Download Neutron diffraction studies at the Puerto Rico Nuclear center

Neutron diffraction studies at the Puerto Rico Nuclear
center
I. Almodovar, H.J. Bielen, B.C. Frazer, M.I. Kay
To cite this version:
I. Almodovar, H.J. Bielen, B.C. Frazer, M.I. Kay.
Neutron diffraction studies at
the Puerto Rico Nuclear center.
Journal de Physique, 1964, 25 (5), pp.442-446.
<10.1051/jphys:01964002505044200>. <jpa-00205801>
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Submitted on 1 Jan 1964
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LE,
JOURNAL DE
PHYSIQUE
TOME
25,
MAI
1964,
442.
NEUTRON DIFFRACTION STUDIES AT THE PUERTO RICO NUCLEAR CENTER
I.
By
ALMODUVAR,
H. J. BIELEN
The Puerto Rico Nuclear
(2), B. C. FRAZER (2) and M. I. KAY
Center, Mayaguez, Puerto Rico.
(1)
(2),
Résumé. 2014 Un programme de diffraction neutronique a été récemment établi au Centre d’Études
Nucléaires de Puerto Rico. Les deux diffractomètres utilisés ont été assemblés avec l’aide de
membres du Laboratoire National de Brookhaven.
Le premier programme de recherche était l’achèvement de l’analyse de la structure d’un monocristal de CaWO4. En choisissant l’origine au site 4a) des atomes W dans la maille quadratique,
groupe I41/a, on trouve les paramètres suivants de l’oxygène dans les sites 16(f) :
0,0861 ± 0,0001.
0,1511 ± 0,0006, z
y
de
vibration
ont été également déterminés pour tous les atomes dans
paramètres anisotropes
l’analyse aux moindres carrés de la structure.
La structure magnétique de CuSO4 a été déterminée en continuation d’une étude, commencee à
Brookhaven en collaboration avec le Dr P. J. Brown. Dans la notation de Wollan-Koehler-Bertaut,
le mode de l’ordre antiferromagnétique des spins dans la maille orthorhombique du groupe Pbnm
x
=
0,2413 ± 0,0005,
=
=
Les
est Ax, c’est-à-dire sur les sites 0 0 0, 0
0 1/2 ; 1/2 1/2 0, 1/2 1/2 1/2, les spins
se
succèdent dans la suite + - +
-,
axe étant selon a. Un moment d’environ 1 03BCB a été trouvé pour l’ion Cu2+.
La structure cristalline de BaNiO2 a été réexaminée et le résultat obtenu par Lander aux rayons X
d’une coordination carré coplanaire de Ni2+ a été confirmé. On a également cherché un ordre
magnétique dans BaNiO2 à 4,2 °K, mais aucun résultat concluant n’a été obtenu.
Une transition magnétique a été trouvée dans Fe2SiO4 au voisinage de 30 °K. Ce composé a une
structure du type olivine avec huit ions Fe2+ dans les positions 4(a) et 4(c) du groupe d’espace
Pbnm. L’analyse de la structure magnétique continue, mais le mode dominant est C dans la
notation W.-K.-B.
leur
A neutron diffraction program was initiated recently at the Puerto Rico Nuclear
Abstract.
Center. The two double crystal spectrometers in use were assembled with the aid of staff members of the Brookhaven National Laboratory.
The first research problem to be completed was a single crystal structure analysis of CaWO4.
Choosing the origin at the 4(a) tungsten site in the tetragonal I41/a cell, the 16(f) oxygen parameters were found to be as follows : x = 0.2413 ± 0.0005, y = 0.1511 ± 0.0006,
z
0.0861 ± 0.0001. Anisotropic temperature parameters were also determined for all atoms
in the least squares analysis of the structure.
