Metallic Re-Re bond formation in different MRe2O6 (M{double bond, long}Fe, Co, Ni) rutile-like polymorphs: The role of temperature in high-pressure synthesis

Article abstract of DOI:10.1016/j.jssc.2008.11.001

Different polymorphs of MRe₂O₆ (M{double bond, long}Fe, Co, Ni) with rutile-like structures were prepared using high-pressure high-temperature synthesis. For syntheses temperatures higher than ～1573 K, tetragonal rutile-type structur

Full text of DOI:10.1016/j.jssc.2008.11.001

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Journal of Solid State Chemistry 182 (2009) 364–373
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Journal of Solid State Chemistry
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Metallic Re–Re bond formation in different MRe2O6 (MQFe, Co, Ni)
rutile-like polymorphs: The role of temperature in high-pressure synthesis
Ã
D. Mikhailova a,b, , H. Ehrenberg b, S. Oswald b, D. Trots c, G. Brey d, H. Fuess a
a Institute for Materials Science, Darmstadt University for Technology, D-64287 Darmstadt, Germany
b Institute for Complex Materials, IFW Dresden, P. O. BOX 270116, D-01171, Dresden, Germany
c Hamburger Synchrotronstrahlungslabor, Notkestr. 85, D-22603 Hamburg, Germany
d Institute for Geosciences, Johann Wolfgang Goethe-University Frankfurt am Main, Altenho¨ferallee 1, D-60438 Frankfurt am Main, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Different polymorphs of MRe2O6 (MQFe, Co, Ni) with rutile-like structures were prepared using high-
pressure high-temperature synthesis. For syntheses temperatures higher than ꢀ1573 K, tetragonal
rutile-type structures (P42/mnm) with a statistical distribution of M- and Re-atoms on the metal
position in the structure were observed for all three compounds, whereas rutile-like structures with
orthorhombic or monoclinic symmetry, partially ordered M- and Re-ions on different sites and metallic
Re–Re-bonds within Re2O10-pairs were found for CoRe2O6 and NiRe2O6 at a synthesis temperature of
1473 K. According to the XPS measurements, a mixture of Re+4/Re+6 and M2+/M3+ is present in both
structural modifications of CoRe2O6 and NiRe2O6. The low-temperature forms contain more Re+4 and
Received 22 August 2008
Received in revised form
21 October 2008
Accepted 3 November 2008
Available online 13 November 2008
Keywords:
3d-metal rhenium oxide MRe2O6
Rutile-like structure
Polymorphism
M
3+ than the high-temperature forms. Tetragonal and monoclinic modifications of NiRe2O6 order with a
ferromagnetic component at ꢀ24 K, whereas tetragonal and orthorhombic CoRe2O6 show two magnetic
transitions: below ꢀ17.5 and 27 K for the tetragonal and below 18 and 67 K for the orthorhombic phase.
Tetragonal FeRe2O6 is antiferromagnetic below 123 K.
High-pressure synthesis
Re–Re bond
Magnetism
& 2008 Elsevier Inc. All rights reserved.
XPS spectra
1. Introduction
The study of MxRe1ꢁxO2 for M-3d transition metals is limited to
only a few works [5–8]. The best known compounds crystallize
either in the rutile-structure with a random distribution of cations
or rutile-like structures with or without cation ordering. MnReO4
Phase relationships and crystal structures of existing com-
pounds in the system Mo–Re–O [1,2] and V–Re–O [3] were
intensively investigated due to their potential catalytical activity.
(Mn0.5Re0.5O2) [5,7] and ZnReO4 (Zn0.5Re0.5O2) [5] adopt
a
wolframite-type structure without any metallic bond. Sc3+
demonstrates similarity with heavy rare earth elements in
Re-containing compounds forming Sc6ReO12 with fluorite-like
structure [9], and with 3d-elements forming ScRe2O6 with a new
rutile-derivated monoclinic structure containing Re2O10-units
with Re–Re bonds [8]. Compounds MReO4 for MQFe, Co, Ni were
obtained for the first time by Sleight [5] using HPHT. FeReO4
crystallizes in the tetragonal rutile-structure with a random
cation occupation, whereas an increasing Fe-content in the
compound causes an ordering of the cations resulting in the
In the quasibinary ReO2–MoO2-system
a MoxRe1ꢁxO2 solid
solution with the orthorhombic ReO2-structure and metallic
M–M-bond is stable for 0pxp0.35 at 1173–1473 K, whereas a
two-phase region consisting of orthorhombic MoxRe1ꢁxO2 and
monoclinic MoO2 was found for 0.35oxp1 [1]. For VxRe1ꢁxO2, the
monoclinic MoO2-type structure was registered for xX0.98 and
the rutile structure was detected for 0.98oxp0.85 [3]. The
increasing Re-content in the phase decreases linearly the
insulator–metal transition temperature and at the same time
increases the molar magnetic susceptibility. The rutile structure
formation of a superstructure: a tri-a-PbO2-type was proposed for
was also stabilized for
V0.5Re0.5O2 by high-pressure high-
Fe2ReO6. For CoReO4 a new rutile-derivated type with an ordering
of two cations was postulated [6], the structure of NiReO4 is not
solved yet [5]. Despite the lack of physical properties investiga-
tions of rutile-like Re- and 3d-containing complex oxides some of
them seem to demonstrate metallic type of conductivity due to
the delocalization of d electrons of Re-cations on Re–O–Re–O and
Re–Re–Re linkages. For example, the decreasing of electrical
resistivity on cooling was measured for FeReO4, MnReO4 [5] and
temperature (HPHT) synthesis [4].
Ã
Corresponding author at: Institute for Materials Science, Darmstadt University
for Technology, D-64287 Darmstadt, Germany. Tel.: +49 6151162894;
fax: +49 6151166023.
