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MATERIALS RESEARCH

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Hydrothermal growth and characterization of NaLa(WO4 )2 crystals K. Byrappaa) and Amita Jain Department of Geology, Mysore University, Manasagangotri, Mysore-570 006, India (Received 20 July 1994; accepted 25 April 1996)

The growth of NaLa(WO4 )2 crystals has been carried out by the hydrothermal method at fairly low P-T conditions. The crystal morphology has been studied with respect to the growth parameters. The crystals obtained were characterized by various techniques such as x-ray diffraction (XRD), differential thermal analysis (DTA), and infrared (IR) spectroscopy.

I. INTRODUCTION

Alkali rare earth tungstates form an important group of technological materials due to their excellent luminescence properties and good electrical conductivity. Although these tungstates were reported during the late 1950s and 1960s, not much has been studied with reference to their crystal growth and phase relations in the respective systems, and there exists some lacunae in the characterization of alkali rare earth tungstates. The probable reason for this may be the nonavailability of good quality single crystals due to the high temperature of synthesis involved. There are several reports on the synthesis and characterization of alkali rare earth tungstates,1–4 but most of them deal with the flux growth and solid state reactions. The crystals are fine grained and poor in quality without a proper morphological development. Similarly there are quite a few reports5,6 on the hydrothermal growth of alkali rare earth tungstates, but the P-T conditions of synthesis were quite high (T ­ 700–900 ±C; P ­ 2–3 kb). In the present work, the authors report the hydrothermal growth of NaLa(WO4 )2 crystals under fairly lower P-T conditions (T ­ 240 ±C; P ­ 80 –100 bar), using a suitable mineralizer solution. Carobbi and Tancredi7 have studied the system Ce2 (WO4 )3 –Na2 WO4 –H2 O at 25 ±C, and this work has formed some basis for the present work to grow NaLn(WO4 )2 (where Ln ­ La, Ca, Nd) at moderately lower P-T conditions. A systematic characterization of these alkali rare earth tungstates has been carried out by various techniques like x-ray diffraction (XRD), differential thermal analysis (DTA), and infrared (IR) spectroscopy. Also, the morphology of the crystals obtained has been studied with respect to the growth conditions. II. CRYSTAL GROWTH

The growth of alkali rare earth tungstates by hydrothermal method is quite complicated due to the high temperature of synthesis involved and the lack of a)

Email: [email protected] J. Mater. Res., Vol. 11, No. 11, Nov 1996

solubility data. Moreover, the crystals obtained by earlier workers were fine grained or small rounded/subrounded. Therefore, the morphological studies have been carried out on these alkali rare earth tungstates. The present authors have made an attempt to synthesize NaLa(WO4 )2 crystals by the hydrothermal method within a wide range of P-T conditions. A systematic study on the search of a suitable mineralizer has been carried