X-ray elastic constants for α-Al2O3

It has recently been discovered that there is a long-standing error in the sign of c14 for α-Al2O3—it is +22 rather than −22GPa. Although c14 is the smallest of the six elastic constants of α-Al2O3 (space group R3¯c), it has a significant effect on the Reuss x-ray elastic constants, particularly for the {022¯4} reflection. Measurements using the Advanced Photon Source have confirmed that the sign of c14 must be positive. Use of the wrong sign of c14 can induce errors in excess of 30% in the measured residual stresses of α-Al2O3 scales.