Abstract
Intermetallic compounds of the typeRFe10Si2 andRCo10Si2 crystallize in theThMn12 structure (space groupI4/mmm) whilst the heavy rareearth series RNi10Si2 crystallizein a maximal subgroup of I4/mmm, P4/nmm. Reported here are neutron powder diffraction investigations forTbNi10Si2 andErNi10Si2 which showthat the P4/nmm structure undergoes a high temperature order–disorder phase transition at approximately930 °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 thelattice parameters, whilst the temperature dependence of the thermal expansion coefficientsα11,α33 and αvolume has been determined from the lattice parameters. Associated with the order–disordertransition is a transition associated with a displacement of the R ion along thec-axis. Both transitions are of second order and the critical exponentassociated with the order–disorder and displacive transitions,β = 0.31, is in excellent agreement with the exponent determined for the three-dimensional Isingmodel.
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