Topotactic reduction as a synthetic route for the preparation of low-dimensional Mn(II) oxide phases: The structure and magnetism of LaAMnO 4-x (A = Sr, Ba)

Article abstract of DOI:10.1039/b923966a

Reaction of LaSrMnO₄ with CaH₂ at 420 °C yields LaSrMnO_3.67(3). Raising the temperature to 480 °C yields the Mn(II) phase LaSrMnO_3.50(2). Neutron powder diffraction data show both phases adopt body-centred orthorhombic crystal structures (LaSrMnO _3.67(3), Immm: a = 3.7256(1) A, b = 3.8227(1) A, c = 13.3617(4) A; LaSrMnO_3.50(2), Immm: a = 3.7810(1) A, b = 3.7936(1) A, c = 13.3974(3) A) with anion vacancies located within the equatorial MnO_2-x planes of the materials. Analogous reactivity is observed between LaBaMnO₄ and CaH₂ to yield body-centred tetragonal reduced phases (LaBaMnO_3.53(3), I4/mmm: a = 3.8872(1)A, c = 13.6438(2) A). Low-temperature neutron diffraction and magnetisation data show that LaSrMnO_3.5 and LaBaMnO_3.5 exhibit three-dimensional antiferromagnetic order below 155 K and 135 K respectively. Above these temperatures, they exhibit two-dimensional antiferromagnetic order with paramagnetic behaviour observed above 480 K in both phases. The origin of the low dimensional magnetic order and ordering of the anion vacancies in the reduced phases is discussed.

Full text of DOI:10.1039/b923966a

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www.rsc.org/dalton | Dalton Transactions
Topotactic reduction as a synthetic route for the preparation of
low-dimensional Mn(II) oxide phases: The structure and magnetism of
LaAMnO4-x (A = Sr, Ba)†
Helen J. Kitchen, Ian Saratovsky and Michael A. Hayward*
Received 20th November 2009, Accepted 30th January 2010
First published as an Advance Article on the web 16th March 2010
DOI: 10.1039/b923966a
Reaction of LaSrMnO4 with CaH2 at 420 ◦C yields LaSrMnO3.67(3). Raising the temperature to 480 ◦C
yields the Mn(II) phase LaSrMnO3.50(2). Neutron powder diffraction data show both phases adopt
˚
body-centred orthorhombic crystal structures (LaSrMnO3.67(3), Immm: a = 3.7256(1) A, b =
˚
˚
˚
˚
3.8227(1) A, c = 13.3617(4) A; LaSrMnO3.50(2), Immm: a = 3.7810(1) A, b = 3.7936(1) A, c =
˚
13.3974(3) A) with anion vacancies located within the equatorial MnO2-x planes of the materials.
Analogous reactivity is observed between LaBaMnO4 and CaH2 to yield body-centred tetragonal
˚
˚
reduced phases (LaBaMnO3.53(3), I4/mmm: a = 3.8872(1)A, c = 13.6438(2) A). Low-temperature
neutron diffraction and magnetisation data show that LaSrMnO3.5 and LaBaMnO3.5 exhibit
three-dimensional antiferromagnetic order below 155 K and 135 K respectively. Above these
temperatures, they exhibit two-dimensional antiferromagnetic order with paramagnetic behaviour
observed above 480 K in both phases. The origin of the low dimensional magnetic order and ordering
of the anion vacancies in the reduced phases is discussed.
series in which ABO3 perovskite type sheets are stacked with AO
Introduction
rock-salt layers to generate phase of composition (AO)(ABO3)n as
shown in Fig. 1. Anion deficient manganates based on the K2NiF4
structure have been reported previously, notably the MnIII phases
Ca2MnO3.5 and Sr2MnO3.5.12,13 We sought to investigate the effect
of introducing significant concentrations of MnII centres on the
structural and physical properties of low dimensional phases of
this type.
Complex oxides containing mixed valent MnIII/IV centres have
received considerable attention due to the large magnetoresistive
ratios observed in these phases.1-3 Interest has been sustained by
the wider observation of strong coupling between spin, charge
and lattice degrees of freedom, driven by the strong competition
between forces which tend to localize the manganese valence
electrons and those which favour delocalized metallic behaviour.4,5
The majority of previous studies of complex manganates have
examined the effect of tuning the manganese oxidation state
and other crystallographic parameters by cation doping. This
has largely restricted investigations to phases with manganese
oxidation states between +3 and +4. Doping phases by the
introduction of large numbers of oxide ion vacancies into their
structures offers the opportunity to study materials with much
lower average manganese oxidation states. Previous work uti-
lizing hydrogen or metal getters as reducing agents suggests
these chemical approaches are limited to manganese oxidation
states of Mn ≥ +3.6-9 Recently however, we have utilized binary
metal hydrides as solid-state reducing agents to prepare phases
containing significant concentrations of MnII centres.10,11
Fig. 1 a) Layered K2NiF4 structure, b) body-centred orthorhombic struc-
tural model of LaSrMnO4-x, c) antiferromagnetic structure of LaAMnO3.5
phases.
Here we report the preparation of mixed valent MnII/III phases
by the reduction of the MnIII phases LaSrMnO4 and LaBaMnO4
which adopt the layered K2NiF4 structure. The K2NiF4 structure
is the n = 1 member of the extended Ruddlesden-Popper structural
Experimental
Department of Chemistry, Inorganic Chemistry Laboratory, University
of Oxford, South Parks Road, Oxford, OX1 3QR. E-mail: michael.
hayward@chem.ox.ac.uk; Fax: +44 1865 272690; Tel: +44 1865 272623
† Electronic supplementary information (ESI) available: Observed, calcu-
lated and difference plots from the structural and magnetic refinements
against neutron powder diffraction data of LaSrMnO3.67, LaSrMnO3.5 and
LaBaMnO3.5. See DOI: 10.1039/b923966a
Synthesis of LaSrMnO4
Samples of LaSrMnO4 were prepared via a citrate gel method.
