Large Negative Thermal Expansion, Thermodynamic Properties, And Temperature-Dependent Raman Scattering of a New Metal–Organic Perovskite Framework [C(NH2)3][Ca(HCOO)3]

We present the synthesis, crystal structure, thermal expansion, Raman spectra, and heat capacity of Ca-guanidinium formate, a new member of the family of metal–organic perovskites ([C(NH2)3][M2+(HCOO)3]), where M2+ is a divalent metal ion. Ca-guanidinium formate shows an extraordinary strong negative thermal expansion between 100 and 400 K in the crystallographic (001) plane with thermal expansion coefficients α11 = α22 = −30.0(7) × 10–6 K–1. Perpendicular to this plane, along a3, the crystal structure of [C(NH2)3][Ca(HCOO)3] expands very strongly with increasing temperature (α33 = 155(2) × 10–6 K –1). This behavior is due to a reduction of the framework distortion on increasing temperature. From temperature-dependent Raman measurements we observed three soft Raman modes. The C–H out-of-plane bending mode (1062 cm–1) of formate groups is influenced by squeezing by guanidinium groups. This squeezing of formate groups has an effect of reducing the framework distortion and is asscociated with a strong negative thermal expansion. From heat capacity measurements we derived the Debye temperature θCp = 217 K. This study contributes to a deeper insight of structure–property relationships of compounds with strong anisotropic and large negative thermal expansion.