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Oxidation behaviour of ytterbium sulphides
Reactioity of Solids, 7 (1989) 143-156 Elsevier Science Publishers B.V., Amsterdam
143 - Printed
in The Netherlands
Oxidation behaviour of ytterbium sulphides L.C. Otero-Diaz, M.J. Torralvo-Fernhdez Dpto de Quimica Inorgcinica, Facultad de Quimicas, 28040 Madrid (Spain)
Universidad Complutense,
and R.M. Rojas * Instituto de Ciencia de Materiales, (Received
November
C.S.I. C., Serrano 113, 28006 Madrid (Spain)
21st, 1988; accepted
February
2&h, 1989)
Abstract Thermal studies (at up to - 1500 o C) carried out on some ytterbium sulphides in air or under nitrogen show that complex decomposition processes undergone before the ultimate formation of the ytterbium oxide. The decompositions take place as overlapping and incomplete reactions to give mixtures of various intermediates, such as Yb,O,S and Yb,O,SO,. Electron microscopy diffraction studies on the intermediate Yb,O,SO, reveals a complex microstructure because of intergrowth with small precipitates of cubic phase Yb,O,.
Introduction
In spite of the general similarities in the physical and chemical properties of the lanthanide elements, the crystal chemistry of their sulphides is complex [l]. Several structural types occur, polymorphy is common and there is great variation in composition for some nominal stoichiometries. For instance in the ytterbium-sulfur system a classic solid solution for the Yb,S, phase has been described [2] with an extended composition range from Single crystal X-ray diffraction studies have been YbS,.,,(Yb,S,) to YbS,,e. carried out [3] for the composition YbS1,38(Yb2,90S4). In fact, the Yb,S, compound has its own ternary structure, Yb2+Yb:+S4, but has been described [4] as being closely related to warwickite which can also be described as NaCl-type ribbons of 4 X 1 X cc octahedra in extent (joined end-to-end), twinned by a glide reflection operation. Recent high resolution transmission electron microscopy studies [5] on this compound reveal an incommensurate extremely lengthy modular structure. 0168-7336/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
144
In order to investigate the stability and reactivity of some ytterbium sulphides, thermal decomposition studies were carried out by means of DTA and TG in air and nitrogen atmospheres up to 1450 o C. Two samples from the Yb,_.S, “solid solution” and the yellow stoichiometric Yb,S, compound [6], having an cy-Al,O,-like structure, have been studied. The several products formed during the thermal decomposition have been characterized by means of powder X-ray diffraction, electron diffraction microscopy and infrared spectroscopy techniques.
Experimental methods 1. Sample preparation Ytterbium sulphides were prepared in induction-heated carbon crucible containing Yb,O, (99.99%) in a stream of H,S (5%) + Ar (95%) at various high temperatures on the basis of previously published results [2] and [4]. Sample I (S-l). Stoichiometric Yb,S, was obtained by heating the oxide under the above conditions at 1500 o C for 3 h, followed by annealing in the gas at 1100” C for 5 h, and then cooled to room temperature by switching off the furnace. The product is yellow colour. Sample 2 (S-2). A black compound from the solid solution YbS, 1.33 < r -C 1.46 [2,4] was prepared by heating the oxide, under the above conditions, at 14OO’C for 100 min. Sample 3 (S-3). A black compound was prepared as sample S-2 but at 1500’ C for 140 min and at 1600°C for 10 min. Both samples S-2 and S-3 were cooled to room temperature by switching off the furnace without annealing at 1100 ’ C. The sulfur ytterbium ratio in samples S-2 and S-3 was determined gravimetrically by heating the sulphides in thoria crucibles up to 1450 o C for 10 h; the product obtained was the corresponding cubic oxide, Yb,O,. 2. Experimental techniques Thermal decomposition experiments were performed in a Stanton STA 1 instrument. The thermograms were obtained in still air or in a stream of nitrogen (0.5 l/mm), at a heating rate of loo/mm. The intermediates formed at the temperatures indicated below were obtained in a DuPont 951 thermobalance, under the same experimental conditions as above and they were quenched by rapidly removing them from the furnace. Powder X-ray diffraction patterns were obtained by use o,f a Siemens D-500 diffractometer with monochromatic Cu-K, (X = 1.5418 A).
145
Electron microscopy diffraction was performed on a Siemens Elmiskop 102 (100 kV) electron microscope, fitted with a double tilting goniometer. Samples were ground under n-butanol and a drop of the suspension was left to evaporate on carbon coated copper grids. IR spectra were registered on a Perkin-Elmer 325 spectrophotometer, as KBr pellets.
