Effect of Aluminum Additions on the Thermodynamic and Structural

23 Effect of Aluminum Additions on the Thermodynamic and Structural Properties of LaNi - Al Hydrides Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 2, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch023

5

x

MARSHALL

x

H.

MENDELSOHN

and

DIETER

M.

GRUEN

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 AUSTIN E. DWIGHT

1

Materials Science D i v i s i o n , Argonne National Laboratory, Argonne, IL 60439

Desorption isotherms for the hydrides of LaNi Al 4.6

LaNi Al 4.5

0.5

0.4

and

are presented and values for the enthalpy and

entropy changes of the hydriding reactions are calculated from the van't Hoff plots of log P vs. 1/T. A crystallographic model of LaNi Al is shown and consideration of the nearest 4

neighbor atom distribution leads to a rationalization of the observed linear relationship between the enthalpy change, ∆H, and the aluminum composition. Brief discussions of methods to predict dissociation pressures or interstitial site occupation are included.

The cubic and hexagonal AB

5

phases are compared and, finally, the application of these alloys in chemical heat pump systems is noted.

great d e a l of interest exists i n alloys o f general c o m p o s i t i o n A B , 5

m a i n l y because o f their

x

remarkable

r a p i d l y a n d r e v e r s i b l y at m o d e r a t e

pressures near r o o m

L a N i , w h i c h crystallizes w i t h t h e C a C u 5

investigated.

A l t h o u g h substitutions

ability to absorb

5

hydrogen

temperature.

structure, has b e e n t h o r o u g h l y

of 2 0 % o f other l a n t h a n i d e s f o r

l a n t h a n u m o r other transition metals f o r n i c k e l h a v e b e e n k n o w n f o r some t i m e to c h a n g e t h e e q u i l i b r i u m h y d r o g e n pressure b y a f a c t o r o f Present address: Department of Physics, Northern Illinois University, Dekalb, IL 60115 1

0-8412-0429-2/79/33-173-279$05.00/0 © 1979 American Chemical Society King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

280

INORGANIC

COMPOUNDS WITH UNUSUAL

PROPERTIES

II

a b o u t f o u r ( 1 ) , i t has b e e n s h o w n r e c e n t l y that a l u m i n u m substitutions are p a r t i c u l a r l y effective i n l o w e r i n g the h y d r o g e n pressure b y a f a c t o r of a b o u t 300 i n g o i n g f r o m L a N i

5

to L a N i A l

(2,3).

4

A l u m i n u m substitutions h a v e b e e n s t u d i e d i n f o u r other A B i n a d d i t i o n to L a N i .

T h e hexagonal C a C u

5

t a i n e d u p to the c o m p o s i t i o n T h N i A l 2

t r a n s i t i o n (4).

Cubic U C u

structure of T h N i

5

Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 2, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch023

at the c o m p o s i t i o n UCU4.5AI0.5 a n d to the h e x a g o n a l C a C u the composition U C u . A l i . 3

5

(5).

5

Cubic ZrNi

5

and cubic U N i

5

5

alloys

is m a i n -

without undergoing a

3

transforms to the h e x a g o n a l M g Z n

5

5

phase

structure

2

s t r u c t u r e at 5

both form

single-phase s o l i d solutions u p to the c o m p o s i t i o n s Z r N i A l a n d U N i A I 4

(6,7).

4

O f these systems, o n l y the h y d r o g e n a b s o r p t i o n p r o p e r t i e s

Th(Ni,Al)

5

ternaries h a v e b e e n s t u d i e d r e c e n t l y ( 8 ) .

of

As with L a ( N i , A l )

5

ternaries, there appears to be a l a r g e decrease i n the e q u i l i b r i u m p l a t e a u pressure for T h ( N i , A l ) , except that T h N i A l 5

any hydrogen

2

3

d i d not a p p e a r to a b s o r b

(8).

T h e present w o r k reports h y d r o g e n d i s s o c i a t i o n pressures o b t a i n e d as a f u n c t i o n of t e m p e r a t u r e

on homogeneous

samples of

LaNisÂl^.

t e r n a r y a l l o y s . T h e d e r i v e d t h e r m o d y n a m i c values are c o n s i d e r e d to b e more

accurate

samples (2).

t h a n those

reported by

us o n less w e l l - c h a r a c t e r i z e d

S o m e a p p a r e n t r e l a t i o n s h i p s of the s t r u c t u r e of the a l l o y s

to the c h e m i c a l p r o p e r t i e s of the h y d r i d e s are also d i s c u s s e d .

