Phase transitions in intermetallic compounds of CrB-type structure during hydrogen sorption

Journal of Alloys and Compounds, 209 (1994) 93-97 JALCOM 1016

93

Phase transitions in intermetallic compounds of CrB-type structure during hydrogen sorption I.E. N e m i r o v s k a y a Institute of Food Substances of the Russian Academy of Sciences, 117813 Moscow (Russian Federation)

V.V. L u n i n Moscow Lomonsov State University, 119899 Moscow (Russian Federation) (Received April 22, 1992; in final form October 13, 1993)

Abstract

Phase transformations in intermetallic compounds (IMCs) of CrB-type structure during hydrogen sorption-desorption are considered. The following phases are identified: ZrNiHl.o, HfNiHl.0, HfNiH3.o, ZrCoH3.o and HfCoH3.o. The effect of IMC composition on the phase transitions during the sorption process is shown.

I. Introduction

Intermetallic compounds (IMCs) with the CrB-type structure are of practical interest in catalytic processes [1]. X-Ray diffractometry can be used to study the nature of the structural transformations during the hydrogenation of IMCs. Hydrogen implantation into an IMC results in either a principle rearrangement of the IMC crystal structure (ZrCo, HfCo) or an increase in its parameters (ZrNi, HfNi). The positions of the metal atoms in the original IMC structures are similar to their positions in the hydride phase [2-13]. Hydrogen atoms have a high mobility in metal matrices and occupy octahedrally or tetrahedrally coordinated sites. The CrB-type structure of ZrNi-H2 and HfNi-HE systems remains unchanged during the sorption process independent of the hydrogen content in the IMC, but a successive increase, in the lattice parameters takes place [2-10, 13-17]. From examination of the P - c - T diagrams and thermoanalytical studies, a monohydride phase was observed for the ZrNi and HfNi systems [3, 4, 13, 18-20]. In studies of ZrNi-H2 by X-ray diffraction [7] and neutron diffraction (T=533 K, PH2=vacuum to 5.0 MPa), a stable monohydride phase ZrNiH is found at PHZ=0.152 MPa and T---533 K, and the monohydride diffractogram shows a slight triclinic distortion of the orthorhombic lattice. Deuteration of ZrCo (CsCl-type structure) results in the formation of ZrCoD3.o isostructural with ZrNiH3.o, with a CrB-type structure, i.e. the metal matrix is rearranged from CsC1 to CrB [7, 11, 12, 14, 21, 22].

In this paper, we have studied the dynamics of the phase transitions in IMC sorption-desorption processes to reveal the nature of the monohydride phase appearance and disappearance in ZrNi-H2 and HfNi-H2 systems.

2. Experimental details 2.1. Sample preparation

The IMCs ZrNi, HfNi, ZrCo and HfCo and their hydrides were prepared as discussed elsewhere [20]. The homogeneity of the alloys was checked by X-ray diffraction.

2.2. Equipment and measurement procedure

High-temperature X-ray diffraction (DRON-2.0 difffactometer) using monochromatic Cu Ka radiation was adopted in the investigations. Phase transformations during specimen heating under hydrogen or an inert gas (helium, argon) were studied in a GPVT 1500 hightemperature chamber. Quantitative X-ray phase analysis [23], based on the relationship between the diffraction line intensities and the proportion of a specific phase in the specimen, was used to determine the pattern of transformation and conversion of the specimens under study. The hydride conversion of the IMC was calculated from the equation a = I/Io × 100%

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I.E. Nemirovskaya, V.V. Lunin / Phase transitions in IMC-Hz systems

94

TABLE 1. Data on the X-ray phase analysis of IMCs and their hydrides

hM

ZrNiH3

d

I

d~

I~

3.30 2.614 2.468 2.224 2.141 1.759 1.658 1.620 1.558

20 100 50 20 80 50 50 70 10

3.33 2.63 2.49 2.25 2.15 1.76 1.66 1.62 1.59

30 100 50 30 80 30 20 30 5

(~)

021 111, 130 041 002, 200 042, 132, 221

040

131 151 061

ZrCoH3

(%)

(.~)

(%)

ZrCo

100 110 111 200

3.34 2.64 2.48 2.24 2.16 1.77 1.66 1.63 1.56

HfCoH3

Ie (%)

do (A)

Io (%)

de (A)

le (%)

20 100 54 20 90 25 12 15 15

3.30 2.60 2.45 2.22 2.13 1.79 1.73 1.64 1.60

90 100 30 20 51 4 10 23 22

3.28 2.59 2.43 2.21 2.11 1.78 1.72 1.64 1.60

50 100 40 7 55 5 7 17 15

HfCo

d (A)

I (%)

de (A)

~ (%)

d (A)

I (%)

~ (A)

(%)

