Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Features of crystalline and electronic structures of Sm 2 MTaO 7 (M=Y, In, Fe) and their hydrogen production via photocatalysis Leticia M. Torres-Martínez a, ⁎ , M.A. Ruíz-Gómez b , E. Moctezuma c a Departamento de Ecomateriales y Energía, Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León UANL, Av. Universidad S/N Ciudad Universitaria, San Nicolás de los Garza, Nuevo León C.P. 64455, México b Centro de Investigación y de Estudios Avanzados del IPN (CINVESTAV), Unidad Mérida, Antigua carretera a Progreso, km 6, Cordemex, Mérida, Yucatán C.P. 97310, México c Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Manuel Nava #6, San Luis Potosí, S.L.P. C.P. 78290, México ARTICLE INFO Keywords: Pyrochlore Rietveld analysis Crystal structure Photocatalysis Hydrogen production ABSTRACT This paper reports on the crystal structure determination of a new phase of Sm 2 YTaO 7 synthesized by a solid- state reaction. Rietveld refinement using X-ray powder diffraction (XRD) data and electron diffraction using transmission electron microscopy (TEM) revealed that Sm 2 YTaO 7 crystallized into an orthorhombic system with space group C222 1 , and according to the crystalline arrangement, it can be considered as a weberite-type phase. A detailed analysis of the crystal chemistry of the family with formula Sm 2 MTaO 7 (M=Y, In, Fe, Ga) was performed, which indicated that all of these complex oxides are composed of corner-sharing octahedral layers of TaO 6 units within a three-, two- or one-dimensional array. In addition, for comparison, the crystal structure, space group and lattice parameters of approximately 100 previously synthesized oxides in the A 2 3+ B 3+ B 5+ O 7 family were collected and analyzed, and a structural map based on the radius ratio rA/rB is reported. According to the photocatalytic results, all oxides in the Sm 2 MTaO 7 (M=Y, In, Fe, Ga) family showed hydrogen production from pure water without any cocatalyst. The highest (62 μmol/g h) and lowest (24 μmol/g h) hydrogen production rates were observed for Sm 2 YTaO 7 and Sm 2 FeTaO 7 , respectively, which reveals that the photoactivity is strongly dependent on the negative potential of the conduction band. 1. Introduction The development of new renewable energy sources is currently one of the biggest challenges in science. The decomposition of water into H 2 and O 2 using solar light and an appropriate photocatalyst is one of the most promising strategies for sustaining the world’s energy supply in the future. Photocatalysts derived from several families of compounds, such as tantalates, niobates and titanium dioxide, have shown inter- esting activities for this reaction. Complex compounds with the general formula A 2 B 2 O 7 are superstructures that are closely related to fluorite (space group Fm-3 m) and can be considered to be ordered defect- fluorite phases with systematic oxygen vacancies [1–10]. It should be mentioned that oxides in the pyrochlore, weberite and layered per- ovskite families are compounds that can possess the same stoichio- metry as A 2 B 2 O 7 [11–16]. The pyrochlore structure is cubic with the space group Fd-3 m, and there are eight molecules per unit cell (Z=8). The A cation (~1 Å) and B cation (~0.6 Å) are 8-fold and 6-fold coordinated to oxygen, respec- tively. These oxides present a wide range of compositions with over 500 different combinations, the A 2 3+ B 2 4+ O 7 -type being the most reported [17–20]. Due to these crystalline features, the chemical stability and the wide variety of cation substitution, A 2 B 2 O 7 pyrochlore oxides have attracted much attention for their interesting potential applications in many technological fields related to solid oxide fuel cells [21], nuclear waste host materials [22], photocatalysis [23–25], geometrically fru- strated magnetism [18,26], luminescence [27], ionic conductivity [28,29], light emitters [8], catalysis [30–32], pigments [33,34], semi- conductors [35,36], superconductors [37,38], ferroelectrics [39], and transmutation targets [2]. In addition, the attractive crystal chemistry has also generated interest in extensive crystallographic studies of pyrochlore-type materials [8]. To maintain charge neutrality, the pyrochlore family can be extended by replacing the two B 4+ cations by a pair of B 3+ and B 5+ cations to give A 2 3+ B 3+ B 5+ O 7 phases [17,40,41]. These types of phases have received little attention, with the magnetic [40–47] and photo- catalytic [48–76] properties having been investigated the most. In particular, A 2 3+ B 3+ B 5+ O 7 -type photocatalysts have been evaluated for the water splitting reaction [48,50,51,53,55–57,59,61– http://dx.doi.org/10.1016/j.ceramint.2016.11.098 Received 9 August 2016; Received in revised form 5 November 2016; Accepted 14 November 2016 ⁎ Corresponding author. E-mail address: leticia.torresgr@uanl.edu.mx (L.M. Torres-Martínez). Ceramics International 43 (2017) 3981–3992 Available online 15 November 2016 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved. MARK