Magnetoelastic effects in rare-earth vanadates YbVO4 and HoVO4

Magnetoelastic anomalies in the thermal expansion and Young modulus, as well as the ΔE-effect in rare-earth vanadates RVO₄ (R = Ho, Yb), are investigated experimentally and theoretically. A considerable softening of the Young modulus is observed for HoVO₄ and YbVO₄ at T < 70 K and T < 150 K, respectively; this effect is adequately described in the framework of the generalized susceptibility formalism. It is shown that the field dependences of the ΔE-effect and their temperature variation in YbVO₄ can also be described using this approach. To compare with experiment, the magnetoelastic contributions to the Young modulus of an isotropic polycrystal from various elastic modes have been averaged. For the Yb vanadate, considerable magnetoelastic anomalies in the thermal expansion along the tetragonal a and c axes have been discovered. The magnetoelastic contributions are used for determining completely symmetric magnetoelastic coefficients; the role of the completely symmetric quadrupole constant for magnetoelastic effects is analyzed.