XRD study of ZrW2O8versus temperature and pressure

International Journal of Inorganic Materials 2 (2000) 123–129

XRD study of ZrW2 O 8 versus temperature and pressure ´ a, Jose´ Manuel Gallardo-Amores a , Ulises Amador b , *, Emilio Moran ´ Miguel Angel Alario-Franco a a

´ Lab. de Altas Presiones, Fac. de Ciencias Quımicas , Universidad Complutense, 28040 -Ciudad Universitaria-Madrid, Madrid, Spain b ´ ´ ´ ´ Inorganica y Materiales, F.C.C. Experimentales y Tecnicas , Universidad San Pablo CEU, Urb. Monteprıncipe , Dep. Quımica 28668 -Boadilla del Monte, Madrid, Spain Received 15 December 1999; accepted 27 December 1999

Abstract ZrW2 O 8 samples have been prepared from ZrO 2 and WO 3 precursors by a ceramic method. Different portions of this material were subjected to non cumulative pressure treatments within the 2–65 Kbar range; the metastable high-pressure phase, g-ZrW2 O 8 , has been isolated by quenching from 6 Kbar. Further pressure increases up to 10 Kbar produced the volume reduction of the g-phase; a minimum volume was found even after having released the pressure. Otherwise, heating above 6008C, followed by quenching at 77 K leads to the high-temperature phase, b-ZrW2 O 8 , which is stable at room temperature. Finally, when g-ZrW2 O 8 was subjected to pressure and temperature, g-ZrW2 O 8 could also be quenched with a still larger volume reduction at 6 Kbar and 6008C.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. ceramics; C. high pressure; C. X-ray diffraction; D. thermal expansion

1. Introduction Most solids expand when heated; this is a direct effect of the increased amplitude of thermal vibration at a higher temperature. Nevertheless, there is increasing evidence of thermal contraction in solids as reported by different authors: for example, the ZrV22x Px O 7 solid solution [1–3], compounds with the general formula A 2 M 3 O 12 (where M is Mo or W and A can be Y and Sc) and materials in the Ta 2 O 5 –WO 3 system [4–6]. These systems present an isotropic thermal contraction, that is, all cell parameters decrease under heating. In more recent studies on aluminosilicates, such as cordierite (Mg 2 Al 2 Si 5 O 18 ) and phosphates such as NZP (NaZr 2 (PO 4 ) 3 ), an anisotropic variation of the cell parameters has been reported, giving rise to a light expansion of the cell volume. In these structures, thermal increase leads to the normal expansion in one or two dimensions causing the polyhedra to rotate in such a way that it results in contraction in another direction [7]. In other cases, as for *Corresponding author. Tel.: 134-1352-0144 ext.229; fax: 134-13510475. E-mail addresses: [email protected] (J.M. GallardoAmores), [email protected] (U. Amador)

the zeolite b-eucryptite, the interstitial cations, when heated, may alter their position changing to other available sites, and thus giving rise to the increase of one or two cell parameters [8,9]. Other families of compounds such as MeW2 O 8 (Me5 Hf, Mo, . . . ) and different solid solutions of these show a similar behaviour [10,11] but, in this case the thermal contraction is greater than in those previously mentioned; it is also remarkable that the contraction takes place in the entire stability range, 0.3–1003 K. These crystals are constituted by linked polyhedra which present, when heated, a balance of forces between the static bending and the dynamic thermal vibration of the oxygen atoms [12]. The result is a whole cell volume reduction. It is obviously of much interest to see what influence pressure has on these types of materials. Evans et al. [10] have shown that a phase transition at about 6 Kbar preceeds the amorphization that takes place at higher pressure and temperature. Meanwhile, when only heated above 473 K, a new phase is obtained, b-ZrW2 O 8 , changing the space group, from P2 1 3 to a centric one Pa-3 [13]. More recently, a new trigonal symmetry polymorph of ZrW2 O 8 has been prepared using a non-hydrolitic sol–gel method [14]. Moreover, ZrW2 O 8 presents a metastable character when synthesised at room pressure, partially decomposing if

1466-6049 / 00 / $ – see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 00 )00004-0

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careful quenching is not performed. In recent works, a new synthesis at low temperature has been proposed to avoid the traditional ceramic method [15]. In order to obtain further insight into the behaviour of this material we have performed a detailed study of the influence of pressure both at room and higher temperatures in some of these materials. We report here some results concerning ZrW2 O 8 .

