LOCAL ORDER IN THE LINBO3 MELT - COMPARISON WITH THE CRYSTALLINE PHASES

From recent structural analysis of the LiNbO3 melt, carried out by mea ns of Small Angle X-ray Scattering [SAXS] and High Temperature Diffrac tion [HTD], it has been possible to describe a local order reduced to the first neighbours. The first three mean interatomic distances, r(1N b-O), r(1Li-O) and r(1O-O) were extracted with the high accuracy Delta r/r = 0.5%. From the first three coordination numbers n(1Nb-O), n(1Li -O) and n(1O-O), it was verified that octahedrally coordinated niobium atoms exist in the melt and the liquid Local order shows only some we ak deviations with the solid states. A comparison is made with the cry stalline phases and particularly with the recent results obtained arou nd the phase transition at T-c similar to 1430 K. The first interatomi c distances in the paraelectric high temperature phase are higher than the values determined in the melt. This fact is explained by a possib le rotation of the Nb-O6 octahedra which are more regular in the liqui d. The small changes, observed in the Nb-O, Li-O and O-O bonds, are de scribed in the model of the smallest aggregat constituted by only two Nb-O-6 octahedra Assuming expansion and tilt of the O-triangles and so me broken O-O bonds, the decrease of the first three mean interatomic distances r(ij), the broadening of their i-j distributions Delta r(ij) and the coordination numbers n(ij) can be explained.