E-mail address: d.mikhailova@ifw-dresden.de (D. Mikhailova).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.11.001

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365
ScRe2O6 [8]. At the same time, there are strong magnetic
interactions in 3d/Re-containing oxides leading to a high tem-
perature of magnetic ordering: TN of 210 K for FeReO4 [5], TN of
275 K and Tc of 225 K for MnReO4 [7], and Tc above room
temperature was measured for ScRe2O6 [8]. In the latter case
only Re-ions (and itinerant electrons) are contributing to the
magnetic moments.
The preparation by HPHT syntheses, crystal structures and
magnetic properties of FeRe2O6, CoRe2O6 and NiRe2O6 are
reported. FeRe2O6 crystallizes in the tetragonal rutile-type with
Fe and Re sharing one crystallographic site. Two more polymorphs
have been observed for each of the other compounds CoRe2O6 and
NiRe2O6, controlled by the temperature of synthesis.
register mass loss and thermal flux curves. About 30 mg of
MRe2O6 were heated in an Al2O3-crucible in Ar-atmosphere with a
rate of 51/min from 20 up to 1150 1C.
X-ray photoelectron spectroscopy (XPS) measurements: The X-ray
photoelectron spectroscopic measurements were carried out at a
PHI 5600 CI system using an Al Ka 350 W monochromatized X-ray
source and a hemispherical analyzer at a pass energy of 29 eV.
When necessary, surface charging was minimized by means of a
low-energy electron flood gun. The system base pressure was
about 10ꢁ9 mbar. ReO3 (Strem Chemicals, 99.9%), Co3O4 (Alfa
Aesar, 99.99%) and NiO (Alfa Aesar, 99.99%) powders were used as
reference materials for the estimation of the valence states of Co,
Ni and Re ions in the ternary phases. The binding energy scale was
calibrated from the carbon contamination using the C 1s peak
at 285.0 eV.
Magnetization measurements: The magnetic properties of
MRe2O6 (MQFe, Co, Ni) have been studied with a superconduct-
ing quantum interference device (SQUID) from Quantum Design.
Measurements were performed upon heating in field-cooled (FC)
and zero-field cooled (ZFC) mode in the temperature range from
1.7 to 350 K and with applied field strengths up to 6.5 T.
2. Experimental
Synthesis: A high-pressure high-temperature (HPHT) synthesis
of MRe2O6 (MQFe, Co, Ni) was performed in a Girdle-Belt
apparatus with pyrophyllite as the pressure-transmitting medium.
A graphite furnace with a platinum capsule inside containing the
reactants was used for the experiment. Mixtures of MO (Alfa Aesar,
99.99%), ReO3 (Strem Chemicals, 99.9%) and Re (Strem Chemicals,
99.99%) were ground in an agate mortar under acetone and filled
in a platinum capsule. The sample holder was pressed up to 5 GPa
before the temperature was raised to 1473 or 1573 K with a rate of
50 K/min. The heating current was switched off after 120 min, and
after cooling to room temperature, pressure was released.
3. Results and discussion
3.1. Synthesis and sample characterisation
Depending on the synthesis temperature different structural
modifications were obtained for NiRe2O6 and CoRe2O6 at
the constant external pressure: phases with the tetragonal
rutile-structure are formed at 1573 K, whereas monoclinic rutile-
derivated structures were obtained from synthesis at 1473 K
(‘‘low-temperature modifications’’). FeRe2O6 was prepared at
1573 K only. Diffraction patterns of low-temperature modifica-
tions were similar to the one of high-pressure form of VO2 [16].
The conditions of synthesis and the lattice parameters are listed in
Table 1. XRD patterns of tetragonal FeRe2O6 and of the low-
temperature modifications of CoRe2O6 and NiRe2O6 are presented
in Fig. 1.
According to thermoanalyses mass losses were observed for
tetragonal CoRe2O6 and NiRe2O6 in Ar-atmosphere above 1153 K,
and CoO or NiO and Re(met.) were found as decomposition
products after the high-temperature treatment during thermo-
analyses.
X-ray powder diffraction (XRD): Phase analysis and determina-
tion of cell parameters at room temperature were carried out
using X-ray powder diffraction with a STOE STADI P diffractometer
˚
(Mo-Ka1-radiation, l ¼ 0.7093 A) in steps of 0.021 for 2 Theta from
31 to 451 in transmission mode for flat samples.
Crystal structure determination: The crystal structures of
MRe2O6, MQFe, Co, Ni, were solved by single-crystal X-ray
diffraction using the Xcalibur system from Oxford Diffraction. The
software packages SHELXS [10] and SHELXL [11] were used for
structure solution and refinement as included in X-STEP32 [12].
High-temperature structure investigations: High-temperature
structure investigations in the temperature range of 295–1173 K
on MRe2O6 powders have been performed by synchrotron
diffraction at beamline B2 [13] of the Hamburger Synchrotron-
strahlungslabor at DESY (Hamburg, Germany) with the on-site
readable image-plate detector OBI [14] and a STOE furnace,
equipped with a EUROTHERM temperature controller and a
˚
3.2. Crystal structure
capillary spinner. The wavelength of 0.49962(1) A was determined
from the positions of 8 reflections from a LaB6 reference material.
After heating up to 1173 K and subsequent cooling, the MRe2O6
samples were analyzed at room temperature again to check the
reversibility of temperature induced changes. All diffraction
patterns have been analyzed by using the software package
WinPLOTR [15]. Only isotropic thermal displacement parameters
have been refined by the Rietveld method.
3.2.1. High-temperature modifications
According to single crystal structural data all high-temperature
modifications of FeRe2O6, CoRe2O6 and NiRe2O6 crystallize in a
tetragonal rutile-type structure (S.G. P42/mnm) with a random
distribution of cations on the 2a-sites and without any metal–
metal bonding (Table 2). Atomic positions and selected intera-
tomic distances are presented in Table 3 and 4. The average
(M,Re)–O–distance is nearly the same for all three compounds
Thermoanalysis: A simultaneous thermal analyzer NETZSCH
STA 429 (CD) operated with dry and purified Ar was used to
Table 1
Synthesis conditions and lattice parameters of MRe2O6 (MQFe, Co, Ni), calculated from powder diffraction data.
