Structure And Magnetic Study Of Transition Metal Fluorophosphate

Transition metal fluorophosphates have attracted extensive research in the fields of structural chemistry and electrochemistry because of their abundant chemical structures.On the one hand,fluorine is incorporated into metal phosphates,it can form MO_xF_yoctahedra or replace oxygen within phosphate to form fluorophosphate PO_4-nF_ntetrahedra,thus changing the structural parameters inside the crystal and causing the system to exhibit different physicochemical properties.On the other hand,unlike other halogens,fluoride atoms are connected in a variety of ways in the crystal structure,either as terminal atoms coordinated to transition metal ions to form octahedral structures;or as corn-sharing,edge-sharing,or surface-sharing connections between an oxygen-fluoride polyhedra of a transition metal and another polyhedra,thus leading to the construction of a variety of structures such as layers(2D),chains(1D),and discrete clusters(0D).These abundant structures provide a large number of experimental models for the magnetic study of transition metal fluorophosphates,which are important for the study of magnetic behavior and quantum phase transition of low-dimensional magnetic materials.Based on this,four examples of transition metal fluorophosphates were synthesized by conventional hydrothermal method in this thesis,mainly using trivalent transition metal ions(Fe³⁺,V³⁺)as the magnetic center and PO₄^3-anion group as the structural framework,while introducing F^-ions and antimagnetic cations(NH₄⁺,K⁺),and their crystal structures and magnetic properties were investigated.1.Two fluorophosphates were synthesized:K₃Fe₄(PO₄)₂F₉and(NH₄)₃Fe₄(PO₄)₂F₉.These two compounds are isostructural,both crystallized in the P2₁/m space group.Two compounds consist of Fe O₂F₄octahedra and PO₄tetrahedra,with corn-sharing F connections between the four Fe O₂F₄octahedra forming tetrahedral cluster chains and multiple ring channels on the bc and ac planes,while the cations are distributed in the channels to form a three-dimensional structure.The formation of strong spin frustration is found in the tetrahedral cluster structure.Although they both show antiferromagnetic ordering at low temperatures and both also show short-range ordering above T_N,the two compounds have different antiferromagnetic transition temperatures of?10 K and?6 K,respectively,due to differences in cations,and compound(NH₄)₃Fe₄(PO₄)₂F₉also shows a field-induced magnetic phase transition at H_C=4.5 T.2.Two new fluorophosphates were synthesized:(NH₄)₂V₂(HPO₄)(PO₄)F₃and(NH₄)₂Fe₂(HPO₄)(PO₄)F₃.Two compounds are isostructural,both crystallized in the P2₁/n space group of the monoclinic crystal system,and the structures of two compounds consist of Fe O₂F₄octahedra and Fe O₄F₂octahedra,and they connected by corn-sharing F to form zigzag chains,and the interchains are connected by PO₄tetrahedra to form a three-dimensional structure.Two compounds are antiferromagnetic ground states,but due to the difference in the spin values of the magnetic ions,(NH₄)₂V₂(HPO₄)(PO₄)F₃shows only an antiferromagnetic transition at low temperatures,while(NH₄)₂Fe₂(HPO₄)(PO₄)F₃shows a short-range ordering above T_N.