Pronounced negative thermal expansion from a simple structure: cubic ScF(3)

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• Scandium trifluoride (ScF3) possesses a cubic ReO(3) type structure.
• Negative thermal expansion (NTE) is a phenomenon observed in certain materials, where they contract upon heating.
• The ReO(3) structure is theoretically linked to NTE via the "rigid unit mode" mechanism.

Purpose of the Study:

• To investigate the structural and thermal expansion properties of scandium trifluoride at low temperatures.
• To experimentally validate the proposed mechanism for negative thermal expansion in ScF3.
• To characterize the phase transition behavior of ScF3 under varying temperature and pressure conditions.

Main Methods:

• Variable temperature and pressure X-ray diffraction studies.
• Dilatometry measurements to determine the coefficient of thermal expansion.
• Analysis of structural changes and phase transitions.

Main Results:

• Scandium trifluoride retains its cubic ReO(3) structure down to 10 K.
• Strong negative thermal expansion (α(l) ≈ -14 ppm K(-1)) was observed between 60-110 K.
• The cubic to rhombohedral phase transition pressure is temperature-dependent, decreasing significantly at low temperatures.
• Room temperature CTE is comparable to ZrW(2)O(8), with positive thermal expansion above 1100 K.

Conclusions:

• Scandium trifluoride provides a clear experimental demonstration of the rigid unit mode mechanism for negative thermal expansion.
• Its unique thermal behavior, including NTE and a tunable phase transition, makes it a significant material for further study.
• The findings contribute to the understanding of NTE phenomena in materials with the ReO(3) structure.