Structural and Magnetic Studies of ABO4-Type Ruthenium and Osmium Oxides

Article abstract of DOI:10.1021/acs.inorgchem.9b03118

Oxides of the form ABO₄ with A = K, Rb, Cs and B = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For A = K the oxides adopted the tetragonal (I4₁/a) scheelite structure. RbOsO₄, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to I4₁/amd near 550 K. RbRuO₄ and CsOsO₄ were found to crystallize in the orthorhombic (Pnma) pseudoscheelite structure, and both displayed discontinuous phase transitions to I4₁/a at high temperatures. CsOsO₄ was determined to undergo a phase transition to a P2₁/c structure below 140 K. CsRuO₄ crystallizes with a baryte-type structure at room temperature. Upon heating CsRuO₄ a first order phase transition to the scheelite structure in I4₁/a is observed at 400 K. A continuous phase transition is observed to P2₁2₁2₁ below 140 K. DC magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except for CsOsO₄, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an S = 1/2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO₄ consisting of predominately antiferromagnetic (AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 μ_B was determined.

Full text of DOI:10.1021/acs.inorgchem.9b03118

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Article
Structural and Magnetic Studies of ABO4‑Type Ruthenium and
Osmium Oxides
Sean Injac, Alexander K. L. Yuen, Maxim Avdeev, Chun-Hai Wang, Peter Turner, Helen E. A. Brand,
and Brendan J. Kennedy*
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ABSTRACT: Oxides of the form ABO4 with A = K, Rb, Cs and B = Ru and Os have been
synthesized and characterized by diffraction and magnetic techniques. For A = K the oxides adopted
the tetragonal (I41/a) scheelite structure. RbOsO4, which crystallizes as a scheelite at room
temperature, underwent a continuous phase transition to I41/amd near 550 K. RbRuO4 and CsOsO4
were found to crystallize in the orthorhombic (Pnma) pseudoscheelite structure, and both displayed
discontinuous phase transitions to I41/a at high temperatures. CsOsO4 was determined to undergo a
phase transition to a P21/c structure below 140 K. CsRuO4 crystallizes with a baryte-type structure at
room temperature. Upon heating CsRuO4 a first order phase transition to the scheelite structure in
I41/a is observed at 400 K. A continuous phase transition is observed to P212121 below 140 K. DC
magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except
for CsOsO4, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an S = 1/
2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an
enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO4 consisting of predominately antiferromagnetic
(AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 μB was
determined.
was possible to stabilize an alternate orthorhombic structure
that contains the same BO4 topology by increasing the size of
the A-type cation.3−6 Since these perrhenates contain the 5d0
ReVII cation, they are diamagnetic. Similar, but not identical,
structures have been reported for the corresponding
diamagnetic pertechnetates that contain the 4d0 TcVII cation.7,8
Silverman described KRuO4 (RuVII 4d1) as having the
tetragonal scheelite structure but did not report its magnetic
properties.9 Marjerrison and co-workers, using powder neutron
diffraction, confirmed the structure of KRuO4 and, from bulk
magnetic susceptibility measurements, showed this has an
effective magnetic moment of 1.73 μB with a large Weiss
constant of −76 K indicative of antiferromagnetism.10 The
ordered magnetic structure was established to have k = (000)
with a small ordered moment for the RuVII of 0.57(7) μB.
Leevason11 reported that KOsO4 is isostructural with KRuO4
and stated that μeff at 298 K was 1.4 μB which is consistent with
a 5d1 configuration of OsVII.
INTRODUCTION
■
Transition metals are vital in many technologically important
materials. In particular, the complex interplay between the
spin, lattice, and orbital degrees of freedom can result in the
emergence of unique properties including superconductivity,
colossal magnetoresistance, and magnetism. Recent focus on
4d and 5d transition metal oxides has revealed a delicate
balance between the spin−orbit interactions and the crystalline
electric field that play a critical role in the generation of novel
magnetic states. The spatial extension of the d orbitals and the
spin−orbit coupling are both considerably larger in the 5d
system than in their 4d analogues.
The majority of magnetic 4d and 5d systems described to
date has the transition metal in an octahedral, or distorted
variant thereof, crystal field. Recently the possibility of using
tetrahedral crystal fields to generate new magnetic materials
incorporating 4d and 5d transition metals has emerged as an
exciting field of research.1 In general, tetrahedral coordination
results in a high spin ground state in oxides, although Zhou et
Shortly after the work of Marjerrison on KRuO4 was
