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The thermal decomposition of the heavier rare earth metal chloride hydrates
J. lnorg. Nu¢l. Chem., 1959, Vol. 9, pp. 136 to 139. Pergamon Press Ltd. Printed in Northern Ireland
THE THERMAL DECOMPOSITION OF THE HEAVIER RARE EARTH METAL CHLORIDE HYDRATES W . W . WENDLANDT
Department of Chemistry and Chemical Engineering Texas Technological College, Lubbock, Texas (Received 5 May 1958; in revised form 30 June 1958)
Abstract--The thermal decomposition of the 6-hydrates of europium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium chlorides was studied on the thermobalance. The 6hydrates began to lose water of hydration in the 65° to 95°C temperature range. Because of the decreasing basicity of the metal ions, horizontal weight levels corresponding to the anhydrous metal chlorides were not found. Instead, the decomposition curves in the 200° to 265°C temperature range corresponded to the composition, MOCI'2MCls. On increasing the temperature, these compounds decomposed to form the metal oxychlorides in the 360° to 425°C temperature range. THE thermal decomposition o f fhe yttrium, scandium, and lighter rare earth metal chloride hydrates has previously been discussed, tl~ It was f o u n d that the pyrolysis, in air, followed the general pattern: MC13.6H20 ~ MCla.nH20 ~ MC1 z ~ MOC1. The m o r e basic metal chloride hydrates decomposed to f o r m weight levels o f almost pure anhydrous chloride; while as the basicity o f the metal ions decreased, the MC1 a levels contained increasingly larger a m o u n t s o f oxychloride content. However, along with decreasing basicity, the tendency to f o r m intermediate metal chloride hydrates, MCla'nH20, increased. It was evident that the less basic ions, which are also smaller in radii, f o r m e d the more stable intermediate hydrates. The terminal decomposition product, except in the case o f scandium and cerium (III), was the metal oxychloride. It was f o u n d that the m i n i m u m oxychloride level temperatures decreased in the order:
La ~> Pr ~ N d ~ Sm ~ Y ~ Gd. With scandium and cerium, the terminal products were the oxides, Sc20 3 and CeOz, respectively. T o complete the study o f this series, this investigation is concerned with the thermal decomposition o f the heavier rare earth metal chloride h y d r a t e s - - t h o s e f r o m e u r o p i u m (excluding gadolinium) to lutetium. It was o f interest to extend the trends revealed in the thermal decomposition patterns o f the lighter rare earths to those o f the heavier rare earth chlorides. EXPERIMENTAL Thermobalance. An automatic recording thermobalance as previously described was used. ~2~ Sample sizes ranged from 90 to 100 mg with a linear heating rate of 5-4°C per rain. A slow stream of air was passed through the furnace during the pyrolysis. Samples were run in duplicate or triplicate. Chemicals. The rare earths were obtained as the oxides of 99.9 per cent purity from the St. Eloi Chemical Corp., Newtown, Ohio, and the Lindsay Chemical Co., West Chicago, Ill. i1) W. W. W~YDLANDT,J. lnorg. Nucl. Chem. 5, 118 (1957). :zl W. W. WENDLANDT,Analyt. Chem. 30, 56 (1958). 136
The thermal decomposition of the heavier rare earth metal chloride hydrates
137
Preparation of the rare earth chloride hydrates. The same general procedure as previously described was used. m DISCUSSION The t h e r m a l d e c o m p o s i t i o n curves for the heavier rare e a r t h m e t a l c h l o r i d e h y d r a t e s are given in Figs. 1 a n d 2. The c o m p o s i t i o n d a t a is given in T a b l e 1. TABLE I . - - T H E
THERMAL DECOMPOSITION OF THE RARE EARTH CHLORIDE H Y D R A T E S - W E I G H T LOSS DATA
Weight loss (%) Rare earth chloride 6-hydrate
MC13 I '
Theor.
Found
MOCI-2MCIa Theor.
