| Zircon-type ABO4ternary oxides are common accessory minerals, and they sharemany physical chemical properties, as well as displaying variable degrees of solidsolutions among end members existing in a wide variety of sedimentary, igneous, andmetamorphic rocks. The research on zircon-type ABO4compounds has drawnconsiderable interest because it could provide a wide range of geochemical andgeophysical investigations, including studies on the evolution of Earth’s crust andmantle. Apart from the geophysical importance, the zircon-type rare-earthorthovanadates RVO4and orthophosphates RPO4currently attract considerable interestby virtue of their wide potential applications and interesting optical/luminescentproperties. Their phosphor powders with nano size are required, because they can meetmultifunctional applications. It is well-known that the thermodynamic properties ofnanomaterials may difer signifcantly from their bulk counterparts due to largesurface-to-volume ratio. Application of extreme conditions of pressure is a veryeffective way to investigate the structure-property relations. Therefore, zircon-typeYPO4nanoparticles，zircon-type YV1–xPxO4:Eu3+(x=0,0.5,0.7,1.0) solid-solutionnanoparticles and zircon-type LaVO4nanorods have been selected to probe thestructural phase transition sequence, compressibility and spectroscopic properties inthis study, and we have obtained the original results as follow.The high-pressure behavior of zircon structured YPO4(with/without Eu3+doping)nanoparticles was examined at room temperature using in situ synchrotron X-raydiffraction (XRD) and photoluminescence (PL) measurements. In contrast to the reported XRD results of bulk YPO4upon compression, the nanoparticles showed adistinct transition sequence: zircon phaseâ†'scheelite phase (~18GPa) without themetastable monazite phase. By the return to ambient pressure, both XRD and PLresults revealed that the scheelite phase could be reserved. Further Raman experimentshelped us to identify the valuable mode ν1(Ag) of the scheelite structure in thequenched samples. The dopants effect, quasi-hydrostatic stress and nanoscale-inducedsurface energy difference are considered to discuss the high-pressure behavior of thenanoparticles. It is proposed that the nanoscale-induced higher surface energycontribution plays a crucial role in the distinctive high-pressure behavior of thenanoparticles.We probed the high-pressure response of the YV1–xPxO4:Eu3+(x=0,0.5,0.7,1.0)solid-solution nanoparticles using angular dispersive synchrotron XRD and Ramantechniques at room temperature. In situ diffraction results showed that the overallnanoparticles underwent an irreversible zircon-to-scheelite structural transformation.The transition pressures were~9.3GPa,~12.1GPa,~14GPa and~18.4GPa for theYV1–xPxO4:Eu3+(x=0,0.5,0.7,1.0) samples, respectively. Coupled with thezircon-to-scheelite transition features, it was proposed that the transition pressure wasprobably governed by the stiffness of VO4/PO4units in the solid solutions. This claimwas verified by further Raman measurements which revealed that the stiffness ofVO4/PO4units was enhanced with increasing P contents. The structural refinementsshowed that the samples with comparable particle size (20–90nm) became lesscompressible with increasing P content (x=0â†'0.7â†'1.0). However, the compressibilityof the YV0.5P0.5O4:Eu3+sample with smaller particle size (10–30nm) was similar tothat of the YV0.3P0.7O4:Eu3+sample. The general compressibility behavior as a functionof P content was ascribed to the special packing style related to the stiffness ofVO4/PO4tetrahedra in zircon structure, and the higher surface energy contribution wasresponsible for the exceptional compressibility in the smaller nanoparticles.A combination of synchrotron powder XRD and Raman spectroscopy has been used to study high-pressure behavior of the zircon-type LaVO4nanorods. In situhigh-pressure XRD results identified an irreversible zircon-to-monazite phasetransition at~5GPa and a reversible transition to an undetermined secondhigh-pressure phase (phase III) at~12.9GPa. Through Le Bail refinements of the XRDpatterns with zircon-type structure, we show that the zircon-type LaVO4nanorodspossess the smallest bulk modulus among zircon-type rare-earth orthovanadates.Furthermore, negative pressure coefficients of external translational T(Eg) and internalÏ…2(B2g) bending modes have been observed in Raman measurements. The Ramanspectra of phase III with distinctive features have been fully recorded for the first time,and a related structure associated with a coordination increase for V is suggested interms of the post-monazite phase in LaVO4nanorods. Finally, analysis of thetransmission electron microscopy both before and after compression indicates that alarge number of nanorods can be recovered in the quenched samples, allowing us toverify the orientation relationship for zircon-to-monazite phase transformation. |
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