Thermodynamic studies in the system SmH2SmH3

Thermodynamic studies in the system SmH2SmH3

Solid State Ionics 43 (1990) 103-111 North-Holland THERMODYNAMIC S T U D I E S I N T H E S Y S T E M SmH2-SmH3 Longmei W A N G l, K. C O N D E R an...

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Solid State Ionics 43 (1990) 103-111 North-Holland

THERMODYNAMIC

S T U D I E S I N T H E S Y S T E M SmH2-SmH3

Longmei W A N G l, K. C O N D E R and E. KALDIS Laboratorium ffir FestkOrperphysik, E T H - HOnggerberg HPF, CH-8093 Ziirich, Switzerland

Received 31 January 1990; accepted for publication 29 March 1990

Equilibrium hydrogen dissociation pressures have been measured as functions of composition and temperature for the samarium-hydrogen system in the range 448 to 623 K. Relative partial molar enthalpies and entropies in the composition region of continuous solid solutions from n= 2 to 2.5 atoms H/atom Sm have been evaluated from van't Hoff plots. The interaction energy between hydrogen atoms and octahedral interstices is negative (4 ~ 8 kJ/mole) indicating attractive interaction. The hydrogen pressures associated with the transition process from cubic to hexagonalsamarium hydride have been determined and the relative molar enthalpy and entropy of formation of hydrogen-deficient hexagonal SmH3 have been calculated from the temperature dependence of the equilibrium pressures in the two-phase region. Due to the extreme high-purity conditions, the hysteresisfound in this work is 6.5 times less than that found in the past. No similarities have been found between the structures of La and Ce trihydrides deviating from the cubic symmetry and the phases observed in the Sm-H system.

1. Introduction

The systems o f C e - H 2 [ 1-3 ], La-H2 [4,5 ], Pr-H2 [6,7], N d - H 2 [8] hydrides have been intensively studied. In these systems the dihydrides MH2 have the cubic fluorite-type structure in which tetrahedral sites are filled with hydrogen. Additional hydrogen can fill octahedral sites (although it was shown that some a m o u n t of octahedral sites can be taken before all tetrahedral ones are filled), up to the composition MH3 for which both types of sites are completely filled. It was believed for long time that in the systems LaH2-LaH3 and CeH2-CeH3 both dihydrides and trihydrides have the same cubic structure contrary to the Sm, Gd, Tb, Dy, Ho, Er and T m which create cubic dihydrides but hexagonal trihydrides. Recently [3,5] we have found by X-ray investigations that also nearly stoichiometric l a n t h a n u m and cerium trihydrides have a structure of a lower symmetry than a cubic F m 3 m . Based on our experience with La a n d Ce hydrides we decided to reinvestigate the t h e r m o d y n a m i c and structural properties of the system SmH2-SmH3. This is the lightest rare earth m e t a l - h y d r o g e n system for which the transformaOn leave of absence from Central Iron and Steel Research Institute, Beijing,P.R. China.

tion from cubic dihydride to hexagonal trihydride has been found. Therefore, we would like to find out if the observed deviation from the cubic structure is characteristic only of the C e - H and L a - H systems or it appears also in the S m - H system. In the S m - H a n d the heavier rare earth-hydrogen systems, dihydrides and trihydrides are separated by a two-phase (cubic and hexagonal ) region; a "second plateau" is, therefore, existing in their pressure-composition isotherms. In the s a m a r i u m - h y d r o g e n system the dissociation pressures and the second plateau have only been measured by Messer et al. [9] and Ohki et al. [ 10 ]. The equilibrium dissociation pressure data of Messer et al. were not well reproducible at temperature below 573 K. Especially, in the two-phase plateau region, the n u m b e r of the measurement points was not sufficient. Thus, the plateau boundaries were only assumed. Ohki et al. did not give the dissociation pressure in the whole composition. We have measured the equilibrium hydrogen dissociation pressures in the SmH2-SmH3 region, paying particular attention to the two-phase plateau region. The results of the present research are interpreted in terms of the regular-solution theory. Interaction energies and other related parameters including relative partial molar enthalpies of /1H I 0 and entropies of SH-- ~SH2, ~HH2 l o were calcu-

