New heavy metal oxide glasses: investigations within the TeO2–Nb2O5–Bi2O3 system

Journal of Alloys and Compounds 347 (2002) 206–212

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New heavy metal oxide glasses: investigations within the TeO 2 –Nb 2 O 5 –Bi 2 O 3 system S. Blanchandin, P. Thomas*, P. Marchet, J.C. Champarnaud-Mesjard, B. Frit ´ ´ Ceramiques ´ Laboratoire Science des Procedes et Traitements de Surface, U.M.R. 6638 CNRS, Faculte´ des Sciences, 123 avenue Albert Thomas, 87060 Limoges, Cedex, France Received 16 March 2001; received in revised form 1 June 2001; accepted 4 March 2002

Abstract The TeO 2 –Nb 2 O 5 –Bi 2 O 3 system was investigated by differential scanning calorimetry and temperature programmed X-ray diffraction. A large glass-forming domain was evidenced and the complex structural evolution with temperature of these glasses was followed. The glass transition and crystallisation temperatures and the nature of crystalline phases formed were determined. Moreover, this study confirmed the existence of the metastable g-TeO 2 polymorph and of a new crystalline compound, BiNbTe 2 O 8 .  2002 Elsevier Science B.V. All rights reserved. Keywords: Amorphous material; Liquid quenching; X-ray diffraction; Thermal analysis

1. Introduction An increasing interest in materials presenting nonlinear optical properties has been induced by the industrial requirement for all optical devices. Tellurium dioxidebased glasses are potential candidates for such applications due to their malleability, their homogeneity over a wide range of compositions, their chemical stability, their low melting temperatures, their high linear and nonlinear refractive indices and their good visible and infrared light transmittance [1–3]. Recent studies concerning these glasses have shown that their nonlinear index n 2 was the highest for oxide glasses and could be from 50 up to 100 times as large as that of SiO 2 [4–7]. The origin of this nonlinearity was attributed to the hyperpolarizability of the Te IV atoms lone pairs which is very often reinforced by addition of either a second lone pair holder (such as Tl 1 , Bi 31 , Pb 21 ) or of cations with empty d-orbitals, such as Ti 41 or Nb 51 . Actually, numerous studies are devoted to the knowledge of the relationships between structure and nonlinear optical response of tellurite glasses [7–15]. However, little reported literature is available on investigations about phase equilibrium and nonequilibrium diagrams of such tellurite systems and especially the *Corresponding author. Tel.: 133-5-5545-7496; fax: 133-5-55457270. E-mail address: [email protected] (P. Thomas).

crystallisation process of tellurite glasses. The knowledge of the first compound to crystallise from glasses is of prime importance if one wants to understand both the glass structure and the glass–crystal transition. Therefore we have recently followed the crystallisation of some heavy metal (thallium [16], tungsten [17], niobium [18]) tellurite glasses. In particular, our investigations within the TeO 2 rich part of the TeO 2 –Nb 2 O 5 system have provided evidence of two new metastable polymorphs (d and g) of TeO 2 [19,20] and confirmed the existence of: (i) three crystalline phases, Nb 2 Te 4 O 13 , Nb 2 Te 3 O 11 and Nb 6 TeO 17 [18,21–24], (ii) a large glassy domain (0–25 mol% NbO 2.5 , under icy-water-quenching conditions [18]) whose composition ranges greatly differed from one author to the other, probably because of different experimental conditions (from 7 to 51 mol% NbO 2.5 [25] and from 5 to 25 mol% NbO 2.5 [26]). The aim of this study was both to investigate the possible extension of the TeO 2 –Nb 2 O 5 glass formation domain within the TeO 2 –Nb 2 O 5 –Bi 2 O 3 system, and to follow the thermal behaviour of the glasses, i.e. to determine their glass transition (T g ), crystallization (T c ) and melting (T l ) temperatures, and the nature of the various crystalline phases formed.

