Scandium(III) fluoride
Encyclopedia
Scandium fluoride, ScF3, is an ionic compound. It is slightly soluble in water but dissolves in the presence of excess fluoride to form ScF63−. ScF3 can be produced by reacting scandium and fluorine. It is also formed during the extraction from the ore thortveitite
by the reaction of Sc2O3 with ammonium bifluoride at high temperature:
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.
Cubic Scandium trifluoride has an unusual Negative thermal expansion
property, meaning it shrinks as it gets hotter. This 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 draws the scandium atoms together throughout the material and it contracts. ScF3 exhibits this property from 10K to 1100K above which it shows the normal positive thermal expansion.
Thortveitite
Thortveitite is a mineral consisting of scandium yttrium silicate 2Si2O7. It is the primary source of scandium. Occurrence is in granitic pegmatites. It was named after Olaus Thortveit, Norwegian engineer. It is greyish-green, black or grey in color....
by the reaction of Sc2O3 with ammonium bifluoride at high temperature:
- Sc2O3 + NH4HF2 → 2ScF3 + 6NH4F + 3H2O
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.
Cubic Scandium trifluoride has an unusual Negative thermal expansion
Negative thermal expansion
Negative Thermal Expansion is a physicochemical process in which some materials contract upon heating rather than expanding as most materials do. Materials which undergo this unusual process have a range of potential engineering, photonic, electronic, and structural applications...
property, meaning it shrinks as it gets hotter. This 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 draws the scandium atoms together throughout the material and it contracts. ScF3 exhibits this property from 10K to 1100K above which it shows the normal positive thermal expansion.