Structure and thermodynamic properties of zirconium hydrides by structure search method and first principles calculations

Abstract The formation of precipitated zirconium (Zr) hydrides is closely related to the hydrogen embrittlement of the claddings in pressured water reactors. In this work, we systematically investigated the structures of Zr hydrides ZrH x ( x  = 0.5, 1, 1.5, 2) and their thermodynamic properties by combining the basin hopping algorithm with first principles calculations. All the experimentally identified structures of ZrH x were reproduced, and new structures that are more stable were found. For ZrH 0.5 , the most stable structure is of Pn 3 ¯ m symmetry. The experimentally discovered P3m1 ζ-ZrH 0.5 was found in this work, which is both dynamically and mechanically stable. The corrected lattice constants and atomic coordinates of the P3m1 ζ-ZrH 0.5 are also clarified. For ZrH, the most stable structure found in this work is of P4 2 /mmc symmetry, which is in agreement with experiments. For ZrH 1.5 , it was found that the P4 2 /nnm structure is most stable at low temperature, while the experimentally observed Pn 3 ¯ m structure is most stable at high temperature. For ZrH 2 , the bistable structures of I4/mmm ( c/a  = 1.25) symmetry is most stable, which is consistent with experiments. Furthermore, through analysis of the structural changes of Zr matrixes companying the phase transitions of Zr hydrides, we suggest that γ → δ → e phase transitions can be explained reasonably using the P4 2 /nnm ZrH 1.5 structure as the reaction intermediate. Additionally, through comparing the formation free energy, we found that δ-ZrH 1.5 is the most stable Zr hydride at high temperature, while e-ZrH 2 is most stable at low temperature. Our results indicate that fast cooling process will promote the formation of δ-ZrH 1.5 , and slow cooling process will promote the formation of γ-ZrH.