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The crystallographic and magnetic structure of Ni(IO3)2.2D2O
Physica 57 (1972) 215-220 0 North-Holland Publishing Co.
THE CRYSTALLOGRAPHIC
AND MAGNETIC
STRUCTURE
OF Ni(IOa)a. 2DsO J. B. A. A. ELEMANS Kamerlingh
Onnes Laboratorium, Leiden, Nederland
and B. VAN
LAAR
and B. 0. LOOPSTRA
Reactor Centrum Nederland, Petten, Nederland (Commun.
Kamerlingh
Received
Onnes Lab.,
26 May
Leiden
No. 387~)
1971
synopsis The crystallographic been determined netic symmetry
structure
by X-ray Pb’c’a
of B-nickel iodate
and neutron
powder
dihydrate,
diffraction.
space group At
Pbca,
has
T = 1.2 K the mag-
is found.
1. Introduction. ,!?-Ni(IOa)s. 2HsO has been studied, performing measurements on nuclear quadrupole resonancei), specific heats) and magnetizationa94). A n t’f1 erromagnetism accompanied by a weak ferromagnetic moment is found below TN = 3.08 K. Maxima in the specific heat are reported to occur at 3.0 K and 2.4 K, and although only the first of these is influenced by a magnetic field, both are interpreted as pertaining to magnetic ordering. At the same temperatures maxima are found in the initial magnetic susceptibility; the maximum at the lower temperature appears only in external magnetic fields less than about 100 Oe. The present investigation is an attempt to determine the magnetic structure by means of neutron diffraction and includes a determination of the complete crystal structure. 2. Exfierimental. Ni(IOa)s. 2HsO was obtained by slow evaporation of a saturated aqueous solution and recrystallization in water at 70-75°C for several days. A deuterated sample for neutron diffraction was prepared by dehydrating the hydrate at approximately 260°C for about 15 h followed by recrystallization in DsO at 70-75°C. For both samples CuKa X-ray diagrams were taken in steps of 0.02” in 28 on a Philips diffractometer. These diagrams were identical to within the accuracy determined by counting statistics. Neutron diagrams of the deuterated sample were taken at temper215
216
J. B. A. A. ELEMANS,
B. VAN
LAAR
AND
B. 0. LOOPSTRA
atures of 297, 4.2 and 1.2 K on the powder diffractometer
at the High Flux
Reactor at Petten. The wavelength, obtained from a Cu( 111) monochromator, was 2.5808 f- 0.0004 A as determined with an a-quartz sample; higher-order radiation was removed by 10 cm of pyrolytic graphite. Soller slits of 15’ angular divergence were placed between the reactor and the monochromator, and in front of the BFa counter. A range of 0 < sin 13/n< 0.35 A-l was scanned in steps of 0.072” in 28. The sample was contained in a vanadium cylinder with a diameter of 20 mm. It was not necessary to correct the measured intensities for absorption. In a later stage of the analysis the low-angle part of the diagrams at 4.2 and 1.2 K was reexamined with a longer counting time and 30’ Soller slits in order to observe the rather weak magnetic scattering. 3. The crystal structure. The X-ray diffraction pattern was indexed on an orthorhombic unit cell of dimensions a = 9.158 (1) A, b = 12.206 (2) k, c = 6.584 (1) A, standard d eviations in units of the last decimal being given in parentheses. Comparison of the neutron diagrams at 300 and 4.2 K revealed no change in crystal structure. By the reflection conditions the space group Pbca, proposed by Woodr), was confirmed. The unit cell contains four formula units, corresponding to a calculated density of 4.01 g/ems. The positions of the Ni and I atoms were derived from a Patterson synthesis based on the X-ray powder intensities. Assuming that the Ni atoms are situated at the 4 (a) sites the I atoms are found at 8 (c) positions with coordinates ), &, i, approximately. A least-squares refinement resulted in two almost identical possibilities : x = 0.274,
y = 0.133,
z = 0.276
or
0.224.
