5.70- HCP " ~
2.52.4. Thermodynamic properties
Lundin and Yamamoto (1967) used the Knudsen effusion technique to determine the thermodynamic activities in the G d - Y alloy system. The vapor pressures of both G d and Y in the solid state are weil below the measuring ability of this technique;
hence it was necessary to employ the molten state. Before analyses of the alloys, the v a p o r pressures of the pure components were measured. Vapors effused from nine liquid G d - Y alloys at 1600°C were collected and subjected to X-ray spectrometer analysis. Activities of both components were determined from the ratio of the
GADOLINIU M-
YTTRIUM
1.0 \ r t i I
\ _ " ~ T = 1600° C ~ O.B-
aGd~" ~ /.,d/~-ay
>- 0.6
>
i-- o
< 0.4
0.2
I I I I
O0 20 4.0 60 80 100
Gd ATOMIC PERCENT YTTRIUM Y
Fig. 91. Activities of gadolinium and yttrium in liquid Gd-Y alloys at 1600°C.
c o m p o n e n t s in the c o n d e n s e d vapor phase using a modified G i b b s - D u h e m relation.
T h e i r activity d a t a are p r e s e n t e d in fig. 91 where it is observed that the data fit a straight line a n d R a o u l t ' s law is obeyed in the liquid state. T h u s the liquid alloy solutions at 1 6 0 0 ° C are t h e r m o d y n a m i c a l l y ideal. T h e e n t h a l p y of mixing is zero.
T h e o n l y difference in these two elements is electronic structure, which does n o t affect the solution ideality in the G d - Y system. This solution ideality would be e x p e c t e d b e c a u s e of the similarity of atomic diameters, electronegativities, valences a n d crystal structures.
References
Bauminger, E.R., A. Diamant, I. Feiner, I. Nowik and S. Ofer, 1975, Phys. Rev. Lett. 34, 962.
Laridjani, M. and J.F. Sadoc, 1981, J. Phys. (Paris) 42, 1293.
Lundin, C.E. and A.S. Yamamoto, 1967, Final report, Denver Research Institute Rept. DRI-2437, University of Denver, Denver, CO.
McWhan, D.B. and A.L. Stevens, 1967, Phys. Rev. 154, 438.
Shiflet, G.J., J.K. Lee and H.I. Aaronson, 1979, Calphad 3, 129.
Spedding, F.H., R.M. Valletta and A.H. Daane, 1962, Trans. Quarterly (Am. Soc. Met.) 55, 483.
Yakel, H.L., 1968, Oak Ridge National Laboratory Rept. ORNL-4370, Oak Ridge, TN.
Yakel, H.L., 1969, Oak Ridge National Laboratory Rept. ORNL-4470, Oak Ridge, TN.
õ
CO l.IJ
_o l--
J o
5.70
5.68
5.66
5.64
TERBIUM- DYSPROSlUM
~ Hc~ ,
5.6501
76 Sir e , o
~
5.62° s.58 1 I I I I
0 20 40 60 80 I00
Tb ATOMIC PERCENT DYSPROSIUM Dy
Fig. 92. Lattice spaclngs in the terbium-dysprosium system. The straight lines show the Vegard's law relationships for the pure metaJs and are based on the data in table 2.
118 K.A. G S C H N E I D N E R and F.W. C A L D E R W O O D
2.53.
Tb-Dy: Terbium-dysprosium
2.53.1.Lattice spacings
Sirota and Semirenko (1976) studied ternary terbium-dysprosium-holmium al- loys and included diagrams that showed lines of equal value of the lattice spacings in this ternary system. Binary lattice spacing data in the terbium-dysprosium system were obtained by scaling the T b - D y side of their diagram. The data shown in fig. 92 have been adjusted to being the lattice spacings observed by these authors into agreement with the accepted values for these pure metals as listed in table 2. Both the a and c lattice spacing data appear to be in agreement with the Vegard's law relationship, which is depicted by the straight lines between the data for pure terbium and dysprosium.
Reference
Sirota, N.N. a n d V.V. Semirenko, 1976, Izv. Akad. N a u k SSSR, Met. [6], 209 [English transl.: Russ.
Metall. [6], 167].
