Structural Modification of the Cation-Ordered Ruddlesden-Popper Phase YSr2Mn2O7 by Cation Exchange and Anion Insertion

Article abstract of DOI:10.1021/acs.inorgchem.7b01525

Calcium-for-strontium cation substitution of the a^-b⁰c⁰/b⁰a^-c⁰-distorted, cation-ordered, n = 2 Ruddlesden-Popper phase, YSr₂Mn₂O₇, leads to separation into two phases, which both retain an a^-b⁰c⁰/b⁰a^-c⁰-distorted framework and have the same stoichiometry but exhibit different degrees of Y/Sr/Ca cation order. Increasing the calcium concentration to form YSr_0.5Ca_1.5Mn₂O₇ leads to a change in the cooperative tilting on the MnO₆ units to a novel a^-b^-c^-/b^-a^-c^- arrangement described in space group P2₁/n11. Low-temperature, topochemical fluorination of YSr₂Mn₂O₇ yields YSr₂Mn₂O_5.5F_3.5. In contrast to many other fluorinated n = 2 Ruddlesden-Popper oxide phases, YSr₂Mn₂O_5.5F_3.5 retains the a^-b⁰c⁰/b⁰a^-c⁰ lattice distortion and P4₂/mnm space group symmetry of the parent oxide phase. The resilience of the a^-b⁰c⁰/b⁰a^-c⁰-distorted framework of YSr₂Mn₂O₇ to resist symmetry-changing deformations upon both cation substitution and anion insertion/exchange is discussed on the basis the A-site cation order of the lattice and the large change in the ionic radius of manganese upon oxidation from Mn³⁺ to Mn⁴⁺. The structure property relations observed in the Y-Sr-Ca-Mn-O-F system provide insight into assisting in the synthesis of n = 2 Ruddlesden-Popper phases, which adopt cooperative structural distortions that break the inversion symmetry of the extended lattice and therefore act as a route for the preparation of ferroelectric and multiferroic materials.

Full text of DOI:10.1021/acs.inorgchem.7b01525

Article
pubs.acs.org/IC
Structural Modification of the Cation-Ordered Ruddlesden−Popper
Phase YSr2Mn2O7 by Cation Exchange and Anion Insertion
Ronghuan Zhang,† Alexandra S. Gibbs,‡ Weiguo Zhang,§ P. Shiv Halasyamani,§
and Michael A. Hayward*,†
†Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K.
‡ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K.
§Department of Chemistry, University of Houston, 112 Fleming Building, Houston, Texas 77204-5003, United States
S
* Supporting Information
ABSTRACT: Calcium-for-strontium cation substitution of the
a−b0c0/b0a−c0-distorted, cation-ordered, n = 2 Ruddlesden−
Popper phase, YSr2Mn2O7, leads to separation into two phases,
which both retain an a−b0c0/b0a−c0-distorted framework and
have the same stoichiometry but exhibit different degrees of Y/
Sr/Ca cation order. Increasing the calcium concentration to
form YSr0.5Ca1.5Mn2O7 leads to a change in the cooperative
tilting on the MnO6 units to a novel a−b−c−/b−a−c−
arrangement described in space group P21/n11. Low-temper-
ature, topochemical fluorination of YSr2Mn2O7 yields
YSr2Mn2O5.5F3.5. In contrast to many other fluorinated n = 2
Ruddlesden−Popper oxide phases, YSr2Mn2O5.5F3.5 retains the
a−b0c0/b0a−c0 lattice distortion and P42/mnm space group
symmetry of the parent oxide phase. The resilience of the a−b0c0/b0a−c0-distorted framework of YSr2Mn2O7 to resist symmetry-
changing deformations upon both cation substitution and anion insertion/exchange is discussed on the basis the A-site cation
order of the lattice and the large change in the ionic radius of manganese upon oxidation from Mn3+ to Mn4+. The structure
property relations observed in the Y−Sr−Ca−Mn−O−F system provide insight into assisting in the synthesis of n = 2
Ruddlesden−Popper phases, which adopt cooperative structural distortions that break the inversion symmetry of the extended
lattice and therefore act as a route for the preparation of ferroelectric and multiferroic materials.
delocalized systems or magnetic exchange interactions in
localized systems.2 As a result much effort has gone into
understanding the electronic and magnetic consequences of
tuning the structural distortions of perovskite phases, typically
via studies that make chemical substitutions on the 12-
coordinate A-site.2,3
In addition to tuning the electronic coupling between
neighboring B-cations, the collective distortions of the
perovskite phases also modify the lattice symmetry of the
framework. As a consequence, if the correct distortion mode is
adopted, the collective deformations of perovskite lattices offer
a mechanism to break the inversion symmetry of the system
and thus the potential to induce ferroelectric behavior.4 In the
specific case of the three-dimensional perovskite lattice, acentric
phases can only be produced when octahedral tilting is
accompanied by cation order.5−8 However, when equivalent
structural distortions are applied to layered analogues of the
perovskite structure, particularly A3B2O7, n = 2 Ruddlesden−
Popper phases, the inversion symmetry of the system can be
INTRODUCTION
■
The chemical diversity of oxide phases that adopt ABO3
perovskite structures can be attributed, in part, to the high
mechanical flexibility of the perovskite framework. This
flexibility allows the apex-linked BO6 octahedra, which
constitute the perovskite framework, to cooperatively tilt and
twist to optimize the packing of the system in response to
changes in the ratio of the A- and B-cation radii (conveniently
parametrized by the structural tolerance factor: t = ⟨A−O⟩/
√2⟨B−O⟩).1 As a result, a wide range of cation combinations
can be incorporated into thermodynamically stable perovskite
lattices.
