Pressure-Induced Octahedron Strain in VF₃-Type Compounds

Abstract

High-pressure X-ray investigations on crystal powders of the TiF3, titanium trifluoride, and FeF3, iron trifluoride, phases with VF3-type structures were performed up to 7.67 (7) and 9.00 (7) GPa, respectively. Both compounds are highly compressible and show strong anisotropic behavior. While the hexagonal a lattice parameters are shortened under pressure, the c parameters are elongated. These changes are associated with distortions of the TiF6 and FeF6 coordination octahedra, whereby the octahedron strain is stronger in TiF3 than in FeF3. The high-pressure behavior is characterized by cooperative tiltings of the octahedra, while simultaneously repulsive interactions between the cations probably cause the octahedral distortions.

Negative thermal expansion and compressibility of Sc1–xYxF3 (x≤0.25)

“…Materials with this structure are typically highly compressible but do not undergo phase transitions on modest compression. [74][75][76][77] In this structure, the M-F-M links are bent, and further reduction of this bond angle on compression provides an energetically low-cost path for volume reduction, leading to much lower bulk moduli than those observed for materials with the cubic ReO 3 -type structure, in which these same links are linear. In Sc 1-x Y x F 3 , insertion of the relatively large Y 3þ into the ScF 3 matrix presumably locally distorts the ideal cubic structure.…”

High-pressure powder diffraction study of TaO2F

“…TiF 3 and FeF 3 have been studied at up to 7.67 and 9.00 GPa, respectively, and no phase transitions were observed for either compound [20]. Volume reduction on compression of these R-3c materials was attributed to the rotation of octahedra along with octahedral distortions.…”

“…Many materials have a VF 3 -type structure under ambient conditions, including AF 3 (A ¼ Al, Cr, Fe, Ga, In, Ti, V), and they typically transform to a cubic ReO 3 -type structure at elevated temperatures [17][18][19][20]. TiF 3 and FeF 3 have been studied at up to 7.67 and 9.00 GPa, respectively, and no phase transitions were observed for either compound [20].…”

Jahn-Teller transition inTiF3investigated using density-functional theory

“…We fix d T i−F = 1.947Å and d T i−F = 1.996 A for LDA and LDA+U calculations respectively. For LDA+U calculations V 0 was fixed to the experimental value [26] V 0 =58.3Å 3 , while for LDA it is reduced to match the optimal volume V 0 =56.2Å 3 to eliminate the strain effects. The results of calculations for the unit cell parameters given in table I are shown on Fig.…”