Magnetic Properties Of Several Potassium-doped Anthracene Molecules

The search for alkali-doped aromatic hydrocarbon organic superconductors and organic magnets has been a hot topic in condensed matter physics,chemistry,and materials science in recent years.In this thesis,potassium doping experiments were performed on a series of anthracene molecules modified with different functional groups.We attempted to synthesize stable molecular crystals and study their physical properties.We have tested and characterized the crystal structure,molecular vibration,and magnetic properties of synthetic materials by a variety of methods,including X-ray diffraction(XRD),Raman spectrometer,and superconducting quantum interference device(SQUID).In addition,we also used the first-principles calculation method based on density functional theory to study the crystal and electronic structures of synthetic materials.The main work of this thesis is as follows:1.Combined with constant temperature ultrasound and low temperature annealing process,potassium-doped 9-methylanthracene molecular crystal with good crystallinity was successfully synthesized.The dc magnetic susceptibility shows that the synthesized samples exhibit pure Curie-Weiss behavior in the temperature range of 1.8-300 K,and each9-methylanthracene molecule has a magnetic moment of 0.2-0.3?_B,which is different from diamagnetism of pristine 9-methylanthracene.Combined with XRD data and first-principles calculation results,it was found that potassium-doped 9-methylanthracene belongs to the P2₁space symmetry group,and the molar ratio of potassium and 9-methylanthracene is 1:1.The calculation of the electronic structure shows that the magnetic moment originates from the conjugated?electrons on the benzene rings,and the magnetic exchange interaction between magnetic moments is weak,which reasonably explains the paramagnetic properties of the doped materials.In addition,multiple modes exhibit redshift in the Raman scattering spectra.It is because the 4s electrons of potassium are transferred to the 2p orbital of carbon,causing a phonon softening effect for the Raman modes.2.Combined with constant temperature ultrasound and low temperature annealing process,potassium-doped 9-phenylanthracene molecular crystal with good crystallinity was successfully synthesized.Magnetic measurements showed that all synthesized samples exhibit an antiferromagnetic transition at 35 K and a weak ferromagnetic transition at 5.2 K.Combined with XRD data and first-principles calculation results,it was found that potassium-doped 9-phenylanthracene belongs to the P1 space symmetry group,and the molar ratio of potassium and 9-phenylanthracene is 2:1.In the Raman scattering spectra,we also observed multiple modal redshifts.The calculation results of the electronic structure show that the magnetic moment is formed by the transfer of 4s electrons from K to the 2p orbital of carbon,and the spin density is distributed on both the phenyl and anthracene molecules of9-phenylanthracene.In each unit cell,the spins on the same molecule tend to be arranged in parallel,and the spins on different molecules tend to be arranged in anti-parallel.This non-collinear arrangement of spins leads to the formation of weak ferromagnetism.3.Through solid phase synthesis,combined with a series of processes such as high vacuum annealing,we performed potassium doping experiments on 9,10-diphenylanthracene,9-(4-biphenyl)anthracene,9-(1-naphthyl)anthracene,and 9-(2-naphthyl)anthracene.XRD measurements showed that only 9-(1-naphthyl)anthracene molecular crystal with a stable structure has been obtained.The magnetic measurements indicated that the zero-field cooling susceptibility of potassium-doped 9-(1-naphthyl)anthracene sample exhibits the Curie-Weiss behavior in the entire temperature region,while the field cooling susceptibility shows a transition similar to weak ferromagnetism near 43 K.The reasons for this phenomenon need to be further studied.Our research shows that organic functional group modification can effectively adjust the crystal and electronic structures of potassium-doped aromatic hydrocarbon materials,as well as the physical properties of doped materials.This provides an important research approach for exploring new organic superconductors and organic magnets based on aromatic hydrocarbons.