Experimental L a N i . A l o . 4 a n d L a N i . A l o . 5 w e r e b o t h p r e p a r e d b y the D e n v e r R e s e a r c h Institute u n d e r a c o n t r a c t to A r g o n n e N a t i o n a l L a b o r a t o r y . T h e alloys w e r e p r e p a r e d b y s t a n d a r d i n e r t a t m o s p h e r e , arc m e l t i n g p r o c e dures. A f t e r m e l t i n g , the alloys w e r e heat t r e a t e d at 1 0 5 0 ° C f o r 2 h r i n a sealed q u a r t z t u b e i n a n i n e r t a t m o s p h e r e . W e i g h t losses d u r i n g m e l t i n g w e r e o n the o r d e r of 0 . 1 % . T h e samples w e r e c h a r a c t e r i z e d m e t a l l o g r a p h i c a l l y as a single-phase m a t e r i a l a n d w e r e also c h e c k e d at A r g o n n e b y x-ray d i f f r a c t i o n . S a m p l e s , w e i g h e d to =b 0.0001 g, w e r e p l a c e d i n a n all-316 stainless steel reactor e q u i p p e d w i t h a w e l d e d 1/x p o r o u s stainless steel filter disk. T h e reactor w a s c o n n e c t e d to a n all-316 stainless steel h i g h pressure m a n i f o l d w i t h connections to 0 - 1 0 0 0 p s i a a n d 0-100 p s i a Sensotec pressure t r a n s d u c e r s ( a c c u r a c y ± 0 . 1 % ) , a v a c u u m p u m p , a n d a h i g h pressure h y d r o g e n gas c y l i n d e r . H y d r o g e n was M a t h e s o n s u l t r a - h i g h p u r i t y g r a d e ( 9 9 . 9 9 9 % m i n ) . T h e reactor w a s i m m e r s e d i n a b a t h of d i x y l y l e t h a n e at t e m p e r a t u r e s u p to 1 3 0 ° C a n d D o w C o r n i n g 710 s i l i c o n e o i l a b o v e 1 3 0 ° C . T e m p e r a t u r e s w e r e m a i n t a i n e d to ± 0 . 2 ° C at the l o w e r t e m p e r a t u r e s to ± 0 . 8 ° C at the h i g h e r t e m p e r a t u r e s w i t h Y S I m o d e l s 71 a n d 72 t e m p e r a t u r e c o n t r o l l e r s . E q u i l i b r a t i o n w a s c o n s i d e r e d a c h i e v e d w h e n the pressure r e m a i n e d constant f o r a p e r i o d of a b o u t 15 m i n . E q u i l i b r a t i o n times w e r e f r o m 30 m i n to 16 h r . T o o b t a i n a n a l l o y that r a p i d l y absorbs a n d desorbs h y d r o g e n , it is necessary i n m o s t cases to use a n a c t i v a t i o n p r o c e d u r e . 4

G

4

5

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

23.

MENDELSOHN E T A L .

LaNi

_ Al

5

x

x

281

Hydrides

A c t i v a t i o n is u s u a l l y a c c o m p l i s h e d b y e x p o s i n g the a l l o y to h i g h e r pressures a n d / o r t e m p e r a t u r e s t h a n s u b s e q u e n t l y r e q u i r e d f o r h y d r o g e n a b s o r p t i o n . I n m a n y cases these c o n d i t i o n s c a n o n l y b e d e t e r m i n e d b y t r i a l a n d error. L a N i 4 . A l . w a s a c t i v a t e d b y s i m p l y e v a c u a t i n g the reactor c o n t a i n i n g the a l l o y f o r several m i n u t e s a n d e x p o s i n g the s a m p l e to 250 p s i H f o r 1 h r . T h e L a N i . A l . 5 s a m p l e w a s s l i g h t l y m o r e d i f f i c u l t to activate. T h i s s a m p l e w a s h e a t e d to a b o u t 9 0 ° C i n 200 p s i H , t h e n e v a c u a t e d a n d p u m p e d o n f o r a b o u t 20 m i n w h i l e c o o l i n g , a n d finally e x p o s e d to 300 p s i H f o r a b o u t 2 h r . D e s o r p t i o n isotherms w e r e determ i n e d b y r e m o v i n g m e a s u r e d a m o u n t s of h y d r o g e n a n d e q u i l i b r a t i n g the system. 6

0

4

2

4

5

0

2


2

Results O n e m e t h o d of d e t e r m i n i n g the t h e r m o d y n a m i c p r o p e r t i e s of a l l o y h y d r o g e n systems is to measure t h e i r h y d r o g e n e q u i l i b r i u m pressures a f u n c t i o n of the h y d r o g e n c o n t a i n e d i n the s o l i d phases.

c o m p o s i t i o n isotherms g e n e r a l l y c a n be d i v i d e d i n t o three regions. i n g w i t h the h y d r i d e d a l l o y , there is first a r e g i o n of r a p i d decrease w i t h o u t m u c h c h a n g e

as

These pressure-

i n hydrogen composition.

Start-

pressure

T h i s is

the

ft phase or h y d r i d e r e g i o n . N e x t there is a p l a t e a u r e g i o n of constant or r e l a t i v e l y constant pressure w h e r e the t w o s o l i d phases

+

co-exist.

F i n a l l y , there is a g a i n a r e g i o n of r a p i d decrease i n pressure, this b e i n g the a p h a s e or s o l i d s o l u t i o n r e g i o n of h y d r o g e n i n the a l l o y . H y d r o g e n d i s s o c i a t i o n pressures f o r the p l a t e a u r e g i o n of L a N i . e 4

A l o ^ H y are s h o w n i n F i g u r e l a at t w o t e m p e r a t u r e s

comparing data

f r o m the h o m o g e n e o u s samples to earlier results. T h e r e p r o d u c i b i l i t y of a d e s o r p t i o n i s o t h e r m is i n d i c a t e d b y the d a t a i n F i g u r e l b t a k e n f r o m t w o separate r u n s . T h e error i n /?-phase c o m p o s i t i o n appears to b e w h i l e the error i n the p l a t e a u pressure r e g i o n appears Tables

I and II

to be

± 2 % ±

g i v e the e x p e r i m e n t a l p r e s s u r e - c o m p o s i t i o n d a t a

1%. for

L a N L ^ A l o ^ H y at six temperatures a n d L a N L i . 5 A l o . 5 H y at five t e m p e r a t u r e s , respectively.

