Characterization and thermal behavior of HGdP2O7·3H2O

Pergamon

Materials Research Bulletin 36 (2001) 2175–2181

Characterization and thermal behavior of HGdP2O7䡠3H2O F. Chehimi-Moumena, M. Feridb, D. Ben Hassen-Chehimia, M. Trabelsi-Ayadia,* a

Laboratoire de Physico-Chimie Mine´rale, Faculte´ des Sciences de Bizerte, 7021 Zarzouna-Bizerte, Tunisie b Laboratoire des Proce´de´s Chimiques, Institut National de Recherche Scientifique et Technique. B.P. 95 Hammam-Lif, Tunisie (Refereed) Received 25 February 2001; accepted 16 May 2001

Abstract Crystals of the trihydrated gadolinium diphosphate HGdP2O7䡠3H2O were obtained from a mixture of diphosphoric acid and gadolinium nitrate solutions. This salt was characterized by X-ray powder diffraction, IR spectroscopy and thermal analysis. IR spectroscopic study reveals the existence of the characteristic band of the P2O7 group, the ␯as(POP) band, at 941 cm⫺1 and the absence of the ␯s(POP) one. The thermal decomposition of HGdP2O7䡠3H2O, which was followed by thermogravimetry and differential scanning calorimetry, shows that the elimination of the crystallization water takes place in three stages between 71 and 500°C. The overall ⌬H of the first two dehydration were found to be 41.06 and 25.88 kJ/mol. New well-crystallized hydrates HGdP2O7䡠1.5H2O and HGdP2O7䡠0.5H2O were obtained through heating HGdP2O7䡠3H2O, respectively, at 145 and 280°C. The anhydrous HGdP2O7 salt, which was obtained by calcination of HGdP2O7䡠3H2O at 530°C, decomposes at 700°C with loss of P2O5 to give the gadolinium diphosphate Gd4(P2O7)3. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compound; C. Infrared spectroscopy; C. Differential scanning calorimetry; C. Thermogravimetric analysis; C. X-ray diffraction

1. Introduction The synthesis of acid lanthanide diphosphates of general formula HLnP2O7䡠xH2O (x ⫽ 3, 3.5) has been reported since 1968 [1– 4]. These compounds were characterized by X-ray * Corresponding author. Fax: ⫹216-2-434-566. E-mail address: [email protected] (F. Chehimi-Moumen). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 8 9 - 4

2176

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

powder diffraction, IR spectroscopy, and thermal analysis. It has been reported that they form two isostructural groups comprising the La–Sm compounds and the Eu–Lu compounds, but no structural studies were mentioned. The study of this type of phosphates has been, then, undertaken in our laboratory to study their crystalline structure and to clarify their decomposition process. Recently the preparation and the study of the thermal decomposition of a new acidic lanthanide diphosphate HPrP2O7䡠3H2O, belonging to the first group, has been reported by the authors [5]. Single crystals of HGdP2O7䡠3H2O have been also synthesized, and their structure was solved in the triclinic system [6]. It has been found that this salt crystallizes in the P1៮ space group with Z ⫽ 2. In this paper we report the study of the thermal behavior of HGdP2O7䡠3H2O. 2. Experimental As reported previously [6], single crystals of HGdP2O7䡠3H2O were obtained by neutralization of an aqueous solution of diphosphoric acid, H4P2O7, with Gd(NO3)3䡠5H2O and evaporation of the solvent at room temperature. The crystallization of triclinic prisms of HGdP2O7䡠3H2O started from the solution after a few days. The crystals were then isolated and washed with distilled water. The aqueous solution of H4P2O7 was obtained from the sodium salt, Na4P2O7, by using an ion exchange resin (Amberlite IR 120). X-ray powder diffraction studies were carried out on a PHILIPS PW 3710 diffractometer using Cu K␣ radiation. The IR spectra were recorded in the 4000 – 400 cm⫺1 range, on a Perkin-Elmer FTIR 1000 spectrophotometer. The powder was finely ground and pressed into KBr pellets. TG-DTA thermograms were obtained with a 47.05-mg sample in a platinum crucible. The differential scanning calorimetry (DSC) was carried out using a weighed 33-mg sample sealed in an aluminum DSC crucible. In both techniques, SETARAM TG-DTA-DSC 92 instruments were used; the samples were heated in argon with a 5°C/mn heating rate.

3. Results and discussion 3.1. IR spectroscopy The IR spectrum of HGdP2O7䡠3H2O is shown in Fig. 1a. Absorption bands that correspond to OH/H2O vibrations are observed in the 3500 –1600 cm⫺1 range. Although the existence of different hydrogen bonding in the HGdP2O7䡠3H2O structure [6], only one broad band appears at around 3432 cm⫺1 corresponding to O—H stretching (␯1) of the water molecules. The O—H bending (␯2) appears as a narrow and intense band at 1624 cm⫺1. The absorption band observed at 2354 cm⫺1 can be due to the O—H vibration of the acidic hydrogen [7].