2. Experimental Glassy samples were prepared by first melting at 800 8C

0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00766-1

S. Blanchandin et al. / Journal of Alloys and Compounds 347 (2002) 206–212

in sealed gold tubes, and then air-quenching intimate mixtures of TeO 2 (prepared in the laboratory by decomposition at 550 8C under flowing oxygen of commercial H 6 TeO 6 (Aldrich, 99.9%)), Nb 2 O 5 (Aldrich, 99.5%) and Bi 2 O 3 (Aldrich, 99.5%). Glass formation domain was determined by using X-ray diffraction (Guinier-De Wolff camera, CuKa radiation) for the analysis of many samples with neighbouring compositions close to the glass–crystal separating line. The structural evolution with temperature of glasses was followed by in situ X-ray powder diffraction (XRPD) with a Siemens D5000 diffractometer (u /u mode, CuKa radiation) fitted with a high temperature furnace (Anton-Parr HTK10), a platinum heating sample holder and an Elphyse sensitive position detector (148 aperture). The heating rate was 5 8C / min and each XRPD pattern was recorded after an annealing time of 10 min at the chosen temperature, in the 2u range 10–908 (step size: 0.029, time range: 18 min). Glass transition (T g ), crystallisation (T c ) and melting (T l ) temperatures were measured by heat flux differential scanning calorimetry (Netzsch STA 409 apparatus). The powdered samples (|30 mg) were introduced into covered gold crucibles and the DSC curves were recorded between 20 and 800 8C using a heating rate of 10 8C / min. The glass transition temperature was taken as the inflection point of the steep change of the calorimetric signal associated with this transition. The crystallisation temperature was taken as the intersection of the slope of the exothermic peak with the baseline. Glasses densities were measured on finely ground powders by helium pycnometry (Accupyc 1330 pycnometer).

207

Fig. 1. Crystalline phases and glass formation domain (at 800 8C, air quenching) in the TeO 2 –Nb 2 O 5 –Bi 2 O 3 system.

3. Results and discussion

3.1. Glass formation Under our experimental conditions, a large glass forming domain is observed within the TeO 2 –Nb 2 O 5 –Bi 2 O 3 system (Fig. 1). The characteristics of some glasses are shown in Table 1. These glasses are all yellow, the intensity of the colour increasing with the Bi 2 O 3 content. The density slightly increases from about 5.2 to 5.8 with

Table 1 Glass-transition temperature T g , crystallisation temperature T c , melting temperature T l and density of some glasses in the TeO 2 –NbO 2.5 –BiO 1.5 system No.

Composition (mol%) TeO 2 –NbO 2.5 –BiO 1.5

Tg (65 8C)

Tc (65 8C)

Tl (65 8C)

Density (g cm 23 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

95–5–0 90–10–0 85–15–0 80–20–0 75–25–0 85–10–5 80–15–5 75–20–5 70–25–5 75–15–10 70–20–10 65–25–10 74–13–13 65–20–15 60–25–15 60–20–20

327 332 347 366 381 337 358 373 394 360 376 395 358 385 404 391

407 400 430 461 480 377 464 479 486 446 465 515 394 446 504 446

708 697 725 751 761 676 720 745 777 695 718 768 667 698 714 736

5.45 5.42 5.36 5.22 5.20 5.47 5.48 5.33 5.13 5.50 5.53 5.37 5.61 5.72 5.60 5.79

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increasing Bi 2 O 3 content (Fig. 2). Refractive indices of 2.2–2.3 have been measured in these glasses. They are comparable to those observed with niobium tellurite glasses [3,8,25,26]. The evolutions with composition of the glass transition (T g ) and crystallisation (T c ) temperatures are shown in Fig. 2. One observes for all the samples a slightly and continuous increase of T g with increasing Bi 2 O 3 content. The evolution of T c is more complex: it weakly decreases for the 20 mol% NbO 2.5 glasses, slightly increases for higher NbO 2.5 content (25 mol% NbO 2.5 ) and decreases for lower NbO 2.5 content (15 mol% NbO 2.5 ). So, addition of Bi 2 O 3 to TeO 2 –Nb 2 O 5 glasses, seems to decrease their thermal stability, and that all the more since they are less rich in Nb 2 O 5 .

