Structural And Magnetic Transitions Of Transition-metal Selenites/Tellurates

Transition-metal selenites and tellurates have attracted great interests in condensed matter physics due to their diverse structures and magnetic properties.Discovering new structures and growing high-quality single crystals,and further analyzing the relationship between the structure and magnetism have became crucial works for studying the magnetism of this system.In this thesis,the transition-metal selenite and tellurate systems are selected as the research objects.We grow single crystals by hydrothermal synthesis and flux method,and study their structural transition and magnetic field-induced transitions.The main works are as follows:In the introduction,we introduce the current research progresses of transition metal selenites and tellurates,structural transitions and related changes of physical properties,as well as some astonishing magnetic field-induced transitions.In first chapter,a new selenite compound Ca Ni₂（Se O₃）₃·2H₂O is synthesized by hydrothermal method,in which the Ni₂O₉dimer units and Se O₃^2-groups constitute a Zemannite-type framework.Ca²⁺ions and water molecules are located in the honeycomb-like channels.During the dehydration,successive crystal-to-crystal transformations occur.This compound firstly changes to Ca Ni₂（Se O₃）₃·1.5H₂O and finally to Ca Ni₂（Se O₃）₃,and slight framework shrink occurs in the former while a new square framework is achieved in the latter.Magnetic susceptibility results suggest that Ca Ni₂（Se O₃）₃·2H₂O and Ca Ni₂（Se O₃）₃·1.5H₂O display a short-range antiferromagnetic（AFM）order at 21 K and18 K,and undergo long-range AFM order at 13.9 K and 12.6 K,respectively.However,in Ca Ni₂（Se O₃）₃a short-range AFM order develops at 39.2 K,and successive AFM transitions are followed at 37.6 K and 6.9 K,respectively.In addition,a spin-flop-like transition is observed in magnetic fields for Ca Ni₂（Se O₃）₃.In the second chapter,we report the results of single-crystal of Fe₂（Te O₃）₂（SO₄）·3H₂O grown by hydrothermal method.The Fe₂O₁₀dimer units are connected with Te O₃^2-anion groups to form a two-dimensional layer in ac plane.Magnetic susceptibility shows that an AFM transition at 32.5 K with easy axis along the c-axis,and a ferromagnetic component is observed both along a and b directions.When the magnetic field is applied parallel to the c direction,a magnetic field-induced spin-flop transition happens at～7.1 T.It is worth noting that this compound has a large magnetic anisotropy with the magnetic anisotropy energy at 2 K reaches to 4.78（3）×10⁸ergs/cm³,which may be resulted from the large single-ion anisotropy and strong spin-orbit coupling due to the significant distortion of Fe O₆octahedra.In the third chapter,two new tellurite compounds Ca M₂Te O₆（M=Co,Ni）are synthesized by flux method.Both compounds have the same structures with two particular M positions.For each M sublattice,the MO₆octahedra are edge-shared to each other and form a chain running along the b axis.The M1 and M2 chains are further corner-shared and construct a three-dimensional structure.Magnetic susceptibility and specific heat results suggest that Ca Co₂Te O₆exhibits successive AFM transitions at 16.5K and 14.4 K,respectively.When the magnetic field is applied along the easy axis（c axis）,two magnetic transitions are observed at H_c1=3.49 T and H_c2=5.38 T,respectively.However,for the Ca Ni₂Te O₆polycrystalline,an AFM order is found at 32 K with a weak ferromagnetic component,suggesting a canted AFM ground state.