Negative thermal expansion, behavior on compression, and other anomalous properties in rhenium trioxide structured mixed metal fluorides

Negative thermal expansion;Metal fluorides;Thermal expansion;Compressibility;Diffraction;Solid state;Inorganic;Perovskite;Crystallography

Hester, Brett R. ; Wilkinson, Angus P. Chemistry and Biochemistry Sadighi, Joseph Sholl, David Xia, Younan Zhang, Z. John ; Wilkinson, Angus P.

University:Georgia Institute of Technology

Department:Chemistry and Biochemistry

关键词: Negative thermal expansion;    Metal fluorides;    Thermal expansion;    Compressibility;    Diffraction;    Solid state;    Inorganic;    Perovskite;    Crystallography;   

Others  :  https://smartech.gatech.edu/bitstream/1853/62649/1/HESTER-DISSERTATION-2019.pdf

美国|英语

来源: SMARTech Repository

【 摘 要 】

In order to gain fundamental insight into the control of thermal expansion, this dissertation examines the thermal expansion and response to pressure of several mixed metal fluorides (M2+M4+F6), using variable-temperature/pressure x-ray and neutron diffraction. The stoichiometric materials CaNbF6, MgZrF6, MgNbF6, CaTiF6, MnNbF6, and ZnNbF6 were examined, and several found to display negative thermal expansion (NTE). Cation substitutions using species with smaller ionic radii and/or higher electronegativity was shown to destabilize the cubic phase, which altered both the occurrence of phase transitions and thermal expansion. On compression the cubic materials undergo a variety of phase transitions at modest pressures. The use of fluoride excess to generate edge-sharing defects, and hence stabilize cubic NTE metal fluorides to higher pressures and gain further control over thermal expansion, was also explored. However, while the density of Ca[Zr(IV)1-xNb(V)x]F6+x (x = 0.25, 0.5, and 0.75) samples indicated the presence interstitial fluoride, local structure probes indicated no edge-sharing defects. This behavior is attributed to the cation ordering in this system and resulted in little change in thermal expansion or compressibility on the introduction of niobium. Finally, the porosity of metal fluorides to compressed gases was examined. Helium was inserted into the NTE material CaZrF6 at high pressures. Other gases, such as nitrogen and neon, did not penetrate into the material at 300 K. The helium could be trapped in CaZrF6 at temperatures below ~200 K. Helium insertion leads to a new perovskite material with helium on the “A-site”. Helium was also shown to penetrate into the smaller CaNbF6 and fluoride excess material CaNbIV0.5NbV0.5F6.5 on compression. The fully helium inserted materials [He2][CaZr]F6, [He2][CaNb]F6, and[He2][CaNbIV0.5NbV0.5]F6.5 are the first perovskites to contain a noble gas on the A-site.