title: Reentrant Structural and Optical Properties and Large Positive Thermal Expansion in Perovskite Formamidinium Lead Iodide.
abstract: The structure of the hybrid perovskite HC(NH2)2PbI3(formamidinium lead iodide) reflects competing interactions associated with molecular motion, hydrogen bonding tendencies, thermally activated soft octahedral rotations, and the propensity for the Pb2+lone pair to express its stereochemistry. High-resolution synchrotron X-ray powder diffraction reveals a continuous transition from the cubic α-phase (Pm3‾ m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 285 K, followed by a first-order transition to a tetragonal γ-phase (retaining P4/mbm, #127) at 140 K. An unusual reentrant pseudosymmetry in the β-to-γ phase transition is seen that is also reflected in the photoluminescence. Around room temperature, the coefficient of volumetric thermal expansion is among the largest for any extended crystalline solid.
authors.affiliation: Materials Research Laboratory, University of California, Santa Barbara, CA, 93106, USA.|Materials Department, University of California, Santa Barbara, CA, 93106, USA.
authors.forename: Douglas H
authors.identifier: 0000-0002-2391-1507
authors.initials: DH
authors.lastname: Fabini
authors.affiliation: Chemistry Department, Northwestern University, Evanston, IL, 60208, USA.
authors.forename: Constantinos C
authors.identifier: 0000-0001-8396-9578
authors.initials: CC
authors.lastname: Stoumpos
authors.affiliation: Materials Research Laboratory, University of California, Santa Barbara, CA, 93106, USA.
authors.forename: Geneva
authors.identifier: 0000-0001-9577-150X
authors.initials: G
authors.lastname: Laurita
authors.affiliation: Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, 15310, Greece.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kaltzoglou
authors.affiliation: Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, 15310, Greece.
authors.forename: Athanassios G
authors.initials: AG
authors.lastname: Kontos
authors.affiliation: Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens, 15310, Greece.
authors.forename: Polycarpos
authors.initials: P
authors.lastname: Falaras
authors.affiliation: Chemistry Department, Northwestern University, Evanston, IL, 60208, USA.
authors.forename: Mercouri G
authors.identifier: 0000-0003-2037-4168
authors.initials: MG
authors.lastname: Kanatzidis
authors.affiliation: Materials Research Laboratory, University of California, Santa Barbara, CA, 93106, USA.|Materials Department, University of California, Santa Barbara, CA, 93106, USA.|Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, 93106, USA.
authors.forename: Ram
authors.identifier: 0000-0001-5858-4027
authors.initials: R
authors.lastname: Seshadri
keywords: perovskite phases; phase transitions; solid-state structures; structure elucidation; thermal expansion