The ternary alkali-metal manganese hydrides A3MnH5 and A3MnD5 (A = K, Rb, Cs) were synthesised by the reaction of the respective alkali-metal hydride (deuteride) with manganese powder at 875 K under a hydrogen pressure of above 3000 bar. X-ray investigations of the powdered samples and elastic neutron diffraction experiments on the deuterated compounds over the temperature range 2–600 K led to the complete determination of their structures. Their atomic arrangements are isotypic with that of Cs3[CoCl4]Cl (space group: I4/mcm, Z=4). The structure contains isolated [MnH4]2− tetrahedra with the remaining hydrogen ions exclusively coordinated by the alkali-metal cations. The magnetic susceptibilities show a Curie-Weiss behaviour. Antiferromagnetic interactions were observed at low temperatures. Three different spin structures result from neutron diffraction experiments.

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K3MnH5 and K3MnD5 were synthesized by the reaction of potassium hydride (deuteride) with manganese powder under a hydrogen pressure above 3000 bar at 875 K. X-ray investigations on powdered samples and elastic neutron diffraction experiments on the deuterated compound at the time-of-flight spectrometer LAD in the temperature range 5 - 600 K led to the complete structure determination. The atomic arrangement is isotypic with that of Cs-3[CoCl4]Cl (space group: I4/mcm, Z = 4). The structure of K3MnH5 contains isolated [MnH4](2-)-tetrahedra and additional hydrogen ions which are exclusively coordinated by potassium cations. The magnetic susceptibilities show Curie-Weiss behaviour. At temperatures below 50 K there are obviously antiferromagnetic interactions.