The location of oxygen atoms in vanadium-oxygen alloys by means of neutron diffraction

THE LOCATION OF OXYGEN ATOMS IN VANADIUM-OXYGEN BY MEANS OF NEUTRON DIFFRACTION*

ALLOYS

A comparison of X-ray and neutron diffraction is made for the purpose of Iocating oxygen atoms in vanadium-oxygen alloys. In the region of low oxygen content, neutron diffraction possesses a large advantage because of the large scattering factor for oxygen and the low value for vanadium with neutrons. In an alloy containing 21.0 atamic per cent oxygen, the oxygen atoms are found in the actahedral positions of a body-centered tetragonal lattice, as mferred by earlier workers using X-ray diffraction.

Une comparaison est f&e entre la diffraction des rayons X et la diffraction des neutrons dans le but de la localisation des atomes d’oxygene dans les alliages vanadium-oxygene. Dans le cas de la diffraction des neutrons, le facteur de dispersion est grand pour I’oxygene et faible pour le vanadium, cette mtithode est, par consequent, beaucoup plus avantageuse clans la region de faibles teneurs en oxygehe. Dana un alliage contenant 21,O pour cent en atomes d’oxyghne, les atomes d’oxyg&ne ont &4 trouv& dans Ies positions ~ta~dr~qne~ d’un &eau t&ragonal cent& ce qui coincide avee ies r&ultats des travaux ant&&n-s gaits par des chercheurs utilisant la diffraction des rayons X, DIE BESTIMMUNG VANADIUM-SAUERSTOFF

DER LAGE DER SAUERSTOFFATOME IN LEGIERUNGEN MITTELS NEUTRONENBEUGUNG

Ein VergIeich zwischen Rontgen- und Neutronenbeugung wird angestellt, urn die Lage der Sauerstoffatome in Vanadium-Sauersto~l~~ie~nngen festzustellen. Im Gebiet der niedrigen Sauers~~~k~n~e~~at~one~ hat die ~e~~~~e~~~~~~~~ wegen des grossen Streufaktors des Sauerstoffs und des kleinen S~eufakto~ des Vanadiums fiir Neutronen grosse Vorteile. In einer 2% pro2ent SauerSXOS enthaltenden Legierung befanden sich die ~~ersto~a~ume in den Ukta~er~l~~n &es tetragonaf raumzentrierten Gitters. wie es bereits Autoren frtiherer Arbeiten auf Grund von Riintgenbeugungsdiagrammen annahmen.

The value of the neutron di~act~o~ te~~~~~~~ in the study of certain types of crystal structure problems has now been amply demonstrated. Such problems usually involve those cases for which the relative X-ray scattering powers of the various atoms in the crystaalare not ~~v~~~~~~ for d~st~~gu~sh~ngthe X-ray scattering by the different atoms involved. We have recently had occasion to study the positions of oxygen atoms in vanadium-oxygen alloys. This problem constitutes an interesting application of the neutron ~chn~~ue since, for neutrons, ~~nad~~rn has a very much smaller scattering amphtude than oxygen, whife with X-rays the opposite is true. The problem arose from some metallurgical studies of vanadium-oxygen solid solutions made by Seyboh and ~~rn~jo~ [I], Their work shows that as oxygen is added to pure vanadium, with its bodycentered cubic structure (a0 = 3.026 A), faint lines of a body-centered tetragonal, structure first appear at 3.2 atomic per cent oxygen. They designated this phase as the &phase and found its lattice parameters “Received March 20, E%%. Atomic Power Laboratory, Schenectady, New York, U.S.A. $Present address: General Electric The Knolls, Schenectady, New York. fXn5%

ACTA

METALLURGICA,

GeneraI Electric Research

VOL. I, JULY

Co,

Laboratory,

1953

to be a@ = 2.99 A, co = 3.26 & giving ZU+I axial ratio of 1 .WO+These parameters remain constant up to 15 atomic per cent oxygen which marks the phase boundary for the pure p-phase. Beyond this and up to 22 atomic per cent oxygen, the structure becomes more tetragonal, reaching limiting values of ao = 2.94 A, cg = 3.50 A with an arriat ratio of l.f9. Above the 2k! per cent region, there is a two-phase region involving the &phase and WI. The compound VO has the rock salt structure with aa = 4.081 A. The problem which naturally a.rose was the location of the oxygen atoms in the tetragonal p-phase. For &hesimple crystal structure of the &phase, the structure factor for every reflection involves either the sum ar the difference of the vanadium and oxygen atomic scattering factors. Thus, for maximum sensitivity in locating oxygen atoms, one compares reflections for which the scattering by the oxygen and vanadium atoms oppose and reinfmce me another, In the ahoy containing the rnax~rn~~rnamount of oxygen in the P-phase, namely 22 atomic per cent oxygen, one compares reflections for which the factors (fV + 0.28 $ .) and (Sy - 0.28 fO), where fv is the vanadium atomic scattering factor and ff, that f&r oxygen are involved. The atomic scattering factars for X-rays vary with (sin a)/‘~, and for a ty@caf value, the above functions would be 18 for “in phase”.and 15 for “out of phase” scattering by the