From the Patterson function one of the oxygens was seen to be situated at x M 0.25, y = 0.30, z m 0.25. Using these results a number of structure models containing all oxygens was constructed from packing considerations. It was not imposed that the iodate group was a regular trigonal pyramid; the Ni atoms were assumed to be octahedrally coordinated by four iodate oxygens and two water oxygens. Calculation of the X-ray intensities and, after insertion of the D atoms, of the neutron intensities showed only one of these models to be consistent with both sets of data. Moreover, for this model the interatomic distances remained acceptable on refinement, and therefore it was adopted for the final calculations using Rietveld’s profile programs). In table I the results for T = 4.2 K are presented; the observed and calculated scattered intensities are shown in fig. 1. A stereoscopic picture of the structure, obtained with an adapted version of the ORTEP programs) is reproduced in fig. 2, while relevant interatomic distances are collected in table II.
STRUCTURE
OF Ni IODATE
217
TABLE I Ni(I03)2.2D30 at 4.2 K Cell constants (A) Atomic positions Ni I 01 02 03 04 Dl D2
B (As) 0.62
9.126(l)
12.138(l)
O(-_)
6.555( 1)
O(-_)
0.268( 1) 0.077( 1) 0.281(l) 0.280( 1) O.Oll(2) 0.095( 1) 0.926( 1)
O(-_)
0.213( 1) 0.194(l) 0.443( 1) 0.325( 1) 0.737(2) 0.757( 1) 0.731(l)
0.134(l) 0.129(l) 0.046( 1) 0.272( 1) 0.101(l) 0.145(2) 0.149( 1)
Residual 7.2
Scattering lengths used: Ni: 1.03, I : 0.52, 0 : 0.577, D: 0.621 (all X lo-rscm). Standard deviations are given in parentheses in units of the last decimal.
Ni //03/~~2~~O at 4-2 K z =
8000.-_._ CALCULATED PROFILE . ....*.
MEFISURED
SCATTERING Fig. 1. Observed
PROFILE
ANGLE 28
and calculated neutron diagram. The peaks not included in the calculation are from the cryostat.
218
J. B. A. A. ELEMANS,
B. VAN LAAR AND B. 0. LOOPSTRA
Fig. 2. Stereoscopic pictures of the structure of nickel iodate dihydrate. The a axis is placed horizontally, the b axis about vertically, and the c axis approximately normal to the plane of the paper. o: Ni; 0: I; o: 0; 0: H.
TABLE II Interatomic distances Atom 1
Atom 2
Distance
Atom 1
Atom 2
(A) Ni Ni I I
I I’ I’ I”
3.26( 1) 3.26( 1) 4.59(2) 4.63(2)
(A) Ni Ni Ni 01 01
0.5 I I I 01 01
02
01
02 03 02 03 03
1.75(2) 1.85( 1) 1.83( 1) 2.67( 1) 2.68( 1) 2.85( 1)
Distance
04 04 0; 0; 0; 0;
01
0;; 04
06
04 04 Di D2
Di D2
04 04
2.13(l) 2.11(l) 2*12(l) 2.98( 1) 2.93( 1) 2.93(2) 0.95(2) 0.97(2) 2.02(2) 1.68(2) 2.97(2) 2.65(2)
Standard deviations are given in units of the last decimal.
4. The magnetic structwe. Initially no magnetic scattering was apparent in the diffraction pattern at 1.2 K and therefore the possible magnetic structures were considered. As nickel iodate dihydrate is known to be a weak ferromagnetl* 4) the magnetic unit cell must be identical to the chemical cell. For the space group Pbca three magnetic space groups allowing ferromagnetism exist: Pb’c’a, Pb’ca’ and Pbc’a’. A calculation of the diffraction pattern for a moment of 2,~s along one of the axes indicated
STRUCTURE
OF Ni IODATE
219
that the intensities would be measurable with improved counting statistics, and therefore it was decided to reexamine the low-angle part of the diagrams, where the influence of the magnetic form factor is not very pronounced, as described under Eqberimental. Weak magnetic intensities were actually observed, compatible with the symmetry Pb’c’a. In this space group the moments have components as given in table III. An x component of about 2~0 and a y component of about 0.5~0 are found, but the present data do not allow an accurate determination of the magnetic moment. TABLE
III
Magnetic space group Pb’c’a’ Atomic position 000 3&O 404 o*+
SZ
S,
sz
+
+
+
+
+
+ +
-
+
5. Discussion. It is seen that the suggestion by Meijer and Van den Handels), that half of the Ni spins order at 3.0 K and the other half at 2.4 K, is invalidated by the observation that all Ni atoms are equivalent. Because the magnetic scattering is rather weak it is not feasible to determine the magnetic structure between 3.0 and 2.4 K by means of powder diffraction. From the fact, however, that Burgiel et aZ.1) do not report a change in the internal magnetic field at 2.4 K one may infer that a magnetic transition at that temperature is rather improbable. A possible explanation for the second peak in the specific heat is afforded by the observation in the X-ray diagram, of impurities in the sample used for the previous measurements. (This sample was available to us through the courtesy of H. C. Meijer). If this peak is to be ascribed to these impurities the occurrence of a maximum in the susceptibility, also seen in the present, purer sample, at the same temperature, is a coincidence. Consequently be highly interesting to repeat the specific-heat measurements.
