- Email: [email protected]
Desorption isotherms of DyFe3 hydrides
Journal of the Less-Common Metals, 71(1980) 311 - 315 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
DESORPTION
ISOTHERMS
311
OF DyFea HYDRIDES
HENRY A. KIERSTEAD Solid State Science Division, (Received
Argonne
National
Laboratory,
Argonne,
III. 60439
(U.S.A.)
July 30, 1979)
Summary Desorption isotherms for the DyFes-H system were obtained at 0 “C and 20 “C. The isotherms are analyzed in terms of a theory of multiplateau hydrogen absorption recently published by the author. Changes observed in the isotherms in successive absorption-desorption cycles are described.
1. Introduction Bechman et al. [l] have reported that DyFes absorbs 3.2 atoms of hydrogen at 28.2 atm pressure and 125 “C. Although no isotherm was published, they reported only a single plateau. In a search for additional phase transitions in this system, we studied the desorption isotherms of DyFesH, at 0 “C and 20 “C. Only one plateau was found, but the structure of the isotherm in the one-phase region indicates that at least one additional phase transition would be found at lower temperatures. The total absorption at 10 000 Torr and 0 “C is 3.9 hydrogen atoms per mole of DyFe,. We also observed changes in the desorption isotherm during absorptiondesorption cycles. This phenomenon is described in Section 2.
2. Experimental
procedure
and observations
DyFe, was prepared by arc melting 99.99% pure iron with 99.9% pure dysprosium in a purified argon atmosphere and was annealed for 9 days at 1000 “C. The lattice parameters for the PuNis-type structure, determined from Debye-Scherrer diffraction patterns, were a = 5.12 A and c = 24.48 a, The lines were sharp, and no other phases were detected. Hydrogen absorption was determined by adding measured amounts of 99.999% pure hydrogen and subtracting the hydrogen remaining in the gas phase. Then the desorption curve was measured by allowing the hydrogen to expand into reservoirs of known volume. Pressures were measured with Texas Instruments quartz Bourdon gauges having a range of 10 000 Torr, and a resolution of 0.04 Torr at the higher pressures and 0.01 Torr at the lower
312
pressures. The sample was maintained at a constant temperature of 20 f 0.05 “C in a thermistor-controlled thermostat or 0 ?r 0.05 “C in an ice bath. The temperature standard was a platinum resistance thermometer. After absorption of hydrogen at 20 “C, the first desorption isotherm was measured at 20 “C. These measurements are plotted as circles in Fig. 1. During the course of these measurements, isopleths (pressure P uersus temperature T curves at constant composition) were measured at hydrogen concentrations x of 2.16 and 0.71. They are shown in Fig. 2. The square at P = 0.98, x = 2.16 in Fig. 1 is the pressure measured at 0 “C on the isopleth. When substantially all the hydrogen had been desorbed, hydrogen was again added at 0 “C and a desorption isotherm was measured at 0 “C. These measurements are plotted as triangles in Fig. 1. The shift in the concentration at the inflection point and at the upper end of the plateau suggested that there had been some change in the sample between the two isotherms. Therefore the sample was reloaded with hydrogen at 0 “C!and a third desorption run was made, measuring pressures alternately at 0 “C and 20 “C. These points are indicated by + symbols in Fig. 1. Finally the hydrogen was completely removed by evacuation at 275 “C!for 4 h and the sample was reloaded with hydrogen at 20 “C. A few desorption measurements at 20 “C gave results identical with the first desorption at 20 “C, indicating that heating to 275 “C had restored the sample to its original condition. Finally the hydrogen concentration was reduced to x = 1.78, where the pressure was 1.7 Torr, and the sample was removed from the apparatus. X-ray diffraction measurements showed that the structure of the metal atoms was the same as in the original DyFes, but the lattice parameters had
H;
DyFe3
Fig. 1. Desorption isotherms of DyFea hydrides: broken second run (0 “C); solid lines, third run (0 “C and 20 “C).
lines,
first run (20
“C) and
313
Fig, 2. Isopleths of DyFe3H0.71 and DyFesH2.16.
increased to a = 5.26 A and c = 25.54 a. Another sample from the same batch of DyFes was hydrided to x = 2.5, where the pressure is 5.1 Torr, and was poisoned with SO%. It also had the same structure as the DyFes with lattice parameters of a = 5.34 a and c = 25.80 8.
