Crystal growth, morphology, structure, and properties of HNaMP 2 O 7 crystals where M = Co and Ni
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A. Clearfield and M. Damodara Poojary Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 (Received 10 October 1993; accepted 15 January 1994)
The HNaMP 2 O 7 (where M = Co and Ni) new Na+ superionic pyrophosphates were obtained by the hydrothermal method at lower PT conditions (T = 250 °C, P = 100 atm). The solubility, complexation with reference to the solvent-solute interaction, and the growth process have been studied in detail. A new investigation on the crystal structure of these compounds showed that the title compounds do not contain Zr as reported previously. The crystal morphology and properties have also been studied.
I. INTRODUCTION Phosphates were studied extensively for the last two decades owing to their widespread applications in modern technology. Phosphates are used, for example, in piezoelectric, luminescent, ceramic, solid state laser, and superionic applications. The synthesis of phosphates began in the previous century.1 Superionic phosphates have been reported for the past 15 years or so. Most of these phosphates are usually ortho- or triorthophosphates and the majority of their structures are directly related to NaZr2(PO4)2.2 Recently, high ionic conductivity in condensed phosphates (pyro- group of phosphates) was reported for the first time by our group.3'* These pyrophosphates were obtained by hydrothermal techniques. In order to grow these crystals using hydrothermal techniques it is very important to know their solubility, internal atomic structure, and crystal morphology. Here, the authors have made an attempt to study the solvent selection, solvent-solute interaction, growth rate, role of volatiles, morphology, internal atomic structure, and physical properties. Such studies are seldom found in the literature for superionics in general. We had earlier refined the crystal structures of these compounds as Na2MZr(P2O7)2 where M = Co and Ni6 in the space group P I . The recent accurate refinement of the structures of the same compounds has posed a question about the presence of Zr in Na 2 MZr (P 2 O 7 ) 2 , where M = Co and Ni, and it has now been generally concluded that there is no Zr in these compounds and the general formula is HNaMP 2 O 7 , where M = Co and Ni, instead of Na 2 MZr(P 2 O 7 ) 2 . II. CRYSTAL GROWTH The crystals of the title compound were obtained by the hydrothermal technique. The growth of condensed phosphates by the hydrothermal method is quite J. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org
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complicated.7'8 The experiments were carried out in Morey autoclaves provided with teflon liners within the temperature range 200-300 °C and in the pressure range 100-200 bars. The starting materials such as oxides/nitrates of cobalt, nickel, zinc, manganese, iron, and zirconium (Analar grade, Reidel de Haen Ag, Germany, BDH, India) were added to the reaction vessel along with the calculated amount of 85% H3PO4 (Analar grade, Ranbaxy, India). The alkaline component (NaOH Analar grade, Wilson Laboratorie
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