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Tetravalent lanthanides—I Sodium praseodymium(IV) fluorides
J. Inorg. Nu¢I. Chem., 1961, Vol. 16, pp. 260 to 262. PergamonPress Ltd. Printedin NorthernIreland
TETRAVALENT LANTHANIDES--I SODIUM PRASEODYMIUM(IV) FLUORIDES*I" L. B. ASPREY a n d T. K . KEENAN Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (Received 29 February 1960)
Abstract--The compounds NaPrF8 and NazPrFe containing tetravalent praseodymium have been prepared by oxidation of the chlorides with elemental fluorine at elevated temperatures. The compounds were identified by X-ray diffraction methods. Further proof of the tetravalency of the praseodymium was obtained by absorption spectrophotometry and by iodometry.
OF the fifteen lanthanide elements, only cerium, praseodymium, and terbium form compounds in which the lanthanide is in the plus four oxidation state. The dioxides of these elements have all been prepared. (1,~,8) The tetrafluorides of cerium and terbium also have been prepared, ~4'5) but similar attempts to prepare PrF 4 were unsuccessful (5,e'~) except in solid solutions with cerium tetrafluoride. (a) In a long series of papers, Professor WILHELMKLEMMand his colleagues(9) have reported the preparation of compounds containing an alkali metal, fluorine, and a transition metal where the transition metal has an oxidation state much higher than usual, i.e. K3NiF~ and KaNiF e. It was felt that such an approach might yield a stable compound containing Pr(IV). The compound KPrF 5 has been reported but has not been completely characterized. (~°l After the completion of this work HOPPEtu) reported in a note the preparation of alkali metal-praseodymium(IV) fluorides where the ratio of alkali metal to praseodymium was two. Few data are given in HOPPE'S brief note although X-ray studies, magnetic susceptibility, and iodide ion oxidation were used to prove the tetravalency of praseodymium in these compounds. In this paper, a study of the sodium-praseodymium fluoride system is reported where the sodium to praseodymium ratios are 1: 1, 2:1 and 4: 1. EXPERIMENTAL The rare earth oxides used in this study were 99.9 per cent pure. All other chemicals were of reagent grade and were used without further purification. The fluorine gas was obtained from * Work done under the authority of the U.S. Atomic Energy Commission, "["Presented in part before the XVIIth International Congress of Pure and Applied Chemistry, 1959. cl) V. M. GOLDSCHM~DT,F. ULgtCH and T. F. W. BARTH, Osloer. Akad. Ber. 5 (1925). Ig) F. SCHERRERand J. PALACIOS,Anal. Fis. Quim. 26, 309 (1928). ca~ D. M. GRUEN, W. C. KOEHLERand J. J. KATZ,J. Amer. Chem. See. 73, 1475 (1951). t~ W. KLEMMand P. HENKrL, Z. Anorg. Chem. 220, 180 (1934). t6~ B. B. CUNMNGHA~, D. C. FLAYand M. G. ROI.LIER,J. Amer. Chem. Soc. 76, 3361 (1954). c6~A. I. PoPov and G. GLOCgI.ER,J. Amer. Chem. Soc. 74, 1357 (1952). ~¢JT. PERROSand C. R. NAESER,J. Amer. Chem. Soc. 74, 3694 (1952). ~a~W. P. BaVAN, UCRL-3894 (Thesis), University of California, Berkeley, Calif. (1957). ~9~For a review, see W. KLEMM,Angew. Chem. 63, 396 (1951); and W. KLEMM,Anorganische Chemie (9th Ed.). W. de Gruyter, Berlin (1956). tl0j T. PERROS,NYO-7597 (1956). tu~ R. HOpPI~,Angew. Chem. 71,457 (1959). 260
Tetravalent lanthanides--I
261
