Hygroscopicity,Phase Transition And Optical Property Of A₂M₃O₁₂ Family

Materials with negative thermal expansion(NTE) have being one of investigation hotspot in material field since the NTE property of ZrW2O8 was reported, which are hoped to solve the problems in thermal expansion coefficient mismatch in modern technology divices. With the investigation development, more and more materials are founded to present NTE property. In open framework NTE materials, A2M3O12 family investigated widely has the most stable structure. The applictions of A2M3O12 family are limited by the two main problems: hygroscopicity and phase transition. In this thesis,the Y2Mo3O12 and In2Mo3O12 were selected as the typical representation of the A2M3O12 family to investigate the problems of hygroscopicity and phase transition. On the other hand, the novel excellent NTE materails of HfScW2PO12 and HfScMo2VO12 were synthesized and characterized with stable photoluminescence property at wide temperature range and no hygroscopicity. The main results and innovations of this work are listed as follows:1. The evolution of surface morphology of Y2Mo3O12 ceramics with temperature had been investigated using atomic force microscopy(AFM). It was found that both the shape and size of individual and the integration morphology of the sample showed the contraction-expansion phenomenon, which is consistent with the macroscopic results measured by the dilotmeter. The anisotropic thermal expansion behavior of Y2Mo3O12 can be judged by the change of grain morphology, which is consistent with that of lattice constant change calculated by X-ray diffraction pattern. AFM can be used as an effective tool to study the thermal expansion properties of ceramics and other materials. The hygroscopic of Y2Mo3O12 was reduced effectively by using(LiMg)3+ partially substitute Y3+ ion and tailored thermal expansion coefficients were obtained also.2. The phase transition temperature of In2Mo3O12 was reduced effectively by partial substitution of(HfMg)6+ or(ZrMg)6+ for In3+. In the solid solutions of In2(1-x)(HfMg)xMo3O12, when x?0.85, the phase transition was not observed from room temperature(RT) to 753 K and near zero thermal expansion was obtained. In the solid solutions of In2(1-x)(ZrMg)xMo3O12, even for x=0.9, the phase transition from monoclinic to orthorhombic structure was still observed from the change of thermal expansion curve from RT to 373 K. Therefore, compared to(ZrMg)6+, it is more effective for the substitution of(HfMg)6+ than that of(ZrMg)6+ to reduce the phase transition. However, the softening temperature at 823 K for In2(1-x)(HfMg)xMo3O12 is much lower than that for In2(1-x)(ZrMg)xMo3O12 because no softening phenomemon till 923 K was observed with substitution of(ZrMg)6+. The solid solution In0.6(HfMg)0.7Mo3O12 was selected to improve its NTE property by introducing oxygen vacancy. The results indicated that the introduction of oxygen vacacy had been not effected on the phase transition but could increase the negative coefficient of thermal expansion.3. A doube ions substitution method was used to design novel NTE material of HfScW2PO12. In A2M3O12, the A site ion was substituted with a tetravalent ion and the hexavalent ion at M site was substituted with a pentavalence ion to compensate the valence balance. HfScW2PO12 has the similar orthorhombic structure to that of Sc2W3O12 with space group Pnca(No. 60) and exhibits stable NTE property with an intrinsic coefficient of thermal expansion-1.27 ppm K-1 from 140 to 1469 K without hygroscopicity or phase transition. Besides, HfScW2PO12 possesses stable and intense blue-green photoluminescence(PL) with wavelength from 350 to 550 nm and chromaticity coordinate of(0.159, 0.083) from RT to 10 K. All the results indicated that HfScW2PO12 as a near ultraviolet phosphor could find a potential application in blue-green LEDs.4. Another novel NTE material of HfScMo2VO12 with intense white-light emission was synthesized using the similar method to prepare HfScW2PO12. HfScMo2VO12 adopts an orthorhombic structure similar to Sc2Mo3O12 with space group Pnca(No. 60). Similar to HfScW2PO12, Hf4+/Sc3+ are located at the center of octahedron of Hf4+/Sc3+-O and Mo6+/V5+ are localted at the center of tetrahedron of Mo6+/V5+-O. The octahedra and tetrahedra sharing vertex of oxygen atom form frame structure. HfScMo2VO12 exhibits stable NTE property from 150 to 775 K with an intrinsic coefficient of thermal expansion ?l=-2.11 ppm K-1(RT-873 K) without hygroscopicity or phase transition. Besides the NTE property, Hf ScMo2VO12 possesses stable wide-band photoluminescence(PL) property(RT-10 K) from 380 to 650 nm with chromaticity coordinate of(0.27, 0.39) belonging to white light at RT. The results suggest potential applications for HfScMo2VO12 as as a white phosphor in white LEDs.