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Thermal stability and phase studies of crystalline Zr2Pd hydrides
Journal
of the Less-Common
Metals,
172-174
(1991) 29-35
29
Thermal stability and phase studies of crystalline Zr,Pd hydrides J. S. Cantrell Department of Chemistry, Miami Uniuersity, Oxford, OH 45056 (U.S.A.)
R. C. Bowman, Aeorjet
Electronic
Jr.
Systems Division,
P.O. Box 296, Azusa, CA 91702 (U.S.A.)
Abstract The crystalline metal hydrides Zr,PdH, for x < 2.0 undergoes an endothermic reaction above 800 K and decomposes to ZrH, and ZrPd on further heating above 800 K. When the crystalline hydride has x > 3.0, it undergoes abrupt endothermic transitions around 550 K ( +50 K) which lead to formation of lower hydrogen content ternary hydrides as indicated by changes in the X-ray diffraction patterns. These ternary hydrides undergo further endothermic reactions to form a mixture of phases that includes ZrH,, ZrPd and ZrO, when heated above 800 K. The crystalline decomposition products are similar to those previously found for amorphous Zr,PdH,.
1. Introduction The Zr,Pd-H,
system
has been
the subject
of several
recent
investiga-
tions [l&8] largely because it has both amorphous and crystalline hydride phases over the composition range l-3.3 for x. Problems with the assignment of hydrogen site occupancy have complicated the interpretation of the hydrogen diffusion [4] and electronic structure [3,7] studies of these hydrides. In addition, there have been difficulties with assignment of the crystal structure for x > 2.9 [3,7] where an orthorhombic second phase is indicated by the splitting of a number of the larger tetragonal diffraction peaks. However, when x < 2, it is well known that the hydride phases retain the Cllb (tetragonal MoSi,) type structure [8] of the parent intermetallic and that hydrogen occupies the T, (tetrahedral) sites formed by four zirconium atoms [9, lo].
2. Sample description
and experimental
procedures
The Zr,PdH, samples used for these studies have been prepared by direct reactions at Allied/Signal Corp. [9, lo] with hydrogen gas as described in detail elsewhere [2,5,8]. Briefly, the ternary metal hydrides were prepared by heating a known quantity of hydrogen gas with the crystalline Zr,Pd intermetallic compound in an enclosed reaction vessel. Samples with x < 2.0
Elsevier Sequoia/Printed
in The Netherlands
30 TABLE
1
Summary of X-ray diffraction Sample
XRD lattice
data and results Lattice
Unit cell volume
parameters
(nm?
x x x x
= = = =
1.84 1.94 3.0 3.4
Tetragonal Tetragonal Orthorhombic Orthorhombic
a (nm)
b (nm)
c (nm)
0.3335(2) 0.3340( 1) 0.3398( 1) 0.3407(6)
0.3393( 1) 0.3429(2)
1.1546(7) 1.1597(5) 1.1454(4) 1.1419(2)
0.1284(2) 0.1294( 1) 0.1321(l) 0.1334(3)
were prepared at temperatures around 750 K while those with hydrogen compositions x > 2.9 were prepared at temperatures in the 510-550 K range. The lattice parameters for these samples, as prepared, are summarized in Table 1. Additional information on these materials is available elsewhere [5,8]. The samples, used for the annealing studies, were hermetically sealed under purified argon gas in gold pans for differential scanning calorimetry (DSC) and quenched to room temperature and opened for the powder X-ray diffraction (XRD) studies. The DSC work was performed with a PerkinElmer model DSC-2 calorimeter with thermal anneals typically at 20 K min-‘. The XRD studies were made with a Rigaku wide angle diffractometer using copper radiation. Typically, the samples were heated to the desired maximum temperature in the differential scanning calorimeter the results being recorded); then the run was stopped and the sample was rapidly cooled to room temperature. The gold boat was opened in an inert atmosphere box and a powder diffraction sample prepared by dusting the powder on double sticky tape mounted on a glass slide. The XRD data were taken at room temperature, with the sample open to the air for only about ten minutes during the XRD run. Repeated XRD scans did not indicate either oxide formation or loss of hydrogen by these samples under these data recording conditions.