Found
Europium
34"49 33"4 33"0
Terbium
33"85 33"8(250°) 34"6 32"89 33"3(235°) 32'9 33 "7(220°) 33"32 33'9 33'12 32"5(220°) 33"2 33"5(225°) 32'97 32'9 32'62 31"7(205°) 32"4 32"46 32'9(200 °) 31"5
Dysprosium Holmium
28 '49
28"3(175 °) 28"0
Erbium Thulium Ytterbium Lutetium
28-20
29'1(185 °) 29"1
MOC1 Theor.
Found
~'48
44'7 ~'1
43 "66 43 "24 42"97 42"70 42"51 42'07 41"86
Other Theor.
Found
26"76 27"5(165°) 26"8 as EuCI3.O.5H20
43'3 44"2 42"8 42"4 42"6 42"4 42"4 42"6 42"8 42'8 41 "4 41 '7 42'7 41"9
Europium (Fig. 1D). The initial c o m p o u n d b e g a n to lose a b s o r b e d w a t e r at a little a b o v e r o o m temperature. A f t e r the loss o f this water, a h o r i z o n t a l weight level was observed f r o m 50 ° to 80°C which c o r r e s p o n d e d to the f o r m u l a , EuClz.6H20. The 6-hydrate b e g a n to evolve water o f h y d r a t i o n at 80°C, giving a b r e a k in the curve at 165°C which c o r r e s p o n d e d a p p r o x i m a t e l y to the c o m p o s i t i o n , EuC13.0-5It20. F u r t h e r weight loss then t o o k place to give a n o t h e r h o r i z o n t a l weight level, f r o m 225 ° to 265°C, which c o r r e s p o n d e d r o u g h l y to a mixed metal o x y c h l o r i d e - c h l o r i d e , EuOC1.2EuCI 3. A b o v e 265°C, further d e c o m p o s i t i o n ensued, resulting in the EuOC1 level at 380°C. It is interesting to n o t e that an a n h y d r o u s m e t a l chloride level was n o t o b t a i n e d ; instead, the c o m p o s i t i o n a p p r o a c h e d t h a t o f the 0.5-hydrate. The existence o f a m i x e d metal o x y c h l o r i d e - c h l o r i d e was also observed, the presence o f which is also i n d i c a t e d in the o t h e r heavier rare e a r t h chlorides. Terbium (Fig. 2B). The h y d r a t e , TbC13.6H20, began to evolve w a t e r o f h y d r a t i o n at 65°C. A f t e r a b r e a k in the curve at 200°C, a h o r i z o n t a l weight level was observed f r o m 250 ° to 300°C. The c o m p o s i t i o n o f the curve in this region c o r r e s p o n d e d
138
W . W . WENDLAND1"
approximately to the formula, TbOC1.2TbC1a. Above 300°C, further decomposition took place to give the TbOC1 level at 425°C. At elevated temperatures, namely 715°C, the oxychloride began to decompose to form the oxide, Tb407. However, since the thermoba.lance has an upper furnace temperature limit of ~850°C, the conversion, TbOC1 ~ Tb407, was not completed. Dysprosium (Fig. 2D) The hydrate, DyCla'6HzO, began to evolve water of hydration at 90°C. Breaks were observed in the curve at two points, 190° and 235°C.
