The crystal structures of VNi2, VPd2 VPt2, and related AB2 phases

JOURNALOF THE LESS-COMMON

THE

STRUCTURES

CRYSTAL

OF VNiz, VPdz, VI%,

METALS

AND RELATED

AB2

PHASES

B. C.

GIESSEN

Department (Received

AND

September

J.

N.

of Metallurgy,

GRANT

Massachusetts

zIst,

Instifute

ofTechnology,

Cambridge, Mass. (U.S.A.)

1964)

SUMMARY Isomorphous AI32 phases of the MoPt~-tripe were found in six of the nine AB2 combinations with A = V, Nb, Ta; and B = Ni, Pd, Pt. Their structures were completely determined and are compared with other known A& phases; interatomic distances are presented; and previous literature data are reviewed and corrected.

INTRODUCTION Recent investigations of the binary systems Nb-Pdi, Nb-Pt2, and Ta-Pd283 revealed the existence of an ordered phase at the composition AB2. In the homologous systems V-N? and V-Pdj ordered AB2 phases had also been found, but the structures had then not been established completely. Since these AB2 phases appeared to be of a similar crystal structure, it was decided to investigate the AB2 phases wherein A = V, Nb, Ta, and B = Ni, Pd, Pt that belong to this structure type to establish systematic variations of their lattice parameters and atomic positions. During the period of our studies, structures for VNi2, VP& and NbJoPdso were suggested by SCHUBERT et ~2.6. These phases were found to be of the MoPtz-type described earlier by SCHUBERTet al.?. The structure of VPtz was later confirmed by DWIGHT~. Precise lattice constants and atomic positions were not given in the latter three reports6-s. In this report, the structures for VNiz, VPd2, and VPtz will be discussed together with those for NbPdz, NbPtz, and TaPd2. In a previous abstract9 by the present authors, NbPts and TaPdz were briefly described; the structure type was designated as NbPte-type instead of MOP&-type. EXPERIMENTAL

METHODS

The procedures of alloy preparation by argon atmosphere arc-melting, heattreating in a vacuum of 10-5 mm Hg, and X-ray diffraction used in this program have been described elsewherelo. V and Ni of 99.8% and Nb, Ta, Pd, Pt of 99.9% purity were used, with oxygen levels per element of < 50 p.p.m. All V-alloys were homogenized at about IIOO”C and then held at about 850°C to permit ordering. (VNi2 and VPds are ordered up to 920°c4 and gog”C5, respectively; VPtg was found

CRYSTAL

OF VNiz,VPdz

STRUCTURES

AXD

VPta

11.5

to be ordered at IIOO’C and 850”C, but probably The alloys were then crushed

disorders

at higher temperatures.)

to powder and stress-relieved.

These heat treatments

were as follows: VNiz : 1.5 h at 1og5”C; powder held 40 h at 875°C. VPdz:

21 h at IIOO"C

-t 5 h at 845°C;

powder held 48 h at 825°C or 2.5 h

at 845°C. \‘Ptz : 21 h at 1100%

+ 5 h at 845°C;

powder held 2.5 h at 845°C.

The heat treatments for the other AR2 phases, which are ordered up to the solidus temperatures, were carried out between IZOO-1500°C. The X-ray diffraction patterns were taken with CrKn radiation for VNi2, and with CuKa radiation for all other phases, employing a 114.6 mm camera. For VNiz and NbPdz long-time diffractometer step scans were carried out, permitting the detection of order lattice lines with an intensity ratio Is/lp -; I/IOOO. Films were calibrated

with an internal

mine the lattice

Si standard;

parameters.

corrected

The error limits for

high angle lines were used to deterUO, bo,

CO,

are .< 5.10-4.

RESULTS The powder patterns of VNi2, VPdz, VPt2, NbPdg, NbPtz, and TaPda could all be indexed with an orthorhombic lattice, Of”, - Immm, with atomic positions; [ooo; t - &] + 2 A (a):

000

+ 4 B (9) : I!E (0~0); Y x 3. The precise

value for the free parameter

the visually estimated

intensities

according

y was obtained

in each case by grouping

to their h index (k = O-IO observed

with

CuK&), since : F=zf~+4fucosz~~ky. . By approximatmg y x 4 the phase of F could be determined; F was plotted and y was varied for best fit. An alternate procedure used is that described by BRADLEY AND Lull for Cr2AI. In Table I, measured sin”0 values and estimated intensities for VNiz are compared with those calculated on the basis of the unit cell given above. The agreement of the intensities is satisfactory; the agreement between lobs and Icarc is even better for the other phases where reflections up to k = IO could be used. This structure type is represented by that of MoPta, as described by SCHUBERT

et ~1.7. Table II lists the lattice constants tems. In addition, the ratios included for MoPtz structures.