The magnetic structure of CuSO4 has been determined in a continuation of a study started at
Brookhaven in collaboration with Dr. P. J. Brown. Using the Wollan-Koehler-Bertaut notation,
the antiferromagnetic spin ordering mode in the orthorhombic Pbnm cell is Ax, i.e., + - +
2014
=
-
on
the
0 0 0, 0 0 1/2, 1/2 1/2 0, 1/2 1/2 1/2 sites, with the spin axis parallel to
a.
A moment of
approximately
found for the Cu2+ ion.
The crystal structure of BaNiO2 was re-examined in a neutron powder diffraction study and
Lander’s x-ray result of a coplanar square coordination of Ni2+ was confirmed. BaNiO2 was also
investigated for magnetic order at 4,2 °K but conclusive results have not been obtained.
A magnetic transition has been found in Fe2SiO4 in the neighborhood of 30 °K. This compound
has an olivine type structure with eight Fe2+ ions in the 4(a) and 4(c) positions of the Pbnm space
group. Analysis of the magnetic structure is still in progress, but the dominant mode is C in the
W.-K.-B. notation.
1 03BCB
was
History of neutron diffraction at the Puerto Rico
While
Nuclear Center and technique employed.
definite plans for a neutron diffraction program at the
Puerto Rico Nuclear Center began to take shape about
one and a half years ago, the first actual research measu---
rcments
were
not made until
January, 1963.
At that
(1) Work performed under the auspices of the U. S.
Atomic Energy Commission.
(2) Guest scientists : H. J. Bielen from the University of
Frankfurt, Frankfurt, Germany ; B. C. Frazer from Brookhaven National Laboratory, Upton, New York, U. S. A. ;
I. Kay from Georgia Institute of Technology, Atlanta,
Georgia, U. S. A.
time the installation and testing of a rather simple specwas completed at the Center’s one megawatt
swimming pool type reactor. The in-pile collimation,
water shutter, monochromator mount, and shield for
this first machine were all patterned after some of the
less elaborate installations at the graphite reactor of
the Brookhaven National Laboratory. The diffractometer was an old instrument that had been used at
Brookhaven for several years until it was replaced by
one of more advanced
design. It was literally
rescued from the scrap heap and rehabilitated for use
in Puerto Rico. In June the Westhinghouse Research
Laboratories donated an excellent diffractometer and
trometer
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505044200
443
several items of accessory equipment, including a
modified Varian magnet, to the University of Puerto
Rico. The original design for this machine was made
at Brookhaven by B. C. Frazer and T. Mullaney, but
modifications were made at Westinghouse by G. Shirane, W. Takei, and R. Stuart. The old Brookhaven
diffractometer was replaced by this fine piece of equipment.
In
July,
installation
was
started of
a
second spec-
only about ~.1 A [1]. The reason for this is that
the B atom is always one of rather high atomic
number, relative to oxygen, and hence the oxygen
contributions to x-ray structure f actors are comparatively small. In the neutron case the oxygen
scattering length does not differ greatly from those
of the other atoms. For CaWO4 the neutron
scattering lengths
are :
trometer. This versatile and highly accurate instrument is almost identical with the new U. S. I units
designed by A. Kevey for the Brookhaven High Flux
Beam Reactor. The cylindrical monochromator shield
rests on a gun mount (obtained as surplus equipment
from the U. S. Navy) which can be rotated to permit
continuous angular settings for wavelength selection
over a range of approximately
500 to + 900. The
diffractometer is on a cantilevered mount which is
bolted to the gun mount, and hence remains in proper
alignment if changes are made in 20 of the monochromator. The monochromator itself is 1 : 2 coupled to the gun mount hy means of an angle divider
so that the crystal stays in reflecting position if a
change in wavelength is made. It is possible to change
the collimation of the primary beam from the reactor
by turning a crank. This rotates a steel cylinder
which has positions for three collimators and has a
fourth closed, or shutter, position. At present only
one collimator is mounted.
This has 24’ soller slits.
