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Structure and magnetism of
the A site scandium perovskite
(Sc0.94Mn0.06)Mn0.65Ni0.35O3
synthesized at high pressure
rsta.royalsocietypublishing.org
Chris I. Thomas, Matthew R. Suchomel,
Giap V. Duong, Andrew M. Fogg, John B. Claridge
Research
Cite this article: Thomas CI, Suchomel MR,
Duong GV, Fogg AM, Claridge JB, Rosseinsky
MJ. 2014 Structure and magnetism of the
A site scandium perovskite
(Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 synthesized at
high pressure. Phil. Trans. R. Soc. A 372:
20130012.
http://dx.doi.org/10.1098/rsta.2013.0012
One contribution of 8 to a Theo Murphy
Meeting Issue ‘Theo Murphy International
Scientific Meeting between the UK and China
on the chemistry and physics of functional
materials’.
Subject Areas:
inorganic chemistry
Keywords:
ceramics, high-pressure phase, magnetic
properties and crystal structure
Author for correspondence:
Matthew J. Rosseinsky
e-mail: [email protected]
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rsta.2013.0012 or
via http://rsta.royalsocietypublishing.org.
and Matthew J. Rosseinsky
Department of Chemistry, University of Liverpool, Crown Street,
Liverpool L69 7ZD, UK
Scandium perovskite (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 ,
synthesized at high pressure and high temperature,
has a triclinic structure (space group P1̄) at room
√
temperature and ambient pressure with a 2ap ×
√
2ap × 2ap structure with α ≈ 90◦ , β ≈ 89◦ , γ ≈ 90◦ .
Magnetic measurements show that the material
displays Curie–Weiss behaviour above 50 K with C =
2.11 emu K mol−1 (μeff = 4.11 µB per formula unit)
and θ = −95.27 K. Bond valence sum analysis of the
crystal structure shows that manganese is present in
three different oxidation states (+2, +3, +4), with
the +2 oxidation state on the A site resulting in a
highly tilted perovskite structure (average tilt 21.2◦
compared with 15.7◦ calculated for LaCaMnNbO6 ),
2+
4+
3+
giving the formula (Sc3+
0.94 Mn0.06 )(Mn0.41 Mn0.09 )
2+
(Mn3+
0.15 Ni0.35 )O3 .
1. Introduction
Multiferroic materials have been receiving an increased
amount of attention recently owing to the possibility of
creating higher-density memory through electrical write
and magnetic read [1]. Some classes of magnetic order
remove the centre of symmetry in the crystal structure
and produce coupling between the magnetization and
polarization, e.g. TbMnO3 [2] and DyMnO3 [3]. A second
approach is to impose polarization chemically on a
structure that displays a permanent magnetization.
This second approach can involve combining a
stereochemically active lone-pair element such as Bi3+
2014 The Author(s) Published by the Royal Society. All rights reserved.
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(a) Synthesis
Polycrystalline Sc2 MnNiO6 and (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 were synthesized under high
pressure and temperature using a Rockland Research 600 tonne press with the Walker octahedral
multianvil design [15,16]. Stoichiometric amounts of Sc2 O3 (Alfa Aesar, 99.998%), MnO2 (Alfa
Aesar, 99.997%) and NiO (Alfa Aesar, 99.998%) were intimately mixed by hand in a pestle and
mortar before being placed in a platinum-lined Al2 O3 crucible inside a cylindrical graphite
resistance furnace. The graphite furnace is then loaded into the centre of a cast octahedron
(Aremco Ceramacast 584-OS). The octahedron is then loaded into the centre of eight truncated
tungsten carbide cubes, with the truncations pressing into the faces of the octahedron, forming
a cube. This cube is then placed between the six anvils in the press and placed under a
hydraulic pressure of 7000 kpsi, which corresponds to a pressure for the sample of 6–7 GPa.