1600
,1400
1200
1000
n--
~ B o o - n.- w
f . . w
~ 6 0 0 -
4 O 0 -
2 0 0 - -
0 0 Tb
TERBIUM-HOLMIUM
I I I I
- LIQUID --
1474 (, - 1 5 5 6 °
~ ~ 1462 °
1 ~ o ~ ' ( ~ T b ) BCC
(«Tb,Ho) HCP
I I I
20 4 0 60 8 0
ATOMIC PERCENT HOLMIUM
100 Fig. 93. Phase diagram of the ter- Ho b i u m - h o l m i u m system.
2.54. Tb-Ho: Terbium-holmium 2.54.1. Phase diagram
The phase diagram for the terbium-holmium system has been reported by Spedding et al. (1973). Impurities present in their holmium metal (in atppm) were:
816 H, 556 O, 59 each N and Si, 30 Fe, 27 C, 26 F, 22 Sc and 20 each Cu and Er.
Their terbium metal contained (in atppm) at 1340 O, 315 H, 66 C, 57 Fe, 53 Ta, 46 N, 33 F and 20 each Ca and Cu.
This system was studied by thermal, X-ray and metallographic methods. Care was tal(en in the preparation of specimens and in the design of equipment and experi- mental runs to assure accuracy in the reported results, which include the phase diagram shown in fig. 93.
Since the thermal arrests were within 1.5°C on both the heating and cooling curves, as is the case for pure metals, the liquidus-solidus line and the solvus line for the bcc ~ hcp transformation were drawn as single lines. The transformation ternperature of terbium was found to be raised linearly by the addition of holmium but at a greater slope than the melting temperature. Extrapolation of the transforma- tion temperature curve to the solidus showed that the two curves intersect at 90 at%
Ho. This confirms the absence of the bcc form at high temperatures in holmium.
5.70
77 5.68
i 0
( / )
uJ 5.66
t O p -
...1
5.64
5.62 o <
d Z 0
co 3.61
I.d F-
~ 3.59
._.1
3.57 0 Tb
TERBIUM - HOLMIUM
I I I I
- 3 . 6 0 5 5
_ 0 _ _
20 4 0 6 0 8 0 100
ATOMIC PERCENT HOLMIUM Ho
Fig. 94. Lattice spacings in the ter- b i u m - h o l m i u m system. The straJght lines represent the Vegard's law relationships in this system based on the data for the pure metals listed in table 2.
120 K.A. GSCHNEIDNER and F.W. CALDERWOOD
Shiflet et al. (1979) have applied the Kaufman approach to the calculation of the T b - H o phase diagram and have compared their results with those obtained experi- mentally by Spedding et al. Their calculated peritectic point in this system fell at a somewhat lower temperature and holmium concentration than did the experimen- tally deterrnined peritectic point.
2.54.2. Lattice spacings
Lattice spacing data from the report by Spedding et al. (1973) and from Sirota and Semirenko (1976) are presented in fig. 94. The data of Sirota and Semirenko were presented in the form of a ternary diagram showing lines of equal value of the lattice parameters in the T b - D y - H o alloy system. The data presented hefe were obtained by scaling the T b - H o side of their diagram, then adjusting this data so that the lattice spacings for the end-members agreed with the accepted values for the pure metals as shown in table 2. No adjustments were required on the data of Spedding et al. The data, as plotted, show good agreement with Vegard's law. The presence of small deviations in the data of Sirota and Semirenko (a, negative; c, positive) is probably due to higher impurity levels in their alloys.
References
Shiflet, G.J., J.K. Lee and H.I. Aaronson, 1979, Calphad 3, 129.
Sirota, N.N. and V.V. Semirenko, 1976, Izv. Akad. Nauk SSSR, Met. [6], 209 [English transl.: Russ.
Metall. [6], 167].
Spedding, F.H., B. Sandeen and B.J. Beaudry, 1973, J. Less-Common Met. 31, 1.
2.55. Tb-Er: Terbium-erbium 2.55.1. Phase diagram
Spedding et al. (1973) have investigated the terbium-erbium system using metallography, X-ray and thermal analysis methods. The terbium used in prepara- tion of their alloys contained the following impurities (in atppm): 1340 O, 315 H, 66 C, 57 Fe, 53 Ta, 46 N, 33 F and 20 each Ca and Cu. Impurities in their erbium (also in atppm) were: 368 O, 331 H, 145 Cu, 97 C, 60 Ta, 28 Y and 26 F. Specially designed thermal analysis equipment was used to precisely establish the melting and transition temperatures of the alloys in this system. Their phase diagram is shown in fig. 95. As anticipated, each component was entirely soluble in the other over the entire composition range. The melting point of T b - E r alloys was observed to incréase linearly with increasing erbium concentration. The bcc phase terminates at 66 at% Er. The transformation temperature for the bcc ~ hcp transition was lowered linearly from 66 at% Er with increasing terbium content to the transition tempera- tute of pure terbium, 1289°C. No bcc ~ hcp transition was observed in pure erbium metal or in alloys containing more than 66 at% Er.
