Elastic constants of FeF2between 1.5 and 298 K

Transit times of 30-MHz ultrasonic waves have been measured in Fe${\mathrm{F}}_2$ single crystals down to 1.5 K for the first time and used to determine ultrasonic velocities and all the elastic stiffness moduli: $C_{11}$, $C_{12}$, $C_{13}$, $C_{33}$, $C_{44}$, and $C_{66}$. Near the Néel temperature (78.4 K) the longitudinal-mode velocities exhibit inverted $\lambda$-type anomalies as in previous work. Our new findings include softening of $\frac{(C_{11}-C_{12})}{2}$ from 298 K on down, suggesting a tendency toward lattice instability, and of $C_{44}$ below 90 K, due to its internal strain component becoming more negative as the frequency of the $E_g$ Raman mode decreases with decreasing temperature in this range. At 1.5 K we find an elastic Debye temperature of 411±5 K which yields an acoustic-mode heat capacity in agreement with that deduced from heat-capacity data.