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The thermodynamics of the γ- and δ-hydrides and deuterides of CaNi5
The thermodynamics of the y- and h-hydrides and deuterides of CaNi, D. M. GRANT,”
J. J. MURRAY,
and
M.
I-. POST
Division of Chemistry. National Research Cotmcil c?f’Canada, Ottawa, Ontario. Canada KIA OR6 ( Received 217 January 1987; in final~fitrtn 6 April 1987) The thermodynamics of CaNi,H, and CaNi,D, were investigated for 4.0.~ < 6.9. Contirmation that the y single-phase region actually consists of two similar structurally related phases. *f’ and y”, was obtained. Enthalpies of the hydrides and deuterides, and pressure hysterescs of the (fl+*f’). (y’+y”). and (y”+S) two-phase regions are detailed. The reverhc isotope effect. hysteresis. enthalpy. and entropy considerations are discussed and the assumption that hydrides and deuterides hate similar bonding is questioned.
1. Introduction CaNi,H, near ambient temperatures and pressuresconsists of a series of hydride phases denoted, for ease of reference, as the r-. p-. y-, and S-phases.“’ The end-member phases, the cl-phase (0
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D. M. GRANT.
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M. L. POST
and to examine the thermochemistry of the anomalous isotope eflect reported”’ for the plateau pressure of the y-to-6 phase conversion. Finally the combined results for the entire system have been used to establish the entropy characteristics of the hydriding of CaNi,. 2. Experimental The experimental apparatus, operating conditions, and procedures for this pressurcagainst-composition (p, x) and calorimetric work are detailed elsewhere.“’ The (p. x) measurements were made using welded 316 stainless-steel manifolds with bellows-sealed valves. and capacitance-type pressure transducers. To obtain the high precision required for this study, the manifold external to the sample region was held at (303.0+0.1) K using a controlled circulating air bath. The sample region within the calorimeter was cooled to (273.15+0.01) K, and as a consequence, the hydrogen pressures of the isotherm were lowered, enabling measurements to be taken with the high-precision transducers. The sensitivity of the calorimeter at this temperature was determined by comparing the tabulated enthalpy increments for NBS standard reference material 720, cl-Al,O,, with those obtained with our calorimeter. The volume-expansion factors for this non-isothermal system were determined using helium, with the sample present, at the experimental temperatures for the manifold and sample. Preliminary and confirmatory results were made using three samples from two independent sources of CaNi,. (3J The sample used here was obtained from a 100 g batch taken from a melt of reagent-grade Ca and Ni prepared specifically for research purposes by Ergenics Ltd. at Wyckoff, New Jersey. Before use, the sample was annealed in a stainless-steel container under 100 kPa of argon at I 17.5 K for 7 d. This material was shown to contain mass fractions < 3 x IO-” of metallic impurities, 5 x lop4 of oxygen, and 4.9 x 10m2 of nickel in excess of that required by the CaNi, stoichiometry. X-ray powder diffractometry confirmed that the excess nickel was present as elemental nickel. Prior to the hydride and deuteride (p, .u) and calorimetric measurements, the sample was cycled to the S-region three times in H, or D, at 300 K. then vacuum degassed at 328 K for 16 h. Detailed results were confined to the region of interest: from the (p+ y’) two-phase region to the 6 single-phase region. The sample size was (7.322+0.001) g and measurements were corrected for the elemental Ni present. This sample size along with the experimental temperature chosen and precise temperature control, enabled increments in x as small as 0.02 to be measured to within 0.6 per cent. 3. Results Figure 1 shows the (p, x) isotherm at 273.15 K for the hydride measurement sequence. A similar measurement sequence was conducted for the deuteride at the same temperature. In desorption, measurements were terminated once a return to the (y’+ p) two-phase region was clearly established, since detailed thermodynamic measurements have already been reported for 0 < x < 2.5.
THERMODYNAMICS
FIGURE 1. The variation [\. y’. f’. 6: 0. absorption;
of hydrogen +, desorption.
OF CaNi,H,
pressure
for CaNi,H,
AND
CaNi,D,
illustrating
1’19
all 5 single-phase
regmns:
2.
The molar enthalpies of absorption obtained here for 0 < s < 2.5 show excellent agreement with the previously reported results. (h) All molar enthalpies are for the reaction: (Z.iG.u)(CaNi,(H
or D),+/&(H
or D), = CaNi,(H
or D),,,,,;.
For the a-to-b conversion, A,H,(273 K) = -(44.3 +0.3) kJ . mol- ’ compared with A,H,(313 K) = -(43.4+0.2) kJ mol- ‘. The difference shows that = (ZA,H,/aT),, for the U-to-S conversion is positive. The errors given arc ~C,.nl appropriate for direct comparison since both were obtained with the same apparatus: a further +0.5 kJ. mol- r is added when total errors are quoted. For the purposes of clarity the relative partial molar enthalpy of single-phase regions and the enthalpy of formation A,H, of one phase from another are represented together and presented here as AH,,,. For graphical and descriptive purposes, it is also convenient to use the absolute value IAH,\ avoiding the change of sign between absorption and desorption. Figures 2 and 3 give the detailed simultaneous (p. X) and IAH,,, against LZ; results in the region 4 < x < 6.9 for the hydride and deuteride respectively, where
D. M. GRANT.
1 I-7Y~ 55,
I,
J. J. MURRAY.
I I I r /,
I,
FIGURE 2. The variation of molar enthalpy during absorption; +, pressure during desorption; of desorption.