Suitable quantities of La2O3 (99.999%, dried at 900 ◦C), SrCO3
(99.994%) and MnO2 (99.999%) were dissolved in a minimum
quantity of a 1 : 1 mixture of 6M nitric acid and distilled water.
6098 | Dalton Trans., 2010, 39, 6098–6105
This journal is
The Royal Society of Chemistry 2010
©

213 mole equivalents of citric acid and 5 ml of ethylene glycol
were added and the solution was heated with constant stirring
to encourage polymerization. The gel thus formed was ground
into a fine powder, placed in an alumina crucible and heated at
was observed by X-ray powder diffraction. Due to the hazards
associated with the production of hydrogen when using CaH2
as a reducing agent, large samples (~4 g) suitable for neutron
diffraction measurements were prepared using a vented reaction
apparatus described previously.16 Specifically, sample SR1 was
prep◦ared by heating a 2 : 1 molar ratio of LaSrMnO4 and CaH2 at
420 C for three periods of 2 days with intermediate regrinding.
Sample SR2 was prepared by heating a 2 : 1 molar ratio of
LaSrMnO4 and CaH2 at 480 ◦C for five periods of 2 days. Sample
BA1 was prepared by heating a 2 : 1 molar ratio of LaSrMnO4
and CaH2 at 475 ◦C for three periods of 2 days. After reaction
samples were washed under nitrogen with 4 ¥ 100 ml of a 0.25M
solution of NH4Cl in methanol to remove the calcium containing
phases (CaH2, CaO). Samples were then washed with 4 ¥ 80 ml
of clean methanol to remove any remaining NH4Cl before being
dried under vacuum.
1
◦C min-1 to 1000 ◦C in air. The resulting black powder was
pressed into 13 mm pellets and heated at 1300 ◦C under a flow of
2% hydrogen in nitrogen for 2 periods of 2 days with intermediate
regrinding. X-ray powder diffraction data confirmed samples were
a single phase and had lattice parameters in good agreement with
literature values.14
Synthesis of LaBaMnO4
LaBaMnO4 was prepared in a manner similar to that described
previously by Bieringer et al.14 Suitable quantities of La2O3
(99.999%, dried at 900 ◦C), BaCO3 (99.997%) and Mn2O3
(prepared by heating MnO2 at 650 ◦C under dynamic vacuum)
were ground together in an argon filled glove box ◦(O2 and
H2O < 1 ppm). The mixture was then calcined at 900 C under
flowing argon for 12 h before being pressed into 13 mm pellets
under 5 tonnes force. The pellets were heated for two periods
of 30 h at 1350 ◦C under argon, with intermediate regrinding.
X-ray powder diffraction data confirmed samples were a single
phase and had lattice parameters in good agreement with literature
values.14
Characterization
X-ray powder diffraction data were collected using a PANalytical
X’Pert diffractometer incorporating an X’celerator position sen-
sitive detector (monochromatic Cu Ka1 radiation). Data were col-
lected from air sensitive samples under an inert atmosphere using
a homemade gas-tight sample holder. Neutron powder diffraction
data were collected from samples contained in vanadium cans
sealed with an indium washer under an argon atmosphere. Data
˚
were collected using the D2b diffractometer (l = 1.59 A) at the
Reduction of LaSrMnO4 and LaBaMnO4
ILL neutron source, Grenoble and the POLARIS diffractometer
at the ISIS neutron source, UK. Rietveld profile refinements
were performed using the GSAS suite of programs.17 Electron
diffraction patterns were collected from samples supported on
lacy carbon grids (deposited from suspension in methanol) using
a JEOL 2000FX microscope operating at 200 KV. Average
manganese oxidation states in all phases were determined by
iodometric titration. Samples were dissolved in HCl containing
an excess of KI and the liberated I2 was titrated with Na2S2O3.
Magnetization data were collected at temperatures in the range
5 < T/K < 300 from powdered samples contained in gelatin
capsules using a Quantum Design MPMS SQUID magnetometer.