Results and discussion X-ray diffraction pattern of S-l shows the characteristic reflections of the stoichiometric Yb,S, phase having a corundum-like structure and unit cell parameters, in hexagonal form, a 6.7463(7), c 18.190(2), V 717.0(l) A3 which are in good agreement with previously reported values [6]. The approximate S/Yb ratio values of S-2 and S-3 calculated from gravimetric analysis are x = 1.42 and 1.44 respectively. The parameters for the basic orthorhombic unit cells are as follows: S-2: a 12.684(2), b 3.8199(6), c 12.862(2) A; V= 623.2(2) A3 and S-3: a 12.746(7), b 3.346(3), c 12.929(5) A; I’= 633.8(4) A3 in an excellent agreement with previously reported values for samples as the solid solution YbS, 1.33 < r < 1.46 [2,3,5]. Differential Thermal Analysis (DTA) and Thermogravimetric (TG) curves recorded for samples S-l, S-2 and S-3 are depicted in Fig. la, b and c respectively. In the stoichiometric ytterbium sulphide experiments, small weight gains with rising temperature were observed up to about 540°C at which a pronounced weight loss starts, continuing until 700 o C. Both processes cause a very broad exothermic effect as can be seen in the DTA curve; the X-ray diffraction diagram obtained for this sample heated at 700 o C, see Fig. 2a, was found to be a mixture of two phases, the starting Yb,S, (corundum type) and the hexagonal Yb,O,S [2]; the diffraction pattern shows broad lines reflecting the poor crystallinity of the material formed under these experimental conditions. If heating is continued the oxidation proceeds further and a new weight gain occurs between 700 and 940 ’ C. The majority of the diffraction maxima observed at 940°C temperature (see Fig. 2b) can be assigned to the orthorhombic phase Yb,O,SO, [7]. On the other hand, the infrared spectrum of this intermediate shows absorption bands at 980 and 1220 cm-’ characteristic of SO:- groups [8,9]. In this sample, at this temperature in the X-ray pattern in addition to the oxysulphate phase is possible to observe some of the main reflections from the ytterbium (Yb,O,) phase [lo], with the cubic c-Ln,O, type, but with very low intensity. In this way our results suggest a very narrow stability range for the ytterbium oxysulphate furthermore it cannot be isolated as a single pure phase; this result is consistent
146
1
1
1000
I
-T(‘C)
700
Fig. 1. Differential Thermal Analysis (DTA) and Thermogravimetric (c) S-3 samples, loo min - ’ heating rate, in still air.
1
400 (TG) for (a) S-l, (b) S-2,
those reported previously [ll]. At higher temperature, - 1100°C the cubic Yb,O, is the only phase present. For samples S-2 and S-3, the thermal decompositions are more complex. with
147
a
,~~~.~~.~~~l~~~~l.~~~l~...I...~I~~~~Ilr,,lr,,,lr,,rlr,r, 60
65
55
58
45
40
45
30
25
20
15 -
10
28
b
,~~~.~1~~~~1~~~.1~~.~1~~..1...~1~.~.1,,,~1,,,,~,,,,1,,,, 65
60
55
50
45
40
35
30
25
20
15 -
Fig. 2. X-ray powder diffraction patterns 940 o C (o Yb,S,, 0 Yb,O,S, A Yb,O,SO,,
from sample •I Yb,O,).
S-l heated
IO
28
at (a) 700” C and (b)
Both samples undergo an appreciable weight increase from room temperature up to 770 o C, the increase becoming especially remarkable between 570 and 770 o C. The X-ray powder diagram of S-2 heated to the temperature of this weight maximum (Fig. 3a) shows that at least two phases co-exist, i.e.: the Yb,O,S phase as well as another group of maxima of which some can be assigned to the starting material, and the rest being due to the same compound that has undergone a significant change in composition because of the large range described for the Yb,_,S, phase [2-41. But the maxima could also be due to a complex oxysulphide analogous to those observed in La and Pr systems [12,13]. Further studies will be carried out. On the other hand, the X-ray diagram recorded for sample S-3 at 850 o C shows the presence of four different phases: Yb,O,S, Yb,O,SO,, Yb,O,, and the starting material (Fig. 4a). Ytterbium oxysulphate is an additional product formed in the thermal decomposition in air of S-2 and S-3 at 915 o C and 931” C respectively (see Figs. 3b and 4b), but the Yb,O,SO, phase seems to have a larger stability temperature range, particularly the one obtained from S-2, which seems to be completely free of Yb,O, (Fig. 3b). At higher temperatures, 1045 and 1050°C for S-2 and S-3 respectively, the powder patterns (see Fig. 3c and 4c) can be indexed as a mixture of the oxysulphate and oxide phases and we note sharp diffraction maxima for the cubic phase in both samples. The amount of the orthorhombic phase seems
a
b
Fig. 3. X-ray powder 1045 o C (0 Yb,O,S,
diffraction patterns from S-2 heated A Yb,O,SO,, q Yb,O,, n Yb,S,).
at (a) 770 o C, (b) 915 o C and (c)
to be greater in the case of S-2 at this temperature, thus confirming the largest temperature stability range and its isolation at almost a pure phase. When the decomposition of these three samples is carried out in a stream of nitrogen, the two phases, Yb,S, (cy-Al,O, type) and Yb,O,S, appeared at 107551090°C independent of the stoichiometry of the starting sample (see
149
Fig. 4. X-ray powder diffraction patterns from S-3 heated 1050°C (m Yb,S,, 0 Yb,O,S, A Yb,O,SO,, 0 Yb,O,).