3.2. Glass crystallisation The crystallisation of glasses was followed by variable temperature XRD studies. Three distinct crystallisation domains (called respectively I, II and III as shown in Fig. 3) were found.

3.2.1. Domain I: TeO2 -rich part In this domain of composition, a particularly complex crystallisation process is observed (see for example the XRD patterns at various temperatures of the 80 mol% TeO 2 –15 mol% NbO 2.5 –5 mol% BiO 1.5 glassy sample (no. 7) shown in Fig. 4). The crystallisation occurs in different steps which are clearly identified and reported in Table 2: • first the crystallisation of a cubic d phase at temperatures ranging from 350 to 400 8C. Such a cubic phase has previously been evidenced during the crystallisation of TeO 2 -rich glasses of the TeO 2 –WO 3 [17] and TeO 2 –Nb 2 O 5 [18] systems. This cubic phase disappears above 420 8C. • according to the composition (see Table 2), simultaneous crystallisation of the g-TeO 2 metastable polymorph, a-TeO 2 and Bi 1 / 2 Nb 1 / 2 Te 3 O 8 compounds at temperature ranging from 440 to 460 8C. The Bi 1 / 2 Nb 1 / 2 Te 3 O 8 compound was previously evidenced ˚ by Meunier et al. [27] (cubic symmetry, a511.275 A; TiTe 3 O 8 -type structure). These two compounds are observed up to about 500–520 8C. At temperatures ranging from 520–540 to 600 8C, a-TeO 2 , g-TeO 2 and Nb 2 Te 4 O 13 are observed with the progressive transformation of g-TeO 2 into a-TeO 2 . • existence of a-TeO 2 and Nb 2 Te 4 O 13 from 600 8C up to the melting temperature of the mixture.

Fig. 2. Evolution of the density with the Bi 2 O 3 content, and of the glass transition (T g ) and crystallisation (T c ) temperatures for some TeO 2 – Nb 2 O 5 –Bi 2 O 3 glasses.

Fig. 3. Three different glass crystallisation domains in the TeO 2 –Nb 2 O 5 – Bi 2 O 3 system.

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˚ which we have previously d-TeO 2 polymorph (5.69 A) evidenced within the TeO 2 -rich part of the TeO 2 –WO 3 and TeO 2 –Nb 2 O 5 systems. This phase could rather correspond to one composition of the Bi 12x Te x O (31x) / 2 (0.5, x,0.85) nonstoichiometric fluorite-type phase previously isolated by El Farissi [28] within the TeO 2 -rich part of the TeO 2 –Bi 2 O 3 system (Fig. 1). Further experiments are in progress in order to confirm such an hypothesis.

3.2.2. Domain II: central part of the glass formation domain Different crystallisation processes are observed according to the glassy samples compositions. As example, the XRPD patterns at various temperatures of the 75 mol% TeO 2 –20 mol% NbO 2.5 –5 mol% BiO 1.5 glassy sample (no. 8) are shown in Fig. 5. The different crystallisation steps are reported Table 2:

Fig. 4. X-ray diffraction patterns at various temperatures of the 80 mol% TeO 2 –15 mol% NbO 2.5 –5 mol% BiO 1.5 glassy sample (no. 7; glass forming domain I; see Fig. 3) (Pt diffraction peaks are those of the sample holder).