A further feature of the tilting and twisting distortions of
perovskite phases is that they can be harnessed to modify and
tune the physical behavior of systems, particularly transition-
metal-containing phases. This is because the collective rotations
of the octahedra act to distort the B−O−B bond angles from
the 180° value observed in the undistorted, aristotype
framework. This tightening of the B−O−B bond angles
modifies the orbital interactions and thus the coupling between
the local electronic states of the transition-metal B-cations,
changing the electronic band widths in electronically
Received: June 16, 2017
© XXXX American Chemical Society
A
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broken in the absence of cation order,9,10 as demonstrated by
the observation of ferroelectric behavior in (Ca, Sr)3Ti2O7.11
Utilizing the distortions of the n = 2 Ruddlesden−Popper
phases to prepare noncentrosymmetric lattices is highly
attractive because it provides a route to polar materials without
the need to include “active distortion centers”, such as d0
transition-metal cations, which break the inversion symmetry
through the action of a second-order Jahn−Teller distor-
tion.12−15 The additional chemical flexibility that comes from
not having to include such centers makes the incorporation of
paramagnetic ions into polar frameworks much easier, and thus
distorted n = 2 Ruddlesden−Popper phases offer a route to the
preparation of magnetoelectric multiferroic materials.
While the distortions of n = 2 Ruddlesden−Popper phases
offer an attractive route for the preparation of noncentrosym-
metric phases in theory, it is hard to realize these concepts in
practice because the desired “A21am”, a−a−c+/a−a−c+ ferro-
electric distortion mode is only stabilized at extremely low
values of the structural tolerance factor. A recent study by
Pitcher et al. highlighted these difficulties when they observed
that it was not possible to induce the desired polar distortion
mode in the Fe3+,Ln2AEFe2O7 (Ln = lanthanide; AE = Ba, Sr,
Ca) system through A-site substitution.16 This was partly
because, in compositions containing very small A-cations,
A3B2O7 n = 2 Ruddlesden−Popper phases tend to be
thermodynamically less stable than a 1:1 mixture of the
corresponding A2BO4 n = 1 Ruddlesden−Popper phase and the
ABO3 perovskite. Thus, in the Ln2AEFe2O7 system, the A21am-
distorted structure could only be stabilized by additional
titanium-for-iron B-site substitution.16
In an attempt to circumvent these problems, we have been
looking at the use of topochemical manipulation as a route to
prepare noncentrosymmetric materials. Specifically, we have
been utilizing the observation that fluorine insertion leads to a
significant enhancement of the lattice distortions of many n = 2
Ruddlesden−Popper systems, as indicated by a general
tightening of the B−O−B bond angles.17−20 Furthermore, it
has also been observed that upon fluorination the largest
enhancement tends to be to the rotations of the BO6 octahedra
around the crystallographic z-axis. This led us to anticipate that
the fluorination of La3Ni2O7, which adopts an Amam, a−a−c0/
a−a−c0 distortion, would lead to the formation of an oxide−
fluoride phase with the desired polar A21am, a−a−c+/a−a−c+-
distorted structure. However, a detailed crystallographic study
of La3Ni2O5.5F3.5 reveals that while fluorination of La3Ni2O7
does induce a z-axis rotation of the NiX6 octahedra, the “sense”
of this rotation (clockwise or anticlockwise) alternates between
adjacent perovskite sheets to yield an a−a−c+/a−a−-(c+)
distortion, which breaks the inversion symmetry of the lattice
locally but not globally, yielding an antiferroelectric lattice with
Pnma space group symmetry.19 While this synthesis did not
achieve the desired result, it shows that the basic concept is
viable and motivates us to study the fluorination of other
systems to see if it is possible to prepare a fluorinated phase
that is ferroelectric rather than antiferroelectric.
EXPERIMENTAL SECTION
■
Synthesis. Samples in the YSr2−xCaxMn2O7 series (x = 0, 0.25, 0.5,
1.0, and 1.5) were prepared via a citrate gel method. Suitable
stoichiometric ratios of Y2O3 (99.99%, dried at 900 °C), SrCO3
(99.99%), CaCO3 (99.99%), and MnO2 (99.9%) were dissolved in a
minimal quantity of 6 M nitric acid. Citric acid and ethylene glycol
were then added, and the solution was heated with constant stirring.
The gel thus formed was heated to 350 °C until a coarse black powder
was obtained. The powder was ground and then heated to 1000 °C in
air at a rate of 1 °C/min to remove the remaining organic
components. The resulting powder was then pressed into pellets
before further heat treatments. For the preparation of YSr2Mn2O7 and
YSr1.75Ca0.25Mn2O7, the sample pellets were heated for three periods of
48 h at 1350 °C in air and two further periods of 48 h at 1450 °C. For
YSrCaMn2O7, the pellets were heated for three periods of 48 h at 1350
°C in air and two further periods of 48 h at 1400 °C. For
YSr0.5Ca1.5Mn2O7, the pellets were heated for five periods of 48 h at
1350 °C in air. Lower synthesis temperatures were employed for the
calcium-substituted samples to avoid melting. The preparation of
YCa2Mn2O7 was also attempted, but the phase failed to form after four
heating cycles of 48 h at 1300 °C, and this composition melts at 1325
°C.
Fluorination of YSr2Mn2O7 and YSr0.5Ca1.5Mn2O7 was carried out
using CuF2 as a fluorination agent, in the same manner as has been
described previously.17,20,21 To avoid contamination of the samples
with CuO, which occurs when the fluorination agent is mixed directly
into the sample, CuF2 was heated separately from the sample at 500
°C under flowing oxygen to liberate fluorine. The resulting O2/F2 gas
mixture was then passed over a sample of YSr2‑xCaxMn2O7 powder
held at 400 °C in an adjacent furnace. Samples were treated in this way
for four periods of 48 h, with the CuF2 reagent being replaced between
heating periods.
Characterization. X-ray powder diffraction data were collected
using a PANalytical X’pert diffractometer incorporating an X’celerator
position-sensitive detector (monochromatic Cu Kα1 radiation).
