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.