The Local Ordering of Molten LiNbOa by X-ray Diffraction

LiNbO a crystal has excellent electro-optical and non-linear optical properties [1, 2]. However local fluctuations of the refractive index and light scattering centers prevent a wider application of this material in optical devices. Since sub-grain boundaries seem to cause such inconvenience, the production of sub-grain free LiNbO a crystals is strongly required. Various properties of molten L i N b 0 3 , such as viscosity, density, and surface tension have recently been measured in order to understand the growth of LiNbOa single crystals in the Czochralski method [3,4]. The viscosity of molten L i N b 0 3 is relatively high just above the melting point (1526 K) and then decreases strongly in the range up to 1553 K [4]. A similar anomalous behaviour has been detected for other physical properties of molten L iNb0 3 and may be related to the appearance of subgrain boundaries during the growth of L i N b 0 3 crystals from the melt [4]. The structure of molten L i N b 0 3 seems not to have been studied as yet. This prompted us to present our results obtained with a high temperature X-ray diffraction method. A miniature furnace composed of four small silicon carbide ceramic heaters in a refractory brick (Fig. 1) used for our measurements. This furnace, set in a high temperature chamber as described in detail in [5], made it possible to maintain the sample temperature