17,
,
AND
M. L. POST
/,
, , , Ty---yJ
and pressure with CaNi,H,r composition: 0, pressure 0. molar enthalpy of absorption. x . molar enthalpy
repeated for absorption and desorption in the deuteride run (see figure 3). The deuteride was chosen because, in this region, there is a reverse isotope effect in which the steady-state pressures are lower than that of the hydride, enabling a more accurate pressure transducer to be used. X-ray powder diffraction of the unhydrided sample gave a hexagonal cell, u = 0.49503, nm and c = 0.39392, nm. The angular full-width half maximum increased from 1.9 x 1O-3 to 2.1 x IO-” in 20 between prepared sample and after This peak broadening is minimal in the hydride and deuteride experiments. comparison with that observed for LaNi,,@’ and indicates little structural damage. which is unusual for hydrides in general.
4. Discussion The results reported here for CaNi 5H x,4.0 were taken at 273 K. This reduced the y”-to-6 transition pressures so that more precise pressure transducers could be used. The previously reported thermodynamic measurements(6’ for CaNi,H,, 2.5 were taken at 313 K so as to raise the a-to-(3 transition pressure to permit accurate (p, .u) measurements in desorption. Because of this temperature difference, the previous pressure and AC, values cannot be combined with the present results to form a single set for the entire composition range. However, p and hence AC, are available for the entire range in absorption at 273 K from the current measurements. The two
THERMODYNAMICS
1 FIGURE 3. The variation of molar during absorption; +, pressure during of desorption.
OF CaNi,H,
s
.Y
AND
h
1221
CaNi,D,
7
enthalpy and pressure with CaNi,D, composition: 0. pressure desorption: 0, molar enthalpy of absorption; x . molar enthalpy
sets of results can be combined for AH,,, with a small correction since, for this temperature range, @AH,/aT), is small compared with the measured enthalpy. In a recent study,‘9’ .it was noted that CaNi, formed peritectically and had a significant range of composition CaNi,,,. CaNi,H,,,,,5 showed clear evidence”’ of this stoichiometric range for single-phase “CaNi,“. The cell parameters and AG, values for the a-to-0 conversion differed for the two samples. However, enthalpies for the a-to-l3 and P-to-y transitions showed no significant variation. Therefore variations in the single-phase cell parameters and hence. presumably, in the stoichiometry of unhydrided phases do not limit the construction of a composite IAH, against (x) diagram. Evidence that disproportionation of CaNi,H, to CaH, and Ni occurs has been reported.“” In the present and previous studies in this laboratory, several CaNi, samples have been cycled up to 8 times in H, and D, at 273 K < T < 3 13 K and 11< 4 MPa, and examined using X-ray powder-diffraction and precision (p, .u) measurements. In all cases the (p, x) diagrams show excellent composition closure for absorption and desorption cycles. All X-ray patterns taken after hydriding and tiehydriding cycles show only CaNi,. and Ni at the levels initially present. The studies in the literature which report disproportionation were at temperatures above it 3 K. The conclusion is that, up to 313 K, the hydrides of CaNi, have negligible rates of disproportionation.
IT?-) ---
D. M. GRANT,
J. J. MURRAY.
AND
M. L. POST
The (p, x) and (AH,, .u) results shown in figures 2 and 3 were obtained over approximately 28 d for a charge/discharge cycle. This allowed pressure-steady states and calorimetric baseline to be established at the 0.1 per cent level after addition of each hydrogen aliquot. Nevertheless, both curves. and particularly the (AH,,,. .Y) curve, are influenced by secondary effects which can significantly obscure the thermodynamic characterization of phase boundaries. One major secondary effect which causes smearing of the phase-boundary discontinuities in both absorption and desorption is a persistence of the preceding two-phase conversion as a single-phase region is entered. In general this effect is much more pronounced in absorption. The other major secondary effect, which occurs predominantly in desorption upon leaving a single-phase region, is the premature occurrence of the two-phase conversion because, during the hydrogen-gas titration, the pressure is momentarily below the desorption-plateau value. Because of the narrow composition range of the y’-to-y” transition, these secondary effects lead to a particularly severe obscuration of the enthalpy and entropy characteristics of this transition. As a result, in view of the multiple phase boundaries present, it is useful to construct (p, s) and (AH,,,. s) diagrams from which these secondary effects are eliminated. Pressure hysteresis in the two-phase regions and the consequent phase-boundary composition shifts and variations in the enthalpy discontinuities are still shown in the idealized diagram. Such an idealized diagram for the y’, (y’+y”), and y” regions of (CaNi, + IxH,) at 273 K is shown in figure 4. A modified version of this diagram can be applied to all other two-phase regions studied. The y region, originally thought to be a single phase, was shown in earlier (p, s) and X-ray-diffraction work (j) to consist of two structurally very closely related phases, y’ and y”, separated by a narrow two-phase region. The present thermodynamic measurements corroborate this conclusion in that, over the range of
FIGURE 4. Schematic of idealized (p, x) and (AH,, x) across a two-phase enthalpy hysteresis. applicable to the (y’-y”) region of CaNi,H, or CaNi,D,.
