A gamma-phase in the plutonium-mercury system

216

JOURNAL

Short

OFTHE

LESS-COMMON

METALS

Communications

A gamma-phase in the plutonium-mercury

system

The existence of two compounds, PuHgs and PuHg4, in the high-Hg end of the Pu-Hg system has been reportedl. The compound PuHg4 is reported to be isostructural with UHg4 r which has a pseudo b.c.c. structure with a = 3.63 A 2 with only two atoms in the pseudo-cell. An investigation of the Pu-Hg system was undertaken to gain a better

understanding

of the structure

of PuHge.

An alloy was prepared by dissolving high-purity Pu in Hg. The excess Hg was distilled by heating the alloy, in vacua, at 105°C for about two weeks. The resulting compound was powdered and sealed in quartz capillaries which were then coated with collodion to prevent the spread of alpha contamination, The powder patterns, prepared with Ni filtered CuKcu-radiation* (35 kV, 20 mA) were of exceptionally fine quality.

A chemical

analysis

of the Pu-Hg

alloy gave a Hg-to-Pu

atom ratio slightly

larger than 4 : I. The powder pattern of this compound, believed to be PuHg4, showed strong reflections at I/& = 0.1556, 0.3093, 0.4624, and 0.6154 in addition to numerous weaker reflections. These strong reflections correspond to a b.c.c. structure two atoms per unit cell. The positions of some higher-angle strong reflections, ticularly

the presence

structure

is a distorted

type

of a strong reflection b.c.c. structure,

at I/G = 0.5643,

suggested

namely the y-brass structure

with par-

that the true

(Strukturbericht

081-3).

The y-brass

structure

has a unit cell containing

52 (or 416) atoms

which is

formed from a body-centered cube by multiplying the cell edge by three (or six) and removing one atom out of every 27 (BRADLEY AND THEWLIS~). A number of compounds having the stoichiometry AsBzi have been reported with the y-brass structureJ-6. In all of these compounds the B element is from group IIB (Zn, Cd, and Hg) or is Be. The A element is a transition metal from the set Mn, Fe, Co, Ni, Rh, Pd, or Pt. Table I compares the observed data for the Pu-Hg compound with published intensity data for the y-phases Ni&Znzi 4, Co&Znzi 4, Pd&rzi 4, N&d21 4, NisBezi 5, and Pt&rsi 4. The powder data for Pt$?& was indexed on the basis of a six-fold multiplication of the pseudo-cell while the other compounds were indexed on the basis of a three-fold multiplication. As a result, for the latter compounds, the published values of h2 + k2 + 12 have been multiplied by 4 to place all data on a comparable basis. The Pu-Hg compound could not be successfully indexed assuming a three-fold multiplication of the pseudo-cell and it was necessary to assume that the unit cell of this compound is a six-fold multiple of the pseudo-cell containing 416 atoms. This is indicated in Table I by the presence of numerous observed weaker reflections for which k2 + K2 + 12 is not a multiple of 4. The excellent agreement among these data, especially for the stronger reflections, is presented as proof of the isomorphism among these phases. If the isomorphism is complete, the Pu-Hg compound should be assigned the formula PugHg2r. The results of the chemical analysis of the Pu-Hg alloy are consistent with this stoichiometry. * l(KOrl) = 1.54050 A; 2(&z) = J. Less-Common

Metals,

I.f+,34

II (1966) zI6+Ig

li;

,i(&a,e)

=

1.54178 A

SHORT

COMMUNICATIONS

TABLE

1

COMPARISON PHASES

217

HAVING

OF

OBSERVED

AgB21

DATA

FOR

PusHgzl

WITH

PUBLISHED

INTENSITY

DATA

FOR

Y-BRASS

STOICHIOMETRY

144 15’ 160 168

0.3093 0.3256

2.148 2.142

‘7’ 176 I80

0.3667

2.145 2.144

184 ‘87 192 LOO 203 208 211 2 16 219 ‘21

0.3774 0.3856

2.142

111

0.4007 0.41’ 0.4283

m 111

0.436~

I

2.143 2.141

2.142 2.149

VW

s w

m

VW

m m m

0.4528 0.4624 0.4688 0.4790

2.146 2.141

2.141 2.138

227 228 232 236

243 244 248 251 25h

m

m

0.4870

2.136

0.5’99

2.140

0.5301

2.193

0.5374

2.14r

VW

0.5540

2. r39

s

m s

0.5643

2.138

272 ‘75 276

m

m

0.5805

2.134

s

0.5887

2. c3.3

280

\v

w

288

m

m

0.0154

2.‘37

291 202 296

m 0.6392

2.138

0.6679

2.141

0.6907

2.138

259 260 264 267

283

299 304 308 372 3’5 320 323 324 328 33’ 3.36 339 344 3.52

w

m

s \\’

m

s

VW

m

m

m VW

m VW

218

SHORT

356 360 372 376 379 384 392

w m In m m vs

COMMUNICATIONS

s VW

0.7677 0.7920

2.132 2.129

“VW

o&73

2.131

Ins

0.8355

2.131

Exact agreement among the data for all reflections for the compounds tabulated is not to be expected because, (I) the relative scattering powers of the A and B atoms is not the same for all the compounds, and (2) for these compounds the values of the radius ratio are different. Different values of the radius ratio result in different atom parameters with corresponding effect on the intensities. Since the compound