of all observed

AR2 phases in Tz-Tro sys-

b/3a and ~/[a2 + (b/3)2]*, and the y parameters These parameters will be discussed below.

are

DISCUSSION

Figure

I illustrates

the MoPtz structure

for NbPtz

in terms

of close-packed

pseudo-hexagonal AR2 layers in which each A atom is surrounded by 6 B atoms. These layers are stacked in a x-layer sequence abc, abc, thus forming a fundamental cell (disregarding

order)

of a slightly

distorted

face-centered-cubic j. Less-Conznzon

Metals,

(A 1) structure. 8 (1965) 114.-119

116 TABLE

B. C. GIESSEN,

N. J. GRANT

I

X-RAY DIFFRACTIONPATTERN OF VNia, MOP&-TYPE (CrKal-radiation;

hkl

020 011 110 101

obs.

talc.

0.08gg

0.0898

0.1267 0.2226

0.1265 0.2225 0.3041

0.5061 0.6386 0.6636 -

0.5059 0.6386 0.6634 0.6654 0.7614

132 220 211

0.7752 0.8002 0.8085 0.8178 0.8902 0.9265

x:4 . 0.8085 0.8183 0.8899 0.9265

oI3

0.9585

0.9585

TABLE

I I.5 0.5

0.29

0.61 0.25 10.00

>IO 0.3593 0.3939 0.4022 0.4161

141 o5I 150 042 200 060

3.549 A)

talc.

est.

0.2

10.00 0.07 0.15 6.65 3.12 0.05 0.12 0.17 0.02 0.02 0.10

3 3 IO 0.2 0.5 0.5

2.45 2.48 10.00 0.07 0.25 0.15

0.3061

0.3943 0.4020 0.4169

I30 002 022 112

7.641 A; co =

Intensity

sinall

0.30461 031 040 121

2.559 A; bo =

ao =

<

0.5 6 3 0.2 0.5 0.5

< -

II

ABa PHASES OF THE MOP&TYPE** A:\B: V

Ni

b= 7.641;

a= 2.559;

2.55 (ref. 6) 7.71 (ref. 6)

c= 3.549;

3.54 (ref. 6)

b/3a= 0.995;

w 1.01 (ref. 6)

Pd

Pt

a= 2.750 a’= 3.890 (ref. 5) b= 8.250 1 C= 3.751; C= 3.740 (ref. 5)

b= 8.323; 8.35 (ref. 6) ; 8.33 (ref. 8)

b/ga= I .oo c/D*= 0.965 ; c/a’= 0.962

b/ga= 1.016; c/D*= 0.977

c/D*=

(ref. 5)

0.984 Y= 0.334

Nb

a=2.730;2.72(ref.6);2.74(ref.8) c=3.800;3.7g(ref.6);3.8o(ref.8) 1.02 (ref. 6)

Y= 0.341

y= 0.340

No ABa phase

a= 2.839;

b= 8.376; c= 3.886;

2.79 (ref. 6) 8.55 (ref. 6) 3.88 (ref. 6)

b/3a=o.g84; c/D*= 0.978

~1.02

(ref. 6)

a= 2.801

b= 8.459 c= 3.951

b/3a= 1.007 c/D*= 0.995 Y= 0.337

Ta

(MoSia-type, ‘I= 3.154 c = 7.905

ref. 13)

a= 2.896 b= 8.397 c= 3.790

(TaPts-type, a= 8.403 b= 4.785

6/3a=

c= 4.744

0.968

ref. 14)

c/D*= 0.941 Y= 0.339 *D = (aa + (b/3)2)* ** All lattice parameters in A; ABs phases of other structure types included in parentheses. J. Less-Common

MetaGs, 8 (1965) II.+-1x9

CRYSTAL STRUCTURES OF

VNi2, VPdz AND VPtz

117

Another way to present this structure, following SCHUBERT~~, is to consider a closepacked face-centered-tetragonal cell with et/at # I, and with 4 B atoms per cell, and to replace the B atoms in every third (110)layer by A atoms. This operation destroys the tetragonal symmetry and leads to a pseudo-monoclinic cell. By choosing the former [IIO] and [IIo] diagonals as new axes a and b, as suggested earlier by PEARSON

AND HUME-ROTHERY~,

at the correct

unit

cell described

and by tripling

one of these short

axes, one arrives

above.

p---? \

/

Fig. I. The structure of NbPtz

3 close-packed AB2 layers in

/0----Q \

(MOP&type).

