A hypothetical polymorph of copper(II) guanidinium formate

The hybrid perovskite copper(ii) guanidinium formate, C(NH2)(3)[Cu(HCO2)(3)], is a close analog of the inorganic material KCuF3, a well-studied exemplar of the Heisenberg spin-1/2 antiferromagnetic chain. However, the polymorphism known in KCuF3, based on different orbital ordering of the d(9) Cu2+ ions, has never been observed in C(NH2)(3)[Cu(HCO2)(3)]. We use density-functional theory calculation to construct a hypothetical polymorph of the hybrid perovskite analogous to the known structure of KCuF3. This polymorph is very similar in geometric and magnetic structures to the known phase of C(NH2)(3)[Cu(HCO2)(3)], but it is no longer polar. It has only a marginally higher internal energy than the known phase but also a lower vibrational entropy and, hence, higher free energy. Furthermore, stacking faults are far more energetically costly in C(NH2)(3)[Cu(HCO2)(3)] than in KCuF3. These results together help to explain why KCuF3 is experimentally polymorphic but C(NH2)(3)[Cu(HCO2)(3)] is not. The detailed comparison between analogous inorganic and hybrid materials illustrates the subtle differences that can arise from replacing atomic ions with molecular ions and that may be exploited in crystal engineering of these materials.