published, Yamaura and Hiroi and Injac and co-workers
al. recently reported the formation of the low spin IrV e4t2 state
0
with tetrahedral coordination in Sr9Ir3O17.2
Received: October 28, 2019
The ABO4 tetragonal scheelite (CaWO4) structure contains
a large low valent A-type cation and a smaller high valent B-
type cation. The B-type cation is bonded to four oxygen atoms
in a distorted tetrahedra. The BO4 tetrahedra are isolated from
each other by AO8 polyhedra. Early structural studies of
AReO4 perrhenates by Rogner and Range established that it
https://dx.doi.org/10.1021/acs.inorgchem.9b03118
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
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Article
a
Table 1. Summary of Structures of the ABO4 Oxides (A = K, Rb, Cs; B = Ru, Os)
composition
KRuO4
RbRuO4
RbRuO4
CsRuO4
CsRuO4
CsRuO4
temperature (K)
space group
a (Å)
b (Å)
c (Å)
volume (Å3)
A x
A y
A z
A Biso (Å2)
Ru x
Ru y
Ru z
Ru Biso (Å2)
O1 x
O1 y
O1 z
O1 Biso (Å2)
O2 x
O2 y
O2 z
O2 Biso (Å2)
O3 x
O3 y
O3 z
O3 Biso (Å2)
O4 x
298
I41/a
298
Pnma
500
I41/a
90
P212121
298
Pnma
500
I41/a
5.60961(2)
5.60961(2)
12.96261(4)
407.903(2)
0
0.25
0.625
2.19(4)
0
0.25
5.50821(7)
5.75022(7)
13.7683(2)
436.090(9)
0.0267(6)
0.25
0.125
2.06(6)
0.9619(6)
0.25
0.9179(3)
2.14(6)
0.1639(9)
0.25
0.62225(2)
2.87(9)
0.6428(7)
0.25
0.9801(4)
3.91(12)
0.9806(8)
0.9945(6)
0.3128(2)
3.53(8)
5.71893(6)
5.71893(6)
13.93628(3)
455.803(2)
0
0.25
0.625
5.16(7)
0
0.25
7.18415(6)
7.85517(5)
8.19231(6)
462.3136(12)
0.2178(1)
0.4091(2)
0.3707(1)
0.809(9)
0.2562(1)
0.9132(2)
0.3770(2)
0.184(1)
0.0632(1)
0.0364(1)
0.3976(2)
2.71(9)
0.4392(1)
0.0417(1)
0.3427(2)
2.71(9)
0.2849(2)
0.7819(1)
0.5414(1)
2.71(9)
0.2052(1)
0.8087(1)
0.2076(3)
2.71(9)
1.734(1)
1.730(1)
1.728(1)
1.729(1)
8.40128(1)
7.17776(1)
8.04343(1)
485.037(9)
0.6297(1)
0.25
0.1468(1)
2.76(3)
0.8789(2)
0.75
0.1531(2)
1.058(3)
0.3813(9)
0.9417(4)
0.2215(4)
2.97(9)
0.4565(8)
0.25
0.5142(7)
5.00(24)
0.7945(7)
0.25
5.88079(3)
5.88079(3)
14.59233(7)
504.6573(8)
0
0.25
0.625
5.219(5)
0
0.25
0.125
0.125
0.125
0.72(2)
0.1073(3)
0.0198(3)
0.1976(2)
2.51(4)
2.94(6)
0.9303(8)
0.4936(5)
0.1701(2)
7.57(8)
3.224(1)
0.9742(8)
0.0257(6)
0.1845(5)
7.99(9)
0.5395(8)
5.14(22)
O4 y
O4 z
O4 Biso (Å2)
Ru−O1 (Å)
Ru−O2 (Å)
Ru−O3 (Å)
Ru−O4 (Å)
Ru−Ru (Å)
A BVS
1.708(2) × 4
1.744(6)
1.757(5)
1.676(3) × 2
1.579(8) × 4
1.706(3)
1.762(6)
1.720(6) × 2
1.587(1) × 4
4.286(1)
1.23
4.408(1), 4.505(3)
1.07
4.520(3)
4.327(1), 4.855(1)
4.481(1), 4.804(2)
0.62
4.679(1)
Ru BVS
χ2 SXRD, NPD
RBragg (SXRD, NPD)
RWP (SXRD, NPD)
composition
8.58
8.49
8.23
1.94, 2.75
3.06, 4.03
3.56, 4.98
KOsO4
1.21, 3.02
2.83, 2.73
3.35, 3.48
RbOsO4
1.63
6.31
7.29
2.66
4.59
6.27
3.26, 2.48
3.02, 3.33
3.86, 4.17
CsOsO4
1.86
4.80
5.88
RbOsO4
CsOsO4
CsOsO4
temperature (K)
space group
a (Å)
b (Å)
c (Å)
β (deg)
volume (Å3)
A x
A y
A z
A Biso (Å2)
Os x
Os y
Os z
Os Biso (Å2)
O1 x
298
I41/a
5.67586(1)
5.67586(1)
12.71914(1)
298
I41/a
5.81846(1)
5.81846(1)
13.34350(2)
573
I41/amd
5.80443(2)
5.80443(2)
13.8860(5)
90
P21/c
298
Pnma
5.75848(1)
5.93846(1)
14.35759(9)
470
I41/a
5.92808(9)
5.92808(9)
14.46877(3)
8.22278(11)
7.921734(7)
8.23025(11)
119.0845(6)
468.506(15)
0.2366(14)
0.1098(4)
0.9928(4)
0.89(12)
0.7581(2)
0.4173(4)
0.8709(3)
3.85(14)
0.6258
409.7531(17)
0
451.737(4)
0
0.25
0.625
2.26(14)
0
0.25
467.841(17)
0
490.984(4)
0.9744(9)
0.25
0.6249(5)
2.94(13)
0.0354(6)
0.25
0.1213(3)
2.76(9)
0.3162(27)
0.25
508.466(13)
0
0.25
0.625
1.85(5)
0
0.25
0.75
0.25
0.625
5.05(15)
0
0.25
0.125
7.64(10)
0
0.25
0.125
0.125
0.375
6.72(6)
0
0.4782*
0.6966*
17.36(62)
0.125
1.41(1)
0.1196(4)
0.0214(4)
0.1978(2)
2.59(4)
1.46(5)
0.9280(43)
0.4609(33)
0.2080(16)
3.48(9)
4.37(9)
0.0173(30)
0.019(9)
0.1877(4)
7.56(29)
O1 y
O1 z
O1 Biso (Å2)
0.9531
0.3158
1.00*
0.0833(9)
4.15(13)
B
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Table 1. continued
composition
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Article
KOsO4
RbOsO4
RbOsO4
CsOsO4
CsOsO4
CsOsO4
O2 x
O2 y
O2 z
0.8386
0.9688
0.7031
1.00*
0.9244
0.1877
0.4652
1.00*
0.6205
0.2511
0.5252
1.00*
0.3473(27)
0.25
0.4688(13)
8.60(24)
0.5064(21)
0.5128(13)
0.6869(5)
4.17(23)
O2 Biso (Å2)
O3 x
O3 y
O3 z
O3 Biso (Å2)
O4 x
O4 y
O4 z
O4 Biso (Å2)
Os−O1 (Å)
Os−O2 (Å)
Os−O3 (Å)
Os−O4 (Å)
Os−Os (Å)
A BVS
1.732(2) × 4
1.704(20) × 4
1.865 × 4
1.738(9)
1.689(10)
1.792(14)
1.779(11)
4.34(3), 4.890(4)
1.14
1.692(17)
1.680(2)
1.707(9) × 2
1.699(8) × 4
4.262(1)
1.28
4.426(1)
0.88
4.525(5)
1.79
4.689(7), 4.591(7))
0.59
4.6784(5)
1.9
Os BVS
7.2
7.8
4.91
6.89
7.31
7.9
χ2 SXRD, NPD
RBragg (SXRD, NPD)
2.96, 1.72
3.27, 2.50
3.79, 3.14
2.29, 2.68
9.74, 3.23
11.0, 4.18
1.14
4.41
4.58
4.53
4.98
5.72
6.64, 7.05
4.04, 4.49
6.25, 7.36
6.56, 4.84
6.10, 4.66
10.2, 6.33
RWP (SXRD, NPD)
a
The structures at room temperature (298 K) were refined against a combined synchrotron X-ray (S-XRD) and Neutron (NPD) data set. All other
structures were refined against S-XRD data alone. Values marked with an (*) were not allowed to vary during the refinement.
