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

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 ZrHx (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 ZrHx were reproduced, and new structures that are more stable were found. For ZrH0.5, the most stable structure is of Pn (3) over barm symmetry. The experimentally discovered P3m1 zeta-ZrH0.5 was found in this work, which is both dynamically and mechanically stable. The corrected lattice constants and atomic coordinates of the P3m1 zeta-ZrH0.5 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 ZrH1.5, it was found that the P4(2)/nnm structure is most stable at low temperature, while the experimentally observed Pn (3) over barm structure is most stable at high temperature. For ZrH2, 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 gamma -> delta -> epsilon phase transitions can be explained reasonably using the P4(2)/nnm ZrH1.5 structure as the reaction intermediate. Additionally, through comparing the formation free energy, we found that delta-ZrH1.5 is the most stable Zr hydride at high temperature, while epsilon-ZrH2 is most stable at low temperature. Our results indicate that fast cooling process will promote the formation of delta-ZrH1.5, and slow cooling process will promote the formation of gamma-ZrH.