Growth Of (Zn_0.9Co_0.1)₂TiO₄?Zn_0.9Co_0.1Nb₂O₆ Single Crystals By Floating Zone Method And Their Structural Stability And Electromagnetic Properties

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(Zn_0.9Co_0.1)₂TiO₄ and Zn_0.9Co_0.1Nb₂O₆,or the physical properties of(Zn_0.9Co_0.1)₂TiO₄ and Zn_0.9Co_0.1Nb₂O₆,there is no relevant report.Therefore,the(Zn_0.9Co_0.1)₂TiO₄ and Zn_0.9Co_0.1Nb₂O₆ 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(Zn_0.9Co_0.1)₂TiO₄ crystal?1?A black translucent(Zn_0.9Co_0.1)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(Zn_0.9Co_0.1)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(Zn_0.9Co_0.1)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(Zn_0.9Co_0.1)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(A_1gg 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(Zn_0.9Co_0.1)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(Zn_0.9Co_0.1)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 2.70?_B.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(Zn_0.9Co_0.1)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(Zn_0.9Co_0.1)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 Zn_0.9Co_0.1Nb₂O₆ crystal?1?The Zn_0.9Co_0.1Nb₂O₆ 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 Zn_0.9Co_0.1Nb₂O₆ 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 Zn_0.9Co_0.1Nb₂O₆ 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 Zn_0.9Co_0.1Nb₂O₆ 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 Zn_0.9Co_0.1Nb₂O₆ 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 Zn_0.9Co_0.1Nb₂O₆ 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 1.47?_B.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 Zn_0.9Co_0.1Nb₂O₆,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 Zn_0.9Co_0.1Nb₂O₆ 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(Zn_0.9Co_0.1)₂TiO₄ and Zn_0.9Co_0.1Nb₂O₆ 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(Zn_0.9Co_0.1)₂TiO₄ and Zn_0.9Co_0.1Nb₂O₆ 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.