Preparation, Structure and Spectra of Some Tetravalent

9 Preparation, Structure and Spectra of Some

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 20, 2015 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch009

Tetravalent Praseodymium Compounds LARNED B. ASPREY, JAMES S. COLEMAN, and MARTIN J. REISFELD University of California, Los Alamos Scientific Laboratory, Los Alamos, Ν. M .

Several fluoride complexes of tetravalent praseodymium have been prepared, and their lattice parameters and ab sorption spectra have been ascertained. The compounds Na Pr F and Na PrF were prepared by reaction of ap propriate mixtures of PrF and NaF in the presence of fluorine gas. PrF was prepared by extracting NaF from Na PrF with liquid anhydrous HF in a fluorine atmos phere. The space groups are: Na Pr F is rhombohedral (R3); Na PrF is orthorhombic (Immm); and PrF is monoclinic (C2/c). The spectra show characteristic absorption peaks at 2893, 2873, and 2889 cm. for the three com pounds, corresponding to the electronic transition F —> F , and lead to a value of ζ = 824 ± 2 cm. . 7

6

31

2

6

3

4

2

6

7

2

6

31

6

4

-1

2

5/2

2

-1

7/2

4f

/^\f a l l t h e l a n t h a n i d e elements, o n l y a f e w f o r m c o m p o u n d s i n w h i c h the l a n t h a n i d e is i n t h e p l u s f o u r o x i d a t i o n state. T h e dioxides of c e r i u m , p r a s e o d y m i u m , a n d t e r b i u m h a v e a l l b e e n p r e p a r e d , as h a v e t h e tetrafluorides of c e r i u m a n d t e r b i u m . n i t r a t e c o m p o u n d s of P r

4 +

ports t h e p r e p a r a t i o n of p u r e P r F spectroscopic data.

Complex

c h l o r i d e , sulfate, a n d

h a v e also b e e n r e p o r t e d ( 2 ) . T h i s p a p e r r e 4

a l o n g w i t h l a t t i c e constants a n d

T h e compounds N a P r F i 7

6

3

and Na PrF 2

have

6

also

been made, a n d similar data are given.

Experimental T h e s o d i u m c o m p o u n d s c o n t a i n i n g tetravalent P r w e r e p r e p a r e d b y h e a t i n g i n t i m a t e m i x t u r e s of P r F a n d t h e a l k a l i fluorides i n t h e d e s i r e d s t o i c h i o m e t r i c ratios i n t h e presence of fluorine gas ( I ). P r F w a s p r e p a r e d b y a d d i n g a q u e o u s H F to a c i d solutions of P r ( I I I ) , f o l l o w e d b y t r e a t i n g t h e a i r - d r i e d s o l i d w i t h fluorine gas at 4 0 0 ° C . f o r several hours. S o d i u m fluoride w a s u s e d w i t h o u t f u r t h e r treatment. T h e t w o a n h y d r o u s 3

3

122 In Lanthanide/Actinide Chemistry; Fields, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

9.

A S P R E Y

E T

Tetravalent

A L .

Praseodymium

123

Compounds

c o m p o u n d s , N a F a n d P r F , w e r e t h e n g r o u n d together i n m u l l i t e m o r t a r s i n a n i n e r t atmosphere. T h e fluorination reactions w e r e c a r r i e d out i n a n i c k e l reactor. Inert a l u m i n u m o x i d e containers w e r e u s e d i n the fluori n a t i o n . T h e m i x t u r e s w e r e fluorinated i n t h e t e m p e r a t u r e range of 3 0 0 5 0 0 ° C . at 1-4 a t m . of fluorine for t w o or m o r e hours. A n a l t e r n a t i v e m e t h o d for p r e p a r i n g N a P r F was also e m p l o y e d . A m i x t u r e of P r O n a n d N a F i n the d e s i r e d s t o i c h i o m e t r i c p r o p o r t i o n s w a s p l a c e d i n t h e reactor. T h e m i x t u r e w a s fluorinated d i r e c t l y at 4 5 0 ° C . for several h o u r s u n d e r a fluorine pressure of 2 a t m . T h e p r o d u c t o b t a i n e d i n this m a n n e r w a s N a P r F , i d e n t i c a l to t h a t p r e p a r e d b y the P r F p l u s N a F t e c h n i q u e . T o p r e p a r e P r F , a different t e c h n i q u e w a s necessary ( 3 ) . A s a m p l e of N a 2 P r F o b t a i n e d b y one of the p r e v i o u s m e t h o d s w a s p l a c e d i n a K e l - F t u b e c o n n e c t e d to the gas h a n d l i n g system. A p l u g of T e f l o n w o o l w a s p l a c e d over the sample. I n the presence of 1/2 a t m . of fluorine gas, H F w a s c o n d e n s e d onto the s a m p l e . A f t e r several m i n u t e s , the t u b e w a s i n v e r t e d , a n d the l i q u i d w a s filtered off t h r o u g h the w o o l p l u g . T h e H F p l u s fluorine treatment w a s r e p e a t e d three m o r e times. T h i s w a s h i n g process w h i c h e x t r a c t e d the N a F f r o m the N a P r F p r o v e d sufficient to generate a s a m p l e w h o s e net w e i g h t c o r r e s p o n d e d w i t h i n 1 % of t h a t e x p e c t e d for a c o n v e r s i o n to P r F . 3


2

2

6

e

6

3

4

6

2

6

4

Results X - r a y p o w d e r patterns w e r e o b t a i n e d f o r the three P r ( I V ) pounds.