pressurized cylinders and passed through a sodium fluoride-potassium fluoride trap to remove HF. The fluorinator has been described previously,u~ Standard solutions of the rare earths were prepared by dissolving weighed amounts of the oxide in hydrochloric acid. Alkali metal chloride standard solutions were also prepared. Aliquots of these solutions were then mixed to prepare different stoicheiometric ratios of alkali metal to lanthanide These chloride solutions were heated and evaporated to dryness in air in a mullite mortar and then ground to a fine powder. This powder was placed in a well in an optical quality calcium fluoride disk. Fluorination of the mixture was then carried out in a nickel fluorinator at temperatures from 2000 to 400°C over a period of hours. After displacement of fluorine gas from the fluorinator by helium, the samples were removed and quickly capped by a thin, fused-silica disk, using a fluorocarbon grease to insure a seal between the calcium fluoride holder and the silica disk. Such a mounting protected the product from the atmosphere and allowed it to be examined in the model 14 Cary Spectrophotometer. The technique used for spectral determinations has been described in connexion with similar studies on the tetrafluorides of americium and curium, tt3~ An inert atmosphere dr) box was used for preparation of X-ray samples. A conventional thinwalled capillary was employed to mount the samples and showed no evidence of attack over a period of many hours. The patterns were obtained by means of a 114-6 mm diameter camera with Eastman Type A or AA film using filtered copper radiation. An iodometric method was developed using CeF, as a standard material. Aluminium chloride was added to the iodide solution to aid in the dissolution of the fluoride. Pure CeF4 was added to a solution 1 M in KI, 1 M in HCI, and 1 M in A l a s , and the solution was stirred for 1 hr in an argon atmosphere. Consistent values corresponding to 97-98 per cent Ce(IV) were obtained upon titration of the liberated iodine by standard thiosulphate. This same technique was used for analysis of the praseodymium samples. RESULTS AND DISCUSSION Visual, o b s e r v a t i o n showed t h a t the f l u o r i n a t e d p r o d u c t s were white in c o m p a r i s o n with the p a l e green starting materials. I n T a b l e 1 the analytical d a t a are given for the oxidized c o m p o u n d s . TABLE 1.--EVIDENCE FOR THE EXISTENCEOF TETRAVALENTPRASEODYMIUM Na/Pr
Absorption spectra
Iodometric analysis
1:1
weak Pr(III) lines > 85 70 Pr(IV)
>75% Pr(IV)
2:1
very weak Pr(IlI) lines, >95 70 Pr(IV)
>=97 % Pr(IV)
4:1
very weak Pr(lll) lines, >95 70 Pr(IV)
>=97 % Pr(1V)
X-Ray diffraction data NaPrF6, rhombohedral, a = 8'96 ~ 0"02 = 107.9 5:0"2 ° Na~PrFe, orthorhombic, a = 5.54 -4- 0-02 ,~ b -- 3.97 + 0.02A c = 11.57 .-4-0.06A good diffraction pattern, not identified
T h e a b s o r p t i o n spectra values were based on the relative absence o f the P r ( I I I ) m a x i m a in the oxidized c o m p o u n d c o m p a r e d to t h e s t r o n g a b s o r p t i o n r e a p p e a r i n g after h y d r o g e n reduction. The l o w value o f a b o u t 85 p e r cent Pr(IV) for the 1 : 1 c o m p o u n d c o u l d be due to r a p i d a t t a c k b y the a t m o s p h e r e d u r i n g transfer o f the sample, since it was o b s e r v e d t h a t this 1:1 c o m p o u n d was c o n s i d e r a b l y less stable t h a n the o t h e r two. N o distinct structure was observed for the oxidized species over the range 2500-20,000 A, a l t h o u g h increased ultra-violet a b s o r p t i o n was noted. Since Pr(IV) is isoelectronic with Ce(III), the absence o f a b s o r p t i o n in the visible a n d ~12~L. B. AspR~v,J. Amer. Chem. Soc. 76, 2019 (1954).