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

2177

Fig. 1. IR spectra of HGdP2O7䡠3H2O at different temperatures: (a) dried at room temperature; (b) calcined at 145°C; (c) calcined at 280°C; (d) calcined at 530°C; (e) calcined at 790°C; (f) calcined at 850°C; (g) calcined at 1050°C.

The P2O74⫺ anion vibration occurs at frequencies below 1200 cm⫺1 [7–9]. Bands in the 1196 –1114 cm⫺1 are attributed to the asymmetric ␯as(PO3). The symmetric ␯s(PO3) stretching is observed as three narrow bands at 1058, 1035, and 1000 cm⫺1. The absorption band at 941 cm⫺1 can be attributed to the asymmetric ␯as(POP) [7]. This band is characteristic of the P2O7 group. But one can note the absence of the symmetric ␯s(POP), which appears normally at about 750 cm⫺1. According to the proposed correlation between the (POP) bridge angles in diphosphates and the difference (␯as ⫺ ␯s) [8], the symmetric (POP) band can be absent only in the case of a (POP) angle equal to 180°. But it is not the case for HGdP2O7䡠3H2O, indeed the POP angle in this diphosphate is found to be 138.8°. The deformation vibrations of the P2O7 group appear as medium intensity bands in the range 624 – 422 cm⫺1. 3.2. Thermal analysis Fig. 2 shows both TG and DTA thermograms of HGdP2O7䡠3H2O from room temperature to 850°C. In the TG curve the weight loss can be divided into four areas 71–150°C, 150 –223°C, 365–500°C, and over 700°C. The total weight loss in 71–500°C temperature

2178

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

Fig. 2. DTA and TG curves of HGdP2O7䡠3H2O.

range is found to be 13.48%, which is in agreement with the calculated value 13.98% corresponding to the loss of three water moles. The TG weight loss in the first stage (7.01%), due to the elimination of 1.5 water molecules, is close to the calculated value 6.99%. It is related to the endothermic peak at 119°C. The second weight loss (4.53%), related to the endotherm peaking at 203°C, is compatible with the theoretical loss of one water mole (4.66%). The last half water mole in the structure is lost in the third temperature area, because the experimental weight loss is similar to the theoretical loss of 0.5 water mole. In this temperature area the DTA curve exhibits two large endotherms at 390 and 457°C. Thus, all the endotherms in the DTA curve are considered to result from the removal of water from the structure. It can be noted that a slow weight loss nonaccompanied by an observable thermal effect takes place from 725°C. It may be due to the decomposition of the anhydrous salt and the removal of the acidic hydrogen. The differential calorimetric curve (Fig. 3) recorded from room temperature to 300°C shows two endothermic effects corresponding respectively to the elimination of 1.5 and one water mole. The first endothermic peak occurs in the temperature range of 76 –172°C, with ⌬H ⫽ 41.06 kJ/mol. The second, occurring in the 172–227°C temperature range, has a ⌬H ⫽ 25.88 kJ/mol. To see the effect of water removal from the HGdP2O7䡠3H2O structure, the elaborated

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

2179

Fig. 3. DSC curve of HGdP2O7䡠3H2O.

product was heated in the DTA unit until a peak was obtained. When this happened, heating was stopped. The sample was cooled to room temperature then subjected to X-ray and IR spectroscopic analysis. Heating was then resumed with a new sample of the same compound, and treatment, as described above, was performed for each effect found in the DTA curve. The results of the heat treatment (RX and IR) are illustrated in Figs. 1 and 4. The IR spectrum recorded for the sample heated at 145°C (Fig. 1b) is completely different from the one of the initial product. Indeed, beside the existence of the ␯as(PO3) band of the P2O7 group at 943 cm⫺1, one can note the appearance of the symmetric P-O-P stretching vibration band and the splitting of the water stretching band to three components at 3551, 3412, and 3260 cm⫺1; which indicates the existence of different hydrogen bonding in the novel obtained diphosphate. The corresponding X-ray patterns (Fig. 4b) show the existence of narrow peaks indicating the formation of a new crystallized product. These patterns are quite different from those of the initial product (Fig. 4a). According to the result of the thermal analysis the hydrate obtained after calcination at 145°C can be HGdP2O7䡠1.5H2O. After calcination at 280°C the heated product shows an IR spectrum (Fig. 1b) similar to that of the heat-treated product at 145°C. In fact, water bands and P2O7 ones are still present. Thus, this spectrum belongs to a second hydrate, which could be HGdP2O7䡠0.5H2O. The X-ray patterns displayed in Fig. 4c show the good crystallinity of the latter. The IR spectrum of the heated product at 530, displayed in Fig. 1d, is different from those in Fig. 1b and 1c. The band around 1200 cm⫺1 in the latter splits into two bands between 1300 and 1200 cm⫺1 and the one observed at 750 cm⫺1 split into three between 800 –700

