Growth of (Zn0.9Co0.1)2TiO4、Zn0.9Co0.1Nb2O6 Single Crystals by Floating Zone Method and Their Structural Stability and Electromagnetic Properties

Author:Liu Ying

Supervisor:xu da peng


Degree Year:2019





The growth of any kind of crystal involves two aspects:one is the basic physical problem of the growth law and growth mechanism of the crystal,the various growth laws and the complicated growth mechanism make each crystal growth difficult.The second is the growth technology,which includes the selection of growth methods and equipment,the exploration and optimization of growth processes,and the mastery of many key technologies.Therefore,exploring the growth of new crystalline materials is very meaningful and valuable,both in basic scientific research and in potential applied research.Zn2TiO4 and ZnNb2O6 have different structures,but all have excellent dielectric and optical properties,and are widely studied functional materials.Zn2TiO4 is an anti-spinel structure,which can be regarded as composed of tetrahedral ZnO4,octahedral ZnO6 and TiO6.In octahedron,Zn and Ti are randomly occupied.In the ZnNb2O6 structure,the Zn2+ion is surrounded by six oxygen atoms to form a ZnO6octahedron,and the Nb5+ion also forms an NbO6 octahedron with the oxygen atom.These materials with tetrahedral and octahedral motifs are interesting because they allow the doping of several ions and have different physicochemical properties compared to the parent compound.In modern information technology,a single semiconductor cannot meet the requirements of storage speed and scale.Many studies have shown that doping magnetic ions in wide band gap semiconductors can form dilute magnetic semiconductors,which is an effective method to obtain ferromagnetic new functional materials.The existing research objects mainly focus on ZnO and TiO2 wide band gap oxide semiconductors.From the comparison of structure and physical properties,Zn2TiO4 and ZnNb2O6 are also wide band gap semiconductors;and there are no magnetic elements in the structure;the microstructure is composed of tetrahedral and octahedral elements,although the connection is different.The doping of magnetic ions into Zn2TiO4 and ZnNb2O6 to study the magnetic difference before and after doping is of great significance from the perspective of dilute magnetic semiconductor research.So far,no matter whether it is the crystal growth of(Zn0.9Co0.1)2TiO4 and Zn0.9Co0.1Nb2O6,or the physical properties of(Zn0.9Co0.1)2TiO4 and Zn0.9Co0.1Nb2O6,there is no relevant report.Therefore,the(Zn0.9Co0.1)2TiO4 and Zn0.9Co0.1Nb2O6 single crystals were grown by optical floating zone method and their structures were characterized.The lattice vibration and structural stability of the samples were investigated by Raman spectroscopy.The electromagnetic properties of the samples were investigated using a comprehensive physical property test system(PPMS-16)and AC impedance spectroscopy.1.Growth and properties of(Zn0.9Co0.1)2TiO4 crystal(1)A black translucent(Zn0.9Co0.1)2TiO4 single crystal having a length of about30 mm and a diameter of about 5 mm was first grown.Mastered the best growing conditions.The(Zn0.9Co0.1)2TiO4 single crystal grown along the(620)crystal plane is still a spinel structure with a lattice parameter of a=b=c=8.46?,which is slightly smaller than the lattice parameter of 8.48?of Zn2TiO4.Elemental analysis showed that Ti ions existed in Ti4+valence state,and no electron emission peak of Ti3+was found.Co exists in the form of divalent ions Co2+.No peak representing oxygen deficiency was observed.(2)Variable temperature Raman studies show that both(Zn0.9Co0.1)2TiO4 and undoped Zn2TiO4 belong to the(Fd-3m)space group.However,after Co2+doping,some Zn2+ions in the tetrahedral and octahedral positions are replaced,so that the three ions of Zn2+,Co2+and Ti4+randomly occupy the original position,and the ion species increase,which leads to the order-disorder enhancement of the structure.The optical phonon mode is stable in the temperature range of-180°C500°C,and the(Zn0.9Co0.1)2TiO4 structure has good thermal stability in this temperature range.Compared with Zn2TiO4,Co2+ion doping hinders the migration of Zn2+and Ti4+cations between tetrahedron and octahedron,resulting in tetrahedral sublattice(A1gg mode)and octahedral sublattice(306cm-1,481cm-11 mode).The local lattice effect in the change is changed,and the effect above room temperature is particularly obvious.(3)In situ Raman study at room temperature and high pressure shows that(Zn0.9Co0.1)2TiO4 undergoes pressure-induced phase transition during pressurization.The phase transition starts from about 25GPa and is completed to about 31.2GPa.The high pressure phase is a high pressure CaTi2O4 structure,which is basically the same as the previously reported Zn2TiO4 pressure-induced phase transition.However,during the pressure relief process,the sample remains unchanged in the high pressure phase and does not return to the spinel structure before pressurization.This is different from Zn2TiO4 and should be related to the doping of Co2+ions.(4)Magnetic studies have found that(Zn0.9Co0.1)2TiO4 undergoes a transition from paramagnetic to antiferromagnetic in the temperature range of 300K5K,and the Neel temperature is 19K.Weak ferromagnetism was observed at temperatures below the Nell temperature(5K)with an effective magnetic moment of 2.70μB.In comparison with Co2TiO4 and ZnCoTiO4,ferromagnetism below the antiferromagnetic and Nell temperatures was observed,but no spin glass state similar to that in Co2TiO4 was observed.Its magnetic properties may be derived from Co2+-Co2+interactions at different positions of tetrahedron and octahedron.Zn2TiO4 exhibits room temperature ferromagnetism and diamagnetism,ferromagnetism dominates at low fields,and diamagnetism dominates at high fields.There is a ferromagnetic mutation below 10K.Theoretical calculations show that Ti3+ions in Zn2TiO4 structure at zero temperature may be the main cause of ferromagnetism.