In addition, magnetization data were also collected from samples
in the temperature range 300 < T/K < 800 from samples sealed
Calcium hydride has been used previously as a solid-state reducing
agent to bring about the low temperature topotactic deintercala-
tion of oxide ions from complex oxides.15 It acts via the reaction:
ABOn + CaH2 → ABOn-x + xCaO + x/2 H2, where ABOn is a
complex oxide as described previously.16 In order to investigate
the reactivity of CaH2 with LaSrMnO4 and LaBaMnO4 as a
function of temperature, small scale reactions (~300 mg) were
performed with 2 : 1 molar ratios of CaH2 : LaAMnO4 (A =
Ba, Sr), which had been ground together in an argon filled
glovebox (O2 and H2O < 1 ppm) and then sealed under vacuum
in silica ampoules. Samples were heated for multiple periods
at temperatures between 200 ◦C and 600 ◦C as described in
Table 1, with intermediate regrinding, until no further change
Table 1 Products of the reaction between LaSrMnO4, LaBaMnO4 and CaH2 as a function of temperature under vacuum
Reaction
Temperature
Result
Composition
LaSrMnO4
LaSrMnO4 + CaH2
T ≤ 375 ◦
C
No reaction
˚
˚
a = 3.80 A, c = 13.15 A
375 ≤ T/◦C ≤ 450
Orthorhombic phase 1
LaSrMnO3.78(4) to
LaSrMnO3.64(4)
˚
˚
a = 3.74 A, b = 3.80 A,
˚
˚
c = 13.29 A to a = 3.71 A,
˚
˚
b = 3.83 A, c = 13.37 A
450 ≤ T/◦C ≤ 550
Orthorhombic phase 2
LaSrMnO3.51(2)
LaBaMnO4
˚
˚
˚
a ª 3.78 A, b ª 3.79 A, c ª 13.39 A
No reaction
LaBaMnO4 + CaH2
T ≤ 250 ◦
C
˚
˚
a = 3.90 A, c = 13.25 A
275 ≤ T/◦C ≤ 350
375 ≤ T/◦C ≤ 500
Tetragonal Phase 1
LaBaMnO3.67(3)
LaBaMnO3.53(3)
˚
˚
a = 3.90 A, c = 13.52 A
Tetragonal Phase 2
˚
˚
a = 3.88 A, c = 13.64 A
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6098–6105 | 6099
©

Table 2 Structure of LaSrMnO3.67(3) (sample SR1) refined from room
The reactivity of LaBaMnO4 with CaH2 follows a similar
pattern to that of LaSrMnO4 as shown in Table 1. It should be
noted however that the products of the reduction of LaBaMnO4
are all tetragonal and the reduction reactions were much more
rapid and reproducible, yielding single phase samples readily. In
common with the strontium analogue reaction of LaBaMnO4
temperature neutron powder diffraction data
Atom
x
y
z
Fraction
Uiso
La/Sr
Mn
O(1)
O(2)
O(3)
0
0
0
0.5
0
0
0
0.3550(1)
0
0.1731(1)
0
0
0.5/0.5
1
1
0.354(4)
0.5
0.00134(1)
0.0015(1)
0.00238(2)
0.00108(2)
0.00108(2)
◦
with CaH2 at temperatures above 500 C lead to decomposition
resulting in complex mixtures of La2O3, BaO, MnO and Mn.
0.028(2)
0.5
0.0783(8)
Space Group Immm
˚
˚
˚
a = 3.7256(1)A, b = 3.8227(1)A, c = 13.3617(4)A
2
Characterization of LaSrMnO3.67(3)
c = 3.64, wRp = 5.10%, Rp = 3.81%
Refined Stoichiometry : LaSrMnO3.708(8)
Titrated Stoichiometry : LaSrMnO3.67(3)
–
Neutron powder diffraction data collected at room temperature
(D2b) from a sample of LaSrMnO4-x (sample SR1), prepared
◦
by the reaction of LaSrMnO4 and CaH2 at 420 C as described
above, could be readily indexed on the basis of an orthorhombic
in within Pyrex tubes, utilizing a high temperature insert to the
magnetometer.
˚
˚
body-centred unit cell (a = 3.7256(1) A, b = 3.8227(1) A,
˚
c = 13.3617(4) A). Iodometric titrations indicated an average
manganese oxidation state of +2.34(6) for this phase, consistent
with an overall stoichiometry of LaSrMnO3.67(3). A structural
model based on an orthorhombically distorted K2NiF4 type
structure as shown in Fig. 1 (space group Immm) was refined
against this data. Atomic positions and displacement parameters
were refined for all atoms. It was observed that the displacement
parameters refined for the two “equatorial” anion sites, O(2)
and O(3), refined to much larger values than the “apical” O(1)
site. This was taken as an indication of a disordered twisting of
the manganese-oxygen coordination polyhedra around the z-axis,
commonly observed in topotactically reduced Ruddlesden-Popper
phases.10,18 To model this feature of the structure more effectively,
the O(2) and O(3) anion positions were displaced from the high
symmetry 2b(12 , 0, 0) and 2d(0, 12 , 0) sites and disordered over the
lower symmetry 4h(12 , y, 0) and 4f (x, 12 , 0) positions respectively.
The occupancy of all the oxygen sites was refined to account for
the lower oxygen stoichiometry of the phase. The occupancies of
the O(1) and O(3) sites were observed to refine to values consistent
with full occupancy, within error, and so were fixed at these values.
The oxide ion vacancies are therefore located exclusively on the
O(2) anion site within the model. The refinement converged readily
to give a good fit to the data. Full details of the refined structural
model are given in Table 2, with selected bond lengths in Table 3.
