Synthesis-Controlled Polymorphism and Magnetic and Electrochemical Properties of Li3Co2SbO6

Article abstract of DOI:10.1021/acs.inorgchem.9b01708

Li₃Co₂SbO₆ is found to adopt two highly distinct structural forms: a pseudohexagonal (monoclinic C2/m) layered O3-LiCoO₂ type phase with honeycomb 2:1 ordering of Co and Sb, and an orthorhombic Fddd phase, isostructural with Li₃Co₂TaO₆ but with the addition of significant Li/Co ordering. Pure samples of both phases can be obtained by conventional solid-state synthesis via a precursor route using Li₃SbO₄ and CoO, by controlling particle size, initial lithium excess, and reaction time. Both phases show relatively poor performance as lithium-ion battery cathode materials in their as-made states, but complex and interesting low-temperature magnetic properties. The honeycomb phase is the first of its type to show A-type antiferromagnetic order (ferromagnetic planes, antiferromagnetically coupled) below T_N = 14 K. Isothermal magnetization and in-field neutron diffraction below T_N show clear evidence for a metamagnetic transition at H ≈ 0.7 T to three-dimensional ferromagnetic order. The orthorhombic phase orders antiferromagnetically below T_N = 112 K and then undergoes two more transitions at 80 and 60 K. Neutron diffraction data show that the ground state is incommensurate.

Full text of DOI:10.1021/acs.inorgchem.9b01708

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Synthesis-Controlled Polymorphism and Magnetic and
Electrochemical Properties of Li Co SbO
3
2
6
†
†
†,‡
Brendan J. Kennedy, and Chris D. Ling*,†
†
Alex J. Brown, Qingbo Xia, Maxim Avdeev,
†
School of Chemistry, The University of Sydney, Sydney 2006, Australia
‡
Australian Centre for Neutron Science, Australian Nuclear Science and Technology Organisation, Kirrawee 2232, Australia
ABSTRACT: Li Co SbO is found to adopt two highly distinct
3
2
6
structural forms: a pseudohexagonal (monoclinic C2/m) layered
O3-LiCoO type phase with “honeycomb” 2:1 ordering of Co and
2
Sb, and an orthorhombic Fddd phase, isostructural with
Li Co TaO but with the addition of significant Li/Co ordering.
3
2
6
Pure samples of both phases can be obtained by conventional solid-
state synthesis via a precursor route using Li SbO and CoO, by
3
4
controlling particle size, initial lithium excess, and reaction time.
Both phases show relatively poor performance as lithium-ion battery cathode materials in their as-made states, but complex and
interesting low-temperature magnetic properties. The honeycomb phase is the first of its type to show A-type antiferromagnetic
order (ferromagnetic planes, antiferromagnetically coupled) below T = 14 K. Isothermal magnetization and in-field neutron
N
diffraction below T show clear evidence for a metamagnetic transition at H ≈ 0.7 T to three-dimensional ferromagnetic order.
N
The orthorhombic phase orders antiferromagnetically below T = 112 K and then undergoes two more transitions at 80 and 60
N
K. Neutron diffraction data show that the ground state is incommensurate.
INTRODUCTION
Zn; M′ = Sb, Bi), A M TeO (A = Li, Na, K; M = Co, Cu, Mg,
6−23
2
2
6
■
Zn, Ni), and Li MTeO (M = Co, Ni, Cu, Zn).
4
6
Layered transition metal oxides of the type A MO , where A is
x
2
However, the above A M M′O stoichiometries do not
3
2
6
an alkali metal and M is a transition metal, are among the most
intensively studied class of solid-state compounds in recent
always give rise to honeycomb-ordered phases. In some cases,
the honeycomb phase competes with a rock-salt type ordered
arrangement commonly referred to as an orthorhombic Fddd
superstructure, of which the first reported example is
1
years. They are the dominant materials used as positive
electrode (cathode) materials for Li-ion batteries due to their
high performance combined with the relative ease of synthesis,
high stability, and high melting points. They also display a wide
variety of other fascinating physical phenomena. For example,
Na CoO shows interesting thermoelectric properties and
2
4
Li Co TaO .
Examples of the Fddd phase include
3
2
6
24−26
Li M M′O (M = Ni, Co, Mg; M′ = Nb, Ta, Sb).
Li Ni SbO has been reported in both honeycomb and
3
2
6
3
2
6
x
2
24
orthorhombic superstructure phases; however, Nalbandyan
and co-workers reported that they were unable to reproduce
the latter despite their comprehensive work on the structure
and magnetism of the honeycomb phase of the same
favorable electrochemical properties for use as a positive
2
−4
electrode in Na-ion batteries and becomes superconducting
5
(
T ≈ 5 K) after intercalating water as Na CoO ·1.4H O.
c
0.3
2
2
The key structural motif of the A MO oxides is hexagonal
x
2
9
stoichiometry, and we were also unsuccessful in preparing
(
or pseudohexagonal) layers of edge-sharing MO octahedra,
alternating with layers of A cations in either octahedral or
trigonal coordination depending on the MO layer stacking
6
this using either conventional reagents or a Li SbO precursor.