Compound
Synthesis conditions
Symmetry
Lattice parameters
˚
˚
FeRe2O6
CoRe2O6
CoRe2O6
NiRe2O6
NiRe2O6
5 GPa, 1573 K
5 GPa, 1573 K
5 GPa, 1473 K
5 GPa, 1573 K
5 GPa, 1473 K
Tetragonal
Tetragonal
Monoclinic
Tetragonal
Monoclinic
a ¼ 4.73954(8) A, c ¼ 2.85881(5) A, Z ¼ 2
˚
˚
a ¼ 4.7414(1) A, c ¼ 2.84731(9) A, Z ¼ 2
˚
˚
˚
b
a ¼ 9.4176(2) A, b ¼ 5.7161(1) A, c ¼ 4.73808(9) A,
¼ 91.388(1)1, Z ¼ 8
˚
˚
a ¼ 4.7313(1) A, c ¼ 2.84552(8) A, Z ¼ 2
˚
˚
˚
b
a ¼ 9.3672(1) A, b ¼ 5.72651(7) A, c ¼ 4.72076(6) A,
¼ 91.7050(8)1, Z ¼ 8

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CoRe O :F222
10000
FeRe O :P4 /mnm
2
6
2
6
2
9000
6000
3000
0
5000
0
27
28
29
30
10
20
30
40
10
20
30
40
2θ,°
2θ,°
CoRe O :C2/m
10000
5000
0
2
6
NiRe O :C2/m
2
6
20000
10000
0
27
28
29
30
10
20
30
40
10
20
30
40
2θ,°
2θ,°
Fig. 1. (a) X-ray diffraction pattern of tetragonal FeRe2O6 (S.G. P42/mnm), observed and calculated profiles together with their difference curve. (b, c, d) X-ray diffraction
patterns of the low-temperature modifications of CoRe2O6 and NiRe2O6. For CoRe2O6, two structural models using F222 (b) and C2/m (c) space groups were refined based on
the same data set (see text). The NiRe2O6 structure was refined in space group C2/m.
assignment of Co+3 and Re+5 than Co+2 and Re+6 [6]. NiReO4, as
reported in [5], adopts also a rutile-derivated structure, but no
structural parameters were provided.
˚
(1.98A). The tetragonal distortion of (M,Re)O6-octahedra, calculated
as the ratio
z ¼ d(M,Re–O)equatorial(4 times)/d(M,Re–O)apical(2
times)
between averaged bond lengths in the equatorial plane and to
apical oxygens, respectively, has the lowest value for NiRe2O6
(
z
¼ 1.008) and the highest one for CoRe2O6 (
z
¼ 1.048). Note that
¼ 0.9827o1 is
for TiO6-octahedra in rutile TiO2
calculated [17].
a
value
z
3.2.2. Low-temperature modifications
Low-temperature modifications of NiRe2O6 and CoRe2O6
crystallize in rutile-like structures with a partial ordering of
cations. NiRe2O6 (Fig. 2, Table 1) adopts monoclinic symmetry,
S.G. C2/m, as other oxides with similar structures, for example,
CrWO4 [20] and the high-pressure modification of VO2 [16]. In
this structure type two crystallographically non-equivalent
[(Ni,Re)O4]n and [ReO4]n strings of edge-sharing ReO6 and
(Ni,Re)O6 octahedra exist. The specific cation distribution on the
metal sites is different in the structures of NiRe2O6 and CrWO4: in
NiRe2O6 the 4g site is exclusively occupied by Re atoms and the 4i
site is occupied by Re and Ni, whereas in the CrWO4 structure
cation positions are completely ordered: Cr ions on the 4i and W
ions on the 4g site. In both structures M2O10 units with metallic
M-M bonds exist between 5d cations (Fig. 3(b)). The Re–Re
Sleight [5] has reported a tetragonal rutile-type modification
(S.G. P42/mnm) in the system Fe–Re–O with the composition
˚
˚
FeReO4. The lattice parameters were: a ¼ 4.663 A, c ¼ 2.927 A.
This means together with our results for FeRe2O6 the existence of
a FexRe1ꢁxO2 solid solution for compositions at least within
0.33pxp0.50. The ‘‘a’’ parameter of the unit cell becomes larger
with increasing Re content in FexRe1ꢁxO2, whereas ‘‘c’’ decreases.
The same effect was also found for CrxRe1ꢁxO2 [18] and
CrxMo1ꢁxO2 [19] solid solutions.
In contrast to FexRe1ꢁxO2 no tetragonal solid solutions are
found in the composition range 0.33pxp0.50 for CoxRe1ꢁxO2 and
NixRe1ꢁxO2. The composition CoReO4 [6] adopts a rutile-like
orthorhombic structure with an ordering of Co and Re on two
distinct crystallographic sites and without Re–Re-bonds. Intera-
tomic metal–oxygen distances Co–O and Re–O suppose rather the
˚
distance in the Re2O10-units is 2.611(2) A and lower than in
˚
˚
metallic Re (2.74 A). The average Re–O distance is 1.95 A and the

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367
Table 2
Details of X-ray single-crystal data collection and structure refinement of polymorph modifications of MRe2O6 (MQFe, Co, Ni).
