Structures, Phase Transition, and Crystal Water of Fe2-xYxMo3O12

Materials with the formula Fe2-xYxMo3O12 (x = 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, and 1.8) have been synthesized, and their structures, phase transitions, hygroscopicity, and thermal expansion properties have been studied by Raman spectroscopy, X-ray diffraction, and differential scanning calorimetry. It is found that Fe2-xYxMo3O12 crystallize in a single monoclinic for x <= 0.4 and a single orthorhombic structure for x >= 0.5. The monoclinic-to-orthorhombic phase transition temperature can be effectively decreased by increasing the contents of Y3+ so that Fe1.5Y0.5Mo3O12 crystallizes already in orthorhombic at room temperature and keeps this structure till very low temperatures (lower than 103 K). Two kinds of water species are found to present in Fe2-xYxMo3O12, one having little influence on the motions of the polyhedra, whereas another interacting strongly with the polyhedra and even causing a cell volume to contract. The amount of crystal water decreases with reducing the content of Y3+ so that the motions of the polyhedra in Fe2-xYxMo3O12 for 0.5 <= x <= 0.8 are not obviously influenced by the crystal water. It is shown that the orthorhombic Fe2-xYxMo3O12 exhibit negative (x > 1.0), near zero (x = 1.0), and low (x < 1.0) thermal expansion properties after the removal of the crystal water.