| Low-dimensional quantum magnets have received widespread attention because of their novelty of the ground state and its relationship with high-temperature superconductivity.For quasi-one-dimensional antiferromagnets,much attention has been paid to the quasi-one-dimensional S=1/2 Heisenberg chain materials,which have undergone the spin-Peierls phase transition to dimerization singlet ground state.The studies on the high quality single crystals of these materials with spin-Peierls phase transition can greatly enrich our research on the properties of quantum ground states,and when these materials are non-magnetic doped,they may undergo metal-to-insulator transitions and may exhibit superconductivity.As a quasi-one-dimensional spin 1/2 quantum magnet with energy gap ground state,titanium oxychloride(TiOCl)is of great value for further research.Raman spectrum is a kind of characterization method that can accurately and conveniently characterize the structure of materials,and even from Raman spectrum,we can get the magnetic information of materials.In this dissertation,TiOCl single crystal is characterized by Raman spectrum,so as to understand its phase transitions under temperature changes and pressure changes.We synthesized the TiOCl single crystal successfully by chemical vapor transport(CVT).The morphology was characterized by scanning electron microscope(SEM),the structure was characterized by X-ray diffraction(XRD),the composition was characterized by energy dispersive spectrometer(EDS),and the results indicated that the quality of the single crystal is better.And then,the magnetic properties of TiOCl single crystals were characterized by the Magnetic Property Measurement System(MPMS)to obtain the magnetic susceptibility curves under different magnetic fields.It was found that there are two phase transitions near the temperature of 67 K and 92 K respectively,of which the former was a first order phase transition and the latter was a second order phase transition.Next,Raman spectrum of TiOCl single crystal was measured at room temperature.And it was found that there was one continuous broad peak along the b-axis direction than the a-axis direction,proving that there was anisotropy,in addition,the peak positions of the other three peaks were basically the same,and their vibration modes are all Ag modes.In addition,we measured the temperature-dependent Raman spectrum of TiOCl single crystal.And it was found in the temperature-dependent Raman spectrum that there will be new peaks appeared or peaks spliting as the temperature decreased.There were phase transitions at 67 K and 92 K respectively,and the continuous broad peak along the bb direction gradually redshifts and becomes very sharp as the temperature decreases.This unusual change may be related to the spin-phonon interaction at low temperature.Meantime,the pressure-dependent Raman spectrum of TiOCl single crystal was measured.And the results show that with the continuous increase of pressure,there will be many new peaks appearing and merging,and at the same time,there will also be splitting or disappearance of the original peaks.The series of changes confirms that there are phase transitions around the pressures of ～7 Gpa,～15 Gpa and ～22 Gpa respectively,meantime,the Raman peaks near the low pressure ～1.62 Gpa and ～5 GPa also exhibit relatively abnormal changes.In this dissertation,through the above series of experimental studies,the structure,magnetism and Raman spectrum information of TiOCl single crystals are obtained.The magnetic susceptibility,phonon and phase transitions of the material are analyzed.Finally,it is found that the phase transition behavior of TiOCl is correlated from room temperature to 92 K and from ambient pressure to 7 GPa,and when the pressure increases to 15 GPa,TiOCl will change from a quasi-one-dimensional system to two-dimensional system,and dimerization will be formed along the a and b axes.After combining our test results with the existing research results,we have drawn and supplemented the temperature-pressure phase diagram of TiOCl. |
PDF Full Text Request