Crystal data
Chemical formula
Formula weight
Crystal system
Space group
Fe1/3Re2/3O2
174.75
Co1/3Re2/3O2
175.78
Co1/3Re2/3O2
175.78
Ni1/3Re2/3O2
175.70
Ni1/3Re2/3O2
175.70
Tetragonal
P42/mnm
Tetragonal
P42/mnm
Orthorhombic
F222
Tetragonal
P42/mnm
Monoclinic
C2/m
˚
Unit cell dimensions (A and 1)
a ¼ 4.741(4)
c ¼ 2.865(7)
a ¼ 4.728(1)
c ¼ 2.847(1)
a ¼ 13.531(3)
b ¼ 13.192(2)
c ¼ 5.7221(9)
a ¼ 4.733(4)
c ¼ 2.845(3)
a ¼ 9.374(4)
b ¼ 5.7225(14)
c ¼ 4.7183(14)
b
¼ 91.72(3)
3
˚
Cell volume (A )
64.40(18)
2
63.64(4)
2
1021.4(3)
32
63.73(10)
2
252.99(15)
Z
8
Calculated density (g/cm3)
9.014
9.173
9.145
9.156
9.226
˚
Radiation type
Temperature
Mo–K
a (l ¼ 0.71073 A)
299(2)
299(2)
299(2)
299(2)
299(2)
Crystal form, color
Crystal size (mm)
Prism, black
Prism, black
0.030 ꢂ 0.025 ꢂ 0.020
Prism, black
0.06 ꢂ 0.04 ꢂ 0.03
Prism, black
0.020 ꢂ 0.015 ꢂ 0.015
Prism, black
0.010 ꢂ 0.025 ꢂ 0.040
0.025 ꢂ 0.025 ꢂ 0.015
Data collection
Diffractometer
Oxford Diffraction Xcalibur (TM); single-crystal X-ray diffractometer with sapphire CCD detector
Rotation method data acquisition using and scans(s)
70.98
Data collection method
Absorption coefficient (mmꢁ1
F(000)
o
j
)
69.75
71.20
71.50
71.93
224
134
2144
135
587
Theta range for data collection
6.08–27.38
ꢁ5php6,
ꢁ5pkp5,
ꢁ2plp3
6.10–27.22
ꢁ3php5,
ꢁ5pkp6,
ꢁ2plp3
174/51
3.01–27.921
ꢁ16php17,
ꢁ13pkp16,
ꢁ7plp7
6.10–30.72
ꢁ6php5,
ꢁ5pkp6,
ꢁ4plp4
4.17–28.27
ꢁ12php11,
ꢁ7pkp7,
ꢁ6plp6
992/312
Range of h, k, l
Reflections collected/unique
227/52
2168/560
[R(int) ¼ 0.0974]
99.7%
178/62
[R(int) ¼ 0.0908]
98.1% (Theta ¼ 27.381)
[R(int) ¼ 0.0644]
[R(int) ¼ 0.0313]
92.5% (Theta ¼ 30.721)
[R(int) ¼ 0.0354]
97.2%
Completeness to Theta ¼ 26.351
98.1%
Refinement method
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-fit on F2
52/0/9
51/0/9
560/0/39
62/0/9
312/0 /23
1.262
1.269
1.208
1.282
1.198
Final R indices [I42sigma(I)]
R1 ¼ 0.0406,
wR2 ¼ 0.0999
R1 ¼ 0.0448,
wR2 ¼ 0.1015
0.46(11)
R1 ¼ 0.0369,
wR2 ¼ 0.0808
R1 ¼ 0.0491,
wR2 ¼ 0.0856
0.06(3)
R1 ¼ 0.0696,
wR2 ¼ 0.2402
R1 ¼ 0.0782,
wR2 ¼ 0.2538
0.0017(3)
R1 ¼ 0.0213,
wR2 ¼ 0.0450
R1 ¼ 0.0329,
wR2 ¼ 0.0477
0.006(5)
R1 ¼ 0.0388,
wR2 ¼ 0.0952
R1 ¼ 0.0432,
wR2 ¼ 0.0977
0.0021(5)
R indices (all data)
Extinction coefficient
3
3
3
3
3
˚
˚
˚
˚
˚
Largest diff. peak and hole
1.823 and ꢁ2.158 e/A
1.623 and –1.382 e/A
4.846 and ꢁ3.674 e/A
0.999 and ꢁ1.192 e/A
2.974 and ꢁ3.576 e/A
˚
average mixed (Re,Ni)–O distance 2.00 A, which is somewhat
shorter than in rhombohedral NiO with Ni2+ at room temperature
[21]. All peaks on the powder diffraction pattern of the low-
temperature NiRe2O6 are well described by this structure model
(Fig. 1(d)). In contrast to the Ni-containing phase all investigated
four single crystals of CoRe2O6 adopt orthorhombic symmetry
(S.G. F222) with a larger unit cell, unambiguously revealed based
on additional superstructure reflections. The same superstructure
cell was also observed for another polymorph of CrWO4 [22]
The monoclinic form is very sensitive to antiphase boundaries,
where 3d-metal and Re-ions interchange site occupations, and the
orthorhombic superstructure can be described as an ordered
sequence of such antiphase boundaries. Therefore, several inter-
mediate distributions of local Re–Re pairs are possible by some
partial disorder in the specific site occupations, and the two
polymorphs are just representing two different arrangements
with a pronounced long-range order. Nevertheless, a high degree
of residual disorder is reflected for both polymorphs by the mixed
occupied sites. The formation of metallic Re–Re bonds seems to
introduce a monoclinic distortion, which can be overcome by
partial order in an orthorhombic superstructure as observed for
all large single crystals. Note that the powder diffraction pattern
of low-temperature CoRe2O6 cannot be satisfactorily explained
based on the F222 space group (Fig. 1(d)), but only in C2/m,
isotypic with NiRe2O6. For example, the (480) reflection at 27.681
in the F222 model (inset in the Fig. 1(b)) is clearly split into at
least two reflections, well explained in the C2/m model (inset in
Fig. 1(c)). However, no monoclinic CoRe2O6 crystal with this
structure model could be isolated for single crystal X-ray
diffraction and are therefore probably limited to small domains,
while large X-ray coherent crystals are only observed for a
partially ordered superstructure.
(Fig. 3(a), Tables 1–3), but with
a slightly different cation
distribution: whereas W and Cr ions show a high degree of order
on different sites (W atoms occupy 4a, 4b and 8h sites, Cr atoms 8i
and 8j), the distribution of Co and Re is disordered (Table 2). The
˚
Re–Re bonds (2.539 A) are due to Re-ions on the 8h-site as the
W–W-bonds in orthorhombic CrWO4. The Re–Re distance in
the low temperature polymorph is shorter for the Co- than for the
Ni-compound, indicating that more electrons of Re, which are not
involved in Re–O bonds, participate in the metallic Re–Re bonds.