conversion
region
with
no
THERMODYNAMICS
OF
CaNi,H,
AND
CaNi,D,
1323
x of the apparent (y’+y”) two-phase region, there is a clear increase in IAH, for both absorption and desorption and for both hydrides and deuterides. In principle. discontinuities in IAH,\ against (x) with (alAH,l/ax) > 0 at the phase boundaries, with a constant IAH,,, within the two-phase region. are expected. The narrowness of this two-phase region and the comparatively sloped plateau {(a In p/ax) is larger than for any other two/phase region in this system} results in comparatively poor resolution of IAH,] against (x). However, an increase in IAH,/ against (x> is apparent in the experimental results (see figures 2 and 3) between the y’ and (y’+ y”) boundary. No such discontinuity could be resolved in these measurements for the boundary
between (y’+y”)
and y” but, for the purposes
of the idealized diagram (see
figure 4) it is assumed that such an effect exists. It is noteworthy that the rate of change of enthalpy with composition in the y” phase is significantly greater than in the y’ phase. This is additional evidence to add to the (p, x) and X-ray-diffraction evidence reported previously’3’ that the y’-to-y” conversion leads to different sites, involving different bond energies, being occupied in these two closely related structures. The (p, x) and (AH,,,, x) behaviour of the &phase region includes a (p. x) anomaly in absorption at x z 6.4 for the deuteride and x z 6.5 for the hydride. with a corresponding anomaly in IAH,I against (x) with IAH, values as high as 48 kJ . mol- ‘. No corresponding anomaly is observed in desorption; however. the IAH, values for the S-to-y” transition observed in desorption are significantly higher than those observed in absorption, particularly for x in the (y” + 6) two-phase region near the boundary between (y”+6) and 6 (see figures 2 and 3). This deviation suggests the S-phase has a narrow phase-separation region, analogous to (y’+ y”), which occurs at pressures just above the y”-to-6 two-phase conversion pressures. In desorption, as a consequence of this small pressure difference, this narrow S-phase separation reverses sluggishly and encroaches on the high x end of the (y”+6) two-phase region causing larger /AH,,,/ values. As a result. the difference in AH,,, between absorption and desorption for the y”-to-b transition is observed to be larger than for the other phase transitions (see figures 2 and 3 and table 2). The interpretation is subject to significant uncertainty because the high pressures and (Q/ax) values, even for the deuteride, substantially limit the accuracy of the enthalpies in this composition range. The underlying thermodynamics of the &phase is very similar to that of which has the same hexagonal structure and approximate hydrogen LaNi,H,,,.,, or deuterium content. At the lower phase boundary, there is a rapid rise in IAH, (probably a discontinuous rise in an ideal situation) from the y”-to-6 conversion value of 32.5 kJ . mol- ’ to 40.5 kJ . mol -I, followed, for the deuteride, by a region of constant jAH,j for 6.3 < x < 6.7, with the exception of the absorption anomaly described above. There is a subsequent rapid decrease in JAH,,I for the deuteride for .v > 6.7. In general, for the compound hydride phases of AB,H, for which precision enthalpy measurements have been reported, including the p-LaNi, hydride.’ 1. ‘I and the f3-, y’-, y”-, and S-CaNi, hydrides. the relative partial molar enthalpy of solution of H, decreases with increasing s. In p-CaNi, hydride, deuteride, and
1224
D. M. GRANT.
J. J. MIJRRAY.
AND
M.
L. POS?
p-LaNi, hydride there is a region near the lower phase boundary where /AH,,/ t\ constant over a significant range of .u. There is evidence of a similar region in &CaNi, deuteride (see figure 3) particularly from the desorption results; in absorption, evidence for this enthalpy behaviour is obscured by the anomaly described above. (AH,/ also decreases with .Y in the initial a-phase regions 01 LaNi,H,,” ‘. IZ’ and CaNi5H,.“‘,“) In these initial phases this is believed to be due, at least partly, to trapping phenomena, that is. the absorption of hydrogen into special high-stability sites associated with impurities and local nonstoichiometry, prior to any absorption into a true hydrogen solution. Such special sites are not normally available in the hydrogen-solution regions of hydrogen compounds. Corresponding thermodynamic characterization for other types of compound hydrides, e.g. Laves phase hydrides. AB and A,B types. is not available. The variation of hysteresis, defined as ln(pabs/Pder), where pa,,, and pdeY are the pressures in absorption and desorption, respectively, with the three two-phase conversions regions investigated here are detailed in table 1. Note the large hysteresis of the (y’ + y”) region compared with the (p + y’) and (y” + 6) regions. From previous’** 3, and current X-ray powder-diffraction work on the hydrides and deuterides of this system, the change in molecular volume per H or D (AV1A.u) which accompanies the phase conversion for each of the (s-t y’). (y’+ y”). and (y” + 6) two-phase regions, has been calculated (see table 1). Notice the (y”+ 6) two-phase region