Crystal structure of a new small-pore zirconosilicate Na 2 ZrSi 2 O 7 $H 2 O and its relation to stoichiometrically and topologically similar compounds Rosica Petrova Nikolova a, * , Keiko Fujiwara a , Noriaki Nakayama a , Vladislav Kostov-Kytin b a Department of Advanced Materials Science and Engineering, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan b Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Science, Acad. G. Bonchev Street, Bl. 107 Sofia, Bulgaria article info Article history: Received 22 April 2008 Received in revised form 6 June 2008 Accepted 5 July 2008 Available online xxx Keywords: Sodium zirconosilicate Powder structure refinement Small-pore material Topological similarity abstract The small-pore zirconosilicate Na 2 ZrSi 2 O 7 $H 2 O crystallizes in monoclinic system with space group C2/c, a ¼ 5.4715(4); b ¼ 9.4111(6); c ¼ 13.0969(8) Å, b ¼ 92.851(7)  . Its framework consists of layers built of ZrO 6 octahedra and SiO 4 tetrahedra and forming condensed [Si 2 O 7 ] pyrogroups by connection. The sodium ions and water molecules are placed in channels set up between the layers. The stoichiometric and structural similarities of the studied phase with anhydrous compounds having general chemical formula A 2(3) MT 2 O 7 (A ¼ Na,K; M ¼ Zr,Lu,Sc; T ¼ Si,Ge) are discussed. The topological relationship of their structures is interpreted in the light of spatial combination of silicon and zirconium polyhedra as basic building units into larger composite building units and their three-dimensional arrangement. Ó 2008 Elsevier Masson SAS. All rights reserved. 1. Introduction Structural and functional similarities to the framework alumi- nosilicate-based zeolites have directed the researchers’ efforts to the less studied subclass of compounds with heterogeneous octa- hedral–tetrahedral (MT) frameworks. Much attention has been paid to the investigation of zirconosilicates and their analogs [1–5]. On the other hand the increasing amount of structural information, concerning natural and synthetic zirconosilicates, is a challenge for systematization and classification of the data aimed at design of new materials. In their study from 2002 Ilyushin and Blatov undertook first attempt of systematic consideration of character- istic structural features of zirconosilicates and their analogs intro- ducing appropriate definitions of the basic concepts for the topological classification of MT frameworks they presented [2]. Later on, considering the formation peculiarities of Na–Zr silicates in terms of the matrix assembling of crystal structures from invariant precursor units, Ilyushin defined M 2 T n ,(n ¼ 2, 4, 6) as one of the subpolyhedral structural units. [6,7] The same approach was applied later for describing the Cs and Li zirconosilicates [8]. Synthesis and thermal behavior of three new sodium zircono- silicates adopting layered, small-pore microporous and dense structures were described by Kostov-Kytin and Kalvachev [9]. Recently we reported the initial structural data for the layered compound Na 2 Zr 7 Si 2.5 O 20 $3H 2 O [10]. This contribution presents the synthesis and crystal structure of the small-pore zirconosilicate Na 2 ZrSi 2 O 7 $H 2 O. There are two waterless sodium zirconosilicates with similar chemical composition, initially found as the minerals keldyshite [(Na,H) 2 ZrSi 2 O 7 , CC20186] and parakeldyshite [Na 2 Zr- Si 2 O 7 , CC24866] as well as their synthetically prepared analogs [11– 13]. Both structures are triclinic and their similarity to each other and to that one of the monoclinic mineral khibinskite [K 2 ZrSi 2 O 7 , CC20100] has also been noticed [2]. Ilyushin explained their crystal structures on the basis of formation and condensation of sub- polyhedral structural units of the type [M 2 T 4 ] and defined para- keldyshite as a basic member of a homologous series involving the synthetic analog of this mineral and the synthetic compounds Na 2 ZrSi 4 O 11 and Na 2 ZrSi 6 O 15 $3H 2 O which natural analog is the mineral elpidite [13]. On the other hand, there are synthetic phases, with similar compositional features and higher symmet- rical structures such as K 3 LuSi 2 O 7 (P6 3 /mmc), Na 2 ScSi 2 O 7 (Pbnm) and K 2 ZrGe 2 O 7 (C2/c) [14–16] which have been described as composed of layer units connected through the pyrogroups and recognized as a-ZrHPO 4 type by the last author of Ref. [16]. In this paper we note the stoichiometric and topological simi- larities of compounds with general chemical formula A 2(3) MT 2 O 7 (A ¼ Na,K; M ¼ Zr,Lu,Sc; T ¼ Si,Ge) and discuss their structural aspects in the light of spatial combination of silicon and zirconium polyhedra as basic building units (BBUs) into larger composite building units (CBUs). In turn, the three-dimensional arrangement of the CBUs is presented as a condensation of layers built from * Corresponding author. E-mail addresses: rosica@yamaguchi-u.ac.jp (R.P. Nikolova), keiko@yama- guchi-u.ac.jp (K. Fujiwara), nakayamn@yamaguchi-u.ac.jp (N. Nakayama), vkytin@ clmc.bas.bg (V. Kostov-Kytin). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie ARTICLE IN PRESS 1293-2558/$ – see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.07.003 Solid State Sciences xxx (2008) 1–7 Please cite this article inpress as: Rosica Petrova Nikolova et al., Crystal structure of a new small-pore zirconosilicate Na 2 ZrSi 2 O 7 $H 2 O and its relation to stoichiometrically and topologically similar compounds, Solid State Sci. (2008), doi:10.1016/j.solidstatesciences.2008.07.003