Power is then fed to the graphite furnace, giving a sample temperature of 1200◦ C, and held
for 30 min. The system is then quenched to room temperature by turning the furnace power
off. The use of lower temperatures was attempted, but this resulted in a greater amount of
impurity phase being formed. Overall, it was found that reproducing synthesis runs required
very tight control of both the temperature and the duration of the heating to limit the
amount of secondary phases. The material comes out of the press as a fragile pellet that
is made up of small (less than 0.5 mm) black shiny crystals that can be broken away from
the main pellet. All samples contained a small amount of an Mn-doped Sc2 O3 phase (less
than 0.01 wt%).
.........................................................
2. Experimental
2
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
or Pb2+ on the A site of a perovskite structure with magnetically ordering transition metals on
the B site, e.g. Bi2 MnNiO6 [4]. Here, the polar rhombohedral derivatives of perovskite structures
based on Bi3+ have been popular platforms for combining magnetism and polarity; in particular,
BiFeO3 -based derivatives have been the source of much work [5]. Other multiferroic efforts have
focused on exploiting large differences in cation size to remove the centre of symmetry. For
example, the coexistence of ferromagnetism and ferroelectricity has been demonstrated in the
high-pressure phase of FeTiO3 , isostructural with LiNbO3 . This same approach has generated
multiferroic interest in ScFeO3 , which can be synthesized under high pressure, adopts a polar
derivative of the corundum structure and has permanent magnetization above room temperature
[6]. The small size of both Fe2+ and Sc3+ drives the distortion from the perovskite to the closepacked, oxide-based corundum and LiNbO3 structures. A site displacement due to mismatched
cation sizes has been proposed as a mechanism for obtaining polar behaviour in ferromagnetic
Ni−Mn double perovskites, such as (La,Lu)2 MnNiO6 [7], and in Y2 NiMnO6 [8], with Y3+ being
used as the small A site ion that drives the polarization-producing distortion. Sc3+ is smaller than
both Lu3+ and Y3+ , so may induce a greater distortion in the perovskite structure if this can be
retained despite the competition with corundum. Park & Parise [9] also noted that the smaller the
M ion, the greater the tilting of the octahedra in the series MCrO3 (M = La–Lu, In, Sc), reaching a
maximum at In.
Few A site scandium perovskites are known, with ScAlO3 [10], ScCrO3 [9] and ScRhO3 [11] all
requiring high-pressure synthesis. Attempts by several groups to synthesize perovskite ScMnO3
at 5 GPa proved unsuccessful, producing instead hexagonal ScMnO3 [12]. However, recently
Chen et al. [13] used 12.5 GPa to produce perovskite ScMnO3 . This paper reports the outcome
of synthesis addressing the target composition Sc2 MnNiO6 , which was selected on the basis of
Sc3+ providing the distortion for ferroelectricity, with B site order of Mn4+ and Ni2+ and the
associated half-filled–empty orbital superexchange providing the ferromagnetism [14]. Attempts
to synthesize Sc2 MnNiO6 were unsuccessful, leading to the perovskite-based material with the
distinct composition described here.
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(b) Characterization
Magnetic characterization was carried out using a Quantum Design MPMS magnetometer, which
was controlled using the MPMS software.
(d) Energy-dispersive X-ray analysis
The energy-dispersive X-ray spectroscopy (EDS) was carried out using a JEOL 2000FX
transmission electron microscope operated at 200 kV.
3. Results
Initial ambient-pressure synthesis attempts at the Sc2 MnNiO6 target composition afforded
hexagonal ScMnO3 , NiMn2 O4 and unreacted oxides. High-pressure synthesis of the target
Sc2 MnNiO6 afforded a multiphase mixture of two Sc−Mn−Ni−O ternary oxides together with
Mn2 O3 and Mn3 O4 related phases. EDS analysis showed that the compositions of the crystallites
clustered into two groups. One of these groups has the Mn : Ni elemental ratio of 2/3 : 1/3,
whereas the other group, with fewer crystallites observed, has the inverse ratio (see electronic
supplementary material, figure S1).