has a (A V/A?c) value twice that of the (b + y’) or (y’ + y”) two-phase regions despite its comparatively small hysteresis. These observations disagree with theories”3’ that equate the extent of hysteresis with volume increase of the hydride or deuteride. Table 2 lists the molar enthalpies and molar entropies for both hydrides and deuterides for the three two-phase conversion regions under examination. The molar entropies have been calculated using the mean pressures of absorption and desorption in these regions to compute the equilibrium A,G, and this value has been combined with the average of the observed molar enthalpies of absorption and desorption. These molar entropies have high confidence levels because AH,,, and A,G, have been measured independently. Although only the two-phase conversion molar entropies are tabulated here, the method can also be used to obtain precise molar entropies throughout all the solution regions. There is a consistent increase in the molar entropy [AS,/. within the assigned
TABLE
1. The variation molecular
of hysteresis In(pabs/pder). the desorption plateau pressure p, and the change volume per H or D for the three two-phase conversion regions P kPa
Two-phase region hydride (P+Y’) (7’ + Y”) (7” + 6)
0.56 0.46 0.19
deuteride 0.67 0.3 1 0.079
hydride
deuteride
hydride
deuteride
10.1 +o.i 109+ 1 785+5
9.3kO.l s1+ I 207+5
3.2+0.1 3.3 +0.7 h.Ok0.3
3.3_+0.1 3.OkO.6 6.1 kO.5
in
THERMODYNAMICS TABLE
2. The
molar
enthalpies
OF CaNi,H,
AND
CaNi,D,
and molar entropies of absorption of hydrides three two-phase conversion regions
Two-phase region
and deuterides
fc)r the
deuteride Aadm kJ mol-
p-to-y f-to-y” y-to-6
T-ABLE
1225
’
-33.7kO.2 -36.2kO.5 -26.4* 1.0
3. The
phase
boundaries
I&A J.K-‘.mol-’
A&L kJ.mol-’ 33.3kO.2 36.5kO.5 27.8+ 1.0
during
105*2 135*3 lIti&
-36.OkO.6 -3X.2*0.5 -33.4kO.3
absorption for CaNi,H, molar enthalpies
IA&n/ J.K.-‘.mol
4,erKn kJ.mol-’ 36.OkO.6 38.5kO.5 34.OkO.5
and CaNi,D,
based
~I
112+2 140+3 129*2
on lp, V) and
Y( k 0.0.5)
Phase region hydride Y’ Y” 6
Aadm kJ.mol-’
4.68 to S. 14 5.28 to 5.50 > 6.46
deuteride 4.72 to s.14 5.26 to 5.34 >6.‘5
errors, of approximately 8 J. K ’ mol ’ between the hydride and deuteride and there is also a significant increase in the molar enthalpy lAH,I. This enthalpy difference is ~2 kJ. mol-r for the (b+y’) and (y’+y”) regions. rising to ~7 kJ.mol~’ for the (y”+6) region. This large molar-enthalpy difference for deuterides compared with hydrides for the (y”+6) region is consistent with the marked reverse isotope effect for this transition. That is, the deuteride plateau pressures are lower than the hydride pressures which is opposite to that normally observed,‘14’ in this case by almost a factor of 4 (see table 1). This abnormally large difference in apparent bond energies strongly suggests that the D atoms occupy different sites, with stronger bonds, than the H atoms occupy in the corresponding hydride. Table 3 lists the phase boundaries, in absorption. for the hydride and deuteride results based on (p, x) and (AH,, x). The hydride and deuteride boundaries are consistent until the y” single-phase region is reached. The reduced span in .Y of the deuteride y” single-phase region in comparison with the corresponding hydride phase indicates a significant variation in the site occupancy factors between the hydride and deuteride, presuming the same site being occupied. Whatever the explanation, for the y”- and &phases of CaNi,H, and CaNi,D,. the relatively large thermochemical differences between deuterides and hydrides and the implied differences in bond strengths brings into question the usual assumption that deuterium and hydrogen must occupy structurally equivalent sites. Also of interest is the start of the &single phase which appears to be nonreversible. Another phase conversion cannot be discounted, but the high pressures. slope, and small region in x make X-ray work and accurate enthalpy measurements difficult.
1226
D. M. GRANT,
J. J. MURRAY.
AND
M. L. POST
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO. Il. 12. 13. 14.
Sandrock. G. D.; Murray. J. J.: Post, M. L.: Taylor, J. B. Mar. RES. Bull. 1982, 17. 887. Gainsford, G. J.: Calvert, L. D.; Murray. J. J.: Taylor. J. B. Ada. X-GUY. Anal. 1983, 26, 163. Murray. J. J.: Grant, D. M.; Akiba. E.; Post, M. L. Mar. R~s. Bull. 1986, 21, 515. Calvert. L. D.; Murray, J. J.; Gainsford. G. J.; Taylor. J. B. Mar. RCJS. BUN. 1984, 19. 107. Catvert. L. D.; Powell, B. M.; Murray. J. J.; Le Page, Y. J. .%/iJSrare C/rem. 1985, 60. 62. Murray. J. J.; Post, M. L.: Taylor. J. B. J. Less-Common Met. 1983, 90, 65. Murray. J. J.; Post, M. L.: Taylor. J. B. J. Less-Common Met. 1981, 80, 201. Nomura, K.; Uruno. H.: One. S.; Shinozuka. H.: Suda, S. J. Less-Common MH. 1985, 107, 221. Saindrenan. G.: Vitant-Barbien, J.; Constantinoff, M. J. Lex4’ommnn Me/. 1986, 1 lg. 227. Goodell, P. D. J. Less-Common Met. 1984, 99. I. Murray. J. J.: Post, M. L.: Taylor, J. B. J. Less-Common Met. 1981, SO, 21 I. Murray, J. J.: Post, M. L. J. Less-Common Met. 1984, 103, 129. Scholtus, N. A.; Hall. W. K. J. C’hem. Phys. 1963, 39. 868. Libowitz, G. G. The Solid S/a/e Chemistry qf Binary Metal Hydrides. Benjamin: New York. 1%~.