˚ Remark: It is worth noting that the unit-cell (a55.67 A) measured for the cubic d phase (accurate measurement of the XRPD pattern of a glassy sample annealed for 12 h at 400 8C) is relatively different from that of the metastable

• at temperatures ranging from 390 to 450 8C and according to the Nb 2 O 5 content, we observe either the crystallisation of the Bi 1 / 2 Nb 1 / 2 Te 3 O 8 compound (sample no. 9), or the simultaneous crystallisation of Bi 1 / 2 Nb 1 / 2 Te 3 O 8 and a-TeO 2 compounds (sample no. 8), or the simultaneous crystallisation of Bi 1 / 2 Nb 1 / 2 Te 3 O 8 and Nb 2 Te 4 O 13 compounds (sample no. 10). • at temperatures ranging from 450–480 8C to 600 8C, either Bi 1 / 2 Nb 1 / 2 Te 3 O 8 and a-TeO 2 or Bi 1 / 2 Nb 1 / 2 Te 3 O 8 , a-TeO 2 and Nb 2 Te 4 O 13 are observed according to the sample composition; • above 600 8C up to the melting temperature of the mixture, a-TeO 2 and Nb 2 Te 4 O 13 are observed.

Table 2 Different steps of crystallisation of some glasses within the glass forming domains I and II (see Fig. 3) (followed by in-situ X-ray powder diffraction) No.

Composition (mol%)

T (8C)

Observed crystalline phases

5

350 460 520 600

cubic d phase Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1g-TeO 2 1a-TeO 2 g-TeO 2 1a-TeO 2 1Nb 2 Te 4 O 13 a-TeO 2 1Nb 2 Te 4 O 13

15

5

400 440 500 540 600

cubic d phase Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 g-TeO 2 1a-TeO 2 1Bi 1 / 2 Nb 1 / 2 Te 3 O 8 g-TeO 2 1a-TeO 2 1Nb 2 Te 4 O 13 a-TeO 2 1Nb 2 Te 4 O 13

75

20

5

420 450 600

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 1Nb 2 Te 4 O 13 a-TeO 2 1Nb 2 Te 4 O 13

9 (II)

75

15

10

390 480 600

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 a-TeO 2 1Nb 2 Te 4 O 13

10 (II)

70

25

5

450 480 600

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1Nb 2 Te 4 O 13 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 1Nb 2 Te 4 O 13 a-TeO 2 1Nb 2 Te 4 O 13

TeO 2

NbO 2.5

6 (I)

85

10

7 (I)

80

8 (II)

BiO 1.5

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3.2.3. Domain III The different steps of crystallisation are reported Table 3. As examples, XRPD patterns at various temperatures of the 70 mol% TeO 2 –20 mol% NbO 2.5 –10 mol% BiO 1.5 (no. 11) and of the 60 mol% TeO 2 –20 mol% NbO 2.5 –20 mol% BiO 1.5 (no. 16) glassy samples are shown respectively in Figs. 6 and 7. For all the samples, we observe first the crystallisation of the cubic Bi 1 / 2 Nb 1 / 2 Te 3 O 8 compound at temperatures ranging from 370 to 450 8C. Then, two different crystallisation processes can be observed according to the TeO 2 content: • TeO 2 -rich samples (nos. 11, 13 and 14): • Bi 1 / 2 Nb 1 / 2 Te 3 O 8 and a-TeO 2 are observed from 480–580 8C (according to the composition) to 600 8C. For the sample no. 11 the Nb 2 Te 4 O 13 compound is also detected. • at temperatures ranging from 600 8C to 640– 660 8C, a new crystalline compound BiNbTe 2 O 8 , Nb 2 Te 4 O 13 and a-TeO 2 compounds are observed. The crystal structure of this BiNbTe 2 O 8 compound has been solved recently from single crystal XRD data [29]. It crystallises with orthorhombic symmetry (space group Pbca) and unit cell parameters: ˚ b58.1277(8) A ˚ and c531.205(3) a55.6109(7) A, ˚A.

Fig. 5. X-ray diffraction patterns at various temperatures of the 75 mol% TeO 2 –20 mol% NbO 2.5 –5 mol% BiO 1.5 glassy sample (no. 8; glass forming domain II; see Fig. 3) (Pt diffraction peaks are those of the sample holder).