clear and colorless. The aqueous layer was decanted into an
evaporating basin and evaporated at 100 °C until yellow-green
crystals were recovered. In the case of CsRuO4, crystals precipitated
without reduction of the solvent volume.
independently reported the structure and magnetic suscepti-
bility of KOsO4.12,13 Both groups reported μeff of ∼1.4 μB and
θ = −44 K showing the material to be an antiferromagnet.
Injac revealed, using powder neutron diffraction, KOsO4 to
have the same AFM structure as KRuO4 with the ordered
moment of the OsVII being 0.5 μB. The experimental studies of
Injac verified the earlier results of Song et al.14 that had
prompted theoretical studies of Hayami and co-workers,15
predicting the emergence of odd parity multipoles associated
with spin and orbital ordering in KOsO4.
Given the paucity of oxides where a magnetic 4d and 5d
cation exists in a tetrahedral coordination, we have prepared
and characterized other members of the ABO4 scheelites where
B = Ru or Os. Neutron diffraction has been used to obtain
accurate and precise structures.
The AOsO4 (A = K, Rb, Cs) perosmates were synthesized by
reduction of OsO4 with the appropriate iodide salt of the A-type
cation. All manipulations were performed using standard Schlenk
techniques. A solution of 1 g of OsO4 dissolved in approximately 20
mL of dried acetone was transferred, via canula, to a solution
containing a 1.5 molar ratio of AI dissolved in a minimum amount of
dried acetone. This solution was stirred at room temperature until it
turned dark red. The solvent volume was reduced until a black
precipitate formed. This precipitate was separated, using a centrifuge,
and washed with cold acetone.
Physical Methods. Neutron powder diffraction (NPD) data were
measured at room temperature using the high-resolution powder
diffractometer Echidna at ANSTO’s OPAL reactor, using a wave-
length of 1.622 Å.17 Variable temperatures were achieved using a
cryofurnace. Synchrotron X-ray powder diffraction data were collected
over the angular range 5 < 2θ < 85°, using X-rays of wavelength
0.56154 Å, as determined by structural refinement of a diluted NIST
SRM660b LaB6 standard, on the powder diffractometer at beamline
BL-10 of the Australian Synchrotron.18 Temperatures were achieved
using a Cyberstar hot-air blower and were increased at a rate of 5 °C/
min data. Temperatures were calibrated using a combination of
thermocouple measurements and a range of melting-point and phase-
transition standards. The samples were housed in 0.2 mm diameter
quartz capillaries that were rotated during the measurements.
Structure refinements, using the Rietveld method, were undertaken
using the FullProf software suite with the WinPlotr interface.19,20 The
peak shapes were modeled using a Thompson-Cox-Hastings pseudo-
Voigt with axial divergence asymmetry profile function, and the
background was modeled using linear interpolation between points
with refinable intensities. The scale factor, detector zero-point, lattice
parameters, atomic coordinates, and atomic displacement parameters
were refined together with the peak profile parameters.
EXPERIMENTAL SECTION
■
Hazards. Caution! RuO4 is an extremely strong oxidizing agent,
which will spontaneously combust or explode in the presence of organic
solvents.16 Heating above 100 °C results in explosive decomposition. RuO4
readily sublimes at room temperature. No attempt was made to isolate
pure RuO4. OsO4 is an extremely toxic, corrosive, and potentially lethal
oxidizing agent. The compound is lethal by inhalation, skin contact, and
ingestion and will cause blindness if in contact with eyes. Appropriate PPE
including sealed goggles and Latex gloves were used for all manipulations
where RuO4 or OsO4 could be present and always in a fumehood.