Shiflet et al. (1979) combined the Kaufman approach with values of the enthalpies and entropies of melting and transformation of the pure rare earth metals and calculated the phase diagram for the T b - E r system. The peritectic point in the
1800
1600
1400
1200
P
kO ù," 1000
IM
o . 8 0 0 - - IM
6 0 0 -
4 0 0 - -
2 0 0 - -
TERBIUM- ERBIUM
I I I I
-- LIQUID 1529 ~
66%
~ ( Æ T b )
(aTb,Er) HCP
_
I I I I
0 0 20 40 60 80
Tb ATOMIC PERCENT ERBIUM
100 Fig. 95. Phase diagram of the terbi- Er um-erbium system.
calculated T b - E r diagram fell at a higher temperature and at a higher erbium concentration than did the experimentally determined peritectic point.
2.55.2. Lattice spacings
Spedding et al. (1973) measured lattice spacings on their carefully prepared T b - E r alloys. Although their spacings, shown in fig. 96, are nearly linear, they observed a distinct deviation from linearity that was positive for the a lattice spacings and sliglitly negative for the c latfice spacings.
McWhan and Stevens (1967), who measured magnetic properties of several rare earth alloys as a function of pressure, reported the lattice spacings at atmospheric pressure for an alloy containing 43 at% Er. The a and c lattice spacings for this alloy are included on fig. 96. The c spacing for their alloy is in fair agreement witli the data of Spedding et al. but has a small positive deviation from linearity. The a spacing for their alloy has a large positive deviation from linearity, whicli would appear to indicate the presence of nonmetallic impurities in their alloy (see section 1.2.2).
122 K.A. GSCHNEIDNER and F.W. CALDERWOOD
TERBIUM-ERBIUM
5.61 I I I I 5.72
5.6055 o , a 75 Spe
x,+ 67 Mcw
5.6C - - 5.70
5.5c~ - o - 5.68
o<~ ~ 0 c9"
C9 Z
5.58 - 5.66
w
õ 5.57 5.64 _o
- - I..-
~ 5.56~ 5.62 o
/
3.55 [ 5.60
3.54 « 5.58
0 2 0 4 0 6 0 8 0 100
Tb A T O M I C PERCENT EIRBIUM Er
Fig. 96. Lattice spacings in the terbium-erbium system. The straight lines illustrate Vegard's law behavior and are based on aceepted values for the pure metals as listed in table 2.
References
McWhan, D.B. and A.L. Stevens, 1967, Phys. Rev. 154, 438.
Shiflet, G.J., J.K. Lee and H.I. Aaronson, 1979, Calphad 3, 129.
Spedding, F.H., B. Sandeen and B.J. Beaudry, J., 1973, Less-Common Met. 31, 1.
2.56. Tb- Yb: Terbium-ytterbium 2.56.1. Lattice spacings
Burgardt et al. (1979) studied magnetic ordering in terbium alloys with several n o n m a g n e t i c diluents and reported lattice spacings for several alloy compositions.
Ytterbium, which exists in the nonmagnetic divalent state, did not form a wide range of solid solutions. The lattice spacing data reported for this system by Burgardt et al.
are presented in fig. 97 along with the accepted values of the lattice spacings for pure t e r b i u m m e t a l from table 2. A negative deviation f r o m Vegard's law behavior is observed in b o t h the a and c lattice spacings in fig. 97. These authors found that the addition of a nonmagnetic solute such as ytterbium, which expands the c lattice spacings of terbium, promotes ferromagnetism, whereas the addition of a solute, which shrinks the c parameter of terbium, favors helical magnetic structure.
Reference
Burgardt, P., S. Legvold, B.J. Beaudry and B.N. Harmon, 1979, Phys. Rev. B 20, 3787.
