Effect of Uniaxial Stress on the Zero-Field Splitting ofCr3+in Alums

The effect of uniaxial stress on the zero-field splitting of ${\mathrm{Cr}}^{3+}$ in five alum crystals is reported. In conjunction with uniaxial stress electron-spin-resonance measurements at room temperature are capable of determining the sign of the zero-field-splitting parameter $2D$ in alum crystals. This method provides a means of determining the sign of $D$ in those alums which undergo crystallographic or phase transitions at low temperature. The positive zero-field splitting in the three investigated $\alpha$-alums is reduced by an externally applied uniaxial stress along the trigonal axis, and the negative zero-field splitting in the two $\beta$-alums is increased by the same stress. These results are in agreement with McGarvey's model of a ${\mathrm{Cr}}^{3+}$ complex in which the zero-field splitting is positive or negative depending on whether the complex is trigonally extended or compressed. If the association is made that $\alpha$-alums have a positive $D$ value, and $\beta$-alums have a negative $D$ value, a result which seems to be generally valid, the effect of stress on the chromium spectrum in alums can be used to differentiate between $\alpha$ and $\beta$ types. Stress-induced changes in the zero-field splitting were used to calculate the $C_{33}$ spin-lattice coefficient for the five alums.