3. Results The DSC results are given in Fig. 1. Typically, the crystalline samples exhibit much greater thermal stability than the corresponding amorphous samples of the same composition [ll]. The crystalline hydride materials fall into two groups. When x < 2.0, the material is stable and has no change in XRD data or unit cell volume up to 800 K. Above 800 K these (x < 2.0) hydrides decompose slowly to form a mixture of phases that includes ZrH,, ZrPd and zirconium oxides. The release of hydrogen is kinetically hindered and results in a variation in the amounts of these phases for separate or independent DSC anneals. When the crystalline hydride has x > 3.0 it undergoes an abrupt endothermic reaction at 550-600 K (which depends on composition, decreasing in T as x increases) to release hydrogen and to form a
l-ZrzPdH1
8,
t-- - Offscale o-ZrzPdHs
3Ii I-
1
__--‘----4
__-j-----____-j_
i
500
1 540
i
A
I 580
I 620
I 660
I 700
Temperature
Fig. 1. DSC traces of crystalline dent DSC scans had been halted room temperature.
,:
I 740
I 760
i
1.ill 620
660
900
940
960
(K)
Zr,I’dH, (c-Zr,PdH,) material. X. temperatures to obtain samples for XRD measurements after
where indepencooling hack to
32 TABLE
2
Summary of major and minor phases of c-Zr,PdH, Sample
T,,,
x = 1.84
300 800 860 900 980
Tetragonal Tetragonal Tetragonal Tetragonal ZrH,, ZrPd
Zr,PdH, Zr,PdH, Zr,PdH, Zr,PdH,
ZrH, ZrH,,
x = 1.94
300 800
Tetragonal Tetragonal
Zr,PdH, Zr,PdH,
ZrO,
x = 3.0
300 700 800 900
Orthorhombic Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH,
ZrH,,
ZrPd
x = 3.4
300 700 800 900
Orthorhombic Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH,
ZrH, ZrH,,
ZrPd
(W
Major phases
Minor phases
ZrPd
crystalline material of lower hydrogen composition, such as c-Zr,PdH,_, where the hydrogen released, in general, is contained by the sealed gold boats. On heating to 800-1000 K the remaining hydride decomposes gradually as a result of the kinetically hindered hydrogen release and ZrH,, ZrO, and ZrPd are formed as the sample decomposes. Table 2 gives the materials found after quenching a number of samples of different compositions from several different temperatures. The amorphous and crystalline materials give similar decomposition products but the crystalline materials are more stable thermally. The hydrogen released by the crystalline material during the DSC run is apparently not absorbed again when the material is quenched to room temperature. In a few cases the gold pans broke open at the higher temperatures. Additional insights into the effects of thermal annealing are provided by the behavior of the unit cell volumes for three c-Zr,PdH, samples. As maintained its volume until the shown in Fig. 2, tetragonal Zr,PdH,,,, annealing temperature exceeded 800 K. However, anneals to higher temperatures lead to volume decreases that almost reach the value of 0.1191(2) nm3 for hydrogen-free Zr,Pd. Thus, a significant decrease in hydrogen content is strongly indicated which corresponds [ll] to the endothermic character of the DSC trace above 800 K for this sample. Figure 2 also reveals that the volumes for the two samples with x > 3 decrease below 700 K and indicates some loss in stoichiometry at much lower temperatures. Nevertheless, these volumes are still significantly larger than unit cell dimensions for unheated which implies that x remains above 2.0 for annealZr,PdH,.,, or Zr,PdH,.,,, ing temperatures up to at least 700 K. However, the volumes for the samples with initial x > 3 are found to decrease much more rapidly above 800 K
33
0.135
I[I -.
-.
I- .
--._ .
--__ .
-.
.
0.130
oE 5 E 2 :
0.125 I-
= s .E 2
0.12c
C-
Zr2Pd
0.115 IL 300
400
500
600 Temperature
Fig.