F i g u r e s 2 a n d 3 s h o w a p l o t of these d a t a f o r three t e m -

peratures. F i g u r e 4 shows a p l o t of l o g P vs. 1/T f o r b o t h a l l o y h y d r i d e s . F o r LaNi4.eAlo.4Hy, the pressures s h o w n i n F i g u r e 4 w e r e o b t a i n e d at a composition

y

o b t a i n e d at y = AH

=

2.75 2.50.

w h i l e for

L a N i 4 . 5 A l o . 5 H y , the

pressures

were

T h e errors i n t r o d u c e d i n t o the d e t e r m i n a t i o n of

r e s u l t i n g f r o m the s o m e w h a t a r b i t r a r y d e c i s i o n to use

pressures.near

the m i d d l e of the p l a t e a u are g r e a t l y r e d u c e d i n the present w o r k o v e r the e a r l i e r d a t a ( 2 )

because of the r e l a t i v e "flatness" of the

plateaus.

F r o m a least squares fit of the l o g P vs. 1/T p l o t , the e n t h a l p y ( A f f ) a n d t h e e n t r o p y ( A S ) of t r a n s i t i o n w e r e c a l c u l a t e d f r o m the slope a n d i n t e r cept, r e s p e c t i v e l y . T h e s e v a l u e s are g i v e n i n T a b l e I I I a n d f o r L a N i . 4

A l o ^ H y are c o m p a r e d w i t h a n e a r l i e r r e p o r t e d v a l u e .

T h e errors


6

are

282

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

II

O THIS WORK -


• PREVIOUS WORK

0

1

3 4 H/mole La NL

2

5

4.6

o

r u n

•

r u n

#

7

0.4

1

1

1

—

6

Al

1

1 O

1 *2

O a

O u 12

—

•

8

—

in i o o 0

00

°

—

dO

o o

0

Figure I .

°

1 I

(a) Comparison

1

I

2 3 H/mole L a N i

I

4 4 5

AI

I

5

6

Q 5

of plateau pressure data; (b) reproducibility separate runs


of two

23.

MENDELSOHN E T AL. Table I.


30° ±

48° ±

0.2

0.2

C

C

6 0 ° ± 0.2°

5

x

x

283

Hydrides

Pressure—Composition D a t a for LaNi4.6Alo.4 (H:M)

(°C)

LaNi _ Al

5.87 5.76 5.59 5.18 4.93 4.48 3.94 3.38 2.81 1.73 1.22 0.83 5.78 5.69 5.60 5.46 5.22 4.79 4.20 3.66 3.13 2.59 2.11 1.57 1.04 0.55 0.29 5.60 5.55 5.47 5.35 5.16 4.90 4.56 4.10 3.59 3.07 2.56 2.03 1.51 1.01 0.52 0.28

(Atm) 7.72 4.18 1.91 0.89 0.54 0.34 0.30 0.29 0.28 0.26 0.24 0.22 7.79 5.34 3.89 2.52 1.48 0.86 0.71 0.66 0.63 0.61 0.58 0.55 0.51 0.43 0.22 9.78 7.89 6.12 4.13 2.89 1.87 1.27 1.07 1.02 0.98 0.95 0.92 0.87 0.80 0.68 0.28

(°C) 8 0 ° ± 0.2° 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

100° ± 0.5° 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

126° ± 0.8° 11 11 11 11 11 11 11 11 11 11 11 11 11

(H:M) 5.50 5.42 5.34 5.23 5.07 4.85 4.57 4.14 3.63 3.13 2.62 2.11 1.58 1.12 0.64 0.40 4.99 4.88 4.73 4.54 4.47 4.18 3.70 3.41 3.04 2.74 2.41 1.82 1.25 0.73 0.43 0.26 0.15 4.90 4.75 4.51 4.24 3.87 3.35 2.77 2.19 1.67 1.26 0.75 0.56 0.38 0.27


(Atm) 12.1 9.69 7.75 6.09 4.56 3.37 2.63 2.27 2.16 2.08 2.01 1.93 1.88 1.67 1.47 0.67 12.3 10.4 8.23 6.54 5.41 5.03 4.54 4.46 4.23 4.22 4.07 3.82 3.64 3.27 2.46 0.95 0.26 16.2 13.8 11.5 10.3 9.59 9.13 8.74 8.42 8.06 7.50 6.53 4.52 1.39 0.24

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES


284


II

MENDELSOHN ET AL.

LaNi .Âl 5

x

Hydrides


23.


285

286

INORGANIC

Table II.