559(
A
\
2_50
\ z25~
zsS~_
3MG
~36o
B
c
sos .'-~-----~
265 X,......580 D
785 675
FIG. 1.--Thermograms of the rare earth metal chloride 6-hydrates. A. H o l m i u m B. Thulium C. Lutetium D. Europium
_
C
~o~
~,.<.~:s6s sks--'-'m 235 " ~ 3 9 0
,
D
790
s?o _w~o
FIG. 2 . - - T h e r m o g r a m s of the rare earth metal chloride 6-hydrates. A. Erbium B. Terbium C. Ytterbium D. Dysprosium
The composition of the curve at the 190°C break is not known but at the 235°C break, it corresponded approximately to the formula, DyOC1.2DyC13. No horizontal weight level was observed but instead, a gradual weight loss took place. This weight loss became more rapid above 300°C, resulting in the DyOC1 level at 390°C. Decomposition to the oxide, Dy203, began to take place above 590°C, however, as in the case of terbium, the process was not completed in the temperature range employed. Holmium (Fig. 1A). The .hydrate, HoCla.6H20 , began to evolve water of hydration at 75°C. Two breaks were observed in the curve, one at 175°C, the other at 220°C. The composition of the curve at the 175°C break corresponded approximately to the formula, HoC13. At the 220°C curve break, the composition approached the formula, HoOC1.2HoC13. A gradual weight loss then took place above 220°C, followed by a more rapid weight loss above 305°C, resulting in the HoOC1 level at 360°C. Decomposition to the oxide, Ho203, began to take place above 510°C. Erbium (Fig. 2A). The hydrate, ErCla.6H20, was the most stable of all of the compounds studied. Water of hydration began to come off at 95°C, giving breaks in the curve at 175° and 220°C. The composition of the curve at the 175°C break is unknown, but presumably, it is the anhydrous chloride. The 220°C curve break corresponded approximately to the composition, ErOC1-2ErC1a. After further weight loss, the ErOC1 level was obtained at 380°C. The decomposition to the oxide, Er~Oa, began to take place above 550°C. It is worthy to note that the erbium curve is remarkably similar to the holimum curve.
The thermal decompositionof the heavierrare earth metal chloride hydrates
139
Thulium (Fig. 1B). The initial compound began to lose absorbed water at about 40°C. This was followed by a horizontal weight level, from 55° to 90°C, which corresponded to the composition, TmC13.6H~O. Above 90°C, the 6-hydrate began to evolve water of hydration, giving breaks in the curve at 185° and 225°C. The curve compositions for these breaks approximated the formulae, TmCl3and TmOC1.2TmC13, respectively. Further weight loss above 225°C resulted in the TmOC1 level at 405°C. The decomposition to the oxide, Tm~Oa, began at 535°C. Ytterbium (Fig. 2C). The hydrate, YbC13.6H20, began to evolve water of hydration at 90°C. Two intermediate breaks were then observed in the curve, one at 145°C, the other at 205°C. The composition of the curve at the 145°C break is unknown, but at the 205°C break, the composition corresponded approximately to the formula, YbOC1-2YbC13. Further weight loss took place above 205°C, restflting in the YbOC1 level at 395°C. The decomposition to the oxide, Yb203, began to take place above 585°C. Lutetium (Fig. 1C). The hydrate, LuCI3"6H20, began to evolve water of hydration at 90°C. Two intermediate breaks were then observed in the curve, one at 135°C, the other at 200°C. The thermogram pattern and the decomposition temperatures closely paralleled the ytterbium curve. The composition of the curve at the 135°C break is unknown, but at the 200°C break, the composition corresponded approximately to the formula, LuOC1.2LuC13. After further weight loss above 200°C, the LuOC1 level began at 390°C. The decomposition to the oxide, Lu203, began at 505°C. It should be noted that lutetium possessed the shortest oxychloride level of all of the rare earths studied. General observations. The thermal decomposition patterns for the heavier rare earth chloride hydrates are those that would be expected for the transition to decreasing basicity and decreasing ionic radii of the metal ions. There was little evidence for the formation of weight levels for stable intermediate hydrates or for the anhydrous metal chlorides. Evidence was observed for the formation of compounds intermediate between the metal chloride and the oxychloride. These compounds were of the general formula, MOC1.2MC13. Horizontal weight levels were found, in all cases, for the metal oxychloride. However, as the temperature was increased, it was found that the oxychlorides began to decompose to form the metal oxides. Unfortunately, the upper limit of the thermobalance furnace did not permit the attainment of the pure metal oxide weight levels. Acknowled¢ement---It is a pleasure to acknowledgeRICHARDMANDLEof the Davison ChemicalCo., and the LindsayChemicalCo., for the samples of the thuliumand lutetiumoxides.
