Quadrupole approximation in the theory of the magnetoelasticity of rare-earth compounds: magnetoelastic contribution to the thermal expansion of the rare-earth anadates RVO4

摘要

Systematic experimental and theoretical investigations of the thermal expansion of the rare-earth vanadates RVO4 (R=Pr, Nd, Gd-Ho, and Tm) are carried out. Considerable magnetoelastic thermal-expansion anomalies are detected, and the magnetoelastic contributions are identified in the Debye model of thermal expansion with consideration of corrections for the variation of the phonon contribution along the RVO4 series. The applicability of the quadrupole approximation to the totally symmetric modes is discussed on the basis of a comparison of calculated multipole moments of all orders for different rare-earth ions in the vanadate structure. It is shown that the magnetoelastic contribution to the thermal expansion of rare-earth vanadates is described well by the temperature dependences of the quadrupole moments of the rare-earth ions. The totally symmetric magnetoelastic coefficients for all the rare-earth ions investigated are determined from the experimental data on the magnetoelastic contributions, and comparisons are made for various rare-earth ions in trie structure under consideration, as well as for the closely related phosphate structure.