J. J. MURRAY,
and
M.
I-. POST
Division of Chemistry. National Research Cotmcil c?f’Canada, Ottawa, Ontario. Canada KIA OR6 ( Received 217 January 1987; in final~fitrtn 6 April 1987) The thermodynamics of CaNi,H, and CaNi,D, were investigated for 4.0.~ < 6.9. Contirmation that the y single-phase region actually consists of two similar structurally related phases. *f’ and y”, was obtained. Enthalpies of the hydrides and deuterides, and pressure hysterescs of the (fl+*f’). (y’+y”). and (y”+S) two-phase regions are detailed. The reverhc isotope effect. hysteresis. enthalpy. and entropy considerations are discussed and the assumption that hydrides and deuterides hate similar bonding is questioned.
1. Introduction CaNi,H, near ambient temperatures and pressuresconsists of a series of hydride phases denoted, for ease of reference, as the r-. p-. y-, and S-phases.“’ The end-member phases, the cl-phase (0
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ILnnltetl
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D. M. GRANT.
J. J. MURRAY.
AND
M. L. POST
and to examine the thermochemistry of the anomalous isotope eflect reported”’ for the plateau pressure of the y-to-6 phase conversion. Finally the combined results for the entire system have been used to establish the entropy characteristics of the hydriding of CaNi,. 2. Experimental The experimental apparatus, operating conditions, and procedures for this pressurcagainst-composition (p, x) and calorimetric work are detailed elsewhere.“’ The (p. x) measurements were made using welded 316 stainless-steel manifolds with bellows-sealed valves. and capacitance-type pressure transducers. To obtain the high precision required for this study, the manifold external to the sample region was held at (303.0+0.1) K using a controlled circulating air bath. The sample region within the calorimeter was cooled to (273.15+0.01) K, and as a consequence, the hydrogen pressures of the isotherm were lowered, enabling measurements to be taken with the high-precision transducers. The sensitivity of the calorimeter at this temperature was determined by comparing the tabulated enthalpy increments for NBS standard reference material 720, cl-Al,O,, with those obtained with our calorimeter. The volume-expansion factors for this non-isothermal system were determined using helium, with the sample present, at the experimental temperatures for the manifold and sample. Preliminary and confirmatory results were made using three samples from two independent sources of CaNi,. (3J The sample used here was obtained from a 100 g batch taken from a melt of reagent-grade Ca and Ni prepared specifically for research purposes by Ergenics Ltd. at Wyckoff, New Jersey. Before use, the sample was annealed in a stainless-steel container under 100 kPa of argon at I 17.5 K for 7 d. This material was shown to contain mass fractions < 3 x IO-” of metallic impurities, 5 x lop4 of oxygen, and 4.9 x 10m2 of nickel in excess of that required by the CaNi, stoichiometry. X-ray powder diffractometry confirmed that the excess nickel was present as elemental nickel. Prior to the hydride and deuteride (p, .u) and calorimetric measurements, the sample was cycled to the S-region three times in H, or D, at 300 K. then vacuum degassed at 328 K for 16 h. Detailed results were confined to the region of interest: from the (p+ y’) two-phase region to the 6 single-phase region. The sample size was (7.322+0.001) g and measurements were corrected for the elemental Ni present. This sample size along with the experimental temperature chosen and precise temperature control, enabled increments in x as small as 0.02 to be measured to within 0.6 per cent. 3. Results Figure 1 shows the (p, x) isotherm at 273.15 K for the hydride measurement sequence. A similar measurement sequence was conducted for the deuteride at the same temperature. In desorption, measurements were terminated once a return to the (y’+ p) two-phase region was clearly established, since detailed thermodynamic measurements have already been reported for 0 < x < 2.5.
THERMODYNAMICS
FIGURE 1. The variation [\. y’. f’. 6: 0. absorption;
of hydrogen +, desorption.
OF CaNi,H,
pressure
for CaNi,H,
AND
CaNi,D,
illustrating
1’19
all 5 single-phase
regmns:
2.
The molar enthalpies of absorption obtained here for 0 < s < 2.5 show excellent agreement with the previously reported results. (h) All molar enthalpies are for the reaction: (Z.iG.u)(CaNi,(H
or D),+/&(H
or D), = CaNi,(H
or D),,,,,;.
For the a-to-b conversion, A,H,(273 K) = -(44.3 +0.3) kJ . mol- ’ compared with A,H,(313 K) = -(43.4+0.2) kJ mol- ‘. The difference shows that = (ZA,H,/aT),, for the U-to-S conversion is positive. The errors given arc ~C,.nl appropriate for direct comparison since both were obtained with the same apparatus: a further +0.5 kJ. mol- r is added when total errors are quoted. For the purposes of clarity the relative partial molar enthalpy of single-phase regions and the enthalpy of formation A,H, of one phase from another are represented together and presented here as AH,,,. For graphical and descriptive purposes, it is also convenient to use the absolute value IAH,\ avoiding the change of sign between absorption and desorption. Figures 2 and 3 give the detailed simultaneous (p. X) and IAH,,, against LZ; results in the region 4 < x < 6.9 for the hydride and deuteride respectively, where
D. M. GRANT.
1 I-7Y~ 55,
I,
J. J. MURRAY.
I I I r /,
I,
FIGURE 2. The variation of molar enthalpy during absorption; +, pressure during desorption; of desorption.
