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PrAlO3 undergoes a first-order phase transformation at 205°K, and a second order transformation at 151°K. Single crystal X-ray measurements at 293°K, 172°K, and 135°K display the Laue symmetries 
m, mmm, and 2/m respectively. With a specimen ground to a thin crystal plate, and mounted with minimal mechanical restraints, the 205°K transformation was single crystal to single crystal; the 151°K transformation was single crystal to twinned crystal. Both transformations were completely reversible with no sign of fatigue; the twinned crystal always transformed back to a single crystal. The true space group symmetries of the low temperature phases are masked by the effects of two types of domains on the observed intensities. The most probable space groups, expressed in unconventional orientations to preserve continuity with the ideal perovskite structure, are: at 293°K F2/c, α = 90°21′, V/8 = 53.56 Å3, at 172°K I112/m, γ = 90°, V/4 = 53.22 Å3, at 135°K I
, α = γ = 90°, β = 90°40′, V/4 = 53.02 Å3. To unravel the true symmetries it was necessary to solve all three crystal structures. The first order transformation is characterized by a shear of (0k0) planes along the rhombohedral [101] direction to form an orthogonal space lattice. The AlO6 groups are rotated into new orientations and Pr atoms are subjected to displacements which are normal to the shear direction. The Pr displacements lower the symmetry from orthorhombic to monoclinic even though the space lattice is orthogonal. The second order transformation is characterized by a shear of (00l) planes along ±[100] in the orthogonal space lattice to form twinned monoclinic space lattices. The AlO6 groups do not assume new orientations. The Pr displacements are increased, again in directions normal to the shear directions. Because of the Pr displacements the symmetry is triclinic even though the space lattice is monoclinic.
m, mmm, and 2/m respectively. With a specimen ground to a thin crystal plate, and mounted with minimal mechanical restraints, the 205°K transformation was single crystal to single crystal; the 151°K transformation was single crystal to twinned crystal. Both transformations were completely reversible with no sign of fatigue; the twinned crystal always transformed back to a single crystal. The true space group symmetries of the low temperature phases are masked by the effects of two types of domains on the observed intensities. The most probable space groups, expressed in unconventional orientations to preserve continuity with the ideal perovskite structure, are: at 293°K F2/c, α = 90°21′, V/8 = 53.56 Å3, at 172°K I112/m, γ = 90°, V/4 = 53.22 Å3, at 135°K I
, α = γ = 90°, β = 90°40′, V/4 = 53.02 Å3. To unravel the true symmetries it was necessary to solve all three crystal structures. The first order transformation is characterized by a shear of (0k0) planes along the rhombohedral [101] direction to form an orthogonal space lattice. The AlO6 groups are rotated into new orientations and Pr atoms are subjected to displacements which are normal to the shear direction. The Pr displacements lower the symmetry from orthorhombic to monoclinic even though the space lattice is orthogonal. The second order transformation is characterized by a shear of (00l) planes along ±[100] in the orthogonal space lattice to form twinned monoclinic space lattices. The AlO6 groups do not assume new orientations. The Pr displacements are increased, again in directions normal to the shear directions. Because of the Pr displacements the symmetry is triclinic even though the space lattice is monoclinic.
