Phase Transition and Negative Thermal Expansion in Guanidinium Magnesium-Hypophosphite Hybrid Perovskite

When the A- and X-sites in three-dimensional (3D) ABX3-type organic–inorganic hybrid perovskites are occupied by organic molecules, the perovskites possess rich features and functionalities. Herein, we report a new hybrid organic–inorganic ABX3-type guanidinium hypophosphite perovskite [Gua][Mg(H2POO)3] (Gua = guanidinium, C(NH2)3+), where its A- and X-sites are occupied by Gua cations and hypophosphite (H2POO–) bridges, respectively, therefore displaying many excellent properties. Results show that it undergoes a second-order irreversible phase transition and exhibits a significant negative thermal expansion (NTE, αb(LT) = −7.7(7) and αb(HT) = −20.5(1) MK–1) effect along the b-axis. In addition, UV–vis absorption spectrum and calculated band structure and density of states demonstrate that the total structure donate to the large direct band gap. These results show that hypophosphite perovskites provide a promising platform for preparation of new functional materials.