3
4
More recently, Li Co TeO was reported to crystallize in an
3
1.06
6
6
orthorhombic Fddd space group with unit cell dimensions
arrangement. Substituting one-third of the M sites with
another element, M′, can give rise to ordered superstructures
with the general formula A M M′O , where each M′O
2
7
similar to other Li M M′O rock salt-type superstructures,
3
2
6
2
+
3+
but with Co and Co mixed valence states.
x
2
6
6
+
More recently, K honeycomb tellurates K M TeO (M =
octahedron is surrounded by six edge-sharing MO octahedra
2
2
6
6
Ni, Mg, Zn, Co, and Cu) have been synthesized and studied as
to form “honeycomb” layers. The honeycomb arrangement is
stabilized when the charge difference between M and M′ is
large but the size difference is relatively small: specifically, M =
2
8
rechargeable K-ion battery cathode materials. K Ni TeO
2
2
6
can reversibly insert K+ at voltages of ∼4 V and has a
−
1
2
+
2+
2+
2+
2+
5+
5+
6+
discharge/charge capacity of ∼70 mAh g . It also shows very
1
28
Mg , Co , Ni , Cu , Zn and M′ = Sb , Bi , or Te . The
−
good ionic conductivity of 0.01 mS cm at 298 K.
first reported A M M′O honeycomb-type phases were
3
2
6
6
2+
Another significant area of interest in the honeycomb oxides
has been their low-temperature magnetic properties. Hex-
Li Zn SbO and Li Zn BiO in 1990. Li Zn SbO has Zn /
3
2
6
3
2
6
3
2
6
5
+
Sb octahedral honeycomb ordering with alternating metal
+
layers separated by Li layers. Since then, the number of
known examples has expanded to include most of the
Received: June 13, 2019
combinations A M M′O (A = Na, Li; M = Mg, Co, Ni, Cu,
3
2
6
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01708
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
agonal arrangements of magnetic M cations around a
nonmagnetic M′ cation can lead to frustrated magnetic ground
states, and the relatively long and weak interlayer magnetic
interactions means that the systems are pseudo-low-dimen-
sional and highly susceptible to competing quantum
was then pressed into a rod at ∼40 MPa using a hydrostatic press.
The rod was heated to 1100 °C in an alumina boat under flowing Ar,
at a rate of 8 °C/min. The temperature was held at 1100 °C for 3 h
and then furnace-cooled to room temperature. We note that the
resulting sample was a lighter-colored pink powder compared to the
honeycomb phase; however, this may be due to changes in the
morphology and/or density, rather than the electronic structure.
Stratan et al. described their ion-exchanged sample of the
honeycomb phase as reddish-brown in color.
Polycrystalline samples were characterized by X-ray powder
diffraction (XRD) at Sydney Analytical, a core research facility at
the University of Sydney. Data were collected on a PANalytical X’Pert
2
9
fluctuations. Honeycomb oxides have been reported to
show spin-glass behavior and spin-flop transitions, in addition
to a long-range-ordered “zigzag” antiferromagnetic (AFM)
3
2
8
,12,13,29
ground state that breaks hexagonal symmetry
preference to the more intuitive Neel state that would preserve
it. Finally, recent theoretical work suggests that Co
honeycomb compounds could stabilize Kitaev exchange and
in
́
2
+
7
d
Pro MPD, using non-monochromated Cu Kα radiation (Kα , λ =
1
30
1.5406 Å; Kα , λ = 1.5444 Å) with a Ni filter. Data were collected in
2
give rise to a quantum spin-liquid state.
The present work concerns Li Co SbO , which has been
Bragg−Brentano θ:2θ geometry, with a voltage of 45 kV and current
of 40 mA, at a scanning rate of 0.045 2θ/s, typically over the range 5 <
2θ < 90°2θ.
3
2
6
relatively under-represented in the literature on the Li M SbO
3
2
6
honeycomb-type phases. An XRD pattern deposited with the
ICDD (Powder Diffraction File 00-58-637) reports a C2/m
unit cell (a common monoclinic distortion of the hexagonal
honeycomb phases), and another (Powder Diffraction File 00-
The orthorhombic phase of Li Co SbO was further characterized
3
2
6
by low-temperature XRD using a STOE Stadi P diffractometer with a
Mo−K source (λ = 0.7093 Å), monochromatized by (111) Ge. The
α1
sample was mounted on an Al-coated bronze sample holder.
Temperatures were achieved using an Oxford Cryosystems PheniX
with data measured to 12 K, in steps of 20 K with collection times of 3
h per step. Data were collected in the range 5 < 2θ < 55° using a
Dectris MYTHEN2 1K microstrip detector.
5
8-770) reports an Fddd unit cell and suggests that the
structure type may be analogous to Li Ni TaO . Magnetic
3
2
6
properties and EPR spectra for a honeycomb phase of
Li Co SbO were reported in a conference proceeding from
3
2
6
Structures were refined using the Rietveld method as implemented
in TOPAS (V5). The background was fit using a eighth-order
Chebychev polynomial. For the honeycomb phase, preferred
3
1
2
013, but the synthetic conditions and electrochemistry were
not described. However, while the present work was under
review, these authors (Stratan et al.) published a full journal
orientation (PO) was modeled using the spherical harmonics function
3
2
35,36
article expanding on their results and describing the synthesis
of the honeycomb phase of Li Co SbO by ion exchange from
(Jar
̈
vinen, 1993) found in TOPAS.
The peaks were modeled
using the pseudo-Voigt TCHZ function.
3
2
6
Magnetic susceptibilities were measured using a Quantum Design
Physical Properties Measurement System (PPMS). Temperature-
dependent DC magnetic susceptibility measurements were taken
under zero field-cooled (ZFC) and field-cooled (FC) conditions over
the range 2−300 K, in an applied field of 0.1 T. Isothermal field-
dependent magnetization data were collected to ±3 T at constant
temperatures.
the Na analogue, Na Co SbO . Where our results overlap, we
therefore now draw comparisons to those published in the
latter article.
Here, we report the solid-state synthesis (via a precursor
route), crystal structure, magnetism, and basic electrochemical
3
2
6
performance of Li Co SbO in two polymorphs: an O3-type
3
2
6
honeycomb phase and an Fddd orthorhombic phase.
Neutron powder diffraction (NPD) measurements were carried out
37
using the high-resolution diffractometer ECHIDNA at the OPAL
EXPERIMENTAL SECTION
facility, Australian Nuclear Science and Technology Organisation
■
(
(ANSTO), using neutrons of wavelength 2.4395 Å. The powder
Li Co SbO was prepared using conventional solid-state methods.