The orthorhombic unit cell of CoRe2O6 can be described as a
superstructure of four monoclinic cells of NiRe2O6, in which
each second [ReO4]n chain of edge sharing ReO6 octahedra is
broken by (Co,Re)O6 octahedra, so that a sequence of octahedra
[ReO6]–[(Co,Re)O6]–[ReO6]–? appears instead of [ReO6]–[ReO6]–
[ReO6]–?. Accordingly, both the monoclinic and the orthorhom-
bic structure type are characterised by the formation of metallic
Re–Re bonds in contrast to the high-temperature form.
The analysis of average interatomic (Re,M)–O distances in the
˚
low-temperature forms (for example, 1.93 and 2.01 A for CoRe2O6)
provide some information about the oxidation states of Co and Re:

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Table 3
2
3
˚
Atomic coordinates and anisotropic thermal displacement parameters (A , ꢂ10 ) for MRe2O6.
Compound
FeRe2O6
S.G.
Atoms
Wyckoff site
x
y
z
Occupancy
U11
7(1)
U22
7(1)
U33
U23
U13
U12
3(1)
P42/mnm
Fe
Re
O
2a
2a
4f
0
0
0
0
0
0.33
0.67
1
31(2)
31(2)
19(5)
0
0
0
0
0
0
7(1)
7(1)
0
0
3(1)
7(4)
0.2931(15)
0.2931(15)
15(6)
15(6)
CoRe2O6
CoRe2O6
P42/mnm
F222
Co
Re
O
2a
2a
4f
0
0
0
0
0
0.33
0.67
1
6(2)
6(2)
6(2)
6(2)
22(2)
22(2)
19(10)
0
0
0
0
0
0
6(1)
6(1)
0
0
0.2870(30)
0.2870(30)
14(7)
14(7)
ꢁ1(10)
Re(1)
Re(2)
Re(3)
Co(3)
Re(4)
Co(4)
Re(5)
Co(5)
O(1)
4a
8h
4b
4b
8i
0
0
0
1
20(2)
13(1)
1(2)
19(2)
16(1)
5(2)
53(6)
10(1)
17(3)
17(3)
28(2)
28(2)
22(2)
22(2)
0
0
0
0
0
0
0.25
0.25
ꢁ0.0281(3)
0.5
1
0
ꢁ6(1)
0
0
0.68(2)
0
0
0
0
0
0
0
0
0
0
0.5
0.32(2)
1(2)
5(2)
0.25
0.0044(2)
0.0044(2)
0.25
0.25
0.46(2)
32(2)
32(2)
12(2)
12(2)
24(4)
33(5)
19(8)
40(11)
24(5)
26(6)
17(2)
17(2)
16(2)
16(2)
0
23(3)
23(3)
0
8i
0.25
0.25
0.54(2)
0
8j
0.0082(2)
0.0082(2)
0.1013(15)
0.250(2)
0
0.25
0.40(2)
15(3)
15(3)
8j
0.25
0.25
0.60(2)
0
16k
16k
8f
0.0029(13)
0.1069(18)
0.143(2)
0.361(3)
0.25
0.251(7)
ꢁ0.008(4)
0
1
1
1
1
1
1
O(2)
O(3)
O(4)
8f
0
0
O(5)
8j
0.155(2)
0.25
O(6)
8j
ꢁ0.137(2)
0.25
0.25
NiRe2O6
NiRe2O6
P42/mnm
C2/m
Ni
Re
O
2a
2a
4f
0
0
0
0
0
0.33
0.67
1
11(1)
11(1)
13(3)
11(1)
11(1)
13(3)
22(1)
22(1)
12(5)
0
0
0
0
0
0
6(1)
6(1)
4(4)
0
0
0.2938(17)
0.2938(17)
Ni(1)
Re(1)
Re(2)
O(1)
O(2)
O(3)
4i
4i
4g
8j
4i
4i
0.2599(2)
0.2599(2)
0
0
0.4928(3)
0.4928(3)
0
0.68(2)
5(1)
5(1)
7(1)
9(2)
8(3)
8(3)
8(1)
8(1)
4(1)
4(1)
6(1)
0
0
0
0(1)
0(1)
4(1)
0
0
0
0
0.32(2)
0.7719(2)
1
1
1
1
10(1)
0.1437(12)
0.1099(17)
0.4066(17)
0.2420(18)
0.291(2)
0.790(3)
0.188(3)
0
0
2
For oxygen atoms in monoclinic NiRe2O6 and orthorhombic CoRe2O6 equivalent isotropic displacement parameters (A ꢂ103) were refined.
˚
Table 4
˚
Selected interatomic distances (A) for the different polymorphs of MRe2O6.
