The compression mechanism of CrF3

Quenchable amorphous glass-like material from VF3

The quite simple but relatively stable VF3-type compounds are known to be of major interest due to their building blocks - octahedra that are extremely important in perovskites as well. Here, we show that the VF6 octahedron in VF3 varies over a fairly wide pressure range (0-50 GPa), maintaining undisturbed rhombohedral crystal symmetry. Half of this pressure, VF6 rotates easily while the other undergoes strong uniaxial deformation in a "super-dense" condition. The congested sphere packing ultimately does not endure and drives the material to amorphize. We observed that the amorphous state could be quenched and acquire a transparent glass-like appearance when unloaded to ambient conditions. Dramatic, pressure-induced changes are clarified by phonon dispersion curves with the imaginary phonon mode, the so-called phonon soft mode, which indicates the structural instability. The distortion of the VF6 octahedra is attributed to the distinctive amorphization that could be further searched for throughout the whole almost identical VF3-type series providing metal trifluorides of various amorphous species.

Density Functional Theory Calculations of Structural, Electronic, and Magnetic Properties of the 3d Metal Trifluorides MF3 (M = Ti-Ni) in the Solid State

We employ density functional theory with Hubbard U correction or hybrid functionals to study the series of magnetic 3d metal trifluorides MF₃ (M = Ti-Ni). Experimental lattice parameters are reproduced with an error margin of 0.5%-4.3%. Cooperative Jahn-Teller distortions are reproduced for MnF₃ , but also found in TiF₃ and CoF₃ at smaller levels compared to MnF₃ . Trends in electronic structure with respect to positions of the d bands are linked to the magnetic properties where M = Ti-Cr are weak magnetic Mott-Hubbard insulators, M = Fe-Ni are strong magnetic charge-transfer insulators and MnF₃ falls in between. Our work contributes to the characterization of the relatively unknown NiF₃ , since FeF₃ and CoF₃ have similar electronic and magnetic properties. However, NiF₃ does not show a Jahn-Teller distortion as present in CoF₃ . © 2019 Wiley Periodicals, Inc.

Negative Thermal Expansion, Response to Pressure and Phase Transitions in CaTiF6

Strong volume negative thermal expansion over a wide temperature range typically only occurs in ReO₃-type fluorides that retain an ideal cubic structure to very low temperatures, such as ScF₃, CaZrF₆, CaHfF₆, and CaNbF₆. CaTiF₆ was examined in an effort to expand this small family of materials. However, it undergoes a cubic ( Fm3̅ m) to rhombohedral ( R3̅) transition on cooling to ∼120 K, with a minimum volume coefficient of thermal expansion (CTE) close to -42 ppm K^-1 at 180 K and a CTE of about -32 ppm K^-1 at room temperature. On compression at ambient temperature, the material remains cubic to ∼0.25 GPa with K₀ = 29(1) GPa and K'₀ = -50(5). Cubic CaTiF₆ is elastically softer and shows more pronounced pressure induced softening, than both CaZrF₆ and CaNbF₆. In sharp contrast to both CaZrF₆ and CaNbF₆, CaTiF₆ undergoes a first-order pressure induced octahedral tilting transition to a rhombohedral phase ( R3̅) on compression above 0.25 GPa, which is closely related to that seen in ScF₃. Just above the transition pressure, this phase is elastically very soft with a bulk modulus of only ∼4 GPa as octahedral tilting associated with a reduction in the Ca-F-Ti angles provides a low energy pathway for volume reduction. This volume reduction mechanism leads to highly anisotropic elastic properties, with the rhombohedral phase displaying both a low bulk modulus and negative linear compressibility parallel to the crystallographic c-axis for pressures below ∼2.5 GPa. At ∼3 GPa, a further phase transition to a poorly ordered phase occurs.

Exploring and exploiting the influence of the compression mechanism on the transport properties of CrF3

We have explored the influence of the compression mechanism and the temperature on the transport properties of CrF₃.