0167-2738/90/$ 03.50 © 1990 - Elsevier SciencePublishers B.V. ( North-Holland )

104

L. Wang et al. / Thermodynamic studies in SmH2-SmH3

lated. The relative molar enthalphy and entropy of formation of the hydrogen-deficient hexagonal SmH3 from hydrogen-rich dihydride were evaluated from the temperature dependence of the equilibrium pressures in the two-phase region. Based on the above investigations, one-phase trihydride samples could be synthesized and were characterized by X-rays. In comparison with the work done before by other authors, we consider as advantages of the present work not only the use of very pure metals (Ames, Iowa), but also the low contamination level of the hydrides (which are extremely sensitive to oxidation) and the exact determination of the hydrogen content.

2. Experimental 2.1. P - T - x

m e a s u r e m e n t s a n d s a m p l e preparation

The apparatus consists of an UHV-tight reactor and a vacuum system connected to the hydrogen pressure line, the details of which have been described previously [1 t ]. The reactor is a doublewalled horizontal quartz tube which can withstand P < 6 bar and T< 1425 K. The hydrogen pressures are monitored with several sensitive piezoresistive transducers with an accuracy of + 0.005 bar. Reactant hydrogen (99.999 vol%) was further purified with a Pd-Ag diffusion apparatus before it was purity stored in a thermostatised reservoir of calibrated volume and subsequently introduced into the reactor via a sapphire UHV-high precision dosimeter value. The leak rate of the hydrogen-pressure line in the system was lower than 1 × 10- 6 mbar s- 1, even when the reactor was subjected to a hydrogen pressure of 6 bar at elevated temperature. The temperature was measured with a Pt 10%Rh-Pt thermocoupie positioned close to the sample center and was controlled to _+0.5 K by a Kontron (model 36) recording controller. The purest samarium (4N) available today was used (Ames Laboratory, Iowa). Metal and hydrides were exposed only to the gettered atmosphere of the glove boxes. After removing mechanically the surface layer, metal pieces were cut from an ingot, weighted with an accuracy of _+0.02 mg, and then put into a tungsten boat which was transferred to the reactor. The UHV-tight reactor was opened, charged

and closed in the train of glove boxes filled with helium continuously gettered with hot cerium turnings (less than 1 ppm H20 and 02). To remove even the trace of H20 from the quartz walls of the reactor, a charge of cerium was heated overnight under vacuum in the reactor and was removed before the actual charge was loaded in the glove box. The metal specimens were carefully degassed under vacuum at 475 K before introducing hydrogen. It was found, that it was not possible to measure reproducibly the equilibrium pressures in the twophase region, if the specimens were not broken into fine pieces. This was done in situ, by alternating hydridation and dehydridation cycles (by changing the temperature), before the plateau was reached. The first measurements were performed by ascending and descending order of temperature i.e. the temperature of the sample was increased stepwise and at desired temperatures (e.g. 448,473, 523, 548, 573, 623 K) the hydrogen pressure in the reactor was recorded till a constant value was obtained. After the highest temperature of the experiment was reached and a new portion of hydrogen was introduced, the same procedure was repeated with decreasing temperature. Considerable hysteresis phenomena were found in this way in the P - x measurements in the composition range in which the transition from cubic to hexagonal phase takes place (see fig. 1 ). The pressure values were much lower in the ascending than in the descending temperature runs. It was observed that the time which was necessary to reach stationary state (pressure change in the tube -<0.5 mbar) was essentially 25 times longer ( 12.5 h compared to 30 min) for descending than for ascending temperature. The time was even shorter when the investigation was performed in this way that, after the highest temperature was reached, the sample was cooled to room temperature and the measuring procedure was repeated increasing stepwise the temperature. Because the dense hydride pieces were cracked during cooling, a new highly disordered surface was created during this procedure. This decreases probably the activation energy for the structural phase transformation. A particular advantage of this procedure is that it decreases the probability of contamination, as the measurements were performed during much shorter time. 12 h equilibrium time with such con-

105

L. Wang et al. / Thermodynamic studies in SmH2-SmH~

The lattice parameters of the samarium hydrides were determined with an accuracy better than + 0.00005 nm, using a Guinier camera with Cu K a i radiation and silicon internal standard (U.S. National Bureau of Standards ( N B S ) ) .

