Study On Magnetic,Optical,and Electrochemical Properties Of Transition Metal Fluorides

Transition metal fluorides have very specific crystal structures and abundant physical properties,especially in terms of magnetic and optical properties.Unlike magnetic oxides or metal alloys,transition metal fluorides are rich in room temperature ferroelectrics,magnetoelectric materials,single-phase multiferroics,et.al..The magnetic research of these materials has great physical significance and applications,such as the potential applications of magnetic ferroelectrics in information storage,energy capture and spintronics,which have received widespread attention.At present,most of the researches attempting to explore multiferroics are focused on oxides with perovskite structure.Many magnetics and ferroelectrics have now been discovered,but unfortunately,there is little overlap between ferroelectricity and magnetics.In the field of perovskite oxides,the different requirements of ferroelectricity and magnetism on the orbital filling of transition metal ions lead to the mutual exclusion of these two ordered states,which makes it difficult to integrat ferroelectricity and magnetism into single-phase materials.However,there are hundreds of natural magnetic ferroelectrics in the fluorides such as BaMF₄?M=Mn,Fe,Co,Ni,Cu?,A₅B₃F₁₉?A=Sr,Ba;B=Fe,V,Cr,Mn,Ga?,et al..The ionic bond formed by the transition metal ions and F is also different from the covalent bond in oxides,which can successfully break the d⁰ criterion.Thus,these fluorides can provide a wide materials field for people to search single-phase multiferroic materials,which is of extremely high research and application value.Optically,fluorides have the potential of being an up-conversion luminescent host material due to the characteristics of small phonon energy,good transparency,and extremely short cut-off-wavelength.Many commercial phosphors are rare earth doped fluorides,such as BaYF₄:N_b⁴⁺,etc.But rare earth elements are very rare and expensive,in contrast,the study of the intrinsic optical properties of transition metal fluorides also show great application value.Therefore,while studying the magnetic property of transition metal fluorides,we also focus on their optical properties and intrinsic optical physics.In the field of energy storage,there are three kinds of energy-supplied devices:traditional capacitors,lithium-ion batteries and supercapacitors,and the latest one can fill the huge blank of energy requirements network between the previous two.Supercapacitors have special performance with high power density,short charge-discharge time,long cycle life,and wide working temperature range.They are generally divided into double electrical layer capacitors and pseudocapacitors which store energy through adsorption/desorption effects and redox reactions,respectively.Thus,the selection of electrode materials is extremely crucial.Actually,many transition metal fluorides can also meet the basic requirements of electrode materials.For example,The quasi-two-dimensional structure of fluorides can provide a larger specific surface area,and transition metal ions can be used as a redox reaction carrier.What's more,the widespread presence of room temperature ferroelectricity in fluorides may be an unexpected motivation for ion motion both on the surface and in the bulk of the electrode materials.In this context,we will take a deep research on three aspects of transition metal fluorides:?1?Magnetic study of transition metal fluorides;?2?Photoluminescence of transition metal fluorides and their physical mechanism;?3?The transition metal fluorides applied in supercapacitors as electrode materials and study of their electrochemical properties.?1?Magnetic study of transition metal fluorides.Transition metal fluoride BaMnF₄ possesses room-temperature ferroelectric property and a unique spin structure at low temperature.Therefore,early theorists have predicted on many occasions that there is a magnetoelectric coupling effect between the ferroelectric polarization and spin structure.In order to verify this prediction,we experimentally prepare BaMnF₄sample,and carry out a variety of characteric methods which revealed the sample is micro-sheets of high quality single crystals.Magnetic measurements show that BaMnF₄exhibits a exchange bias effect at a low temperature.After analysised,the exchange bias is derived from the magnetoelectric coupling generated by the DM interaction between the ferroelectric polarization and canted anti-ferromagnetism,which firstly proves the existence of the previous prediction of magnetoelectric coupling.On the other hand,theoretists have pointed out the existence of magnetoelectric effects in different directions due to the special spin structure of Ba₂Ni₃F₁₀.Therefore,we start to study the magnetic properties of Ba₂Ni₃F₁₀in detail.Instead of traditional high-temperature solid phase method,we produce a low-temperature hydrothermal approach with lower cost and higher controllability to fabricate Ba₂Ni₃F₁₀.During the synthesis,different growth condition can be used to obtained Ba₂Ni₃F₁₀nanowires with different sizes and morphology.Magnetic characterization shows that exchange bias effect is observed below 55 K,and magnetic hysteresis loops are obtained at the temperatures below 60 K,showing weak ferromagnetism caused by surface stress.Analysis shows that:the as-grown Ba₂Ni₃F₁₀0 nanowires have antiferromagnetic/ferromagnetic core-shell structure,and the exchange bias effect is derived from the interface effect because of the magnetic moment pinning.?2?Photoluminescence of transition metal fluorides and their physical mechanisms.To study the photoluminescence of transition metal fluorides(BaMnF₄ and Ba₂Ni₃F₁₀)and their emission mechanisms,we prepared the samples by low temperature hydrothermal method.Under the excitation of ultraviolet light,BaMnF₄ emitts two bands of fluorescence:blue light with the emission peak at 410 nm and ultraviolet light at 385 nm.The absolute quantum yield is up to 67%.In addition,there is a sharp and asymmetric valley at 400 nm in the transition region between the two emission bands,which is a significant anomaly.Interestingly,we found that such an abnomal is originated from the photon self-absorption effect in BaMnF₄.In order to explore the mechanism,we use first-principles-calculations method to calculate the electronic structure of BaMnF₄ and finally found that the two emission peaks are induced byelectrons hopping between the higher energy level Hubbard band t_2gg orbital and the lower e_g level of Mn²⁺ions.In ferroelectrics,the Mott band gap modulated by this d-d orbits provides more freedom for phonon generation and absorption.In addition,the Ba₂Ni₃F₁₀0 nanowires with uniform size are prepared by optimized condition,and the luminescent properties is explored.Ba₂Ni₃F₁₀0 nanowires emit three different wavelengths under 325 nm laser excitation,360 nm,530 nm and 700 nm,respectively.We study the intrinsic luminescence mechanism in Ba₂Ni₃F₁₀0 by using the first-principles-calculations and find the first two emmisions are originated from the electron transitions between the t_2gg orbital and e_g orbital,while the last band is probably form crystalline defect.?3?The transition metal fluorides applied in supercapacitors as electrode materials and the study of their electrochemical properties.Since BaMF₄?M=Mn,Co,Ni?belongs to layered materials,both of the higher specific surface area and valence-variable ions Mn,Co,Ni can meet the common requirements of electrode materials in supercapacitors.Ba MF₄ samples are attached on nickle foam electrodes,which are then added to three-electrode system to test the electrochemical performance.The results show that all the materials in this family have obvious Faradaic redox reactions and absorption/deabsroption effect.The specific capacitance is measured to be 360 F/g under the optimized alkaline electrolyte with solution of 6 mol/L.Through first-principles calculations,we analyze the physical-chemical mechanisms of the electrochemical behaviors in every sample and figure out the role of ferroelectric polarization on ion transport.