3. Results and discussion The isopleth at x = 0.71, in a region where the isotherms are stable through three absorption-desorption cycles, is a straight line the slope of which corresponds to a heat of absorption of -10.91 kcal (mol Hz)-’ for the e--p phase transition. The entropy of absorption is -23.74 cal K-l mol- ‘. The isopleth at x = 2.16 in the one-phase region is slightly curved. The partial molar heat of absorption is -9.68 kcal (mol Ha)- ’ near 0 “C and -10.39 kcal mole1 near 95 “C!.The corresponding partial molar entropies of absorption are -22.20 and -24.47 cal K-l mol-’ respectively. Since this concentration is in the region where the shape of the isotherm changed between the first and second desorptions, the isopleth probably does not refer to the equilibrium state. Since there is a large difference between the 20 “C points in the first and third desorption runs while there is only a small difference between the second and third runs (at 0 “C), it seems likely that whatever change took place in the sample was substantially finished by the third run and that the two isotherms of that run represent the equilibrium state of the material. The shape of these isotherms is well represented by a multiplateau extension of the Lather [2] theory recently proposed by the author [3].
314
The theory views the isotherm as the result of an equilibrium of hydrogen in the gas phase with hydrogen absorbed in several types of sites in the metal lattice. Each type of site is characterized by four parameters: ni, the number of sites of the ith kind per mole of metal; AHi, the heat of absorption on these sites; AS,, the entropy of absorption; Tj, the critical tempera~re for these sites. To fit the DyFe, isotherms, three types of sites are needed. Values of the twelve parameters that give a good fit to the isotherms of the third run are listed in Table 1. The solid lines in Fig. 1 were calculated using these parameters. The first type of site, with a critical temperature of 463 K, accounts for the W-P phase transition. The second type, with a critical temperature of 247 K, is mainly responsible for the inflection point near x = 2.4 and would produce a second phase transition below 247 K. The third group accounts mainly for the upper part of the isotherm. Since its critical temperature is negative it cannot cause a phase transition at any temperature, All three types of site contribute to the absorption below the plateau. The isotherms from the first two desorption runs can be fitted with this same set of parameters, changing only the nj values. These values are also listed in Table 1 and the calculated curves are shown as broken lines in Fig. 1. The heat and entropy of absorption for type 1 sites are in excellent agreement with those obtained from the isopleth at x = 0.71. This is to be expected since the plateau results from a phase transition involving type 1 sites only. At x = 2.16 the type 1 sites are nearly all filled, while the type 2 and type 3 sites are both partially filled and the distribution of hydrogen atoms between them changes with temperature because of their different heats of absorption. Thus we expect the heat of absorption obtained from the isopleth to fall somewhere between the heats of absorption for the two types and to be somewhat temperature dependent. This is exactly what is found. The ABs lattice can be viewed as being composed of alternating units of AB, and ABs lattices. The tetrahedral interstitial sites are grouped in hexagonal rings around A-A bonds. There are, per A atom, 4 hexagonal ring derived from the ABa lattice and $j rings derived from the ABs lattice, TABLE 1 Parametersof DyFea in the multipla~au theory Parameter
AHi( kcal-1 mol-‘2 AS&CalK-l md- ) T&K) ni (run 1) nj (run 2) ni (run 3)
Parameter
values
i=l
i=2
i=3
-10.93 -23.64 463 1.141 1.531 1.634
- 9.07 -20.02 247 1.143 0.665 0.576
- 10.63 - 31.46 -154 1.806 1.916 1.883
315
making a total of 5 rings each conta~~g six ~terstiti~ sites. There is in addition one octahedral site per A atom. Since in run 3 the parameters nl and n3 are both close to $, it seems likely that type 1 sites consist of one hydrogen atom in each hexagonal ring and type 3 sites consist of a second atom in each ring. Then the type 2 sites could be octahedral sites. Comparing runs 1 and 2 with run 3, we see that n3 is substantially the same in all three runs. But some of the sites which are type 2 in run 3 are type 1 in run 1 and to some extent in run 2, This shortens the plateau and moves the inflection point toward smaller values of 3~.These changes apparently occur at some point during the absorption-desorption cycle. They are unaffected by temperature cycling between 0 “C and 95 “C, as in measuring an isopleth, but are reversed by heating the hydrogen-free metal to 275 “C. These phenomena are at present unexplained.
We wish to express our thanks to D. Niarchos for preparing the DyFes sample and making the X-ray measurements on the metal and the hydrides. This work was supported by the U.S. Department of Energy.
References 1 C. A. Bechman, A. Goudy, T. Takeshita, W. E. Wallace and R. S. Craig, Znorg. Chem., 15 (1976) 2184. 2 J. R. Lather, Proc. R. Sot. London, Ser. A, 161 (1937) 525. 3 H. A. Kierstead, J. Less-Common Met., 71 (1980) 303.
