Negative Thermal Expansion Mechanism And Phase Transition Temperature Scaling Law In Framework Oxides

Negative thermal expansion(NTE)is an abnormal physical phenomenon,and its complex physical mechanism is an important subject of condensed matter physics.In addition,exploring NTE materials and near-zero expansion materials with wide temperature is of great applications in aerospace equipment,precision optical instruments,and electronic devices.A2M3O12-based NTE compounds have received great attention due to their easy preparation,convenient adjustment of expansion coefficient and wide NTE temperature range.However,the higher phase transition temperature and hydroscopicity are two major obstacles to their application.This dissertation mainly studies the NTE mechanism and phase transition temperature scaling law of A2M3O12-based compounds.In addition,in order to understand the NTE mechanism of oxides with different structures and develop single-structure near-zero expansion compounds,we also study the NTE property and mechanism of Zn2GeO4 with three-coordinated oxygen atoms.1.In order to reduce the phase transition temperature of Sc2Mo3O12,Zr4+ and V5+are used as a unit to compensatorily replace Sc3+ and Mo6+ in Sc2Mo3O12 so as to prepare a new NTE compound Zr0.3Sc1.7Mo2.7V0.3O12.Its phase temperature is 45 K lower than that of SC2Mo3O12,and its NTE temperature range is broadened.The structure,phase transition,NTE property and mechanism of Zr0.3Sc1.7Mo2.7V0.3O12 are studied by high-resolution SXRD,variable temperature XRD,ultra-low wavenumber Raman and first-principles calculations.Zr0.3Sc1.7MO2.7V0.3O12 has an orthorhombic structure(Pbcn)at room temperature,where V occupies the position of Mol and Zr occupies the position of Sc.In addition,temperature-dependent Raman is used to extract the total anharmonicity of low-frequency phonons below 50 cm-1.The two Raman modes of 27.4 cm-1 and 49.3 cm-1 have the highest and second highest total anharmonicity.The calculation results show that the two optical phonon molds of 38.5 cm-1 and 45.8 cm-1 have the largest and second largest negative Grüneisen parameters.Both theory and experiment show that the transverse and librational vibrations of low-frequency phonons are the origin of NTE.Zr0.3Sc1.7Mo2.7V0.3O12 has been studied on the structure and NTE mechanism,and laid the foundation for understanding the phase transition law in A2M3O12-based materials.2.In order to further reduce the phase transition temperature and study the influence of the substitution of A and M on the phase transition temperature,a new low-cost NTE compound Zr2MoVPO12(αV=-6.98×10-6 K-1,100～700 K)has been prepared by increasing the doping ratio on the basis of Zr0.3Sc1.7Mo2.7V0.3O12 and completely replacing the expensive Sc3+in Sc2Mo3O12 with Zr4+.A combined study of SXRD,Raman spectroscopy,dilatometer and first-principles calculations is used to investigate the crystal structure,phase transition and NTE property of Zr2MoVPO12.The results show that Zr2MoVPO12 has an orthorhombic structure at room temperature and NTE with a wide temperature range.Its phase transition temperature is lower than 83 K.In addition,Zr2MoVPOi2 has low thermal expansion anisotropy without hydroscopicity,which is beneficial to application.It is found by first-principles calculation studies that most of the phonon modes below 300 cm-1 have negative Gruneisen parameters.In particular,the two lowest-frequency optical phonons(47.0 cm-1 and 57.6 cm-1)contribute most to the NTE.This work not only provides a new NTE material,but also analyzes the influence of M-site on the phase transition temperature and NTE property of A2M3O12-based materials,laying a foundation for the design and regulation of other NTE materials.3.Aiming at the mechanical performance degradation caused by phase transition and hydroscopicity in A2M3O12-based compounds,we propose a concept of average effective electronegativity(AEE)and establish a linear relationship between the Tt and AEE for A2M3O12-based compounds,thus realizing the quantitative prediction of the phase transition temperature.In addition,this work reveals that the hygroscopicity is a local effect resulting from dipole-dipole interactions between water molecules and atoms,and proposes a criterion for designing NTE materials without hygroscopicity.The linear scaling law is validated by first principles calculations of the effective charge on oxygen.The high-entropy compounds SCx1Zrx2Hfx3Fex4Moy1Vy2O12 have been fabricated experimentally,and their phase transition temperatures and hygroscopicity are consistent with the expected results.We also generalized this linear scaling law to other existing NTE oxide materials,acting as a simple and effective approach to exploring NTE compounds with desirable Tt and hydroscopicity.4.In order to understand the NTE mechanism of oxides with different structures,the intrinsic thermal expansion and local vibration of Zn2GeO4 are studied.Zn2GeO4 exhibits a very low thermal expansion near room temperature(αV＝-2.02×10-6 K-1,100～300 K;αV=+2.54×10-6 K-1,300～475 K).A combined study of neutron powder diffraction and extended X-ray absorption fine structure spectroscopy shows that the NTE of Zn2GeO4 originates from the transverse vibrations of O atoms in the four-and six-membered rings of ZnO4-GeO4 tetrahedra.Its oxygen atom is not flexible enough.In addition,the results of temperature-and pressure-dependent Raman spectra show that the low frequency phonon modes(50～150 cm-1)contribute most to the NTE.This study not only reports the thermal expansion behavior of Zn2GeO4,but also reveals the NTE mechanism of different oxygen coordination.5.In order to control the expansion coefficient of Zn2GeO4 and the temperature range of near zero expansion,Zn2-xMnxGeO4(x=0,0.2,0.4,0.6)are prepared by solid state sintering.The apparent and intrinsic thermal expansion properties are studied by dilatometer and variable temperature SXRD.The effects of Mn2+ions on thermal expansion,microstructure,band gap,and luminescence intensity are studied.The NTE coefficients of Zn2-xMnxGeO4(x=0.2,0.4,0.6)below room temperature are stronger than Zn2GeO4,and their positive thermal expansion coefficients above room temperature are lower than that of Zn2GeO4,and the expansion coefficients are nearly zero.Mn2+ion doping broadens the temperature range of near-zero thermal expansion.