While there is a clear trend, the exact relationship between the shear piezoelectric coefficients and the corresponding compliance values varies, especially across material class. For instance, C[NH 2 ] 3 BF 4 has c 55 value of 230 TPa -1 and d 25 of 10.9 pC/N. So while the shear compliance values of C[NH 2 ] 3 BF 4 are larger than those of [F -Q]ReO 4 , the piezoelectric response is lower.