Magnetization data collected from LaSrMnO3.67(3) are shown in
Fig. 2. Data in the temperature range 100 < T/K < 300 can be
fitted to the Curie–Weiss law (c = C/(T-q)) yielding values of C =
3.36(1) cm3 K mol-1 and q = -258(2)K. This corresponds to a
meff = 5.18 mB intermediate between the spin only values expected
for MnIII (s = 2, mexp = 4.89 mB) and MnII (s = 5/2, mexp = 5.91 mB),
Results
Reactivity of LaSrMnO4 and LaBaMnO4 with CaH2
The products of the reaction between LaSrMnO4 and CaH2 evolve
as a simple function of temperature as detailed in Table 1. At
reaction temperatures below 375 C there is no observable reac-
tion, as indicated by X-ray powder diffraction. In the intermediate
temperature range 375 < T/◦C < 450, tetragonal LaSrMnO4 is
readily reduced to a series of body-centred orthorhombic phases
◦
˚
˚
with lattice parameters varying in the range a = 3.80 A, b = 3.74 A,
˚
˚
˚
˚
c = 13.29 A to a = 3.83 A, b = 3.71 A, c = 13.37 A consistent with
a topotactic reduction process. Iodometric titration measurements
indicate overall product compositions varying from LaSrMnO3.78(4)
to LaSrMnO3.64(4) with degree of reduction achieved depending on
the exact reaction temperature and duration employed. On raising
the reaction temperature into the range 450 < T/◦C < 550, a
further reduction occurs to form a phase which appears almost
˚
˚
˚
tetragonal (a = 3.78 A, b = 3.79 A, c = 13.39 A). Close inspection
of X-ray diffraction data collected from materials prepared in
this temperature range revealed multiple heating cycles (≥5) were
required to achieve complete reaction, with multiple phase samples
resulting if this synthesis procedure was not followed. Iodometric
titration of samples prepared in this temperature range yields
average manganese oxidation states in the range +2 to +2.05,
consistent with an overall composition of LaSrMnO3.5+d. On
raising the reaction temperature above 550 ◦C, decomposition
occurs resulting in a complex mixture of La2O3, SrO, Mn, MnO
and other phases.
˚
Table 3 Selected bond lengths (A) and angles (degrees) from the refined structures of LaAMnO4-x phases
LaSrMnO3.67 (SR1)
LaSrMnO3.5+d (SR2)
LaBaMnO3.5+d (BA1)
Mn
O(1)
O(2)
O(3)
2.313(1)
1.866(1)
1.933(1)
2.314(1)
1.901(1)
1.924(1)
2.323(1)
1.964(1)
—
La/A
O(1) ¥ 1
O(1) ¥ 4
O(2)
2.430(2)
2.695(1)
2.647(5)/2.798(6)
2.494(2)/2.898(2)
173.4(1)
2.433(2)
2.702(1)
2.588(3)/2.863(3)
2.504(3)/2.950(3)
168.1(1)
2.513(1)
2.769(1)
2.588(1)/2.984(1)
—
163.4(1)
—
O(3)
Mn-O(2)-Mn
Mn-O(3)-Mn
162.6(1)
160.7(1)
6100 | Dalton Trans., 2010, 39, 6098–6105
This journal is
The Royal Society of Chemistry 2010
©

Table 4 Structure of LaSrMnO3.50(2) (sample SR2) refined from room
temperature neutron powder diffraction data
3
˚
Atom
x
y
z
Fraction
Uiso (A )
La/Sr
Mn
O(1)
O(2)
O(3)
0
0
0
0.5
0
0
0
0.3543(1)
0
0.1727(1)
0
0
0.5/0.5
1
1
0.339(4)
0.415(5)
0.0093(1)
0.0088(1)
0.0181(1)
0.0065(3)
0.0065(3)
0.052(1)
0.5
0.085(1)
Space Group Immm
˚
˚
˚
a = 3.7810(1) A, b = 3.7936(1) A, c = 13.3974(3) A
2
c = 1.57, wRp = 4.43%, Rp = 5.63%
Refined Stoichiometry : LaSrMnO3.50(2)
Titrated Stoichiometry : LaSrMnO3.52(3)
–
Fig. 2 Zero-field cooled and field cooled magnetisation data collected
from LaSrMnO3.67(3) (SR1) as a function of temperature. Inset shows
magnetisation isotherm collected at 6 K.
although it should be noted that the large value of the Weiss
constant with respect to the fitted range means these numbers
should be treated with caution. The zero-field cooled and field
cooled data diverge at T~75 K with an additional distinct inflection
point in both data sets at T~60 K. Magnetization-field isotherms
collected at 6 K (inset to Fig. 2) are clearly displaced from the
origin indicating glassy behaviour. Due to a technical problem it
was not possible to collect low temperature neutron diffraction
data from LaSrMnO3.67(3) to investigate if this glassy behaviour
was accompanied by any long range magnetic order.
Structural characterization of LaAMnO3.5+d (A = Sr, Ba)
Neutron powder diffraction data collected at 298 K (POLARIS)
from a sample of LaSrMnO3.5+d (sample SR2) could be readily
indexed using a body-centred orthorhombic unit cell (a =
Fig. 3 Electron diffraction data collected from a) the [001] and b) the [010]
zones of LaSrMnO3.5; c) the [001] and d) the [010] zones of LaBaMnO3.5
.
˚
˚
˚
3.7810(1) A, b = 3.7936(1) A, c = 13.3974(4) A). Electron
diffraction data were collected to confirm this cell and showed no
evidence for any cell expansion (Fig. 3). A structural model, anal-
ogous to that used to describe LaSrMnO3.67, was refined against
these data. The positional and displacement parameters of all the
atoms in the structural model were refined and in common with the
structural refinement of sample SR1, a description of the MnO2-x
equatorial plane with a disordered displacement of the O(2) and
O(3) anion sites was adopted. To locate the anion vacancies within
the structure, the occupancy factors of all the anion sites were
refined and it was observed that the occupancy of the O(1) axial
anion site refined to full occupancy, within error, and was therefore
fixed at this value, localizing the anion vacancies in the equatorial
MnO2-x plane on sites O(2) and O(3). The refinement converged
observable in the data which were not accounted for by this cell. In
order to determine if these additional features were due to a unit
cell expansion, electron diffraction data were collected from the
sample. As shown in Fig. 3, all the data collected could be indexed
using the body-centred tetragonal cell detailed above, indicating
the additional diffraction features observed in the neutron diffrac-
tion data were due to impurities. Further analysis of the neutron
diffraction data revealed the additional features were consistent
with the presence of a cubic perovskite impurity (Pm-3m, a =
4.003(2) A) and La2O3.19 A three phase model was therefore refined
˚
against the diffraction data. The bulk phase model was based on
the body-centred tetragonal structure of LaBaMnO4. Refinement
of the oxygen occupation factors revealed anion vacancies were
only present in the equatorial anion sites, in common with
the LaSrMnO4-x phases. In addition the refined displacement
parameters of the equatorial anion site were significantly larger
than the axial anion site, suggestive of static disorder, so the O(2)
site was displaced from the high symmetry 4c(0, 12 , 0) and site
and disordered over the lower symmetry 8j(x, 12 , 0) position in
common with the LaSrMnO4-x phases. The low concentration of
2
readily to give a good fit (c = 1.57). Full details of the final refined
structure are given in Table 4, with selected bond lengths in Table 3.