3
2
6
samples were sealed in vanadium cans, and low-temperature data were
collected at 3 and 20 K for the honeycomb phase, and 3 and 125 K for
the orthorhombic phase. The magnetic structures were analyzed by
Rietveld refinement using TOPAS. For the magnetic structure
refinements of the monoclinic phase, the nuclear structure model
refined against 20 K NPD data was selected as the parent structure
Starting materials were obtained from commercial suppliers. Li CO
99.9%, Sigma-Aldrich) was dried at 120 °C prior to reaction to avoid
moisture contamination. Five gram samples of Li SbO precursor
2
3
3
4
were prepared based on methods previously described in the
33,34
literature.
Li CO3 and Sb O (99%, BDH Chemicals) were
2 2 3
mixed in the relevant stochiometric ratios and ground with acetone
using an agate mortar and pestle. The ground paste was dried before
being pressed into a 20 mm pellet at ∼35 MPa. The pellet was then
reacted at an initial temperature of 650 °C for 2 h, before calcining at
(
undistorted), and symmetry representation analysis was applied
38
using ISODISTORT based on the observed magnetic supercell
reflections.
Both Li Co SbO phases were tested as Li-ion battery cathode
9
50 °C for 12 h. Li SbO was obtained as a cream-colored
3
2
6
3
4
materials. Composite electrodes were prepared by mixing with carbon
black and polyvinylidene fluoride (PVDF) binder (80:10:10 wt %)
and casting onto an aluminum foil current collector. Coin cells were
polycrystalline sample and characterized by X-ray diffraction
(
(
XRD). 5−6 g samples of CoO were synthesized from Co O
99.9%, Sigma-Aldrich) by heating finely ground. Co O as a loose
powder at 1050 °C under flowing Ar in a tube furnace for 12 h. The
resulting brown powder of CoO was confirmed by XRD.
−3 g samples of the honeycomb phase of Li Co SbO were
prepared from a stochiometric 1:2 mixture of Li SbO and CoO. The
mixture was ground using an agate mortar and pestle before being
pressed into a rod at ∼40 MPa using a hydrostatic press. The rod was
placed in an alumina boat, heated to 1100 °C under flowing Ar at a
rate of 6 °C/min, held at that temperature for 90 min, and then
furnace-cooled to room temperature. These short reaction times were
found to produce the most crystalline samples and minimize
evaporative loss of Li. The resulting compound was a pinkish-red
powder.
3
4
3
4
made in an Ar-filled glovebox, with 1 M LiPF in ethylene carbonate
6
and diethyl carbonate (1:1 volume ratio) used as the electrolyte. Li
metal was the counter electrode, separated from the Li Co SbO
6
2
3
2
3
2
6
electrode by a fiber glass separator. Electrochemical performance of
the coin-cells was tested with a Neware BTS 4000 battery cycler, at 25
3
4
°C, with a constant current of 0.04 mA and voltage ranging from 2 to
4.2 V.
RESULTS AND DISCUSSION
■
Synthesis. Initial attempts to prepare Li Co SbO using a
3
2
6
solid-state method similar to that described for the Na
8
analogue, Na Co SbO , were unsuccessful. Reaction in air or
2
−3 g samples of the orthorhombic phase of Li Co SbO were
3
2
6
3
2
6
prepared in a similar manner. Li SbO and CoO were mixed with
under flowing O
crystalline honeycomb phases with very pronounced peak
2
at temperatures ≤1000 °C resulted in poorly
3
4
Li CO in 5% excess by ball-milling for 3 h at 400 rpm. The mixture
2
3
B
DOI: 10.1021/acs.inorgchem.9b01708
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Rietveld refinement against XRD data (λ = 1.5406 Å) for the C2/m honeycomb phase of Li Co SbO at room temperature. Black circles
3
2
6
are experimental data, the red line is the calculated pattern, and the gray line is the difference. Bragg peaks are indicated by blue tick marks; the
upper purple tick mark at 19° indicates the sole observed peak from a trace fraction of the orthorhombic phase. The inset highlights peaks due to
Co/Sb honeycomb superlattice ordering, with Miller indices in parentheses.
broadening and intensity loss due to layer-stacking faults, and
Li2 was initially fixed to the value from the Na Co SbO
3 2 6
8
an LiCoO impurity, suggesting evaporative loss of Li and Sb.
starting model, due to the relative insensitivity of XRD data to
light Li atoms, and refined in the final cycles. Li2 did not move,
within error, from its position in the initial Na Co SbO
2
Higher reaction temperatures caused complete decomposition,
with only deep blue CoAl O remaining on the alumina
2
4
3
2
6
crucible walls.
We therefore developed an alternative synthesis route where
model. A trace amount of the orthorhombic phase (the
structure of which is described below) was included in the
refinement. The final Rietveld fit is shown in Figure 1, and the
refined structure is shown in Figure 2. Details of the refinement
3
3,34
a precursor phase Li SbO4
was reacted with freshly
3
prepared CoO. This precursor route has a number of favorable
aspects. Sb is already in the 5+ oxidation state (vs 3+ in the
conventional solid-state reagent, Sb O ), which allows the
2
3
reaction to be performed in the absence of O (in this case,
2
under Ar). In turn, the absence of O allows us to use CoO
2
with Co in the desired 2+ oxidation state, avoiding the
formation of Co O . Finally, Li SbO has a higher melting
3
4
3
4
point than either Li CO or Sb O , which minimizes the
2
3
2
3
evaporative loss of Li and Sb and allows us to use higher
reaction temperatures for shorter times (90 min). Longer
annealing times at these temperatures led to decomposition, as
for the direct (nonprecursor) route.
The orthorhombic phase, which we first identified as an
impurity in the synthesis of the honeycomb phase, was isolated
after empirically testing different reaction conditions. We
found two factors that favored the formation of the
orthorhombic phase: a small (5%) excess of Li CO , and
Figure 2. Crystal structure of the C2/m O3-type honeycomb phase of
Li Co SbO . CoO octahedra are blue, SbO octahedra are bronze, O
3
2
6
6
6
atoms are red, and Li atoms are green.
