research papers J. Appl. Cryst. (2016). 49, 213–221 http://dx.doi.org/10.1107/S1600576715023663 213 Received 3 September 2015 Accepted 9 December 2015 Edited by K. Chapman, Argonne National Laboratory, USA Keywords: equations of state; high pressure; Rietveld method; density functional theory (DFT) calculations; X-ray diffraction. Pressure-induced polymorphism in nanostructured SnSe Sergio Michielon de Souza, a * Hidembergue Ordozgoith da Frota, a Daniela Menegon Triche ˆs, a Angsula Ghosh, a Puspitapallab Chaudhuri, a Marta Silva dos Santos Gusmao, a Aercio Filipe Franklim de Figueiredo Pereira, a Mariana Couto Siqueira, b Kleber Daum Machado b and Joao Cardoso de Lima c a Departamento de Fı ´sica, UFAM, 3000-Japiim, Manaus, AM 69077-000, Brazil, b Departamento de Fı ´sica, Centro Politecnico, Universidade Federal do Parana ´, 81531-990 Curitiba, Parana ´, Brazil, and c Departamento de Fı ´sica, Universidade Federal de Santa Catarina, Campus Trindade, Floriano ´ polis, SC 88040-900, Brazil. *Correspondence e-mail: s.michielon@gmail.com The pressure-induced phase transitions in nanostructured SnSe were investi- gated using angle-dispersive X-ray diffraction in a synchrotron source along with first-principles density functional theory (DFT) calculations. The variation of the cell parameters along with enthalpy calculations for pressures up to 18 GPa have been considered. Both the experimental and the theoretical approaches demonstrate a phase transition at around 4 GPa. Below 8.2 GPa the X-ray diffraction patterns were fitted using the Rietveld method with space group Pnma (No. 62). The lattice parameters and atomic positions for the above-mentioned symmetry were used in DFT calculations of thermodynamic parameters. The enthalpy calculations with the computationally optimized structure and the proposed Pnma structure of SnSe were compatible. The variations of the cell volume for the high-pressure phases are described by a third-order Birch–Murnaghan equation of state. 1. Introduction IV–VI semiconductor nanostructures, such as GeS, GeSe, SnS and SnSe, show several types of structural complexity formed mainly by two-dimensional or three-dimensional frameworks of polyhedral atomic arrangements. These materials are characterized by many interesting thermoelectric and optical properties depending on the shape and size of the nano- structures. At ambient conditions, SnSe crystallizes in a Pnma space group structure (GeS B16-type), forming perpendicular double layers in the direction of the largest axis of the unit cell. The unit cell contains eight atoms organized in two adjacent double layers. The atoms in each layer bond to their three nearest neighbors and form a chain in a zigzag along the direction of the minor axis of the crystal. Owing to its layered structure and the high negative value of the enthalpy of formation, which lies between 95 and 76.5 kJ mol 1 (Boone & Kleppa, 1992; Achimovicova ´ et al., 2011; Bletskan, 2005), SnSe can be synthesized by a relatively easy and controlled process (Zhao et al. , 2015). The bi- dimensional character of SnSe leads to anisotropy in (i) microstructural and morphological (Baumgardner et al., 2010; Boscher et al., 2008, Li et al., 2000, Shikha et al., 2012), (ii) optoelectronic (Zhao et al. , 2015; Baumgardner et al., 2010; Mariappan et al., 2010; Solanki et al., 2014), and (iii) high- temperature (Zhao et al. , 2014; Carrete et al. , 2014; Sassi et al., ISSN 1600-5767 # 2016 International Union of Crystallography