For Na Pr F i, 7

6

a rhombohedral

3

isostructural w i t h N a U F i . 7

orthorhombic y - N a U F 2

6

6

3

Na PrF 2

(Immm).

6

(R3)

structure w a s

com found,

w a s f o u n d to b e i s o s t r u c t u r a l w i t h

T h e m a t e r i a l i d e n t i f i e d as P r F

f o u n d to b e i s o s t r u c t u r a l w i t h m o n o c l i n i c U F

4

(C2/c).

4

was

The derived

lattice parameters a n d u n i t c e l l v o l u m e s are g i v e n i n T a b l e I.

Table I. Compound PrF 4

Na Pr F 7

6

3 1

Structure and Lattice Parameters for P r Structure and Space Group Monoclinic ( C 2 / c )

Rhombohedral (R3)

4 +

Compounds

Lattice Parameters 12.47 A . h = 10.54 A . β = 126.4° c = 8.18 A . a =

Volume (A ) 865.4 3

583.6

8.96 A .

. a — 107.9°

Na PrF 2

A

6

Orthorhombic (Immm)

computation was made

254.5

5.54 A . a = b = 3.97 A . c = 11.57 A .

of the nearest-neighbor

distances

and

b o n d angles i n v o l v i n g the c e n t r a l p r a s e o d y m i u m i o n i n o r d e r to ascertain the s t r u c t u r e a n d s y m m e t r y of the first c o o r d i n a t i o n spheres for the three compounds.

T h e s e structures are d e p i c t e d i n F i g u r e 1.

In Lanthanide/Actinide Chemistry; Fields, P., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

124

L A N T H A N I D E / A C T I N I D E

I n a l l cases the p r a s e o d y m i u m m a y be coordinated.

For Na PrF 2

considered

C H E M I S T R Y

as

eight-fold

the l o c a l s y m m e t r y is c u b i c , w i t h e i g h t

6

rines a p p r o x i m a t e l y e q u i d i s t a n t at a distance of 2.23 A .

For PrF

s t r u c t u r e is that of a t e t r a g o n a l a n t i p r i s m a g a i n c o n t a i n i n g eight

fluo 4

the

fluorines

a p p r o x i m a t e l y e q u i d i s t a n t f r o m the p r a s e o d y m i u m at distances r a n g i n g f r o m 2.20 to 2.28 A . T h e s i t u a t i o n for N a P r F i is s l i g h t l y m o r e c o m p l e x .


7

6

3

T h e s t r u c t u r e m a y b e v i e w e d as a d i s t o r t e d a n t i p r i s m w i t h eight

fluorines

at v a r y i n g distances b e t w e e n 2.08 a n d 2.23 A . A n i n t h

a t o m is

fluorine

also b o n d e d to the p r a s e o d y m i u m a n d is l o c a t e d o n t o p of one of the faces of t h e a n t i p r i s m at a d i s t a n c e of 3.02 A . A l t h o u g h w e h a v e h e r e a t r u e case of n i n e - f o l d c o o r d i n a t i o n , the l o n g n i n t h P r - F b o n d s t i l l a l l o w s us to v i e w the s t r u c t u r e as e i g h t - f o l d c o o r d i n a t e .

Figure

1.

Structure of first coordination sphere of around tetravalent praseodymium ion

I n contrast to the t y p i c a l l y green c o l o r of P r

3 +

fluorides

c o m p o u n d s , the m a t e

rials r e p o r t e d here are w h i t e , w i t h the N a P r F i s h o w i n g a s l i g h t t a n 7

6

3

cast. A b s o r p t i o n spectra w e r e t a k e n o n a l l three c o m p o u n d s i n b o t h the v i s i b l e a n d i n f r a r e d regions. Samples w e r e g r o u n d i n a n i n e r t a t m o s p h e r e w i t h F l u o r o l u b e oils of v a r y i n g viscosities to o b t a i n g o o d m u l l s . mulls were placed between C a F spectrophotometer

The

flats for o b s e r v a t i o n . A C a r y M o d e l 14

2

w a s u s e d for measurements i n the range

2000-25000

Α., a n d a P e r k i n - E l m e r M o d e l 521 s p e c t r o p h o t o m e t e r w a s u t i l i z e d for the r e g i o n of 1500-4000 temperatures.

cm." . 1

S p e c t r a w e r e also t a k e n at l i q u i d n i t r o g e n

T h e l o w t e m p e r a t u r e spectra i n t h e r e g i o n of

2800-3000

c m . " are s h o w n i n F i g u r e 2. 1

W h e r e a s the spectra of P r

3 +

t y p i c a l l y e x h i b i t three b a n d s i n the

r e g i o n of 4000 A . a r i s i n g f r o m transitions to the P i , 2 m u l t i p l e t , n o n e of 3

0 )

t h e o x i d i z e d P r ( I V ) samples s h o w e d s u c h t r a n s i t i o n s — f u r t h e r e v i d e n c e f o r t h e existence of the p l u s f o u r state of the P r . P r

4 +

has a 4 /

1

electronic

c o n f i g u r a t i o n , a n d therefore the a b s o r p t i o n s p e c t r u m s h o u l d consist of a


9.