3.161 2.249 1.840 1.591

30 100 10 50

3.19 2.25 1.83 1.61

13 I00 5 20

3.118 2.218 1.817 1.574

30 100 20 30

3.16 2.24 1.83 1.58

40 100 16 13

ZrNi

021 041, 111 130 041 002 131 200 042, 151

do (A)

HfNiH3

HfNi

d

I

de

1,

de

l,

(A)

(%)

(A)

(%)

(A)

(%)

3.131 2.459 2.312 2.110 2.041 2.015 1.624 1.575

10 100 20 18 25 45 14 16

3.17 2.48 2.33 2.12 2.05 2.02 1.64 1.59

20 100 14 23 55 36 10 19

3.12 2.46 2.32 2.11 2.02 -

85 100 15 25 38 -

d and de are the interplanar spacings (h,k,/) according to ref. 25 and the experimental data respectively. I and le are the relative intensities of the phase lines according to ref. 25 and the experimental data respectively.

where Io is the line intensity of the phase in the original specimen and I is the line intensity of the phase during the transformation process. Metal matrices of HfCoH3.o and HfNiH3.o hydrides are isostructural with ZrNiH3.o and ZrCoHa.o [3, 12, 18, 24]. The lattice parameters of these compounds are similar since the percentage differences in the atomic radii of Hf and Zr and Ni and Co are less than 1%. X-Ray spectra can be identified, even if no data are available in the ASTM file [25], from the classical pattern of line splitting according to the structural type of the system [26] (Table 1).

3. Results and discussion

The desorption study was carried out under various atmospheres (hydrogen, helium, argon) and in vacuum.

In the absence of oxygen-containing impurities in the reacting gas, only the temperatures of onset and termination of hydrogen sorption-desorption are affected by the gas atmosphere. For the HfNi-H2 system, phase transformations in the bulk phase take place over the following temperature ranges: 413-570 K under hydrogen; 420-560 K under helium; for the ZrNi-H2 system: 420-540 K under inert gas; 530-570 K under hydrogen; for the HfCo-H2 system: 470-590 K under helium; 490-590 K under hydrogen; for the ZrCo-H2 system: 600-720 K under helium; 470-570 K in a vacuum of about 0.1-1.0 kPa (Figs. l(a)-l(c)). X-Ray phase analysis demonstrates the complete reversibility of the sorption processes under hydrogen. The X-ray diffractograms for initial hydrogen desorption from an IMC hydride, under both inert gas and hydrogen, are identical with the diffractogram obtained for the decomposition of a previously hydrogenated specimen.

LE. Nemirovskaya, F.F. Lunin, / Phase transitions in I M C - H 2 systems

95

T,K

573 T,K

T,K

533

,,~

595

573

473

,.~

573

455

,~

445

i#

423

.,/

~ ~

543

~

523

j

498

298 ,

25 (Q)

35

513

425

298

298

i

45

20

25 (b)

35

4.5

28

25 (c)

35

45

28

Fig. 1. Diffraction pattern variation on hydrogen desorption from HfNiH3. 0 (a), HfCoHa.0 (b) and ZrCoH3.0 (c) at atmospheric hydrogen pressure (a, b) and under vacuum (0.1-1.0 kPa) (c).

Repeated hydrogenation--dehydrogenation of an IMC results in slight line broadening and a decrease in intensity, which correlate with the observations reported for the LaNis-H2 system [27]. These changes in the diffraction pattern are caused by disordering of the lattice after the cyclic sorption process. The positions of the lines in the diffractograms of an IMC and its hydride do not change during the process. The degree of dispersion of an IMC hydride almost doubles after cyclic hydrogenation. Hysteresis of the sorption process is manifested by a shift of absorption to lower temperature compared with hydrogen desorption (e.g. for the HfCo-Ha system, desorption proceeds as the temperature increases from 490 to 590 K, while absorption proceeds as the temperature decreases from 540 to

350 I9. Changes in the diffraction pattern during the sorption process of HfNiH3.o are shown in Fig. l(a). Investigations were carried out at heating rates of 2 and 5 K min-1. Hydrogen desorption starts at 313 K. As this takes place, an increase in pressure (for scans in vacuum) and a continuous shift of the diffractogram lines to large angles occur, which corresponds to a decrease in the lattice parameters on hydrogen desorption from a metal matrix. Temperature expansion of a specimen usually results in an increase in the lattice parameters manifested as a shift in the lines to small angles in the diffractogram. Thus, constant line positions indicate that hydrogen desorption occurs, since the decrease in the lattice parameters is dynamically compensated by