17,
,
AND
M. L. POST
/,
, , , Ty---yJ
and pressure with CaNi,H,r composition: 0, pressure 0. molar enthalpy of absorption. x . molar enthalpy
repeated for absorption and desorption in the deuteride run (see figure 3). The deuteride was chosen because, in this region, there is a reverse isotope effect in which the steady-state pressures are lower than that of the hydride, enabling a more accurate pressure transducer to be used. X-ray powder diffraction of the unhydrided sample gave a hexagonal cell, u = 0.49503, nm and c = 0.39392, nm. The angular full-width half maximum increased from 1.9 x 1O-3 to 2.1 x IO-” in 20 between prepared sample and after This peak broadening is minimal in the hydride and deuteride experiments. comparison with that observed for LaNi,,@’ and indicates little structural damage. which is unusual for hydrides in general.
4. Discussion The results reported here for CaNi 5H x,4.0 were taken at 273 K. This reduced the y”-to-6 transition pressures so that more precise pressure transducers could be used. The previously reported thermodynamic measurements(6’ for CaNi,H,, 2.5 were taken at 313 K so as to raise the a-to-(3 transition pressure to permit accurate (p, .u) measurements in desorption. Because of this temperature difference, the previous pressure and AC, values cannot be combined with the present results to form a single set for the entire composition range. However, p and hence AC, are available for the entire range in absorption at 273 K from the current measurements. The two
THERMODYNAMICS
1 FIGURE 3. The variation of molar during absorption; +, pressure during of desorption.
OF CaNi,H,
s
.Y
AND
h
1221
CaNi,D,
7
enthalpy and pressure with CaNi,D, composition: 0. pressure desorption: 0, molar enthalpy of absorption; x . molar enthalpy
sets of results can be combined for AH,,, with a small correction since, for this temperature range, @AH,/aT), is small compared with the measured enthalpy. In a recent study,‘9’ .it was noted that CaNi, formed peritectically and had a significant range of composition CaNi,,,. CaNi,H,,,,,5 showed clear evidence”’ of this stoichiometric range for single-phase “CaNi,“. The cell parameters and AG, values for the a-to-0 conversion differed for the two samples. However, enthalpies for the a-to-l3 and P-to-y transitions showed no significant variation. Therefore variations in the single-phase cell parameters and hence. presumably, in the stoichiometry of unhydrided phases do not limit the construction of a composite IAH, against (x) diagram. Evidence that disproportionation of CaNi,H, to CaH, and Ni occurs has been reported.“” In the present and previous studies in this laboratory, several CaNi, samples have been cycled up to 8 times in H, and D, at 273 K < T < 3 13 K and 11< 4 MPa, and examined using X-ray powder-diffraction and precision (p, .u) measurements. In all cases the (p, x) diagrams show excellent composition closure for absorption and desorption cycles. All X-ray patterns taken after hydriding and tiehydriding cycles show only CaNi,. and Ni at the levels initially present. The studies in the literature which report disproportionation were at temperatures above it 3 K. The conclusion is that, up to 313 K, the hydrides of CaNi, have negligible rates of disproportionation.
IT?-) ---
D. M. GRANT,
J. J. MURRAY.
AND
M. L. POST
The (p, x) and (AH,, .u) results shown in figures 2 and 3 were obtained over approximately 28 d for a charge/discharge cycle. This allowed pressure-steady states and calorimetric baseline to be established at the 0.1 per cent level after addition of each hydrogen aliquot. Nevertheless, both curves. and particularly the (AH,,,. .Y) curve, are influenced by secondary effects which can significantly obscure the thermodynamic characterization of phase boundaries. One major secondary effect which causes smearing of the phase-boundary discontinuities in both absorption and desorption is a persistence of the preceding two-phase conversion as a single-phase region is entered. In general this effect is much more pronounced in absorption. The other major secondary effect, which occurs predominantly in desorption upon leaving a single-phase region, is the premature occurrence of the two-phase conversion because, during the hydrogen-gas titration, the pressure is momentarily below the desorption-plateau value. Because of the narrow composition range of the y’-to-y” transition, these secondary effects lead to a particularly severe obscuration of the enthalpy and entropy characteristics of this transition. As a result, in view of the multiple phase boundaries present, it is useful to construct (p, s) and (AH,,,. s) diagrams from which these secondary effects are eliminated. Pressure hysteresis in the two-phase regions and the consequent phase-boundary composition shifts and variations in the enthalpy discontinuities are still shown in the idealized diagram. Such an idealized diagram for the y’, (y’+y”), and y” regions of (CaNi, + IxH,) at 273 K is shown in figure 4. A modified version of this diagram can be applied to all other two-phase regions studied. The y region, originally thought to be a single phase, was shown in earlier (p, s) and X-ray-diffraction work (j) to consist of two structurally very closely related phases, y’ and y”, separated by a narrow two-phase region. The present thermodynamic measurements corroborate this conclusion in that, over the range of
FIGURE 4. Schematic of idealized (p, x) and (AH,, x) across a two-phase enthalpy hysteresis. applicable to the (y’-y”) region of CaNi,H, or CaNi,D,.
