Magnetic Properties Of Potassium-and Rubidium-intercalated Non-fused Aromatic Hydrocarbon Molecular Crystals

Molecular-based magnets have received great attention because of their potential applications in high-density information storage,wave absorbing materials,and spintronics.Recent studies have shown that polycyclic aromatic hydrocarbon molecular crystals withπ-conjugated structure can display abundant magnetic properties and novel superconductivity when intercalated by alkali metal atoms,which has attracted great research interest.In this thesis,several non-fused aromatic hydrocarbons,such as biphenyl-like molecules,terphenyl-like molecules and phenyl-metal compounds were chosen for potassium and rubidium intercalation experiments.Based on the synthesis of structurally stable intercalated crystals,we investigate the possible magnetic properties and superconductivity in them.We have characterized the crystal structure,molecular vibrations and magnetic properties of the synthesized samples by various measurements such as X-ray diffractometer,Raman spectrometer and physical property measurement system.Additionally,we have used first-principles calculation method based on density functional theory to perform an in-depth analysis of the crystal structure,electronic structure and magnetic properties of partial synthesized samples.The main innovative research works of this thesis include:1.Structurally stable potassium-intercalated biphenyl molecular crystals and well-crystallized rubidium-intercalated biphenyl molecular crystals were successfully synthesized by a two-step solid-phase method combining constant-temperature ultrasonication and low-temperature annealing.Magnetic measurements show that potassium-intercalated samples containing a small amount of new crystalline phases may have a 120 K diamagnetic transition,while well-crystalline rubidium-intercalated samples show highly reproducible high temperature weak ferromagnetism with a T_c of about 48.5 K.Combining the XRD data and first-principles calculation results,it was found that rubidium atoms and biphenyl molecules formed the Rb₁C₁₂H₁₀ phase with the P2₁ space group in a 1:1 ratio.The calculation of the electronic structure and the study of Raman spectra reveal that the magnetic moments are formed by the transfer of Rb-5s electrons to theπ-orbitals of biphenyl,and the blue and red shifts of Raman modes provide evidence for the formation of monovalent biphenyl anion.In each unit cell,the magnetic moments on adjacent molecules within the ab layer are canted antiferromagnetically aligned,while the ones between layers are ferromagnetically aligned.Such an arrangement of magnetic moments results in a net magnetic moment of 0.0015μ_B in each unit cell,and leads to the formation of weak ferromagnetism.2.Rubidium-intercalated 4-phenylpyridine,2,2?-bipyridine and 2,4?-bipyridine molecular crystals with good crystallinity were successfully synthesized by the solid-phase synthesis method.The magnetic measurements show that the three synthetic crystals have low-dimensional antiferromagnetism.Fitting and computational analysis of experimental measurements through the Heisenberg spin-chain model,the Heisenberg spin-square lattice model and the mean-field approximation revealed that they are all consistent with the magnetic characteristics of the spin-square lattice with magnetic coupling|J|/k _B of 82.0,26.5,and 38.3 K,respectively.Combining the XRD data and first-principles calculation results,it was found that rubidium atoms and 4-phenylpyridine molecules formed the Rb₁C₁₁H₉N phase with the P1 space group in a 1:1 ratio.The calculation of the electronic structure and the study of Raman spectra show that the magnetic moments are formed by the transfer of Rb-5s electrons to theπ-orbitals of 4-phenylpyridine,which is confirmed by the anomalous blue and red shifts of Raman modes.In each unit cell,the magnetic moments on adjacent molecules within the ab layer are antiferromagnetically aligned,while there is almost no magnetic interaction between the layers,and the net magnetic moment is zero in each unit cell,which reasonably explains the experimental measurements.In addition,we also observed anomalous blue and red shifts of Raman modes in rubidium-intercalated 2,2?-bipyridine and 2,4?-bipyridine,suggesting that Rb-5s electrons are also transferred to theπ-molecular orbitals of 2,2?-bipyridine and 2,4?-bipyridine and form local magnetic moments.3.Structurally stable rubidium-intercalated p-terphenyl,potassium-and rubidium-intercalated o-terphenyl,and potassium-and rubidium-intercalated m-terphenyl molecular crystals were successfully synthesized by solid-phase synthesis method.The XRD measurements showed that almost all of the synthesized samples contained a large amount of impurity and amorphous phases,which indicates that it was difficult to obtain well-crystallized intercalated crystals by solid-phase methods.Magnetic measurements showed that all the synthesized samples exhibited typical Curie-weiss behavior with a maximum effective magnetic moment of about 0.68μ_B·mol^-1,which indicates that after alkali metal intercalation,local magnetic moments are formed on the molecule.Our intercalation experimental studies suggest that the formation of local magnetic moments may be the underlying cause of the poor reproducibility of superconductivity at 123 K in potassium-intercalated p-terphenyl.4.Well-crystallized potassium-and rubidium-intercalated diphenylzinc,potassium-intercalatedzinkdibenzoatandpotassium-intercalated tris-（dibenzoylmethanato）iron molecular crystals were successfully synthesized by solid-phase synthesis.DC and AC magnetic measurements show that only potassium-intercalated tris-（dibenzoylmethanato）iron has superparamagnetism with a blocking temperature T_B of about 8.0 K,accompanied by room temperature ferromagnetism.Test experiments on environmental stability show that the room temperature ferromagnetic signal originates from the impurity phase generated by the decomposition during the intercalation process,while the superparamagnetic phase at 8.0 K originates from the molecular crystals formed after the intercalation.Raman spectra studies showed that K-4s electrons were successfully transferred to the tris-（dibenzoylmethanato）iron molecule and formed a local magnetic moment.Combining the XRD measurements and the results of structural refinement showed that the distances between adjacent iron atoms before and after the intercalation are far away,and thus it is difficult to form significant magnetic interactions.This confirms that the superparamagnetism is formed by the magnetic interactions between molecular magnetic moments.Our research works extend the research scope of molecular-based magnets to non-fused aromatic hydrocarbon compounds such as biphenyl-like molecules withπ-conjugated structure.This not only greatly enriches the research system of organic molecular magnets,but also provides an important route for the development of high-temperature organic molecular magnets based on s-and p-electron spins.