Neutron powder diffraction data collected at 298 K (POLARIS)
from LaBaMnO3.5+x (Sample BA1) could be indexed using a body-
˚
˚
centred tetragonal cell (a = 3.8872(1) A, c = 13.6438(2) A). There
were however a series of additional weak diffraction features
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6098–6105 | 6101
©

Table 5 Structure data from the refinement of LaBaMnO3.50(2) (sample
BA1) against room temperature neutron powder diffraction data
3
˚
Atom
x
y
z
Fraction
Uiso (A )
La/Ba
Mn
O(1)
O(2)
0
0
0
0.5
0
0
0
0.35448(4)
0
0.17027(5)
0
0.5/0.5
1
1
0.0083(1)
0.0065(1)
0.0407(1)
0.0123(2)
˚
0.0729(2)
0.382(2)
˚
LaBaMnO3.5+x, Space Group I4/mmm, a = 3.8872(1) A, c = 13.6438(2) A
Phase fraction = 91.5(5) mole%
Refined Stoichiometry : LaBaMnO3.50(2)
Titrated Stoichiometry : LaBaMnO3.52(1)
–
3
˚
Atom
La/Ba
Mn
x
0
0.5
0.5
y
0
0.5
0.5
z
0
0.5
0
Fraction
0.5/0.5
1
1
˚
Uiso (A )
0.02
0.02
0.02
O
La0.5Ba0.5MnO3-x, Space Group Pm-3m, a = 4.003(2) A
Phase Fraction = 4.2(5) mole%
˚
˚
La2O3, Space Group P-3m, a = 3.937(1) A, c = 6.172(1) A
Phase Fraction = 4.3(5) mole%
2
c = 1.66, wRp = 3.53%, Rp = 5.55%
impurity phases in the sample prevented their detailed structural
characterisation. The perovskite impurity phase was modelled as
La0.5Ba0.5MnO3-x consistent with the observed large cubic unit cell.
The only structural parameters refined for the perovskite impurity
phase and the phase modelling La2O3 were the lattice parameters.
2
The 3-phase refinement converged readily (c = 1.66). Complete
structural details are given in Table 5.
The good agreement between the oxygen stoichiometries refined
from the neutron diffraction data for both LaSrMnO3.5 and
LaBaMnO3.5 (Tables 4 and 5) and that determined by chemical
titration confirms that samples SR2 and BA1 are simple anion
deficient oxide phases rather than oxide-hydride phases such as
those observed during the reaction of LnACoO4 phases and
CaH2.15,20
Fig. 4 Zero-field cooled and field cooled magnetisation data collected
from LaSrMnO3.5 (SR2) as a function of temperature (Top). Ordered
magnetic moment extracted from neutron diffraction data as a function of
temperature (Bottom).
to room temperature data. These features could be indexed by
˚
˚
a tetragonal cell (a ª 5.48 A, c ª 13.61 A) directly analogous
to the magnetic cell of LaSrMnO3.5. Therefore a multiple phase
refinement employing a structural model (based on the room
temperature structure of LaBaMnO3.5), a magnetic model (based
on the magnetic structure of LaSrMnO3.5) and models to account
for the La2O3 and perovskite impurity was performed against the
data, which converged readily to yield an ordered moment of
3.58(5)mB per manganese at 5 K. Analogous models were refined
against neutron diffraction data collected in the temperature range
5 < T/K < 140. Fig. 5 shows a plot of the refined ordered moment
as a function of temperature, indicating a magnetic ordering
temperature TN ~ 135 K.
Magnetic characterisation of LaAMnO3.5+x (A = Sr, Ba)
Low-temperature neutron powder diffraction data (POLARIS)
collected from LaSrMnO3.5 (sample SR2) exhibit a series of
additional, large d-spacing diffraction reflections relative to anal-
ogous data collected at room temperature. These extra diffraction
reflections can be indexed on the basis of a◦monoclinic unit cell
˚
˚
(a ª 5.33 A, b ª 5.33 A, c ª 13.37, g = 90.2 ) which represents a
doubling of the area of the ab plane relative to the orthorhombic
cell required to index the data collected at room temperature.
The additional diffraction features were taken to be magnetic in
origin so the low temperature diffraction data were fitted with a
combination of an orthorhombic structural model (as described
for the room temperature data) and a monoclinic magnetic model.
It was found that the additional magnetic diffraction intensity
could be accounted for using a simple G-type antiferromagnetic
model with moments aligned parallel to the z-axis (Fig. 1). The
refinement converged readily to yield an ordered moment of
3.15(3)mB per manganese at 5 K. Analogous models were refined
against neutron diffraction data collected in the temperature range
5 < T/K < 160. Fig. 4 shows a plot of the refined ordered moment
as a function of temperature, indicating a magnetic ordering
temperature TN ~ 155 K.