2. XRD unit
cell volumes
us. temperature
700
0
x = 1.84
A
x = 3.1
0
x = 3.4
800
900
II 30
(K)
of c-Zr,PdH,
compared with Zr,PdH,,,, . This behaviour higher initial hydride compositions.
suggests greater instability
for the
4. Discussion
Previous studies have shown that c-Zr,PdH, (X < 2.0) materials are tetragonal and that the hydrogen or deuterium atoms occupy exclusively the Zr, tetrahedral sites designated as T, [12]. Above x = 2 (e.g. for x = 3.0) samples have been found to occupy octahedral sites (probably 0, [S]) in addition to the T, tetrahedral sites. The octahedral sites are apparently less
34
stable than the tetrahedral sites as shown by the loss of hydrogen by the lattice at temperatures above 800 K that results from vacating these octahedral sites first [B]. The orthorhombic splitting of some tetragonal XRD peaks [B] produced by occupancy of octahedral site for x > 2.8 is relaxed and a tetragonal lattice occurs when x < 2.0 on the loss of hydrogen, as shown by Fig. 2. Samples (x > 2.8) that had been stored in unsealed containers for a year or more also gave the tetragonal lattice by XRD and indicated a loss of hydrogen which was confirmed by our gas analysis. Rigid lattice secondmoment calculations [2,13,14] are consistent with tetrahedral T, sites (Zr,) being preferred sites and pseudotetrahedral T, and octahedral sites being occupied for higher hydrogen concentration (x > 2.8). Amorphous Zr,PdH, (a-Zr,PdH,) with x < 2.4 undergoes [II] exothermic decomposition around 790-810 K to produce mixtures of the crystalline ZrH, and ZrPd phases while endothermic release of hydrogen gas occurs above 900 K. On the contrary, a-Zr,PdH, material with x > 2.7 endothermically releases hydrogen above 590 K without observation of significant crystallization or decomposition until the temperature exceeds 800 K. c-Zr,PdH, remains unchanged and has flat DSC traces to 800 K when x < 2.0 and on additional heating to 900-1000 K it decomposes to form ZrH, and ZrPd. Orthorhombic material for x > 2.8 undergoes endothermic reactions at 550600 K to release hydrogen and to form tetragonal ternary phases corresponding to lower x values and upon further heating to BOO-1000 K it undergoes decomposition to ZrH, and ZrPd. Some ZrO, is also formed if oxygen has been allowed to reach the sample during preparation or storage. These reactions are summarized as follows (where t indicates tetragonal and o indicates orthorthombic crystalline material): t-Zr,PdH,
(x = 1.84-1.94)
o-Zr,PdH,
(x = 3.0-3.4)
(BOO-1000 K) +ZrPd (600-700
K) + t-Zr,PdH,
+ ZrH, (BOO-1000 K) +ZrH,
+ ZrPd
Acknowledgments We wish to acknowledge A. J. Maeland for preparation of many of these materials and J. E. Wagner for many of the DSC runs and some of the XRD results. We also wish to acknowledge E. F. Jendrek and D. B. Sullenger of EG&G Mound Applied Technologies for helpful discussions and XRD and computer support.
References 1 2 3 4
K. Suzuki, J. Less-Common Met., 89 (1983) 183. R. C. Bowman, Jr., M. J. Rosker and W. L. Johnson, J. Non-Cryst. Solids, 53 (1982) 105. R. C. Bowman, Jr., W. L. Johnson, A. J. Maeland and W.-K. Rhim, Phys. Lett. A, 94 (1983) 181. R. C. Bowman, Jr., A. Attalla, A. J. Maeland and W. L. Johnson, Solid State Commun., 47 (1983) 779.
35 5 R. C. Bowman, Jr., J. S. Cantrell and D. E. Etter, Ser. Metall., 18 (1984) 61. 6 A. Williams, J. Eckert, X. L. Yeh, M. Atzman and K. Samwer, J. Non-Cry&. Solids, 61 (1984) 643. 7 R. C. Bowman, Jr., in G. Bambakidis and R. C. Bowman, Jr. (eds.), Hydrogen in Disordered and Amorphous Solids, Plenum, New York, 1986, p. 237. 8 F. E. Spada, R. C. Bowman, Jr., and J. S. Cantrell, J. Less-Common Met., 129 (1987) 197. 9 A. J. Maeland, J. Less-Common Met., 89 (1983) 173. 10 A. J. Maeland and G. G. Libowitz. J. Less-Common Met., 74 (1980) 295. 11 J. E. Wagner, R. C. Bowman, Jr., and J. S. Cantrell, J. Appl. Phys., 58 (1985) 4570. 12 A. J. Maeland, E. Lukacevic, J. J. Rush and A. Santoro, J. Less-Common Met., 129 (1987) 77. 13 R. C. Bowman, Jr., and W.-K. Rhim, J. Mugn. Reson.. 49 (1982) 93. 14 R. C. Bowman. Jr., and W. E. Tadlock, Solid-State Commun., 32 (1979) 313.
of the Less-Common
Metals,
172-174
(1991) 29-35
29
Thermal stability and phase studies of crystalline Zr,Pd hydrides J. S. Cantrell Department of Chemistry, Miami Uniuersity, Oxford, OH 45056 (U.S.A.)