Temperature (°c) 60° ± 0.2°

8 0 ° ± 0.2

100° ± 0.6°

COMPOUNDS

WITH

UNUSUAL PROPERTIES

II

Pressure—Composition D a t a for L a N i ^ A l o . s

Composition (H:M) 5.63 5.59 5.53 5.46 5.32 5.12 4.81 4.38 3.78 3.19 2.60 1.96 1.35 0.79 0.35 5.36 5.28 5.15 4.98 4.73 4.22 3.69 3.11 2.50 1.91 1.33 0.79 0.40 5.35 5.30 5.24 5.12 4.98 4.80 4.52 3.96 3.30

Pressure (Attn) 12.2 10.2 8.07 6.24 3.91 2.23 1.19 0.78 0.67 0.63 0.59 0.55 0.50 0.44 0.18 10.5 8.12 5.86 3.99 2.55 1.65 1.47 1.38 1.31 1.22 1.12 0.99 0.57 16.7 13.9 11.2 8.98 6.84 5.09 3.82 3.12 2.86

Temperature (°C) -• 100° ± 0.6° V yy

yy

120° ± 0.8° )) yy

V yy yy

V

yy

V

yy

yy yy

138° ± 1 ° ° )) yy yy yy yy yy yy yy yy yy yy yy yy yy

Composition (H:M) 2.64 1.97 1.34 0.77 0.51 0.38 5.16 5.07 4.96 4.81 4.58 4.30 3.96 3.48 3.08 2.52 2.02 1.50 1.04 0.65 0.44 0.32 4.97 4.87 4.72 4.56 4.32 3.89 3.38 2.91 2.40 1.95 1.54 1.11 0.72 0.53 0.38

" R u n #1.


Pressure (Attn) • • 2.69 2.43 2.23 1.84 0.94 0.17 19.3 16.3 13.4 10.6 8.13 6.60 5.84 5.43 5.17 4.90 4.63 4.29 3.88 3.22 1.76 0.63 20.0 17.3 14.7 12.6 10.9 9.59 8.97 8.54 8.12 7.69 7.20 6.56 5.40 3.14 0.97

23.

LaNi _ Al

MENDELSOHN ET A L .

Table -

Alloy 4

6

III. A

H

x

287

Hydrides

x

Derived Thermodynamic

Data

( \ - A S ( ° \ \ mol H J \deg — mol H ) k

0

a

l

a l

2

LaNi . Al .4 LaNi .6Al .4 LaNi4. Al . LaNi

8.7 9.1 9.21 7.2

0

4

5

0

5

0

5


5

2.2

± ± ± ±

1

1

2.4

2.6

Figure 4.

0.1 0.2 0.04 0.1

Log V

R f,

2

26.1 28.1 26.6 26.1

1

1

2.8 3.0 IOOO/T (°K) p l a t e a u s

vs.

± ± ± ±

0.3 0.7 0.1 0.4

e

This work 2 This work 2

r

3.2

3.4

1000/temperature


3.6

INORGANIC


288

-20.

COMPOUNDS

0.5

I

X IN Figure 5.

Temp. (°C) 4

0

yy yy yy

LaNi . Alu.5 4

5

yy

yy yy

LaNi * a

1

LaNi

UNUSUAL PROPERTIES

1.5 5 - x

Al

A H vs. x in LaNi

5

x

_ Al x

x

Hysteresis in L a N i s - o A l * Alloys

Table I V .

LaNi .6Al .4

WITH

H:M

(atm)

(atm)

P .P 4

30 48 60 80 100

3.09 3.10 3.10 2.84 2.75

0.31 0.71 1.16 2.41 4.77

0.28 0.64 1.00 2.05 4.20

1.11 1.11 1.16 1.18 1.14

60 80 100 120

2.64 2.48 2.83 2.40

0.65 1.36 2.99 5.27

0.59 1.29 2.73 4.80

1.10 1.05 1.10 1.10

3.0

2.0

1.6

1.25

20

From Ref. 9.


II

23.


LaNi _ Al 5

Table V . Compound 0

LaNi4.eAlo.4H5

LaNi . Alo.5


4

5

LaNi4.5Al . H4. 0

0

5

5

289

Hydrides

x

Crystallographic Data

a (A)

c (A)

5.018 (5.037)• 5.358 (5.343) 5.049 (5.037) 5.340

4.030 (4.017) 4.207 (4.233) 4.021 (4.032) 4.201

0

LaNi4. Al .4 6

x

AV/V

V(A')

0

87.89 (88.24) 104.6 (104.7) 88.77 (88.59) 103.7

0.19 (0.19)

— —

0.17

Values in parenthesis from Ref. 21.

d e v i a t i o n s f r o m a least squares fit of t h e d a t a .

A plot of the available

d a t a f o r A H v s . t h e a l u m i n u m c o m p o s i t i o n is s h o w n i n F i g u r e 5 . T h e a p p a r e n t l i n e a r r e l a t i o n s h i p w i l l b e d i s c u s s e d later. I n g e n e r a l , a b s o r p t i o n pressures d o n o t a p p e a r t o b e as r e p r o d u c i b l e as d e s o r p t i o n pressures, a n d n o extensive d a t a of t h e a b s o r p t i o n isotherms w e r e o b t a i n e d . H o w e v e r , f o r p r a c t i c a l a p p l i c a t i o n s a b s o r p t i o n pressures n e e d to b e k n o w n at least a p p r o x i m a t e l y . T h e r e f o r e , s i n g l e d a t a p o i n t s o n t h e a b s o r p t i o n p r e s s u r e - c o m p o s i t i o n d i a g r a m w e r e t a k e n at s e v e r a l t e m p e r a t u r e s f o r b o t h alloys a n d are l i s t e d i n T a b l e I V . A s c a n b e seen f r o m this t a b l e , t h e m a g n i t u d e of t h e hysteresis as m e a s u r e d b y t h e r a t i o F b orption:F e orption is less t h a n that o b s e r v e d f o r L a N i . a