Zero-field cooled and field cooled magnetisation data collected
from LaSrMnO3.5 and LaBaMnO3.5 are shown in Fig. 4 and 5
respectively. Curie–Weiss fits (c = C/T-q) to the zero-field cooled
data yield values of C = 0.31(3) cm3 K mol-1 for LaSrMnO3.5
and 0.12(2) cm3 K mol-1 for LaBaMnO3.5 compared to a value
of C = 4.37 cm3 K mol-1 expected for a simple MnII, s = 5/2
paramagnetic system. In addition it can be clearly seen that neither
data set show any feature at the transition temperature indicated
by the respective neutron diffraction data (Fig. 4 and 5). In order
to investigate the magnetic behaviour further, high-temperature
magnetisation data (300 < T/K < 700) were collected. These
data, shown in Fig. 6, reveal a divergence between field cooled
and zero-field cooled data, or a significant change in gradient at
Neutron powder diffraction data collected from LaBaMnO3.5
(sample BA1) also exhibit additional diffraction features relative
6102 | Dalton Trans., 2010, 39, 6098–6105
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©

Table 6 Bond valence sum contributions calculated for the anion sites in
LaSrMnO4
O(1) axial
O(2) equatorial
La/Sr BVS
Mn BVS
1.37
0.25
0.94
1.40
Mn BVS–La/Sr BVS
-1.12
+0.46
type structure. This localization of anion vacancies onto the
equatorial anion sites is not unique to LaSrMnO4-x phases and
has also been observed in a number of related layered anion-
deficient manganates such as Ca2MnO3.5, Sr2MnO3.5, Sr3Mn2O6
10,12,13,21
and YSr2Mn2O5.5-d
.
The observed preference for localizing anion vacancies on the
equatorial anion sites within these phases can be rationalized
by considering the different coordination requirements of the
various metals in the system, as has been described previously
for other topotactically reduced phases.10 Following the logic
described previously it is clear that in order to best satisfy
the competing coordination requirements of La/A-cations and
manganese cations, oxide ions will be removed from sites which
form the strongest bonds to the manganese (shortest contacts)
and the weakest bonds (longest contacts) to the La/A sites.
We can evaluate this idea using bond valence sums (BVS)22 as
has be demonstrated previously for the Sr7Mn4O15-x phases.16
Contributions made by the manganese and lanthanum/strontium
sites to the BVS of the anion sites in LaSrMnO4 are shown in
Table 6. It should be noted that in this instance, bond valence sums
are not being used to calculate the ‘valence’ of the cation in the
structure. They are being used to crudely compare the contribution
each anion site makes to the lattice energy of the phase (although
not calculating it explicitly) by taking into account not only the
number and length of anion-cation contacts involved, but also the
non-linear relation between bond-length and strength. Examining
the data in Table 6 it is clear the axial O(1) anion site has a much
stronger interaction with the La/Sr A-cation site and a much
weaker interaction with the Mn site than O(2). As a result, removal
of oxide ions from the O(2) site will have a large effect on the BVS
of Mn with a relatively small change to the BVS of the La/Sr
site, providing a simple explanation for the removal of oxide ions
exclusively from the equatorial MnO2 layers. A similar process can
be followed for LaBaMnO4.
Fig. 5 Zero-field cooled and field cooled magnetisation data collected
from LaBaMnO3.5 (BA1) as a function of temperature (Top). Ordered
magnetic moment extracted from neutron diffraction data as a function of
temperature (Bottom).
The refined structure of LaSrMnO3.67 (sample SR1) reveals that
the oxide ions, deintercalated on the reduction of LaSrMnO4,
are removed exclusively from the O(2) anion sites which are
aligned along the x-axis (Fig. 1). Associated with this alignment
of the anion vacancies, the a cell parameter contracts and the
MnO6-x coordination polyhedra rotate and relax, presumably to
better satisfy the La/Sr site coordination requirements. This
anion vacancy ordering scheme and associated orthorhombic
Fig. 6 Zero-field cooled and field cooled magnetisation data collected
from LaSrMnO3.5 (SR2) and LaBaMnO3.5 (BA1) in the temperature range
300 < T/K < 700.
structural distortion is strongly reminiscent of those reported for
23,24
490 K and 480 K for LaSrMnO3.5 and LaBaMnO3.5 respectively,
suggesting magnetic transitions occur at these temperatures.
the nickelates La1.6Sr0.4NiO3.47 and LaSrCr0.1Ni0.9O3.75
.
In these
examples the electronic favourability of forming planar NiO4 units
containing NiI/II or NiII/III centres provides an obvious explanation
for the structures adopted. However, the driving force in the case
of the MnII/III phase LaSrMnO3.67 is not immediately apparent.
Further reduction of LaSrMnO3.67 with CaH2 continues to
extract oxide ions from the equatorial MnO2-x plane. The re-
sulting material, LaSrMnO3.5, retains orthorhombic symmetry
Discussion
The structures of the topotactically reduced LaSrMnO4-x phases
refined in this study all contain anion vacancies located exclu-
sively within the equatorial MnO2-x layers of the host K2NiF4
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6098–6105 | 6103
©

accompanied by the same preference to locate the anion vacancies
within the structure along the short in-plane axis, although the
ordering in this case is not complete with 65% of the vacancies
along a and 35% along b. In contrast, the LaBaMnO4-x phases
exhibit tetragonal structures with anion vacancies disordered
throughout the MnO2-x equatorial planes. The difference in
vacancy arrangements between the phases can be rationalized by
considering the strain induced into the LaAMnO4-x structures on
reduction. As noted above, the equatorial Mn-O in-plane bonds
expand on reduction. This leads to a co-operative twisting of the
MnO6-x polyhedra to accommodate the mismatch between Mn-O
and A-O bond lengths in a manner analogous to the tolerance
factor driven octahedral tilting and twisting distortions observed
in cubic perovskite phases. The structural strain induced as a result
of the relatively small size of the La/Sr A-cation site drives the
alignment of the anion vacancies in the LaSrMnO4-x phases to
relieve this strain. The large ionic radius of Ba2+ compared to Sr2+
leads to significantly less strain and fully disordered structures for
LaBaMnO4-x phases.