100 [10]

Ohki

This

work

10

/F

m~ S 3. Results and discussion

A ..Q v Q.

1.0

3. I. Dissociation pressures and directly related thermodynamic parameters

523 K v 0.10

5

2 /

3

~

¢

/ / 0.01 2.0

2.5

3.0

n

The pressure-composition isotherms are shown in fig. 2. The plateaus in the isotherms show the transition from cubic to hexagonal and the coexistence of the hydrogen-rich dihydride SmH2+x (cubic) and trihydride SmH3 (hexagonal) phases. Our pressure values in the cubic region (n < 2.5 ) are close to those ofMesser et al. [ 9 ], but in the cubic-hexagonal two-

Fig. 1. Hysteresis of the dissociation pressures P(bar) in the SmH2-SmH3 system. --,ascending; ,--descending temperature. Comparison of the data of this work with ref. [ 10].

tamination sensitive samples, would create danger of surface oxidation of the hydrides. As Ohki et al. [ 10 ] had a full set of data in the two-phase region, it is interesting to compare the hysteresis shown in fig. 1 with the hysteresis they have found for 523 K. We extrapolate from fig. l a difference of 60 mbar between ascending and descending temperature. Their difference is 400 mbar i.e factor 6.5 larger! The same is true for 573 K with AP=0.7 bar (this work) and A P = 6 bar [10]. We attribute this tremendous decrease of the hysteresis to the much purer conditions of our experiments. All the measurements presented in this paper were, therefore performed in this way.

2.2. Hydrogen analysis and X-ray analysis The samples were directly analyzed for hydrogen content according to a very accurate volumetric method described previously [ 12 ]. A series of recent experimental improvements led to an accuracy of x_+ 0.005 in SmHx. The analytical results were used to check the composition determined by P V T meas u r e m e n t s . The results agreed with the P V T meas u r e m e n t s to +_0.03.

1.0

i

0.0

-1.0

a

A

,.,,473~ -~o

J~ v o. o

-2.0

/

-3.0

-4.0 2.0

i

~

i

i

i

2.2

2.4

2.6

2.8

3.0

n (H/Sm) Fig. 2. Pressure-composition isotherms in the hydrogen rich range of the SmH2-SmH3 system: ( O ), ( • ) ascending temperature in two different runs; (I~) descending temperature.

106

L. Wang et al. / Thermodynamic studies in S m H e - S m H ~

phase region the agreement is poor. The estimations of these authors were based only on few points, and are probably too low. On the other hand, our measurements are also different from the results for desorption runs given by Ohki et al. [10], i.e. lower than his pressures at 573 K but higher at 523 K. The composition ranges of the plateaus determined by our dissociation pressure measurements are approximately n=2.5-2.99 (448-523 K), n--2.52-2.85 (548 K) and n=2.58-2.8 (573 K), they are much broader than the range 2.5-2.7 (523-573 K) obtained by Messer et al. [9], and different from the range 2.3-2.8 (523-623 K) measured by Ohki et al. [ 10] (fig. 1 ). Exact determination of hydrogen content was obviously missing in this later work. The composition range of the existence of twophase system decreases with increasing temperature. This means that the thermodynamic stability of the hexagonal phase decreases with increasing temperature. The pressure plateau for transition from cubic to hexagonal phase seems to disappear at 623 K, although we could not extend our isotherms at pressure higher than 6 bar. This indicates that in contradiction to ref. [10] the hexagonal phase is probably not sufficiently stable above 623 K. High pressure (PH2 > 6 bar) measurements at higher temperatures ( T > 623 K), as well as structural investigations under these conditions are intended in future to clarify this problem. Both methods used to establish thermodynamic equilibrium (see section 2.1 ) gave the same dissociation pressure in the composition range n<2.5 (cubic phase). Van't Hoff plots of the data were made for the cubic range compositions n = 2.1-2.5 and are shown in fig. 3. The slopes and intercepts were evaluated by the least squares method and were used to determine the relative partial molar enthalpies HH-1 o ~HH2 and entropies SH-- ½S°2, which are given in table 1. For any given temperature and composition, the partial molar Gibbs free energies can be calculated from the relation [13] GH-I~GHz°= _ ½RTlnPH2 (fig. 6). The relative partial molar enthalpies become less negative with increasing hydrogen content, and the relative partial molar entropies become more negative which may be explained by the higher vibrational frequency of hydrogen in the hydride [ 14 ] and the partial ordering in the octahedral hydrogen positions.