Neutron powder diffraction data were collected from samples
contained within vanadium cans, using the HRPD diffractometer
(ISIS neutron source, Oxon, U.K). Rietveld profile refinement was
performed using the GSAS suite of programs.22 Magnetization data
were collected using a Quantum Design MPMS SQUID magneto-
meter. Average manganese oxidation states were determined by
iodometric titration. Titrations were performed by dissolving samples
in a dilute HCl solution containing an excess of KI and titrating the
amount of liberated I2 with a standardized Na2S2O3 solution. Powder
second-harmonic-generation (SHG) measurements were performed
on a modified Kurtz−Perry instrument.23 A description of the
equipment and methodology was published previously.24
RESULTS
■
Structural Characterization of YSr2Mn2O7 (0 < x < 1.5).
X-ray powder diffraction data collected from samples of
YSr2−xCaxMn2O7 (x = 0, 0.25, 0.5, 1, 1.5), shown in Figure
1, were initially indexed on the basis of tetragonal unit cells,
consistent with the formation of n = 2 Ruddlesden−Popper
phases. Structural models based on the a−b0c0/b0a−c0-distorted
n = 2 Ruddlesden−Popper structure of YSr2Mn2O7 (space
group P42/mnm),25 but with calcium partially replacing
strontium as appropriate, were refined against these data. The
refinements proceeded smoothly; however, close inspection
revealed that, with the exception of the x = 0 sample, none of
the fits to the data were satisfactory.
Detailed inspection of the data from the x = 0.25, 0.5, 1
samples revealed that the poor fitting of the P42/mnm structural
model was due to a peak-shape asymmetry of the 00l diffraction
reflections. This asymmetry could not be modeled by the
addition of hkl-dependent broadening terms; however, the fits
to the data were significantly improved by the introduction of a
In this contribution, we report the effects of A-cation
substitution and topochemical fluorine insertion on the
structure of the A-cation-ordered, n = 2 Ruddlesden−Popper
phase YSr2Mn2O7. It was hoped that these chemical
modifications would enhance the collective distortions of the
layered perovskite phase, leading to a noncentrosymmetric,
polar material.
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variously ascribed to either A-cation inhomogeneity within
samples (i.e., the two phases have different chemical
compositions) or to a difference in the A-cation distribution
on the 9- and 12-fold A-cation coordination sites in the
different phases (i.e., the two phases have the same composition
but different A-cation orderings).26 In order to determine
which of these scenarios is responsible for the phase separation
in the YSr2−xCaxMn2O7 system, the structural refinement of
YSrCaMn2O7 was initially performed with the constraint that
the two tetragonal phases in the structural model had the same,
ideal, chemical composition: YSrCaMn2O7 (neutron scattering
lengths: Ca, 4.70 fm; Y, 7.75 fm; Sr, 7.02 fm).29 The refinement
proceeded smoothly with this constraint to give a good
statistical fit (χ2 = 5.79), producing a structural model in which
the Y/Sr/Ca distributions over the 9-fold (phase 1 = 50:4:46;
phase 2 = 50:10:40) and 12-fold (phase 1 = 0:91:9; phase 2 =
0:80:20) A-cation sites differed between the two phases. The A-
cation distribution constraint was then released so that the
occupancies of the A-cation sites in the two phases could vary
freely and thus the chemical compositions of the two phases
could vary freely. With the removal of this constraint, it was
observed that none of the A-site neutron scattering powers
changed by more than 1%, indicating that the principal
difference between the two phases of YSrCaMn2O7 is their A-
cation ordering, not their chemical composition. A full
description of the refined two-phase structural model of
YSrCaMn2O7 is given in Table S1, with selected bond lengths
and angles in Table S2 and plots of the observed and calculated
data shown in Figure S1.
Structural Characterization of YSr0.5Ca1.5Mn2O7. De-
tailed inspection of the X-ray powder diffraction data collected
from YSr0.5Ca1.5Mn2O7 revealed that the poor fitting of the
P42/mnm structural model was due to a splitting of diffraction
peaks, consistent with a lowering of the crystallographic
symmetry from tetragonal to orthorhombic. As noted above,
calcium substitution into YSr2−xCaxMn2O7 will reduce the
structural tolerance factor of the system, leading to an
enhancement of the collective tilting and twisting distortions
of the MnO6 octahedra. The lower crystallographic symmetry
of the x = 1.5 sample, therefore, suggests that it has a more
distorted structure than the x < 1.5 samples. Starting from the
a−b0c0/b0a−c0 distortion of the 0 < x < 1 phases, the symmetry
analysis performed by Aleksandrov and Bartolome30 indicates
there are six possible collective distortions that
YSr0.5Ca1.5Mn2O7 could adopt, as detailed in Table 2. Structural
models were constructed for each of these six distorted n = 2
Ruddlesden−Popper frameworks and then refined against
neutron powder diffraction data collected from
YSr0.5Ca1.5Mn2O7. It was assumed that the 12-coordinate A-
sites contained a 1:1 mixture of Sr:Ca and the 9-coordinate A-
Figure 1. X-ray powder diffraction data collected from
YSr2−xCaxMn2O7.
second tetragonal n = 2 Ruddlesden−Popper phase to the
models.
In order to stabilize these two-phase refinements, the peak
shapes and atomic positions of the two phases were constrained
to be the same, with only the lattice parameters and phase
fractions of the two phases allowed to refine freely. Table 1
shows the refined values of these parameters.
Table 1. Lattice Parameters from Two-Phase Refinements of
a
YSr2−xCaxMn2O7
x
a (Å)
c (Å)
volume (Å3) fraction (%)
0
5.40456(6)
5.4064(1)
5.3997(1)
5.4106(1)
5.4006(1)
5.4254(1)
5.4123(1)
19.9054(2)
19.7608(3)
19.7883(5)
19.5848(1)
19.6388(5)
19.255(1)
19.325(1)
581.26(2)
577.26(1)
576.98(2)
573.33(1)
572.81(2)
566.79(1)
566.11(3)
100
66.5(6)
0.25
0.5
1
P1
P2
P1
P2
P1
P2
33.5(6)
83.7(2)
16.3(2)
65.8(1)
34.2(1)
a
Parameters from the x = 0 composition were extracted from ref 25.