2
3
ball-milling the reagents prior to pressing into rods.
Significantly longer annealing times under the same conditions,
or postannealing of the orthorhombic phase, led to
decomposition rather than conversion to the honeycomb
phase. Taken together, these observations suggest that the
orthorhombic phase is favored when it can initially form Li-
rich, but is also more susceptible to Li and Sb loss at high
temperatures.
and the refined structure are given in Table 1, with selected
bond data including bond valence sums (BVS) in Table 2.
39
The structure is not significantly different to that reported in
32
the parallel work of Stratan et al., except that they report a
small but statistically significant degree of Li/Co intersite
mixing, and their unit cell at room temperature is slightly
3
3
Structural Refinements. The structure of each
smaller (230.610(11) Å vs 234.02(4) Å ). These two points
of difference may be related and a consequence of the different
synthetic routes, but are also within the range of variations
commonly seen between refinements using data sets from
different instruments. A crystallographic information file (CIF)
for the final refined structure has been deposited with the
CCDC.
Li Co SbO phase was analyzed by Rietveld-refinement against
3
2
6
XRD data. The honeycomb phase was indexed in the
monoclinic space group C2/m, isostructural with Na Co SbO ,
3
2
6
which has an O3-type layer stacking arrangement. Co/Sb
intermixing on both sites was refined but did not deviate from
the fully ordered model within error. The free y coordinate for
C
DOI: 10.1021/acs.inorgchem.9b01708
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Unit Cell Dimensions, Fractional Coordinates, and Isotropic Atomic Displacement Parameters for the Honeycomb
a
Phase of Li Co SbO , from Rietveld-Refinement against XRD Data
3
2
6
honeycomb Li Co SbO
6
crystal system
monoclinic
C (Å)
space group
C2/M
volume (Å3)
234.02(5)
Wyckoff
3
2
GOF = 1.31, RWp = 3.21%
radiation
Cu Kα
atom
T (K)
298
X
A (Å)
5.2717(6)
Y
B (Å)
9.0871(10)
Z
Β (deg)
110.1553(14)
BIso (Å2)
5.2039(6)
occ.
Co
Sb
O1
O2
Li1
Li2
0
0
2/3
0
0 348(2)
1/2
1/2
0
0
1
1
1
1
1
1
0.3(2)
0.2(2)
0.9(4)†
0.9(4)†
0.5(2)‡
0.5(2)‡
4g
2a
8j
4i
2d
4h
0.274(3)
0.246(4)
0
0.772(5)
0.225(9)
1/2
1/2
0.330(9)
1/2
a
Superscript symbols indicate constraints.
Table 2. Selected Interatomic Distances (Å) and Bond
Valence Sums (BVS) for the Honeycomb Phase of
Li Co SbO at Room Temperature
contents, all of which were refined. The 8b site is the most
distorted of these, with the longest M−O bond distances, and
was mostly occupied by Li with a small refined fraction of Co,
while the two 16g sites were fully occupied by Co or Li within
the standard deviations of the refinement and were fixed as
such. This is a notable departure from the starting model of
Li Co TaO , which had Ta on the 8a site but a statistical
occupancy of all three Li/Co sites. Calculated BVS for the
cations are reasonable, all falling within 10% of their nominal
values with the exception of the small fraction of Co2 (BVS =
3
2
6
Co−O1 (x2)
Co−O1 (x2)
Co−O2 (x2)
average
BVS (Co)
Sb−O1 (x4)
Sb−O2 (x2)
average
2.138(6)
2.168(14)
2.075(10)
2.127(18)
1.9
1.937(6)
2.059(19)
1.98(4)
5.1
Li1−O1 (x4)
Li1−O2 (x2)
average
BVS (Li1)
Li2−O1 (x2)
Li2−O1 (x2)
Li2−O2 (x2)
average
2.142(8)
2.239(18)
2.17(2)
0.8
2.147(13)
2.30(6)
2.22(6)
2.22(2)
0.9
3
2
6
24
1
.7+) on the mixed site dominated by Li2 (BVS = 0.9+). Note
that the refined occupancies suggest a slightly Co-rich
BVS (Sb)
BVS (Li2)
composition Li2.82(7)Co2.18(7)SbO . This is consistent with our
6
observations from synthesis that the orthorhombic phase forms
are slightly Li-rich, but is subsequently susceptible to Li and Sb
loss. Nevertheless, given that this nonstoichiometry is only
barely statistically significant (and may be compensated for by
a degree of Li substitution onto the Co1 site that is too small
to be refined), we will continue to refer to the phase as
Li Co SbO , except where that nonstoichiometry is potentially
The honeycomb phase of Li Co SbO has a smaller unit cell
3
2
6
than that of Na Co SbO , with volumes of 234.02(5) and
3
2
6
3
2
66.43(18) Å respectively. The difference in unit cell
dimensions is most pronounced along the layer-stacking c
axis (5.637(2) and 5.2717(6) Å), consistent with the larger
+
+
ionic radius of Na versus Li (1.02 vs 0.76 Å). The
superlattice reflections due to 2:1 Co/Sb honeycomb ordering
are sharp, demonstrating the absence of appreciable stacking
3
2
6
significant (as in the discussion of magnetic properties, below).
Magnetic Properties. Temperature-dependent magnetic
susceptibility data for the honeycomb phase of Li Co SbO
2
2,40,41
faults.