Synthesis. The ARuO4 (A = K, Rb, Cs) perruthenates were
synthesized by reduction of a chloroform solution of RuO4 using an
aqueous solution of the appropriate AOH hydroxide. RuCl3·2H2O
was reacted with 4 mol equiv of NaIO4 in a mixture of 50 mL of
distilled CHCl3 (free from any ethanol or amylene stabilizer) and 100
mL of H2O in order to yield 1 g of RuO4. The mixture was stirred
vigorously overnight during which time the aqueous layer turned clear
and the chloroform layer became a golden color. The chloroform
layer was separated and decanted into a 100 mL flask, using a 0.1 M
aqueous solution of the appropriate hydroxide. This mixture was again
stirred vigorously overnight at which time the chloroform layer was
Single crystal diffraction data were measured using a Rigaku Oxford
Diffraction SuperNova Dual diffractometer equipped with an Atlas
CCD detector and employing mirror monochromated Mo−Kα or
C
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agreement with that reported by Marjerrison et al.10 for
KRuO4 at 280 K of 1.742(4) Å and is approximately equal to
the sum of the ionic radii of four-coordinate RuVII (0.38 Å) and
three-coordinate O2− (1.36 Å) reported by Shannon.21
Shannon does not list an ionic radius for four-coordinate
OsVII, but the present results suggest a value of about 0.38 Å
would be appropriate. We will return to this shortly. That the
ionic radius of the 5d OsVII cation is approximately equal to
that of the 4d RuVII cation is a consequence of the occupancy
of the intervening 4f levels, the lanthanide contraction. The
remaining three oxides, RbRuO4, CsOsO4, and CsRuO4, are all
orthorhombic in space group Pnma. However, they are not
isostructural. RbRuO4 and CsOsO4 have the pseudoscheelite
structure that has been described for the corresponding Re and
Tc oxides, CsReO4 and CsTcO4.7,8 The formation of the
pseudoscheelite structure is favored as the ratio of the ionic
radii rA/rB increases.3,5,6 The average Os−O distance in
CsOsO4 is 1.697 Å and is similar to that seen for KOsO4 and
RbOsO4. Likewise, the Ru−O distance of 1.713 Å in RbRuO4
is similar to that of KRuO4.
Cu−Kα radiation generated from a microsource used for data
collection. The structures were solved by direct methods. Further
details are provided in the SI.
Magnetic susceptibilities were measured using a Quantum Design
Physical Properties Measurement System (PPMS). Temperature-
dependent DC magnetic susceptibility measurements were taken
under zero field-cooled (ZFC) upon heating, and field-cooled (FC)
upon cooling conditions over the range 2−300 K, in an applied field
of 0.1 T. Isothermal field-dependent magnetization data were
collected to 9 T at constant temperatures. Heat capacity
measurements were undertaken on pressed pellets using the same
PPMS platform. Variable temperature heat capacity data were
analyzed by modeling the lattice contribution to heat capacity using
a fifth order polynomial fit, which was subtracted from the total heat
capacity about the magnetic anomaly. In the case of KRuO4 and
KOsO4, the lattice contribution to heat capacity was subtracted by the
heat capacity of the isostructural and diamagnetic KReO4. Values of
ΔSmag were effectively equivalent using either technique to model the
lattice contribution.
RESULTS AND DISCUSSION
■
1. Room Temperature Structures. The three oxides,
KOsO4, RbOsO4, and KRuO4, display a tetragonal scheelite-
type structure described in space group I41/a at room
temperature. In scheelite the A- and B-type cations are at the
4a (0 1/4 1/8) and 4b (0 1/4 1/8) special positions,
respectively. The results of the Rietveld refinements against the
neutron and synchrotron diffraction data are summarized in
Table 1, representative Rietveld plots are shown in Figure 1,
and others are available in the SI Figures S1−S3. The oxygen
occupies the 16f general position with x ∼ 1/8, y ∼ 0, and z ∼
1/4. In this structure there is a single B−O distance of
1.732(2), 1.704(20), and 1.708(2) Å for KOsO4, RbOsO4, and
KRuO4, respectively. The latter value is in reasonable
We have recently demonstrated for the analogous AReO4
oxides that in tetragonal scheelite-type structures the distortion
of the BO4 tetrahedra correlates with the size of the A-type
cation with the bond angles moving away from the ideal value
of 109° as the size of the A-cation increases.22 This trend is
followed for the pair of oxides KOsO4 and RbOsO4.
The unit cell parameters (a = 8.40128(1), b = 7.17776(1), c
= 8.04343(1) Å) of CsRuO4 are distinctly different than those
of scheelite a ∼ 5.9, c ∼ 14.6 Å. These parameters are similar to
that of baryte (BaSO4) although the b-axis is noticeably longer.
The structure is practically equivalent to that reported for
PbSO4 anglesite by Santamaria-Perez et al.23 Since the
synthesis of CsRuO4 produced small, diffraction-quality single
crystals, the structure was solved using single crystal methods.
In CsRuO4, as in baryte, the two cations are on a mirror plane;
however, the three oxygen atoms all occupy 8d general
positions. Like scheelite, the CsRuO4 structure contains
isolated RuO4 tetrahedra. The average Ru−O distance is
1.727 Å. The cesium cation is coordinated by a total of nine
oxygen atoms with distances between 3.036 and 3.294 Å and
has three longer Cs−O bonds, one at 3.449 and two at 3.746
Å. This arrangement is different than that of the Ba cation in
baryte which is 12-coordinate with six short Ba−O contacts.24
The relationship between the CsRuO4 structure and that of
scheelite was explored using the program AMPLIMODES, and
we could not identify any structural relationship.25 The
structures are illustrated in Figure 2.
The differences between the various structures can be
rationalized by considering the relative size of the cations; with
the exception of CsRuO4 the oxides have either the tetragonal
scheelite or orthorhombic pseudoscheelite structure. That
RbRuO4, but not RbOsO4, has the orthorhombic Pnma
pseudoscheelite structure implies that the effective ionic radius
of four-coordinate RuVII must be smaller than that of OsVII
since increasing rA/rB favors the orthorhombic structure.5
Structures obtained by analysis of neutron diffraction data are
available for the four potassium scheelites KBO4 B = Tc, Os,
Re, and Ru. Based on the unit cell volumes, 411.73(2),
409.753(2), 409.17(1), and 407.903(2) Å3, respectively, and
the B−O distances, 1.739(14), 1.732(2), 1.723(2), and
1.708(2) Å, it indeed appears that RuVII is the smallest of
these cations. The B−O distance is sensitive to the size of the
A-type cation. Except for B = Ru, the B−O distance decreases
Figure 1. Examples of the S-XRD and NPD Rietveld fits for RbRuO4
and RbOsO4, with data collected at room temperature. In this figure
the insets are the NPD profiles. Black circles represent the collected
data, the red line indicates the fit, and black vertical marks indicate the
locations of allowed reflections.