Structural Characterization of YSrCaMn2O7. Phase
separation analogous to that observed for YSr2−xCaxMn2O7
(0.25 < x < 1) has been observed in other manganese-
containing n = 2 Ruddlesden−Popper phases.26−28 To further
investigate the phase separation of the YSr2−xCaxMn2O7
system, high-resolution neutron powder diffraction data were
collected from YSrCaMn2O7.
In common with the X-ray data, satisfactory fits to the
neutron powder diffraction data collected from YSrCaMn2O7
could only be achieved using two-phase models; however, the
high resolution of the latter data set allowed the atomic
coordinates and site occupancies of the two phases to be
refined independently. To assist refinement stability, the
thermal displacement factors of corresponding atoms in the
two phases were constrained to be the same.
Table 2. Fitting Statistics from the Structural Refinement of
YSr0.5Ca1.5Mn2O7 Using Models Based on the Distortions of
n = 2 Ruddlesden−Popper Phases
space group
distortion
χ2
wRp (%)
Rp (%)
P21/n11 (No. 14)
P21nm (No. 31)
Pnnm (No. 58)
A21am (No. 36)
A2/a (No. 15)
Amam (No. 63)
a−b−c−/b−a−c−
a−b−c+/b−a−c+
a−b−c0/b−a−c0
a−a−c+/a−a−c+
a−a−c−/a−a−c−
a−a−c0/a−a−c0
7.63
7.71
4.74
4.77
4.92
7.14
7.28
7.15
4.95
5.01
5.08
6.30
6.42
6.31
8.19
17.09
17.79
17.11
The origin of the observed phase separations in manganese-
containing n = 2 Ruddlesden−Popper phases has been
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sites contained a 1:1 mixture of Y:Ca. Subsequent refinement of
these parameters showed this to be the case, so all models were
fixed with these A-site compositions. In addition, displacement
parameters were constrained by element. All six refinements
converged smoothly.
Given the very similar neutron scattering lengths of oxide and
fluoride (O, 5.80 fm; F, 5.65 fm)29 no attempt was made to
differentiate between the two anions at this stage, with all anion
sites treated as if they were occupied by oxide. The refinement
converged smoothly to give a good statistical fit (χ2 = 5.43).
The majority of previous studies that have examined
analogous fluorinations of n = 2 Ruddlesden−Popper oxides
observe a lowering of crystallographic symmetry upon fluorine
insertion.17−19 Therefore, to confirm that the model in space
group P42/mnm is the best description of the structure of
YSr2Mn2O5.5F3.5, a series of lower-symmetry models, with more
distorted Mn(O/F)6 networks and also with disordered
Mn(O/F)6 rotations, were refined against the neutron powder
diffraction data. As shown in Table S5, none of these models
achieves as good a statistical fit as the P42/mnm model,
confirming that this is the best structural description of
YSr2Mn2O5.5F3.5.
The goodness-of-fit parameters listed in Table 2 indicate that
the three A-centered models describing a−a−c?/a−a−c? dis-
tortions give much poorer fits to the data than the three
primitive models that describe a−b−c?/b−a−c? distortions,
eliminating the former models from consideration. Of the
three remaining models, the a−b−c−/b−a−c−-distorted P21/n11
model has the best fitting-statistics, considerably better than the
a−b−c0/b−a−c0-distorted Pnnm model but only marginally
better than the a−b−c+/b−a−c+-distorted P21nm model. In
order to help to differentiate between the P21nm and P21/n11
models, we investigated the SHG activity of YSr0.5Ca1.5Mn2O7
as an a−b−c+/b−a−c+-distorted P21nm structure would be
noncentrosymmetric, and thus SHG-active, while a a−b−c−/
b−a−c−-distorted P21/n11 structure would be centrosymmetric,
and thus SHG-inactive. No SHG activity was observed for
YSr0.5Ca1.5Mn2O7, consistent with this phase adopting an
a−b−c−/b−a−c−-distorted P21/n11 structure. Full details for the
refined structure of YSr0.5Ca1.5Mn2O7 are given in Table S3,
with selected bond lengths and angles in Table S2 and a plot of
the observed and calculated data shown in Figure S2.
Fluorination of YSr2−xCaxMn2O7. The fluorination of
YSr2Mn2O7, by the procedure outlined above, yielded a single-
phase sample with an expanded c-lattice parameter, consistent
with topochemical fluorination to form a phase of composition
YSr2Mn2OxFy. In contrast, when the same procedure was
applied to YSr0.5Ca1.5Mn2O7, a mixture of apparently unreacted
material and a new tetragonal phase, nominally assigned as
YSr0.5Ca1.5Mn2OxFy, was observed with an approximate
stoichiometric ratio of 3:1. Further fluorination cycles led to
no further reaction, and raising the reaction temperature led to
decomposition and the formation of binary fluorides (SrF2,
CaF2, etc.), indicating that the calcium-doped phase could not
be readily fluorinated via this route.
Chemical and Structural Characterization of
YSr2Mn2O5.5F3.5. The crystallographic characterization of
YSr2Mn2OxFy, described below, indicates that this is a
fluorine-intercalated n = 2 Ruddlesden−Popper phase in
which all of the anion sites were fully occupied and thus x +
y = 9, as observed for other fluorinated n = 2 Ruddlesden−
Popper phases.17−20,31 This information, combined with
iodometric titration results, which revealed an average
manganese oxidation state of Mn +3.75, indicates that the
fluorinated phase has a chemical formula of YSr2Mn2O5.5F3.5.
Neutron powder diffraction data collected from
YSr2Mn2O5.5F3.5 can be readily indexed using a tetragonal
unit cell (a = 5.326 Å; c = 22.631 Å) with extinction conditions
consistent with the P42/mnm space group symmetry of the all-
oxide starting material YSr2Mn2O7. A structural model was
constructed in the space group P42/mnm, based on the
reported structure of YSr2Mn2O7 but with additional anions
inserted into the “tetrahedral” coordination sites within the
rock-salt layers of the phase, in an analogy to the structures of a
series of other fluorine-intercalated n = 2 Ruddlesden−Popper
oxide phases.17,19,20 The Y/Sr distribution was set to be the
cation-ordered arrangement observed for the YSr2Mn2O7
parent phase. Refinement of this model against the neutron
powder diffraction data proceeded smoothly. All atomic
positional and displacement parameters were refined freely.