5.80 o« 5./8
,9"
Z U 5.76
5.74
I -
J 5.72
u
5.70 o ~ 3.65
(.9 Z
3.63
o l h l ( J
F- 3.61
/
o 3.59
TERBIUM - YTTERBIUM
I I I I I I
5.6966
_
~~%ó~_
o
-o~~~ ~ ~ °
I I I I I I
0 2 4 6 8 I0 12 14
Tb ATOMIC PERCENT YTTERBIUM
Fig. 97. Lattice spacings in the terbium-rich portion of the terbium-ytterbium system. The Vegard's lines are based on the accepted values for the pure metals as listed in table 2.
2.57. Tb-Lu: Terbium-lutetium 2.57.1. Phase relationships
Smidt (1962) and Smidt and Daane (1963) investigated electrical resistivity of several rare earth alloy systems and reported the existence of complete solubility in the t e r b i u m - l u t e t i u m system as confirmed by X-ray diffraction and resistivity methods. This result was not unexpected since these metals satisfy the H u m e - R o t h e r y requirements for formation of extensive solid solutions: similar valence, electronega- tivities and crystal structures, and have a difference in metallic radii of less than 3%.
2.57.2. Lattice spacings
Smidt and D a a n e (1963) reported lattice spacings for four alloy compositions and for the pure metals in the T b - L u system. Their observed lattice spacings have been adjusted on the basis of composition to bring the observed values for the end-mem- bers into agreement with the accepted values for the p u r e metals as shown in table 2.
These adjusted values are presented in fig. 98 along with a set of lattice spacings for a 32.5 at% Lu alloy from M c W h a n and Stevens (1967) and a set for a 10 at% Lu alloy f r o m Burgardt et al. (1979). The a lattice spacings of Smidt and Daane have a
124 K.A. GSCHNEIDNER and F.W. CALDERWOOD 5.66
5.64
5.62
3.60 (
Z ( J
5.58
W _o
F-
~ 5.56
:5.54
TERBIUM - LUTETI UM
1 ..... I I
t . 6 0 5 5
X
I
o,e 6:3 Smi +, x 67 Mcw
o,m 79 Bur
5.70
5.68
5.66
5.64 (.9
Z (_)
5.62 O3 W 0 F- F-
5.60 ~
5.58
5.52 -- " ~ 5 . 5 6
5,5494-
.3.50 - - 5.5052 5.54
0 20 40 60 80 |00
Tb ATOMIC PERCENT LUTETIUM Lu
Fig. 98. Lattice spacings in the terbium-lutetium system.
The straight lines illustrate the Vegard's law relationships and are based on the accepted val- ues for the lattice spacings of the pure metals, shown in table 2.
positive deviation from Vegard's law behavior over m o s t of this composition range.
Their c lattice spacing data points scatter about the Vegard's law line and generally are in good agreement. The lattice spacings for the alloy reported by M c W h a n and Stevens h a d a negative deviation in the a spacing. T h e data for the 10 at% Lu alloy reported b y Burgardt et al. had an a spacing that agreed with Vegard's law and a c spacing that showed a negative deviation.
References
Burgardt, P., S. Legvold, B.J. Beaudry and B.N. Harmon, 1979, Phys. Rev. B 20, 3787.
McWhan, D.B. and A.L. Stevens, 1967, Phys. Rev. 154, 438.
Smidt, F.A., Jr., 1962, Ph.D. thesis, Iowa State University, Ames, IA.
Smidt, F.A., Jr. and A.H. Daane, 1963, J. Phys. Chem. Solids 24, 361.
2.58. Tb-Sc." Terbium-scandium 2.58.1. Lattice spacings
Chatterjee a n d Corner (1971) measured lattice spacings of T b - S c single crystal specimens containing 11.0, 17.5 and 30.5 at% Sc as weil as polycrystalline specimens containing 50 and 75 at% Sc. The latter were prepared b y repeated arc-melting of the
raw materials to attain homogeneity. Absorption spectroscopy showed the composi- tions to be accurate to within +_ 0.5 at%. Lattice spacings at room temperature were determined by the X-ray powder method. The data shown in fig. 99 have been adjusted so that the spacings for terbium and scandium agree with the accepted values for these pure metals as shown in table 2. The terbium-scandium alloys have hcp structure and the a lattice spacings appear to follow Vegard's law within experimental error. A slight negative deviation from Vegard's law behavior is observed for most of the c spacing data. Burgardt et al. (1979) reported the lattice spacings for a 10at% Sc alloy that also had been prepared by arc-melting the constituent metals. Their lattice spacing data agreed with those of Chatterjee and Corner. Cavin et al. (1966) reported lattice spacings for five compositions in the T b - S c system. Since no spacing data were included for the end-members, no adjustments could be made on the data for the alloys. However, with one exception, these data agree well with those from the other sources. Their c lattice spacing for the 75 at% Sc alloy is so far out of line (it cannot be plotted in fig. 99) as to suggest a typographical error, particularly when the a lattice spacing for this composition and all of their other c lattice data lie about where anticipated.