2. Experimental

2.1. Synthesis ZrW2 O 8 powders were synthesised by a conventional ceramic method. Zirconium and tungsten oxides, supplied by Aldrich (99%), were calcined at 6008C for 2 h, mixed and well ground in an agate mortar for 1 h. Then, they were pressed giving rise to pellets of about 2 g and finally, they were calcined at between 1200 and 14008C for 5 h. To prevent decomposition back to the precursors due to the metastable character of ZrW2 O 8 , different procedures were carried out: (i) a continuous oxygen flow was passed during the reaction, (ii) time calcination was increased, (iii) temperature calcination was varied in the 1200–1400 range and (iv) samples were quenched either at room temperature or at 77 K. In Fig. 1, some X-ray diffraction patterns of different preparations are compared: (a) the precursors were calcined at 12508C for 15 h in a continuous oxygen flow followed by quenching at room temperature, (b) the same method, except that the temperature was raised up to

13508C and (c) the precursors were calcined at 12008C for 8 h followed by quenching at 77 K. In Table 1 their cell parameters and phase percentages are shown. All ZrW2 O 8 powders prepared in the 1200–13508C temperature range show XRD patterns corresponding to a cubic phase (cell ˚ identified as a-ZrW2 O 8 (IDDE no. parameter59.1477 A), 13-557) with little traces of ZrO 2 (baddeleyite, IDDE no. 72-1669) and WO 3 (IDDE no. 20-1323). The best results were obtained at 12008C for 8 h without oxygen flow and quenched at 77 K, obtaining a light green powder, with |4% WO 3 present as an impurity. Several portions were pressed at room temperature in a 2–65 Kbar range in a non-cumulative way in a belt-type apparatus or a piston-cylinder press; samples were also calcined in a 20–10008C range, and some of them were also treated under the combined effect of pressure and temperature.

2.2. Powder X-ray diffraction Powder X-ray diffraction patterns were recorded for all samples using a Siemens D-500 diffractometer at 35 KV and 35 mA. This instrument is equipped with a CuK a radiation source and a Ni-filter. For all of the reported data sets, a step-size of 0.038 and step-time of 5 s were employed in the entire range of analysis. All samples were analysed after their respective treatments. The X-ray diffraction data were analysed using the program FULLPROF [16] to determine the cell parameters and weight ratios of the phases present in the samples. The compositions of the samples were obtained from the scale factors corre-

Fig. 1. X-ray patterns of several ZrW2 O 8 samples synthesised by: (a) calcination at 12508C for 15 h in continuous oxygen flow followed by quenching at room temperature, (b) method (a) except that the temperature was varied up to 13508C and (c) calcination at 12008C for 8 h followed by quenching at 77 K.

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Table 1 Cell parameters, volume and phase percentage for samples treated under pressure P (Kbar) 1 atm (a) 1 atm (b) 1 atm (c) 2 4 6 8 9 10 12 ..12 a

˚ a (A)

˚ b (A)

91431(4) 9.1443(7) 9.1477(3) 9.1220(8) 9.0495(9) 26.873(3) 9.1464(7) 9.0808(2) 27.065(9) 9.1139(13) 26.995(4) 9.0767(12) 27.035(4) 9.0927(11) 26.981(4) 9.0719 (15) 26.987(5) 9.0825(13) 27.041(4) Strong increase of amorphization

3

˚ c (A)

8.8763(8) 8.929(3) 8.9258(9) 8.9216(12) 8.9217(12) 8.9119(16) 8.9235(15)

˚ ) V (A

Phase (%)

764.3 764.6 765.5 759.1 2159 765 2195 2196 2189 2187 2182 2190

a(91)–Z(3)–W(6) a(89)–Z(5)–W(6) a(96)–W(4) a-ZrW2 O 8 (61) g-ZrW2 O 8 (34) a-ZrW2 O 8 (45) g-ZrW2 O 8 (50) g-ZrW2 O 8 g-ZrW2 O 8 g-ZrW2 O 8 g-ZrW2 O 8 g-ZrW2 O 8 a-ZrW2 O 8

a

a5a-ZrW2 O 8 , Z5ZrO 2 , W5WO 3 .

sponding to the different phases, assuming a similar crystallinity.