THE THERMAL DECOMPOSITION OF THE HEAVIER RARE EARTH METAL CHLORIDE HYDRATES W . W . WENDLANDT
Department of Chemistry and Chemical Engineering Texas Technological College, Lubbock, Texas (Received 5 May 1958; in revised form 30 June 1958)
Abstract--The thermal decomposition of the 6-hydrates of europium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium chlorides was studied on the thermobalance. The 6hydrates began to lose water of hydration in the 65° to 95°C temperature range. Because of the decreasing basicity of the metal ions, horizontal weight levels corresponding to the anhydrous metal chlorides were not found. Instead, the decomposition curves in the 200° to 265°C temperature range corresponded to the composition, MOCI'2MCls. On increasing the temperature, these compounds decomposed to form the metal oxychlorides in the 360° to 425°C temperature range. THE thermal decomposition o f fhe yttrium, scandium, and lighter rare earth metal chloride hydrates has previously been discussed, tl~ It was f o u n d that the pyrolysis, in air, followed the general pattern: MC13.6H20 ~ MCla.nH20 ~ MC1 z ~ MOC1. The m o r e basic metal chloride hydrates decomposed to f o r m weight levels o f almost pure anhydrous chloride; while as the basicity o f the metal ions decreased, the MC1 a levels contained increasingly larger a m o u n t s o f oxychloride content. However, along with decreasing basicity, the tendency to f o r m intermediate metal chloride hydrates, MCla'nH20, increased. It was evident that the less basic ions, which are also smaller in radii, f o r m e d the more stable intermediate hydrates. The terminal decomposition product, except in the case o f scandium and cerium (III), was the metal oxychloride. It was f o u n d that the m i n i m u m oxychloride level temperatures decreased in the order:
La ~> Pr ~ N d ~ Sm ~ Y ~ Gd. With scandium and cerium, the terminal products were the oxides, Sc20 3 and CeOz, respectively. T o complete the study o f this series, this investigation is concerned with the thermal decomposition o f the heavier rare earth metal chloride h y d r a t e s - - t h o s e f r o m e u r o p i u m (excluding gadolinium) to lutetium. It was o f interest to extend the trends revealed in the thermal decomposition patterns o f the lighter rare earths to those o f the heavier rare earth chlorides. EXPERIMENTAL Thermobalance. An automatic recording thermobalance as previously described was used. ~2~ Sample sizes ranged from 90 to 100 mg with a linear heating rate of 5-4°C per rain. A slow stream of air was passed through the furnace during the pyrolysis. Samples were run in duplicate or triplicate. Chemicals. The rare earths were obtained as the oxides of 99.9 per cent purity from the St. Eloi Chemical Corp., Newtown, Ohio, and the Lindsay Chemical Co., West Chicago, Ill. i1) W. W. W~YDLANDT,J. lnorg. Nucl. Chem. 5, 118 (1957). :zl W. W. WENDLANDT,Analyt. Chem. 30, 56 (1958). 136
The thermal decomposition of the heavier rare earth metal chloride hydrates
137
Preparation of the rare earth chloride hydrates. The same general procedure as previously described was used. m DISCUSSION The t h e r m a l d e c o m p o s i t i o n curves for the heavier rare e a r t h m e t a l c h l o r i d e h y d r a t e s are given in Figs. 1 a n d 2. The c o m p o s i t i o n d a t a is given in T a b l e 1. TABLE I . - - T H E
THERMAL DECOMPOSITION OF THE RARE EARTH CHLORIDE H Y D R A T E S - W E I G H T LOSS DATA
Weight loss (%) Rare earth chloride 6-hydrate
MC13 I '
Theor.
Found
MOCI-2MCIa Theor.
