Local order in the LiNbO3 melt: Comparison with the crystalline phases

From recent structural analysis of the LiNbO3 melt, carried out by means of Small Angle x-ray Scattering [SAXS] and High Temperature Diffraction [HTD], it has been possible to describe a local order reduced to the first neighbours. The first three mean interatomic distances, r1 Nb-O, r1 Li-O and r1 O-O were extracted with the high accuracy Δr/r = 0.5%. From the first three coordination numbers n1 Nb-O, n1 Li-O and n1 O-O, it was verified that octahedrally coordinated niobium atoms exist in the melt and the liquid local order shows only some weak deviations with the solid states. A comparison is made with the crystalline phases and particularly with the recent results obtained around the phase transition at Tc similar 1430K. The first interatomic distances in the paraelectric high temperature phase are higher than the values determined in the melt. This fact is explained by a possible rotation of the Nb-O6 octahedra which are more regular in the liquid. The small changes, observed in the Nb-O, Li-O and O-O bonds, are described in the model of the smallest aggregat constituted by only two Nb-O6 octahedra. Assuming expansion and tilt of the O-triangles and some broken O-O bonds, the decrease of the first three mean interatomic distances rij, the broadening of their i-j distributions Δrij and the coordination numbers nij can be explained.

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