Both groups of crystallites appeared to be slightly scandium-deficient, with an Sc : (Mn/Ni)
ratio of 0.88 : 1, leading to the ratio Sc : Mn : Ni of 0.88 : 0.66 : 0.33. An ABO3 perovskite with
transition metals on the B site alone would have the composition Sc0.88 Mn0.66 Ni0.33 O3−d , i.e. there
would be A site vacancies, whereas full occupancy of both A and B sublattices would correspond
to Sc0.94 Mn0.71 Ni0.35 O3 , with 0.06 of manganese on the A site. Manganese is known to be present
on the A site of the high-pressure perovskites MnMO3 , where M = Ge [17], Sn [18], Si [19], Ti [20]
or V [21], and in the B site ordered (In1−y Mny )MnO3 [22], where the B site ordered structure
is realized through ordering of Mn3+ and Mn4+ . The mixed valence Mn2+ A site perovskite
polymorph of Mn2 O3 has recently been isolated by high-pressure synthesis [23]. It appears that a
high-pressure synthesis route is desirable if Mn2+ is to go on the A site of ABX3 -type perovskites.
High-pressure synthesis was subsequently carried out at the new Sc0.94 Mn0.71 Ni0.35 O3 target
composition (reaction conditions specified in the Experimental section) and yielded a material
that appeared single-phase in the electron microscope. The EDS average composition was
Sc0.94 Mn0.68 Ni0.36 O3−d (figure 1).
Laboratory X-ray powder diffraction was consistent with a single-phase material based on a
perovskite superstructure unit cell consistent with the presence of two distinct B sites. The initial
.........................................................
(c) Magnetic measurements
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
The phase purity was checked by powder X-ray diffraction (XRD) data with a Panalytical
X’pert Pro Multi-Purpose X-ray diffractometer (Co Kα1 radiation, λ = 1.78901 Å) collected at
ambient temperature. A sample from a single high-pressure run was loaded in a 0.5 mm diameter
amorphous fused silica capillary (Markröhrchen Quarzkapillaren) and measured by synchrotron
powder diffraction (SPD) at station I11 of the Diamond Light Source, Harwell. Data were
collected stepwise over 1.821–141.953◦ 2θ (λ = 0.949831 Å). Another sample from a separate single
high-pressure run (≈30 mg) was loaded in a 1.5 mm diameter amorphous fused silica capillary
(Markröhrchen Quarzkapillaren) and measured by powder neutron diffraction (PND) on the
Polaris station at ISIS, Harwell. Rietveld refinements were performed with the TOPAS program.
In combined SPD and PND refinements, the lattice parameters were refined independently,
whereas the site occupancies and fractional coordinates were constrained to have the same values.
Bond length calculations were made using the PND values. The lattice parameters were refined
separately, as the PND and XRD samples came from two separate synthesis runs and so will have
differing residual strains from the high-pressure synthesis. A slight difference is also expected
due to the differing ambient conditions present for the PND and XRD measurements.
3
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0
1.00
O
0.50
0.75
nO 2
Ni
M
0.50
0.25
1.00
0
0
0.25
0.50
0.75
1.00
ScO1.5
Figure 1. EDS for 30 crystallites of (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 . The filled light grey square shows the ideal point for
(Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 , whereas the dark grey square shows the average value (Sc0.94(3) Mn0.68(2) Ni0.36(2) O3 ) for the 30
crystallites. White crosses give the error of the average value.
refinement of synchrotron and neutron data was carried out using structural parameters from
LuNiO3 [24] in P21 /n (see electronic supplementary material, figure S2a), with initially complete
Sc occupancy of the A site and a disordered B site composition 0.66Mn/0.33Ni. Refinement of the
B site occupancy showed that Ni was ordered uniquely on one of the two possible sites, so a B site
ordered model was used from there on. Reducing the symmetry to P1̄ further improved the fit
(see electronic supplementary material, figure S2b). This can most clearly be seen by comparing
the P21 /n and P1̄ refinements (see electronic supplementary material, figure S2a,b) over the 14.9◦
and 21.7–21.9◦ 2θ ranges. The weighted profile R-factor Rwp decreases from 6.29% to 5.17%.
The symmetry lowering is associated with the creation of two distinct A sites and four B sites.