conversion
region
with
no
THERMODYNAMICS
OF
CaNi,H,
AND
CaNi,D,
1323
x of the apparent (y’+y”) two-phase region, there is a clear increase in IAH, for both absorption and desorption and for both hydrides and deuterides. In principle. discontinuities in IAH,\ against (x) with (alAH,l/ax) > 0 at the phase boundaries, with a constant IAH,,, within the two-phase region. are expected. The narrowness of this two-phase region and the comparatively sloped plateau {(a In p/ax) is larger than for any other two/phase region in this system} results in comparatively poor resolution of IAH,] against (x). However, an increase in IAH,/ against (x> is apparent in the experimental results (see figures 2 and 3) between the y’ and (y’+ y”) boundary. No such discontinuity could be resolved in these measurements for the boundary
between (y’+y”)
and y” but, for the purposes
of the idealized diagram (see
figure 4) it is assumed that such an effect exists. It is noteworthy that the rate of change of enthalpy with composition in the y” phase is significantly greater than in the y’ phase. This is additional evidence to add to the (p, x) and X-ray-diffraction evidence reported previously’3’ that the y’-to-y” conversion leads to different sites, involving different bond energies, being occupied in these two closely related structures. The (p, x) and (AH,,,, x) behaviour of the &phase region includes a (p. x) anomaly in absorption at x z 6.4 for the deuteride and x z 6.5 for the hydride. with a corresponding anomaly in IAH,I against (x) with IAH, values as high as 48 kJ . mol- ‘. No corresponding anomaly is observed in desorption; however. the IAH, values for the S-to-y” transition observed in desorption are significantly higher than those observed in absorption, particularly for x in the (y” + 6) two-phase region near the boundary between (y”+6) and 6 (see figures 2 and 3). This deviation suggests the S-phase has a narrow phase-separation region, analogous to (y’+ y”), which occurs at pressures just above the y”-to-6 two-phase conversion pressures. In desorption, as a consequence of this small pressure difference, this narrow S-phase separation reverses sluggishly and encroaches on the high x end of the (y”+6) two-phase region causing larger /AH,,,/ values. As a result. the difference in AH,,, between absorption and desorption for the y”-to-b transition is observed to be larger than for the other phase transitions (see figures 2 and 3 and table 2). The interpretation is subject to significant uncertainty because the high pressures and (Q/ax) values, even for the deuteride, substantially limit the accuracy of the enthalpies in this composition range. The underlying thermodynamics of the &phase is very similar to that of which has the same hexagonal structure and approximate hydrogen LaNi,H,,,.,, or deuterium content. At the lower phase boundary, there is a rapid rise in IAH, (probably a discontinuous rise in an ideal situation) from the y”-to-6 conversion value of 32.5 kJ . mol- ’ to 40.5 kJ . mol -I, followed, for the deuteride, by a region of constant jAH,j for 6.3 < x < 6.7, with the exception of the absorption anomaly described above. There is a subsequent rapid decrease in JAH,,I for the deuteride for .v > 6.7. In general, for the compound hydride phases of AB,H, for which precision enthalpy measurements have been reported, including the p-LaNi, hydride.’ 1. ‘I and the f3-, y’-, y”-, and S-CaNi, hydrides. the relative partial molar enthalpy of solution of H, decreases with increasing s. In p-CaNi, hydride, deuteride, and
1224
D. M. GRANT.
J. J. MIJRRAY.
AND
M.
L. POS?
p-LaNi, hydride there is a region near the lower phase boundary where /AH,,/ t\ constant over a significant range of .u. There is evidence of a similar region in &CaNi, deuteride (see figure 3) particularly from the desorption results; in absorption, evidence for this enthalpy behaviour is obscured by the anomaly described above. (AH,/ also decreases with .Y in the initial a-phase regions 01 LaNi,H,,” ‘. IZ’ and CaNi5H,.“‘,“) In these initial phases this is believed to be due, at least partly, to trapping phenomena, that is. the absorption of hydrogen into special high-stability sites associated with impurities and local nonstoichiometry, prior to any absorption into a true hydrogen solution. Such special sites are not normally available in the hydrogen-solution regions of hydrogen compounds. Corresponding thermodynamic characterization for other types of compound hydrides, e.g. Laves phase hydrides. AB and A,B types. is not available. The variation of hysteresis, defined as ln(pabs/Pder), where pa,,, and pdeY are the pressures in absorption and desorption, respectively, with the three two-phase conversions regions investigated here are detailed in table 1. Note the large hysteresis of the (y’ + y”) region compared with the (p + y’) and (y” + 6) regions. From previous’** 3, and current X-ray powder-diffraction work on the hydrides and deuterides of this system, the change in molecular volume per H or D (AV1A.u) which accompanies the phase conversion for each of the (s-t y’). (y’+ y”). and (y” + 6) two-phase regions, has been calculated (see table 1). Notice the (y”+ 6) two-phase