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Figure 2. Representations of the crystal structures of KRuO4 at room temperature, which adopts the scheelite structure in space group I41/a (a), 2
× 2 unit cells are shown along the c-axis (top) and a-axis (bottom). A representation of the baryte-like structure of CsRuO4 at room temperature in
space group Pnma is shown in (b), similar to (a), 2 × 2 unit cells are shown along the c-axis (top) and a-axis bottom. A representation of the low
temperature P21/c structure of CsOsO4 is shown in (c), 2 × 2 unit cells are shown projected along the b-axis (top) and c-axis (bottom). In all
representations BO4 tetrahedra are shown with omitted oxygen atoms at the vertices, and the A cation is indicated by blue spheres.
Figure 3. (a) Temperature dependence of the field cooled and zero field cooled magnetic susceptibilities (χ), (b) magnetization at 2 K, and (c)
heat capacity for RbRuO4 and RbOsO4. The insets in (a) for both compounds are plots of the inverse susceptibility with the solid lines illustrating
the fit of the high temperature data to the Curie−Weiss Law.
as the size of the A-type cation increases for the Tc, Os, and Re
scheelites with Rb and Cs counterions. Based on our current
work, we propose an effective ionic radius for four-coordinate
OsVII of 0.379 Å. This implies the effective bond valence
parameters for four-coordinate OsVII are R0 = 1.92 and B =
0.37. An apparent outlier in this discussion is CsRuO4 which
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Figure 4. (a) Temperature dependence of the field cooled and zero field cooled magnetic susceptibilities (χ), (b) magnetization at 2 K, and (c)
heat capacity for CsRuO4 and CsOsO4. The insets in (a) for both oxides are plots of the inverse susceptibility with the solid lines illustrating the fit
of the high temperature data to the Curie−Weiss Law. The inset in (c) for CsOsO4 highlights the minimum at low temperatures.
a
Table 2. Summary of the Magnetic Properties of the ABO4 Oxides
composition
TN (K)
KRuO4
RbRuO4
CsRuO4
KOsO4
RbOsO4
CsOsO4
21
4
35
20
Tχmax (K)
37
12
9
61
32
μeff (μB)
θ (K)
ΔSmag (5.76 J K−1 mol−1
1.42
−43.8
1.35
1.29
−15.2
1.67
−6.9
1.64
1.24
−76.2
1.39
1.29
−35.8
1.38
1.4
−1.1
)
a
TN refers to the magnetic ordering temperature as determined from heat capacity data, and Tχmax refers to the temperature corresponding to the
maximum in magnetic susceptibility curves.
displays a baryte-type structure. The ABO4 family of
compounds encompasses a large number of structural types
that contain tetrahedral BO4 groups including scheelite, zircon,
baryte, and fergusonite. A number of phenomenological rules
for predicting structures have been employed to develop phase
diagrams based on the size of the ions.26−28 These suggest that
the baryte structure will be favored over the scheelite structure
as the size of the B-site cation is reduced, in keeping with our
experimental studies.
moment. The small Jahn−Teller-type distortion of the BO4
tetrahedra evident in the structural analysis, that will remove
the degeneracy of the e-type states, is neglected here. That the
observed moments of the three perruthenates are larger than
the corresponding perosmates reflects the smaller spin−orbit
coupling of 4d electrons compared to 5d electrons.
There is relatively good correlation between the Weiss
́
constants (θ) and the Neel temperature (TN) estimated from
the heat capacity measurements, and this demonstrates the
absence of magnetic frustration.29 In all cases the frustration
index f = |θ|/TN is between 1 and 2, which is consistent with a
2. Magnetic Susceptibility. Temperature dependent
magnetic susceptibility and heat capacity measured between
300 and 2 K and isothermal magnetization measurements at 2
K are shown for the Rb and Cs compounds in Figures 3 and 4.
The corresponding plots for the potassium salts are in the SI,
Figure S4. For all samples except CsOsO4, which remains
paramagnetic to 2 K, a broad maximum and divergence of the
ZFC and FC curves are apparent at low temperatures. Curie−
Weiss fits, using data between 150 and 300 K, yielded effective
magnetic moments of around 1.4 μB, see Table 2, which are
reduced from the spin-only value of a simple d1 system, 1.73
μB. In a tetrahedral field the d1 ion will have an e1t2 electron
́
lack of frustration. In general, the Neel temperatures estimated
by plotting the Fisher heat capacity, d(χT)/dT vs T, are in
good agreement with the values estimated from the molar heat
capacity plots.30 For CsRuO4 the Fisher heat capacity suggests
́
the Neel temperatures to be near 6 K. That TN is larger in
RbRuO4 than Tχmax is puzzling; however, the variable
temperature neutron diffraction data unequivocally demon-
strates the presence of long-range antiferromagnetic ordering
at low temperatures in RbRuO4.
A feature in the heat capacity for CsOsO4 is observed at
approximately 150 K, and an anomaly is observed in the
inverse susceptibility plots at about the same temperature, see
Figure 4. The feature in the heat capacity was independent of
2
configuration which gives a E ligand field ground term in
which the orbital contribution is expected to be quenched.