it would
Acknowledgements. The authors are indebted to Professor J. van den Handel of the Kamerlingh Onnes Laboratory, Leiden, for the suggestion of the problems and his continued interest in the work, to Dr. E. H. P. Cordfunke of the R.C.N. chemistry department for his investigation of the chemical behaviour of nickel iodate hydrates, to the technical staff of the R.C.N. diffraction group who actually performed the measurements, and to Dr. H. M. Rietveld for his help with calculating problems.
220
STRUCTURE
OF Ni IODATE
REFERENCES 1) Burgiel, J. C., Jaccarino, V. and Schawlow, A. L., Phys. Rev. 122 (1961) 429. 2) Du Chatenier, F. J., Boerstoel, B. M. and De Nobel, J., Physica 30 (1964) 1625 (Commun. Kamerlingh Onnes Lab., Leiden No. 340a). 3) Meijer, H. C. and Van den Handel, J., Physica 30 (1964) 1633 (Commun. Kamerlingh Onnes Lab., Leiden No. 340b). 4) Meijer, H. C., Pimmelaer, L. M’. W. A., Elemans, J. B. A. A. and Van den Handel, J., Physica 43 (1969) 569 (Commun. Kamerlingh Onnes Lab., Leiden No. 372~) ; Meijer, H. C., Thesis, Leiden 1970. 5) Rietveld, H. M., J. appl. Cryst. 2 (1969) 65. 6) Johnson, C. K., ORTEP: a fortran thermal-ellipsoid plot program for crystal structure illustrations. ORNL 3794.
THE CRYSTALLOGRAPHIC
AND MAGNETIC
STRUCTURE
OF Ni(IOa)a. 2DsO J. B. A. A. ELEMANS Kamerlingh
Onnes Laboratorium, Leiden, Nederland
and B. VAN
LAAR
and B. 0. LOOPSTRA
Reactor Centrum Nederland, Petten, Nederland (Commun.
Kamerlingh
Received
Onnes Lab.,
26 May
Leiden
No. 387~)
1971
synopsis The crystallographic been determined netic symmetry
structure
by X-ray Pb’c’a
of B-nickel iodate
and neutron
powder
dihydrate,
diffraction.
space group At
Pbca,
has
T = 1.2 K the mag-
is found.
1. Introduction. ,!?-Ni(IOa)s. 2HsO has been studied, performing measurements on nuclear quadrupole resonancei), specific heats) and magnetizationa94). A n t’f1 erromagnetism accompanied by a weak ferromagnetic moment is found below TN = 3.08 K. Maxima in the specific heat are reported to occur at 3.0 K and 2.4 K, and although only the first of these is influenced by a magnetic field, both are interpreted as pertaining to magnetic ordering. At the same temperatures maxima are found in the initial magnetic susceptibility; the maximum at the lower temperature appears only in external magnetic fields less than about 100 Oe. The present investigation is an attempt to determine the magnetic structure by means of neutron diffraction and includes a determination of the complete crystal structure. 2. Exfierimental. Ni(IOa)s. 2HsO was obtained by slow evaporation of a saturated aqueous solution and recrystallization in water at 70-75°C for several days. A deuterated sample for neutron diffraction was prepared by dehydrating the hydrate at approximately 260°C for about 15 h followed by recrystallization in DsO at 70-75°C. For both samples CuKa X-ray diagrams were taken in steps of 0.02” in 28 on a Philips diffractometer. These diagrams were identical to within the accuracy determined by counting statistics. Neutron diagrams of the deuterated sample were taken at temper215
216
J. B. A. A. ELEMANS,
B. VAN
LAAR
AND
B. 0. LOOPSTRA
atures of 297, 4.2 and 1.2 K on the powder diffractometer
at the High Flux