at (a) 850 o C, (b) 930 o C and (c)
Fig. 5). Under these conditions the oxidation to Yb,O,S occurs slowly and at the same time the oxidation from Yb’+ to Yb3+ takes place to give the stoichiometric Yb, S, phase; SO, gas phase is also formed. The small amount of oxygen gas remaining in the apparatus or carried by the nitrogen (N2 99.998 was used without any further purification) must have been responsible for the above process. From the above the changes undergone by the ytterbium sulphides as they decompose in air can be as follows: The initial weight gain, especially in samples with lower sulphur content (S-2 and S-3) is caused by an oxidation process that is probably accompanied or immediately followed by the elimination of sulphur dioxide (SO,) to give the formation of Yb,O,S. Ytterbium oxysulphide readily oxidizes to ytterbium oxysulphate, (this reaction causes the second weight gain observed in TG curves) to finally
150
b
r~~~~~~..l....l~l~~l..~~1~~..1~,..l....l~...I.,..l....l....l..,
66
66
60
45
ho
35
30
n
20
n
lo
-22 C
1~~~.~~~~.~.~11111.11111111111111,,II,,0,,,1l111111,,II,( 65 60 60 a 5
1!iI
30
25
20
15
m
-20 Fig. 5. X-ray powder diffraction patterns from (a) S-l, (b) S-2 and (c) S-3, heated 1100 o C under nitrogen (O Yb,S,, 0 Yb,O,S, 0 Yb,O,).
up to
transform into Yb,O, with the subsequent evolution of the remaining sulphur as sulphur dioxide. The significant differences existing between the calculated and experimental weight losses in the intermediate steps of the decomposition clearly indicate that it proceeds through a series of overlapping and incomplete reactions, as has been previously reported [ll], giving rise to the formation of a mixture of phases, except for the final product Yb,O,. It is worth mentioning at this stage that the stability range of the oxysulphate seems to
151
depend not only on the atomic number of the lanthanide on the stoichiometry of the starting material.
involved but also
Electron microscopy studies Although ytterbium oxysulphate obtained from S-2 seems to be a pure phase according to the X-ray data (see Fig. 3b), the diffraction maxima are not so well defined. This suggests poor crystallinity of the material as confirmed by electron microscopy diffraction observations. Figure 6a depicts an electron micrograph of sample S-2 heated up to 915°C taken at low magnification, which shows that the crystals of Yb,O,SO, phase are different and of varying particle size, (from 0.1 to 0.5 pm). Figures 6b and c show two electron diffraction patterns from the crystal indicated by the arrow. Two types of reflections are present: the first, of strong intensity can be indexed in the orthorhombic unit cell of Yb,O,SO, phase along [OOl], and [loll, *. The second which are weak in intensity (or satellite reflections) and located close to (0 k/4 O),; (0 3k/4 0), with k = 2n and (hk/4 O),; ( h 3k/4 0), with k = 2n, and cannot be indexed in terms of the orthorhombic cell. In addition to these there are also some very faint spots a (0 k 0) k = 2n + 1. These faint spots are forbidden reflections for the space group Izz2 suggested by Fahey [14] for the La,O,SO, phase. When the three sulphides are heated in thoria crucibles in air at 1350-1450°C, the product is the expected oxide, Yb,O,, as a pure phase in the cubic form as the well known c-Ln,O, structure-type [lo]. For instance, in the case of S-2, after it is heated in air at 1400°C, an X-ray powder pattern with 35 diffraction maxima which are all indexed as coming from the ytterbia phase, with a unit cell parameter a, = 10.4371(6) A, I/= 1136.9(2) A3 in good agreement with values reported in the literature [lo]. Transmission electron microscopy studies reveal it to be the only phase present. Figure 7a shows a typical electron high resolution micrograph along [ill], ** orientation, see inset b, presenting cross fringes due to {Oil}, planes and homogeneous contrast, characteristic of a well ordered compound, the white arrows indicate changes in crystal thickness. Figure 7c and d show two electron diffraction patterns from the same crystal, along the zone axes as indicated. On the other hand, some of the Ln,O,SO, have been found to be good matrixes for the cathodoluminescence process of europium [15,16]. X-ray single crystal data are not available owing to difficulties in obtaining single crystals, but the structure was determined by refinement of powder data [13]. As mentioned before, in the thermal decomposition of the three
* Subscript o referred to orthorhombic Yb,O,SO, unit cell. * * Subscript c referred to the cubic Yb,O, unit cell.
152
a
Fig. 6. (a) Low magnification image showing several microcrystals from sample S-2, heated to 915 o C corresponding to the Yb,O,SO, phase. (b, c) electron diffraction patterns of the crystal indicated by an arrow on the micrograph.
sulphides, the oxysulphate phase was found to be present mixed with the cubic oxide and with very poor crystallinity. In order to improve the crystallinity we carried out the decomposition of sample S-3 in air by heating it in a thoria crucible at 820 o C and carried out further annealing at this temperature for 72 h. X-ray powder pattern from this product up to 20 = 90 O, gave 40 diffraction maxima, of these 4 reflections could not be assigned to the orthorhombic Yb,O,SO,, because they are forbidden reflections in the space group I,,, p ro posed for the structure [14], but they were
Fig. 7. Electron diffraction patterns (b, c, d) from the same crystal flake of sample S-2 heated at 14OO*C and corresponding to the cubic Yb,03 oxide; zone axes are indicated. (a) Micrograph showing the high resolution image of the same crystal taken with the incident beam parallel to [ill], orientation, shown in inset b.
ascribed to the cubic oxide Yb,O,. It is noteworthy that many other reflections, up to 20, could also be assigned to the cubic phase. From these data the unit cell parameters obta$ed are: orthorhombic (majority phase): a 4.110(l), b 3.972(l), c 12.471{4) A; V 203.61(S) A3, and cubic (minor phase) a 10.427(6) A; V 1133.66(2) A3, in good agreement with previously reported values [14 and 10, respectively]. In order to determine the microstructure of the oxys~phate some present transmission electron microscopy observations from the annealed material are shown in Fig. 8. In (a) a low magnification image from crystals of this sintered material can be seen, we selected the elongated A plate and made a series of diffraction patterns to explore
Fig. 8. (a) Low magnification image from a crystal of S-3 sample annealed in air at 820°C for 72 h. (b, c) and (d) correspond to a series of diffraction patterns from the same crystal, shown in (a). b’ composite pattern of (b).