Second order spin−orbit coupling will, however, reduce the
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Figure 5. Results of the Rietveld refinement of neutron powder diffraction data for RbRuO4 at 3 K. The black circles are the data, and the red line
and blues lines are the calculated and difference profiles, respectively. The upper set of vertical tick marks is the allowed nuclear Bragg reflections,
and the lower set is the allowed magnetic reflections. The k = (000) magnetic structure is also illustrated.
the applied magnetic field up to 5 T. This lack of magnetic
field dependence indicates that it is due to a change in lattice
enthalpy, that can be attributed to a crystallographic phase
transition. The anomaly in the magnetic susceptibility is
consistent with a slight change in the density of states of the Os
cation, which accompanies a structural transition. Yamaura and
Hiroi also observed a change in the heat capacity near 150 K
and concluded that this was a result of a transition from the
pseudoscheelite structure in space group Pnma to a monoclinic
fergusonite-type structure in space group I2/a.13 These
workers, however, noted that they were unable to refine a
structure in this space group. An alternate model in P21/c is
proposed below.
orthorhombic in Pnma. For both KRuO4 and KOsO4 a single
very weak reflection that could be indexed as (110) appeared
́
below the Neel temperature consistent with a magnetic
structure with k = (000). As described in our earlier discussion
of the magnetic structure of the KOsO4 group theory analysis,
performed with the ISODISTORT software suite, four possible
irreducible representations namely Γ1, Γ3, Γ5, and Γ7 were
indicated.31,32 Of these only Γ5 allows for intensity in the
(110) but not the (002) reflection showing the magnetic
structure to be antiferromagnetic with the moments parallel to
the c-axis. For orthorhombic RbRuO4 eight possible irreps
were identified. Of these, Γ2+, Γ3+, and Γ4+ show ferromagnetic
order and are therefore inconsistent with the magnetization
data. The remaining representations show AFM ordering. For
irreps Γ1+, Γ2−, and Γ3− the moments are required to lie
parallel to the b-axis. Irreps Γ1− and Γ4+ show AFM ordering
along the c-axis, with canting of moments within the 100 plane.
All AFM irreps resulted in a similar quality of Rietveld fits to
the NPD data, indicating that better data quality is required in
order to unequivocally determine the magnetic structure of
RbRuO4. Nevertheless, a magnetic cell is tentatively proposed
using Γ1−, as this magnetic structure is effectively equal to that
determined for KRuO4 and KOsO4, with a small AFM canting
of magnetic moments within the 100 plane. A refinement
profile using this model is shown in Figure 5. The magnetic
moments were refined to be 0.77(24) μB along the c direction
In all cases superexchange, which would involve the AO8
polyhedra and entail a long B-O-A-O-B pathway, is expected to
be very weak, rather direct exchange should be dominant. As
suggested by Marjerrison et al.,10 nearest neighbor interactions
are expected to be the strongest, and this suggestion is
apparently verified by noting how the increase in B−B
́
separation correlates with the decrease in the Neel temperature
in both the Ru and Os scheelites. CsOsO4 is an outlier in this
series reflecting the different configurations of the OsO4
tetrahedra.
3. Magnetic Structures. Powder neutron diffraction data
for the three oxides KRuO4, KOsO4, and RbRuO4 measured at
low temperatures showed that KRuO4 and KOsO4 retained the
tetragonal I41/a structure and that RbRuO4 remained
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Figure 6. Temperature dependence of the lattice parameters for RbRuO4 and RbOsO4 determined from Rietveld refinements using S-XRD.
RbRuO4 exhibits a first order Pnma-I41/a transition near 475 K, and RbOsO4 shows a continuous I41/a to I41/amd transition near 550 K.
and 0.21(9) μB along the a direction. A representation of this
magnetic structure is also shown in Figure 5.
The NPD profile of RbOsO4 measured at 3 K using the
Echidna diffractometer did not reveal any intensity at the
(110) reflection; however, we note that the very weak moment
of OsVII 5d1 means that this reflection is expected to be
extremely weak, and it appears that it is below the sensitivity of
Echidna.17 Likewise, it was not possible to observe a (110)
reflection in the Echidna data for KOsO4; this reflection, was,
however, observed using the ToF diffractometer WISH.12 This
implies that the maximum ordered Os moment of RbOsO4 is
∼0.4 μB. Unfortunately, the CsOsO4 sample showed evidence
for decomposition, and we were unable to measure a
satisfactory low temperature ND pattern for this. Studies of
CsRuO4 are ongoing.
4. Variable Temperature Structures. The structures of
the six oxides were investigated using S-XRD from 90 K to
high temperatures. KOsO4 was found to remain tetragonal in
space group I41/a between 90 and 720 K, and there was no
evidence for any thermally induced phase transitions. Scrutiny
of Table 1 reveals that the atomic displacement parameters
(ADPs) for the anions are invariably larger than that for the
cations and that the ADPs are larger for the larger A-type
cation than the smaller B-type cation. The anomalously large
values for the oxygen in the refined structure of RbOsO4 are
believed to be a consequence of the proximity to the
decomposition of the material. Despite the higher than typical
values for the A-site cations, there is no reason to suspect that
the atoms are displaced from the corresponding Wykoff sites.
Both RbRuO4 and RbOsO4 were found to undergo a
structural phase transition upon heating. In the case of
RbRuO4, this was a Pnma to I41/a transition near 475 K
similar to that seen for CsTcO4,7 and the temperature
dependence of the lattice parameters is illustrated in Figure
6. For RbOsO4 a noticeable change in the anisotropic thermal
expansion is seen at approximately 553 K, see Figure 6. The
temperature dependence of the c parameter is very similar to
what we described recently for RbTcO4 and RbReO4 where it
was established that the change was due to a softening at the Γ
point resulting in a I41/amd to I41/a phase transition.7,22
Figure 7. Representations of the ABO4 structures identified in this
These two structures differ in their orientation of the BO4
work. In all cases the smaller B-type cation is at the center of the
tetrahedra; in I41/a the tetrahedra are rotated about the c-axis,
tetrahedra, and the larger A-type cation is represented by the spheres.
see Figure 7, and the x parameter for the oxygen in the general
The oxygen atoms are at the corners of the tetrahedra. The solid lines
position 16f is a variable, whereas this is constrained to x = 0
for the equivalent 16h position in l41/amd. The cations are on
special positions in both structures. The I41/a to I41/amd
transition is allowed, by group theory, to be continuous, and
the diffraction data is consistent with this. In the
represent observed phase transitions that are allowed to be
continuous, and the dashed lines observed transitions that must be
first order.