o <
TERBIUM-SCANDIUM
5.70, ~ I I
5.60 -
(.9 Z
õ 5 . 5 O -
t.r)
o 5 . 4 0 -
M
o 5 . 3 0 -
W
3.60 ~
(ù.9 Z 0 ~< 3.5o
O0 UJ
o_ 3.40
I-- _1
o 3.30 0 Tb
I œ l
o,e 71 Cha +,x 79 Bur
o" 5.268i.
ù, r
[ [ I 3 i 3 0 8 8 ~
20 40 60 80 100
ATOMIC PERCENT SCANDIUM Sc
Fig. 99. Lattice spacings in the terbium-scandium system. The straight lines, based on the accepted values for the pure metals as listed in table 2, show the Vegard's law rela- tionships for this system.
126 K.A. GSCHNEIDNER and F.W. CALDERWOOD
Chatterjee and Corner measured thermal expansion on their 11 at% Sc alloy and found that the a spacing contracted from 300 K down to the Curie temperature (136 K) where there was a sharp contraction and a continued contraction in the ferromagnetic state. The c spacing contracted down to the Neel point then expanded u p o n further cooling. It was observed by Burgardt et al. that scandium diluents in terbium suppress the ferromagnetic state relative to the helical state.
References
Burgardt, P., S. Legvold, B.J. Beaudry and B.N. Harmon, 1979, Phys. Rev. B 20, 3787.
Cavin, O.B., R.M. Steele, L.A. Harris and H.L. Yakel, 1966, Oak Ridge National Laboratory Rept.
ORNL-3970 (October), Oak Ridge, TN.
Chatterjee, D. and W.D. Comer, 1971, J. Phys. (Paris), Suppl. 32, C1-243.
2.59. Tb- Y: Terbium-yttrium 2.59.1. Phase diagram
Markova et al. (1967) investigated the terbium-yttrium system by means of microscopy, X-ray diffraction, thermal analysis, hardness and electrical resistance measurements. Their starting materials were distilled yttrium of 99.6 to 99.7 (wt?)%
purity and terbium of 98.5 to 99% purity. Impurities in their terbium included yttrium, gadolinium, dysProsium, calcium, copper, iron and tantalum. Both metals contained gaseous impurities. Alloys were melted in an arc furnace under a helium atmosphere and annealed at 850°C for 70 hr.
All alloys in the system were single phase and had hcp structure at room temperature. The solidus temperature for each alloy was determined in vacuum using a calibrated optical pyrometer focused on a 1 m m hole drilled to a depth of 3 - 4 mm in an attempt to achieve black body conditions (normally a 10 : 1 ratio of depth to diameter is required for black body conditions).
The phase diagram for this system is presented in fig. 100. Since their value for the melting point of yttrium (1502°C vs. the accepted value of 1522°C) and for the transition (bcc ~ hcp) temperature of both metals (Tb: 1317°C and Y: 1490°C) did n o t agree with the accepted values for the pure metals (1289 and 1478°C, respec- tively) as listed in table 1, the melting and transition temperatures of the alloys were adjusted on the basis of composition. These discrepancies may be due to the gaseous materials present in their metals and suggested that purities of their metals are significantly lower than reported. As noted above, only the solidus line was estab- lished by experimental data. The other phase boundaries in their diagram are presented as dashed lines, which conform to the anticipated phase relationships.
Complete solid solubility exists at all compositions below the solidus including a narrow single-phase bcc region at high temperatures and hcp structure at lower temperatures.