A S P R E Y

Tetravalent

E T A L .

Praseodymium

125

Compounds

single l i n e , c o r r e s p o n d i n g t o a / = 5 / 2 —» 7 / 2 t r a n s i t i o n a r i s i n g f r o m t h e s p i n - o r b i t c o u p l i n g r e m o v i n g t h e d e g e n e r a c y of t h e F l e v e l . I f a p l o t is 2

m a d e f o r t h e l a n t h a n i d e ions of f i n the region from C e

to E u

3 +

vs. Ζ -\- I, a straight l i n e is o b t a i n e d

4 /

(f

3 +

4 /

is t h e s p i n - o r b i t c o u p l i n g constant,

Ζ t h e a t o m i c n u m b e r , a n d 7 t h e o x i d a t i o n state.) v a l u e of f

4 /

for P r

4 +

1

m a t i o n leads to a n e x p e c t e d a b s o r p t i o n f o r P r

at 2870 c m . " .

4 +

Experi

1

m e n t a l l y , f o r N a P r F , N a P r F i , a n d P r F , strong absorptions are f o u n d 2


T h i s line predicts a

of 820 c m . " . T h i s v a l u e , u n d e r t h e f r e e - i o n a p p r o x i

6

7

6

3

4

at 2873, 2893, a n d 2889 c m . " , r e s p e c t i v e l y .

A n average v a l u e of 2885

1

cm."

1

Pr

with a £

4 +

is therefore t a k e n as t h e energy of t h e F 2

4 /

2

5 / 2

F

7 / 2

transition i n

v a l u e of 824 c m . " , as c o m p a r e d w i t h a p r e d i c t e d v a l u e of 1

820 c m . " . I n a d d i t i o n to t h e peaks s h o w n i n F i g u r e 2, a n u m b e r of w e a k 1

b a n d s w e r e o b s e r v e d , r a n g i n g f r o m 403 to 3910 c m . " . T h e a b s o r p t i o n at 1

403 c m . " p r o b a b l y arises f r o m t h e a s y m m e t r i c P r - F s t r e t c h i n g v i b r a t i o n , 1

b u t t h e o r i g i n of t h e other peaks ( p r o b a b l y c r y s t a l - f i e l d components ) has not b e e n a s c e r t a i n e d as yet. A n i n t e r e s t i n g feature of t h e a b o v e d a t a m a y

3000

2900

2800

3000

2900

2800

3000

2900

2800

CM"'

Figure 2.

Infrared

spectra taken at 80°K. of complexes

praseodymium-fluoride

be f o u n d b y e x a m i n i n g t h e s p e c t r u m of N a P r F . 2

6

F o r this structure, i n

w h i c h a site c o n t a i n i n g a n i n v e r s i o n center is present, t h e electric d i p o l e m o m e n t m e c h a n i s m cannot b e operative. I n this case w e w o u l d o r d i n a r i l y expect a m a r k e d d i m i n u t i o n i n i n t e n s i t y f o r t h e t r a n s i t i o n . A l t h o u g h t h e presence of a s p l i t t i n g i n t h e m a i n p e a k of 12 c m . " m i g h t suggest t h e exist 1

ence of t w o different s y m m e t r y sites i n t h e s a m p l e , x-ray i n v e s t i g a t i o n , as p r e v i o u s l y n o t e d , gave n o s u c h i n d i c a t i o n . U t i l i z i n g t h e m u l l t e c h n i q u e , w e h a v e n o i d e a as to t h e r e l a t i v e oscillator strengths of t h e t r a n s i t i o n f o r the three c o m p o u n d s .

T h e most l i k e l y m e c h a n i s m f o r t h e a p p a r e n t e n

h a n c e m e n t of t h e i n t e n s i t y of t h e t r a n s i t i o n i n N a P r F 2

6

is a v i b r o n i c c o u

p l i n g of one of t h e f u n d a m e n t a l i n f r a r e d v i b r a t i o n a l modes. F u t u r e w o r k u t i l i z i n g single crystals a n d p o l a r i z a t i o n d a t a s h o u l d resolve t h e q u e s t i o n .


126

L A N T H A N I D E / A C T I N I D E

C H E M I S T R Y

Literature Cited (1) Asprey, Larned B., Keenan, Thomas K., J. Inorg. Nucl. Chem. 16, 260 (1961). (2) Pajakoff, S. W., Monatsh 94, 482 (1963). (3) Soriano, J., Givon, M., Shamir, J., Inorg. Nucl. Chem. Letters 2, 13 (1966).


RECEIVED October 14, 1966. This work was done under the auspices of the U. S. Atomic Energy Commission.


Preparation, Structure and Spectra of Some Tetravalent

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