an increase in the parameters during temperature expansion of the specimens. The diffractogram taken at a temperature of 423 K shows, in addition to the HfNiH3.o phase, lines with distinct splitting (d = 3.16 and 3.12; 2.52 and 2.46; 2.15 and 2.12 ~). Line splitting disappears at a temperature of 573 K on formation of HfNi (d=3.12; 2.46; 2.11 ,~). IMC cooling under hydrogen results in the hydrogenation of the specimen at 563 K; the 563-418 K region shows line splitting; the HfNiH3.o phase is completely formed at 393 K. In this study, each line in successive diffractograms was obtained at temperatures differing by a constant value of AT. This made it possible to plot the value of the relative change in integrated line intensity (d = 3.3 (HfNiH3.o), d = 2 . 0 2 / ~ (HfNi) and d=3.16 and 3.12 /~) as a function of temperature (Fig. 2(a)). The temperature range of 473-533 K corresponds to the "plateau" for curve 3 in the relative line intensity vs. temperature plot; this supports the existence of an intermediate phase during hydrogenationdehydrogenation of HfNi-HfNiH3.o. The process of ZrNiH3.o decomposition takes place at higher temperatures compared with HfNiH3.o. The temperature interval of monohydride phase existence is considerably shorter (563-573 K). However, the plot of the IMC, monohydride and trihydride relative line intensities is similar to that presented in Fig. 2(a) for the HfNi-Ha system. The structure of the monohydride shows a triclinic distortion of the orthorhombic lattice,

96

I.E. Nemirovskaya, V.V. Lunin / Phase transitions in IMC-Hz systems

I/Io% I00

50 I-

o

2- •

o (o)

t> 400

500

,

, T,K

I/Io% I00

5O

o

400 500 600 I',K (b) Fig. 2. Relative line intensity vs. temperature during hydrogen desorption from RTHa.0 (R-=Hf, T---Ni (a), Co (h)): 1, RTH3.0 phase; 2, RT phase; 3, RTHL0 phase. as found previously for ZrNiH1.0. The ZrNiHl.o phase can be identified by the splitting of the following lines of the ZrNi IMC diffractogram: d=3.17, 2.50, 2.15 and 2.058/~; HfNiHl.o can be identified by the splitting of the following lines of the HfNi IMC diffractogram: d = 3.12, 2.46 and 2.11/~. HfNiHz.o and ZrNiH2 o phases were not found. Dehydrogenation of HfCoH3.o under hydrogen starts at a temperature of 500 K (Fig. l(b)); as this takes place, the diffraction lines shift to large angles. At the initial stage (400-490 K), hydrogen desorption is from the /3-phase of the hydride and is not associated with the formation of a new phase having a different structural type. The most intense hydrogen desorption starts on formation of the HfCo IMC phase and at the beginning of the phase transition (temperature, 543 K). The diffractogram at 573 K shows mainly HfCo phase lines. At a temperature of 593 K, complete removal of dissolved hydrogen from the IMC phase takes place. On the basis of the relative change in integrated line intensity for HfCoH3.o (d= 3.28/k) and HfCo (d=3.16

/~) as a function of temperature, the plot shown in Fig. 2(b) has been constructed. It is evident that the decrease in hydride phase content corresponds to an increase in IMC phase content. The X-ray data obtained indicate that the phase transition is single stage. When ZrCoH3.o is heated to 473 K (in a vacuum of 0.1-1.0 kPa), the diffractogram lines shift to large angles, i.e. hydrogen desorption begins (Fig. 1(c)). The hydrogen content of the fl-hydride decreases. The vacuum decreases sharply at 498 K and the IMC phase appears in the diffractogram. At a temperature of 573 K, the transition is complete. The diffractogram obtained corresponds to the ZrCo phase. It is known [3, 12, 20] that the Z r C o - H 2 system has two phase transition regions. X-Ray phase analysis has failed to reveal the differences between the/31 and/32 phases, which must be due to the similarity of the lattice parameters of the two phases. Hydrogen desorption from ZrCoH3.o begins at a higher temperature compared with HfCoH3.o. However, the plot of the relative line intensities for the IMC and its hydride is similar to that presented in Fig. 2(b) for HfCo-H2. Cobalt-containing IMCs (HfCo, ZrCo) have a cubic structure. The lattice parameter has been calculated [28]. For HfCo the value of a = 3 . 1 6 / ~ was obtained for lines with different hkl, and for ZrCo the lattice parameter varies from 3.17 to 3.22 ~, which is probably because of defects in the structure of the ZrCo specimen used in the study. The data obtained, within experimental accuracy, agree well with the values available in the literature: 3.16 /~ for HfCo [13]; 3.196 ~ [11] and 3.200/~ [12] for ZrCo. 4. Conclusions In situ high-temperature X-ray analysis of sorption processes in IMC-H2 systems can be used to identify the phase transitions (appearance of stable monohydride phases in nickel-containing systems and the single-stage character of sorption-desorption processes in cobaltcontaining systems) and phases of IMCs and their hydrides.

Acknowledgment

The authors wish to thank Dr. A.N. Grechenko (Novomoskovsk, Russian Federation) for help with the X-ray measurements. References 1 V.V. Lunin and O.V. Krujkov, 1987, p. 86.

Catalysis,

MGU, Moscow,

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