A single crystal collimator and one with 18’ soller slits
will be installed very soon. In addition to the mechanical shutter on the rotating collimator changer, which
puts three feet of steel in the beam, an in-pile water
shutter is also available.
The U. S. I diffractometer is very similar to the
Westinghouse machine, since it is also an improved
version of the original Frazer and Mullaney design.
Both are very sturdy machines capable of taking over
2 000 pounds of direct loading, and have maximum
accumulated angular errors of less than I’ of arc.
Data are collected semi-automatically by pre-determined count monitor controlled step scanning. Data
output is printed on adding machine tape. All electronic components are transistorized. Eventually the
spectrometers will be controlled by automatic programmers, probably of the Brookhaven type, but
because of the reactor operating schedule semi-autoinatic operation is adequate for present work. The
reactor now operates on an 8-hour day and 5-day week
schedule. A 16-hour day and an increase in power to
two megawatts is expected.
Plans are being made
for 24-hour operation at five megawatts some time in
1965. The efficiency of data collection can be increased
greatly at that time by converting to fully automatic
-
systems.
’
CaW04
~
The first neutron diffractioll problem undertaken was a single crystal structure analysis of
CaW04 (scheelite). While this structure is usually
considered to be typical of that adopted by an
important series of AB04 compounds, the oxygen
coordinates have been known with an accuracy of
Accordingly, the structure analysis briefly described here was undertaken. In the course of this
analysis it was found that Zalkin and Templeton [2] were engaged in an independent study
of the structure using very accurate x-ray data
and modern refinement techniques. Their results
are in excellent agreement with those reported
here.
CaWO4 crystallizes in the tetragonal space group
I41 la and has cell dimensions a = 5 . 243 -+- 0. 002~.
and c
11.376 ± 0.003 A[2]. The Ca and W
atoms are in the four-fold symmetry-fixed positions 4(b) and 4(a), respectively, and oxygen is in
the 16( f ) general position.
Least squares refinement calculations, carried
out on (h 0 1) and (h h l) data collected from samples cut from a large synthetic crystal, led to a
final I~ value of 0.041. The oxygen parameters
(with the cell origin chosen at the tungsten site) are
=
These
yield
a
nearly regular,
but
measurably
distorted, WO’4 group with a W-0 distance of
1.784 ~ 0.003 A. The latter has been corrected
by 0.003 A for vibrational motion. Anisotropic
temperature parameters were determined for all
atoms,
as
will be reported elsewhere in
of results.
a
detailed
publication
Cuss 4
The anhydrous sulfates of the divalent transition elements form a very interesting group of
magnetic compounds. Magnetic structures have
already been determined for FeSÛ4, NiSO~,
and
[3-5]. Discussed here are
the results of a neutron diffraction study of CUS04This has been a continuation of a study begun by
two of the present authors (I. A. and B. C. F.) at
Brookhaven with Dr P. J. Brown, and a full
account of this work is to be published later under
joint author ship.
Because of the low Cu2+ moment, and because
of troublesome peak overlaps in a neutron powder
pattern, the magnetic structure of CuSO4 should
really be investigated using single crystals. Single
444
crystals of suitable size are not easily grown,
however, so an attempt has been made to solve the
problem using powder data. What can be said
directly from the neutron data is not very satisfying, but if these data are considered along with
the magnetic measurements of Kreines [6] a structure can be proposed that is almost certainly
correct.
CUS04 has the orthorhombic ZnS04 type crystal
structure with space group Pbnm
The four
ions are loca1 11I
I 11
sites. For
0
ted on the 0 0 0 0
2222
’ 2 22 ’
the case where the magnetic cell is identical with
the chemical cell the possible collinear spin configurations for antiferromagnetic ordering are
+20132013+,++2013-2013, and -~-- -- ~- -. In the
Wollan-Koehler-Bertaut notation these arrangements are labeled G, C and A, respectively [8].
Each of these leads to a distinct set of magnetic
reflections.