2.59.2. Lattice spacings
McWhan and Stevens (1967) studied the effect of high pressures on several rare earth alloys including compositions ranging from 100% Tb to Tb-70 at% Y. They reported lattice spacings in terms of the ratio of the lattice spacing of the alloy to the
o
IJJ r e ZD
n -
I-- bJ
1800
1600
1 4 0 1
1200
I000
800
600
400
200
TERBIUM - YTTRIUM
F I 1 I I
LIQUID ~ ~ ~
-- u
(ŒTb,QY) HCP
0 I I I I
0 20 40 60 80 I00
Tb ATOMIC PERCENT YTTRIUM Y
Fig. 100. Phase diagram of the terbium- yttrium system.
corresponding lattice spacing for pure terbium. The values for the lattice constants of pure terbium given in table 2 were used to obtain the data for the alloys. Their samples were prepared by arc-melting appropriate mixtures of terbium and yttrium [stated purity 99.9(wt?)%]. Weight loss during preparation ranged from 0.02 to 0.24% and this was taken as the maximum deviation in composition that may have occurred. Lattice spacings were determined from Guinier X-ray powder patterns.
Burgardt et al. (1979), who investigated magnetic ordering in terbium alloys, reported lattice spacings for a Tb-10 at% Y alloy. This alloy, made from high purity metals, was also prepared by arc-melting, followed by a three day anneal and a cold watet quench.
Belovol et al. (1975) investigated crystal structure of terbium-yttrium alloys in the 0 to 50 at% Y composition range over the 77 to 300 K temperature range. A plot of lattice spacings vs. temperature for six compositions was reported and it was necessary to scale the plot to retrieve their data, then adjust the retrieved data to bring the lattice spacings for terbium and yttrium into agreement with the accepted values for the pure metals as listed in table 2. The alloy data were then adjusted
128 K.A. G S C H N E I D N E R a n d F.W. C A L D E R W O O D
accordingly by prorating the differënces of the pure metals by the respective compositions.
Finkel' and Vorob'ev (1967) studied the structure of several Tb-Y solid solutions over the temperature range - 196 to 57°C (77 to 330 K) by X-ray diffraction. Their alloys were made from terbium of 99.5 (wt?)% purity and yttrium of 99.8(wt?)%
purity. After arc-melting, their samples were cut to size then annealed at 1200°C in vacuum for 10 to 50hr. Their data, also retrieved by scaling their plots, have been adjusted to bring the lattice spacings for terbium and yttrium into agreement with the table 2 values. Cavin et al. (1966) reported lattice spacings for four compositions in the T b - Y system, but did not report spacings for the end-members and did not indicate the purity of the metals involved or the measurement methods employed.
The lattice spacing data from all five references are shown in fig. 101. The data from McWhan and Stevens show a positive deviafion from Vegard's law behavior in both the a and c lattice spacings. Several of the adjusted data points from Belovol
5.65
3 . 6 4
TERBIUM - Y T T R I U M
I I t
o n 6 7 M c w +, x 7 9 B u r v , , 75 Bei e , • 67 Fin , • 6 6 C0v
[]
[] • []
A V
ò.63 ]-- o
Z / æ "~
õ •
tu 3 . 6 2 | ~ A
_o o
"J :3.61 HCP o
3 . 6 0
[]
• 0 O • A
I
5 . 7 ö l 8
t I t I
0 2 0 4 0 6 0 8 0 I 0 0
Tb ATOMIC PERCENT Y T T R I U M Y
1
] 5.7:.3 5.74(.9 z 5.72 o
O»
5.71 tu
» V--
..J
[5.70
o-- 5.69
Fig. 101. Lattice spacings in the t e r b i u m - y t t r i u m system. The straight lines represent Vegard's law behavior and are based on the accepted values for the pure metals as listed in table 2.
et al. fall directly on corresponding data points from McWhan and Stevens. The lattice spacings for the 90 at% T b - 1 0 at% Y alloy reported by Burgardt et al. lie on the Vegard's law line and may thus reflect higher purity in this alloy. As can be seen in fig. 101, the addition of yttrium expands the c lattice spacing of terbium. Unlike l a n t h a n u m and ytterbium, which also expanded the c lattice and promoted ferro- magnetism in the aUoys, Burgardt et al. observed that yttrium additions favored helical magnetic structure. The data of Finkel' and Vorob'ev, even after adjustment, show more scatter than the data from the other sources, particularly along the c axis where three of their compositions have a negative deviation from the Vegard's law relationship. T h e unadjusted data of Cavin et al. follow the same trend as the data f r o m the other sources with the exception of the a-lattice spacing for the T b - 1 0 at%
Y alloy, which has a negative rather than a positive deviation from Vegard's law behavior.