DESORPTION
ISOTHERMS
311
OF DyFea HYDRIDES
HENRY A. KIERSTEAD Solid State Science Division, (Received
Argonne
National
Laboratory,
Argonne,
III. 60439
(U.S.A.)
July 30, 1979)
Summary Desorption isotherms for the DyFes-H system were obtained at 0 “C and 20 “C. The isotherms are analyzed in terms of a theory of multiplateau hydrogen absorption recently published by the author. Changes observed in the isotherms in successive absorption-desorption cycles are described.
1. Introduction Bechman et al. [l] have reported that DyFes absorbs 3.2 atoms of hydrogen at 28.2 atm pressure and 125 “C. Although no isotherm was published, they reported only a single plateau. In a search for additional phase transitions in this system, we studied the desorption isotherms of DyFesH, at 0 “C and 20 “C. Only one plateau was found, but the structure of the isotherm in the one-phase region indicates that at least one additional phase transition would be found at lower temperatures. The total absorption at 10 000 Torr and 0 “C is 3.9 hydrogen atoms per mole of DyFe,. We also observed changes in the desorption isotherm during absorptiondesorption cycles. This phenomenon is described in Section 2.
2. Experimental
procedure
and observations
DyFe, was prepared by arc melting 99.99% pure iron with 99.9% pure dysprosium in a purified argon atmosphere and was annealed for 9 days at 1000 “C. The lattice parameters for the PuNis-type structure, determined from Debye-Scherrer diffraction patterns, were a = 5.12 A and c = 24.48 a, The lines were sharp, and no other phases were detected. Hydrogen absorption was determined by adding measured amounts of 99.999% pure hydrogen and subtracting the hydrogen remaining in the gas phase. Then the desorption curve was measured by allowing the hydrogen to expand into reservoirs of known volume. Pressures were measured with Texas Instruments quartz Bourdon gauges having a range of 10 000 Torr, and a resolution of 0.04 Torr at the higher pressures and 0.01 Torr at the lower
312
pressures. The sample was maintained at a constant temperature of 20 f 0.05 “C in a thermistor-controlled thermostat or 0 ?r 0.05 “C in an ice bath. The temperature standard was a platinum resistance thermometer. After absorption of hydrogen at 20 “C, the first desorption isotherm was measured at 20 “C. These measurements are plotted as circles in Fig. 1. During the course of these measurements, isopleths (pressure P uersus temperature T curves at constant composition) were measured at hydrogen concentrations x of 2.16 and 0.71. They are shown in Fig. 2. The square at P = 0.98, x = 2.16 in Fig. 1 is the pressure measured at 0 “C on the isopleth. When substantially all the hydrogen had been desorbed, hydrogen was again added at 0 “C and a desorption isotherm was measured at 0 “C. These measurements are plotted as triangles in Fig. 1. The shift in the concentration at the inflection point and at the upper end of the plateau suggested that there had been some change in the sample between the two isotherms. Therefore the sample was reloaded with hydrogen at 0 “C!and a third desorption run was made, measuring pressures alternately at 0 “C and 20 “C. These points are indicated by + symbols in Fig. 1. Finally the hydrogen was completely removed by evacuation at 275 “C!for 4 h and the sample was reloaded with hydrogen at 20 “C. A few desorption measurements at 20 “C gave results identical with the first desorption at 20 “C, indicating that heating to 275 “C had restored the sample to its original condition. Finally the hydrogen concentration was reduced to x = 1.78, where the pressure was 1.7 Torr, and the sample was removed from the apparatus. X-ray diffraction measurements showed that the structure of the metal atoms was the same as in the original DyFes, but the lattice parameters had
H;
DyFe3
Fig. 1. Desorption isotherms of DyFea hydrides: broken second run (0 “C); solid lines, third run (0 “C and 20 “C).
lines,
first run (20
“C) and
313
Fig, 2. Isopleths of DyFe3H0.71 and DyFesH2.16.
increased to a = 5.26 A and c = 25.54 a. Another sample from the same batch of DyFes was hydrided to x = 2.5, where the pressure is 5.1 Torr, and was poisoned with SO%. It also had the same structure as the DyFes with lattice parameters of a = 5.34 a and c = 25.80 8.