As noted above, the poor neutron scattering contrast
between oxide and fluoride ions prevents the anion distribution
in YSr2Mn2O5.5F3.5 from being determined directly. However, it
is possible in some cases to deduce the location of fluoride ions
within oxide−fluoride lattices by examining the local bonding
environments of the anion sites through bond valence sums
(BVS).32−34 This approach has proven to be effective in
locating the fluoride ions in a number of fluorinated n = 2
Ruddlesden−Popper phases.18−20
Table 3 lists the BVS of the seven crystallographically distinct
anion sites in the structure of YSr2Mn2O5.5F3.5. These BVS
Table 3. Anion BVS from YSr2Mn2O7 and YSr2Mn2O5.5F3.5
YSr2Mn2O7
YSr2Mn2O5.5F3.5
BVS(O)
anion
BVS(O)
anion
BVS(F)
O(1)
O(2)
O(3)
O(4)
O(5)
1.711
2.225
2.227
2.236
1.974
X(1)/O/F(1)
X(2)/O(1)
X(3)/O(3)
X(4)/O(4)
X(5)/O(5)
X(6)/F(6)
X(7)/F(7)
1.509
2.069
2.157
1.854
2.122
1.460
1.498
1.237
1.756
1.833
1.569
1.775
1.103
1.132
values were calculated on the basis of occupation by oxide or
fluoride, and the BVS values of the corresponding anion sites in
the structure of YSr2Mn2O7 were also calculated for
comparison. The anion-site labeling scheme is shown in Figure
2. These data reveal that the two smallest BVS values were
calculated from the tetrahedral interstitial anion sites X(6) and
X(7), consistent with the occupation of fluoride ions.
Occupation of the “interstitial” anion sites exclusively by
fluoride ions has been observed widely in other topochemically
fluorinated n = 2 Ruddlesden−Popper oxide phases.17−20 The
composition of the fluorinated phase requires fluoride ions to
have been exchanged with oxide ions of the “parent” oxide
phase.
The data in Table 3 show that the BVS of the “apical” X(1)
site is the lowest of the remaining anion sites, suggesting that it
is occupied by a 3:1 mixture of F−/O2− to make up the overall
YSr2Mn2O5.5F3.5 stoichiometry. Similarly, preferential substitu-
tion of fluoride for oxide at the apical anion site has been
observed in other topochemically fluorinated phases such as
Sr3(Fe0.5Ru0.5)2O5.5F3.5 and La3Ni2O5.5F3.5.18,19 Full details for
the refined structure of YSr2Mn2O5.5F3.5 are given in Table S6,
with selected bond lengths and angles in Table S2 and a plot of
D
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focused on RESr2Mn2O7 phases observed that it was generally
only possible to prepare single-phase samples for compositions
containing small RE3+ cations (Tb−Er and Y) and that systems
containing large RE3+ cations (La−Gd) split into two-phase
mixtures.
This difference in behavior between the large and small A-
cation systems has been attributed to the differing degrees of
cation order that these phases exhibit. Compositions containing
small RE cations (Tb−Er and Y) order to place Sr2+ on the 12-
coordinate A-sites, and a 1:1 mixture of Sr2+ and RE3+ on the 9-
coordinate A-sites in the framework. As the size of RE3+
increases, the degree of A-cation order diminishes, and this is
associated with samples separating into two or more different n
= 2 Ruddlesden−Popper phases. The decline in the degree of
A-cation order can be attributed to the reduction in the size
difference between Sr2+ and RE3+ as the size of RE3+ increases.
The results of the calcium-for-strontium cation exchange of
YSr2Mn2O7 reported here support the association between the
degree of A-cation order and single-phase/multiphase behavior.
Low levels of calcium substitution (25−50%) lead to separation
into two phases that appear to have similar compositions but
different degrees of A-cation order. In addition, the two
observed phases become more dissimilar as the concentration
of calcium increases (Table 1). This behavior can be
rationalized by observing that partial substitution of Sr2+ by
Ca2+ makes the average size of the AE2+ (AE2+ = Ca2+, Sr2+)
cations smaller, and therefore more similar to the size of Y3+,
disrupting the A-cation order of the host phase and leading to
multiphase behavior. Thus, the phase separation observed for
YSr2−xCaxMn2O7 (0.25 < x < 1) compositions can be seen as
analogous to that observed for the RESr2Mn2O7 phases because
both are associated with the loss of A-cation order as the
difference in the average radii of the RE3+ and AE2+ cations
decreases.
Figure 2. Structures of YSr2Mn2O7 and YSr2Mn2O5.5F3.5.
the observed and calculated data shown in Figure S3. A
representation of the structure of YSr2Mn2O5.5F3.5 is shown in
Figure 2.
Magnetism. Zero-field-cooled and field-cooled magnet-
ization data collected from YSr0.5Ca1.5Mn2O7 in an applied field
of 100 Oe, shown in Figure 3, are qualitatively very similar to
Structural Influence of Calcium Doping. It is surprising
at first sight that replacing of 50% of Sr2+ with Ca2+ leads to
only a modest increase in the magnitude of the cooperative
distortions of the YSr2−xCaxMn2O7 framework, increasing the
MnO6 rotation angle from 3.52° at x = 0 to 7.12° in the
majority phase at x = 1 (Figure 4) but leading to no change in
structural symmetry, despite the fact that the structural
tolerance factor, calculated using ionic radii,35 changes from t
= 0.98 for YSr2Mn2O7 to t = 0.95 for YSrCaMn2O7.
This modest structural response to cation substitution can be
rationalized by observing that the majority of the substituted
calcium cations reside on the 9-coordinate A-sites in the lattice.
