The Synthesis Of CaMoO₄:Eu³⁺ And High-pressure Studies Of CaMoO₄ And CaMoO₄:Eu³⁺

Metal molybdates?AMoO₄?and tungstates?AWO₄?of relatively large bivalent cations?ionic radius > 0.99 ?;A = Ca,Ba,Sr,Pb and Eu,in addition to Cd only for the molybdates?crystallize belongs to the tetragonal scheelite structure with space group of I41/a.These nanomaterials are excellent luminescent material owing to their good chemical and thermal stability.Rare earth luminescent materials have unique luminescent properties because of containing rare earth elements of spectral term structure.Rare-earth luminescence materials are used as lighting light source,display,imaging and scintillation crystals due to the unique luminescent characteristics.High-pressure research has proven to be a valid tool to intensively understanding the main physical properties of AWO₄ and AMoO₄ compounds.Pressure is considered to be a cleaner probe for investigating the delicate balance between longand short-range forces by changing interatomic distances only.A large number of experiments confirm that the different performance between nano and bulk materials is reflected by the following two aspects: the critical pressure for phase transformation and the phase transformation mechanism.The high pressure research of CaMoO₄ is only limited to the low pressure of bulk material.In this paper,hyperfine red phosphors composed of Eu³⁺ ion-doped CaMoO₄ with varying morphologies and size were prepared via a facile precipitation method at room temperature without any organic additives.At the same time,the high-pressure investigations of the dried persimmon shape structure of CaMoO₄:Eu³⁺ were conducted via angle-dispersive X-ray diffraction,Raman scattering and luminescence spectroscopy in diamond anvil cells at room temperature in this paper,and explores the physical reasons of the similarities and differences of CaMoO₄:Eu³⁺ nanomaterial and CaMoO₄ bulk at the high-pressure.The details contents of this paper are as follows:?1?The product of CaMoO₄:Eu³⁺ were prepared with ammonium heptamolybdate as the source of molybdenum,and the concentration of the reactant was 0.2 mol � L^-1.X-ray diffraction?XRD?,scanning electron microscopy?SEM?,and transmission electron microscopy?TEM?were used to characterize the synthesized products.The results demonstrated that the morphology of the as-synthesized CaMoO₄:Eu³⁺ is a uniform dried persimmon shape structure with an average width of 1.7 ?m and a thickness of 668 nm,consisting of an average thickness of 10 nm multiple two dimensional nanosheets.?2?The product of CaMoO₄:Eu³⁺ were prepared with ammonium heptamolybdate as the source of molybdenum,and the concentration of the reactant was 0.1 mol � L^-1,the other experimental conditions were consistent with?1?.The XRD and SEM investigations were used to characterize the synthesized products.The results demonstrated that the morphology of the as-synthesized CaMoO₄:Eu³⁺ is a uniform disklike structure with an average width of 650 nm and a thickness of 150 nm.?3?The product of CaMoO₄:Eu³⁺ were prepared with ammonium heptamolybdate as the source of molybdenum,and the concentration of the reactant was 0.05 mol � L^-1,the other experimental conditions were consistent with?1?.The XRD and SEM investigations were used to characterize the synthesized products.The results demonstrated that the morphology of the as-synthesized CaMoO₄:Eu³⁺ is a uniform spindle-like particles with a length of approximately 2.2 ?m and a diameter of approximately 725 nm.?4?The product of CaMoO₄:Eu³⁺ were prepared with ammonium heptamolybdate as the source of molybdenum,and prolonging the aging time to 8 days,the other experimental conditions were consistent with?1?.The XRD and SEM investigations were used to characterize the synthesized products.The results demonstrated that the morphology of the as-synthesized CaMoO₄:Eu³⁺ is a uniform cauliflower shape structure with a length of approximately 4.5 ?m and a diameter of approximately 3.3 ?m.?5?At room temperature,in high-pressure synchrotron x-ray diffraction and Raman scattering studies of bulk CaMoO₄ were carried out to study the behaviors of high pressure phase transition.The first phase transition of bulk CaMoO₄ occurs at 15.2 GPa,which are a tetragonal?I41/a?scheelite to a monoclinic?I21/a?fergusonite structure.With increasing pressure to 22.8 GPa,the second phase transition of bulk CaMoO₄ were observed.By the theoretical predictions,the second high-pressure phase is predicted to the monoclinic post-fergusonite structure.This phase is the result of the monoclinic fergusonite structure enduring further torsion distortion.The scheelite structure bulk modulus B0 = 83.9?6?GPa of bulk CaMoO₄ is reasonably consistent with the theoretical value.Pressure induced changes in the Raman spectra and in the phonon frequencies are attributed to two phase transitions observed around 11.2 and 15.3 GPa,which are reversible.The Bg mode at 113 cm-1,associated with the MoO₄^2--MoO₄^2-vibration along the c-axis,exhibits a softenmg in the scheelite structure,while in the high pressure phases its slope becomes positive.The stretching modes of the MoO₄^2-group exhibit drastic changes in their pressure dependencies at second phase transitions.?6?At room temperature,in high-pressure synchrotron x-ray diffraction,Raman scattering and luminescence spectroscopy studies of CaMoO₄:Eu³⁺ dried persimmon shape structure were carried out to study the behaviors of high pressure phase transition.Two phase transitions of CaMoO₄:Eu³⁺ dried persimmon shape structure observed around at 17.3 GPa and 23.8 GPa by high-pressure synchrotron x-ray diffraction spectroscopy study,which are also a tetragonal scheelite to monoclinic fergusonite and then to the monoclinic post-fergusonite structure.Moreover,for nanocrystalline materials,the bulk modulus usually increases compared to their respective bulk material on account of a higher surface energy contribution.The scheelite structure bulk modulus of CaMoO₄:Eu³⁺ dried persimmon shape structure is B0 = 82.3?4?GPa,which is in agreement with bulk CaMoO₄.Such an abnormal phenomenon is ascribed to the unique intrinsic geometry in the dried persimmon shape structure.Pressure induced changes in the Raman spectra and in the phonon frequencies are attributed to two phase transitions observed around 12.1 and 16.2 GPa,which is also in agreement with bulk CaMoO₄.The transformation pressure is slightly enhanced in dried persimmon shape structure CaMoO₄:Eu³⁺ as compared to the bulk due to the difference in surface energy between the two samples of different sizes.The high PL spectroscopy of CaMoO₄:Eu³⁺ dried persimmon shape structure occured a phase transitions at 11.7 GPa.In the fergusonite structure,Eu³⁺ ions sites changing from S₄ to C₂ symmetry.