1.0

0.0

o -1.0

..Q v -2.0

~

C~ mO

'~.,n=2.5

"~n=2.4 -3.0 \ ~'n=2.3 -4.0

~'~n=2.2

-S.0

1.5

I

i

i

1.7

1.9

2.1

1/T

( K}

i 2.3

n=2.1 2.5

xl000

Fig. 3. Van't Hoff plots, log P(bar) versus 1 / T for the solid solution range SmH2-SmH2.5. The data are shown in table 1.

A plot of the logarithm of the plateau pressure versus reciprocal absolute temperature is shown in fig. 4. It is seen that the points can be described by the linear regression: log Pb,r = 590 + 0.20-- (3.40_+ 0.20) × 103/T. From this equation, thermodynamic functions for the enthalpy and the entropy of formation of hydrogen-deficient hexagonal SmH3 from hydrogen-rich dihydrides were calculated. The calculated values of AH and AS are listed in table 2 and compared with literature. The values of - A H is less than half of that for the dihydride formation, indicating that SmH2 is thermodynamically much more stable than SmH3.

3.2. Interaction energy, entropy and thermodynamic functions The tendency to create solid solutions in the systems rare earth metal-hydrogen can be expressed by the value of the interaction energy ~, (calculated per

107

L. Wang et al. / Thermodynamic studies in SmHz-SmH3

Table 1 Relative partial molar enthalpies and entropies of the system SmH2-SmHzs. Atoms of H/ atom of Sm

Ref. [9] Temp. region 523-723 K

This work Temp. region 448-623 K m

2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50

m

(HH_~~HH2) o kJ(molH)-'

J ( m o l H ) - J K -l

( H . _ I] H H 2o kJ (mol H) -l

34.8 34.0 33.2 32.3 31.5 30.7 29.9 29.1 28.2

34.6 36.1 37.6 39.1 40.7 42.2 43.8 45.3 46.8

38.9 36.0 34.4 32.6 (32.7)

-

-

(S.

-

1.0

1

o

~SH,

)

--

)

-

(SH -

1

0

~S.2

)

J (mol H)-I K-l

54.0 52.8 53.6 53.6 57.4

where e is the interaction energies between different site pairs (H is a hydrogen atom in octahedral site and A vacant octahedral site). The occupation of a vacant octahedral site by hydrogen can be described, therefore, by the reaction:

0.5

0.0

A + ½H2(g)-+H.

(2)

The equilibrium constant for this reaction is: -0.5

K = p~(2 aHa ,

=..

(3)

-1.0

where

O) O

an = ( n - - 2 ) ' y n

-1.5

(4)

and -2.0

a~= (3-n).y~,

-2.5

-3.0 1.5

i

t

i

i

1.7

1.9

2.1

2.3

1 / T (K)

2.5

x 1000

Fig. 4. Van t'Hoffplots for the two-phase region. The thermodynamic data for the fcrmation of SmH3 are given in table 2. mole H atoms) which is defined as [9 ]: a~= 2~HA-- ~ H H

--

EAA

,

(1)

(5)

n is the molar ratio of hydrogen to metal atoms in the lattice and an is the activity of hydrogen dissolved in the lattice. The activity coefficients, from regular-solution theory, are: Yn = e x p [ ( , ~ / R T ) ( 3 - n ) 2] ,

(6)

y a = e x p [ ( , ~ / R T ) ( n - 2 ) 2] .