R. C. Bowman, Aeorjet
Electronic
Jr.
Systems Division,
P.O. Box 296, Azusa, CA 91702 (U.S.A.)
Abstract The crystalline metal hydrides Zr,PdH, for x < 2.0 undergoes an endothermic reaction above 800 K and decomposes to ZrH, and ZrPd on further heating above 800 K. When the crystalline hydride has x > 3.0, it undergoes abrupt endothermic transitions around 550 K ( +50 K) which lead to formation of lower hydrogen content ternary hydrides as indicated by changes in the X-ray diffraction patterns. These ternary hydrides undergo further endothermic reactions to form a mixture of phases that includes ZrH,, ZrPd and ZrO, when heated above 800 K. The crystalline decomposition products are similar to those previously found for amorphous Zr,PdH,.
1. Introduction The Zr,Pd-H,
system
has been
the subject
of several
recent
investiga-
tions [l&8] largely because it has both amorphous and crystalline hydride phases over the composition range l-3.3 for x. Problems with the assignment of hydrogen site occupancy have complicated the interpretation of the hydrogen diffusion [4] and electronic structure [3,7] studies of these hydrides. In addition, there have been difficulties with assignment of the crystal structure for x > 2.9 [3,7] where an orthorhombic second phase is indicated by the splitting of a number of the larger tetragonal diffraction peaks. However, when x < 2, it is well known that the hydride phases retain the Cllb (tetragonal MoSi,) type structure [8] of the parent intermetallic and that hydrogen occupies the T, (tetrahedral) sites formed by four zirconium atoms [9, lo].
2. Sample description
and experimental
procedures
The Zr,PdH, samples used for these studies have been prepared by direct reactions at Allied/Signal Corp. [9, lo] with hydrogen gas as described in detail elsewhere [2,5,8]. Briefly, the ternary metal hydrides were prepared by heating a known quantity of hydrogen gas with the crystalline Zr,Pd intermetallic compound in an enclosed reaction vessel. Samples with x < 2.0
Elsevier Sequoia/Printed
in The Netherlands
30 TABLE
1
Summary of X-ray diffraction Sample
XRD lattice
data and results Lattice
Unit cell volume
parameters
(nm?
x x x x
= = = =
1.84 1.94 3.0 3.4
Tetragonal Tetragonal Orthorhombic Orthorhombic
a (nm)
b (nm)
c (nm)
0.3335(2) 0.3340( 1) 0.3398( 1) 0.3407(6)
0.3393( 1) 0.3429(2)
1.1546(7) 1.1597(5) 1.1454(4) 1.1419(2)
0.1284(2) 0.1294( 1) 0.1321(l) 0.1334(3)
were prepared at temperatures around 750 K while those with hydrogen compositions x > 2.9 were prepared at temperatures in the 510-550 K range. The lattice parameters for these samples, as prepared, are summarized in Table 1. Additional information on these materials is available elsewhere [5,8]. The samples, used for the annealing studies, were hermetically sealed under purified argon gas in gold pans for differential scanning calorimetry (DSC) and quenched to room temperature and opened for the powder X-ray diffraction (XRD) studies. The DSC work was performed with a PerkinElmer model DSC-2 calorimeter with thermal anneals typically at 20 K min-‘. The XRD studies were made with a Rigaku wide angle diffractometer using copper radiation. Typically, the samples were heated to the desired maximum temperature in the differential scanning calorimeter the results being recorded); then the run was stopped and the sample was rapidly cooled to room temperature. The gold boat was opened in an inert atmosphere box and a powder diffraction sample prepared by dusting the powder on double sticky tape mounted on a glass slide. The XRD data were taken at room temperature, with the sample open to the air for only about ten minutes during the XRD run. Repeated XRD scans did not indicate either oxide formation or loss of hydrogen by these samples under these data recording conditions.