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Figure 8. Portions of the S-XRD profiles and the temperature dependence of the lattice parameters for CsRuO4 and CsOsO4 determined from
Rietveld refinements. CsRuO4 exhibits a first order Pnma-I41/a transition near 450 K and a continuous Pnma-P212121 transition near 260 K, and
CsOsO4 shows two first order transitions from the Pnma structure to I41/a near 400 K and to P21/c near 110 K.
pseudoscheelite Pnma structure observed for RbRuO4 the BO4
tetrahedra are rotated in the ab plane, and this rotation is
associated with softening at the M-point of the Brillouin zone.
Group theory does not allow a continuous transition between
these Pnma and I41/a structures. It is possible that RbRuO4
may realize the same I41/amd structure as RbOsO4 at higher
temperatures; however, this could not be experimentally
verified with the sample decomposing above 600 K. A number
of ABO4-type oxides undergo a pressure induced I41/amd to
I41/a transition. However, as illustrated by the example of
YCrO4, invariably in these the low pressure phase has the
l41/amd zircon structure with a ∼ 7.089 and c ∼ 6.281 Å, while
the high pressure phase has the I41/a scheelite structure with a
∼ 5.0004 and c ∼ 11.259 Å.33 In YCrO4 the transition occurs
near 3 GPa, and it is possible to recover the high pressure
scheelite structure by quenching.34 Thus, although increasing
pressure can mimic the effect of decreasing temperature
suggesting the I41/a scheelite structure will be favored at high
pressure or low temperatures, the reconstructive nature of the
zircon to scheelite transition is clearly different than the
continuous transition observed here for RbOsO4.
A feature of Figure 6 is the weak negative thermal expansion
(NTE) of the a-axis for RbOsO4. A number of oxides that have
been shown to display NTE including ZrW2O8,35 ZrV2O7,36
and Ag2O,37 and this is generally associated with low energy
transverse motions of atoms that link together essentially rigid
units.38 The scheelite structure lacks obvious linear links, but
there is evidence that transverse motions of more flexible
linkers can facilitate NTE; and we postulate that the RbO8
units that link the otherwise isolated BO4 tetrahedra fulfill this
role.39
single phase tetragonal scheelite structure is observed.
Refinement profiles of the SXRD data at 410 K are shown in
Figure S7, and both the Pnma and I41/a structures are
observed at this temperature. This behavior is very similar to
that reported recently for CsTcO4 and CsReO4.7,22 CsOsO4
rapidly decomposed above 600 K. The decomposition of this,
and of the other Ru and Os oxides studied here, at high
temperatures contrasts that seen for the ATcO4 and AReO4
compounds, where the room temperature structures can be
recovered from the high temperature melts upon cooling.7
As anticipated from the heat capacity measurements,
CsOsO4 undergoes a second phase transition upon cooling,
see Figure 8. We initially tested the I2/a fergusonite-type
model, suggested by Yamaura and Hiroi;13 however, it was
immediately realized that this was wrong. The S-XRD profile
measured at 90 K could be indexed to a primitive monoclinic
structure with a = 8.2228, b = 7.921734, c = 8.2303 Å, and β =
119.085(6)°, and this was identified as being in space group
P21/c. The structure was then solved using charge flipping and
refined. The lattice parameters are inconsistent with a scheelite
related structure but are similar to the parameters of the
orthorhombic baryte-type CsRuO4 structure (a = 8.40128(1),
b = 7.17776(1), c = 8.04343(1) Å). Scrutiny of the diffraction
data showed evidence of the coexistence of the high
temperature Pnma and low temperature P21/c phases between
140 and 120 K, indicating that the transition between these
two structures is first order. A refinement profile of S-XRD
data collected at 90 K for CsOsO4 is shown in Figure S6. An
increase in heat capacity is also observed at approximately 3 K,
shown in the inset of Figure 4c. Although there is insufficient
data in this temperature region to characterize this feature, it
may indicate long-range magnetic order, and future character-
ization is required.
Like RbRuO4, CsOsO4, which displays the distorted
pseudoscheelite Pnma structure at room temperature, under-
goes a first order phase transition to the undistorted I41/a
scheelite structure upon heating. The transition occurs at a
slightly lower temperature than in RbRuO4 with the
orthorhombic and tetragonal phases coexisting over a limited
temperature range ∼410−420 K, Figure 8. Above 420 K a
The situation for CsRuO4 is more complex. As stated above,
at room temperature CsRuO4 is found in the orthorhombic
baryte-type structure. Heating results in a first order transition
to the tetragonal scheelite structure near 420 K. Cooling below
270 K results in the appearance of additional reflections in S-
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XRD data, and the cell could be indexed to a second
orthorhombic space group P212121. Representative Rietveld
profiles are shown in Figure S6. This structure consists of
chains of edge sharing CsO10 and RuO4 polyhedra. This low
temperature structure was confirmed by single crystal XRD
data collected at 90 K. BaSO4 itself has been reported to
undergo a Pnma → P212121 transition at high pressures around
27 GPa.40 The same transition has been observed at high
pressure in PbSO4.41 The high pressure or postbaryte phase is
described in space group P212121 (No. 19) which is a subgroup
of the Pnma (No. 62) space group of the initial baryte-type
structure. Whereas this transition has been described as first
order for both BaSO4 and PbSO4, in the present case the
transition is clearly continuous.40,41 The two structures differ in
the orientation of the RuO4 tetrahedra (see Figure 2), and the
transformation between them can be described by Γ1−. The
high magnetic entropy determined from heat capacity for this
compound suggests strong magnetic interactions for this
structure.
