Structural transitions in RNi(10)Si(2) intermetallics.

Intermetallic compounds of the type RFe(10)Si(2) and RCo(10)Si(2) crystallize in the ThMn(12) structure (space group I4/mmm) whilst the heavy rare earth series RNi(10)Si(2) crystallize in a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations for TbNi(10)Si(2) and ErNi(10)Si(2) which show that the P4/nmm structure undergoes a high temperature order-disorder phase transition at approximately 930 °C above which the ordered Ni and Si fractions revert to a random distribution on 4d and 4e sites. The volume expansion has been tracked in detail via the temperature dependence of the lattice parameters, whilst the temperature dependence of the thermal expansion coefficients α(11), α(33) and α(volume) has been determined from the lattice parameters. Associated with the order-disorder transition is a transition associated with a displacement of the R ion along the c-axis. Both transitions are of second order and the critical exponent associated with the order-disorder and displacive transitions, β = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Ising model.