Acceptor-doped BaZrO₃ (BZO) and BaCeO₃ (BCO) both exhibit considerable bulk proton conductivity, which makes them suitable as electrolytes in electrochemical devices. However, these materials display high grain-boundary (GB) resistance that severely limits the overall proton transport in polycrystalline samples. This effect has been attributed to the presence of space charges at the GBs, which form because of segregation of protons and charged oxygen vacancies. This blocking behavior is less prominent in BCO, but in contrast to BZO, BCO suffers from poor chemical stability. The aim with the present work is to elucidate why GBs in BZO are more resistive than GBs in BCO. We use density-functional theory (DFT) calculations to study proton and oxygen vacancy segregation to several GBs and find that the oxygen vacancy segregation energy is quite similar in both materials while the tendency for proton segregation is larger in BZO compared with that in BCO. This is not related to the GBs, which display similar proton formation energies in both materials, but because of different formation energies for protons in the bulk regions. This can be understood from a stronger hydrogen bond formation in bulk BCO compared with that in bulk BZO. Furthermore, segregation energies are evaluated in a space-charge model, and the resulting space-charge potentials provide a consistent explanation of the experimentally observed difference in GB conductivity.

中文翻译：

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质子传导性BaZrO ₃和BaCeO ₃晶界处空间电荷形成的比较

掺杂受体的BaZrO ₃（BZO）和BaCeO ₃（BCO）均表现出相当大的质子电导率，这使其适合用作电化学装置中的电解质。但是，这些材料显示出高的晶界（GB）抵抗力，严重限制了多晶样品中整体质子的传输。这种影响归因于在GB处存在空间电荷，这是由于质子和带电的氧空位的隔离而形成的。这种阻塞行为在BCO中不太明显，但与BZO相比，BCO具有较差的化学稳定性。当前工作的目的是阐明为什么BZO中的GB比BCO中的GB具有更高的电阻性。我们使用密度泛函理论（DFT）计算来研究质子和氧空位偏析到几个GB，发现两种材料中的氧空位偏析能量非常相似，而BZO中的质子偏析趋势比BCO中的大。这与GBs无关，后者在两种材料中均显示相似的质子形成能，但是由于在大部分区域中质子的形成能不同。从本体BCO中的氢键形成要比本体BZO中的氢键形成强，这可以理解。此外，在空间电荷模型中评估了分离能，并且所产生的空间电荷电位为实验观察到的GB电导率差异提供了一致的解释。这与GBs无关，后者在两种材料中均显示相似的质子形成能，但是由于在大部分区域中质子的形成能不同。从本体BCO中的氢键形成比本体BZO中的氢键形成更强，可以理解这一点。此外，在空间电荷模型中评估了分离能，并且所产生的空间电荷电位为实验观察到的GB电导率差异提供了一致的解释。这与GBs无关，后者在两种材料中均显示相似的质子形成能，但是由于在大部分区域中质子的形成能不同。从本体BCO中的氢键形成比本体BZO中的氢键形成更强，可以理解这一点。此外，在空间电荷模型中评估了分离能，并且所产生的空间电荷电位为实验观察到的GB电导率差异提供了一致的解释。