505

IOI

15I 1\ 251

30I

35I SCATTERING

FIGURE 1. vanadium

and oxygen

the relatively

atoms.

cross section

for vanadium,

(aside from a scale factor)

and -

diffraction

21 for “out of phase” make

X-ray

case,

they

factor

of 2 in the case of neutrons.

discussion, atomic

per cent

that

oxygen

could be expected

the structure

P-phase

require

very accurate

_A neutron obtained niques

which

Wollan

and Shull

diffraction basis

National

have

previously [a]. The

peaks

has

by comparison

been

placed

with

the

nickel for which the coherent has been accurately barns.

The

except that with neutrons

tion factor.

Phkl

(1)

formulas

For cylindrical

power

are

by

in the

on an absolute pattern

for

cross section

Nicoh = 13.4 f

X-rays

the

same

0.2

as for

there is no polariza-

samples

7%” p

sin

Chemical

tions

intensity

in counts

B = absorption

per minute;

though

the

To settle

IZ = number

of molecules

per unit cell;

tetragonal

cubic

structure

is entirely

reasonable, question,

further

with the structure

in

cm X 1012; j = multiplicity factor; e = Bragg angle. The oxygen amplitude has been taken as 0.58 X lo-l2 cm from the work of Shull and Wollan [3] and

(2) The

but estabal-

plausibility

later found by

martensite. of the oxygen pattern

atom

posi-

of the 21.0 atomic

alloy was made covering

the range

angle,

8, from 2” to 31” with neutrons 0 of 1.163 A. The pattern is shown

length

in this region. peaks

X-ray

I. The observed from equation

short

structure

showed

vertical

show the position A comparison

lines of the of the

structure

factors

is shown

structure

factors

were cal-

(1) which reduces

pnrr = g go calculated

The

reflections.

and calculated

of in

e = 7” to 12.5” is not

1. The region between

the neutron

culated

posi-

of vana-

estimates

beyond

the

diffraction

per cent oxygen of Bragg

the

structure

take the octahedral

has

the question

tions, a neutron

by

to locate

[5] for this phase is one

structure

[6] for nitrogen

in Table

p’,p = powder and solid densities, respectively; F = structure amplitude per unit cell

atoms

structure

that it is identical Jack

observed

correction; per cm3;

The

and Grimm

lished the tetragonal

corresponding

constant;

of molecules

definitely.

measure-

low scattering

it was not felt that their intensity

no structure

N = number

diffraction

of 21.0

tetragonal structure 0 A and an axial ratio

it was not possible

dium metal. The structure

Figure

e sin 28

and calibration

X-ray

in the body-centered

above

P = observed

atoms

cm

by arc

gave a composition

of the relatively

atoms,

given by Klemm

X lo-l2

[4].

shown in the figure since the measurements

where y2 = apparatus

analysis

Because

oxygen

0.48

was prepared

showed

the oxygen

-

of V and V203 in an argon atmos-

per cent oxygen.

ments

wave

as used here,

&J%‘jhatF2hn-t

=

of the &phase

a mixture

in which the oxygen was

described

scattering

determined,

powder

would

by the tech-

powder

sample

of 1.180.

22

and W. C. Koehler been

as

and Levy

the above

answer regarding

integrated

amplitude,

of the

the neutrons

Laboratory

The melting atomic

of the P-phase

for us by E. 0. Wollan

of the Oak Ridge

vanadium

in the

measurements.

pattern

the

from the work of Peterson

phere.

by a

I 60

alloy.

a body-centered 0 with a0 = 2.948 A, co = 3.478

whereas the X-rays

intensity

diffraction

case

alloy

to give a decisive

the oxygen atom positions,

be-

factor

From

in the

55I

28

while

Thus,

only 18 per cent difference

it is clear

due to

11 for “in phase”

scattering.