(7)

Yn is assumed to be equal 1 for MH3 and ya= 1 for the hypothetical dihydride MH~ in which no octahedral but all tetrahedral intersites are occupied [ 9 ]. Introducing into eq. (3) the expressions for activities (4) and (5) with (6) a n d (7) one can obtain:

L. Wang et al. / Thermodynamic studies in SmHz-SmH~

108

Table 2 Enthalpies and entropies o f f o r m a t i o n of the hydrogen deficient hexagonal SmH3 from hydrogen-rich dihydride. - AH kJ ( m o l HE) -1

- AS J ( m o l H 2 ) - ] K -1

Temp. region (K)

Ref.

65.3_+4.0 74.6_+3.7 a) 79.6 _+4.0 b) 105 (est.)

113.0_+4.0 136.2_+6.8 a) 133.2 _+6.7 b) 175 (est.)

448-573 523-623 523 - 623 523-573

this work

[lO1 [lO1 [91

a) O b t a i n e d from a b s o r p t i o n experiments. b) O b t a i n e d from d e s o r p t i o n experiments.

7.0

2.303'{log PH2 + 2 log[ ( 3 - - n ) / ( n - - 2 ) ] } = -- ( 4 ~ / R T ) . n +

(IO,~/RT) -2(2.303) logK.

623 K

(8) If Y - 2.303.{log PH2 + 2 log[ ( 3 - - n ) / ( n - - 2 ) ] ) then Y is a linear function of n, ,, can be calculated from the slope, and log K from the intercept. In the case o f SmHz-SmH3 system, because the trihydride has different structure than the dihydride (no continuous solid solution is existing) it is formally better to consider as a standard state o f the octahedral sites occupied by hydrogen those existing in the lattice o f MH~ i.e. infinite diluted. In this case eq. (6) can be expressed as: yH=exp[(~/RT)(3-n)2].exp(-u,/RT)

,

= --Xl

mSmi x --

E

'X2"

(d~/dT) ] ;

E ASmix + R - I n x~,

A a m i x = X ] "X2 'u~,

4.0 0..

>'-

3.0

2.0

( 8 ~ / R T ) - 2 ( 2 . 3 0 3 ) log/f*.(10)

Using a graph of Yversus n (fig. 5 ), ,~ can be derived from the slope. Table 3 gives a comparison of our ,~ with those o f Messer et al. [9 ]. The interaction energy ~ is negative at all temperatures. This indicates an attractive interaction between octahedral hydrogen atoms and vacant octahedral sites. The excess Gibbs free energy of mixing and the total and excess entropies o f mixing were calculated from the value of-, and d ~ / d T w i t h the standard formulas [ 14 ]: E Agrnix

5.0

(9)

(when n--,2, 7H--' 1 ). In this case, similar calculations as above (8) give: Y= - ( 4~/ RT)n+

6.0

1.0

i

2.1

2.2

i

2.3

n

i

i

2.4

2.5

2.6

(H/Sm)

Fig. 5. D e r i v a t i o n parameters: Y, In PTorr + 2 In [ (3 -- n ) / ( n -- 2 ) ] versus n, ( H / S m ) for S m H z - S m H 3 .

for Xl=X2=0.5 ( n = 2 . 5 ) . Log K, log K* and excess Gibbs free energies of mixing at n = 2.5 were calculated in table 4. Log K is obtained from logPn2 versus 1 / T (K) at n = 2 . 5 , log K* were calculated from the relationship log K * = l o g K + ( - ~ , / R T ) / 2 . 3 0 3 which one can obtain introducing to the equilibrium constant eq. (3) different activity coefficients (6) or (9).