S

d

5

S

Crystallographic

d a t a o b t a i n e d o n b o t h alloys a n d b o t h c o r r e s p o n d i n g h y d r i d e s are c o l lected

i n Table

V

a n d are c o m p a r e d

with

p r e v i o u s results

where

available . Discussion T h e c o n c l u s i o n that s l o p i n g plateaus are a t t r i b u t a b l e t o c o m p o s i t i o n a l i n h o m o g e n e i t i e s i n t h e i n i t i a l a l l o y s a m p l e ( J O ) seems r e a s o n a b l e carefully prepared alloys w i t h CaCu

phase

5

field,

different L a : N i

since

ratios, b u t w i t h i n t h e

d i s p l a y d i f f e r e n t p l a t e a u pressures

(10).

Therefore,

t h e " f l a t t e r " p l a t e a u s of the present series of samples ( s h o w n i n F i g u r e la)

are i n d i c a t i v e o f a b e t t e r degree of h o m o g e n e i t y t h a n t h e e a r l i e r

samples. T h e r e h a v e b e e n several discussions i n t h e l i t e r a t u r e o f t h e r e l a t i o n ships o f structure t o h y d r i d e s t a b i l i t y (11,12).

I t is o f interest t o discuss

t h e i n f l u e n c e of a l u m i n u m o n t h e structure a n d o n t h e p l a t e a u pressure of A B alloys. U p to the c o m p o s i t i o n L a N i . A l i . , t h e L a N i s - a A l * a l l o y s 5

3

crystallize w i t h the C a C u

5

structure.

5

5

T w o v i e w s of a m o d e l o f this

s t r u c t u r e f o r L a N i A l are s h o w n i n F i g u r e 6. F i g u r e 6 a is a v i e w l o o k i n g 4

d o w n t h e c axis a n d shows t h e c o n f i g u r a t i o n of n i c k e l a n d a l u m i n u m atoms a r o u n d t h e c e n t r a l l a n t h a n i u m atoms.

F i g u r e 6b shows the layer-


COMPOUNDS

WITH

UNUSUAL

PROPERTTES-


INORGANIC

ure 6.

Two

views of a model of

LaNiÂl


23.

MENDELSOHN

LaNi _ Al

ET AL.

5

x

x

291

Hydrides

t y p e structure p a r a l l e l to t h e b a s a l p l a n e . T h i s v i e w a l l o w s o n e to d i s c e r n the d i s t i n c t 2c a n d 3 g sites of n i c k e l . O n t h e basis of x - r a y d i f f r a c t i o n intensities ( 2 ) , i t has b e e n s h o w n t h a t t h e a l u m i n u m atoms p r e f e r e n t i a l l y o c c u p y t h e 3 g sites.

I n the L a N i A l 4

m o d e l s h o w n i n F i g u r e 6, t h e

a l u m i n u m atoms also h a v e b e e n o r d e r e d i n t h e 3 g sites w i t h o u t e x p e r i mental evidence.

O n t h e basis of this m o d e l , a c o m p a r i s o n of nearest

n e i g h b o r d i s t r i b u t i o n s is g i v e n i n T a b l e V I f o r six different types of i n t e r -


stitial sites. S e v e r a l w o r k e r s r e c e n t l y h a v e r e p o r t e d results o n n e u t r o n d i f f r a c t i o n investigations of L a N i D 5

6

(13,14,15).

I n e a c h of these p a p e r s , i t is

c o n c l u d e d that t h e s y m m e t r y of t h e a l l o y is l o w e r e d to space g r o u p p 3 1 m o n d e u t e r a t i o n a n d that t h e d e u t e r i u m atoms o c c u p y 3c a n d 6d t e t r a h e d r a l sites. T h e s e sites i n t h e d e u t e r i d e are closely r e l a t e d s p a t i a l l y to t h e 12n a n d 12o t e t r a h e d r a l sites of t h e a l l o y h a v i n g P 6 / m m m s y m metry.

C o n c e n t r a t i n g o n t h e n a n d o sites o n l y , o n e notes that t h e

f r a c t i o n of those sites c o n t a i n i n g o n e a l u m i n u m nearest n e i g h b o r is o n e half for L a N i A I a n d one-fourth for L a N i . A l . 5 . 4

4

5

0

T h u s , i f o n e assumes

the a l u m i n u m atoms to r e p l a c e n i c k e l i n the o r d e r e d m a n n e r d e p i c t e d i n F i g u r e 6, t h e n t h e f r a c t i o n of n a n d o sites w i t h one a l u m i n u m nearest n e i g h b o r is a l i n e a r f u n c t i o n of t h e a l u m i n u m c o m p o s i t i o n . I f o n e f u r t h e r assumes t h a t h y d r o g e n s i n n a n d o sites are b o u n d w i t h energies a v e r a g e d over a l l a v a i l a b l e sites, t h e n t h e " a v e r a g e " i n t e r s t i t i a l site b o n d i n g e n e r g y also w o u l d b e e x p e c t e d to b e p r o p o r t i o n a l to t h e a l u m i n u m c o m p o s i t i o n , thus p r o v i d i n g a r a t i o n a l e f o r t h e o b s e r v e d l i n e a r r e l a t i o n s h i p b e t w e e n AH a n d a l u m i n u m c o m p o s i t i o n s h o w n i n F i g u r e 5. S h i n a r et a l . (16)

h a v e p r o p o s e d that specific i n t e r s t i t i a l site o c c u p a -

tions c a n b e d e t e r m i n e d b y associating different b i n d i n g energies

Table V I .

