Stabilization of LnB₁₂ (Ln = Gd, Sm, Nd, and Pr) in Zr_1–<i>x</i>Ln_<i>x</i>B₁₂ under Ambient Pressure

Abstract

We report ambient pressure stabilization of a previously synthesized high-pressure (6.5 GPa) phase, GdB₁₂, in a Zr_1-xGd_xB₁₂ solid solution (with ∼54 at. % Gd solubility, as determined by both powder X-ray diffraction and energy-dispersive spectroscopy). Limited solubilities of Sm (∼15 at. % Sm), Nd (∼7 at. % Nd), and Pr (∼4 at. % Pr), in ZrB₁₂ were also achieved. Previous attempts at preparing these rare-earth borides were unsuccessful even under high pressure. On the basis of insights provided from the unit cell sizes observed via solid solutions, at least 6.5 GPa of pressure would be needed to synthesize these rare-earth borides since Sm, Nd, and Pr atomic radii are larger than that of Gd. The solid-solution formation for Zr_1-xGd_xB₁₂ and Zr_1-xSm_xB₁₂ can be seen in the change of the unit cell of each of the solid solutions relative to their pure parent compounds as well as in the change of color of the respective alloys. For Zr_0.45Gd_0.55B₁₂ and Zr_0.70Sm_0.30B₁₂, the cubic unit cell parameter (a) reached a value of 7.453 and 7.428 Å, respectively, compared to 7.412 Å for pure ZrB₁₂.