L. Wang et al. / Thermodynamic studies in SmH2-SmH3

AS* are compared with literature (see table 5). The apparent excess entropy of mixing - 9 . 2 1 J / m o l K may be interpreted similarly to the charge of the relative partial molar entropies mentioned above. The data listed in table 1 can be described with a very good accuracy by a linear equation:

Table 3 I n t e r a c t i o n energies f o r s a m a r i u m - h y d r i d e , - . , [ k J m o l -~ ] at various temperatures. T

This work

Ref. [ 9 ]

(K) 448 473 523 548 573 598 623 673 723

7.33 6.87 7.16 7.58 7.12

109

/~H - - ]1H H02 - --- - 6 9 . 3 3 + 16.45-n(kJ/mol H ) . 8.79 9.00 9.34 9.25 10.26 8.42 6.82

4.90

(13)

This is the evidence that the SmH2-SmH3 system (n < 2.5 ) satisfies the regular solution behavior as can be demonstrated by the following calculations: Using eqs. (8), (10) and the well known equation / t H - - ~HH21 0 = -- ½ R T 2 ( O l n P / O T ) , ,

one can show that: ItH - l~HH2° = - ( 2 n - 4 ) [ , ~ - T( d ~ / d T ]

Log K and log/(* are linear function of 1 / T ( K ) and the following equations resulted from a least squares fit: logK=2.44+_O.44-(1.47+_O.44).lO3/T,

(11)

log K* = 2.84_+ 0 . 4 4 - (2.05 _+0.44). 103/T.

(12)

+ R T Z ( d lnK*/dT) = -

(2n-

4) [,~- T(d~/dT]

+AH*

= - - 8 . A / - / m i x ' n + 1 6 " z~g/mi x + A n *

.

(14)

Here the term - 2 . [ , , , - T ( d ~ / d T ) ] is equal to 8"AHm~x (n=2.5). The value of AH*= - 3 9 . 4 kJ/mol

The values of AHm~x, mamix, E ASmix, Z~/-/, AS, A n * a n d

Table 4 T h e r m o d y n a m i c p a r a m e t e r s o f SmHz.s as a f u n c t i o n o f t e m p e r a t u r e .

T (K)

448 473 523 548 573 623

Z~knmix kJ ( m o l H ) - l

5.95 -6.07 -6.62 -6.95 -7.08 -6.95

log K

log K*

K= aH/p~2 aa

K* = a ~t/P~[2

AG mixE

(bar -1/2)

(bar -1/2)

(n= 2.5)

AGmix (n=2.5)

kJ ( m o l H ) -~

kJ ( m o l H ) -~

0.850 0.676 0.362 0.271 0.117 -0.053

-

1 . 6 9 8

1.429 1.071 0.953 0.761 0.355

-

-

1.84 1.72 1.80 1.88 1.76 1.21

-4.44 -4.44 -4.81 - 5.07 -5.11 -4.81

Table 5 Excess e n t r o p i e s o f i n t e r a c t i o n a n d o t h e r t h e r m o d y n a m i c p a r a m e t e r s d e r i v e d f r o m s o l u t i o n t h e o r y . A S v a l u e in J ( m o l H - 1 K - ~; A H in kJ ( m o l H ) -1. AHm~,,

-

-

1.88 7.91

AS~i,,

- 9.21 - 8.58

,5Sm~,,

- 3.43 - 2.81

S m H ~ + ½H2 = S m H 3

~H2=H (in SmH$)

AH

AS

z~d/*

AS*

- 28.1 - 40.6

- 46.9 - 69.9

- 39.4 - 73.7

- 54.4 - 107.2

Ref.

this w o r k [9 ]

110

L. Wang et al. / Thermodynamic studies in SmH2--SmH3

Table 6 Lattice parameters of samarium hydrides. Hydride

Structure

SMH2.95

hex.

SMH2.97

hex.

SMH2.82

hex.

SMD2.84

hex.

SmH3 (H/Sm> 2.90)

hex.

Parameter (nm)

Anmi

x =

--

1.97 ( k J / m o l H ) .