262
L.B. AspgeY and T. K. K ~ , N
near infra-red is not surprising. In the ultra-violet, due to the increased fight scattering by the crystals, the weaker intensity of the fight source and the greater general absorption by the compounds in the ultra-violet, it is possible that weak narrow peaks may exist, although none were found. Iodometric analysis of the compound containing a 1 : 1 ratio of sodium to praseodymium gave variable results, the highest corresponding to 76 per cent Pr(IV). It was observed that this compound dissolved much more rapidly than the others with a considerable evolution of gas. It seems probable that the low value of the oxidation number obtained was due to the rapid oxidation of water by the Pr0V). X-Ray powder patterns were obtained for the three compounds, and for the products of their reduction. In the case of the sodium to praseodymium ratio of l : I, a rhombohedral structure was found, isostrnctural with NaUFs. c14~ The pattern of this same material after hydrogen reduction at 400°C showed it to be primarily hexagonal NaPrF4 with a = 6.11 ± 0.01 A and c = 3.74 ~ 0.01 A , isostructural with NaPuF4. a~ The compound with a sodium to praseodymium ratio of 2 : 1 was identified as being ?-N~PrF s, isostructural with orthorhombic ?-Na~UFe. a4~ The reduction product was composed of NaPrF4 and NaF. The compound with a sodium to praseodymium ratio of 4 : 1 could not be identified but showed NaPrF4 and NaF to be present after reduction, with more NaF than in the 2 : 1 case. OTHER TETRAVALENT RARE EARTHS
Some evidence has been obtained for the existence of tetravalent neodymium and dysprosium in complex fluorides with the heavy alkali metals, rubidium and cesium. Further work is being carried out on these compounds in collaboration with R. HoPrE and W. K L E ~ at the University of Mtinster. Acknowledgements--Thanks are especially due to Professor JOHN C. B ~ , JR., who originally called our attention to the work of ~ et al. We wish also to thank F. H. ELL~Or.R of this Labora-
tory for the preparation and interpretation of the X-ray powder patterns. (u) W. H. 7_~¢~n~s~N, J. Amer. Chem. Soc. 70, 2147 (1948).
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TETRAVALENT LANTHANIDES--I SODIUM PRASEODYMIUM(IV) FLUORIDES*I" L. B. ASPREY a n d T. K . KEENAN Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (Received 29 February 1960)
Abstract--The compounds NaPrF8 and NazPrFe containing tetravalent praseodymium have been prepared by oxidation of the chlorides with elemental fluorine at elevated temperatures. The compounds were identified by X-ray diffraction methods. Further proof of the tetravalency of the praseodymium was obtained by absorption spectrophotometry and by iodometry.
OF the fifteen lanthanide elements, only cerium, praseodymium, and terbium form compounds in which the lanthanide is in the plus four oxidation state. The dioxides of these elements have all been prepared. (1,~,8) The tetrafluorides of cerium and terbium also have been prepared, ~4'5) but similar attempts to prepare PrF 4 were unsuccessful (5,e'~) except in solid solutions with cerium tetrafluoride. (a) In a long series of papers, Professor WILHELMKLEMMand his colleagues(9) have reported the preparation of compounds containing an alkali metal, fluorine, and a transition metal where the transition metal has an oxidation state much higher than usual, i.e. K3NiF~ and KaNiF e. It was felt that such an approach might yield a stable compound containing Pr(IV). The compound KPrF 5 has been reported but has not been completely characterized. (~°l After the completion of this work HOPPEtu) reported in a note the preparation of alkali metal-praseodymium(IV) fluorides where the ratio of alkali metal to praseodymium