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.