Nearest Neighbor Distribution around Alloy Interstitial Sites

LaNi

LaNi Al

5

J?. Designation

Neighbors

k

AT

7

Number of Sites

La

Ni

f

2 2

6 4

1 3

h m

0 2

4 2

4 6

n

1

3

12

0

1

3

12

b

with

La 2 2 2 0 2 2 1 1 1 1

Neighbors Ni 4 2 4 3 2 1 2 3 2 3

Al

, Number of Sites

2 2 0 1 0 1 1 0 1 0

1 1 2 4 2 4 4 8 8 4

A r


292

INORGANIC COMPOUNDS W I T H UNUSUAL

PROPERTIES

II

h y d r o g e n atoms i n different i n t e r s t i t i a l sites. T h e b i n d i n g energies w o u l d b e p r o p o r t i o n a l to A H , the s u m of the heats of f o r m a t i o n of

(imaginary)

b i n a r y h y d r i d e s f o r m e d w i t h the A a n d B atoms s u r r o u n d i n g a p a r t i c u l a r site.

O n the basis of this scheme, one w o u l d p r e d i c t t h a t the h y d r o g e n

atoms p r e f e r sites h a v i n g a n a l u m i n u m nearest n e i g h b o r since t h e heat of f o r m a t i o n of a l u m i n u m h y d r i d e is c o n s i d e r e d to b e m o r e negative t h a n that of n i c k e l h y d r i d e (17).

H o w e v e r , b e c a u s e the m o s t recent

measured


values of A S f o r the a l u m i n u m - c o n t a i n i n g alloys are almost i d e n t i c a l to those of L a N i

5

(see

T a b l e I I I ) , c o n f i g u r a t i o n a l e n t r o p y calculations

i n d i c a t e that the d i s t r i b u t i o n of o c c u p i e d sites is the same f o r L a N i

5

(12) as f o r

the L a N i s ^ A l z alloys. It s h o u l d b e n o t e d that a l t h o u g h the present A S values are c o n s i d e r e d to b e m o r e a c c u r a t e t h a n those p r e v i o u s l y r e p o r t e d ( 2 ) , m o r e precise A S values s h o u l d b e a v a i l a b l e soon f r o m p r e c i s i o n calor i m e t r i c experiments b e i n g p e r f o r m e d i n c o l l a b o r a t i o n w i t h the A r g o n n e National Laboratory C h e m i c a l Engineering Division calorimetry group. C a l c u l a t i o n s f r o m S h i n a r et a l . f o r L a N i H 5

6

s h o w that the o c t a h e d r a l

3/ a n d t e t r a h e d r a l 6 m sites w o u l d b e p r e f e r r e d for h y d r o g e n o c c u p a t i o n , a n d c o n t e n t i o n is m a d e that this result is s u p p o r t e d b y n e u t r o n d i f f r a c t i o n data.

T h e y state t h a t t h e 3c a n d 6d t e t r a h e d r a l i n t e r s t i t i a l sites f o r the

d e u t e r i d e "are analogous to the 3/ a n d 6m c o m p o u n d ( P 6 / m m m space g r o u p ) " (16).

sites i n the ( L a N i ) 5

mother

H o w e v e r , the occupied

3c

sites i n the d e u t e r i d e are d i s p l a c e d ~ 0.08 A f r o m 3/ i n t e r s t i t i a l sites i n LaNi

5

a n d are, i n fact, closer to 12n i n t e r s t i t i a l sites ( 1 5 ) , as a l r e a d y n o t e d .

F u r t h e r m o r e , 6d sites are never e q u i v a l e n t to 6 m i n t e r s t i t i a l sites except i n one s p e c i a l case w h i c h does not p e r t a i n to the d e u t e r i d e , b u t t h e y are closely r e l a t e d to the 12o i n t e r s t i t i a l sites. It seems t h e r e f o r e m o r e reasona b l e to c o n c l u d e that the h y d r o g e n s o c c u p y t e t r a h e d r a l sites i n L a N i H 5

a n d that the n e u t r o n d i f f r a c t i o n results s u p p o r t a n " a v e r a g e "

6

interstitial

site b o n d i n g energy m o d e l o v e r m o d e l s w h i c h assume discrete b o n d i n g energies associated w i t h e v e r y c r y s t a l l o g r a p h i c a l l y d i s t i n c t site o c c u p i e d b y h y d r o g e n . I n a n y event, the q u e s t i o n of site o c c u p a t i o n of h y d r o g e n i n the A B

5

h y d r i d e s r e m a i n s a n i n t r i g u i n g one a n d w i l l r e q u i r e

more

precise s t r u c t u r a l a n d t h e r m o d y n a m i c measurements o n w h i c h to base m o r e a c c u r a t e t h e o r e t i c a l c a l c u l a t i o n s f o r its s o l u t i o n . A scheme f o r c o r r e l a t i n g h y d r i d e stabilities, t h e so c a l l e d " r u l e of r e v e r s e d s t a b i l i t y " (see

e.g., 18,19),

states t h a t f o r a series of

analogous

alloys, the m o r e stable the a l l o y , the less stable (i.e., h i g h e r d i s s o c i a t i o n p r e s s u r e ) the c o r r e s p o n d i n g h y d r i d e .

