Layered Hydride CaNiGeH with a ZrCuSiAs-type Structure: Crystal Structure, Chemical Bonding, and Magnetism Induced by Mn Doping

Stimulated by the discovery of the iron oxypnictide superconductor
with ZrCuSiAs-type structure in 2008, extensive exploration of its
isostructural and isoelectronic compounds has started. These compounds,
including oxides, fluorides, and hydrides, can all be simply recognized
as valence compounds for which the octet rule is valid. We report
herein the first example of a ZrCuSiAs-type hydride, CaNiGeH, which
violates the octet rule. This hydride was synthesized by hydrogenation
of the CeFeSi-type compound CaNiGe under pressurized hydrogen. Powder
diffraction and theoretical simulation confirm that H enters into
the interstitial position of the Ca4 tetrahedron, leading
to notable anisotropic expansion of the unit cell along the c axis. Density functional theory calculations indicate
the modification of the chemical bonding and formation of ionic Ca–H
bond as a result of hydrogen insertion. Furthermore, CaNiGeH shows
Pauli paramagnetism and metallic conduction similar to that of CaNiGe,
but its carrier type changes to hole and the carrier density is drastically
reduced as compared to CaNiGe. Mn-doping at the Ni site introduces
magnetism to both the parent compound and the hydride. The measurement
demonstrates that hydrogenation of CaNi1–xMnxGe reduces ferromagnetic ordering
of Mn ions and induces huge magnetic hysteresis, whereas the spin
glass state observed for the parent compound is preserved in the hydride.
The hydrogenation-induced changes in the electric and magnetic properties
are interpreted in terms of development of two-dimensionality in crystal
structure as well as electronic state.

2008年具有ZrCuSiAs型结构的铁氧磷化物超导体（iron oxypnictide superconductor）被发现后，学界对其同结构等电子化合物展开了广泛探索。这类涵盖氧化物、氟化物与氢化物的化合物，原则上均可归为服从八隅体规则的价键化合物。本文首次报道了一例ZrCuSiAs型氢化物CaNiGeH，其打破了八隅体规则。该氢化物通过在高压氢气氛围中对CeFeSi型化合物CaNiGe进行氢化反应合成得到。粉末衍射与理论模拟结果证实，氢原子占据了Ca4四面体的间隙位点，使得晶胞沿c轴方向发生显著的各向异性膨胀。密度泛函理论（density functional theory）计算表明，氢原子的插入改变了体系的化学键合方式，并形成了离子型Ca–H键。此外，CaNiGeH与母体化合物CaNiGe类似，均表现出泡利顺磁性（Pauli paramagnetism）与金属导电特性，但其载流子类型转变为空穴型，且相较于CaNiGe，其载流子浓度大幅降低。在Ni位点进行Mn掺杂，可同时为母体化合物与该氢化物引入磁性。测试结果显示，对CaNi1–xMnxGe进行氢化处理，会削弱Mn离子的铁磁有序性并诱发显著的磁滞效应，而母体化合物中观测到的自旋玻璃态在氢化产物中得以保留。氢化反应引发的电学与磁学性质变化，可通过晶体结构与电子态的二维性增强得到合理解释。