155
the reciprocal lattice. As can be observed in three of these, Fig. 8b, c and d, they contain two types of reflections: main reflections with strong intensity characteristic of the orthorhombic phase along [302],, [OOl], and [loll, and weak or satellite reflections which cannot assigned to the referred cell. A tentative way of indexing these weak satellite reflections as Yb,O, was made assuming that small precipitates of this phase had grown inside the oxysulphate matrix. In Fig. 8b*, we present a composite pattern for diffraction shown in b. The large circles represent spots from the orthorhombic unit cell, small circles and crosses the majority of the weak spots and were attributed to a small precipitate twinned particle of the cubic oxide phase in a high index zone orientation, [J 1 151,. Both unit cells -- have been outlined. We noted coincidence of many spots, e.g., (020), = (251),; (213), = (451),; twin plane being parallel to [lo 5 31,. In addition, very weak reflections which could be due to double diffraction from both crystals [17] were observed. The presence of many small particles inside the area of the diffraction aperture selected (100 pm diameter) cannot be neglected, but note the contrast of the crystal studied in Fig. 8a. For the diffraction patterns shown in Fig. 8c and d, to be similar to those presented in Fig. 6b and c, the incident beam must be parallel to [OOl], and [loll,, respectively of the orthorhombic Yb,O,SO, matrix. The d-spacing for the first weak satellites spots along g’ (020), in both diffraction patterns are 7.4 A which correspond to [Oil],; these are indicated by small arrows on the figures; but many of the very weak spots are probably due to small oxide particles that were not properly oriented with the incident beam. On the other hand we believe that a structural relationship may exist between the orthorhombic Yb,O,SO, and the cubic Yb,OzO-Yb,O,-, phases: both cells are type I, the coordination of the metal atoms is six in both cases, and some bond lengths Yb-0 are almost equal, and the substitution of an 02- anion for the tetrahedral SOd2- groups, in which each oxygen is bonded to a different ytterbium atom, form the layered structure reported by Fahey [13] for the oxysulphate phase. From our X-ray and electron-diffraction data and from electron microscopy we propose that a phase transformation from Yb,O,SO, to Yb,O, can take place involving the loss of SO, (gas) and this probably could occur via an intermediate modular structure. Hot-stage transmission electron microscopy will be carried out to check whether this possible phase transformation takes place.
Conclusions When ytterbium sulphides, ranging in composition from YbS,,,,(Yb,S,) to YbS,.,,(Yb,S,) are decomposed in air, ytterbium oxysulphide (Yb202S), ytterbium oxysulphate (Yb,O,SO,), and ytterbium oxide (Yb,O, (c-Ln,O,
156
type)) are formed, however, they cannot be isolated as a single phase, except at 1300” C or higher temperatures at which the pure cubic Yb,O, is obtained. Under nitrogen, decomposition yields the ytterbium oxysulphide (Yb,O,S) and Yb,S, as the intermediates and the formation of ytterbium oxysulphate Yb,O,SO, does not take place. A microstructural study of the Yb,O,SO, phase by transmission electron microscopy, reveals a very small particle size and a marked reactiveness towards the electron beam when this product is obtained from the thermal decomposition of the sulphides. Electron diffraction patterns confirm the basic orthorhombic structure for the Yb,O,SO, compound as obtained by X-ray diffraction powder [14]. The microstructure becomes complex owing to the existence of small precipitates of the cubic oxide, Yb,O,, intergrown with the orthorhombic matrix and its formation cannot be avoided in our experiments.
Acknowledgements We are gratefully electron microscopy
indebted studies.
