Temperature-induced phase transformations of the ‘‘glaserite’’ type zirconosilicate Na 3 HZrSi 2 O 8 0.4H 2 O Vladislav Kostov-Kytin a, *, Rosica Nikolova a , Thomas Kerestedjian b , Petr Bezdicka c a Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Science, Acad. G. Bonchev Str., Bl. 107 Sofia, Bulgaria b Geological Institute, Bulgarian Academy of Science, 24 Acad. G. Bonchev Str., 1113 Sofia, Bulgaria c Institute of Inorganic chemistry, Academy of Science of the Czech Republic, 250 68 Rez, Czech Republic 1. Introduction Zirconosilicates with mixed octahedral–tetrahedral frame- works have attracted attention for more than two decades, because of their structural and functional similarities to other zeolite and zeolite-like materials. Some of these compounds exhibit technologically important alkali-ion mobility and ion- exchange properties. Advances considering the synthesis, crystal chemistry, classification, properties, and applications of these materials have been reviewed by Rocha and Anderson (2000), Byrappa and Masahiro (2001), Ilyushin and Blatov (2002), Anderson and Rocha (2002), Chukanov et al. (2004), Roch and Lin (2005), Zubkova and Pushcharovsky (2008) [1–7]. Recently we described synthesis and crystal structure determi- nation of a hydrous sodium zirconosilicate material with ‘‘glaserite’’ type structure and generalized formula Na 3x H 1+x ZrSi 2 O 8 yH 2 O, 0 < x < 3, 0 < y < 1 [8]. It is found that with synthesis duration the crystal structure gradually transforms from higher symmetrical into triclinic one, however preserving its topology. The structure of the triclinic form – Na 3 HZrSi 2 O 8 0.4H 2 O was refined from powder diffraction data. It crystallizes in the space group P-1 with lattice parameters a = 9.0523(4), b = 5.5612(2), c = 6.9622(3) A ˚ , a = 92.178(3), b = 90.839(3), g = 90.288(2)8. To the best of our knowledge the studied compound is the only water-containing material with ‘‘glaserite’’ type structure. In addition, upon treatment with hydrochloric acid the entire amount of extra-framework Na atoms could be leached out. These facts are indicative that the structure of this material is rather an open-framework than a dense- packed one. Furthermore, the TG-DTA measurements registered that the weight loss of 1.77% occurring between 215 and 400 8C was accompanied by an endo-effect immediately followed by a markedly expressed exo-effect. The latter has been assigned to the release of water positioned in voids within the structure which possibly causes certain phase transition. One of the end products from the thermal treatment of the title compound has been recognized as a NASICON type sodium zirconosilicate Na 4 Zr 2 Si 3 O 12 . In their survey of the literature of the solids with high ionic conductivity Kumar and Yashonath [9] pointed out the pioneering work of Hong [10] demonstrating that it is possible to synthesize a series of materials of the general formula Na 1+x Zr 2 Si x P 3x O 12 with 0  x  3 some of which are excellent ionic conductors. Suchlike materials obtained a general name – NASICON (for Na SuperIonic CONductor). The same authors encountered also various techniques for synthesis of NASICONs e.g. conventional ceramic methods including solid state reaction method (or powder mixing), solution-sol-gel method, hydrother- mal method, and ion exchange. Here, we present the results of a study aiming at elucidation of the structural changes that take place with the title compound Materials Research Bulletin 48 (2013) 2029–2033 A R T I C L E I N F O Article history: Received 15 December 2012 Accepted 11 February 2013 Available online 27 February 2013 Keywords: A. Inorganic compounds C. Thermogravimetric analysis (TGA) C. X-ray diffraction D. Crystal structure A B S T R A C T DTA-TG and powder XRD analyses are used to study the occurring upon heating structural transformations of the ‘‘glaserite’’ type zirconosilicate Na 3 HZrSi 2 O 8 0.4H 2 O. The title compound undergoes energetically favored irreversible topotactic dehydration within the temperature range 200 and 400 8C. Essential structural transformations occur only after 700 8C, however, the phase composition of the run-product strongly depends on the conditions of thermal treatment. In all cases the predominant phase at 1000 8C is the NASICON type zirconosilicate – Na 4 Zr 2 Si 3 O 12 . The concomitant compound is either Na 2 SiO 3 or the parakeldyshite synthetic analog – Na 2 ZrSi 2 O 7 . The mechanism of structural transformation and certain structural relationships are discussed in terms of the crystal chemical peculiarities of the initial compound and the run-products. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +359 2 9797055; fax: +359 2 9797056. E-mail address: vkytin@abv.bg (V. Kostov-Kytin). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.02.039