In comparing diffraction patterns taken at
4.2 OK and 77 ~K only one clearly resolved new
peak was found in the low temperature pattern.
This indexed as the (0 01 ) reflection, which is characteristic of type A ordering. A small but definitely measurable change in intensity was also
found for the combination (0 2 1), (1 1 1) peak,
again characteristic of type A. No other inten-
[7].
crystallographically equivalent
.
sity changes were statistically significant.
This small amount of information is sufficient to
identify the ordering scheme (although a multiaxis spin structure with small cant angles cannot
be directly ruled out), but it is insufficient to yield
reliable information on the magnitude and orientation of the Cu2+ moment. Here it is useful to
consider the magnetic measurements of Kreines [6].
Kreines used small single crystals which were
oriented by morphology and x-rays. While the
setting was not specified in her paper, it can be
identified from the cell dimensions as Pbmn. Her
figure 4 is drawn incorrectly, since the Cu sites are
improperly assigned for this setting, but this does
not affect the conclusions which may be drawn
from the measurements. According to Kreines
xd
Xc and ZI, = xa. This of course sugxl
gests a collinear spin structure with the spin axis
parallel to a. With this assumption the CU2 +
moment calculated from the neutron diffraction
The spin
as expected for this ion.
data is 1
ordering mode is Ax, i.e., + - + - on the 0 0 0,.
=
=
001 2110,22111 ’222
sites,
tions
along
’
with the moment orienta-
the a axis of the Pbnm cell.
BaNi02
While the results obtained
on
BaNiO2 have been
largely negative in character, it still may be worth-
while to make a few remarks on this rather unusual
compound. There were several reasons why it
was chosen for study.
In the first place Lander’s
trial-and-error determination of the oxygen positions from visually estimated x-ray data did not
seem overly convincing since the atomic number
of Ba is quite high and even that of Ni is relatively
high [9]. Some preliminary packing calculations
revealed other possible structures which were not
mentioned by Lander, and one of these in particular seemed more logical than the one he reported.
Also the square planar oxygen coordination by
Ni2+ was most unusual in view of Lander’s observation of a nearly " normal " paramagnetic
moment corresponding to 1.8 unpaired electrons.
The explanation suggested byLander for the observed moment is that there are additional bonds
between neighboring nickel ions, as evidenced by
the very short Ni-Ni distance of 2.36 A. Finally,
the orthorhombic Cmcm structure is similar in
many respects to the anhydrous sulfate structures,
and it was of interest to investigate possible
magnetic ordering at low temperatures.
Essentially the structure investigation led to
confirmation of Lander’s structure. The refinement is still in progress, and it may be that some
significant parameter changes will be found (this
seems to be the case with the oxygen y parameter),
but there is no doubt that Lander’s general configuration is correct.
The magnetic studies have not been so conclusive. In a separate paper from his report on the
DalVi02 structure, Lander quotes unpublished susceptibility measurements of F. Morin [10]. The
temperature range is not given, but it is stated
that the Curie-Weiss law is obeyed with A
1.80~,
and that the calculated number of unpaired electrons per nickel ion is 1.83. From this one might
expect to find a Neel point well above the temperature of liquid helium. However, there are no
obvious changes in the neutron powder patterns in
going down to this temperature. Through the
courtesy of Drs W. J. Takei and D. E. Cox [11], of
the Westinghouse Research Laboratories, the
authors were able to obtain magnetic susceptibility
data down to 4.2 OK. These data disagreed somewhat with Morin’s in giving A
720 and approximately one unpaired electron per nickel ion. The
Curie-Weiss law was obeyed very well down to
liquid N~, but the 1 jx curve began to fall off from
a straight line at lower temperatures, suggestive of
an approaching transition.