˚
Distances (A) and angles (1)
FeRe2O6
CoRe2O6 (F222)
CoRe2O6 (P42/mnm)
NiRe2O6 (C2/m)
NiRe2O6 (P42/mnm)
Re(1)/M(1)–O(1)
Re(1)/M(1)–O(1)
M(Re)–M(Re)
Re(1)–O(1)
1.965(10) ( ꢂ 2)
1.994(7) ( ꢂ 4)
2.865(7)
1.92(2) ( ꢂ 2)
2.01(2) ( ꢂ 4)
2.847(1)
1.99(1) ( ꢂ 2)
2.00(1) ( ꢂ 2)
1.967(11) ( ꢂ 2)
1.982(8) ( ꢂ 4)
2.845(3)
1.99(3) ( ꢂ 4)
1.89(3) ( ꢂ 2)
2.8610(5)
Re(1)–O(3)
Re(1)–Re(3)
Re(2)–O(2)
1.89(2) ( ꢂ 2)
1.98(2) ( ꢂ 2)
2.04(2) ( ꢂ 2)
2.539(4)
1.95(1) ( ꢂ 2)
Re(2)–O(6)
Re(2)–O(5)
Re(2)–Re(2)
2.611(2)
Re,Co(3)–O(4)
Re,Co(3)–O(1)
Re,Co(4)–O(1)
Re,Co(4)–O(2)
Re,Co(4)–O(2)
Re,Co(5)–O(3)
Re,Co(5)–O(4)
Re,Co(5)–O(5)
Re,Co(5)–O(4)
Re,Ni(1)–O(2)
Re,Ni(1)–O(3)
Re(2)–O(1)
1.84(4) ( ꢂ 2)
1.98(3) ( ꢂ 4)
2.01(2) ( ꢂ 2)
2.00(2) ( ꢂ 2)
2.02(2) ( ꢂ 2)
2.01(2) ( ꢂ 2)
2.05(3) ( ꢂ 2)
1.97(3)
1.99(3)
2.02(2)
2.02(2)
1.90(1) ( ꢂ 2)
2.01(1) ( ꢂ 2)
Re(2)–O(3)
2+
The Co–O distance of 1.93 A is too low for Co (0.75 A, high-spin)
2.01 A would be expected for Re4+–O. A similar assignment holds
for NiRe2O6.
˚
˚
˚
and O2ꢁ (1.40 A) and corresponds rather to the sum of ionic radii
˚
3+
for Co3+ (0.55 A, low-spin state) or Co (0.61 A, high-spin) and
High-temperature structural investigations did not show any
phase transitions below the phase decomposition temperatures.
˚
˚
O2ꢁ. The Re–O distance of 1.93 A indicates Re or Re6+, whereas
5+
˚

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D. Mikhailova et al. / Journal of Solid State Chemistry 182 (2009) 364–373
369
ternary oxides do not change with an increase of the average
oxidation state of Co from +2 to +3 or +4 as long as the oxygen
coordination remains the same, while the position and the
intensity of the satellite peaks dependent significantly on the
metal oxidation state, see for example [23]. In our spectra there is
a systematic shift of the Co2p and Ni2p cores towards higher
binding energies, maybe as a result of a high positive charge of the
sample due to photoelectron emission, so that the determination
of the positions of the satellites is not very reliable. It should be
noted here explicitly that XPS represents the electronic structure
of the surface region, influenced by contaminations or surface
effects. For the bulk material the Re4f7/2 core peak is more
representative, because the kinetic energies of the detected and
analyzed photoelectrons are about 700 eV for Co2p and 1440 eV
for Re4f7/2, allowing a photoelectron escape from about 1.5 times
deeper regions for Re4f7/2 than for Co2p and Ni2p. The shifts in
binding energies between Re4f7/2 and Re4f5/2 peaks of Re+4 in
ReO2 and Re+6 in ReO3 are significant [24]. The authors of [25]
have established an exponential dependence of the binding
energy on the oxidation state of Re in the compounds NH4ReO4,
Re2O7, ReO3, ReO2 and metallic Re without consideration of the
underlying structures. However, the difference in the positions of
the Re4f7/2 peaks for Re+7 in NH4ReO4 and Re2O7 points out an
influence of the structural features on the XPS spectra: in
ammonium perrhenate only corner-sharing ReO4-tetrahedra
exist, in contrast to both corner-sharing ReO4-tetrahedra and
ReO6-octahedra in Re2O7. Our measurements on ReO3 (Fig. 4a)
reveal the Re4f7/2 peak of ReO2 at 43.6 eV, which indicates the
presence of Re+4 due to a partial reduction in the surface-near
region. The broadening of Re peaks in MRe2O6 can also reflect Re
in different oxidation states within one phase, but is also typically
observed for strong-correlated electron systems. Each Re4f
spectrum of MRe2O6 (MQCo, Ni) contains three peaks (Fig. 4c
and a) which can be represented as a combination of Re+4 and
Re+6, although ‘‘Re+6’’-peaks are shifted to higher energies and
‘‘Re+4’’-peaks to lower energies. The relative area of the Re+4 peak
is higher for the low-temperature modifications of the Co- and
Ni-compound, while the area below the satellite peak of Re+6 is
higher for the tetragonal modifications. This means, taking charge
balance into account, the presence of higher concentrations of
Co3+ and Ni3+ in the low-temperature modifications than in
the tetragonal high-temperature forms1. The Re4f spectrum of
FeRe2O6 (Fig. 4e) is more complex and exhibits an additional peak
at 41.0 eV, which might indicate the presence of some metallic Re,
although not observed in XRD.
c
a
Fig. 2. Projection of monoclinic NiRe2O6 (C2/m) structure along [0–10]. Small
white spheres are O2ꢁ ions, large dark gray ones are Re ions (4g). In gray octahedra
Re and Ni ions (4i) are statistically distributed.
a
b
Re (4a)
b
a
Fig. 3. (a) Projection of the orthorhombic CoRe2O6 (F222) structure along [001].
Small white spheres are O2ꢁ ions, large dark gray ones are Re ions. gray octahedra
are statistically occupied by Re and Co ions. (b) [ReO4]n chains built up from ReO6
edge-shared octahedra in the low-temperature modifications of CoRe2O6 and
NiRe2O6. Re–Re bonds are represented by the thick black lines.
From the XPS valence spectra of MRe2O6 (MQFe, Co, Ni) one
can expect a metallic behavior (Fig. 4f, d, b) of these compounds.
The density of states at EF is a little higher for orthorhombic
CoRe2O6 than for tetragonal CoRe2O6 and significantly higher for
monoclinic NiRe2O6 compared with tetragonal NiRe2O6. It corre-
lates with the relative amount of Re ions in metallic Re–Re bonds:
in the orthorhombic modification of CoRe2O6 only 37% of all Re
atoms form metallic Re–Re bonds in contrast to 75% in the
monoclinic form of NiRe2O6. The shape of MRe2O6 (MQFe, Co, Ni)
valence peaks is quite different from that of ReO3 (Fig. 4f, d, b).