was two. Few data are given in HOPPE'S brief note although X-ray studies, magnetic susceptibility, and iodide ion oxidation were used to prove the tetravalency of praseodymium in these compounds. In this paper, a study of the sodium-praseodymium fluoride system is reported where the sodium to praseodymium ratios are 1: 1, 2:1 and 4: 1. EXPERIMENTAL The rare earth oxides used in this study were 99.9 per cent pure. All other chemicals were of reagent grade and were used without further purification. The fluorine gas was obtained from * Work done under the authority of the U.S. Atomic Energy Commission, "["Presented in part before the XVIIth International Congress of Pure and Applied Chemistry, 1959. cl) V. M. GOLDSCHM~DT,F. ULgtCH and T. F. W. BARTH, Osloer. Akad. Ber. 5 (1925). Ig) F. SCHERRERand J. PALACIOS,Anal. Fis. Quim. 26, 309 (1928). ca~ D. M. GRUEN, W. C. KOEHLERand J. J. KATZ,J. Amer. Chem. See. 73, 1475 (1951). t~ W. KLEMMand P. HENKrL, Z. Anorg. Chem. 220, 180 (1934). t6~ B. B. CUNMNGHA~, D. C. FLAYand M. G. ROI.LIER,J. Amer. Chem. Soc. 76, 3361 (1954). c6~A. I. PoPov and G. GLOCgI.ER,J. Amer. Chem. Soc. 74, 1357 (1952). ~¢JT. PERROSand C. R. NAESER,J. Amer. Chem. Soc. 74, 3694 (1952). ~a~W. P. BaVAN, UCRL-3894 (Thesis), University of California, Berkeley, Calif. (1957). ~9~For a review, see W. KLEMM,Angew. Chem. 63, 396 (1951); and W. KLEMM,Anorganische Chemie (9th Ed.). W. de Gruyter, Berlin (1956). tl0j T. PERROS,NYO-7597 (1956). tu~ R. HOpPI~,Angew. Chem. 71,457 (1959). 260
Tetravalent lanthanides--I
261
pressurized cylinders and passed through a sodium fluoride-potassium fluoride trap to remove HF. The fluorinator has been described previously,u~ Standard solutions of the rare earths were prepared by dissolving weighed amounts of the oxide in hydrochloric acid. Alkali metal chloride standard solutions were also prepared. Aliquots of these solutions were then mixed to prepare different stoicheiometric ratios of alkali metal to lanthanide These chloride solutions were heated and evaporated to dryness in air in a mullite mortar and then ground to a fine powder. This powder was placed in a well in an optical quality calcium fluoride disk. Fluorination of the mixture was then carried out in a nickel fluorinator at temperatures from 2000 to 400°C over a period of hours. After displacement of fluorine gas from the fluorinator by helium, the samples were removed and quickly capped by a thin, fused-silica disk, using a fluorocarbon grease to insure a seal between the calcium fluoride holder and the silica disk. Such a mounting protected the product from the atmosphere and allowed it to be examined in the model 14 Cary Spectrophotometer. The technique used for spectral determinations has been described in connexion with similar studies on the tetrafluorides of americium and curium, tt3~ An inert atmosphere dr) box was used for preparation of X-ray samples. A conventional thinwalled capillary was employed to mount the samples and showed no evidence of attack over a period of many hours. The patterns were obtained by means of a 114-6 mm diameter camera with Eastman Type A or AA film using filtered copper radiation. An iodometric method was developed using CeF, as a standard material. Aluminium chloride was added to the iodide solution to aid in the dissolution of the fluoride. Pure CeF4 was added to a solution 1 M in KI, 1 M in HCI, and 1 M in A l a s , and the solution was stirred for 1 hr in an argon atmosphere. Consistent values corresponding to 97-98 per cent Ce(IV) were obtained upon titration of the liberated iodine by standard thiosulphate. This same technique was used for analysis of the praseodymium samples. RESULTS AND DISCUSSION Visual, o b s e r v a t i o n showed t h a t the f l u o r i n a t e d p r o d u c t s were white in c o m p a r i s o n with the p a l e green starting materials. I n T a b l e 1 the analytical d a t a are given for the oxidized c o m p o u n d s . TABLE 1.--EVIDENCE FOR THE EXISTENCEOF TETRAVALENTPRASEODYMIUM Na/Pr