Reactor at Petten. The wavelength, obtained from a Cu( 111) monochromator, was 2.5808 f- 0.0004 A as determined with an a-quartz sample; higher-order radiation was removed by 10 cm of pyrolytic graphite. Soller slits of 15’ angular divergence were placed between the reactor and the monochromator, and in front of the BFa counter. A range of 0 < sin 13/n< 0.35 A-l was scanned in steps of 0.072” in 28. The sample was contained in a vanadium cylinder with a diameter of 20 mm. It was not necessary to correct the measured intensities for absorption. In a later stage of the analysis the low-angle part of the diagrams at 4.2 and 1.2 K was reexamined with a longer counting time and 30’ Soller slits in order to observe the rather weak magnetic scattering. 3. The crystal structure. The X-ray diffraction pattern was indexed on an orthorhombic unit cell of dimensions a = 9.158 (1) A, b = 12.206 (2) k, c = 6.584 (1) A, standard d eviations in units of the last decimal being given in parentheses. Comparison of the neutron diagrams at 300 and 4.2 K revealed no change in crystal structure. By the reflection conditions the space group Pbca, proposed by Woodr), was confirmed. The unit cell contains four formula units, corresponding to a calculated density of 4.01 g/ems. The positions of the Ni and I atoms were derived from a Patterson synthesis based on the X-ray powder intensities. Assuming that the Ni atoms are situated at the 4 (a) sites the I atoms are found at 8 (c) positions with coordinates ), &, i, approximately. A least-squares refinement resulted in two almost identical possibilities : x = 0.274,
y = 0.133,
z = 0.276
or
0.224.
From the Patterson function one of the oxygens was seen to be situated at x M 0.25, y = 0.30, z m 0.25. Using these results a number of structure models containing all oxygens was constructed from packing considerations. It was not imposed that the iodate group was a regular trigonal pyramid; the Ni atoms were assumed to be octahedrally coordinated by four iodate oxygens and two water oxygens. Calculation of the X-ray intensities and, after insertion of the D atoms, of the neutron intensities showed only one of these models to be consistent with both sets of data. Moreover, for this model the interatomic distances remained acceptable on refinement, and therefore it was adopted for the final calculations using Rietveld’s profile programs). In table I the results for T = 4.2 K are presented; the observed and calculated scattered intensities are shown in fig. 1. A stereoscopic picture of the structure, obtained with an adapted version of the ORTEP programs) is reproduced in fig. 2, while relevant interatomic distances are collected in table II.
STRUCTURE
OF Ni IODATE
217
TABLE I Ni(I03)2.2D30 at 4.2 K Cell constants (A) Atomic positions Ni I 01 02 03 04 Dl D2
B (As) 0.62
9.126(l)
12.138(l)
O(-_)
6.555( 1)
O(-_)
0.268( 1) 0.077( 1) 0.281(l) 0.280( 1) O.Oll(2) 0.095( 1) 0.926( 1)
O(-_)
0.213( 1) 0.194(l) 0.443( 1) 0.325( 1) 0.737(2) 0.757( 1) 0.731(l)
0.134(l) 0.129(l) 0.046( 1) 0.272( 1) 0.101(l) 0.145(2) 0.149( 1)
Residual 7.2
Scattering lengths used: Ni: 1.03, I : 0.52, 0 : 0.577, D: 0.621 (all X lo-rscm). Standard deviations are given in parentheses in units of the last decimal.
Ni //03/~~2~~O at 4-2 K z =
8000.-_._ CALCULATED PROFILE . ....*.
MEFISURED
SCATTERING Fig. 1. Observed
PROFILE
ANGLE 28
and calculated neutron diagram. The peaks not included in the calculation are from the cryostat.
218
J. B. A. A. ELEMANS,
B. VAN LAAR AND B. 0. LOOPSTRA
Fig. 2. Stereoscopic pictures of the structure of nickel iodate dihydrate. The a axis is placed horizontally, the b axis about vertically, and the c axis approximately normal to the plane of the paper. o: Ni; 0: I; o: 0; 0: H.
