Study On The Phase Transition Of Columbite A²⁺Nb₂O₆(A²⁺=ca、zn、mg) At High Pressure

High-pressure and low-temperature research can discover some newphenomenon which substances cannot exhibit at normal pressure androom temperature, disclose new rules and properties, and even find newsubstances. It provides new approaches for investigating substances’properties at high pressure and low temperature and explaining physicalphenomenon at normal pressure and room temperature. With rapiddeveloping of static high pressure measuring technology, diamond anvilcells （DAC） has been widely applied to the high-pressure study of manymaterials. It is well-known that the application of pressure on crystalsresults in decrease of interatomic distance and increase of the overlappingof the adjacent electron orbit，which will change the electronic structureand the forces of the adjacent atoms. Consequently a forced equilibriumstate under pressure is formed. If there are the rearrangement of atomsand the change of symmetry in it, the forced equilibrium state is a newcrystal structure phase, and the phase transition is named as thepressure-induced phase transition. Raman spectroscopy and x-raydiffraction combined with the high-pressure technology can make us convenient to investigate the structural changes of materials underpressure, so the high-pressure in-situ Raman spectroscopy is one of themost useful high-pressure experimental technologies, and supplies asensitive and effective probing method for investigating phase transitionand chemical reaction of the substance in DAC.Recently, high pressure study on AB2O₆of columbite structure waswidely applied in the production for industrial chemicals and polymers.Raman spectroscopy and x-ray diffraction combined with thehigh-pressure technology can make us convenient to investigate thestructural changes of materials under pressure. At present, high pressurestudy on niobate had been reported, but so far no has accurate transitionpoint or the structural characterization of their high-pressure phase.Binary niobate ceramies, with the formula A²⁺Nb₂O₆where A=Ca, Mg,Zn, Fe, Ni, Cd, Co，Mn, have the orthorhombic columbite structure.There is a growing interest in the columbites as microwave dielectricceramics, due to their lower processing temperatures, less complicatedprocessing, the lower cost of niobium compared with tantalum andlow-temperature cofired ceramics （LTCC） with Cu2+.Up to now, there are few researches on the crystal structural evolutionof columbites under high pressure. Yosuke Shiratori et al. reported onpressure-induced phase transitions in NaNbO₃by high-pressure Ramanspectroscopy, and found different successive transitions below16GPa. Pistorino et al. and Serena et al. reported on crystal structure of （Fe,Mn）（Nb, Ta）2O₆under high pressure by single-crystal X-ray diffraction,and did not observed phase transition below7GPa. On the crystalstructural evolution of A²⁺Nb₂O₆under high pressure, there have been noreports. The in-situ high pressure Raman spectroscopy is a very powerfultechnology for dynamically investigating the pressure-induced phasetransition of materials, and the in-situ XRD spectrum is intuitive tocharacterize the crystal structure. In the present work, we have performedin-situ high pressure Raman spectroscopy and in-situ XRD measurementswith a DAC to investigate the pressure-induced phase transition inCaNb₂O₆up to23GPa, ZnNb₂O₆and MgNb₂O₆up to30GPa respectively.Besides, the comparison of crystalline phases obtained by high pressureand by low-temperature crystallization is an important direction of highpressure research. We also studied the Raman peaks of these three niobateat low temperature by in-situ Raman spectroscopy associated withLinkam variable-temperature experimental system, in comparison withtheir pressure-induced phase transition.The main contents and conclusions as follows:1. Pressure-induced phase transition in CaNb₂O₆.We have performed the in-situ high pressure studies of CaNb₂O₆using Raman spectroscopy and Synchrotron X-ray diffraction in aDAC up to23GPa. Both Raman and X-ray diffraction data provide evidence of a reversible phase transition from Pbcn to P21/c,beginning from about8GPa and completing at15GPa. Thecompression is mainly on the a direction of stacking ofCaNbNbCaNbNb layers, and the structure expands along the bdirection. The phase transition involves the deformation andrearrangement of the NbO₆and CaO₆octahedra, as well as thedistortion of the NbO₆and CaO₆octahedral chains. Thehigh-pressure phase keeps stable below22.9GPa. All of thesubsequent XRD patterns above15.1GPa are well described withmonoclinic symmetry, and the indexed lattice parameters area=8.2857, b=5.92358, c=4.69152, β=104.41°by the Pawleyrefinements of the X-ray diffraction data.Temperature dependence Raman spectra of all the crystals showthat there is no phase transition in the whole temperature range（-190-560℃）.The temperature-dependent shift of Raman phononsis also discussed in the context.2. Pressure-induced phase transition in ZnNb₂O₆.We have performed the in-situ high pressure studies of ZnNb₂O₆using Raman spectroscopy in a DAC up to30GPa. Raman spectraprovide evidence of a reversible phase transition, beginning fromabout10GPa and completing at16GPa. The high-pressure phase keeps stable below20GPa. With the pressure over20GPa, adisordered state formed.3. Pressure-induced phase transition in MgNb₂O₆.In situ x-ray diffraction and Raman spectroscopy were used toexplore the pressure-induced phase transformation of MgNb₂O₆single crystal powder with orthorhombic columbite structure29.4GPa and35.0GPa, respectively. Results indicate that MgNb₂O₆initially transforms to the monoclinic structure at about8GPa, andsuch a phase is completed at about15GPa. Also we identified thephase transition as a space group change from Pbcnto P2/C. Thehigh-pressure phase remained stable up to about22.3GPa. With thepressure over22GPa, a disordered state formed.Besides, we carefully studied the crystal structure of1%molEu3+doped MgNb₂O₆under high pressure. A complete phasetransition initially occurred at about8GPa and ultimatelycompleted at around15GPa was observed, which was slightly lessthan pristine MgNb₂O₆. This is reasonably ascribed to higherelectrons deriving from the substitution of Mg2+by Eu3+. Aftersubstitution, the Mg2+tetrahedron vacancy is formed based uponelectronic compensation principle. Furthermore, the volume ofMgO₆octahedraon was also varied by Eu3+substitution and theexistence of vacancy made the phase transition easily happened. As mentioned above, a slightly decrease of phase transition wasinvestigated in our study.In summary, our results not only provide an experimental meansfor deeply understanding the phase transition of A²⁺Nb₂O₆andmicrostructure under different pressure with columbite structurebut also are great significance for its Raman peaks identificationunder normal pressure. Using these experimental methods, it couldbe greatly helpful for the application of columbite structurematerials and novel functional species.