Solid solubility, phase transitions, thermal expansion, and compressibility in Sc1−xAlxF3

• The solubility limit of AlF3 in ScF3 at ~1340 K is ~50%.
• The phase transition temperature of Sc1−xAlxF3 increases smoothly with x.
• The cubic-phase volume CTE varies from negative to positive with increasing x.
• The cubic solid solutions elastically stiffen on heating.
• Al3+ substitution causes softening at a given temperature.

With the goal of thermal expansion control, the synthesis and properties of Sc1−xAlxF3 were investigated. The solubility limit of AlF3 in ScF3 at ~1340 K is ~50%. Solid solutions (x≤0.50) were characterized by synchrotron powder diffraction at ambient pressure between 100 and 900 K and at pressures <0.414 GPa while heating from 298 to 523 K. A phase transition from cubic to rhombohedral is observed. The transition temperature increases smoothly with Al3+ content, approaching 500 K at the solid solubility limit, and also upon compression at fixed Al3+ content. The slope of the pressure–temperature phase boundary is ~0.5 K MPa−1, which is steep relative to that for most symmetry-lowering phase transitions in perovskites. The volume coefficient of thermal expansion (CTE) for the rhombohedral phase is strongly positive, but the cubic-phase CTE varies from negative (x<0.15) to near-zero (x=0.15) to positive (x>0.20) between ~600 and 800 K. The cubic solid solutions elastically stiffen on heating, while Al3+ substitution causes softening at a given temperature.

The cubic-phase coefficient of thermal expansion for Sc1−xAlxF3(solubility limit ~50% at ~1340 K) becomes more positive with increased Al3+ substitution, but the average isothermal bulk modulus decreases (elastic softening).Figure optionsDownload as PowerPoint slide