to Mr. A. Garcia
for his assistance
with the
References 1 J. Flahaut, in K.A. Gschneider and L.R. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, North-Holland Publishing Co., Amsterdam, Vol. 4, 1979, p. 1. 2 J. Flahaut, L. Domange, Mlle M. Guittard and L. Loriers, Bull. Sot. Chim. Fr., (1961) 102. 3 R. Chevalier, P. Laruelle and J. Flahaut, Bull. Sot. Fr. Mineral. Cristallogr., (1967) 564. 4 B.G. Hyde, S. Andersson, M. Bakker, C.M. Plug and M. O’Keeffe, Prog. Solid State Chem., 12 (1980) 273. 5 L.C. Otero-Diaz and B.G. Hyde, Acta Cryst., B39 (1983) 569. 6 J. Flahaut, L. Domange and M.P. Pardo, C.R. Acad. Sci. Paris, 258 (1964) 594. 7 R. Ballestracci and J. Mareschal, Mater. Res. Bull., 2 (1967) 993. 8 G. Pannetier and A. Dereigne, Bull. Sot. Chim. Fr., (1963) 1850. 9 M. Leskela, Thermochim. Acta, 92 (1985) 739. 10 R.S. Roth and S.J. Schneider, J. Res. Nat. Bur. Stand., 64A (1960) 309. 11 M. Leskela and L. Niinisto, in W. Hemminger (Ed.), Thermal Analysis ICTA 80, 2 (1980) 247. Birkhauser Verlag, M. Leskela and L. Niinistro, in K.A. Gschneider and L.R. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, North-Holland Publishing Co., Amsterdam, Vol. 8, 1986, p. 203. 12 P. Besarqon, J. Solid State Chem., 7 (1973) 232. 13 P. Besaqon, D. Carre and P. Laruelle, Acta Cryst., B29 (1973) 1064. 14 J.A. Fahey, Proc. Rare Earths Res. Conf. 12th, (1976) 762. 15 P. Percher, D.R. Svoronos, M. Leskela and J. HBlsa, J. Solid State Chem., 46 (1983) 101. 16 P. Percher and P. Caro, J. Less-Common Met., 93 (1983) 151. 17 P.B. Hirsh, A. Howie, R.B. Nicholson, D.W. Pashey and M.J. Whelan, Electron Microscopy of Thin Crystals, R.E. Krieger Pub., 2nd ed., 1977.
143 - Printed
in The Netherlands
Oxidation behaviour of ytterbium sulphides L.C. Otero-Diaz, M.J. Torralvo-Fernhdez Dpto de Quimica Inorgcinica, Facultad de Quimicas, 28040 Madrid (Spain)
Universidad Complutense,
and R.M. Rojas * Instituto de Ciencia de Materiales, (Received
November
C.S.I. C., Serrano 113, 28006 Madrid (Spain)
21st, 1988; accepted
February
2&h, 1989)
Abstract Thermal studies (at up to - 1500 o C) carried out on some ytterbium sulphides in air or under nitrogen show that complex decomposition processes undergone before the ultimate formation of the ytterbium oxide. The decompositions take place as overlapping and incomplete reactions to give mixtures of various intermediates, such as Yb,O,S and Yb,O,SO,. Electron microscopy diffraction studies on the intermediate Yb,O,SO, reveals a complex microstructure because of intergrowth with small precipitates of cubic phase Yb,O,.
Introduction
In spite of the general similarities in the physical and chemical properties of the lanthanide elements, the crystal chemistry of their sulphides is complex [l]. Several structural types occur, polymorphy is common and there is great variation in composition for some nominal stoichiometries. For instance in the ytterbium-sulfur system a classic solid solution for the Yb,S, phase has been described [2] with an extended composition range from Single crystal X-ray diffraction studies have been YbS,.,,(Yb,S,) to YbS,,e. carried out [3] for the composition YbS1,38(Yb2,90S4). In fact, the Yb,S, compound has its own ternary structure, Yb2+Yb:+S4, but has been described [4] as being closely related to warwickite which can also be described as NaCl-type ribbons of 4 X 1 X cc octahedra in extent (joined end-to-end), twinned by a glide reflection operation. Recent high resolution transmission electron microscopy studies [5] on this compound reveal an incommensurate extremely lengthy modular structure. 0168-7336/89/$03.50
0 1989 Elsevier Science Publishers
B.V.
144
In order to investigate the stability and reactivity of some ytterbium sulphides, thermal decomposition studies were carried out by means of DTA and TG in air and nitrogen atmospheres up to 1450 o C. Two samples from the Yb,_.S, “solid solution” and the yellow stoichiometric Yb,S, compound [6], having an cy-Al,O,-like structure, have been studied. The several products formed during the thermal decomposition have been characterized by means of powder X-ray diffraction, electron diffraction microscopy and infrared spectroscopy techniques.
Experimental methods 1. Sample preparation Ytterbium sulphides were prepared in induction-heated carbon crucible containing Yb,O, (99.99%) in a stream of H,S (5%) + Ar (95%) at various high temperatures on the basis of previously published results [2] and [4]. Sample I (S-l). Stoichiometric Yb,S, was obtained by heating the oxide under the above conditions at 1500 o C for 3 h, followed by annealing in the gas at 1100” C for 5 h, and then cooled to room temperature by switching off the furnace. The product is yellow colour. Sample 2 (S-2). A black compound from the solid solution YbS, 1.33 < r -C 1.46 [2,4] was prepared by heating the oxide, under the above conditions, at 14OO’C for 100 min. Sample 3 (S-3). A black compound was prepared as sample S-2 but at 1500’ C for 140 min and at 1600°C for 10 min. Both samples S-2 and S-3 were cooled to room temperature by switching off the furnace without annealing at 1100 ’ C. The sulfur ytterbium ratio in samples S-2 and S-3 was determined gravimetrically by heating the sulphides in thoria crucibles up to 1450 o C for 10 h; the product obtained was the corresponding cubic oxide, Yb,O,. 2. Experimental techniques Thermal decomposition experiments were performed in a Stanton STA 1 instrument. The thermograms were obtained in still air or in a stream of nitrogen (0.5 l/mm), at a heating rate of loo/mm. The intermediates formed at the temperatures indicated below were obtained in a DuPont 951 thermobalance, under the same experimental conditions as above and they were quenched by rapidly removing them from the furnace. Powder X-ray diffraction patterns were obtained by use o,f a Siemens D-500 diffractometer with monochromatic Cu-K, (X = 1.5418 A).