X-ray density (g/cm 3)

0.042067

6.049

0.042100

6.047

0.042184

6.030

[16]

0.041682

6.103

[16]

0.041987

6.065

[17]

V'= V / Z

a=0.37823(9) c=0.67888(1) a=0.37842(3) c=0.67893(1) a=0.37870(3) c=0.67926(8) a=0.37726(4) c=0.67632(9) a=0.3782 c=0.6779

H, at n = 2.0, (see table 5 ). Comparing eqs. ( 13 ) and (12), AHmix can be evaluated:

(nm3)

ments over s a m a r i u m hydrides enabled us to establish the c o m p o s i t i o n range o f the existence o f twophase c u b i c - h e x a g o n a l area in the phase diagram. This extends between 2 . 5 < H / S m _ < 2 . 9 9 and dis-

Since the t e m p e r a t u r e dependence o f the enthalpy o f mixing could be neglected within the experimental error, then AHmix can be estimated by using the value o f the interaction energy ~ and eq. (14). This gives AHmix= - 1.88 ( k J / m o l H ) , which agrees well with the value derived above. 3.3. L a t t i c e p a r a m e t e r s

-3.0

O -3.5

-1-

"6 We found no similarity between the diffraction patterns obtained for s a m a r i u m trihydrides and those reported before by us for La [5] and Ce [3] trihydrides. The s a m a r i u m t r i h y d r i d e structure is hexagonal - space group P 3 m 1. In our samples we did not notice any other lines than those characteristic for the hexagonal phase. This means that within the experimental error o f the X-ray measurements our SmH3 samples are one-phase. Lattice p a r a m e t e r s for the samples o f the experimentally d e t e r m i n e d compositions 5mH2.95 and 5mH2.97 are presented in table 6 together with results from the literature. (See also fig. 6).

Ref.

E

-4.0

v

O v

-4.5

o.~

O v--I¢~

-5.0

i

Io v -5.5

-6.0

2.0

4. Conclusions

(1)

Hydrogen dissociation pressure measure-

2.1

2.2

2.3

2.4

2.5

2.6

n ( atomic ratio H/Sm ) Fig. 6. Partial molar free energy of the solid solution SmH2SmHz5 versus composition

L. Wang et al. / Thermodynamic studies in SmHz-SmHs

a p p e a r s at t e m p e r a t u r e a b o u t 573 K. In c o n t r a s t to the results [9,10] o b t a i n e d before, it s e e m s t h a t especially at relatively low t e m p e r a t u r e s ( b e l o w 523 K ) , the area o f the stability o f h e x a g o n a l t r i h y d r i d e is v e r y n a r r o w i.e. o n e can e x p e c t t h a t s a m a r i u m trihydride is at these temperatures nearly stoichiometric. ( 2 ) T h e S m H 2 - H s y s t e m has a regular s o l u t i o n b e h a v i o u r in the c o m p o s i t i o n range S m H 2 - S m H z s . T h e t h e r m o d y n a m i c f u n c t i o n s r e l e v a n t to the form a t i o n o f this s o l u t i o n h a v e b e e n e s t i m a t e d . T h e diff e r e n c e s in c o m p a r i s o n w i t h the literature v a l u e s result p r o b a b l y f r o m the h i g h e r p u r i t y o f o u r s a m p l e s a n d the d i f f e r e n t t e m p e r a t u r e region in w h i c h the measurements were performed. (3) Exact values o f the dissociation pressure in the " s e c o n d p l a t e a u " w e r e m e a s u r e d a n d the t h e r m o d y n a m i c f u n c t i o n s o f the f o r m a t i o n o f s a m a r i u m t r i h y d r i d e f r o m the h y d r o g e n rich d i h y d r i d e h a v e been determined. ( 4 ) Last b u t n o t least, it was s h o w n t h a t t h e r e is no s i m i l a r i t y b e t w e e n the structures o f the La a n d Ce t r i h y d r i d e s a n d those o b s e r v e d in the S m - H system.

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