2.3. XDS measurements

air up to 1173 K using a Seiko 320U system at a heating rate of 10 K / min.

3. Results

The composition of some of the phases obtained was confirmed by energy dispersive X-ray (XDS) analysis using a Link scanning electron microscope equipped with a Link energy-dispersive X-ray detector.

2.4. Thermal analysis Differential thermal analyses (DTA) were performed on

3.1. Behaviour upon the increase of pressure The X-ray patterns of a a-ZrW2 O 8 sample as prepared by method (c) (see Section 2.1) and treated at different pressures (within a 2–12 Kbar range) are compared in Fig. 2. When the pressure is increased, a-ZrW2 O 8 (the cubic phase) transforms progressively into g-ZrW2 O 8 (the tetragonal phase, P2 1 2 1 2 1 space group) reaching a g:a ratio of

Fig. 2. X-ray patterns of a-ZrW2 O 8 after treated at non-cumulative different pressures.

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about 34% at 2 Kbar with a volume contraction for both phases, this is greater for g-ZrW2 O 8 , with about a 21.7% of volume reduction. The orthorhombic phase proportion becomes |50% at 4 Kbar showing cell parameters similar to those of the initial phases; however, the g-phase appears pure at 6 Kbar. Until this pressure, there are not significant variations in the amount of WO 3 and ZrO 2 by-products, indicating that at high-pressure the g-phase does not decompose. These data are in agreement with those ˚ 3 for reported elsewhere [17], where a volume of 2186 A the g-phase was obtained. Subsequently, at higher pressures the g-phase tends to reduce the volume notably until reaching a minimum value ˚ 3 at 10 Kbar. It means a volume contraction of of 2182 A about 0.6% with respect to that one at 6 Kbar and a significant amount of amorphous material appears. Further pressure increases show the metastable character of this structure leading to a significant volume increase up to ˚ 3 . These results are in agreement with a previous 2190 A work [17], where the g-phase compressibility was studied under pressure; our data refer to the quenched phases after releasing the pressure. From 13 Kbar onwards, a very strong amorphization process is observed and only the main reflections of a-ZrW2 O 8 , which are progressively weakened, can be detected in the XRD patterns (Fig. 2). The diffraction peaks become broader above 18 Kbar and at 40 Kbar they could not be distinguished anymore, indicating that the samples had undergone full amorphization. No evidence of new transformations was observed up to 65 Kbar. After releasing pressure, the high-pressure amorphous phase was retained at room temperature. According to Perottoni et al. [18], this behaviour could be the result of a large number of overlapped Bragg peaks from a low-symmetry structure, which were not resolved due to

the intrinsically low resolution of the experimental technique. On the basis of these results, it can be concluded that this structure shows a very flexible framework. There is a minimum volume for g-ZrW2 O 8 when subjected to a pressure of 10 Kbar after which the structure is very unstable and tends to transforms into a-ZrW2 O 8 and finally, it suffers pressure-induced amorphization.