Refinement shows that Mn is only present on one of the two A sites. The Sc distribution was
then investigated by comparing an A site-deficient model with the Mn on the Sc/Mn mixed
A site replaced with vacancies. In this case, two of the four B sites are solely occupied by Mn
and the other two are mixed Mn/Ni at an overall composition (Sc0.88 Vac0.12 )Mn0.66 Ni0.33 O3 (see
electronic supplementary material, figure S2c). This was then compared with the refinement for
a fully occupied Mn-containing A site (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 model. The A site-deficient
model would require an average oxidation state for the Mn of +4.09 or an Ni state of +2.18.
However, the filled A site model only requires the average Mn oxidation state to be +3.49. This
is achievable through putting Mn2+ on the A site and having close to 2 : 1 Mn4+ : Mn3+ on the B
site (full 2 : 1 gives 3.52). The model with a full A site with a small amount of Mn gave a clearly
better fit (Rwp = 5.17% (see electronic supplementary material, figure S2b) versus Rwp = 6.15%
(see electronic supplementary material, figure S2c)). The preference for a completely occupied
Mn-containing A site in the (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 model is consistent with the expectation
that high-pressure synthesis would produce dense phases (table 1).
Figure 2 shows the refined structure of (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 . The final joint SPD and
PND refinements are shown in figure 3. The 30% Mn/70% Ni mixed sites and Mn-only B sites
order in a rock-salt manner. The symmetry lowering to triclinic does not change the basic B site
ordering pattern from the LuNiO3 starting model—although there are four crystallographically
.........................................................
0.75
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
0.25
4
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(a)
(b)
5
.........................................................
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
a
b
c
a
c
b
(c)
(d)
b
c
a
c
a
b
Figure 2. The structure of (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 (tilt system a+ b− c− ), showing the two types of A site: large lighter
grey (green online) spheres are pure Sc sites, whereas black spheres are mixed Sc/Mn sites. Lighter grey (red online) octahedra
represent pure Mn sites, whereas darker grey (blue online) octahedra show mixed Mn/Ni sites. The small light grey spheres show
the oxygen positions. (a) The view down [001] (pseudocubic a-axis). (b) The view along [110] (pseudocubic b-axis). (c) The view
down [11̄0] (pseudocubic c-axis). (d) The A site ions only shown in a projection close to the view down the a-axis. (Online version
in colour.)
Table 1. Comparison of refinement statistics for refinements described in the main text.
refinement
P21 /n (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3
Rwp (%)
6.29
figure
electronic supplementary material, figure S2a
P1̄ (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3
5.17
electronic supplementary material, figure S2b
P1̄ with A site vacancies (Sc0.88 Vac0.12 )Mn0.66 Ni0.33 O3
6.15
electronic supplementary material, figure S2c
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
distinct sites, they divided into two pairs of equivalent occupancies (table 2). The A site ordering
corresponds to corrugated layers of the distinct Sc-only and 12% Mn/88% Sc A sites in the ac
plane stacked along the b-axis (figure 2d).
The reduction in symmetry from P21 /n to P1̄ increases the number of crystallographically
distinct A sites from one to two, and the number of B sites from two to four. The final refined
values (table 2) reveal that manganese is observed on only one of the two possible A sites and
that nickel is present on only two of the four possible B sites. Bond lengths and angles and bond
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(a)
(b)
intensity (arb. units)
2000
1000
4000
2000
0
0
10
20
30
40
50
60
2000
4000
6000
2q
8000 10 000 12 000 14 000 16 000 18 000
time of flight
Figure 3. Combined refinement of (a) XRD λ = 0.949831 Å and (b) neutron data (backscattering bank Polaris) Rwp = 5.25%.
Tick marks are given for allowed reflections; in the XRD pattern the lower tick marks refer to 0.01 wt% of an Mn-doped Sc2 O3
impurity phase.