Absorption spectra
Iodometric analysis
1:1
weak Pr(III) lines > 85 70 Pr(IV)
>75% Pr(IV)
2:1
very weak Pr(IlI) lines, >95 70 Pr(IV)
>=97 % Pr(IV)
4:1
very weak Pr(lll) lines, >95 70 Pr(IV)
>=97 % Pr(1V)
X-Ray diffraction data NaPrF6, rhombohedral, a = 8'96 ~ 0"02 = 107.9 5:0"2 ° Na~PrFe, orthorhombic, a = 5.54 -4- 0-02 ,~ b -- 3.97 + 0.02A c = 11.57 .-4-0.06A good diffraction pattern, not identified
T h e a b s o r p t i o n spectra values were based on the relative absence o f the P r ( I I I ) m a x i m a in the oxidized c o m p o u n d c o m p a r e d to t h e s t r o n g a b s o r p t i o n r e a p p e a r i n g after h y d r o g e n reduction. The l o w value o f a b o u t 85 p e r cent Pr(IV) for the 1 : 1 c o m p o u n d c o u l d be due to r a p i d a t t a c k b y the a t m o s p h e r e d u r i n g transfer o f the sample, since it was o b s e r v e d t h a t this 1:1 c o m p o u n d was c o n s i d e r a b l y less stable t h a n the o t h e r two. N o distinct structure was observed for the oxidized species over the range 2500-20,000 A, a l t h o u g h increased ultra-violet a b s o r p t i o n was noted. Since Pr(IV) is isoelectronic with Ce(III), the absence o f a b s o r p t i o n in the visible a n d ~12~L. B. AspR~v,J. Amer. Chem. Soc. 76, 2019 (1954).
262
L.B. AspgeY and T. K. K ~ , N
near infra-red is not surprising. In the ultra-violet, due to the increased fight scattering by the crystals, the weaker intensity of the fight source and the greater general absorption by the compounds in the ultra-violet, it is possible that weak narrow peaks may exist, although none were found. Iodometric analysis of the compound containing a 1 : 1 ratio of sodium to praseodymium gave variable results, the highest corresponding to 76 per cent Pr(IV). It was observed that this compound dissolved much more rapidly than the others with a considerable evolution of gas. It seems probable that the low value of the oxidation number obtained was due to the rapid oxidation of water by the Pr0V). X-Ray powder patterns were obtained for the three compounds, and for the products of their reduction. In the case of the sodium to praseodymium ratio of l : I, a rhombohedral structure was found, isostrnctural with NaUFs. c14~ The pattern of this same material after hydrogen reduction at 400°C showed it to be primarily hexagonal NaPrF4 with a = 6.11 ± 0.01 A and c = 3.74 ~ 0.01 A , isostructural with NaPuF4. a~ The compound with a sodium to praseodymium ratio of 2 : 1 was identified as being ?-N~PrF s, isostructural with orthorhombic ?-Na~UFe. a4~ The reduction product was composed of NaPrF4 and NaF. The compound with a sodium to praseodymium ratio of 4 : 1 could not be identified but showed NaPrF4 and NaF to be present after reduction, with more NaF than in the 2 : 1 case. OTHER TETRAVALENT RARE EARTHS
Some evidence has been obtained for the existence of tetravalent neodymium and dysprosium in complex fluorides with the heavy alkali metals, rubidium and cesium. Further work is being carried out on these compounds in collaboration with R. HoPrE and W. K L E ~ at the University of Mtinster. Acknowledgements--Thanks are especially due to Professor JOHN C. B ~ , JR., who originally called our attention to the work of ~ et al. We wish also to thank F. H. ELL~Or.R of this Labora-
tory for the preparation and interpretation of the X-ray powder patterns. (u) W. H. 7_~¢~n~s~N, J. Amer. Chem. Soc. 70, 2147 (1948).
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