Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom Crystal structure of the high-temperature polymorph of C(NH 2 ) 3 PbI 3 and its thermal decomposition Sandra Dimitrovska-Lazova a, ⁎ , Miha Bukleski a , Peter Tzvetkov b , Slobotka Aleksovska a , Daniela Kovacheva b a University “Ss Cyril and Methodius”, Faculty of Natural Sciences and Mathematics, Institute of Chemistry, Arhimedova 5, 1000 Skopje, Macedonia b Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str, Bd.11, 1113 Sofia, Bulgaria article info Article history: Received 14 September 2020 Received in revised form 20 November 2020 Accepted 23 November 2020 Available online xxxx Keywords: GUAPbI 3 High-temperature polymorph structure Simulated annealing Rietveld structure refnement abstract The synthesis of guanidinium lead iodide, C(NH 2 ) 3 PbI 3 (GUAPbI 3 ), was conducted by slow evaporation of the mixture obtained by dissolving PbI 2 and C(NH 2 ) 3 I in acetonitrile. When the evaporation is done at 40 ºC, a yellow needle-like crystals are being formed. The sample was characterized by elemental analysis, density measurements, scanning electron microscopy, thermal analyses, high-temperature X-ray powder diffraction and infrared spectroscopy measurements. The elemental analysis of the obtained crystals confrmed the proposed stoichiometry. The performed thermal analyses showed an endothermic peak associated with structural transition around 160 ºC. On the other hand, the endothermic temperature effects above 300 ºC are accompanied with mass loss and were interpreted as compound degradation. The crystal structure of high temperature polymorph between 160 ºC and 300 ºC was determined using high-temperature powder diffraction data measurements at 280 ºC using simulated annealing technique in order to obtain initial structural model. The structure was refned using the Rietveld method. At temperatures higher than 160 ºC, C(NH 2 ) 3 PbI 3 crystallizes in hexagonal space group P6 3 mc with unit cell parameter a increasing from 9.269 Å to 9.337 Å between 160 ºC and 300 ºC and c parameter increasing from 15.211 Å to 15.287 Å in the same temperature range. The structure consists of PbI 6 octahedra couples sharing a common face, linked with corners. Guanidinium cations are situated in the channels between Pb 2 I 9 couples in a manner that the plane of the molecule is perpendicular to the c-axis. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Over the past decade, organic-inorganic perovskites (hybrid perovskites) emerged as one of the most promising materials for solar cell fabrication combining both high efciency and low cost [1,2]. Starting with the work of Kojima et al. [3] on CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite solar cell, which showed 3.8% power con- version efciency (PCE), the intense work on this feld resulted in increasing PCE up to 22% [4,5] in just a few years. Hybrid per- ovskites have shown unique properties suitable for photovoltaic applications, such as high light absorption coefcients [3,6], long charge carrier (electron-hole) diffusion lengths [7], tunable band gaps [8,9] etc. However, organic-inorganic perovskites for solar cell applications suffer from instability under external conditions such as humidity, heat, light etc. [10–12]. Therefore, the further development of perovskite solar cells has been focused on in- creasing both PCE and their stability [13]. Recent investigation on organic-inorganic perovskites, has shown that the organic cation in the structure has a great infuence on both the stability and performances [14], so a number of attempts were made on exchanging and modifcation of the monovalent organic cation [9,14,15]. A promising candidate among the organic cations, is guani- dinium cation [(NH 2 ) 3 C] + (GUA) [16,17]. Unfortunately, GUA has a sig- nifcantly larger ionic radius (278 pm), and is assumed as not suitable to form a 3D perovskite structure as the sole A-cation [14,16]. However, Giorgi et al. [18] have theoretically characterized the guanidinium lead iodide system, GUAPbI 3 using density functional theory calculations (DFT). The results of their calculations pointed out to better thermo- dynamic stability, low formation energy and low band gap compared to MAPbI 3 and FAPbI 3 . The authors suggest that this is due to a high symmetry of guanidinium cation and zero dipole moment as a result of its D 3 h symmetry. In addition, the three NH 2 groups are expected to frustrate the bulkier GUA cation rotation in the semiconductor cavity of the proposed perovskite structure [18]. https://doi.org/10.1016/j.jallcom.2020.158104 0925-8388/© 2020 Elsevier B.V. All rights reserved. ⁎ Corresponding author. E-mail address: sandra@pmf.ukim.mk (S. Dimitrovska-Lazova). Journal of Alloys and Compounds xxx (xxxx) xxx Please cite this article as: S. Dimitrovska-Lazova, M. Bukleski, P. Tzvetkov et al., Crystal structure of the high-temperature polymorph of C(NH 2 ) 3 PbI 3 and its thermal decomposition, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2020.158104i