Solid circles: Nb atoms;

open circles: Pt atoms;

(~oi-)planes indicated in broken lines.

TO illustrate the dimensional changes accompanying this ordering process, the parameter b/p has been listed in Table II. This parameter indicates whether a contraction or dilation of the lattice occur in the direction of the long b-axis by insertion of the B atoms. A systematic variation of b/3a with the position of the alloy in this table is found: in horizontal rows (A = constant), b/3a increases uniformly on passing from Ni to Pd to Pt (expansion in b); in vertical columns (B = constant), b/3a decreases strongly on passing from V to Nb to Ta (contraction in b). At the position of VPdz: b/3a = 1.00; no contraction occurs in the ordered lattice. This may mean an absence of micro-stresses produced on the occurrence of ordered domains in their and directions. For VPd2, slightly broadened lines were always observed, signifying very small ordered regions or regions of stresses that could not be eliminated by the normal stress-relieving procedure. To obtain a measure of the tetragonal distortion occurring in the fundamental face-centered-tetragonal cell on ordering, the parameter c/[a2 + (b/3) “]* zz G/U, was introduced in Table I ; however, no systematic dependence was observed. It should be noted that ctjat was not constant for VNiz,* depending on the heat treatment, values of et/at = 0.978 (powder held 48 h at 825°C) to 0.984 (powder held 40 h at 875°C) were observed. J, Less-Common

Metals, 8 (1965) 114-119

118

B. C. GIESSEN,

N. J. GRANT

The interatomic distances calculated with the values of Table II are presented in Table III for five AB2 structures; in the case of NbPdz the similarity of the scattering factors of the components made a quantitative determination of y impossible. The error limits, caused mostly by uncertainties in y, are f 0.02 A. It will be noted TABLE

III

INTERATOMIC

DISTANCES

IN

AB2 PHASES OF THE hfoPt~-TYPE

Atoms

VNiz

VP&

VP&

NbPtz

ApzA A-8B A-zB

2.56

2.75 2.67

2.73 2.69 2.84

2.80

2.90

2.53 2.55

2.79 2.85

2.74 2.85

2.84 2.80 2.76

2.90 2.70

2.79 2.85

2.74 2.85

=4B B-ZB B-IB =4A B-IA

2.54 2.56 2.54 2.53 2.55

2.80

2.76

2.78

2.75 2.64 2.67

2.73 2.65 2.69 2.84

2.80

Tap&

2.82

that one short B-B bond and 8 rather shortened A-8B bonds, the latter probably being responsible for the bonding strength, occur in all phases except VNi2, which has nearly ideal pseudo-tetragonal atom positions (y N 4). In this phase, as well as in NbPtz, a strong compression of the A-ZA bond occurs. This may affect the space available for the A atoms and cause a smaller relative contraction of the A-8B bonds in these phases, making y more nearly Q. The larger ratio c/[(b/3)2+ a214 in VNi2 and NbPt2 will tend to act in the same direction. A further remark concerns the sequence TaNiz, TaPdz, TaPtz, in which all AB2 structure types occurring in Ts-Tro combinations are found. In TaNi (MoSiatyper3) pseudo-hexagonal AB2 layers are perfectly stacked in trigonal positions in z-layer sequences AB, AB, approximating an AZ-type fundamental structure; in TaPW pseudo-hexagonal layers are stacked in z-layer sequence AB, AB in positions intermediate between tetrahedral (close-packing) and trigonal, but closer to the trigonal positions. TaPd2 as described above has pseudo-hexagonal layers stacked in almost tetrahedral positions, but in 3-layer sequence ABC, ABC. The dimensions of this unit layer and the pseudo-hexagonal c-axis of TaPd2 are very similar to the lattice constants of TaPt2: TaPtz : a = 8.403 A;

b = 4.785 A;

TaPd2: b = 8.397 A; [a” + c2]* = 4.77

c = 4.744 A;

A; dlol =

1a2u~~2]t = 4.60 A.