Using Miedema's formula

the c a l c u l a t e d heat of f o r m a t i o n f o r L a N i is — 42.1 k j / m o l . S i n c e L a A l

5

5

(20),

is — 11.2 k j / m o l a n d f o r L a A l

5

is m o r e stable ( m o r e n e g a t i v e A H ) t h a n

L a N i , the r u l e of r e v e r s e d s t a b i l i t y p r e d i c t s the L a N i s . ^ A l ^ h y d r i d e s to 5

be less stable t h a n L a N i H 5

6

c o n t r a r y to o b s e r v a t i o n .

S i m i l a r l y , S h i n a r et


23.

LaNi . Al

MENDELSOHN E T AL.

5 x

x

293

Hydrides

a l . (16) h a v e f o u n d disagreement w i t h t h e r u l e o f r e v e r s e d s t a b i l i t y f o r the LaNis.^Cutf h y d r i d e s . I t appears that i n t h e case o f t h e a l u m i n u m a n d c o p p e r ternaries,

as i n m a n y other A B h y d r i d e systems,

t h e factors

5

d e t e r m i n i n g t h e h y d r o g e n d i s s o c i a t i o n pressures

a r e closely

correlated

w i t h c e l l v o l u m e s (2) o r i n t e r s t i t i a l h o l e sizes ( I I ) . T h e t w o most c o m m o n l y o b s e r v e d structure types f o r c o m p o u n d s having the composition A B Downloaded by UNIV OF MICHIGAN ANN ARBOR on October 2, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch023

agonal C a C u

5

structure.

5

are t h e c u b i c U N i

5

structure a n d t h e hex-

T h e relationships these structures b e a r t o o n e

another a r e i m p o r t a n t because, t o o u r k n o w l e d g e , alloys of c o m p o s i t i o n AB

w h i c h h a v e b e e n r e p o r t e d t o absorb large quantities of h y d r o g e n

5

are e x c l u s i v e l y of the C a C u

5

structure t y p e , a l t h o u g h u p t o n o w i t appears

that t h e o n l y c u b i c c o m p o u n d tested f o r h y d r o g e n a b s o r p t i o n is Y N i M n 4

T h e C a C u h e x a g o n a l phase, i n a c c o r d a n c e w i t h D w i g h t ' s e m p i r i c a l

(21).

5

r u l e (22),

is o n l y stable i f t h e r a d i u s r a t i o r :r

w i t h r :r

< 1.30 c r y s t a l l i z e i n the c u b i c U N i

A

B

A

1.30.

Compounds

structure.

However, it

>

B

5

has b e e n f o u n d that t h e l i m i t 1.30 is n o t c r i t i c a l f o r t h e c u b i c structure, a n d i n d e e d c u b i c c o m p o u n d s a r e k n o w n - w i t h r a d i u s ratios u p t o

1.42

I f one examines the i o n i c c r y s t a l r a d i i f o r l a n t h a n u m , n e o d y m i u m ,

(23).

t h o r i u m , e r b i u m , a n d u r a n i u m , a n d t h e structures a n d v o l u m e s of t h e i r corresponding A N i UNi

5

5

c o m p o u n d s , as s h o w n i n T a b l e V I I , t h e v o l u m e of

seems t o b e a n o m a l o u s l y s m a l l . H o w e v e r , t h e m e t a l l i c r a d i u s o f

u r a n i u m is s m a l l e r t h a n a l l o f t h e rare earth m e t a l l i c r a d i i .

This implies

that u r a n i u m is n o t t r i - v a l e n t i n this m e t a l l i c c o m p o u n d b u t rather o f higher valency.

L a c k i n g a n estimate o f t h e m e t a l l i c r a d i u s o f u r a n i u m

i n U N i , a d v a n t a g e has b e e n t a k e n o f t h e a p p a r e n t l i n e a r r e l a t i o n s h i p 5

b e t w e e n i o n i c c r y s t a l r a d i i a n d a l l o y c e l l v o l u m e f o r several A N i

com-

5

p o u n d s . I n fact, b y e x t r a p o l a t i n g s u c h a p l o t , one w o u l d expect t h e i o n i c r a d i u s of u r a n i u m t o b e a b o u t 0.98 A f o r t h e c e l l v o l u m e to b e 77.9 A . 3

T h u s , u r a n i u m c o u l d b e either tetra-valent

(CR =

1.03,

R e f . 24) o r

p e n t a - v a l e n t ( C R = 0.90, R e f . 24) o r p o s s i b l y o f i n t e r m e d i a t e v a l e n c y . Table V I I . Ion La Nd Th Er U

3 + 3 + 4 +

3 +

+ 3

Crystal Radius (A) 1.172 1.123 1.08 1.030 1.165

ANi

5

ANi

5

Structure

hexagonal hexagonal hexagonal hexagonal cubic

Volume (A ) 3

86.7 84.3 84.r 81.0

b 6

6

77.9

A l l values from Ref. 24. From Ref. 25. From Ref. 4. * From Ref. 6. a

b

0


d

294

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

F i n a l l y , a t w o - m e t a l h y d r i d e c o n c e p t o p e r a t i n g as a c h e m i c a l heat p u m p f o r storage a n d r e c o v e r y of t h e r m a l energy f o r h e a t i n g , c o o l i n g , a n d energy c o n v e r s i o n has b e e n p r o p o s e d ( 2 6 ) a n d is c u r r e n t l y b e i n g tested

(27).