As described above, the three scheelite related structures in
space groups I41/amd, I41/a, and Pnma differ in their
orientation of the BO4 tetrahedra. The diffraction data are
consistent with the conclusions from group theory that the
transition between the two tetragonal structures, I41/amd and
I41/a, as seen in the two Rb containing oxides is continuous,
whereas the structure between the tetragonal I41/a and
orthorhombic Pnma structures seen in CsOsO4 is first order.
The formation of a second structural type that we describe as a
distorted baryte-type is surprising but is consistent with
Bastide’s analysis of ABO4-type structures.26 Despite both
CsRuO4 and CsOsO4 being found in the same space group,
there is no relationship between the structures, and the
transformation between the distorted baryte and scheelite as
observed in CsRuO4 is first order.
analogous to the high pressure behavior displayed by other
ABO4 oxides including PbSO4.41 Interestingly, heating CsRuO4
induces a reconstructive first order transition to the tetragonal
scheelite structure observed in the other Ru and Os oxides
described here. Errandonea and co-workers have reported
pressure dependent structural studies for a number of ABO4-
type oxides but no examples with Ru or Os, and a reoccurring
feature of this work is the susceptibility of these materials to
undergo reconstructive phase transitions.33 While resulting in
considerable structural diversity, the preponderance of first
order phase transitions in the ABO4 oxides makes elucidation
of the systematics controlling the formation of particular
structural motifs challenging and suggests the diagram of
Bastide needs revision.26
The oxides display long-range antiferromagnetic ordering of
́
the S = 1/2 spins at low temperatures with the Neel
temperature of the Os oxides generally being higher, reflecting
the more diffuse nature of the 5d vs 4d orbitals. Direct, rather
than superexchange, exchange appears to be dominant with an
increase in the metal−metal separation resulting in a decrease
́
in the Neel temperature in both the Ru and Os scheelites.
CsOsO4 is an outlier in this series reflecting the different
configurations of the OsO4 tetrahedra. The high magnetic
entropy determined from heat capacity for CsRuO4 suggests
strong magnetic interactions for this structure. Low temper-
ature neutron diffraction measurements would prove valuable
in determination of the magnetic structure of this compound.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03118.
■
sı
Rietveld profiles of the S-XRD and NPD fits for KOsO4,
KRuO4 and CsOsO4 at room temperature, representa-
tions of refined structures, magnetic susceptibility and
heat capacity curves for KRuO4 and KOsO4, variable
temperature S-XRD profiles of CsRuO4 and CsOsO4,
and single crystal X-ray structural details (PDF)
CONCLUDING REMARKS
■
Crystal structures of six ABO4 oxides (A = K, Rb, or Cs; B =
Ru or Os) at room temperature have been determined using a
combination of powder neutron and synchrotron X-ray
diffraction, and their magnetic properties were established.
Except for CsRuO4, all the samples exhibit either a tetragonal
scheelite or orthorhombic pseudoscheelite structure at room
temperature, with the symmetry lowering resulting from a
decrease in the ratio of the ionic radii rA/rB. Comparing the
structures of KRuO4 and KOsO4 provides the first direct
experimental evidence for the small increase in the ionic radius
of four-coordinate OsVII over that of RuVII. RbRuO4 undergoes
a first order Pnma-I41/a transition upon heating as is also
observed in CsOsO4. Group theory does not allow a
continuous Pnma-I41/a transition for these oxides reflecting
the different orientation of the BO4 tetrahedra. RbOsO4
displays anomalous thermal expansion behavior associated
with a continuous I41/a to I41/amd transition as seen recently
in both RbTcO4 and RbReO4.12,22 While the high temperature
phase transition behavior seen for CsOsO4 is similar to that
observed in the isostructural Tc and Re oxides, the first order
transition observed upon cooling has no analogy in these
oxides, rather it involves a reconstructive transition to a new
P21/c polymorph rather than the I2/a fergusonite structure
observed in oxides such as LaNbO4.42 At room temperature
CsRuO4 has a structure that is more closely related to baryte
(BaSO4) than scheelite. This undergoes a continuous
transition to a postbaryte-type structure upon cooling,
Accession Codes
CCDC 1964054−1964062 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Brendan J. Kennedy − School of Chemistry, The University of
Sydney, Sydney 2006, Australia; orcid.org/0000-0002-
7187-4579; Email: Brendan.Kennedy@Sydney.edu.au
Authors
Sean Injac − School of Chemistry, The University of Sydney,
Sydney 2006, Australia
Alexander K. L. Yuen − School of Chemistry, The University of
Sydney, Sydney 2006, Australia
Maxim Avdeev − School of Chemistry, The University of Sydney,
Sydney 2006, Australia; Australian Centre for Neutron
Scattering, Australian Nuclear Science and Technology
Organisation, ANSTO, , Lucas Heights 2234, Australia;
orcid.org/0000-0003-2366-5809
J
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Chun-Hai Wang − School of Chemistry, The University of
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Peter Turner − School of Chemistry, The University of Sydney,
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Helen E. A. Brand − Australian Synchrotron, Australian
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
B.J.K. acknowledges the support of the Australian Research
Council for this work that was, in part, performed at the
powder diffraction beamline at the Australian Synchrotron and
at the Australian Centre for Neutron Scattering. This research
was facilitated by access to Sydney Analytical, a core research
facility at the University of Sydney
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