3. Results and discussion The isopleth at x = 0.71, in a region where the isotherms are stable through three absorption-desorption cycles, is a straight line the slope of which corresponds to a heat of absorption of -10.91 kcal (mol Hz)-’ for the e--p phase transition. The entropy of absorption is -23.74 cal K-l mol- ‘. The isopleth at x = 2.16 in the one-phase region is slightly curved. The partial molar heat of absorption is -9.68 kcal (mol Ha)- ’ near 0 “C and -10.39 kcal mole1 near 95 “C!.The corresponding partial molar entropies of absorption are -22.20 and -24.47 cal K-l mol-’ respectively. Since this concentration is in the region where the shape of the isotherm changed between the first and second desorptions, the isopleth probably does not refer to the equilibrium state. Since there is a large difference between the 20 “C points in the first and third desorption runs while there is only a small difference between the second and third runs (at 0 “C), it seems likely that whatever change took place in the sample was substantially finished by the third run and that the two isotherms of that run represent the equilibrium state of the material. The shape of these isotherms is well represented by a multiplateau extension of the Lather [2] theory recently proposed by the author [3].
314
The theory views the isotherm as the result of an equilibrium of hydrogen in the gas phase with hydrogen absorbed in several types of sites in the metal lattice. Each type of site is characterized by four parameters: ni, the number of sites of the ith kind per mole of metal; AHi, the heat of absorption on these sites; AS,, the entropy of absorption; Tj, the critical tempera~re for these sites. To fit the DyFe, isotherms, three types of sites are needed. Values of the twelve parameters that give a good fit to the isotherms of the third run are listed in Table 1. The solid lines in Fig. 1 were calculated using these parameters. The first type of site, with a critical temperature of 463 K, accounts for the W-P phase transition. The second type, with a critical temperature of 247 K, is mainly responsible for the inflection point near x = 2.4 and would produce a second phase transition below 247 K. The third group accounts mainly for the upper part of the isotherm. Since its critical temperature is negative it cannot cause a phase transition at any temperature, All three types of site contribute to the absorption below the plateau. The isotherms from the first two desorption runs can be fitted with this same set of parameters, changing only the nj values. These values are also listed in Table 1 and the calculated curves are shown as broken lines in Fig. 1. The heat and entropy of absorption for type 1 sites are in excellent agreement with those obtained from the isopleth at x = 0.71. This is to be expected since the plateau results from a phase transition involving type 1 sites only. At x = 2.16 the type 1 sites are nearly all filled, while the type 2 and type 3 sites are both partially filled and the distribution of hydrogen atoms between them changes with temperature because of their different heats of absorption. Thus we expect the heat of absorption obtained from the isopleth to fall somewhere between the heats of absorption for the two types and to be somewhat temperature dependent. This is exactly what is found. The ABs lattice can be viewed as being composed of alternating units of AB, and ABs lattices. The tetrahedral interstitial sites are grouped in hexagonal rings around A-A bonds. There are, per A atom, 4 hexagonal ring derived from the ABa lattice and $j rings derived from the ABs lattice, TABLE 1 Parametersof DyFea in the multipla~au theory Parameter
AHi( kcal-1 mol-‘2 AS&CalK-l md- ) T&K) ni (run 1) nj (run 2) ni (run 3)
Parameter
values
i=l
i=2
i=3
-10.93 -23.64 463 1.141 1.531 1.634
- 9.07 -20.02 247 1.143 0.665 0.576
- 10.63 - 31.46 -154 1.806 1.916 1.883
315
making a total of 5 rings each conta~~g six ~terstiti~ sites. There is in addition one octahedral site per A atom. Since in run 3 the parameters nl and n3 are both close to $, it seems likely that type 1 sites consist of one hydrogen atom in each hexagonal ring and type 3 sites consist of a second atom in each ring. Then the type 2 sites could be octahedral sites. Comparing runs 1 and 2 with run 3, we see that n3 is substantially the same in all three runs. But some of the sites which are type 2 in run 3 are type 1 in run 1 and to some extent in run 2, This shortens the plateau and moves the inflection point toward smaller values of 3~.These changes apparently occur at some point during the absorption-desorption cycle. They are unaffected by temperature cycling between 0 “C and 95 “C, as in measuring an isopleth, but are reversed by heating the hydrogen-free metal to 275 “C. These phenomena are at present unexplained.
We wish to express our thanks to D. Niarchos for preparing the DyFes sample and making the X-ray measurements on the metal and the hydrides. This work was supported by the U.S. Department of Energy.
References 1 C. A. Bechman, A. Goudy, T. Takeshita, W. E. Wallace and R. S. Craig, Znorg. Chem., 15 (1976) 2184. 2 J. R. Lather, Proc. R. Sot. London, Ser. A, 161 (1937) 525. 3 H. A. Kierstead, J. Less-Common Met., 71 (1980) 303.