Found
Europium
34"49 33"4 33"0
Terbium
33"85 33"8(250°) 34"6 32"89 33"3(235°) 32'9 33 "7(220°) 33"32 33'9 33'12 32"5(220°) 33"2 33"5(225°) 32'97 32'9 32'62 31"7(205°) 32"4 32"46 32'9(200 °) 31"5
Dysprosium Holmium
28 '49
28"3(175 °) 28"0
Erbium Thulium Ytterbium Lutetium
28-20
29'1(185 °) 29"1
MOC1 Theor.
Found
~'48
44'7 ~'1
43 "66 43 "24 42"97 42"70 42"51 42'07 41"86
Other Theor.
Found
26"76 27"5(165°) 26"8 as EuCI3.O.5H20
43'3 44"2 42"8 42"4 42"6 42"4 42"4 42"6 42"8 42'8 41 "4 41 '7 42'7 41"9
Europium (Fig. 1D). The initial c o m p o u n d b e g a n to lose a b s o r b e d w a t e r at a little a b o v e r o o m temperature. A f t e r the loss o f this water, a h o r i z o n t a l weight level was observed f r o m 50 ° to 80°C which c o r r e s p o n d e d to the f o r m u l a , EuClz.6H20. The 6-hydrate b e g a n to evolve water o f h y d r a t i o n at 80°C, giving a b r e a k in the curve at 165°C which c o r r e s p o n d e d a p p r o x i m a t e l y to the c o m p o s i t i o n , EuC13.0-5It20. F u r t h e r weight loss then t o o k place to give a n o t h e r h o r i z o n t a l weight level, f r o m 225 ° to 265°C, which c o r r e s p o n d e d r o u g h l y to a mixed metal o x y c h l o r i d e - c h l o r i d e , EuOC1.2EuCI 3. A b o v e 265°C, further d e c o m p o s i t i o n ensued, resulting in the EuOC1 level at 380°C. It is interesting to n o t e that an a n h y d r o u s m e t a l chloride level was n o t o b t a i n e d ; instead, the c o m p o s i t i o n a p p r o a c h e d t h a t o f the 0.5-hydrate. The existence o f a m i x e d metal o x y c h l o r i d e - c h l o r i d e was also observed, the presence o f which is also i n d i c a t e d in the o t h e r heavier rare e a r t h chlorides. Terbium (Fig. 2B). The h y d r a t e , TbC13.6H20, began to evolve w a t e r o f h y d r a t i o n at 65°C. A f t e r a b r e a k in the curve at 200°C, a h o r i z o n t a l weight level was observed f r o m 250 ° to 300°C. The c o m p o s i t i o n o f the curve in this region c o r r e s p o n d e d
138
W . W . WENDLAND1"
approximately to the formula, TbOC1.2TbC1a. Above 300°C, further decomposition took place to give the TbOC1 level at 425°C. At elevated temperatures, namely 715°C, the oxychloride began to decompose to form the oxide, Tb407. However, since the thermoba.lance has an upper furnace temperature limit of ~850°C, the conversion, TbOC1 ~ Tb407, was not completed. Dysprosium (Fig. 2D) The hydrate, DyCla'6HzO, began to evolve water of hydration at 90°C. Breaks were observed in the curve at two points, 190° and 235°C.