Calculated BVS for the cations are all very close to
3
2
6
their nominal values, further validating the structural changes
from the Na Co SbO starting model.
show an AFM ordering transition at a Nee
4 K (Figure 5). There was no significant divergence between
data collected under zero field-cooled (ZFC) and field-cooled
FC) conditions using an applied field of 0.1 T. A Curie−
Weiss fit to ZFC data over the range 150−300 K yielded a
Weiss constant θ = 18.1 K and an effective magnetic moment
́
l temperature T =
N
3
2
6
1
The orthorhombic phase was indexed in the orthorhombic
24
space group Fddd and is isostructural with Li Co TaO . For
3
2
6
(
the structural refinements, it was assumed that each cation site
was fully occupied. Biso values for O1 and O2 diverged slightly,
indicating some degree of correlation in the refinement, so
were constrained to the same value. The free z coordinate for
Li1 was initially fixed at the value from the Li Co TaO
6
w
2
+
of μeff = 5.04 μ per Co ion. The latter value is consistent
with high-spin Co (d , S = 3/2, μ = 3.87 μ ), with an
unquenched orbital contribution in a T1g (using the
octahedral notation) ground state and is comparable to μ
.22 μ reported for Na Co SbO . The small deviation in
B
2
+
7
so
B
3
2
4
starting model, due to the relative insensitivity of XRD to
2
4
=
eff
light Li atoms, and only refined in the last cycles. Li1 was
found to move only slightly from its position in the
Li Co TaO model, improving the fit from R = 23.57%,
8
5
B 3 2 6
susceptibility observed around 75−80 K is due to a trace
amount of the orthorhombic phase (the magnetic properties of
3
2
6
wp
GOF = 1.66 to Rwp = 22.67%, GOF = 1.59 in the final
refinement. The final Rietveld fit is shown in Figure 3 and a
representation of the structure in Figure 4. Details of the
refinement and the refined structure are given in Table 3, with
selected bond data including BVS in Table 4. A CIF for the
final refined structure has been deposited to the CCDC.
which are described below). The values T
K for Li Co SbO can be compared to θ = 8 K and T
for Li Ni SbO
In all cases, the positive Weiss constants θ
= 14 K, θ
= 18.1
= 15 K
N
w
3
2
6
w
N
9
, and T
= 8.3 K, θ
= 2.2 K for Na Co SbO .
3 2 6
3
2
6
N
w
indicate the
w
presence of significant ferromagnetic contributions above the
AFM transition temperature.
In this orthorhombic phase, SbO octahedra share 12 edges
6
with (Li,Co)O octahedra. Placing Sb on the 8a Wykoff site
The magnetic constants are slightly different than those
6
32
maximizes Sb−Sb distances to minimize electrostatic repulsion
reported in the parallel work of Stratan et al., who obtained
5
+
between Sb cations. Li and Co are distributed over three
crystallographically independent sites with varying Li/Co
lower values for T (9.9 K), θ (14 K), and μ (3.3 μ per
N
w
eff
B
2
+
Co ion). This suggests that the solid-state precursor synthetic
D
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Inorganic Chemistry
Article
Figure 3. Rietveld refinement of the orthorhombic Fddd phase of Li Co SbO against XRD data from (A) a Cu Kα source (λ = 1.5418 Å) at 298 K
3
2
6
and (B) a Mo Kα source (λ = 0.7093 Å) at 12 K under a vacuum. Observed data are black circles, the calculated fit is a red line, and the difference
1
is a gray line. Blue tick marks indicate Bragg reflections.
route produces a slightly better-ordered sample than the ion-
exchange route, as would be expected. However, the qualitative
behavior is the clearly the same, and the Weiss constant is
positive in both cases.
Temperature-dependent magnetic susceptibility data for the
orthorhombic phase of Li Co SbO under ZFC conditions
3
2
6
show a pronounced AFM transition on cooling through T =
N
113 K followed by more transitions at 80 and 60 K (Figure 7).
Noting that no impurity phases were evident in XRD data, the
series of transitions suggest close competition between at least
three ground states in this compound. A Curie−Weiss fit to the
ZFC data for T > 160 K yields μ = 5.06 μ , slightly larger
Field-dependent magnetization data for the honeycomb
phase of Li Co SbO were collected over the field range −3 T
3
2
6
≤
H ≤ 3 T (Figure 6). At T = 20 K (above T ), the behavior is
N
close to linear. At 2 K (below T ), a metamagnetic transition is
N
eff
B
apparent at H = ∼0.7 T. Other honeycomb oxides have been
reported to undergo similar field-induced transitions, e.g.,
Na Co SbO has a magnetization shoulder above H = 1 T,
than for the honeycomb phase of Li Co SbO , while if the
3 2 6
slight refined nonstoichiometry of the sample, Li2.82Co2.18SbO6,
is correct, the expected spin-only moment μso would be
reduced from μ = 3.87 to 3.78 μ due to the presence of some
3
2
6
which has been attributed to a field-induced spin-flop
so
B
8
,13
1+
transition;
Na Co TeO has an upturn in magnetization
reduced Co . Regardless, there are significant contributions
2
2
6
1
3,29,42
at ∼5 T,
and Na Ni BiO shows evidence of
from an unquenched orbital component, as is typical for HS
3
2
6
2
+
transition(s) from 4 to 7 T depending on measurement
Co oxides. The highly negative Weiss constant θ = −181 K
w
1
7
temperature. These results collectively underline the finely
balanced and competing/frustrated nature of magnetic
exchange in honeycomb antiferromagnets. Again, the field-
dependent magnetization of honeycomb Li Co SbO is
indicates dominant AFM exchange. The FC data strongly
diverge from the ZFC data below T , with the applied field of
N
0.1 T appearing to be sufficient to induce an FM state.
Field-dependent magnetization data for the orthorhombic
phase of Li Co SbO were collected over a field range −3 T ≤
3
2
6
qualitatively the same as that reported in the parallel work of
3
2
6
3
2
Stratan et al., leading to the same conclusion of a probable
H ≤ 3 T (Figure 8) at T = 2 and 90 K. Both data sets show
spin-flop transition.
predominantly AFM behavior with a narrow hysteresis
E
DOI: 10.1021/acs.inorgchem.9b01708
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. Selected Interatomic Distances (Å) and Bond
Valence Sums (BVS) for the Orthorhombic Phase of
Li Co SbO at 12 K
3
2
6
Co1−O1 (x2)
Co1−O2 (x2)
Co1−O2 (x2)
average
BVS (Co1)
Sb−O1 (x2)
Sb−O2 (x4)
average
2.0841(7)
2.132(7)
2.186(3)
2.134(4)
1.8
2.014(7)
2.017(14)
2.016(14)
4.5
Li1−O1 (x2)
Li1−O2 (x2)
Li1−O2(x2)
average
2.118(7)
2.1506(3)
2.174(7)
2.15(4)
0.9
2.316(14)
2.094(7)
2.17(5)
1.7/0.9
BVS (Li2)
Co2/Li2−O1 (x2)
Co2/Li2−O2 (x4)
average
BVS (Sb)
BVS (Co2/Li2)
Figure 4. Crystal structure of the orthorhombic Fddd phase of
Li Co SbO in two different orientations. CoO octahedra are blue,
3
2
6
6
SbO octahedra are bronze, O atoms are red, and Li atoms are green.