TABLE II Interatomic distances Atom 1
Atom 2
Distance
Atom 1
Atom 2
(A) Ni Ni I I
I I’ I’ I”
3.26( 1) 3.26( 1) 4.59(2) 4.63(2)
(A) Ni Ni Ni 01 01
0.5 I I I 01 01
02
01
02 03 02 03 03
1.75(2) 1.85( 1) 1.83( 1) 2.67( 1) 2.68( 1) 2.85( 1)
Distance
04 04 0; 0; 0; 0;
01
0;; 04
06
04 04 Di D2
Di D2
04 04
2.13(l) 2.11(l) 2*12(l) 2.98( 1) 2.93( 1) 2.93(2) 0.95(2) 0.97(2) 2.02(2) 1.68(2) 2.97(2) 2.65(2)
Standard deviations are given in units of the last decimal.
4. The magnetic structwe. Initially no magnetic scattering was apparent in the diffraction pattern at 1.2 K and therefore the possible magnetic structures were considered. As nickel iodate dihydrate is known to be a weak ferromagnetl* 4) the magnetic unit cell must be identical to the chemical cell. For the space group Pbca three magnetic space groups allowing ferromagnetism exist: Pb’c’a, Pb’ca’ and Pbc’a’. A calculation of the diffraction pattern for a moment of 2,~s along one of the axes indicated
STRUCTURE
OF Ni IODATE
219
that the intensities would be measurable with improved counting statistics, and therefore it was decided to reexamine the low-angle part of the diagrams, where the influence of the magnetic form factor is not very pronounced, as described under Eqberimental. Weak magnetic intensities were actually observed, compatible with the symmetry Pb’c’a. In this space group the moments have components as given in table III. An x component of about 2~0 and a y component of about 0.5~0 are found, but the present data do not allow an accurate determination of the magnetic moment. TABLE
III
Magnetic space group Pb’c’a’ Atomic position 000 3&O 404 o*+
SZ
S,
sz
+
+
+
+
+
+ +
-
+
5. Discussion. It is seen that the suggestion by Meijer and Van den Handels), that half of the Ni spins order at 3.0 K and the other half at 2.4 K, is invalidated by the observation that all Ni atoms are equivalent. Because the magnetic scattering is rather weak it is not feasible to determine the magnetic structure between 3.0 and 2.4 K by means of powder diffraction. From the fact, however, that Burgiel et aZ.1) do not report a change in the internal magnetic field at 2.4 K one may infer that a magnetic transition at that temperature is rather improbable. A possible explanation for the second peak in the specific heat is afforded by the observation in the X-ray diagram, of impurities in the sample used for the previous measurements. (This sample was available to us through the courtesy of H. C. Meijer). If this peak is to be ascribed to these impurities the occurrence of a maximum in the susceptibility, also seen in the present, purer sample, at the same temperature, is a coincidence. Consequently be highly interesting to repeat the specific-heat measurements.
it would
Acknowledgements. The authors are indebted to Professor J. van den Handel of the Kamerlingh Onnes Laboratory, Leiden, for the suggestion of the problems and his continued interest in the work, to Dr. E. H. P. Cordfunke of the R.C.N. chemistry department for his investigation of the chemical behaviour of nickel iodate hydrates, to the technical staff of the R.C.N. diffraction group who actually performed the measurements, and to Dr. H. M. Rietveld for his help with calculating problems.
220
STRUCTURE
OF Ni IODATE
REFERENCES 1) Burgiel, J. C., Jaccarino, V. and Schawlow, A. L., Phys. Rev. 122 (1961) 429. 2) Du Chatenier, F. J., Boerstoel, B. M. and De Nobel, J., Physica 30 (1964) 1625 (Commun. Kamerlingh Onnes Lab., Leiden No. 340a). 3) Meijer, H. C. and Van den Handel, J., Physica 30 (1964) 1633 (Commun. Kamerlingh Onnes Lab., Leiden No. 340b). 4) Meijer, H. C., Pimmelaer, L. M’. W. A., Elemans, J. B. A. A. and Van den Handel, J., Physica 43 (1969) 569 (Commun. Kamerlingh Onnes Lab., Leiden No. 372~) ; Meijer, H. C., Thesis, Leiden 1970. 5) Rietveld, H. M., J. appl. Cryst. 2 (1969) 65. 6) Johnson, C. K., ORTEP: a fortran thermal-ellipsoid plot program for crystal structure illustrations. ORNL 3794.