Hydrogen absorption by Pd exhibits hysteresis loops (provided the temperature is not too hight) and represents one of the classical examples of the first-order phase transitions in metals. The experiments indicate that addition of even small amount of Au is able to suppress hysteresis. From this perspective, we analyze the energetic of hydrogen in a Pd-Au alloy by using extensive density-functional-theory (DFT) calculations. The dependence of the hydrogen binding energy on the number ($n$) of Au atoms forming an adsorption site is found to be appreciably non-linear. With the DFT input for statistical calculations, we reproduce special features of the hydrogen absorption isotherms and explain rapid decrease of the corresponding critical temperature with increasing the Au fraction. The key factor here is that the phase transition is related primarily with absorption in sites formed only by Pd. With increasing the Au amount, the fraction of such sites rapidly decreases, the distance between H atoms located there becomes on average larger, the interaction between them becomes weaker, and accordingly the driving force for the phase transition decreases. It is of interest that all these effects can be illustrated by taking only the configurations with $n\le 2$ into account. This means that in the context under consideration the fine details of the dependence of the hydrogen binding energy on $n$ are in fact not too important.

中文翻译：

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Pd-Au合金在氢吸收中的磁滞抑制

Pd吸收的氢表现出磁滞回线（假设温度不太高），并且代表金属中一阶相变的经典例子之一。实验表明，即使添加少量的Au也能抑制磁滞现象。从这个角度出发，我们通过使用广泛的密度泛函理论（DFT）计算来分析Pd-Au合金中的氢能。氢结合能对数量的依赖性（$ñ$发现形成吸附位的Au原子的α）明显是非线性的。利用DFT输入进行统计计算，我们重现了氢吸收等温线的特殊特征，并解释了随着Au含量的增加相应临界温度的快速降低。此处的关键因素是，相变主要与仅由Pd形成的位点的吸收有关。随着Au量的增加，这些位点的比例迅速减小，位于该位置的H原子之间的距离平均变大，它们之间的相互作用变弱，因此用于相变的驱动力减小。有趣的是，可以通过仅采用以下配置来说明所有这些效果$ñ\le 2$考虑在内。这意味着在所考虑的背景下，氢结合能对$ñ$ 实际上并不是太重要。