145
Electron microscopy diffraction was performed on a Siemens Elmiskop 102 (100 kV) electron microscope, fitted with a double tilting goniometer. Samples were ground under n-butanol and a drop of the suspension was left to evaporate on carbon coated copper grids. IR spectra were registered on a Perkin-Elmer 325 spectrophotometer, as KBr pellets.
Results and discussion X-ray diffraction pattern of S-l shows the characteristic reflections of the stoichiometric Yb,S, phase having a corundum-like structure and unit cell parameters, in hexagonal form, a 6.7463(7), c 18.190(2), V 717.0(l) A3 which are in good agreement with previously reported values [6]. The approximate S/Yb ratio values of S-2 and S-3 calculated from gravimetric analysis are x = 1.42 and 1.44 respectively. The parameters for the basic orthorhombic unit cells are as follows: S-2: a 12.684(2), b 3.8199(6), c 12.862(2) A; V= 623.2(2) A3 and S-3: a 12.746(7), b 3.346(3), c 12.929(5) A; I’= 633.8(4) A3 in an excellent agreement with previously reported values for samples as the solid solution YbS, 1.33 < r < 1.46 [2,3,5]. Differential Thermal Analysis (DTA) and Thermogravimetric (TG) curves recorded for samples S-l, S-2 and S-3 are depicted in Fig. la, b and c respectively. In the stoichiometric ytterbium sulphide experiments, small weight gains with rising temperature were observed up to about 540°C at which a pronounced weight loss starts, continuing until 700 o C. Both processes cause a very broad exothermic effect as can be seen in the DTA curve; the X-ray diffraction diagram obtained for this sample heated at 700 o C, see Fig. 2a, was found to be a mixture of two phases, the starting Yb,S, (corundum type) and the hexagonal Yb,O,S [2]; the diffraction pattern shows broad lines reflecting the poor crystallinity of the material formed under these experimental conditions. If heating is continued the oxidation proceeds further and a new weight gain occurs between 700 and 940 ’ C. The majority of the diffraction maxima observed at 940°C temperature (see Fig. 2b) can be assigned to the orthorhombic phase Yb,O,SO, [7]. On the other hand, the infrared spectrum of this intermediate shows absorption bands at 980 and 1220 cm-’ characteristic of SO:- groups [8,9]. In this sample, at this temperature in the X-ray pattern in addition to the oxysulphate phase is possible to observe some of the main reflections from the ytterbium (Yb,O,) phase [lo], with the cubic c-Ln,O, type, but with very low intensity. In this way our results suggest a very narrow stability range for the ytterbium oxysulphate furthermore it cannot be isolated as a single pure phase; this result is consistent
146
1
1
1000
I
-T(‘C)
700
Fig. 1. Differential Thermal Analysis (DTA) and Thermogravimetric (c) S-3 samples, loo min - ’ heating rate, in still air.
1
400 (TG) for (a) S-l, (b) S-2,
those reported previously [ll]. At higher temperature, - 1100°C the cubic Yb,O, is the only phase present. For samples S-2 and S-3, the thermal decompositions are more complex. with
147
a
,~~~.~~.~~~l~~~~l.~~~l~...I...~I~~~~Ilr,,lr,,,lr,,rlr,r, 60
65
55
58
45
40
45
30
25
20
15 -
10
28
b
,~~~.~1~~~~1~~~.1~~.~1~~..1...~1~.~.1,,,~1,,,,~,,,,1,,,, 65
60
55
50
45
40
35
30
25
20
15 -
Fig. 2. X-ray powder diffraction patterns 940 o C (o Yb,S,, 0 Yb,O,S, A Yb,O,SO,,
from sample •I Yb,O,).
S-l heated
IO
28
at (a) 700” C and (b)
Both samples undergo an appreciable weight increase from room temperature up to 770 o C, the increase becoming especially remarkable between 570 and 770 o C. The X-ray powder diagram of S-2 heated to the temperature of this weight maximum (Fig. 3a) shows that at least two phases co-exist, i.e.: the Yb,O,S phase as well as another group of maxima of which some can be assigned to the starting material, and the rest being due to the same compound that has undergone a significant change in composition because of the large range described for the Yb,_,S, phase [2-41. But the maxima could also be due to a complex oxysulphide analogous to those observed in La and Pr systems [12,13]. Further studies will be carried out. On the other hand, the X-ray diagram recorded for sample S-3 at 850 o C shows the presence of four different phases: Yb,O,S, Yb,O,SO,, Yb,O,, and the starting material (Fig. 4a). Ytterbium oxysulphate is an additional product formed in the thermal decomposition in air of S-2 and S-3 at 915 o C and 931” C respectively (see Figs. 3b and 4b), but the Yb,O,SO, phase seems to have a larger stability temperature range, particularly the one obtained from S-2, which seems to be completely free of Yb,O, (Fig. 3b). At higher temperatures, 1045 and 1050°C for S-2 and S-3 respectively, the powder patterns (see Fig. 3c and 4c) can be indexed as a mixture of the oxysulphate and oxide phases and we note sharp diffraction maxima for the cubic phase in both samples. The amount of the orthorhombic phase seems
a
b
Fig. 3. X-ray powder 1045 o C (0 Yb,O,S,
diffraction patterns from S-2 heated A Yb,O,SO,, q Yb,O,, n Yb,S,).