PusHgzi

has a radius

ratio

nearest

that

for Pt&nai

7 one would expect

the data for these two compounds to give the most favorable agreement. Table I suggests that this is the case. These two compounds are the only ones tabulated which are indexed on the basis of the larger unit cell. From the above considerations it is concluded that the compound reported by ELLINGER~ to be PuHg4 is actually PusHgzi and has the y-brass structure (081-a). The unit cell edge, as determined by a least-square extrapolation to cos2 0 = o, is 21.78 A with IJ = 0.01 .&. The X-ray density is 13.90 g/cm3. There are 416 atoms (16 formula units) in the unit cell. The y structure is formed when the electron to atom ratio is 21/13 and may exist for a wide variety of stoichiometries and a wide range of values of the radius ratios. For PugHgzi to satisfy the electron to atom ratio of 21/13 it is necessary to assign to plutonium a valence of zero. The transition metals in all other known y-phases with AsB2i stoichiometry are likewise assigned a valence of zero. For PugHgzi the edge of the pseudo-cell is 3.63 A while for the isomorphous UHg4 the pseudo-cell has a = 3.627 if 2. This is consistent with the observation that for isomorphous U and Pu compounds the Pu compound has a slightly larger unit cellr,6. Addition of Pu to n-U results in a small increase in the volume of the unit cellg. In view of the reported isomorphism between UHg4 and the compound which is herein shown to be PugHgzi, it does not seem unreasonable to predict that the compound

UHg4 is probably

y-phase

UbHg21.

The author wishes to thank Dr. R. HIGHT for reviewing the manuscript. The Pu-Hg alloy was made, and the powder patterns were prepared at the Argonne National Laboratory under the auspices of the U.S. Atomic Energy Commission. Metallurgy Division, Argonne National Laboratory, Argonne, Ill. (U.S.A.) ; and Chew&y Departnzent, University of Missouri at St. Louis, St. Louis, MO. (U.S.A.)* * Presentaddress. J. Less-Common

Metals,

II (1966) 216-219

A.F.

BERNDT

SHORT

I L 3 4 5 b

7 8 9

COMMUNICATIONS

219

F. H. ELLINGER, U.S. At. Energy Comm., Rept. AECU-4629, 196. R. E. KUKDLE AND A. S. WILSON, Acta Cryst., 2 (1949) 148. _I. J. BRADLEY AND J. THEWLIS, Proc. Roy. Sot. (Londolz), Ar12 (1926) 678. W. ERMAN,~. Phys. Chew, BIG (1931) 57. L. MISCH, Z. Phys. Chem., Bzg (1935) 42, A. TAYLOR AND B. J. EAGLE, Crystallographic Ikzta on M&l mzd Alloy Structures, Do\-cr, New York, 1963. E. TE~TU~I, K. GSCHNEIDNER,JR.,AND J.WABER, U.S.;~t.E?z~~~~'Corv2nz., Rept.Lrl-23~5,1900. H. JONES, Proc. Roy. Sot. (London), il144 (1934) 225. A. F. UERNDT, U.S. At. Energy Comnz., Rept. .d2\‘L-6460, 1902.

Received

Niobium

April zest,

1966

alloys containing beryllium

The attractiveness

of strengthening

niobium alloys by reactive

metal additions,

such as zirconium or hafnium, has long been recognized. In brief, the strengthening effects are usually attributed to a dispersed-phase mechanism relying on association between the reactive addition and either residual or intentionally-added interstitial elements. Because of the great solution affinity of niobium for interstitially dissolved elements at high temperatures, commercial alloys in the Nb-(W, MO, Ta, V)- (Zr, Hf) systems respond to thermal treatment, and may be prone to structural instability under certain conditions in the usual service temperature regimes (~ooo-rjoo”(‘). The thermodynamic stabilities of some beryllium compounds suggest that beryllium may indeed be a meritorious alternative or supplemental addition to zirconium- or hafnium-containing niobium alloys for improved structural properties. *A series of alloys was prepared by non-consumable arc melting on a chilled crucible,

and fabricated

by rolling to sheet,

roughly

30 mils thick.

The alloys were

Nb-r5W, Nb-r5WP3Zr, Nb-r5W-o.rHe, and Nb-r5W--zCro.rBe (compositions in weight percent), where the tungsten addition was selected for solid-solution strength ening. During preparation and fabrication, selected laboratory procedures insured against interfering contamination effects. The finished alloys typically containetl from 150 to 250 p.p.m. total interstitials. The alloys were studied in a condition wherein they had been cold-rolled 60 percent after a process-recrystallization treatment, and then stress-relief annealed. All alloys were easily fabricated. The final anneal did not induce metallographically detectable recrvstallization in any of the allovs. Metallographic examination of the alloys in several conditions (cast, hot-worked, process-recrystallized at 165o”C, cold-worked, stress-relief annealed at 12ooY‘) showed the following : (I) Nb-r5W-single phase; grain structure showed the usual response to working and annealing treatments. (2) Nb-r5W-3Zr-two phase; quantity, shape, and distribution of second phase were highly sensitive to annealing at both x200 and r45o”C. (3) Nb-r5W-o.rBe and Ni-r5W-zCr-o.rBe-two phase; quantity, size, shape, and distribution of the second phase were quite insensitive to annealing at either temperature. Dispersed phases were not identified in these alloys. Further evaluation of the alloys included tensile tests at room temperature