Synthesis And Property Studies Of Tellurite

The transition metal tellurite WTe₂ has been widely studied due to the observed large non-saturating magnetoresistance effect?XMR?.Although there is an opinion that the perfect compensation of electrons and holes induces this large non-saturating magnetoresistance effect,the underlying physical mechanism of this effect is still controversial.In this thesis,the effects of tungsten vacancies and selenium doping on the crystal structure,crystal morphology,Raman spectrum and magnetoelectric properties have been studied to explore the potential factors affecting magnetoresistance in WTe₂.Polycrystalline WTe₂ has been successfully synthesized through solid state reaction method.The compound WTe₂ crystallizes in the orthorhombic system with space group Pmn2₁.Observed by scanning electron microscopy,the samples'grains mainly consists of irregular blocks as well as some small particles.EDS energy spectrum analysis show that the atomic ratios of the sample is close to stoichiometry.The Raman spectrums show that there exist eight Raman peaks corresponding to different vibrational modes,including three inner vibrations and five interlayer vibrations.A large positive magnetoresistance has been observed in polycrystalline WTe₂ sample with metallic property.The magnetoresistance gradually decreases as the temperature rises and basically disappears above 100 K.Tungsten vacancies were successfully introduced in WTe₂ to prepare polycrystalline W_0.9Te₂.The introduction of tungsten vacancies does not change the sample's crystal system,Raman peak position and metal properties.However,the grain size increases and the interlayer spacing decreases for W_0.9Te₂,in contrast to WTe₂.It is also found that the intensity of the Raman spectrum peak decreases in W_0.9Te₂.More interesting,the introduction of a small amount of tungsten vacancies significantly increases the magnetoresistance effect.Se doped polycrystalline WTe_2-xSe_x were prepared successfully.WTe_2-xSe_x?x?0.1,0.2?are single-phase samples with orthorhombic crystal structure like WTe₂.The lattice constant gradually decreases with the increase of Se doping.When x?0.30,the samples are composed of mixed phases of Td and 2H.Raman peaks were found to broaden with the increase of Se doping,indicating that crystallization degree reduces and crystal defect increases.Both conductivity and magnetoresistance decrease with Se doping increase for WTe_2-xSe_x?x=0.1,0.2?.For x=0.3 sample,an insulator property has been observed and the magnetoresistance effect basically disappear.In summary,the enhanced magnetoresistance effect has been observed in W_0.9Te₂,even the introduction of tungsten vacancy has broken the perfect compensation of electrons and holes.The doping of Se weakens the magnetoresistance effect although there is no change in the proportion of electrons and holes.Therefore,the perfect compensation of electrons and holes is not entirely the intrinsic mechanism of the large magnetoresistance effect in tellurite.Crystal structure and sample quality might also be the main factors affecting the magnetoresistance effect.