In contrast to the 12-coordinate A-cation sites, the A−O bond
lengths of the 9-coordinate A-cation sites can be contracted, to
accommodate a decline in the average A-cation radius, by
simply contracting the rock-salt layers of the Ruddlesden−
Popper framework along the z-axis. Such a contraction brings
the perovskite blocks closer together without deforming the
MnO6 octahedra or requiring them to cooperatively rotate,
leading to the observed contraction in the c-lattice parameters
upon calcium substitution (Table 1), but only a modest change
to the tilting distortion of the Mn−O framework. Thus, it is not
until the cation-substitution level exceeds 50%, and a significant
concentration of Ca2+ resides on the 12-coordinate A-cation
site, that a large increase in the magnitude of the MnO6 tilts,
and a change in lattice symmetry, is observed.
Figure 3. Zero-field-cooled and field-cooled magnetization data
collected from YSr2Mn2O7 and YSr0.5Ca1.5Mn2O7 in an applied field
of 100 Oe.
the analogous data collected from YSr2Mn2O7, also shown in
Figure 3. The latter phase has been characterized as a spin-
glass,25 suggesting that YSr0.5Ca1.5Mn2O7 is also a spin-glass
with a slightly elevated freezing temperature compared to
YSr2Mn2O7.
DISCUSSION
■
Phase Separation in YSr2−xCaxMn2O7. The phase
separation observed upon calcium doping of YSr2Mn2O7 is
analogous to that reported for other RESr2Mn2O7 (RE = Y, Ln)
n = 2 Ruddlesden−Popper phases.26−28 Previous studies
Crystal Structure of YSr0.5Ca1.5Mn2O7. As shown in
Figure 4, when 75% of the strontium is replaced by calcium, the
system changes symmetry to adopt an a−b−c−/b−a−c−-distorted
E
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Figure 4. Structures of YSr2‑xCaxMn2O7 phases highlighting how the A-cation distribution and the collective tilting and twisting of the MnO6
octahedra change with the calcium content. Octahedral rotation angles were determined via O−O−O−O torsion angles, as described in the
Supporting Information.
lattice, in space group P21/n11. To the best of our knowledge,
this is the first report of an n = 2 Ruddlesden−Popper phase
adopting this distortion pattern. Typically, as the tolerance
factor of a P42/mnm a−b0c0/b0a−c0-distorted n = 2 Rud-
dlesden−Popper phase is reduced, the arrangement of titled
octahedra changes to an Amam a−a−c0/a−a−c0 distortion in
which a rotation is added around the y-axis with the same
magnitude as the x-axis, as observed during the cation
substitution of LnAE2Fe2O7 phases.16 In contrast, the lattice
distortion observed for YSr0.5Ca1.5Mn2O7 maintains the large
a−b0c0/b0a−c0 distortion component observed for
YSr2−xCaxMn2O7 x < 1 and adds small a0b−c0/b−a0c0 and
a0b0c−/b0a0c− components such that the average rotation angles
are 8.74(12)°, 1.72(14)°, and 0.89(25)° for a, b, and c,
respectively. In terms of an irreducible representation (irrep)
description, the octahedral tilting in the P42/mnm a−b0c0/
b0a−c0 structures (symmetry D4−h14) is described by a distortion
which fluoride ions have been inserted into tetrahedral
coordination sites in the rock-salt layers of the framework
and also exchanged with “apical” oxide ions in the perovskite
sheets.
An unusual feature of this chemical transformation is that,
unlike many analogous fluorination reactions, it is not
accompanied by a change in crystallographic symmetry.
Specifically, fluorination of YSr2Mn2O7 does not induce a
cooperative twist of the Mn(O/F)6 units around the z-axis, a
structural deformation that is observed in a number of
analogous fluorination reactions.17−20
The tendency for fluorination to induce a “z-twist” to the
perovskite framework of n = 2 Ruddlesden−Popper oxide
phases can be rationalized by observing that insertion of
fluoride ions into the rock-salt layers of the structure leads to a
contraction of these blocks as the inserted fluoride ions pull the
A-cations closer together. This contraction acts to constrict the
ab-plane of the perovskite sheets, as indicated by a contraction
of the a- and b-lattice parameters.17,18,20 In response, the BO6
octahedra cooperatively rotate around the z-axis to relive the
bond strain induced by this ab-plane compression.
When the structures of YSr2Mn2O7 and YSr2Mn2O5.5F3.5 are
compared, it can be seen that a similar contraction has
occurred, with the area of the ab-plane declining from 29.20(1)
to 28.37(1) Å2 upon fluorination. However, this contraction
does not induce a z-twist into the perovskite framework
because concomitant oxidation of Mn3.5+ to Mn3.75+, which
occurs upon fluorination, leads to a contraction in the average
Mn−O bond length, so the bond strain induced by the ab-
plane contraction is much reduced and can be relieved by
enhancing the x-twist of the parent oxide phase. This
mechanism of strain relief is particularly effective for
YSr2Mn2O5.5F3.5 because there is a large change in the ⟨Mn−
O⟩ bond length upon oxidation (6CN radii: Mn3+, 0.645 Å;
Mn4+, 0.530 Å),35 compared to other elements, for example,
mode that transforms as X3 (a;a). Upon lowering of the
−
symmetry to P21/n11 (C2h5), this X3
mode becomes an
(a;a)
−
X3
mode, leading to a change to an a−b−c0/b−a−c0
(a;b)
−
distortion, which, upon the addition of an X1 distortion
mode, becomes an a−b−c−/b−a−c− distortion.
The reason for YSr0.5Ca1.5Mn2O7 adopting this unusual
distortion pattern is not immediately clear. However, it should
be noted that, despite the extensive cation substitution, the
phase retains a high level of cation order such that the average
radii of the cations on the 9- and 12-coordinate A-sites remain
significantly different. It seems likely that this unusual feature is
responsible for the novel octahedral tilting arrangement,
although the microscopic mechanism by which it stabilizes
this distortion is unclear.
