发表机构
The Pennsylvania State University; The American University in Cairo（宾夕法尼亚州立大学; 开罗美国大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究摒弃NaCa(BH$_4$)$_3$的钙钛矿假设，通过多源候选构建、机器学习筛选与第一性原理计算，确定其最稳定结构为单斜Cm型，解释了其无法形成钙钛矿的原因并给出模拟衍射特征。
AI 中文摘要
等摩尔NaCa(BH$_4$)$_3$的理论氢容量为11.24 wt.%，分解焓预计介于NaBH$_4$与Ca(BH$_4$)$_2$之间，但该物质从未被制备出来，也未报道过其晶体结构。此前对它的预测均基于其重同系物所属的钙钛矿家族。本文摒弃了这一假设，从三个独立来源构建了六个候选框架：实验确定的ABX$_3$型硼氢化物、数据驱动结构预测得到的畸变钙钛矿，以及对结构数据库的无偏搜索（共生成2238个候选并全部弛豫，未排除任何一个）。在固定电中性组成下，对每个框架的阳离子排布进行了穷举枚举，共420种排布简化为118种对称不等价构型，通过机器学习势筛选后，再经第一性原理计算确定。最稳定结构并非钙钛矿型，而是单斜晶系、空间群为Cm的框架，仅通过无限制搜索得到，能量为-4.185640 eV/atom，比此前最佳框架低7.67 meV/atom；两种化学无关的供体原型收敛至该结构的能量差为0.076 meV/atom，其有序阳离子排布是自身系列的基态。其声子谱在超胞可精确分辨的任意波矢处均无虚频，弛豫离子弹性张量为正定，而正交晶系钙钛矿候选结构则存在-2.401 THz的不稳定性。这一对比为该组成无法以钙钛矿形式制备提供了物理解释，本文报道的模拟衍射图谱通过七个反射峰识别出该预测框架，而这七个峰在两种母体相及竞争框架中均不存在。
英文摘要
Equimolar NaCa(BH$_4$)$_3$ offers a theoretical hydrogen capacity of 11.24 wt.% and a decomposition enthalpy expected to fall between those of NaBH$_4$ and Ca(BH$_4$)$_2$, but it has never been prepared and no crystal structure has been reported. Predictions for it have so far been built within the perovskite family that its heavier homologues adopt. Here that assumption is removed. Six candidate frameworks were assembled from three independent sources --- experimentally determined ABX$_3$ borohydrides, a distorted perovskite from data-driven structure prediction, and an unbiased search over a structural database in which all 2238 generated candidates were relaxed with none excluded --- and at fixed charge-neutral composition the cation arrangement of every framework was enumerated exhaustively, 420 decorations reducing to 118 symmetry-inequivalent configurations, screened with a machine-learning potential and settled from first principles. The most stable structure is not a perovskite. It is a monoclinic framework of space group Cm, reached only by the unrestricted search, at $-$4.185640 eV/atom, 7.67 meV/atom below the best framework available beforehand; two chemically unrelated donor prototypes converge on it to 0.076 meV/atom, and its ordered cation arrangement is the ground state of its own series. Its phonon spectrum carries no imaginary mode at any wavevector the supercell resolves exactly and its relaxed-ion elastic tensor is positive definite, whereas the orthorhombic perovskite candidate is unstable to $-$2.401 THz. That contrast supplies a physical reason for the reported failure to obtain this composition in perovskite form, and the simulated diffraction pattern reported here identifies the predicted framework by seven reflections that neither parent phase nor the competing framework produces.