Table 2. Crystallographic data for (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 at room temperature from the combined refinements shown in
figure 3.
atom
Wyckoff
position
oxidation
x
y
z
Biso
occupancy
Sc1
2i
+3
0.9734(5)
0.9230(6)
0.7473(4)
0.35(4)
1.00
Sc2
2i
+3
0.5231(5)
0.4264(6)
0.7582(4)
0.91(5)
0.88(8)
Mn2
2i
+2
0.5231(5)
0.4264(6)
0.7582(4)
0.91(5)
0.12(8)
Mn3
1f
+4
0.5
0
0.5
0.49(6)
1
Mn4
1c
+4
0
0.5
0
0.22(6)
1
Mn5
1g
+3
0
0.5
0.5
0.27(6)
0.307(10)
Mn6
1d
+3
0.5
0
0
0.27(6)
0.293(10)
Ni5
1g
+2
0
0.5
0.5
0.27(6)
0.693(10)
Ni6
1d
+2
0.5
0
0
0.27(6)
0.707(10)
O1
2i
−2
0.6832(9)
0.6740(9)
0.9462(6)
0.35(4)
1
O2
2i
−2
0.7990(9)
0.1886(10)
0.5715(6)
0.01(1)
1
O3
2i
−2
0.3739(10)
0.0550(1)
0.7386(7)
0.05(5)
1
O4
2i
−2
0.1454(10)
0.5657(1)
0.7560(7)
0.03(3)
1
O5
2i
−2
0.8325(9)
0.1906(10)
0.9337(6)
1.39(28)
1
O6
2i
−2
0.6888(10)
0.7085(10)
0.5830(6)
0.42(17)
1
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
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..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
..........................................................................................................................................................................................................
valence sums are quoted in table 3. Bond valence sums are consistent with the presence of Mn2+
on the A site. The Mn-only B site bond lengths correspond to an Mn oxidation state of +4, whereas
the site shared with Ni corresponds to an Mn oxidation state of +3. As the mixed Mn/Ni sites
and Mn-only sites order in a rock-salt manner (figure 2), the Mn3+ and Mn4+ oxidation states are
thus also rock-salt-ordered. As a result of the synchrotron, X-ray and neutron refinement being
combined, the Biso values for the oxygen sites vary significantly from what is expected. This is
due to the synchrotron X-ray data having vastly more counts and so dominating the thermal
parameter calculation. So, as the values are more XRD-based, the values for low-scattering atoms
such as oxygen become less precise.
.........................................................
3000
6
6000
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intensity (arb. units)
4000
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Table 3. Bond lengths (Å), selected angles (deg) and bond valence sums (BVS) in (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 at room
temperature.
BVS
2.44(9)
2.22(1)
Sc 2.83
bond (B1−O−B2)
angle (deg)
3.08(2)
2.67(5)
2.12(6)
3.55(4)
3.54(1)
2.13(4)
2.10(5)
3.59(0)
2.64(9)
2.10(1)
..........................................................................................................................................................................................................
average
2.68(4)
Sc2/Mn2−O
2.02(5)
2.84(1)
..........................................................................................................................................................................................................
Sc 2.90
Mn 2.4
3.59(6)
3.44(6)
2.14(4)
2.31(9)
3.20(6)
2.08(9)
2.39(0)
3.52(9)
2.46(8)
2.12(4)
..........................................................................................................................................................................................................
average
Mn3−O
2.67(6)
..........................................................................................................................................................................................................
4 × 1.88(9)
Mn 4.02
.......................
2 × Mn3−O3−Ni6/Mn6
139.2(3)
..........................................................................................
1.89 (8)
2 × Mn3−O2−Ni5/Mn5
2 × Mn3−O6−Ni5/Mn5
137.2(3)
2 × 1.92(5)
2 × Mn4−O1−Ni6/Mn6
140.3(3)
2 × 1.92(6)
139.8(3)
..........................................................................................................................................................................................................
average
Mn4−O
..........................................................................................................................................................................................................
Mn 3.76
.......................
..........................................................................................
2 × Mn4−O4−Ni5/Mn5
2 × 1.88(5)
.......................
132.4(2)
..........................................................................................
2 × Mn4−O5−Ni6/Mn6
2 × 1.97(2)
137.5(3)
..........................................................................................................................................................................................................
average
1.92(4)
..........................................................................................................................................................................................................