3.2. Behaviour upon the increase of temperature In Fig. 3, X-ray patterns of a-ZrW2 O 8 powders calcined at different temperatures are shown. It is well-known that temperature increase produces a volume contraction of a-ZrW2 O 8 , an effect usually known as negative thermal expansion (NTE), which is in fact a thermal contraction [7] but, in all cases, smaller than that reached by pressure. Moreover, when a-ZrW2 O 8 is heated in air, an endothermic peak at 2108C together with negligible weight losses is observed in the DTA experiments (Fig. 4). It corresponds to the conversion of a- to b-ZrW2 O 8 with a centric space group Pa-3. This phase was up to now only detected by X-ray diffraction experiments during the heating. Basically, it is characterised by the progressive intensity decrease of the peaks 111, 221 and 310. These are practically absent at 2408C. Otherwise, the material is stable up to temperatures lower than 8008C, then it decomposes in monoclinic ZrO 2 and triclinic WO 3 . If the X-ray diffraction experiment is carried out during the heating the tetragonal ZrO 2 (IDDE no. 71-1282) is identified, instead. We have prepared a b-ZrW2 O 8 sample by calcining a-ZrW2 O 8 at 6008C for 4 h and quenching at 77 K (even at 3008C there is unequivocal evidence of its stabilisation, appearing as a a1b-phase mixture). As

Fig. 3. X-ray patterns of a-ZrW2 O 8 calcined at several temperatures followed by quenching at 77 K.

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shown in Fig. 3, there are variations in the intensities of the above peaks named before. In spite of the fact that these peaks should be absent at 6008C, we find a better fitting for the model based in the space group Pa-3 (Table 2). In fact, these data can be explained taking into account the small differences between both space groups (a symmetry centre) linked to the quenching effect which can retain the symmetry of the local site within the average one. Due to the well-known thermal properties of this phase, the cell parameter is increased with respect to that ˚ calculated from the XRD pattern at 6008C to 9.1269 A [13].

3.3. Treatments under pressure and temperature

Fig. 4. Thermal analysis (DTA) of a ZrW2 O 8 powder.

Table 2 Cell parameters, volume and phase percentage for samples treated under temperature a T (8C)

˚ a (A)

25 (c) 120, 2h 300, 2 h, Q

9.1477(3) 9.1471(2) 9.1475(3) 9.1439(5) 9.1468(3) Decomposition in WO 3 (t) and

600, 2 h, Q 800, 2 h, Q a

˚ b (A)

˚ c (A)

˚ 3) V (A

Phase (%)

765.5 765.3 765.4 764.5 765.2 ZrO 2 (m)

a-ZrW2 O 8 a-ZrW2 O 8 a-ZrW2 O 8 (80) b-ZrW2 O 8 (11) b-ZrW2 O 8

t5Triclinic, m5monoclinic, Q5quenched phase and (c)5as in Fig. 1.

After studying the separate effects of pressure and temperature on a-ZrW2 O 8 , the combined effect of both was studied. In Fig. 5, the X-ray diffraction patterns of samples treated simultaneously under pressure and temperature are compared and their structural features are listed in Table 3. Samples treated at 6 Kbar and 6008C are constituted mainly by |72% g-ZrW2 O 8 , 6% a-ZrW2 O 8 , as well as a 9% monoclinic ZrO 2 and 13% triclinic WO 3 as by-products with a DV|20.7% for the g-phase. These data show the g-ZrW2 O 8 tendency to a reduction volume when the temperature is increased, in good agreement with Sleight et al. [17]; however, in this case, the pressure allows the structure to be quenched and even at room temperature. Otherwise, the increase in the amount of the by-products and the appearance of a-ZrW2 O 3 indicate that the g high-pressure phase is metastable and starts to decompose at room temperature and pressure. This is indeed confirmed by increasing the temperature by 2008C,

Fig. 5. X-ray patterns of a-ZrW2 O 8 after subjected to the combined effects of pressure and temperature.

