Structures, Phase Transition And Hygroscopicity Of Fe_2-xY_xMo₃O₁₂

Of the NTE materials, compounds with general formula A2M3O12, where'A'is one of the trivalent cations ranging in size from Al (radius of 0.675 A) to Gd (radius of 1.075 A) and M is W or Mo, are an important class of materials exhibiting NTE. A2M3O12 may crystallize either in monoclinic or orthorhombic structure depending on the A cation size. Only the orthorhombic corner shows significant negative thermal expansion behavior.The materials of the A2M3O12 family with smaller A3+ cation size give rise to smaller NTE or even crystallize in a monoclinic structure at room temperature whose NTE is only possible after monoclinic-to-orthorhombic phase transition at higher temperatures. On the other hand, those with larger A3+cation size exhibit larger NTE over a wide temperature range, and the openings in the network become large enough to admit water molecules which inhibit the flexibility as well as the internal vibrations of the polyhedra which are essential for negative thermal expansion.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 and hygroscopicity have been studied. It is found that Fe2-xYxMo3O12 crystallized 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 with increasing the contents of Y3+so that of Fe1.5Y0.5Mo3O12 crystallizes already in orthorhombic at room temperature and keeps this structure till to very low temperature (lower than 103 K).There are two kinds of water species presenting in Fe2-xYxMo3O12 for x> 0.8, one having little influence on the motions of the polyhedra and another interacting strongly with the polyhedra and hence deteriorating the negative thermal expansion properties of the materials. However, only one kind of water species which does not affect the motions of the polyhedra is found in Fe2-xYxMo3O12 for 0.4<x≤0.8.The materials of the A2M3O12 family are found to be suitable hosts for transition metal and lanthanide ions and have unusually high trivalent ion conduction as well. These features of the compounds are useful for many applications such as fuel cell, phosphor, luminescent materials, superconducting material, optical fiber, gas sensors and laser materials, etc. In this paper, the phosphor of Y2Mo3O12:Eu3+ and Y2Mo3O12:Er3+ are synthesized by high temperature solid-state reaction and their luminescence properties are investigated. It shows that the phosphor Y2Mo3O12:Eu3+ has four major emission peaks locating at 594 nm,613 nm,655 nm and 701 nm and the phosphor Y2Mo3O12:Er3+ has major emission around 655-675 nm.