Unf ortunately, Takei
and Cox were unable to take data between 4.2 OK
and approximately 50 °K, so there was no definite
evidence of a transition. Also unfortunately, the
magnetic sample was found to be somewhat contaminated with impurities, so not too much faith can
be placed in the differences from Morin’s measureas
ments. If the Ni moment is as low as 1
=
==
445
indicated by the data of Takei and Cox, it is
possible that the neutron diffraction statistics were
not sufficiently good to observe ordering. This
would be particularly true if the ordering is type A,
since peak overlaps happen to be very bad for this
There
are
two four-fold sets of Fe2+ ions :
case.
This is as much as can be said on this problem
for the moment. Further magnetic measurements
must be made at low temperatures on a pure
sample before continuing with neutron diffraction
work.
Fe2SiO 4
Until very recently very little has been known
about the magnetic properties of the transition
metal olivine type compounds. Early in this year
Kondo and Miyahara [12] published magnetic susceptibility data on the orthosilicates of divalent
Mn, Fe, Co, and Ni between 77 oK and 300 OK.
The curves suggest that magnetic transitions will
be found in all of these compounds at lower temperatures. A transition in Fe2SiO 4 has been found
by the present authors by neutron diffraction measurements, and by Takei and Cox [11.] by magnetic
susceptibility measurements. Fayalite mineral
samples kindly supplied by Dr Clifford Frondel of
Harvard~University, were used for both measurements. The Néel point is not known accurately,
but it is in the vicinity of 30 OK. The magnetic
measurements also showed some evidence of weak
ferromagnetism, with about 0.05
although
this may not be real since the mineral sample was
rather impure.
The first diffraction data were taken using the
powdered mineral sample. Several single crystals
which appeared to be of fairly good quality were
removed from the bulk material before grinding
the sample. The purity of the crystals should be
considerably better than the powdered material,
and one of these has been cut and mounted for
study. So far the only measurements made have
been of powder, samples. While this showed
only very small percentages of substitutional
impurities in a chemical analysis, the diffraction patterns disclosed an appreciable contamination of FeSiO,. The latter material also has a
magnetic transition at low temperatures, so its
effects on the data cannot be completely eliminated
by subtraction of liquid N2 and liquid He patterns.
This should not be a problem in the future, however, for in addition to the natural single crystals
already mentioned, some very good synthetic
powder samples have been obtained.
In the orthorhombic olivine structure [131, space
group Pbnm(D§g), there are four formula units per
iinit cell. The cell of Fe2SIO, has dimensions (14]
In the ideal olivine structure x
0 and y = 1 /4,
but the values observed [13] in olivine itself are
0.99 and 0.281, respectively. Each of the Fe 2+
ions is in octahedral coordination with oxygen, and
one may expect magnetic interactions of the same
nature as the B-B interaction in spinel structures.
In addition there are superexchange paths available via the covalent Si01+ groups.
If the magnetic and chemical cells are identical,
as proves to be the case in Fe2S’01, there are eight
spin vectors in the cell, and hence there are very
many possible antif erromagnetic configurations.
These can be reduced considerably by symmetry
considerations, but even then there are 19 possible
collinear spin ordering modes, and of course there
are many more combination modes that become
possible for canted spin structures. The problem
of sorting through all of these possibilities to
satisfy observed neutron intensities is complicated
by the f ree x and y parameters of Fell. For one
thing these parameters must be accurately determined as part of the analysis, but a more important
difficulty is that the magnetic extinction rules
which would apply for Feri spins in the ideal
olivine structure are no longer valid. In this preliminary stage of the problem, however, the ideal
structure is being assumed. This should be reasonably safe in determining the ,principal features of
the magnetic structure, since the intensity contributions arising from non-ideal shifts should be
small.
Let the spins on the Fei 4(a) sites be numbered
1 through 4 in the same order as given above.
Assuming ideal positions for Fell, let the spins on
the 4(c) sites be numbered 5 through 8 in the
following order :
=
If one calculates magnetic extinction conditions
for the various possible coliinear antiferromagnetic
spin configurations it is found that none of these
Hence there
can account for the observed data.
must be at least two spin directions in the structure. The proper spin vectors may be constructed
from their components by using combinations of
the collinear ordering modes.