at (a) 770 o C, (b) 915 o C and (c)
to be greater in the case of S-2 at this temperature, thus confirming the largest temperature stability range and its isolation at almost a pure phase. When the decomposition of these three samples is carried out in a stream of nitrogen, the two phases, Yb,S, (cy-Al,O, type) and Yb,O,S, appeared at 107551090°C independent of the stoichiometry of the starting sample (see
149
Fig. 4. X-ray powder diffraction patterns from S-3 heated 1050°C (m Yb,S,, 0 Yb,O,S, A Yb,O,SO,, 0 Yb,O,).
at (a) 850 o C, (b) 930 o C and (c)
Fig. 5). Under these conditions the oxidation to Yb,O,S occurs slowly and at the same time the oxidation from Yb’+ to Yb3+ takes place to give the stoichiometric Yb, S, phase; SO, gas phase is also formed. The small amount of oxygen gas remaining in the apparatus or carried by the nitrogen (N2 99.998 was used without any further purification) must have been responsible for the above process. From the above the changes undergone by the ytterbium sulphides as they decompose in air can be as follows: The initial weight gain, especially in samples with lower sulphur content (S-2 and S-3) is caused by an oxidation process that is probably accompanied or immediately followed by the elimination of sulphur dioxide (SO,) to give the formation of Yb,O,S. Ytterbium oxysulphide readily oxidizes to ytterbium oxysulphate, (this reaction causes the second weight gain observed in TG curves) to finally
150
b
r~~~~~~..l....l~l~~l..~~1~~..1~,..l....l~...I.,..l....l....l..,
66
66
60
45
ho
35
30
n
20
n
lo
-22 C
1~~~.~~~~.~.~11111.11111111111111,,II,,0,,,1l111111,,II,( 65 60 60 a 5
1!iI
30
25
20
15
m
-20 Fig. 5. X-ray powder diffraction patterns from (a) S-l, (b) S-2 and (c) S-3, heated 1100 o C under nitrogen (O Yb,S,, 0 Yb,O,S, 0 Yb,O,).
up to
transform into Yb,O, with the subsequent evolution of the remaining sulphur as sulphur dioxide. The significant differences existing between the calculated and experimental weight losses in the intermediate steps of the decomposition clearly indicate that it proceeds through a series of overlapping and incomplete reactions, as has been previously reported [ll], giving rise to the formation of a mixture of phases, except for the final product Yb,O,. It is worth mentioning at this stage that the stability range of the oxysulphate seems to
151
depend not only on the atomic number of the lanthanide on the stoichiometry of the starting material.
involved but also
Electron microscopy studies Although ytterbium oxysulphate obtained from S-2 seems to be a pure phase according to the X-ray data (see Fig. 3b), the diffraction maxima are not so well defined. This suggests poor crystallinity of the material as confirmed by electron microscopy diffraction observations. Figure 6a depicts an electron micrograph of sample S-2 heated up to 915°C taken at low magnification, which shows that the crystals of Yb,O,SO, phase are different and of varying particle size, (from 0.1 to 0.5 pm). Figures 6b and c show two electron diffraction patterns from the crystal indicated by the arrow. Two types of reflections are present: the first, of strong intensity can be indexed in the orthorhombic unit cell of Yb,O,SO, phase along [OOl], and [loll, *. The second which are weak in intensity (or satellite reflections) and located close to (0 k/4 O),; (0 3k/4 0), with k = 2n and (hk/4 O),; ( h 3k/4 0), with k = 2n, and cannot be indexed in terms of the orthorhombic cell. In addition to these there are also some very faint spots a (0 k 0) k = 2n + 1. These faint spots are forbidden reflections for the space group Izz2 suggested by Fahey [14] for the La,O,SO, phase. When the three sulphides are heated in thoria crucibles in air at 1350-1450°C, the product is the expected oxide, Yb,O,, as a pure phase in the cubic form as the well known c-Ln,O, structure-type [lo]. For instance, in the case of S-2, after it is heated in air at 1400°C, an X-ray powder pattern with 35 diffraction maxima which are all indexed as coming from the ytterbia phase, with a unit cell parameter a, = 10.4371(6) A, I/= 1136.9(2) A3 in good agreement with values reported in the literature [lo]. Transmission electron microscopy studies reveal it to be the only phase present. Figure 7a shows a typical electron high resolution micrograph along [ill], ** orientation, see inset b, presenting cross fringes due to {Oil}, planes and homogeneous contrast, characteristic of a well ordered compound, the white arrows indicate changes in crystal thickness. Figure 7c and d show two electron diffraction patterns from the same crystal, along the zone axes as indicated. On the other hand, some of the Ln,O,SO, have been found to be good matrixes for the cathodoluminescence process of europium [15,16]. X-ray single crystal data are not available owing to difficulties in obtaining single crystals, but the structure was determined by refinement of powder data [13]. As mentioned before, in the thermal decomposition of the three
* Subscript o referred to orthorhombic Yb,O,SO, unit cell. * * Subscript c referred to the cubic Yb,O, unit cell.
