Stéphanie Blanchandin, Philippe Thomas, Pascal Marchet, Jean Claude ... Limoges, Faculté des Sciences, 123 avenue Albert-Thomas, 87060 Limoges Cedex, ...
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JOURNAL OF
Ste´phanie Blanchandin, Philippe Thomas, Pascal Marchet, Jean Claude Champarnaud-Mesjard and Bernard Frit
Materials
Equilibrium and non-equilibrium phase diagram within the TeO -rich 2 part of the TeO –Nb O system 2 2 5
CHEMISTRY
Science des Proce´de´s Ce´ramiques et de Traitements de Surface, UMR 6638 CNRS, Universite´ de Limoges, Faculte´ des Sciences, 123 avenue Albert-Thomas, 87060 Limoges Cedex, France
Downloaded on 21 February 2013 Published on 01 January 1999 on http://pubs.rsc.org | doi:10.1039/A900788A
Received 29th January 1999, Accepted 18th May 1999
The TeO -rich part of the TeO –Nb O system has been investigated by temperature programmed X-ray diffraction 2 2 2 5 and differential scanning calorimetry. Three invariant equilibria have been observed: one eutectic reaction (8 mol% NbO , T =690±5 °C, L = TeO +Nb Te O ), one peritectic reaction (incongruent melting, at 766±5 °C, of 2.5 E E 2 2 4 13 the Nb Te O compound) and one congruent melting reaction (congruent melting, at 810±5 °C, of the Nb Te O 2 4 13 2 3 11 compound ). A large glass-forming domain has been evidenced (0 to 25 mol% NbO ). The thermal behaviour of 2.5 these glasses has been followed. The glass transition, crystallization temperatures and the nature of crystalline phases formed have been determined.
Introduction Tellurium dioxide-based glasses are very interesting materials for non-linear optical applications, especially optoelectronic devices (such as ultra-fast switching devices and optical transistors), because of their high linear and non-linear refractive indices and of their good visible and infrared light transmittance.1–3 Numerous recent studies concerning these glasses have shown that their non-linear index n was the highest found 2 for oxide glasses and could be from 50 up to 100 times as large as that of SiO .4–6 Accurate values of n were measured at 2 2 1.5 mm for various tellurite glasses, using a time-resolved interferometric technique (n (Herasil )=0.12(9)×10−19 m2 W−1; 2 n (90% TeO –10% Nb O )=6.9(7)×10−19 m2 W−1).6 The 2 2 2 5 origin of this non-linearity was attributed to the hyperpolarizability of the TeIV lone pairs which is very often reinforced by addition of either a second lone pair holder (such as Tl+, Bi3+, Pb2+) or of cations with empty d orbitals, such as Ti4+ or Nb5+.6–10 From this point of view, knowledge of the relationships between the structure and non-linear optical response of niobium tellurite glasses was of prime importance. So the TeO –Nb O system was investigated. Previous studies con2 2 5 cerning this system have revealed the existence of: (i) three crystalline phases, Nb Te O , Nb Te O and Nb TeO ,11–14 2 4 13 2 3 11 6 17 (ii) a large glassy domain whose composition range greatly differed from one study to another probably because of different experimental conditions (from 7 to 51 mol% NbO 15 and from 2.5 5 to 25 mol% NbO 16). Investigations on the structure of 2.5 these TeO –Nb O glasses have shown that, with increasing 2 2 5 Nb O content, the TeO structural entities observed in pure 2 5 4 TeO glass were progressively transformed into TeO units 2 3+1 ˚ ) and one medium (three short bonds ( Te–O