According to a simplified model of the ReO3 band structure
without covalence contributions and based on a formal separation
of the total electronic structure into an O2p and a Re5d band, the
peak with ꢀ2 eV-width just below the Fermi energy is interpreted
as the Re5d-derived conduction band, and the major part of the
p-band signal (between 3 and 10 eV) is also due to the Re-d
The average expansion coefficients along the ‘‘c’’ axes ac for the
tetragonal modifications of MRe2O6 are more than five times
higher than those along the ‘‘a’’ axes aa
, for example
a
a
c(NiRe2O6) ¼ 1.31 ꢂ10ꢁ51/K,
a
a
a(NiRe2O6) ¼ 2.53 ꢂ10ꢁ6 1/K, or
c(FeRe2O6) ¼ 1.68 ꢂ 10ꢁ51/K,
a(FeRe2O6) ¼ 6.44 ꢂ10ꢁ7 1/K. The
thermal expansion of the low-temperature forms calculated for
the same structural fragment as for the tetragonal ones is in the
same order:
3.05 ꢂ10ꢁ6 1/K.
a
‘‘c’’(CoRe2O6) ¼ 1.34 ꢂ10ꢁ51/K,
a‘‘a’’(CoRe2O6) ¼
3.3. XPS measurements
The Re4f7/2 and Re4f5/2 XPS spectra of the orthorhombic,
monoclinic and tetragonal modifications of MRe2O6 together with
ReO3 as a reference material are presented in Fig. 4. All spectra
were calibrated with respect to the C1s peak. XPS spectra of Fe, Co
and Ni are not shown, but have also been measured. It is known
that the positions of the main Co2p photoemission peaks in
1
Note that all experiments were performed at room temperature. ‘‘Low-
temperature modification’’ and ‘‘high-temperature form’’ refers to the relative
temperatures during syntheses.

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D. Mikhailova et al. / Journal of Solid State Chemistry 182 (2009) 364–373
6+
4f , Re
6+
Re 4f
ReO
7/2
CoRe O (P4 /mnm)
4f , Re
2
6
2
5/2
CoRe O (F222)
2
6
6+
0.8
0.4
0.0
4f , Re
7/2
3
CoRe O
2
6
(P4 /mnm)
2
CoRe O (F222)
2
6
ReO3
52
48
44
40
10
50
0 = EF
Binding energy [eV]
Binding energy [eV]
6+
NiRe O (P4 /mnm)
4f , Re
2
6
2
7/2
6+
Re 4f
4f , Re
5/2
NiRe O (C2/m)
2
6
0.8
0.4
0.0
4+
4f , Re
7/2
ReO
3
NiRe O
2
6
(P4 /mnm)
2
NiRe O
2
6
(C/2m)
ReO
3
52
48
44
40
10
50
0 = EF
Binding energy [eV]
Binding energy [eV]
6+
4f , Re
7/2
FeRe O (P4 /mnm)
Re 4f
2
6
6
6+
4+
4f , Re
4f , Re
5/2
7/2
0.8
0.4
0.0
ReO
3
0
4f , Re
FeRe O
7/2
2
6
(P4 /mnm)
2
ReO
3
55
50
45
40
10
5
0
Binding energy [eV]
Binding energy [eV]
Fig. 4. (a) Re4f7/2 and 4f5/2 core peaks of ReO3 and CoRe2O6 (orthorhombic and tetragonal modifications). (b) XPS valence spectra of orthorhombic and tetragonal CoRe2O6
and ReO3. (c) Re4f7/2 and 4f5/2 core peaks of ReO3 and NiRe2O6 (monoclinic and tetragonal modifications). (d) XPS valence spectra of monoclinic and tetragonal NiRe2O6 and
ReO3 as external standard. (e) Re4f7/2 and 4f5/2 core peaks of ReO3 and tetragonal FeRe2O6.
component [26]. In our XPS spectra the 3d band of the M element
must also be taken into consideration, which makes a more
specific interpretation of the spectra without theoretical band
structure calculations impossible.
Field dependences of magnetization for tetragonal CoRe2O6 and
NiRe2O6 are presented in Fig. 6. The coercivity field strength for
the Co-compound is 50 G at 5 K. The pronounced differences
between field-up and field-down branches at 5 K at higher field
strengths (up to 6 T) is probably due to first-order field-induced
transitions into the intermediate magnetic structure, which is
stable between 17.5 and 27 K in zero-field. At 21 K, this magnetic
structure exists already in zero-field, so that no such transition
can be induced and no anomaly is observed in M(H) at this
3.4. Magnetic properties
The temperature dependences of the inverse specific suscept-
ibility wꢁm1 of MRe2O6 (MQFe, Co and Ni) are shown in Fig. 5.
Tetragonal FeRe2O6 orders antiferromagnetically below T ¼ 123 K
in contrast to FeReO4 [5] with a higher TN ¼ 194 K, in agreement
with the higher concentration of Fe-ions in the latter compound.
Tetragonal and monoclinic modifications of NiRe2O6 order with a
ferromagnetic component at ꢀ24 K, whereas both modifications
of CoRe2O6 show two magnetic transitions at ꢀ17.5 and 27 K for
the tetragonal one and at 18 and 67 K for the orthorhombic phase.
temperature. Magnetic moments
meff(exp) per formula unit
MxRe1ꢁxO2 are calculated from the Curie constants in the
paramagnetic region, obtained from fitting Eq. (1) to the observed
data (Table 5).
