IJSRST162626 | Received: 16 Nov 2016 | Accepted: 29 Nov 2016 | November-December-2016 [(2)6: 283-295] © 2016 IJSRST | Volume 2 | Issue 6 | Print ISSN: 2395-6011 | Online ISSN: 2395-602X Themed Section: Science and Technology 283 On the Formation of Solid Solutions with Blödite and Kröhnkite- Type Structures. II. Structural and Thermal Investigations of Solid Solutions Na 2 Zn 1-x Cu x (SO 4 ) 2 ⋅4H 2 O(0 <x< 0.14) D. Marinova 1 , M. Georgiev 2 , Tsv. Bancheva 2 , D. Stoilova 1* * 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria 2 Department of Inorganic Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria ABSTRACT Structural and thermal investigations of solid solutions Na 2 Zn 1-x Cu x (SO 4 ) 2 ⋅4H 2 O (0 <x< 0.14) of a blödite-type structure are reported in the present paper. The replacement of the zinc ions by copper ones in the crystals of the blödite-type structure do not change considerably the lattice parameters and the volumes of the unit cells of the solid solutions due probably to the buffer effect of the sodium cations. However, the degree of distortion of the [M(H 2 O) 4 (SO 4 ) 2 ] clusters in the solid solutions is larger than that in Na 2 Zn(SO 4 ) 2 ⋅4H 2 O as deduced from the single crystal X-ray diffraction due to the Jahn-Teller effect (DI has value of 0.011 and 0.018 for the neat zinc blödite and solid solutions, respectively). On the basis of TG-DTA-DSC methods combined with X-ray powder diffraction at elevated temperature, it has been found that the dehydration of blödite phases occurs stepwise, thus forming intermediate dehydrate (at about 130 ºC) and two polymorphic modifications of anhydrous compounds. The form (I) of Na 2 Zn(SO 4 ) 2 and Na 2 Zn 0.86 Cu 0.14 (SO 4 ) 2 is stable in the temperature interval of about 200−280 ºC and the from (II) − in the temperature interval of about 330−380 ºC. The incorporation of copper ions in the zinc blödite results in increasing in the values of ∆H deh and decreasing in the values of ∆H f with increasing in the copper content. The water molecule in the kröhnkite compound Na 2 Cu(SO 4 ) 2 ⋅2H 2 O are separated in one step (the dihydrate is stable up to 250 ºC) and after heating at higher temperature it transforms into an anhydrous compound (300−380 ºC). Additionally, the anhydrous Na 2 Zn(SO 4 ) 2 , Na 2 Cu(SO 4 ) 2 and Na 2 Zn 0.86 Cu 0.14 (SO 4 ) 2 were characterized structurally using both X-ray diffraction and spectroscopic techniques. The infrared spectroscopic experiments add an additional information about the structural properties of the anhydrous compounds. For example, the sulfate tetrahedra in the anhydrous compounds are distorted in a higher degree in comparison to those in the hydrated ones. Keywords: Na 2 Zn 1-x Cu x (SO 4 ) 2 ∙4H 2 O solid solutions; X-ray powder diffraction; Enthalpy of dehydration; Enthalpy of formation; Infrared spectra. I. INTRODUCTION In a previous paper we provided for the first time results on the synthesis and spectroscopic investigations (infrared, Raman and EPR) of solid solutions Na 2 Zn 1-x Cu x (SO 4 ) 2 ⋅4H 2 O (0 <x< 0.14) [1]. The preparation procedure – crystallization from mixed sodium copper zinc solutions using the method of isothermal decrease of supersaturation guarantees the homogeneity of the solid phases obtained. The other advantage of the method used is the possibility to prepare solid solutions with a desired amount of copper ions incorporated in the zinc blödite structure. The main conclusion of these studies is the finding that the copper ions are included isodirmorphously in the crystals of the zinc blödite up to 14 mol%, thus forming solid solutions of a blödite-type structure. Contrary, as the solubility diagram of the