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Table 3 Cell parameters, volume and phase percentage for samples treated under pressure and temperature a Conditions 6 Kbar16008C

6 Kbar18008C

10 Kbar16008C a

˚ a (A)

˚ b (A)

˚ c (A)

˚ 3) V (A

Phase (%)

26.968(16)

8.932(5)

759 2180

5.2334(7) b 5101.7(1) 7.3717(8) 7.5092(7) a 589.27(1) b 591.17(1) Pressure-induced amorphization

5.368(1)

134 146

a-ZrW2 O 8 (6) g-ZrW2 O 8 (72) Z(9)–W(13) ZrO 2 (c,8) ZrO 2 (m,1)

425

WO 3 (t,91)

9.123(4) 9.053(5) 5.127 (5) 5.315(1)

7.6784(7) g 591.08(1)

a-ZrW2 O 8

c5Cubic, m5monoclinic, t5triclinic, Z5ZrO 2 , W5WO 3 .

where we did find a material constituted by ZrO 2 (cubic, IDDE no. 27-0997), ZrO 2 (monoclinic, IDDE no. 371484) and WO 3 (triclinic, no. 83-0951) (Fig. 5). In fact, the ZrO 2 cubic phase is unstable at room temperature, but in this sample, the cell parameter is notably enhanced from ˚ (IDDE no. 27-0997) to a55.1223(3) A, ˚ a55.0900 A which could be indicative of some tungsten / zirconium replacement. Nevertheless, the percentage for this phase is below 10%. Experiments carried out at 10 Kbar and 6008C give rise to almost completely amorphous materials comparable to those obtained at 45 Kbar.

g-ZrW2 O 8 . The X-ray diffraction patterns of a sample subjected to 6 Kbar after several treatments are compared in Fig. 6 and the cell parameters and percentages of all phases are listed in Table 4. After 7 days at room temperature, the sample remarkably changes (Fig. 6), converting to a phase mixture with a a-ZrW2 O 8 :g-ZrW2 O 8 ratio of 30:64, but if pressure again is applied, it tends to transform back into the g-phase. In this case, the sample reaches about 84% of g-ZrW2 O 8 after being treated at 6 Kbar for 1 h. This fact is indicative that the a→g phase transition of ZrW2 O 8 is reversible and that g-ZrW2 O 8 shows a metastable behaviour [17].

3.4. Relaxation of the sample with time after the effect of pressure

4. Conclusions

A study was performed with some samples that seem to decompose with time after releasing the pressure. We show the results obtained for the sample at 6 Kbar formed by

• a-ZrW2 O 8 shows thermal contraction in the range of 25–8008C and transforms into the b-phase at about 2108C, as detected by an endothermic peak in the thermal analysis and the extinction of several reflec-

Fig. 6. X-ray pattern evolution of g-ZrW2 O 8 with time and subsequent treatment upon pressure at 6 Kbar.

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Table 4 Cell parameters, volume and phase percentage for a sample treated at 6 Kbar, its evolution with time and newly treated at 6 Kbar Conditions

˚ a (A)

˚ b (A)

˚ c (A)

˚ 3) V (A

Phase (%)

6 Kbar

9.1139(13)

26.995(4)

8.9258(9)

2196

g-ZrW2 O 8

6 Kbar17 days

9.1465(5) 9.0694(12)

27.025(4)

8.9118(12)

765 2184

a-ZrW2 O 8 (30) g-ZrW2 O 8 (64)

9.1396(13) 9.0686(18)

27.029(6)

8.9112(18)

763.5 2184

a-ZrW2 O 8 (13) g-ZrW2 O 8 (81)

6 Kbar1 7 days 1 6 Kbar

tions in the X-ray diffraction pattern. b-ZrW2 O 8 was synthesised by calcining a-ZrW2 O 8 at 6008C followed by quenching at 77 K. • g-ZrW2 O 8 can be compressed up to 10 Kbar, where a volume reduction even after releasing the pressure was found. From 12 Kbar onwards, a strong pressure-induced amorphization is seen. g-ZrW2 O 8 , transforms in a g1a-phase mixture after 7 days of releasing the pressure and tends to revert to gZrW2 O 8 if the pressure is again applied, showing the reversible character of this transition. • When pressure and temperature are simultaneously applied to a-ZrW2 O 8 , g-ZrW2 O 8 is obtained with a ˚ 3 ) than that observed larger volume reduction (2180 A when only pressure is applied. Thus, the pressure acts as a stabilising factor for this metastable phase.

Acknowledgements Authors would like to thank CICYT, Project MAT 98-1053-V04-01, for financial support.

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