (2005) reported the solvothermal synthesis of Rb 2 Mo 4 O 13 , but did not provide any crystallographic details. …”

Hydrothermal Synthesis of Molybdenum Oxide Based Materials: Strategy and Structural Chemistry

“…We have widely investigated the complex hydrothermal structure–synthesis relationships among binary and ternary alkali polyoxomolybdates containing potassium, rubidium and caesium. Two key results brought us to devise an extensive field study:82…”

“…The hydrothermal treatment of Mo VI ‐based starting materials in aqueous MF (M=K–Cs) solutions is a convenient preparative short‐cut to highly fluorinated molybdates with structural motifs based on isolated octahedral moieties or on their connection into dimers or chains, respectively. Their preparation is facilitated with respect to conventional high‐temperature methods, because the hydrothermal syntheses proceed under mild conditions (temperatures of 220 °C and below and no need for hydrogen fluoride) 82. Furthermore, a multitude of alkali difluorooctamolybdates of the M 6 Mo 8 O 26 F 2 ⋅ n H 2 O type (M=K–Cs) is readily available from this synthetic pathway through control of the reaction conditions.…”

“…Up to now, little is known about the structural chemistry of fluorinated POMs: apart from [Mo 4 O 12 F 2 ] 2− ,87 the difluorooctamolybdate ion [Mo 8 O 26 F 2 ] 6− has only been the second fully characterised fluorinated polyoxomolybdate anion with K 6 Mo 8 O 26 F 2 ⋅6 H 2 O as its single representative (Figure 3) 88. The triclinic M 2 Mo 4 O 13 tetramolybdates83 and the M 6 Mo 8 O 26 F 2 ⋅ n H 2 O‐type alkali difluorooctamolybdates (M=K–Cs)82, 88 are structurally closely related (Figure 4): when the hydrothermal synthesis temperature is raised from 180 °C to 220 °C, the [Mo 4 O 13 ] 2− chain is cut up into discrete [Mo 8 O 26 F 2 ] 6− units.…”

Hydrothermal synthesis of anisotropic alkali and alkaline earth vanadates

“…46 Contrary to the anisotropic vanadate formation discussed in this study, most of the analogous molybdenum oxide-based reactions either led to millimeter-sized molybdate crystals through the reaction with the ionic compound 72 or to the production of additive-free MoO 3 fibers. In other words, the concept of initiating the growth of anisotropic products in a given hydrothermal system through the presence of ionic auxiliary substances did not work for the investigated molybdate systems.…”