559(
A
\
2_50
\ z25~
zsS~_
3MG
~36o
B
c
sos .'-~-----~
265 X,......580 D
785 675
FIG. 1.--Thermograms of the rare earth metal chloride 6-hydrates. A. H o l m i u m B. Thulium C. Lutetium D. Europium
_
C
~o~
~,.<.~:s6s sks--'-'m 235 " ~ 3 9 0
,
D
790
s?o _w~o
FIG. 2 . - - T h e r m o g r a m s of the rare earth metal chloride 6-hydrates. A. Erbium B. Terbium C. Ytterbium D. Dysprosium
The composition of the curve at the 190°C break is not known but at the 235°C break, it corresponded approximately to the formula, DyOC1.2DyC13. No horizontal weight level was observed but instead, a gradual weight loss took place. This weight loss became more rapid above 300°C, resulting in the DyOC1 level at 390°C. Decomposition to the oxide, Dy203, began to take place above 590°C, however, as in the case of terbium, the process was not completed in the temperature range employed. Holmium (Fig. 1A). The .hydrate, HoCla.6H20 , began to evolve water of hydration at 75°C. Two breaks were observed in the curve, one at 175°C, the other at 220°C. The composition of the curve at the 175°C break corresponded approximately to the formula, HoC13. At the 220°C curve break, the composition approached the formula, HoOC1.2HoC13. A gradual weight loss then took place above 220°C, followed by a more rapid weight loss above 305°C, resulting in the HoOC1 level at 360°C. Decomposition to the oxide, Ho203, began to take place above 510°C. Erbium (Fig. 2A). The hydrate, ErCla.6H20, was the most stable of all of the compounds studied. Water of hydration began to come off at 95°C, giving breaks in the curve at 175° and 220°C. The composition of the curve at the 175°C break is unknown, but presumably, it is the anhydrous chloride. The 220°C curve break corresponded approximately to the composition, ErOC1-2ErC1a. After further weight loss, the ErOC1 level was obtained at 380°C. The decomposition to the oxide, Er~Oa, began to take place above 550°C. It is worthy to note that the erbium curve is remarkably similar to the holimum curve.
The thermal decompositionof the heavierrare earth metal chloride hydrates
139
Thulium (Fig. 1B). The initial compound began to lose absorbed water at about 40°C. This was followed by a horizontal weight level, from 55° to 90°C, which corresponded to the composition, TmC13.6H~O. Above 90°C, the 6-hydrate began to evolve water of hydration, giving breaks in the curve at 185° and 225°C. The curve compositions for these breaks approximated the formulae, TmCl3and TmOC1.2TmC13, respectively. Further weight loss above 225°C resulted in the TmOC1 level at 405°C. The decomposition to the oxide, Tm~Oa, began at 535°C. Ytterbium (Fig. 2C). The hydrate, YbC13.6H20, began to evolve water of hydration at 90°C. Two intermediate breaks were then observed in the curve, one at 145°C, the other at 205°C. The composition of the curve at the 145°C break is unknown, but at the 205°C break, the composition corresponded approximately to the formula, YbOC1-2YbC13. Further weight loss took place above 205°C, restflting in the YbOC1 level at 395°C. The decomposition to the oxide, Yb203, began to take place above 585°C. Lutetium (Fig. 1C). The hydrate, LuCI3"6H20, began to evolve water of hydration at 90°C. Two intermediate breaks were then observed in the curve, one at 135°C, the other at 200°C. The thermogram pattern and the decomposition temperatures closely paralleled the ytterbium curve. The composition of the curve at the 135°C break is unknown, but at the 200°C break, the composition corresponded approximately to the formula, LuOC1.2LuC13. After further weight loss above 200°C, the LuOC1 level began at 390°C. The decomposition to the oxide, Lu203, began at 505°C. It should be noted that lutetium possessed the shortest oxychloride level of all of the rare earths studied. General observations. The thermal decomposition patterns for the heavier rare earth chloride hydrates are those that would be expected for the transition to decreasing basicity and decreasing ionic radii of the metal ions. There was little evidence for the formation of weight levels for stable intermediate hydrates or for the anhydrous metal chlorides. Evidence was observed for the formation of compounds intermediate between the metal chloride and the oxychloride. These compounds were of the general formula, MOC1.2MC13. Horizontal weight levels were found, in all cases, for the metal oxychloride. However, as the temperature was increased, it was found that the oxychlorides began to decompose to form the metal oxides. Unfortunately, the upper limit of the thermobalance furnace did not permit the attainment of the pure metal oxide weight levels. Acknowled¢ement---It is a pleasure to acknowledgeRICHARDMANDLEof the Davison ChemicalCo., and the LindsayChemicalCo., for the samples of the thuliumand lutetiumoxides.