Preparation And Study Of The Negative-thermal-expansion Material Zr₂P₂MO₁₂

Negative thermal expansion (NTE) materials have been receiving increasing attention for their potential applications in engineering materials with controlled coefficients of thermal expansion (CTE) by compositing them with positive thermal expansion materials. A large family of such materials is tungstates and molybdates with the formula A2M3O12, this type of compounds shows negative thermal expansion behavior only in orthorhombic structure. An interesting feature of these structures is the large chemical flexibility. The A3+ cation can not only be a transition metal or rare earth that occupies an octahedral position, but also be substituted by a mixture of tetravalent and bivalent cations, or by a tetravalent cation with partial replacing of M6+ by quinquevalent cations. Zr2P2MO12 (M=W or Mo) have been reported to maintain orthorhombic structure in a wide temperature range.Zr2P2MO12 display anisotropic volume NTE in a wide range of temperature (60-300K,122-400K respectively).In this work, a range of Zr2P2WxMo1-xO12(0≤x≤1) and Zr2PVMo12 samples were prepared by the solid state reaction. The structure changes and properties of Zr2P2WxMo1-xO12 were studied by X-ray diffraction and Raman spectroscopy. The ceramic of Zr2P2WO12 and ZrW2O8 were synthetized which can find use as substrates for athermalization of fiber Bragg gratings.The main results obtained are as follows.1 Zr2P2WxMo1-xO12(0≤x≤1) solid solutions were synthesized by the solid state reaction. The starting compounds of Zr2P2WO12 were sintered at 1200-1400℃for not less than 20min. The starting compounds of Zr2P2MoO12 and Zr2P2WxMo1-xO12 (0<x<1) were sintered at 850-900℃for 0.5-1h. This treatment was followed by grinding and sintering for another 0.5h at 1000-1100℃. The starting compounds of Zr2PVMoO12 were sintered at 850℃for 2h.2 The XRD analyses show that all samples maintain orthorhombic structure and contain a network of corner-sharing AO6 octahedra and MO4/PO4 tetrahedra. Raman spectroscopic studies of the samples at different temperatures reveal that there is no phase transition between 20-600℃3 There are more Raman bands between 1200-1060cm-1 and 760-460cm-1 in the Raman spectra of Zr2P2WxMo1-xO12 compared with the Raman spectra of A2(MO4)3. Raman spectra on the vibrational properties of three kinds of tetrahedra (MoO4, WO4 and PO4 tetrahedra) are discussed by analyzing the difference of the spectra of Zr2P2WxMo1-xO12.4 Zr2PVMO12 and ceramic composites of Zr2P2WO12 and ZrW2O8 were synthesized by the solid state reactions. The XRD analyses show that the ceramic composites can be prepared by rapid cooling to prevent ZrW2O8 from decomposing into ZrO2 and WO3.