The most intense reflection in the pattern is
the (100). This peak can arise from two different
modes but only one of these can yield a high inteny
·
sity :i
~
1
2
3
4
5
6
7
8
446
°
In this ordering scheme the spin components
form alternating parallel sheets on the (200) planes.
This mode, wich is the dominent one in the
structure, is associated with the cristallographic b axis. Analysis of the data indicates
a + - + - + - + - arrangement of spin
components parallel to a. The components in
the c direction are small, if indeed they exist at
all. The Fe2+ moment was found to be approximately 4 ~B for both Fei and Fell. In both cases
the spin vectors are approximately parallel to
the XY plane and are tilted from the b axis by
about 300, with the tilt alternating in sign as
prescribed by the a axis ordering mode.
Discussion
Pr JAMES.
-
Pouvez-vous
me
pr6ciser l’impor-
tance de l’impuret6 FeSi03 dans votre Fe2SiO4 et
comment la d6terminez-vous ?
Dr KAY. - Environ 15 % et peut-être 20 %,
par diagrammes aux neutrons et aux rayons X.
Pr BERTAUT.
11 y a des tables des repr6sentations irr6ductibles pour les sites b) et c) dans le
groupe Pbnm qui est aussi le groupe des p6rovskites
des terres rares FeR03 ~R
terre rare). Avez-vous
6tudl6 si les configurations de FeI et Fei, sont
eoupl6es en quel cas elles doivent appartenir a la
même representation ?
-
==
Jusqu’ici nous avons suppose lay
les repr6sentations de groupes
la structure approch6e. Mais nous
Dr KAY. -
In closing, the authors
Acknowledgements.
would like to acknowledge the valuable assistance
and advice of the Brookhaven National Laboratory
staff members in starting this program.
-
,
nication.
FRAZER (B.
C.)
and BROWN
avec
pour trouver
devrons d’abord raffiner la structure avec des
donn6es meilleures avant de r6pondre d6finitivement a la question.
REFERENCES
[1] SILLEN (L. G.) and NYLANDER (A.), Arkiv Kemi Min.
Geol., 1943, 17 A, n° 4.
[2] ZALKIN (A.) and TEMPLETON (D. H.), Private commu-
[3]
eompatibilité
(P. J.), Phys. Rev., 1962,
125, 1283.
[4] BROWN (P. J.) and FRAZER (B. C.), Phys. Rev. 1963,
129, 1145.
[5] BERTAUT (E. F.), COING-BOYAT (J.) and DELAPALME
(A.), Phys. Letters, 1963, 3, 178.
[6] KREINES (N. M.), Zhur. Eksp. i Teoret., Fiz., 1958,
35, 1391 (Eng. transl : Sov. Phys., JETP, 1959, 8,
972).
KOKKOROS
(P. A.) and RENTZPERIS (P. J.), Acta
Cryst., 1958, 11, 361.
[8] BERTAUT (E. F.), J. Appl. Phys., Suppl., 1962, 33,
[7]
1138.
[9]
[10]
[11]
[12]
LANDER (J. J.), Acta Cryst., 1951, 4, 148.
LANDER (J. J.), J. Am. Chem. Soc., 1951, 73, 2450.
TAKEI (W. J.) and Cox (D. E.), Unpublished data.
KONDO (H.) and MIYAHARA (S.), J. Phys. Soc. Japan,
[13]
BELOV (N. V.), BELOVA (E. N.), ANDRIANOVA (N. H.)
and SMIRNOVA (P. F.), Dokl. Akad. Nauk, SSSR,
1951, 81, 399.
WINCHELL (H.), Amer. Cryst. Assoc. Meeting Abstr.,
1963, 18, 305.
[14]
p.14 (April 10-12, 1950).