6
Colored wedges show mixed occupancies.
opening, but only the 2 K data show a (small) remanent
magnetization. The magnetization is approximately 2 orders of
magnitude smaller than for the honeycomb phase at the same
fields.
Figure 5. Temperature-dependent DC magnetic susceptibility (χm)
and inverse magnetic susceptibility (1/χ ) as functions of temper-
m
ature for the honeycomb phase of Li Co SbO , using an applied field
3
2
6
of 0.1 T. The ZFC curve is black, the FC curve is red, and the inverse
susceptibility is blue. The dotted black line shows the Curie−Weiss fit
to the ZFC inverse susceptibility.
Low-Temperature Neutron Diffraction Data. NPD
data for the honeycomb phase of Li Co SbO at 3 K (below
3
2
6
T ) versus 20 K (above T ) revealed magnetic Bragg peaks
N
N
that could be indexed to a propagation vector k = (0 0 1/2).
Representational analysis unambiguously led to the best fit
being obtained for the same irreducible representation (irrep),
has not previously been observed in the honeycomb phases.
The spins are oriented along the b axis. The final Rietveld fit is
shown in Figure 9, yielding a refined magnetic moment of
32
Γ , found in the parallel work of Stratan et al. The analysis
1
procedure and table of basis vectors are identical to those
reported in that work and are therefore not reproduced here.
The Γ irrep corresponds to the magnetic space group C 2/m,
2.53(3) μ /Co along b at 3 K, close to the fully ordered value
B
2
+
32
of 3 μ /Co for HS Co . Note that Stratan et al. report M =
1
c
B
which has ferromagnetic (FM) honeycomb planes with AFM
coupling between them. This so-called A-type AFM structure
3.74(4) μ /Co at 1.5 K or 1.75(2) μ /Co along b. This is
B
B
consistent with the magnetometry results discussed above,
Table 3. Unit Cell Dimensions, Fractional Coordinates, Occupancies and Isotropic Atomic Displacement Parameters for the
Orthorhombic Phase of Li Co SbO , from Rietveld-Refinement against XRD data at 12 K and Unit Cell Dimensions Only (in
3
2
6
a
Italics) from Rietveld-Refinement against XRD Data at 298 K Superscript Symbols Indicate Constraints
orthorhombic Li Co SbO
crystal system
orthorhombic
space group
Fddd
3
2
6
radiation
T (K)
a (Å)
b (Å)
c (Å)
α = β = γ (deg)
volume (Å3)
Cu Kα
Mo Kα1
atom
298
12
5.937(2)
5.918(2)
8.703(2)
8.666(4)
17.944(5)
17.884(7)
90
90
927.4(4)
917.1(6)
Wyckoff
x
y
z
occ.
Biso (Å2)
Sb
Co1
Li1
Li2/Co2
O1
O2
1/8
1/8
1/8
1/8
1/8
0.1156(17)
1/8
1/8
1
1
1
0.2(1)
0.3(1)
1.0(4)
1.0(3)
0.40(15)†
0.40(15)†
8a
1/8
5/8
5/8
0.358(2)
0.377(2)
0.2934(2)
0.2907(2)
1/8
1/8
0.2954(7)
16g
16g
8b
16f
32h
0.82/0.18 (4)
1
1
a
GOF = 1.34, Rwp = 3.46% (Cu Kα); GOF = 1.59, Rwp = 22.67% (Mo Kα1)
F
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Inorganic Chemistry
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While the A-type AFM state has not previously been
observed for a A M M′O honeycomb phase, it is interesting
x
2
6
to note that this state was originally predicted for the
9
6
honeycomb phase of Li Ni SbO prior to NPD experiments
3
2
that showed it has a zigzag ground state. The Ni−O−Ni
magnetic exchange pathway in this compound is ∼90°, which
is expected to yield FM in-plane interactions based on
Goodenough-Kanamori rules. A positive Weiss temperature
(
θ = 8 K) supported this prediction, so the onset of long-range
AFM below TN = 15 K was assumed to be due to AFM
interlayer coupling. The Co analogue reported here has an
almost identical Neel temperature, Weiss temperature, and
́
M−O−M bond anglebut in this case A-type AFM is indeed
the resulting ground state. The biggest difference between
2
+
8
these compounds is their electronic ground state: Ni is d ,
3
which in octahedral coordination has a A ground term with
2
g
2
+
7
4
filled t orbitals, while Co is d , with a T ground term and
2
g
1g
an unpaired electron in one t2g orbital. This additional
unpaired electron contributes additional unquenched angular
orbital momenta, which may be a factor in the emergence of A-
type ordering.
