Anomalies in thermal expansion of rare-earth diborides in the temperature range of magnetic phase transformations

The variations with temperature of the lattice parameters a(T) and c(T) and the thermal expansion coefficients of rare-earth diborides RB2(R=Tb, Dy, Ho, Er, Tm, Lu) have been investigated in the temperature range 4.2—300 K. It has been revealed that the dependences a(T) and c(T) exhibit an anomalous behavior in the vicinity of the ferromagnetic transformation temperatures T c which correlates with the temperature dependences of the heat capacity. It has been established that the magnetic contributions to the thermal expansion of the diborides Δa m(T) increase with increasing temperature and tend to constant values at T≫T c and that the magnetic components Δc m(T) decrease for all the diborides studied, also tending to constant values at elevated temperatures. The exchange integrals Y a and Y c for the paramagnetic metal ions in the rare-earth diborides are determined from the dependences Δa m(T) and Δc m(T).