C
MðTÞ ¼
þ M0
y
(1)
T ꢁ

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D. Mikhailova et al. / Journal of Solid State Chemistry 182 (2009) 364–373
371
Co Re O2 - P4 /mnm
CoRe O - F222
2 6
1/3
1/3
2
3x104
2x104
3x103
2x103
T
2
~ 17.5 K
T
~ 18 K
2
1x104
0
1x103
0
T
~ 27 K
1
ZFC
T
~ 67 K
1
FC
0
20
40
Temperature [K]
60
80
0
50
100
Temperature [K]
150
200
Ni Re
1/3
O - P4 /mnm
2 2
NiRe O - C2/m
2/3
2
6
1.2x105
8.0x104
9.0x104
6.0x104
4.0x104
0.0
3.0x104
0.0
T
~ 23.5 K
Tc ~ 24 K
100
c
0
50
100
150
200
250
300
350
0
50
150
200
250
300
350
Temperature [K]
Temeprature [K]
Fe Re
1/3
O
- P4 /mnm
2/3
2
2
1.4x105
1.2x105
1.0x105
8.0x104
6.0x104
FC
123 K
0
50
100
150
200
250
300
350
Temperature [K]
Fig. 5. (a) The temperature dependences of the inverse specific susceptibility wꢁm1 for the two modifications of CoRe2O6. (b) The temperature dependences of the inverse
specific susceptibility wꢁm1 for the two modifications of NiRe2O6. (c) The temperature dependence of the inverse specific susceptibility wꢁm1 of tetragonal FeRe2O6.
Note that paramagnetic moments of the low-temperature forms
of MRe2O6 are significantly higher than those for the tetragonal
forms, which can be connected either with the more itinerant
character of d electrons in the tetragonal forms or with a
partial charge transfer from M to Re and therefore the formation
calculated from the spin only contributions of the 3d-ions,
distinguished for the low-spin and high-spin states of M2+
and M3+
,
respectively, according to the equation
´
meff(cal) ¼
g[0.33S(S+1)]1/2, where g is the Lande splitting factor and S is
the total spin. This simple model is based on delocalized Re
electrons, supported by the XPS valence band measurements,
which do not give a significant contribution to the paramagnetic
of M-ions in
a
higher oxidation states. The theoretical
eff(cal), were
effective magnetic moments per M0.33Re0.67O2 unit,
m

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D. Mikhailova et al. / Journal of Solid State Chemistry 182 (2009) 364–373
Co Re
O
- P4 /mnm
5 K
Ni Re
1/3
O - P4 /mnm
2 2
1/3
2/3
2
2
2/3
6
4
3
2
21 K
2
1
0
0
5 K
-1
-2
-3
-2
-4
-6
5 K
-6
-4
-2
0
2
4
6
-6
-4
-2
0
2
4
6
B [H]
B [H]
Fig. 6. Field dependence of magnetization for tetragonal CoRe2O6 and NiRe2O6.
Table 5
Magnetic properties of MRe2O6.
Compound
Space group
Tc (K)
w0 ꢂ106 (Gg/emu)
Y
(K)
Temperature range for fit (K)
meff(exp) (mB)
CoRe2O6
F222
67, 18
27, 17.5
24
0
ꢁ18.9(2)
ꢁ17.4(5)
ꢁ18.5(4)
ꢁ4.1(7)
80–200
25–350
50–300
30–350
160–300
2.74(1)
2.06(1)
2.00(1)
1.59(2)
3.06(3)
Co1/3Re2/3O2
NiRe2O6
P42/mnm
C2/m
0
0
Ni1/3Re2/3O2
Fe1/3Re2/3O2
P42/mnm
P42/mnm
23.5
5.0(2)
9.1(3)
123 (TN)
ꢁ2.0(1.5)
Magnetic moments are normalized to one formula unit MxRe1ꢁxO2. The standard deviations in brackets are determined as the limits, for which an up to 10% higher residual
is obtained in the least-square fit than for the optimum fit for Eq. (1). w0 was calculated from M0 in Eq. (1) as w0 ¼ M0/(Mx H).
Table 6
4. Conclusion
Spin-only calculated magnetic moments of MxRe1ꢁxO2 (MQFe, Co,Ni) for low-spin
and high-spin states of M.
Different modifications with rutile-derivated crystal structures
of compounds with composition MRe2O6, MQFe, Co and Ni,
can be prepared, controlled by the temperature of synthesis.
A random distribution of M and Re on one site results for
higher temperatures. These compounds should be labelled as
(M1/3Re2/3)O2 to distinguish them from the compounds MRe2O6
with a partial order of M and Re on distinct sites, obtained at
lower temperatures. The symmetry of the crystal structures is
reduced by this partial order and metallic Re-Re bonds are formed,
accompanied by changes in the electronic configurations of the
transition metals.
Cation
Ground state
term
Calculated magnetic moment per
M0.33Re0.67O2 (mB)
Low-spin
High-spin
configuration
configuration
Fe2+, d6
Fe3+, d5
Co2+, d7
Co3+, d6
Ni2+, d8
Ni3+, d7
5D4
0.00
1.00
1.00
0.00
0.00
1.00
2.81
3.40
2.22
2.81
1.62
2.22
6S5/2
4F9/2
5D4
3F4
4F9/2
References
moments. The same approximation was previously applied to
similar systems, for example CrxMo1ꢁxO2 [26]. From this point of
view, the experimental paramagnetic moment of orthorhombic
¨
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CoRe2O6 (2.74
for high-spin Co3+ (2.81
(2.22 B) (Table 6). The measured paramagnetic moment of
monoclinic NiRe2O6 (2.00 B) is lower than the one calculated
for high-spin Ni3+ (2.22 B) and+, but higher than the one for Ni2+
(1.62
m
B) is in a good agreement with the calculated one
¨
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m
m
m
B), so that an intermediate electronic state of Ni between
Ni2+ and Ni3+ with electron delocalization might be indicated. The
lower magnetic moments in the tetragonal modifications of
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3672–3680.
CoRe2O6 (2.06 mB) and NiRe2O6 (1.59 mB) may be due to the
[10] G.M. Sheldrick, Acta Crystallogr. A 46 (1990) 467–473.
[11] G.M. Sheldrick, SHELXL97, Program for the Refinement of Crystal Structures,
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328–334.
presence of some amount of high-spin M2+ in the compounds,
which is indicated by the XPS measurements. The paramagnetic
moment of Fe in the tetragonal modification corresponds to the
oxidation state of Fe between +2 (high-spin) and +3 (high-spin).

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