| Summary: | We report a first-principles study of hybrid organic–inorganic perovskites with formula [A]Cu(H<sub>2</sub>POO)<sub>3</sub> (A = triazolium (Trz) and guanidinium (Gua), and H<sub>2</sub>POO<sup>−</sup> = hypophosphite), and [HIm]Cu(HCO<sub>2</sub>)<sub>3</sub> (HIm = imidazolium cation, <inline-formula><math display="inline"><semantics><mrow><msubsup><mrow><mi>HCO</mi></mrow><mn>2</mn><mo>−</mo></msubsup></mrow></semantics></math></inline-formula> = formate). The triazolium hypophosphite and the formate have been suggested as possible ferroelectrics. We study the fully relaxed structures with different magnetic orderings and possible phonon instabilities. For the [Trz]Cu hypophosphite, the Trz cation is shown to induce large octahedral distortions due to the Jahn-Teller effect, with Cu-O long-bond ordering along two perpendicular directions, which is correlated with antiferromagnetic ordering and strongly one-dimensional. We find that the structure is dynamically stable with respect to zone-center distortions, but instabilities appear along high symmetry lines in the Brillouin zone. On the other hand, for the [HIm]Cu formate, large octahedral distortions are found, with large Cu-O bonds present in half of the octahedra, in this case along a single direction, and correspondingly, the magnetism is almost two-dimensional.
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