Fluorination of YSr2Mn2O7. In common with a number of
other n = 2 Ruddlesden−Popper phases, topochemical
fluorination of YSr2Mn2O7 proceeds via a mixture of anion
insertion and anion exchange to yield YSr2Mn2O5.5F3.5, in
F
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Article
ruthenium (6CN radii: Ru4+, 0.620 Å; Ru5+, 0.565 Å),35 which
is present in many of the other oxide−fluoride systems
previously studied. Thus, fluorination of YSr2Mn2O7 to
YSr2Mn2O5.5F3.5 leads to only a modest change in the tilting
distortion of the perovskite network, with the rotation of the
MnX6 units around the x-axis increasing from 3.52(3)° to
4.58(13)°, without a change in the crystallographic symmetry
(Figure 2).
Facilities Council. W.Z. and P.S.H. thank the Welch
Foundation (Grant E-1457) and the NSF (Grant DMR-
1503573).
REFERENCES
■
(1) Goldschmidt, V. M. Naturwissenschaften 1926, 14, 477.
(2) Goodenough, J. B. Localized to Itinerant Electronic Transition in
Perovskite Oxides; Springer-Verlag: Berlin, 2001.
(3) Tilley, R. J. D. Perovskites: Structure−Property Relationships; John
CONCLUSION
■
Wiley and Sons: Chichester, U.K., 2016.
The cation-ordered structure of YSr2Mn2O7 incorporating an
a−b0c0/b0a−c0 distortion is very robust to cation substitution
and anion insertion. In order to modify the distortion mode of
the structure by calcium-for-strontium exchange, 75% of the
strontium must be exchanged to drive a change in the
crystallographic symmetry. Principally, this is because the small
Ca2+ cations are substituted preferentially onto the 9-coordinate
A-site in the structure, which has only a small influence on the
collective tilting distortions of the perovskite layers. Thus, it
could be argued that the A-cation order of the phase “protects”
the a−b0c0/b0a−c0-distorted lattice from modification by cation
exchange, which further suggests that, in general, it will be hard
to utilize cation doping of cation-ordered n = 2 Ruddlesden−
Popper phases to induce noncentrosymmetric lattice dis-
tortions.
YSr2Mn2O7 also undergoes only minimal structural deforma-
tion upon fluorine insertion because of the large contraction of
the ⟨Mn−O⟩ bond lengths, which compensates for the lattice
contraction that occurs when fluoride ions are intercalated into
the rock-salt layers of the structure. These two effects mean that
the Y−Sr−Ca−Mn−O−F system retains a−b0c0/b0a−c0-dis-
torted perovskite blocks over a wide range of composition.
(4) Benedek, N. A.; Rondinelli, J. M.; Djani, H.; Ghosez, P.;
Lightfoot, P. Understanding Ferroelectricity in Layered Perovskites:
New Ideas and Insights From Theory and Experiments. Dalton Trans.
2015, 44, 10543−10558.
(5) Rondinelli, J. M.; Fennie, C. J. Octahedral Rotation-Induced
Ferroelectricity in Cation Ordered Perovskites. Adv. Mater. 2012, 24,
1961−1968.
(6) Knapp, M. C.; Woodward, P. M. A-site Cation Ordering in
AA′BB′O6 Perovskites. J. Solid State Chem. 2006, 179, 1076−1085.
(7) King, G.; Woodward, P. M. Cation Ordering in Perovskites. J.
Mater. Chem. 2010, 20, 5785−5796.
(8) Bousquet, E.; Dawber, M.; Stucki, N.; Lichtensteiger, C.; Hermet,
P.; Gariglio, S.; Triscone, J. M.; Ghosez, P. Improper Ferroelectricity in
Perovskite Oxide Artificial Superlattices. Nature 2008, 452, 732−U4.
(9) Benedek, N. A.; Fennie, C. J. Hybrid Improper Ferroelectricity: A
Mechanism for Controllable Polarization-Magnetization Coupling.
Phys. Rev. Lett. 2011, 106, 107204.
(10) Mulder, A. T.; Benedek, N. A.; Rondinelli, J. M.; Fennie, C. J.
Turning ABO3 Antiferroelectrics into Ferroelectrics: Design Rules for
Practical Rotation-Driven Ferroelectricity in Double Perovskites and
A3B2O7 Ruddlesden-Popper Compounds. Adv. Funct. Mater. 2013, 23,
4810−4820.
(11) Oh, Y. S.; Luo, X.; Huang, F. T.; Wang, Y. Z.; Cheong, S. W.
Experimental demonstration of hybrid improper ferroelectricity and
the presence of abundant charged walls in (Ca, Sr)3Ti2O7 crystals. Nat.
Mater. 2015, 14, 407−413.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01525.
■
S
(12) Cohen, R. E. Origin of Ferroelectricity in Perovskite Oxides.
Nature 1992, 358, 136−138.
(13) Kang, S. K.; Tang, H.; Albright, T. A. Structures for d0 Ml6 and
Ml5 Complexes. J. Am. Chem. Soc. 1993, 115, 1971−1981.
(14) Kunz, M.; Brown, I. D. Out-of-Center Distortions around
Octahedrally Coordinated d0 Transition-Metals. J. Solid State Chem.
1995, 115, 395−406.
Structural parameters, selected bond lengths and
observed and calculated data from the structural
refinements of YSrCaMn2O7, YSr0.5Ca1.5Mn2O7, and
YSr2Mn2O3.5F5.5, and a description of the method used
for determining octahedral rotation angles from O−O−
O−O torsion angles (PDF)
(15) Pearson, R. G. The Second-Order Jahn-Teller Effect. J. Mol.
Struct.: THEOCHEM 1983, 103, 25−34.
(16) Pitcher, M. J.; Mandal, P.; Dyer, M. S.; Alaria, J.; Borisov, P.;
Niu, H.; Claridge, J. B.; Rosseinsky, M. J. Tilt Engineering of
Spontaneous Polarization and Magnetization Above 300 K in a Bulk
Layered Perovskite. Science 2015, 347, 420−424.