region has a (A V/A?c) value twice that of the (b + y’) or (y’ + y”) two-phase regions despite its comparatively small hysteresis. These observations disagree with theories”3’ that equate the extent of hysteresis with volume increase of the hydride or deuteride. Table 2 lists the molar enthalpies and molar entropies for both hydrides and deuterides for the three two-phase conversion regions under examination. The molar entropies have been calculated using the mean pressures of absorption and desorption in these regions to compute the equilibrium A,G, and this value has been combined with the average of the observed molar enthalpies of absorption and desorption. These molar entropies have high confidence levels because AH,,, and A,G, have been measured independently. Although only the two-phase conversion molar entropies are tabulated here, the method can also be used to obtain precise molar entropies throughout all the solution regions. There is a consistent increase in the molar entropy [AS,/. within the assigned
TABLE
1. The variation molecular
of hysteresis In(pabs/pder). the desorption plateau pressure p, and the change volume per H or D for the three two-phase conversion regions P kPa
Two-phase region hydride (P+Y’) (7’ + Y”) (7” + 6)
0.56 0.46 0.19
deuteride 0.67 0.3 1 0.079
hydride
deuteride
hydride
deuteride
10.1 +o.i 109+ 1 785+5
9.3kO.l s1+ I 207+5
3.2+0.1 3.3 +0.7 h.Ok0.3
3.3_+0.1 3.OkO.6 6.1 kO.5
in
THERMODYNAMICS TABLE
2. The
molar
enthalpies
OF CaNi,H,
AND
CaNi,D,
and molar entropies of absorption of hydrides three two-phase conversion regions
Two-phase region
and deuterides
fc)r the
deuteride Aadm kJ mol-
p-to-y f-to-y” y-to-6
T-ABLE
1225
’
-33.7kO.2 -36.2kO.5 -26.4* 1.0
3. The
phase
boundaries
I&A J.K-‘.mol-’
A&L kJ.mol-’ 33.3kO.2 36.5kO.5 27.8+ 1.0
during
105*2 135*3 lIti&
-36.OkO.6 -3X.2*0.5 -33.4kO.3
absorption for CaNi,H, molar enthalpies
IA&n/ J.K.-‘.mol
4,erKn kJ.mol-’ 36.OkO.6 38.5kO.5 34.OkO.5
and CaNi,D,
based
~I
112+2 140+3 129*2
on lp, V) and
Y( k 0.0.5)
Phase region hydride Y’ Y” 6
Aadm kJ.mol-’
4.68 to S. 14 5.28 to 5.50 > 6.46
deuteride 4.72 to s.14 5.26 to 5.34 >6.‘5
errors, of approximately 8 J. K ’ mol ’ between the hydride and deuteride and there is also a significant increase in the molar enthalpy lAH,I. This enthalpy difference is ~2 kJ. mol-r for the (b+y’) and (y’+y”) regions. rising to ~7 kJ.mol~’ for the (y”+6) region. This large molar-enthalpy difference for deuterides compared with hydrides for the (y”+6) region is consistent with the marked reverse isotope effect for this transition. That is, the deuteride plateau pressures are lower than the hydride pressures which is opposite to that normally observed,‘14’ in this case by almost a factor of 4 (see table 1). This abnormally large difference in apparent bond energies strongly suggests that the D atoms occupy different sites, with stronger bonds, than the H atoms occupy in the corresponding hydride. Table 3 lists the phase boundaries, in absorption. for the hydride and deuteride results based on (p, x) and (AH,, x). The hydride and deuteride boundaries are consistent until the y” single-phase region is reached. The reduced span in .Y of the deuteride y” single-phase region in comparison with the corresponding hydride phase indicates a significant variation in the site occupancy factors between the hydride and deuteride, presuming the same site being occupied. Whatever the explanation, for the y”- and &phases of CaNi,H, and CaNi,D,. the relatively large thermochemical differences between deuterides and hydrides and the implied differences in bond strengths brings into question the usual assumption that deuterium and hydrogen must occupy structurally equivalent sites. Also of interest is the start of the &single phase which appears to be nonreversible. Another phase conversion cannot be discounted, but the high pressures. slope, and small region in x make X-ray work and accurate enthalpy measurements difficult.
1226
D. M. GRANT,
J. J. MURRAY.
AND
M. L. POST
REFERENCES I. 2. 3. 4. 5. 6. 7. 8. 9. IO. Il. 12. 13. 14.
Sandrock. G. D.; Murray. J. J.: Post, M. L.: Taylor, J. B. Mar. RES. Bull. 1982, 17. 887. Gainsford, G. J.: Calvert, L. D.; Murray. J. J.: Taylor. J. B. Ada. X-GUY. Anal. 1983, 26, 163. Murray. J. J.: Grant, D. M.; Akiba. E.; Post, M. L. Mar. R~s. Bull. 1986, 21, 515. Calvert. L. D.; Murray, J. J.; Gainsford. G. J.; Taylor. J. B. Mar. RCJS. BUN. 1984, 19. 107. Catvert. L. D.; Powell, B. M.; Murray. J. J.; Le Page, Y. J. .%/iJSrare C/rem. 1985, 60. 62. Murray. J. J.; Post, M. L.: Taylor. J. B. J. Less-Common Met. 1983, 90, 65. Murray. J. J.; Post, M. L.: Taylor. J. B. J. Less-Common Met. 1981, 80, 201. Nomura, K.; Uruno. H.: One. S.; Shinozuka. H.: Suda, S. J. Less-Common MH. 1985, 107, 221. Saindrenan. G.: Vitant-Barbien, J.; Constantinoff, M. J. Lex4’ommnn Me/. 1986, 1 lg. 227. Goodell, P. D. J. Less-Common Met. 1984, 99. I. Murray. J. J.: Post, M. L.: Taylor, J. B. J. Less-Common Met. 1981, SO, 21 I. Murray, J. J.: Post, M. L. J. Less-Common Met. 1984, 103, 129. Scholtus, N. A.; Hall. W. K. J. C’hem. Phys. 1963, 39. 868. Libowitz, G. G. The Solid S/a/e Chemistry qf Binary Metal Hydrides. Benjamin: New York. 1%~.