Thus, on passing from TaPdz to TaPt2, there is a slight increase in the size of the basal plane parameters a (TaPt2) and b (TaPtz), and a large increase in the lattice parameter normal to the basal plane c(TaPt2) due to the deviation from close-packing. It is of interest that NbPtz bears much less similarity to TaPt2 than TaPd2, indicating that the A element Ta or Nb has a stronger influence on the structure than B. Finally, the present results are compared with those of prior work (see Table II). There are significant differences in the data for VNiz and NbPd2, taken from SCHUBERT et a1.6, and the present ones, especially in the b-values for VNiz and NbPd2 J. Less-Common

Metals, 8 (1965)

I I,.-Irg

CRYSTAL STRUCTURES OF VNi2, VP&

and in the better.

u-value

Therefore,

calculated

for NbPdz.

The

it seems likely

by tripling

AND VPts

119

agreement

that

in the

other

the b-parameters

lattice

one of the short axes of the “fundamental

which were measured

by evaluating

the strong

constants

is

of SCHUBERT et al.6 were

“fundamental

cell” (2 atoms/cell) lines” of the powder

pattern, and that the wrong axis was tripled in the case of VNia and NbPda. If one takes these values’jand transforms according to a’ = b/3, b’ == 3a, good agreement is obtained. Since in the present are preferred. agreement

investigation

For VPt2,

there

this aspect was carefully is fair agreement

checked,

the new values

with SCHUBERT et al.6 and good

with DWIGHT~. For VPd2, KOESTER AND HAEHL~ give lattice parameters

for a face-centered-tetragonal

cell that are in very good agreement

with the present

a = a’,‘v’z. They failed, however, to ones, after transforming their u-parameter: detect the ordering in VPdz leading to a larger cell. Approximate lattice constants for the fundamental

cell of VNiz have been given by PEARSOX AND HUME-ROTHERY~,

who first described DARBY,

DOWNEY

this cell. i\n unindexed AND NORTON~;

can be well described

powder

the line positions

in terms of the present

pattern

for TaPdx

and intensities

was given by

of their X-phase

cell.

ACKNOWLEDGEMENTS

The Contract

authors

SD-90.

Nickel Company

are grateful

They

for financial

wish to thank

support

Engelhard

of this

Industries

work under

ARPA

and The International

for their gifts of Pd and Pt metal.

REFERENCES I D. P. PARKER, S. B. Thesis, Dept. Metallurgy, Mass. Inst. Tech., June, 1963 2 B. C. GIESSEN, R. KOCH AND N. T. GRANT, unpublished work. 3 J. B. DARBY, J. W. DOWNEY AND L. J, NOR&N, Trans. AIME, 227 (7963) 1028. 4 W. B. PEaRSON AND W. HUME-ROTHERY, J. Inst. Met., 80 (1952) 641, 5 W. KOESTER AND W. D. HAEHL. Z. Met., 49 (1958) 647. 6 K. SCHUBERT, K. FRANK, R. GOHLE, A. MALDONADO, H. G. MEISSNER, ,4. RAMAN AND W. ROSSTEUTSCHER, Naturwissen., 50 (1903) 41. 7 K. SCHUBERT. W. BURKHARDT. P. ESSLINGER. E. GUENZEL. H. G. MEISSNER, W. SCHUETT. J. WEGST AND M. WILKENS, N&rwissen., 43 (1956) 248. 8 A. E. DWIGHT, Argonne Natl. Lab. Rep. ANL 6868, (1963) 305. 9 B. C. GIESSEN AND N. J. GRANT, Acta Cryst., 17 (1964) 615. IO I). L. RITTER, B. C. GIESSEN AND N. J. GRANT, Trans. A/ME, 230 (1964) 1~50. II A. J. BRADLEY AND S. S. Lu, 2. K&t., 96 (19x7) 20. IL R. SCHUBERT, Kristallstrukturen Zweikomponentiger Phusen, Springer, Berlin, 1964, p. 104. 13 B. C. GIESSEN AND N. J. GRANT, Trans. AIME, 230 (1964) 1730. 14 B. C. GIESSEN, R. S. KANE AND N. J. GRANT, On the constitution diagram tantalum~platinum bctwecn 50-100 atomic percent platinum, Tvans. AIME, accepted for publication. 15 ;\. MALDONADO AND K. SCHUBERT, Z. Metallk., 55 (1964) 619. This paper was published after the present one was submitted ; it contains lattice parameters for six MloF’tz-type phases. For VNis and NbPdz numerical differences remain ,I.

Less-Common

filet&,

8 (1965) 114~119