The lack of long range anion vacancy order in LaSrMnO3.5
and LaBaMnO3.5 is in marked contrast to the iso-stoichiometric
phases Ca2MnO3.5 Sr2MnO3.5.12,13 These MnIII phases, prepared
by the reduction of Ca2MnO4 and Sr2MnO4 respectively, and
the structurally related phase Sr3Mn2O6 derived from the n = 2
Ruddlesden-Popper phase Sr3Mn2O7,21 have structures which can
be considered as the stacking of AO rock-salt layers and MnO1.5
anion deficient layers, in common with the LaAMnO3.5 phases
reported here. However the MnO1.5 layers present in Ca2MnO3.5,
Sr2MnO3.5 and Sr3Mn2O6 contain highly ordered arrays of anion
vacancies resulting in ordered sheets of corner-sharing, distorted
MnIIIO5 units. The lack of any evidence for long range anion
vacancy order in the LaAMnO3.5 phases is therefore a little
surprising. While none of the diffraction data collected from the
LaAMnO3.5 phases showed any evidence for three-dimensional
anion vacancy order, they do not rule out the possibility of two-
dimensional ordering within individual perovskite layers. In this
situation there would be rigorous order of the anion vacancies
within individual MnO1.5 layers, but no structural registry between
layers, in an analogy to the paracrystalline cation ordering model
proposed for La4LiMnO8.25 Such a two-dimensional ordering
scheme would be consistent with the larger inter-layer separa-
order at 155 K and 135 K for LaSrMnO3.5 and LaBaMnO3.5
respectively. The lack of any feature in the magnetisation data
collected from either phase at these transition temperatures, and
the very low absolute magnitude of the magnetisations observed,
suggests that rather than a transition from 3D antiferromag-
netism at low-temperature to paramagnetism at high-temperature,
these are transitions from 3D antiferromagnetic order to 2D
antiferromagnetically ordered states. Such a transition would
involve a change from a rigorously ordered 3D antiferromag-
netic arrangement of spins to a state where there was rigorous
antiferromagnetic order within each equatorial MnO1.5 sheet,
but no long range order in the relative arrangement of spins
between sheets. Such a 3D to 2D transition would have no
signature in a magnetisation measurement as the number of
unpaired spins does not change, only their relative orientation.
This model is supported by the observation that the 3D to 2D
transition occurs at a lower temperature in LaBaMnO3.5 than
LaSrMnO3.5 (135 K vs. 155 K), consistent with the larger interlayer
separation (6.82 A vs. 6.69 A) and thus weaker interlayer magnetic
coupling in the former phase. Parallels can be drawn between
the observed change in dimensionality of the magnetic order in
LaAMnO3.5 phases and that of the hexagonal BaMnO3-x phases
which also exhibit no, or very weak, magnetisation anomalies at
their 3D antiferromagnetic ordering temperatures.27 On raising
the temperature further, magnetisation data show a transition at
T~480 K for both LaAMnO3.5 phases. This is characterised as the
change from 2D antiferromagnetic to paramagnetic behaviour.
Such high magnetic ordering temperatures are not unexpected for
d5 MnII phases. For example BaMnO2 and Ba0.5Sr0.5MnO2 become
antiferromagnetically ordered at 350 K and 355 K respectively.28,29
In contrast the reported antiferromagnetic transition in Sr2MnO3.5
is 280 K.13 High magnetic ordering temperatures are known in per-
ovskite derived phases containing well connected arrays of other d5
transition metal centres such as FeIII (LnFeO3 620 < TN < 74030).
The ordering temperature of the FeIII phase structurally analogous
to LaAMnO3.5 phases, LaSrFeO4, is 350 K.31
˚
˚
In conclusion, the introduction of large numbers of MnII
centres into manganates with the K2NiF4 structure leads to a
significant rise in the magnetic coupling temperatures observed
in these phases. However, the large inter-layer separations in the
LaAMnO4-x phases result in 2D ordering, both magnetically and
crystallographically, with 3D magnetic order only observed at
much lower temperatures.
˚
tions observed in LaSrMnO3.5 and LaBaMnO3.5 (6.69 A and
˚
6.82 A respectively) compared with three-dimensionally ordered
˚
˚
˚
Ca2MnO3.5, Sr2MnO3.5 and Sr3Mn2O6 (6.11 A, 6.28 A and 6.22 A
respectively).12,13,21
Acknowledgements
The magnetization data collected from LaSrMnO3.67(3) are
consistent with paramagnetic behaviour at room temperature,
with a transition to glassy behaviour at T ~60–75 K. Spin glass
behaviour is consistent with the expected magnetic superexchange
coupling in a mixed MnII/III phase. Assuming the MnIII centres are
Jahn–Teller distorted and align their dz2 orbitals parallel to the
crystallographic z-axis, the in plane 180◦ exchange interactions
given by the Goodenough-Kanamori rules26 would be: MnIII-O-
MnIII antiferromagnetic; MnII-O-MnII antiferromagnetic; MnIII-
O-MnII ferromagnetic. In the absence of an ordered array of MnII
and MnIII centers, these magnetic couplings will lead to a frustrated
system and thus the glassy behaviour observed.