H y d r o g e n gas is t r a n s f e r r e d

t h e r m a l energy i n p u t at a characteristic

f r o m one h y d r i d e b e d b y

temperature

to a s e c o n d b e d

w h e r e h y d r o g e n is a b s o r b e d , a n d t h e r m a l energy is released at another characteristic

temperature.

T h i s c o n c e p t is i l l u s t r a t e d d i a g r a m m a t i c a l l y


i n F i g u r e 7. T h e heat p u m p a p p l i c a t i o n becomes most efficient f o r t h e case w h e n t h e values o f A S f o r t h e h y d r i d i n g reactions h y d r i d e p a i r are e q u a l L a N i . . A l . alloys. 5

a

( Z

of t h e m e t a l

( 2 8 ) , a p r o p e r t y closely a p p r o x i m a t e d b y t h e

T h i s p r o p e r t y together w i t h t h e a b i l i t y t o v a r y t h e

h y d r o g e n d i s s o c i a t i o n pressure

o v e r w i d e ranges m a k e t h e

LaNi . A\ 5 w

a l l o y system of p a r t i c u l a r interest f o r c h e m i c a l heat p u m p a p p l i c a t i o n s .

IOOO/T(°K) Figure 7.

Representation

of a two metal hydride chemical heat pump


x

23.


LaNi

5

_ Al x

x

Hydrides

295

Acknowledgment W e w i s h t o t h a n k S. P e t e r s o n f o r h e l p f u l discussions.


Literature Cited 1. van Mal, H. H., Buschow, K. H. J., Miedema, A. R., J. Less-Common Met. (1974) 35, 65. 2. Mendelsohn, M. H., Gruen, D. M., Dwight, A. E., Nature (1977) 269, 45. 3. Achard, J.C.,Percheron-Guegan, A., Diaz, H., Briancourt, F., Denany, F., Int. Congr. on Hydrogen in Metals, 2nd, Paris, France, 1977. 4. Ban, Z., Sikirica, M., Raseta, R., J. Less-Common Met. (1967) 12, 478. 5. Blazina, Z., Ban, Z., Z. Naturforsch. (1973) 28b, 561. 6. Blazina, Z., Ban, Z., J. Less-Common Met. (1973) 33, 321. 7. Blazina, Z., Ban, Z., Croat. Chem. Acta (1971) 43, 59. 8. Takeshita, T., Wallace, W. E., J. Less-Common Met. (1977) 55, 61. 9. Kuijpers, F. A., van Mal, H. H., J. Less-Common Met. (1971) 23, 395. 10. Buschow, K. H. J., van Mal, H. H., J. Less-Common Met. (1972) 29, 203. 11. Lundin, C. E., Lynch, F. E., Magee, C. B., J. Less-Common Met. (1977) 56, 19. 12. Gruen, D. M., Mendelsohn, M. H., J. Less-Common Met. (1977) 55, 149. 13. Andresen, A. F., Proc. Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August 1977. 14. Fischer, P., Furrer, A., Busch, G., Schlapbach, L., Proc. Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August, 1977. 15. Bowmann, A. L., Anderson, J. L., Nereson, N. G., Proc. Rare Earth Res. Conf., 10th, Carefree, Arizona, 1973. 16. Shinar, J., Jacob, I., Davidov, D., Shaltiel, D., Proc. Int. Symp. on Hydrides for Energy Storage, Geilo, Norway, August, 1977. 17. "Metal Hydrides," W. M. Mueller, J. P. Blackledge, G. G. Libowitz, Eds., Academic, New York, 1968. 18. vanMal,H. H., Buschow, K. H. J., Miedema, A. R., J. Less-Common Met. (1974) 35, 65. 19. Buschow, K. H. J., vanMal,H. H., Miedema, A. R., J. Less-Common Met. (1975) 42, 163. 20. Miedema, A. R., J. Less-Common Met. (1976) 46, 67. 21. Mendelsohn, M. H., Gruen, D. M., Dwight, A. E., Proc. Rare Earth Res. Conf., 13th, Wheeling, West Virginia, October, 1977. 22. Dwight, A. E., Trans. ASM (1961) 53, 479. 23. Buschow, K. H. J., van der Goot, A. S., Birkham, J., J. Less-Common Met. (1969) 19, 433. 24. Shannon, R. D., Acta Crystallogr. (1976) A32, 751. 25. Wernick, J.H.,Geller, S., Acta Crystallogr. (1959) 12, 662. 26. Gruen, D. M., et al., Proc. Intersoc. Energy Conver. Engin. Conf., 11th, Stateline, Nevada, 1976. 27. Gruen, D. M., et al., Argonne National Laboratory Report, ANL-77-39, June, 1977. 28. Gruen, D. M., Mendelsohn, M. H., Sheft, I., Sol. Energy (1978) 21 (2). RECEIVED March 7, 1977. Work performed under the auspices of the Division of Basic Energy Sciences of the Department of Energy.


Effect of Aluminum Additions on the Thermodynamic and Structural

Recommend Documents