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Magnetic Phase Transition in Synthetic Cobalt-Olivine
A.P. Sazonov 1* , M. Meven 2 , V. Hutanu 1 , G. Heger 1 , M. Merz 1 , V.V. Sikolenko 3
1 Institute of Cristallography, RWTH Aachen, D-52056 Aachen, Germany
2 ZWE FRM-II, TU Munich, D-85747 Garching, Germany
3 BENSC, HMI, D-14109 Berlin, Germany
*E-mail: andrew.sazonov@frm2.tum.de
Olivine-type silicates, M 2 SiO 4 (M – divalent cation), are a major and important component of the upper Earth’s mantle.
Therefore, the properties of these materials are of considerable interest in physics, geology and crystal chemistry. Olivine
compounds are used as an important composition in some refractory materials, additives in cement concrete, acid-resistant
containers, ceramic pigments, etc. Well known examples of natural olivine-type silicates are fayalite (Fe 2 SiO 4 , with
paramagnetic Fe 2+ ions) and forsterite (Mg 2 SiO 4 , with diamagnetic Mg 2+ ions). There are also a few natural members with
another transition (Mn, Ni) or alkaline-earth (Ca) metal ions and their mixtures, e.g. tephroite (Mn 2 SiO 4 ), kirschsteinite
(CaFeSiO 4 ), etc.
A remarkable feature of the orthorhombic olivine-type structure (space group Pnma, no. 62 [1]) consists in two
crystallographically non-equivalent M positions. Moreover, these systems are interesting due to the peculiarities of their
magnetic structures. The magnetic properties of olivine compounds are quite complex and depend on type of M cation.
Synthetic Co 2 SiO 4 also crystallizes in the olivine-type structure. An antiferromagnetic phase transition occurs in this
compound. However, the magnetic properties of Co 2 SiO 4 were not yet well understood. Therefore, in order to determine the
nature of magnetism in this system we have performed both X-ray and neutron diffraction studies as well as magnetization
measurements of cobalt-olivine.
A large Co 2 SiO 4 single crystal (length ~ 1.5 cm, diameter ~ 0.5 cm) was grown by the zone melting method using a mirror
furnace (Inst. of Cryst., RWTH, Aachen). The phase purity was checked using the high resolution X-ray powder diffraction
(MILIDI, Inst. of Cryst., RWTH, Aachen) in the temperature range from 19 to 300 K with Cu Kα radiation. The unpolarized
neutron diffraction measurements were done using the single crystal diffractometer HEiDi [2] at the hot source of the FRM-II
(TU Munich, Germany). Data were collected at 2 K, 55 K and 300 K with wavelength of 0.55 Å up to about sinΘ/λ ≈ 1.1 Å -1 .
We have measured 3021 reflections all together with 1223 unique reflections (891 reflections with I > 3σ(I)) at 2 K.
Likewise, 2232 reflections all together with 1390 unique reflections (1026 reflections with I > 3σ(I)) were collected at 55 K.
At room temperature we have measured 2357 reflections with 1465 unique reflections (1092 reflections with I > 3σ(I)).
Temperature stability was better than 0.1 K. The neutron diffraction data were analyzed with the Rietveld method using the
FullProf program [3]. The dc magnetization measurements were performed using a Quantum Design MPMS-5 SQUID
magnetometer (HMI, Berlin). The temperature dependencies of the magnetization M(T) were measured on warming from 4 to
300 K in a field of 5 T.
As was already pointed out, Co 2 SiO 4 have an olivine-type orthorhombic crystal structure with the space group Pnma in
which four formula units are contained in the unit cell (figure 1). The silicon atoms are coordinated with the four oxygen
atoms to form SiO 4 tetrahedra. The cobalt atoms are surrounded by the six oxygen atoms and form CoO 6 octahedra. There are
two crystallographically non-equivalent Co sites, where Co I (4a) ions are sites of inversion symmetry (the smaller and more
distorted sites), and Co II (4c) ions are in the plane of mirror symmetry (the lager and less distorted sites).
Preliminary studies of the sample at/below room temperature were carried out using X-ray diffraction, and the crystal
structure is confirmed to be orthorhombic. The results indicate that the general trend of the thermal expansion appears to be
normal; the unit cell parameters and the cell volume were found to gradually increase with temperature (figure 2).
a (A)
10.31
10.30
10.29
10.28
b (A)
6.00
5.99
c (A)
4.785
4.780
Figure 1. A schematic representation of Co 2 SiO 4
crystal structure
V (A 3 )
4.775
296
295
294
0 50 100 150 200 250 300
T (K)
Figure 2. Temperature dependency of the lattice
parameters and cell volume of Co 2 SiO 4
According to experimental data, an antiferromagnetic phase transition occurs in this compound at T N ≈ 50 K (figures 3 and
4 and ref. [4]). On the other hand, no significant anomalies were observed in the cell parameters at temperatures near T N . The
investigation of any subtle changes should be performed with smaller temperature steps, but this is outside the scope of the
present work.
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