152
a
Fig. 6. (a) Low magnification image showing several microcrystals from sample S-2, heated to 915 o C corresponding to the Yb,O,SO, phase. (b, c) electron diffraction patterns of the crystal indicated by an arrow on the micrograph.
sulphides, the oxysulphate phase was found to be present mixed with the cubic oxide and with very poor crystallinity. In order to improve the crystallinity we carried out the decomposition of sample S-3 in air by heating it in a thoria crucible at 820 o C and carried out further annealing at this temperature for 72 h. X-ray powder pattern from this product up to 20 = 90 O, gave 40 diffraction maxima, of these 4 reflections could not be assigned to the orthorhombic Yb,O,SO,, because they are forbidden reflections in the space group I,,, p ro posed for the structure [14], but they were
Fig. 7. Electron diffraction patterns (b, c, d) from the same crystal flake of sample S-2 heated at 14OO*C and corresponding to the cubic Yb,03 oxide; zone axes are indicated. (a) Micrograph showing the high resolution image of the same crystal taken with the incident beam parallel to [ill], orientation, shown in inset b.
ascribed to the cubic oxide Yb,O,. It is noteworthy that many other reflections, up to 20, could also be assigned to the cubic phase. From these data the unit cell parameters obta$ed are: orthorhombic (majority phase): a 4.110(l), b 3.972(l), c 12.471{4) A; V 203.61(S) A3, and cubic (minor phase) a 10.427(6) A; V 1133.66(2) A3, in good agreement with previously reported values [14 and 10, respectively]. In order to determine the microstructure of the oxys~phate some present transmission electron microscopy observations from the annealed material are shown in Fig. 8. In (a) a low magnification image from crystals of this sintered material can be seen, we selected the elongated A plate and made a series of diffraction patterns to explore
Fig. 8. (a) Low magnification image from a crystal of S-3 sample annealed in air at 820°C for 72 h. (b, c) and (d) correspond to a series of diffraction patterns from the same crystal, shown in (a). b’ composite pattern of (b).
155
the reciprocal lattice. As can be observed in three of these, Fig. 8b, c and d, they contain two types of reflections: main reflections with strong intensity characteristic of the orthorhombic phase along [302],, [OOl], and [loll, and weak or satellite reflections which cannot assigned to the referred cell. A tentative way of indexing these weak satellite reflections as Yb,O, was made assuming that small precipitates of this phase had grown inside the oxysulphate matrix. In Fig. 8b*, we present a composite pattern for diffraction shown in b. The large circles represent spots from the orthorhombic unit cell, small circles and crosses the majority of the weak spots and were attributed to a small precipitate twinned particle of the cubic oxide phase in a high index zone orientation, [J 1 151,. Both unit cells -- have been outlined. We noted coincidence of many spots, e.g., (020), = (251),; (213), = (451),; twin plane being parallel to [lo 5 31,. In addition, very weak reflections which could be due to double diffraction from both crystals [17] were observed. The presence of many small particles inside the area of the diffraction aperture selected (100 pm diameter) cannot be neglected, but note the contrast of the crystal studied in Fig. 8a. For the diffraction patterns shown in Fig. 8c and d, to be similar to those presented in Fig. 6b and c, the incident beam must be parallel to [OOl], and [loll,, respectively of the orthorhombic Yb,O,SO, matrix. The d-spacing for the first weak satellites spots along g’ (020), in both diffraction patterns are 7.4 A which correspond to [Oil],; these are indicated by small arrows on the figures; but many of the very weak spots are probably due to small oxide particles that were not properly oriented with the incident beam. On the other hand we believe that a structural relationship may exist between the orthorhombic Yb,O,SO, and the cubic Yb,OzO-Yb,O,-, phases: both cells are type I, the coordination of the metal atoms is six in both cases, and some bond lengths Yb-0 are almost equal, and the substitution of an 02- anion for the tetrahedral SOd2- groups, in which each oxygen is bonded to a different ytterbium atom, form the layered structure reported by Fahey [13] for the oxysulphate phase. From our X-ray and electron-diffraction data and from electron microscopy we propose that a phase transformation from Yb,O,SO, to Yb,O, can take place involving the loss of SO, (gas) and this probably could occur via an intermediate modular structure. Hot-stage transmission electron microscopy will be carried out to check whether this possible phase transformation takes place.
Conclusions When ytterbium sulphides, ranging in composition from YbS,,,,(Yb,S,) to YbS,.,,(Yb,S,) are decomposed in air, ytterbium oxysulphide (Yb202S), ytterbium oxysulphate (Yb,O,SO,), and ytterbium oxide (Yb,O, (c-Ln,O,
156
type)) are formed, however, they cannot be isolated as a single phase, except at 1300” C or higher temperatures at which the pure cubic Yb,O, is obtained. Under nitrogen, decomposition yields the ytterbium oxysulphide (Yb,O,S) and Yb,S, as the intermediates and the formation of ytterbium oxysulphate Yb,O,SO, does not take place. A microstructural study of the Yb,O,SO, phase by transmission electron microscopy, reveals a very small particle size and a marked reactiveness towards the electron beam when this product is obtained from the thermal decomposition of the sulphides. Electron diffraction patterns confirm the basic orthorhombic structure for the Yb,O,SO, compound as obtained by X-ray diffraction powder [14]. The microstructure becomes complex owing to the existence of small precipitates of the cubic oxide, Yb,O,, intergrown with the orthorhombic matrix and its formation cannot be avoided in our experiments.
Acknowledgements We are gratefully electron microscopy
indebted studies.
to Mr. A. Garcia
for his assistance
with the
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