Scandium(III) fluoride, ScF₃, is an ionic compound. This salt is slightly soluble in water but dissolves in the presence of excess fluoride to form the ScF₆³⁻ anion.

Production

ScF₃ can be produced by reacting

scandium Scandium is a chemical element; it has Symbol (chemistry), symbol Sc and atomic number 21. It is a silvery-white metallic d-block, d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the lantha ...

and

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

.S.A.Cotton, Scandium, Yttrium and the Lanthanides: Inorganic and Coordination Chemistry, Encyclopedia of Inorganic Chemistry, 1994, John Wiley & Sons, . It is also formed during the extraction from the ore thortveitite by the reaction of Sc₂O₃ with

ammonium bifluoride Ammonium bifluoride is an inorganic compound with the formula or . It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass-industrial etching, etchant and an intermediate in a once-contemplated route to hydrofluoric acid ...

at high temperature:Pradyot Patnaik, 2003, ''Handbook of Inorganic Chemicals'', McGraw-Hill Professional, . : Sc₂O₃ + 6 NH₄HF₂ → 2 ScF₃ + 6 NH₄F + 3 H₂O The resulting mixture contains a number of metal fluorides and this is reduced by reaction with calcium metal at high temperature. Further purification steps are required to produce usable metallic scandium.

Properties

Scandium trifluoride exhibits the unusual property of

negative thermal expansion Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. The most well-known material with NTE is water at 0 to 3.98 °C. Also, the d ...

, meaning it shrinks when heated. This phenomenon is explained by the quartic oscillation of the fluoride ions. The energy stored in the bending strain of the fluoride ion is proportional to the fourth power of the displacement angle, unlike most other materials where it is proportional to the square of the displacement. A fluorine atom is bound to two scandium atoms, and as temperature increases the fluorine oscillates more perpendicularly to its bonds. This motion draws the scandium atoms together throughout the bulk material, which contracts. ScF₃ exhibits this property from at least 10 K to 1100 K above which it shows the normal positive thermal expansion; furthermore, the material has cubic symmetry over this entire temperature range, and up to at least 1600 K at ambient pressure. The negative thermal expansion at very low temperatures is quite strong (coefficient of thermal expansion around -14 ppm/K between 60 and 110 K). At ambient pressures scandium trifluoride adopts the cubic crystal system, using the perovskite structure with one metal position vacant. The unit cell dimension is 4.01 Å. Under pressure scandium trifluoride also forms different crystal structures with rhombohedral, and above 3 GPa tetrahedral. Scandium fluoride (ScF₃) features high transparency across UV to IR wavelengths, low optical absorption, and a low

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

(~1.5). It exhibits nonlinear optical properties for frequency conversion and can luminesce when doped with rare-earth ions.

References

{{fluorides Scandium compounds Fluorides Metal halides