Figure 6. Field-dependent magnetization of the honeycomb phase of
Li Co SbO at 2 K (black) and 20 K (red), below and above T =
3
2
6
N
1
4.5 K, respectively.
Considering the magnetic structure of honeycomb
Li Co SbO , the spin-flop transition observed in field-depend-
3
2
6
ent magnetization data (Figure 6) almost certainly corresponds
to a change from AFM to FM interactions between the FM
planes, i.e., a change from A-type AFM (magnetic space group
Cc2/m) to 3-D FM (magnetic space group C2/m). In order to
test this, we collected field-dependent NPD data between 0
and 6 T at 1.8 K. The data are shown in Figure 10. The
characteristic A-type AFM Bragg peaks disappear as the field is
increased, while the (0 0 l) nuclear reflections (perpendicular
to the honeycomb planes) show the strongest growth in
intensity. Unfortunately, complex preferred orientation due to
partial particle reorientation in the field meant that we were
unable to refine the magnetic structure to obtain a precise
value and orientation for the ordered moment at 6 T (note
that the (−3 3 1) nuclear reflection, equivalent to (1 1 0) in
the hexagonal parent phase, i.e., parallel to the honeycomb
planes, weakens as the field increases). Nevertheless, these
intensity trends confirm a metamagnetic transition at high
fields to an in-plane 3-D FM state.
Figure 7. Magnetic susceptibility as a function of temperature for the
orthorhombic phase of Li Co SbO in an applied field of 0.1 T. ZFC
data are black and FC data are red. The inset shows the Curie−Weiss
fit to ZFC inverse susceptibility above 160 K.
3
2
6
NPD data for the orthorhombic phase of Li Co SbO at 125
3
2
6
K (above T ) and 3 K are shown in Figure 11. The difference
between them highlights the magnetic Bragg peaks, proving
that this phase undergoes long-range spin ordering. However,
N
which indicate a slightly better-ordered sample from solid-state
precursor synthesis compared to ion-exchange synthesis.
Figure 8. Field-dependent magnetization data for the orthorhombic phase of Li Co SbO at 2 K (black) and 90 K (red).
3
2
6
G
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Inorganic Chemistry
Article
Figure 9. Rietveld fit to 3 K NPD data (λ = 2.4395 Å) for the honeycomb phase of Li Co SbO using the nuclear plus Γ (C 2/m) magnetic model.
3
2
6
1
c
Black circles are the observed diffraction pattern, the red line is the calculated pattern, and the gray line gives the difference profile. Blue ticks mark
reflections for the magnetic and nuclear peak positions. Green ticks are a minor orthorhombic Li Co SbO impurity.
3
2
6
Figure 10. NPD data (λ = 2.4395 Å) for the honeycomb phase of Li Co SbO at 1.8 K in increasing magnetic fields. Note that the doublets at 63°
3
2
6
and 74.5° are Al peaks from the front and back side of the cryomagnet bore.
the majority could not be indexed with a rational k-vector,
indicating that the ground-state is incommensurate. A k-vector
search failed to identify a convincing result due to an
insufficient number of observed magnetic peaks relative to
the nuclear symmetry. Further insights into the magnetic
structure of the orthorhombic phase of Li Co SbO will
3
2
6
require single-crystal and/or polarized neutron diffraction
experiments to obtain more unambiguous observed magnetic
peaks.
Electrochemical Properties. The honeycomb and ortho-
rhombic phases of Li Co SbO were both tested as active
3
2
6
positive electrode (cathode) materials in Li-ion coin cell
batteries. Charge and discharge cycles are shown in Figures 12
and 13. Upon the first charge to 4.2 V, the honeycomb phase
has a capacity of 32 mAh/g, while the orthorhombic phase has
a capacity of 20 mAh/g. These low capacities fade after the first
cycle, to 3.4 mAh/g and 3.7 mAh/g, respectively, and continue
to decrease with each subsequent cycle. Note that the
stoichiometry Li Co SbO gives a calculated theoretical
Figure 11. NPD data (λ = 2.4395 Å) of orthorhombic Li Co SbO at
3
2
6
3
K (red) and 125 K (blue). The difference curve (black) shows the
possible magnetic Bragg peaks, highlighted with green arrows.
3
2
6
H
DOI: 10.1021/acs.inorgchem.9b01708
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Inorganic Chemistry
Article
Figure 12. (A) Charge/discharge voltage profiles of honeycomb-type L Co SbO , with the first cycle in red and the next 99 cycles in black. (B)
i3
2
6
Capacity retention of charge (black) and discharge (red) for the first 10 cycles.
Figure 13. (A) Charge/discharge voltage profiles for the orthorhombic phase of Li Co SbO , with the first cycle in red and the next 99 cycles in
3
2
6
black. (B) Capacity retention of charge (black) and discharge (red) for the first 10 cycles.
2
+
3+
capacity of 75.2 mAh/g based on Co /Co redox alone, at
which point two-thirds of the Li has been removed.
excess, and reaction time. Conventional grinding, stoichio-
metric reagents, and short reaction times yield a layered
honeycomb phase isostructural with Na Co SbO , while ball-
The capacity is inferior to a number of other honeycomb
oxides, notably, the Ni compounds. For example, Na Ni SbO
6
has been shown to have reversible capacities between 110 and
30 mAh/g with 70% retention over 500 cycles.
Na Ni TeO has a reversible capacity of 110 mAh/g. Of the
3
2
6
milling, a slight Li excess, and longer reaction times yield an
Fddd orthorhombic phase isostructural with Li Co TaO but
3
2
3
2
6
43,44
1
Similarly,
with much more well-defined Li/Co ordering. Both phases are
challenging to make as pure samples, with very narrow ranges
of thermodynamic stability between the minimum required
synthesis temperature and decomposition due to evaporative
loss of Li and/or Sb. Future work using low-temperature chimie
douce synthetic routes may help address these challenges, but
at the expense of crystallinity, which was the priority in the
present work aimed at solving and refining the fully ordered
structures.
Neither phase shows promising electrochemical perform-
ance as the lithium-ion battery cathode in their as-made states,
although it may be possible to improve this in the future by
ball-milling to micron scale before casting onto Al foil, to
increase the surface area. The poor performance might be due
to irreversible structural changes, which could potentially be
explored through future in situ diffraction studies and/or ex
situ studies of charged samples.
4
5
2
2
6
Li compounds, Li NiTeO has a reversible capacity of 110−
4
6
4
6,47
1
20 mAh/g.