Ni5/Mn5−O
2 × 2.00(6)
Ni 2.17
.......................
2 × 2.07(5)
.......................
2 × 2.01(2)
2 × Ni5/Mn5−O2−Mn3
139.8(3)
..........................................................................................
Mn 2.89
2 × Ni5/Mn5−O4−Mn4
132.4(2)
..........................................................................................
2 × Ni5/Mn5−O6−Mn3
137.2(3)
..........................................................................................................................................................................................................
average
2.02(4)
..........................................................................................................................................................................................................
(Continued.)
.........................................................
lengths (Å)
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
bond (X−O)
Sc1−O
7
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Table 3. (Continued.)
8
bond (B1−O−B2)
2 × Ni6/Mn6−O1−Mn4
BVS
Ni 2.20
.......................
2 × 2.06(9)
angle (deg)
140.3(3)
.........................................................................................
2 × Ni6/Mn6−O3−Mn3
Mn 2.93
.......................
139.2(3)
.........................................................................................
2 × Ni6/Mn6-O5−Mn4
2 × 2.00(4)
137.5(3)
..........................................................................................................................................................................................................
average
2.02(7)
..........................................................................................................................................................................................................
(a)
(b)
c
c
b
b
Figure 4. A site environments for the Sc ions in the unit cell (a) Sc1 and (b) the mixed Sc2/Mn6 site. Light grey spheres are
oxygen, black spheres are Sc, dark grey spheres are the calculated centres of the oxygen coordination spheres, and the joining
black arrows are the displacement vectors from the centres of the oxygen positions. Both figures are viewed along the triclinic
a-axis [1̄00].
The 1 : 3 A site ordered compounds AA3 Mn4 O12 (A = Mn3+ ) can also simultaneously show
differing Mn oxidation states on the A and B sites, with the proportion of valence ordering
on the B site being determined by the A cation. For example, in the case of A = Na [25], the
Mn3+ and Mn4+ oxidation states order fully in distinct crystallographic sites. When A = Ca
[26], the compound has 1 : 3 Ca : Mn3+ on the A site as well as 1 : 3 Mn4+ : Mn3+ order on
the B site, with distinct crystallographic sites. However, when A = Bi [27], all Mn is present
as Mn3+ . Thus, the high-pressure phases (In1−y Mny )MnO3 [22] and Mn2 O3 [23] and the new
(Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 composition reported here are rare in showing both A and B site
Mn with differing oxidation states and the A site being Mn2+ .
Within the Glazer–Woodward scheme, the P1̄ space group is described as a+ b− c− , where the
rotations around the x-, y- and z-axes have differing magnitudes, with x-rotations being in phase
and y- and z-rotations being in anti-phase. This relates to the monoclinic P21 /c tilting system
a+ b− b− (when the P21 /n setting is used, the tilt system is described as b− b− c+ [28]). The tilting
angle of the octahedra can be defined as φ = (180 − Φ)/2, where Φ is the Ni/Mn−O−Mn bond
angle (Φ = 180◦ for a cubic perovskite, making φ = 0). Mean tilt angles of 22.1(2)◦ about 001
(figure 2a), 20.9(2)◦ about 110 (figure 2b) and 20.5(2)◦ about 11̄0 (figure 2c) are calculated. The
average tilt of 21.2◦ compared to 15.7◦ calculated for LaCaMnNbO6 [29] demonstrates the high
degree of tilting in (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 . As the b and c tilts are the same within error,
there is no observed deviation from a+ b− b− (P21 /c), consistent with the tilting being controlled
by the number of distinct B sites.
The tilt angles signal a strong deviation from the cubic perovskite structure that is confirmed
by four Sc−O bonds for each site being longer than 3 Å. Figure 4 shows the A site coordination
.........................................................
lengths (Å)
2 × 2.00(8)
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
bond (X−O)
Ni6/Mn6−O
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c–1 (emu–1 mol)
0.05
120
80
40
0
0.04
0
0.03
9
400
100
200
temperature (K)
300
200
0
–200
0.02
–400
0.01
0
20
40
60
temperature (K)
80
100
–10 000
5000
0
field (Oe)
5000
10 000
Figure 5. (a) Zero field cooled (empty squares) and field cooled (filled squares) magnetization measurement with 100 Oe
measuring field. Inset shows inverse susceptibility with Curie–Weiss behaviour, which was fitted between 170 and 280 K.
