Crystal growth and phase formation of high‐entropy rare‐earth monoclinic aluminates

Abstract For the first time, high‐entropy rare‐earth monoclinic aluminate crystals were grown via directional solidification using the micro‐pulling‐down method. Five high‐entropy compositions were formulated with a general formula RE 4 Al 2 O 9 , where RE is an equiatomic mixture of five rare‐earth elements. The rare‐earth elements included were Lu, Yb, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, and La. High‐temperature powder X‐ray diffraction and Rietveld structure refinement indicated that all crystals were a single monoclinic phase and that rare‐earth average ionic radius did not affect phase purity. At room temperature, the refined lattice parameters increased consistently with increasing average ionic radii of the five compositions. One of the crystals had a typical high‐temperature phase transition of single‐RE RE 4 Al 2 O 9 in the range of 1100–1150°C, which consisted of a lattice contraction upon heating. Differential scanning calorimetry indicated a thermal event corresponding to that phase transition. Electron probe microanalysis revealed Al‐rich inclusions on the surface of the crystals. Crystals containing Tb had dark surface features that became lighter after annealing in a reducing atmosphere, which indicated that Tb 4+ may be responsible for the dark features.