2180

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

Fig. 4. X-ray patterns of HPrP2O7䡠3H2O at different temperatures: (a) dried at room temperature; (b) calcined at 145°C; (c) calcined at 280°C; (d) calcined at 530°C; (e) calcined at 790°C; (f) calcined at 850°C; (g) calcined at 1050°C.

cm⫺1. In fact, all the crystallization water is lost below 500°C, so this spectrum can be attributed to the anhydrous salt. The corresponding X-ray diagram, shown in Fig. 4d, is similar to those in Fig. 4c. The comparison of this pattern with those given by Afonin [2] for HLnP2O7 (Ln ⫽ Dy, Er, Yb, and Y) shows that the obtained anhydrous salt, HGdP2O7 is isostructural with the latter compounds. HGdP2O7 seems to be stable until 700°C. Indeed, after heating at 790°C (Figs. 1e and 4e) the crystallinity of the product decreases and supplementary lines are observed, which indicates that a decomposition takes place at this temperature. After 10 h calcination in a furnace at 850°C, the heated product exhibits a more complex IR spectrum (Fig. 1f). The X-ray study shows that the patterns of the obtained salt (Fig. 4f) agree with those given by Kizilyalli for the high temperature form of Gd4(P2O7)3 [10]. When the product was heated at 1050°C for 36 h only GdPO4 (Monazite) bands and lines were observed respectively in Figs. 1g and 4g [11]. During the calcinations at 850 and 1050°C, P2O5 is lost by volatization to yield single phase solids. Taking into account the results of the thermal analysis, the X-ray powder diffraction and the IR spectroscopic studies the loss of weight taking place over 725°C can be attributed to the reaction: 4HGdP 2O 7 3 2H 2O ⫹ Gd 4共P 2O 7兲 3 ⫹ P 2O 5 共 g兲

F. Chehimi-Moumen et al. / Materials Research Bulletin 36 (2001) 2175–2181

2181

The gadolinium monophosphate can be obtained from the decomposition of the gadolinium diphosphate: Gd 4共P 2O 7兲 3 3 4GdPO 4 ⫹ P 2O 5 共 g兲 4. Conclusion The study of the thermal behavior of HGdP2O7䡠3H2O shows that its decomposition process is different from that found for HPrP2O7䡠3H2O [5]. The observed transformations for HGdP2O7䡠3H2O can be represented schematically by the following steps: HGdP 2O 7 䡠 3H 2O

3 HGdP 2O 7 䡠 1.5H 2O ⫹ 1.5H 2O

71–150°C

HGdP 2O 7 䡠 1.5H 2O 3 HGdP 2O 7 䡠 0.5H 2O ⫹ H 2O

150–223°C

HGdP 2O 7 䡠 0.5H 2O 3 HGdP 2O 7 ⫹ 0.5H 2O

365–500°C

4HGdP 2O 7

3 2H 2O ⫹ Gd 4共P 2O 7兲 3 ⫹ P 2O 5 共 g兲

700–850°C

Gd 4共P 2O 7兲 3

3 4GdPO 4 ⫹ P 2O 5 共 g兲

1050°C

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

I.V. Tananaev, V.G. Kuznetsov, V.P. Vasileva, Izv. Akad. Nauk SSSR. Neorg. Mater. 3 (1967) 107–113. E.G. Afonin, N.I. Pechurova, Russ. J. Inorg. Chem. 35 (1990) 783–785. M. Kizilyalli, J. Less-Common Metals 127 (1987) 147–154. S. Ben Moussa, I. Sobrados, J.E. Iglesias, M. Trabelsi-Ayadi, J. Sanz, J. Mater. Chem. 10 (2000) 1973–1978. F. Chehimi-Moumen, D. Ben Hassen-Chehimi, M. Ferid, M. Trabelsi-Ayadi, J. Therm. Anal. Cal., in press. F. Chehimi-Moumen, D. Ben Hassen-Chehimi, M. Ferid, M. Trabelsi-Ayadi Mater. Res. Bull. 36 (1–2) (2001) 365–373. D.E.C. Corbridge, E.J. Lowe, J. Chem. Soc. (1954) 493–502. A. Rumont, R. Cahay, M. Liegeois-Duyckaerts, P.Tarte. Eur. J. Solid State Inorg. Chem. 28 (1991) 207–2019. B.C. Cornilsen, R.A. Condrate, J. Solid State Chem. 23 (1978) 375–382 M. Kizilyalli, J. Inorg. Nucl. Chem. 38 (1976) 483– 486. J.C.P.D.S Card No. 32-386.