Table 3 Different steps of crystallisation of some glasses within the glass forming domain III (see Fig. 3) (followed by in situ X-ray powder diffraction) No.

13

Compositions (mol%) TeO 2

NbO 2.5

BiO 1.5

74

13

13

T (8C)

Observed crystalline phases

370 480 600 640

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 a-TeO 2 1Nb 2 Te 4 O 13 1Bi 1 / 2 Nb 1 / 2 Te 3 O 8 (very small amount) Nb 2 Te 4 O 13 1BiNbTe 2 O 8

11

70

20

10

410 520 600 660

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 1Nb 2 Te 4 O 13 Nb 2 Te 4 O 13 1BiNbTe 2 O 8 1 a-TeO 2 Nb 2 Te 4 O 13 1BiNbTe 2 O 8

14

65

20

15

430 580

640

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1a-TeO 2 (very small amount) Nb 2 Te 4 O 13 1BiNbTe 2 O 8 1 a-TeO 2 (very small amount) Nb 2 Te 4 O 13 1BiNbTe 2 O 8

600

12

65

25

10

450 540 600

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1Nb 2 Te 4 O 13 Nb 2 Te 4 O 13 1BiNbTe 2 O 8

15

60

25

15

430 580

600

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 (very small amount)1 Nb 2 Te 4 O 13 1BiNbTe 2 O 8 Nb 2 Te 4 O 13 1BiNbTe 2 O 8

420 540 580

Bi 1 / 2 Nb 1 / 2 Te 3 O 8 Bi 1 / 2 Nb 1 / 2 Te 3 O 8 1BiNbTe 2 O 8 BiNbTe 2 O 8

16

60

20

20

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• above 640–660 8C, BiNbTe 2 O 8 and Nb 2 Te 4 O 13 are still observed. • samples with composition close to BiNbTe 2 O 8 (nos. 12, 15 and 16): • the crystallisation of a-TeO 2 is no more observed; • at temperatures ranging from 540–580 to 600 8C, we observed Bi 1 / 2 Nb 1 / 2 Te 3 O 8 with either Nb 2 Te 4 O 13 (no. 12), or BiNbTe 2 O 8 (no. 16), or these two latter compounds (no. 15). • above 600 8C up to the melting temperature of the mixtures, BiNbTe 2 O 8 is observed. Nb 2 Te 4 O 13 can also be detected when the content of Nb 2 O 5 increases.

4. Conclusion

Fig. 6. X-ray diffraction patterns at various temperatures of the 70 mol% TeO 2 –20 mol% NbO 2.5 –10 mol% BiO 1.5 glassy sample (no. 11; glass forming domain III; see Fig. 3) (Pt diffraction peaks are those of the sample holder).

The existence of a large glass-forming domain has been observed within the TeO 2 –Nb 2 O 5 –Bi 2 O 3 system. Addition of Bi 2 O 3 seems to decrease the thermal stability of TeO 2 –Nb 2 O 5 glasses. The complex thermal behaviour of those glasses has been followed using both differential scanning calorimetry and temperature-programmed XRD. Three distinct domains of crystallisation have been detected according to the composition. These investigations have confirmed the existence of the metastable form of TeO 2 , g-TeO 2 , which has been observed during the crystallisation of TeO 2 -rich glasses and have provided evidence in richer Bi 2 O 3 contents for a previously unknown compound, BiNbTe 2 O 8 . Further experiments are in progress in order to master the precipitation of a crystalline phase within the glassy matrix and to obtain transparent optical samples which could be efficient nonlinear materials.

References

Fig. 7. X-ray diffraction patterns at various temperatures of the 60 mol% TeO 2 –20 mol% NbO 2.5 –20 mol% BiO 1.5 glassy sample (no. 16; glass forming domain III; see Fig. 3) (Pt diffraction peaks are those of the sample holder).

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