3. Results The DSC results are given in Fig. 1. Typically, the crystalline samples exhibit much greater thermal stability than the corresponding amorphous samples of the same composition [ll]. The crystalline hydride materials fall into two groups. When x < 2.0, the material is stable and has no change in XRD data or unit cell volume up to 800 K. Above 800 K these (x < 2.0) hydrides decompose slowly to form a mixture of phases that includes ZrH,, ZrPd and zirconium oxides. The release of hydrogen is kinetically hindered and results in a variation in the amounts of these phases for separate or independent DSC anneals. When the crystalline hydride has x > 3.0 it undergoes an abrupt endothermic reaction at 550-600 K (which depends on composition, decreasing in T as x increases) to release hydrogen and to form a
l-ZrzPdH1
8,
t-- - Offscale o-ZrzPdHs
3Ii I-
1
__--‘----4
__-j-----____-j_
i
500
1 540
i
A
I 580
I 620
I 660
I 700
Temperature
Fig. 1. DSC traces of crystalline dent DSC scans had been halted room temperature.
,:
I 740
I 760
i
1.ill 620
660
900
940
960
(K)
Zr,I’dH, (c-Zr,PdH,) material. X. temperatures to obtain samples for XRD measurements after
where indepencooling hack to
32 TABLE
2
Summary of major and minor phases of c-Zr,PdH, Sample
T,,,
x = 1.84
300 800 860 900 980
Tetragonal Tetragonal Tetragonal Tetragonal ZrH,, ZrPd
Zr,PdH, Zr,PdH, Zr,PdH, Zr,PdH,
ZrH, ZrH,,
x = 1.94
300 800
Tetragonal Tetragonal
Zr,PdH, Zr,PdH,
ZrO,
x = 3.0
300 700 800 900
Orthorhombic Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH,
ZrH,,
ZrPd
x = 3.4
300 700 800 900
Orthorhombic Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH, Tetragonal Zr,PdH,
ZrH, ZrH,,
ZrPd
(W
Major phases
Minor phases
ZrPd
crystalline material of lower hydrogen composition, such as c-Zr,PdH,_, where the hydrogen released, in general, is contained by the sealed gold boats. On heating to 800-1000 K the remaining hydride decomposes gradually as a result of the kinetically hindered hydrogen release and ZrH,, ZrO, and ZrPd are formed as the sample decomposes. Table 2 gives the materials found after quenching a number of samples of different compositions from several different temperatures. The amorphous and crystalline materials give similar decomposition products but the crystalline materials are more stable thermally. The hydrogen released by the crystalline material during the DSC run is apparently not absorbed again when the material is quenched to room temperature. In a few cases the gold pans broke open at the higher temperatures. Additional insights into the effects of thermal annealing are provided by the behavior of the unit cell volumes for three c-Zr,PdH, samples. As maintained its volume until the shown in Fig. 2, tetragonal Zr,PdH,,,, annealing temperature exceeded 800 K. However, anneals to higher temperatures lead to volume decreases that almost reach the value of 0.1191(2) nm3 for hydrogen-free Zr,Pd. Thus, a significant decrease in hydrogen content is strongly indicated which corresponds [ll] to the endothermic character of the DSC trace above 800 K for this sample. Figure 2 also reveals that the volumes for the two samples with x > 3 decrease below 700 K and indicates some loss in stoichiometry at much lower temperatures. Nevertheless, these volumes are still significantly larger than unit cell dimensions for unheated which implies that x remains above 2.0 for annealZr,PdH,.,, or Zr,PdH,.,,, ing temperatures up to at least 700 K. However, the volumes for the samples with initial x > 3 are found to decrease much more rapidly above 800 K
33
0.135
I[I -.
-.
I- .
--._ .
--__ .
-.
.
0.130
oE 5 E 2 :
0.125 I-
= s .E 2
0.12c
C-
Zr2Pd
0.115 IL 300
400
500
600 Temperature
Fig.
2. XRD unit
cell volumes
us. temperature
700
0
x = 1.84
A
x = 3.1
0
x = 3.4
800
900
II 30
(K)
of c-Zr,PdH,
compared with Zr,PdH,,,, . This behaviour higher initial hydride compositions.
