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 Delta a (m)(T) increase with increasing temperature and tend to constant values at Ta parts per thousand << T (c) and that the magnetic components Delta c (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 Delta a (m)(T) and Delta c (m)(T).