A local order study of molten LiNbO₃by neutron diffraction

Abstract

The total structure factor S(Q) and the total pair correlation function G(r) of liquid LiNbO3 have been investigated by neutron diffraction. The temperature dependence of the scattered intensity has been measured in the range of momentum transfer 0.3 AA-1 < Q < 16.2 AA-1 and the range of temperature 1623 K > T > 1490 K including an undercooling domain. A large variation of S(Q) with temperature was never observed. From an analysis of G(r) made with reference to the crystalline LiNbO3 structure, it was possible to confirm a local ordering similar to the crystalline one. The assumption of a change in the Nb coordination in the liquid near solidification is unfounded. The Nb atom remains octahedrally coordinated up to one hundred degrees above the melting point. The values of the first-neighbour distances r1Nb-O, r1O-O and r1Li-O and the coordination numbers nLi-O and nO-O can be explained by the presence of NbO6 octahedra bonded together by Li atoms in the melt.

Local Order Refinement in Liquid Lithium Niobate Using a Two Radiation Method (X-Rays and Neutrons)

“…The presence of clusters in the melt has been recently confirmed by small angle X-ray scattering [16] and the local order evolution with temperature obtained from neutron scattering [18] using samples of natural isotopic abundance. Evolution of clusters and local order have to be described more precisely to explain the high thermal expansion in a melt which contains clusters having a mean density identical to the solid one.…”

“…The experiments are described in [17,18]; here we only recall the ranges of the temperature (T) and wave vector (Q) explored with both radiations.…”

“…Following the evolution of the first Bragg peak of LiNb0 3 crystal in situ during the initial temperature rise in the neutron diffraction study [18], and assuming an isotropic continuous expansion in the rhombohedral phase, it was possible to determine an approximate mean expansion coefficient a m % 3.3 x 10" 5 per degree. The X-ray experiments were carried out on the diffractometer "theta-theta" at the University of Senda'i (SENKEN).…”

“…With this in mind, recent structural studies of the LiNb0 3 melt have been carried out by means of small angle scatter- ing using synchrotron radiation [16] and by means of the high temperature diffraction method with X-rays [17] and neutrons [18]. Six experiments with different contributions W^ of the i-j pairs are necessary to extract the S {j (Q) functions which determine the local order.…”

Cluster Formation in Liquid Lithium Niobate A Small Angle X-ray Scattering Investigation

“…This behaviour, near the melting point Tm, might be correlated with the appearance of subgrain boundaries [4,5] affecting the crystal growth since the single crystals are synthesized from the melt by the pulling method of Czochralski [6]. The Nb coordination in the melt has recently been determined from structural studies carried out by high temperature X-ray and neutron diffraction [7,8]. These results confirm a local ordering in the melt similar to the crystalline one up to the highest explored temperature with a persistent octahedral Nb coordination as observed in the crystal, which is classified in the family of ilmenite.…”