The stability and electronic structure of perovskite hydrides ABH 3 were investigated by means of first-principles density functional calculations. Two types of perovskite hydrides are distinguished: (1) When A and B are alkali and alkaline earth metals, the hydrides are ionic compounds with calculated band gaps of around 2 eV and higher. Their stability trend follows basically the concept of Goldschmidt's tolerance factor. (2) When A is one of the heavier alkaline earth metals (Ca, Sr, Ba) and B a transition metal, stable compounds ABH3 result only when B is from the Fe, Co, or Ni groups. This stability trend is basically determined by effects associated with d band filling of both the transition metal and the hydride. In contrast to group (1) perovskites, the transition metal-containing compounds are metals. The synthesis of CaNiH3 and its structure determination from CaNiD3 is reported. This compound is a type (2) perovskite hydride with a fully occupied hydrogen position (CaNiD3: a=3.551(4) Å, dNi-D=1. 776(2) Å). Its stability is discussed with respect to transition metal hydrides with complex anions (e.g., Mg2NiH4, Na 2PdH2, Sr2PdH4).
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