50I

of 21.0 atomic per cent oxygen-vanadium

of oxygen

the functions +

the oxygens

change

ANGLE,

pattern

For neutrons,

high scattering

and the low value come

Neutron

45I

401

to

_&&&r_ sm

e sin 28

factors

are

based

on a

tetragonal unit cell in space group Ddh17 (Internationale Tabellen [7]) with two vanadium atoms at 000 and $33 and 0.532

oxygen

at 0 0 h and + 3 0. This produces

distributed a factor

randomly (0.266f,

+

ACTA

392

METALLURGICA,

fY) in the structure factor for reflections with 1 even and the factor (0.266 f0 -fv) for reflections with I odd. TABLE

I

OBSERVED AND CALCULATEDVALUES FOR Phkl AND Fhkl IN A 21.0 ATOMIC PER CENT OXYGEN ALLOV OF THE &PHASE

hkl

P

P

(observed)

(calculated)

(101) (110) (002) (200)

100 133 21 10 10

(112) (211)

::

99.7 12 4 6 10 66

F (observed) 0.23 0.47 0.28 0.32 0.27 0.27 0.46

F (calculated)

0.41 0.21 0.21 0.21 0.21 0.41

The agreement between the observed and calculated structure factors in Table I is seen to be only fair on an absolute basis. However, on a relative basis the agreement is quite satisfactory. Since the reflections with 1 odd have structure factors of 1.7 times those with I even (compared to a value of 2.0 for the assumed structure), the data serve very well to establish the fact that the positions of the oxygen and vanadium atoms are as given by the Klemm and Grimm structure [5]. The fact that the agreement is only fair between the 1.7 value for the ratio of the observed structure factors for “out of phase” and “in phase” reflections and the 2.0 value for the calculated value, is probably due to the “in phase” reflections being just barely observable and their intensity values, therefore, considerably less accurate. There are two points regarding the data in Table I which need discussion. First, there is an extra reflection which occurs at a Bragg angle of 4” 24’, the first in the table, which is apparently not a part of the structure. This extra reflection may be due to an impurity of some type, possibly from a trace of a higher oxide of vanadium. However, assuming that it is a reflection of low index with j = 2, it actually has a structure factor somewhat less than the weak reflections which are just observable. The fact that it appears so important in Figure 1 is due to the factor l/(sin 0 sin 28) in equation (1). This factor greatly enhances the apparent importance of the reflections with very low Bragg angles. An alternative explanation for this peak is that it is associated with short range order of the oxygen atoms. However, the

VOL.

1, 1953

large interplanar spacing for the peak, 7.6 A, probably eliminates this possibility. The further possibility that the oxygen atoms are ordered on planes perpendicular to the c-axis in alternate unit cells (thus producing a spacing of d = 7.0 A) can also be ruled out, as it would produce other reflections which are not observed. The possibility that the peak may be of magnetic origin has not yet been fully investigated. The fact that the observed structure factors in Table I are about 20 per cent higher than the calculated values is more difficult to reconcile. By assuming that the true composition of the alloy is 25 rather than 21 atomic per cent oxygen the agreement is much better. However, examination of the X-ray data of Seybolt and Sumsion [l] definitely indicates that the p-phase cannot retain more than 22 per cent oxygen, unless there is some (and the same) systematic error in the two methods of chemical analysis used on their alloys. But, since the relative values of the structure factors for the P-phase are in good agreement, and the calculated intensities are highly sensitive to small variations in oxygen content, it does not seem necessary to question the validity of the structure of Klemm and Grimm [5] for the P-phase. This work is another example of the usefulness of neutron diffraction in determining atomic positions in special cases with relatively simple structures and favorable values of the coherent scattering amplitudes. Vanadium compounds constitute a rather special case because of the anomolously small coherent scattering amplitude of vanadium. The neutron method is applicable when a majority of the atoms are vanadium< whereas X-ray diffraction would have been employed for determining the position of a small fraction of vanadium atoms in the presence of a large fraction of oxygen atoms. The work confirms the structure suggested by Klemm and Grimm [5] for the P-phase. In this structure the oxygen atoms take the octahedral positions in the body-centered cubic vanadium structure and thereby deform it into the observed tetragonal structure.

Acknozwledgement It is a pleasure to acknowledge the assistance .of Dr. E. 0. Wollan and W. C. Koehler of the Oak Ridge National Laboratory, who arranged for and obtained the neutron diffraction pattern of the p-phase.

4.

References 1. SEYB0I.I. .I. c. and

Saiwos,

H. T.

submitted for publication, 1952. 2. \~'OLL.AX,E:.O. and SHC-LL, C. G. 830. 3. SHULL, C. G. and N'OLLSS, E. 0. 527.

Phys.

Trans.

.-\.I.M.E.,

Rev.,

73 (1948)

Whys. Rev.,

81 (1951)

PETERSOX, S. Iv. and LEVY, H. .I. 1'hy% Itcv..87 (1!)52)

462. 5. KLEMM,

IV. and GRIMM, L.

%. ancrg.

Chcm.

43) 12. 6. JACK, K. II. I'roc.Roy. Sot., A208 (1051) 7. Internationale Tabellen Zur Bestimmung structuren (Berlin, Borntracger, 1935).

200. \‘on

250 (1942-

Kristall-