Growth And Dielectric Properties Of Ferroelectric La₂Ti_2-xTa_xO₇ Single Crystal By Floating Zone Method

La₂Ti₂O₇?LTO?crystal,with perovskite-like layered structure,is an excellent ferroelectric for high temperature transducer and electro-optic devices.For a long time,research on LTO has been focused on polycrystalline ceramics and thin films,while less attention is paid to LTO single crystal.In recent years,research subject has been re-paid to the crystal structure and electrical structure of LTO,via studying the microelectronic to design and improve its performance.It's reported that LTO is a potential multiferroic material,nonetheless,high quality LTO single crystal is difficult to obtain,and the modified LTO study is rare.La₂Ti₂O₇??5×70 mm?and Ta-doped La₂Ti₂O₇?Ta-LTO???5×40 mm?single crystals have been successfully carried out by the optical floating zone technique using a high power halogen lamp image furnace.The growth parameters have been optimized,such as temperature,growth speed,temperature field,growth atmosphere.The factors that affect the growth of single crystal were discussed.The dielectric properties of LTO and Ta-LTO single crystals were also studied systematically.In addition,La₂Ti_2-xTa_xO₇?x=0,0.05,0.2?ceramic were prepared by solid-state sintering,the dielectric properties were analyzed and compared with the dielectric properties of LTO single crystal.The study manifests that the most suitable synthesis condition is 1250?for 8 h.The as-grown crystals are transparent after annealing in air.X-ray diffraction shows that this compound possessed a monoclinic structure with P2₁ space group.X-ray rocking curve reveals that the crystal of LTO has excellent crystalline quality.The profile of the?400?Bragg-reflections has a symmetrical peak with a full width at half maximum?FWHM?of 19arcsec.Generally,Ta ions replace Ti ions,and Ta ion?0.65??radius are large than Ti ion?0.604??,which induce expansion of the crystal structure.The positions of XRD corresponding peak of Ta-LTO move to a large diffraction angle.EDS demonstrates that Ta are doped in LTO lattice uniformly,and pure phase Ta-LTO single crystal is formed.The optical properties of the annealed LTO and Ta-LTO wafers are investigated at room temperature,both of the LTO and Ta-LTO crystal are transparent in visible and near-infrared region and the transmittance up to 80%.The dielectric properties of LTO and Ta-LTO crystal are investigated as functions of temperature?25³00??and frequency?10⁴ Hz¹0⁶ Hz?.Dielectric spectra indicates a high plateau in the room-temperature dielectric constant accompanied by a drop in the loss tangent with the Ta doping.At the low-temperature,the dielectric constant of Ta-LTO crystals are almost independent of frequency and manifest an intrinsic dielectric response.At high temperature,the frequency-dependent dielectric permittivity of Ta-LTO remarkably climbs as the temperature increases above 200?.Although the color of Ta-LTO crystal changes from black to transparent after annealing at 1200°C for 1 h,oxygen vacancies are still existed.With the rising temperature the movement speed of the oxygen vacancy elevates rapidly,leading to the increase in current.One relaxation is observed in the spectra of electric modulus,which is ascribed to the oxygen-vacancy.The activation energy of Ta-LTO single crystal is calculated as1.16 eV,which is close to the activation energy of conductivity of 0.96 eV.It is preliminarily judged that the dielectric anomaly is caused by the jumping motion of oxygen vacancy holes.Meanwhile,we find another dielectric anomaly at 150?²50?.A dielectric constant peak is observed at certain point temperature,and the dielectric loss peak is found at the corresponding temperature.The positions of dielectric peak are shift to high temperature with decreasing frequency.High temperature Raman study shows that the LTO crystals ranged 20?²50?Raman peaks change,some peak gradually disappear,indicates the structure of the phase change caused by temperature increasing is an ongoing process,not a remark of the process.At last,for the Ta doped LTO ceramic samples,a-and b-dimensions increase and c-dimension decrease with increasing Ta content.All of the compounds have the same structure as LTO,which is monoclinic with space group P2₁ at room temperature.The dielectric properties of LTO and Ta-LTO ceramics manifest that there is no phase transition at100?³00?,further prove the necessity of study LTO single crystal,provide reliable reference for more research to follow.