Structural And Physical Properties Of Several Low-dimensional Compounds Containing Cr

In this dissertation,the focus is on the exploration of new magnetic materials in low-dimensional compounds containing transition metals,and the main results are the crystal structures and physical properties of several quasi-one-dimensional and layered chromium chalcogenides and chromium pnictides.In the experiments,crystal structures of new compounds were determined based on the polycrystalline samples synthesized by high-temperature solid-state reaction method,and then detailed physical property studies were carried on with the corresponding single crystals obtained by flux method.The main research results of this dissertation are shown below.First,a new compound Rb₂Cr Se₂ was successfully synthesized,and its structural and magnetic properties,especially the 1/3 magnetization plateau,were studied systematically.The crystal structure analysis based on the powder X-ray diffraction data reveals that Rb₂Cr Se₂ owns linear Cr Se₂ chains parallel to the c axis.Magnetic measurements indicate the strong magnetic anisotropy,and the a axis is the magnetic easy axis.Rb₂Cr Se₂ is an antiferromagnet with the transition temperature T_N=5 K.When the magnetic field is applied along the easy axis at temperature lower than T_N,M-H curve shows the 1/3 magnetization plateau and the saturation plateau,which reflect the occurrence of magnetic field induced phase transitions.Magnetic structures corresponding to the ground antiferromagnetic state,the 1/3 magnetization plateau and the saturation plateau were given by analyzing neutron powder diffraction data collected under different conditions,helping us understand these magnetic states from the microscopic view.In addition,magnetic,heat capacity and neutron measurements all show that there is another magnetic transition at T^*=8.5 K,which maybe arise from an incommensurate magnetic ordering but needs more exploration.Subsequently,K and Cs substitutions of Rb in Rb₂Cr Se₂ were achieved,and evolutions of structure and magnetic properties in the series of compounds(x?A=K,Cs,0?x?1?were studied.From K to Rb and to Cs containing compounds,the crystal structure remains all the same while the unit cell volume gradually expands and their lattice parameters change monotonously.Magnetic properties also evolve continuously,but two end compounds,K2Cr Se2 and Cs2Cr Se2,have quite different properties.In(Rb_1-x?0?x?1?,as x increases,the interchain distance increases and the 1/3 magnetization plateau becomes narrower,decreasing from more than 50 k Oe in Rb2Cr Se2 to less than 20 k Oe in Cs2Cr Se2.In(Rb1-x?0?x?1?,as x increases,the magnetic anisotropy is weakened and the1/3 magnetization plateau fades out.It is deduced that the magnetic structures determined in Rb2Cr Se2 is only applicable to Cs2Cr Se2 but not to K2Cr Se2.The(x?A=K,Cs,0?x?1?system provides adjustable structural parameters and rich controllable magnetic properties,offering a rare platform for the study of quasi-one-dimensional magnetism,especially the 1/3 magnetization plateau.Next,the first reported Th Cr2Si2-type chromium chalcogenides Cs Li Cr Se2,Rb Li Cr S₂ and Cs Li Cr S₂ were discovered.In this ALi Cr Ch₂ system,the Cr/Li-Ch bond length shrinks as the size of intercalated alkali atom decreases.Phases containing smaller atoms,such as K,cannot exist stably because of the limited Cr/Li-S bond length,which is very close to its minimum.Their electrical transport behaviors can be well fitted with the thermal activation model,showing correlation with the anion electronegativity and the unit cell volume.These compounds have anisotropic spin-glass behaviors.It is found that the spin-glass transition temperature depends on the Cr concentration x in A(Li_1-xCr_x)₂Ch₂?0.45?x?0.55?,instead of structural parameters proposed before,extending the understanding of AM'MCh₂.Finally,the effects of Co doping in Sr Cr₂As₂ on electrical transport properties were investigated.In?0?x?1?,a controllable adjustment of TCR over a wide range can be achieved by controlling the doping amount,and Sr?Cr0.2Co0.8?2As2owns the near zero temperature coefficient of resistivity?TCR?.Further,we found that the common effect of magnetic doping on the electrical transport properties in the Thtype compound Ae M2As2?Ae is an alkaline earth metal and M is a 3d transition metal?.