suggests greater instability
for the
4. Discussion
Previous studies have shown that c-Zr,PdH, (X < 2.0) materials are tetragonal and that the hydrogen or deuterium atoms occupy exclusively the Zr, tetrahedral sites designated as T, [12]. Above x = 2 (e.g. for x = 3.0) samples have been found to occupy octahedral sites (probably 0, [S]) in addition to the T, tetrahedral sites. The octahedral sites are apparently less
34
stable than the tetrahedral sites as shown by the loss of hydrogen by the lattice at temperatures above 800 K that results from vacating these octahedral sites first [B]. The orthorhombic splitting of some tetragonal XRD peaks [B] produced by occupancy of octahedral site for x > 2.8 is relaxed and a tetragonal lattice occurs when x < 2.0 on the loss of hydrogen, as shown by Fig. 2. Samples (x > 2.8) that had been stored in unsealed containers for a year or more also gave the tetragonal lattice by XRD and indicated a loss of hydrogen which was confirmed by our gas analysis. Rigid lattice secondmoment calculations [2,13,14] are consistent with tetrahedral T, sites (Zr,) being preferred sites and pseudotetrahedral T, and octahedral sites being occupied for higher hydrogen concentration (x > 2.8). Amorphous Zr,PdH, (a-Zr,PdH,) with x < 2.4 undergoes [II] exothermic decomposition around 790-810 K to produce mixtures of the crystalline ZrH, and ZrPd phases while endothermic release of hydrogen gas occurs above 900 K. On the contrary, a-Zr,PdH, material with x > 2.7 endothermically releases hydrogen above 590 K without observation of significant crystallization or decomposition until the temperature exceeds 800 K. c-Zr,PdH, remains unchanged and has flat DSC traces to 800 K when x < 2.0 and on additional heating to 900-1000 K it decomposes to form ZrH, and ZrPd. Orthorhombic material for x > 2.8 undergoes endothermic reactions at 550600 K to release hydrogen and to form tetragonal ternary phases corresponding to lower x values and upon further heating to BOO-1000 K it undergoes decomposition to ZrH, and ZrPd. Some ZrO, is also formed if oxygen has been allowed to reach the sample during preparation or storage. These reactions are summarized as follows (where t indicates tetragonal and o indicates orthorthombic crystalline material): t-Zr,PdH,
(x = 1.84-1.94)
o-Zr,PdH,
(x = 3.0-3.4)
(BOO-1000 K) +ZrPd (600-700
K) + t-Zr,PdH,
+ ZrH, (BOO-1000 K) +ZrH,
+ ZrPd
Acknowledgments We wish to acknowledge A. J. Maeland for preparation of many of these materials and J. E. Wagner for many of the DSC runs and some of the XRD results. We also wish to acknowledge E. F. Jendrek and D. B. Sullenger of EG&G Mound Applied Technologies for helpful discussions and XRD and computer support.
References 1 2 3 4
K. Suzuki, J. Less-Common Met., 89 (1983) 183. R. C. Bowman, Jr., M. J. Rosker and W. L. Johnson, J. Non-Cryst. Solids, 53 (1982) 105. R. C. Bowman, Jr., W. L. Johnson, A. J. Maeland and W.-K. Rhim, Phys. Lett. A, 94 (1983) 181. R. C. Bowman, Jr., A. Attalla, A. J. Maeland and W. L. Johnson, Solid State Commun., 47 (1983) 779.
35 5 R. C. Bowman, Jr., J. S. Cantrell and D. E. Etter, Ser. Metall., 18 (1984) 61. 6 A. Williams, J. Eckert, X. L. Yeh, M. Atzman and K. Samwer, J. Non-Cry&. Solids, 61 (1984) 643. 7 R. C. Bowman, Jr., in G. Bambakidis and R. C. Bowman, Jr. (eds.), Hydrogen in Disordered and Amorphous Solids, Plenum, New York, 1986, p. 237. 8 F. E. Spada, R. C. Bowman, Jr., and J. S. Cantrell, J. Less-Common Met., 129 (1987) 197. 9 A. J. Maeland, J. Less-Common Met., 89 (1983) 173. 10 A. J. Maeland and G. G. Libowitz. J. Less-Common Met., 74 (1980) 295. 11 J. E. Wagner, R. C. Bowman, Jr., and J. S. Cantrell, J. Appl. Phys., 58 (1985) 4570. 12 A. J. Maeland, E. Lukacevic, J. J. Rush and A. Santoro, J. Less-Common Met., 129 (1987) 77. 13 R. C. Bowman, Jr., and W.-K. Rhim, J. Mugn. Reson.. 49 (1982) 93. 14 R. C. Bowman. Jr., and W. E. Tadlock, Solid-State Commun., 32 (1979) 313.