(5)At a temperature of 600℃or less,the conductivity of(Zn0.9Co0.1)2TiO4 is extremely small,the conductivity at 600℃is 1.9×10-6Scm-1.In the range of600℃800℃,the conductivity increases with temperature,which satisfies the linear relationship of Arrhenius,and reflects the heat-activated electron transport process with an activation energy of 1.71 eV.The conductivity of(Zn0.9Co0.1)2TiO4 at 800℃is1.1×10-4Scm-1,which is lower than that of undoped Zn2TiO4(1×10-5Scm-1)is an order of magnitude higher,and Co doping with Zn2TiO4 improves conductivity.2.Growth and properties of Zn0.9Co0.1Nb2O6 crystal(1)The Zn0.9Co0.1Nb2O6 single crystal was first grown,with a length of about27 mm and a diameter of 7 mm,and the whole appeared black-blue.Mastered the optimal conditions for crystal growth.The single crystal grows along the(102)crystal plane and remains a Neodymium iron structure,but the XRD peaks are slightly offset from the high angle direction compared with ZnNb2O6.Elemental analysis shows that Zn is in the+2 valence state,Nb is in the+5 valence state,and is not affected by Co ion doping;Co exists in the form of divalent ion Co2+,and Co2+replaces part of Zn2+ion to form CoO6 octahedron.No peak representing oxygen deficiency was observed.(2)Variable temperature Raman studies show that Zn0.9Co0.1Nb2O6 belongs to the space group Pbcn(60).The optical phonon mode is stable in the range of-180°C to 500°C,showing that the structure has good thermal stability in this temperature range.The Raman spectra changes with temperature of Zn0.9Co0.1Nb2O6 and ZnNb2O6single crystals can be attributed to the non-harmonic interaction between phonons.The CoO6 octahedron formed by the substitution of Co2+ions instead of some Zn2+ions does not affect the NbO6 octahedron.(3)In situ Raman study at room temperature and high pressure showed that pressure-induced phase transition occurred in Zn0.9Co0.1Nb2O6 during pressurization.The phase transition starts from about 9.93GPa and is completed at about 15.5GPa.The original NbO6 octahedron in the formed high pressure phase loses Raman activity.When the pressure increases to about 23.78GPa,the high pressure phase disappears into a disordered state.The pressure relief process indicates that the phase change is reversible.Compared with the pressure effect of ZnNb2O6,the pressure range of the phase change process is the same,and the original structure is restored after pressure relief.However,the NbO6 octahedron in the high pressure phase formed by Zn0.9Co0.1Nb2O6 above 15.5GPa loses the Raman activity,which is different from the pressure-induced high pressure phase of ZnNb2O6.The reason may be that Co2+ions replace the CoO6 octahedron formed by some Zn2+ions,which indirectly affects the vibration state of NbO6 octahedron under pressure.(4)Magnetic studies have found that Zn0.9Co0.1Nb2O6 shows obvious paramagnetism in the range of 300K20K.When the temperature dropped to 5K,a very weak hysteresis loop was observed and ferromagnetism was present.No antiferromagnetic properties were observed in the ZFC and FC curves,and the effective magnetic moment was 1.47μB.The M-H curves of undoped ZnNb2O6 in the range of 300K5K show a superposition of weak ferromagnetism and weak paramagnetism.The low field ferromagnetism is obvious,and the high field paramagnetism is obvious.It is found that the doping of Co causes the weak ferromagnetism of the original ZnNb2O6 to disappear,while the paramagnetism is enhanced,and the magnetic properties of the sample after doping are closer to the magnetic properties of CoNb2O6.Due to the presence of a part of CoO6 octahedron in Zn0.9Co0.1Nb2O6,a one-dimensional ferromagnetic chain similar to that in CoNb2O6structure can be formed,but there is no antiferromagneticity of a two-dimensional isosceles triangular lattice.(5)Under the temperature of 700°C,the conductivity of Zn0.9Co0.1Nb2O6 is very small.The conductivity increases with the increase of temperature in the range of700°C900°C,which satisfies the Arrhenius linear relationship.The activation energy is 1.94eV,which reflects the heat activate the electron transport process.The conductivity at 900°Cis 6.5×10-5Scm-1,which is higher than the conductivity of ZnNb2O6,but lower than the conductivity of CoNb2O6.The conductivity of Co-doped ZnNb2O6 increases,and the Co content should also affect high-temperature conductivity.The above research results have both scientific significance and application value.The(Zn0.9Co0.1)2TiO4 and Zn0.9Co0.1Nb2O6 single crystals were grown for the first time,which provided a material basis for in-depth study of its intrinsic properties and practical applications.Mastered the key technologies of these two single crystal growth,providing a technical reference for the growth of similar single crystals.Some physical properties of(Zn0.9Co0.1)2TiO4 and Zn0.9Co0.1Nb2O6 single crystals and their intrinsic physical mechanisms have been studied.They are important in basic science,and they are expanding their application range and designing and developing new functional materials.The aspect has important reference value and guidance.