We thank R. Smith and E. Suard for assistance collecting the
neutron diffraction data. Experiments at the ISIS pulsed neutron
facility were supported by a beam time allocation from the Science
and Technology Facilities Council. We thank the EPSRC for
funding this work.
References
1 R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz and K. Sanwer,
Phys. Rev. Lett., 1993, 71, 2331.
2 K. Chahara, T. Ohno, M. Kasao and Y. Kozono, Appl. Phys. Lett.,
1993, 63, 1990.
3 P. Schiffer, A. P. Ramirez, W. Bao and S.-W. Cheong, Phys. Rev. Lett.,
1995, 75, 3336.
Low temperature neutron diffraction data show a clear tran-
sition to three-dimensional (3D) long range antiferromagnetic
6104 | Dalton Trans., 2010, 39, 6098–6105
This journal is
The Royal Society of Chemistry 2010
©

4 P. G. Radaelli, M. Marezio, H. Y. Hwang, S.-W. Cheong and B. Batlogg,
Phys. Rev. B: Condens. Matter, 1996, 54, 8992.
5 M. R. Ibarra, P. A. Algarabel, J. Biasco and J. Garcia, Phys. Rev. Lett.,
1995, 75, 3541.
6 R. Cortes-Gil, M. L. Ruiz-Gonzalez, J. M. Alonso, M. Vallet-Regi,
M. Hernando and J. M. Gonzalez Calbet, Chem.–Eur. J., 2007, 13,
4246–4252.
17 A. C. Larson, R. B. Von Dreele, General Structure Analysis System,
Los Alamos National Laboratory Report LAUR 86-748: 2000.
18 M. A. Hayward and M. J. Rosseinsky, Chem. Mater., 2000, 12, 2182–
2195.
19 P. Aldebert and J. P. Traverse, Mater. Res. Bull., 1979, 14, 303–323.
20 A. Bowman, J. B. Claridge and M. J. Rosseinsky, Chem. Mater., 2006,
18, 3046–3056.
7 J. M. Gonzalez-Calbet, E. Herrero, N. Rangavittal, J. M. Alonso, J. L.
Martinez and M. Vallet-Regi, J. Solid State Chem., 1999, 148, 158–
169.
8 J. E. Millburn and J. F. Mitchell, Chem. Mater., 2001, 13,
1957.
9 J. F. Mitchell, J. E. Millburn, M. Medarde, D. N. Argyriou and J. D.
Jorgensen, J. Appl. Phys., 1999, 85, 4352.
10 M. A. Hayward, Chem. Mater., 2006, 18, 321.
11 P. S. Casey, D. Barker and M. A. Hayward, J. Solid State Chem., 2006,
179, 1375–1382.
12 M. E. Leonowicz, K. R. Poeppelmeier and J. M. Longo, J. Solid State
Chem., 1985, 59, 71.
13 L. J. Gillie, A. J. Wright, J. Hadermann, G. Van Tendeloo and C.
Greaves, J. Solid State Chem., 2002, 167, 145.
21 L. J. Gillie, A. J. Wright, J. Hadermann, G. Van Tendeloo and C.
Greaves, J. Solid State Chem., 2003, 175, 188.
22 N. E. Brese and M. O’Keefe, Acta Crystallogr., Sect. B: Struct. Sci.,
1991, B47, 192–197.
23 M. Crespin, J. M. Bassat, P. Odier, P. Mouron and J. Choisnet, J. Solid
State Chem., 1990, 84, 165.
24 J. E. Millburn and M. J. Rosseinsky, Chem. Mater., 1997, 9, 511.
25 J. C. Burley, P. D. Battle, D. J. Gallon, J. Sloan, C. P. Grey and M. J.
Rosseinsky, J. Am. Chem. Soc., 2002, 124, 620.
26 J. B. Goodenough, Magnetism and the Chemical Bond, Wiley, New
York, 1963.
27 J. J. Adkin and M. A. Hayward, Chem. Mater., 2007, 19, 755–762.
28 J. Hadermann, A. Abakumov, J. J. Adkin and M. A. Hayward, J. Am.
Chem. Soc., 2009, 131, 10598–10604.
14 M. Bieringer and J. E. Greedan, J. Mater. Chem., 2002, 12, 279.
15 M. A. Hayward, E. J. Cussen, J. B. Claridge, M. Bieringer, M. J.
Rosseinsky, C. J. Kiely, S. J. Blundell, I. M. Marshall and F. L. Pratt,
Science, 2002, 295, 1882.
16 M. O’Malley, M. A. Lockett and M. A. Hayward, J. Solid State Chem.,
2007, 180, 2851–2858.
29 J. J. Adkin and M. A. Hayward, Inorg. Chem., 2008, 47, 10959–10964.
30 J. B. Goodenough, J. M. Longo, Landolt-Bo¨rnstein Tabellen, Springer,
Berlin, Hidelberg, New York, 1970, p 228.
31 M. H. Jung, A. M. Alsmadi, S. Chang, M. R. Fitzsimmons, Y. Zhao,
A. H. Lacerda, H. Kawanaka, S. El-Katib and H. Nakotte, J. Appl.
Phys., 2005, 97, 10A926.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6098–6105 | 6105
©