Preparation,Crystal Strucuture,Negative Thermal Expansion And Magnetism In Rhenium Oxide-type Fluorides

Negative thermal expansion(NTE)is peculiar physical property of materials,which includes the coupled interaction among charge,spin,phonon and lattice degrees of freedom.NTE is significant not only for the fundamental research but also for the practical applications.Cubic ReO3-type fluorides are open framework structures,composed of corner-sharing MF6 octahedra.ScF3 and CaZrF6 with single and double ReO3-type structure respectively,demonstrate prominent negatvie thermal expansion(NTE).This dissertation focuses on chemical modifications on the crystallographic structures,electronic band structures of ScF3-and CaZrF6-based compounds.Further,from the perspective of local and phonon structures,we attempt to derive the mechanisms of controllable thermal expansion,semiconductor features and high-temperature ferromagnetism.By the substitution of Ga and Fe for Sc to form(Sc,Ga,Fe)F3 solid solutions with different compositions,we achieve controllable thermal expansion with the linear coefficient of thermal expansion,αl=-3.11～+ 3.27×10-6/K(300～900 K).And the introduction of Fe enables the transformation of diamagnetic insulator of undoped ScF3 into ferromagnetic semiconductor of(Sc,Ga,Fe)F3.Synchrotron X-ray pair distribution funciton(XPDF)technique sheds light on the significant role of local distortion in controllable thermal expansion.The combined analyses of XPDF and extended X-ray absorption fine structure(EXAFS)spectra demonstrate the great thermal expansion of the Sc-F chemical bond with increasing temperature.This experimental finding is contrary to the conventional rigid unit mode(RUM)model.EXAFS spectra provide the mean square relative displacements of Sc-F and Sc-Sc atomic correlations with increasing temperature,which precisely captures the anisotropy of relative thermal vibration of Sc-F atomic correlation,γ.And local vibrational dynamics shows that larger γ is critical for NTE,instead of the rigid unit.By utilizing the single-source pyrolysis method,we prepare ScF3 nanocrystals with different crystalline sizes.These ScF3 nanocrystals demonstrate controllable thermal expansion with αl =-3.88～+ 0.63×10-6/K(325～675K).XPDF analysis demonstrates local rhombohedral distortion in the ScF3 nanocrystals.Local vibration dynamics analysis shows the constricted rotation model in ScF3 nanocrystals.And the γ value of Sc-F atomic correlation in ScF3 nanocrystals also decreases greatly,compared to that of bulk ScF3.The theoretical calculations support that rhombohedral distortion eliminates the phonons with negative Gruneisen paraters.Consequently,localized symmetry breaking plays a critical role in stiffening ScF3 lattice and further suppressing NTE.With the introduction of Fe element into ScF3 host matrix,The high-Tc(Tc =545 K)ferromagnetic semiconductor with smaller band gap(Eg = 1.87 eV)has been achieved in(Sc,Fe)F3.The ferromagnetic order is closely correlated with the chemical valence of Fe.The ferromagnetism could be explained by the bound magnetic polaron model.The fluorine vacancy,captured by the EXAFS analysis,acts as the postive charge center and traps an electon,which behaves like hydron-like electron.The orbital of hydron-like is electron correlates with the 3d orbital of magnetic cation,which finally produces the spin polarization.The replacement of Ca in CaZrF6 with a series of 3d transition metal elements gives rise to controllable thermal expansion with αl =-6.69～+ 18.23×10-6/K(300～675 K).XPDF demonstrates that the flexibility of M…F atomic linkage plays a significant role in the cooperated vibration of MF6 and ZrF6 octahedra,and finally produces distinct thermal expnasion.This dissertation focuses on the cubic ReO3-type ScF3 and CaZrF6.By the introduction of alien atoms into host matrix and the design of the crystalline size,we could obtain controllable thermal expansion in ScF3-and MZrF6-based compounds.Especially,isotropic zero thermal expansion has been achieved in(Sc0.85Ga0.05Fe0.1)F3,ScF3 nanocrystal(8.1nm)and ZnZrF6.The combined analyses of XPDF,EXAFS and theoretical calculations shed light on that the local distortion and the stiffness of lattice vibration suppress the NTE.Besides,the high-Te semiconductors with smaller band gaps are achieved in(Sc,Ga,Fe)F3 and(Sc,Fe)F3.This dissertation not only elucidate the NTE mechanism in ScF3,but also extends the scope of controllable thermal expansion materials,which provides a clear reference for the deep understanding of NTE mechanism and the design of novel NTE multifunctional materials.