(17) Li, R. K.; Greaves, C. Double-Layered Ruthenate Sr3Ru2O7F2
Formed by Fluorine Insertion into Sr3Ru2O7. Phys. Rev. B: Condens.
Matter Mater. Phys. 2000, 62, 3811−3815.
(18) Romero, F. D.; Bingham, P. A.; Forder, S. D.; Hayward, M. A.
Topochemical Flulorination of Sr3(M0.5Ru0.5)2O7 (M = Ti, Mn, Fe) n
= 2 Ruddlesden-Popper Phases. Inorg. Chem. 2013, 52, 3388−3398.
(19) Zhang, R.; Senn, M. S.; Hayward, M. A. Directed Lifting of
Inversion Symmetry in Ruddlesden-Popper Oxide-Fluorides: Toward
Ferroelectric and Multiferroic Behavior. Chem. Mater. 2016, 28, 8399−
8406.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michael.hayward@chem.ox.ac.uk. Tel: +44 1865
272623. Fax: +44 1865 272690.
ORCID
P. Shiv Halasyamani: 0000-0003-1787-1040
Michael A. Hayward: 0000-0002-6248-2063
Author Contributions
■
All authors have given approval to the final version of the
(20) Zhang, R.; Read, G.; Lang, F.; Lancaster, T.; Blundell, S. J.;
Hayward, M. A. La2SrCr2O7F2 - A Ruddlesden-Popper Oxyfluoride
Containing Octahedrally Coordinated Cr4+ Centers. Inorg. Chem.
2016, 55, 3169−3174.
manuscript.
Notes
The authors declare no competing financial interest.
(21) Luo, K.; Tran, T. T.; Halasyamani, P. S.; Hayward, M. A.
Synthesis and Selective Topochemical Fluorination of the Cation and
Anion-Vacancy Ordered Phases Ba2YCoO5 and Ba3YCo2O7.5. Inorg.
Chem. 2013, 52, 13762−13769.
ACKNOWLEDGMENTS
Experiments at the ISIS pulsed neutron facility were supported
by a beam time allocation from the Science and Technology
■
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DOI: 10.1021/acs.inorgchem.7b01525
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(22) Larson, A. C.; Von Dreele, R. B. General Structure Analysis
System; Los Alamos National Laboratory: Los Alamos, NM, 2000;
Report LAUR 86-748.
(23) Kurtz, S. K.; Perry, T. T. A Powder Technique for the
Evaluation of Nonlinear Optical Material. J. Appl. Phys. 1968, 39,
3798−3813.
(24) Ok, K. M.; Chi, E. O.; Halasyamani, P. S. Bulk characterization
methods for non-centrosymmetric materials: second-harmonic gen-
eration, piezoelectricity, pyroelectricity, and ferroelectricity. Chem. Soc.
Rev. 2006, 35, 710−717.
(25) Battle, P. D.; Millburn, J. E.; Rosseinsky, M. J.; Spring, L. E.;
Vente, J. F.; Radaelli, P. G. Neutron Diffraction study of the structural
and electronic propoerties of Sr2HoMn2O7 and Sr2YMn2O7. Chem.
Mater. 1997, 9, 3136−3143.
(26) Battle, P. D.; Green, M. A.; Laskey, N. S.; Millburn, J. E.;
Murphy, L.; Rosseinsky, M. J.; Sullivan, S. P.; Vente, J. F. Layered
Ruddlesden-Popper manganese oxides:synthesis and cation ordering.
Chem. Mater. 1997, 9, 552−559.
(27) Battle, P. D.; Cox, D. E.; Green, M. A.; Millburn, J. E.; Spring, L.
E.; Radaelli, P. G.; Rosseinsky, M. J.; Vente, J. F. Antiferromagnetism,
ferromagnetism, and phase separation in the GMR system
Sr2‑xLa1+xMn2O7. Chem. Mater. 1997, 9, 1042−1049.
(28) Battle, P. D.; Green, M. A.; Laskey, N. S.; Millburn, J. E.;
Radaelli, P. G.; Rosseinsky, M. J.; Sullivan, S. P.; Vente, J. F. Crystal
and magnetic structures of the colossal magnetoresistance manganates
Sr2‑xNd1+xMn2O7 (x = 0.0, 0.1). Phys. Rev. B: Condens. Matter Mater.
Phys. 1996, 54, 15967−15977.
(29) Sears, V. F. Neutron Scattering Lengths and Cross Sections.
Neutron News 1992, 3, 26−37.
(30) Aleksandrov, K. S.; Bartolome, J. Octahedral Tilt Phases in
Perovskite-like Crystals with Slabs Containing an Even Number of
Octahedral Layers. J. Phys.: Condens. Matter 1994, 6, 8219−8235.
(31) Gurusinghe, N. N. M.; Fones, J. C.; Marco, J. F.; Berry, F. J.;
Greaves, C. Fluorine Insertion into the Ruddlesden-Popper Phase
La2BaFe2O7: The Structure and Magnetic Properties of La2BaFe2O5F4.
Dalton Trans. 2014, 43, 2038−2043.
(32) Case, G. S.; Hector, A. L.; Levason, W.; Needs, R. L.; Thomas,
M. F.; Weller, M. T. Syntheses, powder neutron diffraction structures
and Mossbauer studies of some complex iron oxyfluorides:
Sr3Fe2O6F0.87, Sr2FeO3F and Ba2InFeO5F0.68. J. Mater. Chem. 1999,
9, 2821−2827.
(33) Brown, I. D.; Altermatt, D. Bond-Valence Parameters Obtained
from a Systematic Analysis of the Inorganic Crystal Structure
Database. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247.
(34) Brese, N. E.; O’Keeffe, M. Bond-Valence Parameters for Solids.
Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197.
(35) Shannon, R. D. Revised effective ionic radii. Acta Crystallogr.,
Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767.
H
DOI: 10.1021/acs.inorgchem.7b01525
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