(b) Magnetization versus field measured at 5 K.
environments at the two sites, with the dark grey spheres showing the calculated centroids of the
oxygen polyhedra. The Sc1 site is displaced by 0.540(2) Å, whereas the mixed Sc2/Mn6 site has
the smaller displacement of 0.515(6) Å—in both cases, the displacements are antiferrodistortive.
These distortions compare to ScCrO3 [9], where the displacement is 0.503(1) Å, and to ScAlO3
[10], where the displacement is 0.452(1) Å. Both ScCrO3 and ScAlO3 are Pbnm perovskites with
the same b− b− c+ tilt system, close to that found in the present B site ordered material. Analysis of
the displacement directions reveals that both the Sc1 and Sc2 sites move predominantly along the
b-axis. Although the difference between the sites is relatively small (5% of the total displacement),
the reduced displacement of the Sc2/Mn6 site coincides with the Mn occupancy of this site. Both
Mn2+ and Sc3+ cations are too small to occupy a conventional perovskite A site, and thus drive
extensive tilting to reduce their coordination number.
The relatively small difference between the refined tilts along 1̄1̄0 and 110 indicates that the
deviation from the monoclinic structure where the tilts would be equal is minor. The driving
force behind this deviation is thus the preferential ordering of the manganese cations on one of
the two A sites in the P1̄ structure, which is not possible in the single A site P21 /n structure.
Although the number of B sites also increases, there are effectively only two distinct sites in the
lower-symmetry structure, consistent with the creation of two distinct A sites as the reason for
the observed triclinic symmetry.
The Goldschmidt tolerance factor [30] for (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 is 0.81, which is below
the suggested stability limit for perovskites of t ≈ 0.85 [29], consistent with the requirement for
synthesis under high pressures [31].
Consistent with the absence of magnetic Bragg scattering in the room-temperature
neutron diffraction data, (Sc0.94 Mn0.06 )Mn0.65 Ni0.35 O3 is a paramagnet to low temperatures.
Curie–Weiss analysis of the susceptibility (figure 5a) between 170 and 280 K affords C =
2.11 emu K mol−1 (μeff = 4.11 µB per f.u. (formula unit)) and θ = −95.27 K. This compares well
with the calculated C of 2.09 emu K mol−1 (µeff = 4.09 µB per f.u.) for the charge distribution
2+
4+
3+
3+
2+
of (Sc3+
0.94 Mn0.06 )(Mn0.41 Mn0.09 ) (Mn0.15 Ni0.35 )O3 assigned on the basis of the refined crystal
structure using bond valence sum considerations. The negative Weiss temperature is expected
due to the presence of dominant antiferromagnetic interactions. Divergence of the zero field
cooled and field cooled magnetizations below 18 K can be associated with the B site cation
disorder and the presence of competing ferromagnetic Mn4+ –Ni2+ exchange interactions.
Commensurate with the transition to a cooperative magnetic state, the hysteresis measurement
at 5 K (figure 5b) shows a small coercive field (72 Oe) with remanent magnetization of 6 emu mol−1
(0.001 µB per f.u.) but the loop does not saturate in a 1 T field.
.........................................................
0.06
160
rsta.royalsocietypublishing.org Phil. Trans. R. Soc. A 372: 20130012
0.07
c (emu mol–1)
(b)
200
0.08
M (emu mol–1)
(a)
Downloaded from http://rsta.royalsocietypublishing.org/ on June 18, 2017
4. Conclusion
to Diamond, where we thank Dr C. Tang, Dr J. Parker and Dr S. Thompson for assistance on the I11
diffractometer, and access to ISIS, where we thank Dr R. Smith and Dr S. Hull for assistance on the Polaris
instrument.
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