However, the closest analogue to Li Co SbO
3 2 6
for which electrochemical data have been reported,
Li Ni BiO , has a similarly low capacity of 22 mAh/g.
48
3
2
6
CONCLUSIONS
■
We have shown that Li Co SbO can be prepared by
conventional solid-state synthesis from preprepared Li SbO
3
2
6
3
4
and CoO. These precursor phases were found to be necessary
so that Sb and Co would be present in their desired oxidation
states (Sb and Co , at the upper and lower ends of their
normal ranges in oxides, respectively) prior to high-temper-
ature reaction under a protective Ar flow. Two distinct phases
can be obtained by controlling particle size, initial lithium
5
+
2+
I
DOI: 10.1021/acs.inorgchem.9b01708
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Inorganic Chemistry
Article
Both phases show complex and interesting low-temperature
(6) Greaves, C.; Katib, S. M. A. The Structural Chemistry of
Li Zn MO (M = Sb, Bi) and Related Phases. Mater. Res. Bull. 1990,
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3
2
6
N
2
(
5 (9), 1175−1182.
7) Seibel, E. M.; Roudebush, J. H.; Ali, M. N.; Ross, K. A.; Cava, R.
J. Structure and Magnetic Properties of the Spin-1/2-Based
Honeycomb NaNi BiO and Its Hydrate NaNi BiO ·1.7H O.
adopts A-type AFM order (FM planes, AFM coupled), which
has been predicted but not previously observed for any
A M M′O honeycomb phase. Isothermal magnetization and
x
2
6
2
6‑δ
2
6‑δ
2
in-field NPD below T reveal a metamagnetic “spin-flop”
N
Inorg. Chem. 2014, 53 (20), 10989−10995.
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in Honeycomb-Layered Na Co SbO . J. Solid State Chem. 2016, 243,
transition to a 3-D FM phase above H ≈ 0.7 T.
The orthorhombic phase also undergoes long-range AFM
(
3
2
6
order, at the significantly higher T = 112 K, but in this case
N
18−22.
with a strongly negative Weiss constant θ = −181 K that
(9) Zvereva, E. A.; Evstigneeva, M. A.; Nalbandyan, V. B.; Savelieva,
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Compound Li Ni SbO : Preparation, Crystal Structure and Magnetic
w
points to somewhat frustrated AFM intersections in the
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2
6
Properties. Dalton Trans. 2012, 41 (2), 572−580.
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Novel Rare-Earth Free Red-Emitting Li Mg SbO :Mn4+ Phosphor-
(
hysteresis in isothermal magnetization, below T suggest a fine
N
3
2
6
balance between local AFM and FM exchange interactions.
Low-temperature NPD data point to an incommensurate AFM
ground state, which could not be confidently indexed based on
the limited number of observed reflections. Further detailed
measurements (preferably on single crystals, which are not
presently available) will be required to unravel this complex
behavior.
in-Glass for Warm w-LEDs: Synthesis, Structure, and Luminescence
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ASSOCIATED CONTENT
Accession Codes
■
2
2
6
(13) Viciu, L.; Huang, Q.; Morosan, E.; Zandbergen, H. W.;
CCDC 1937386−1937387 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Greenbaum, N. I.; McQueen, T.; Cava, R. J. Structure and Basic
Magnetic Properties of the Honeycomb Lattice Compounds
Na Co TeO and Na Co SbO . J. Solid State Chem. 2007, 180 (3),
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Rocksalt Superstructure Type Layered Oxides Li M TeO (M-
4
.5 0.5
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III)Cr, Mn, Al, Ga. Mater. Res. Bull. 2016, 76, 118−123.
(15) Smirnova, O. A.; Nalbandyan, V. B.; Petrenko, A. A.; Avdeev,
M. Subsolidus Phase Relations in Na O−CuO−Sb O System and
AUTHOR INFORMATION
Corresponding Author
E-mail: chris.ling@sydney.edu.au.
■
2
2
n
Crystal Structure of New Sodium Copper Antimonate Na Cu SbO . J.
3
2
6
*
Solid State Chem. 2005, 178 (4), 1165−1170.
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Synthesis of Li Cu SbO , a New Partially Ordered Rock Salt
Structure. J. Solid State Chem. 1997, 131 (1), 115−120.
17) Seibel, E. M.; Roudebush, J. H.; Wu, H.; Huang, Q.; Ali, M. N.;
ORCID
Maxim Avdeev: 0000-0003-2366-5809
Brendan J. Kennedy: 0000-0002-7187-4579
Chris D. Ling: 0000-0003-2205-3106
3
2
6
(
Ji, H.; Cava, R. J. Structure and Magnetic Properties of the A-
Notes
NaFeO -Type Honeycomb Compound Na Ni BiO . Inorg. Chem.
2
3
2
6
The authors declare no competing financial interest.
2013, 52 (23), 13605−13611.
(18) Nalbandyan, V. B.; Avdeev, M.; Evstigneeva, M. A. Crystal
ACKNOWLEDGMENTS
Structure of Li
Chem. 2013, 199, 62−65.
19) Kumar, V.; Gupta, A.; Uma, S. Formation of Honeycomb
Ordered Monoclinic Li M TeO (M = Cu, Ni) and Disordered
4 6 3 2 6
ZnTeO and Revision of Li Cu SbO . J. Solid State
■
C.D.L. and B.J.K. received funding for this work from the
Australian Research Council − Discovery Projects. A.J.B.
received funding from the Australian Institute for Nuclear
Science and Engineering − AINSE Honours Scholarships.
(
2
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Orthorhombic Li Ni TeO Oxides. Dalton Trans. 2013, 42 (42),
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4992−14998.
20) Kumar, V.; Bhardwaj, N.; Tomar, N.; Thakral, V.; Uma, S.
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DOI: 10.1021/acs.inorgchem.9b01708
Inorg. Chem. XXXX, XXX, XXX−XXX