Single crystal X-ray structure study of the Li(2-x)Na(x)Ni[PO4]F system.

Abstract

The new compounds Li(2-x)Na(x)Ni[PO(4)]F (x = 0.7, 1, and 2) have been synthesized by a solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data. Li(1.3)Na(0.7)Ni[PO(4)]F crystallizes with the orthorhombic Li(2)Ni[PO(4)]F structure, space group Pnma, a = 10.7874(3), b = 6.2196(5), c = 11.1780(4) Å and Z = 8, LiNaNi[PO(4)]F crystallizes with a monoclinic pseudomerohedrally twinned structure, space group P2(1)/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) Å, β = 90° and Z = 4, and Na(2)Ni[PO(4)]F crystallizes with a monoclinic twinned structure, space group P2(1)/c, a = 13.4581(8), b = 5.1991(3), c = 13.6978(16) Å, β = 120.58(1)° and Z = 8. For x = 0.7 and 1, the structures contain NiFO(3) chains made up of edge-sharing NiO(4)F(2) octahedra, whereas for x = 2 the chains are formed of dimer units (face-sharing octahedra) sharing corners. These chains are interlinked by PO(4) tetrahedra forming a 3D framework for x = 0.7 and different Ni[PO(4)]F layers for x = 1 and 2. A sodium/lithium disorder over three atomic positions is observed in Li(1.3)Na(0.7)Ni[PO(4)]F structure, whereas the alkali metal atoms are well ordered in between the layers in the LiNaNi[PO(4)]F and Na(2)Ni[PO(4)]F structures, which makes both compounds of great interest as potential positive electrodes for sodium cells.

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PAPER
Single crystal X-ray structure study of the Li2−xNaxNi[PO4]F system†
Hamdi Ben Yahia,* Masahiro Shikano,* Shinji Koike, Kuniaki Tatsumi and Hironori Kobayashi
Received 22nd November 2011, Accepted 1st March 2012
DOI: 10.1039/c2dt12375d
The new compounds Li2−xNaxNi[PO4]F (x = 0.7, 1, and 2) have been synthesized by a solid state reaction
route. Their crystal structures were determined from single-crystal X-ray diffraction data. Li1.3Na0.7Ni
[PO4]F crystallizes with the orthorhombic Li2Ni[PO4]F structure, space group Pnma, a = 10.7874(3),
b = 6.2196(5), c = 11.1780(4) Å and Z = 8, LiNaNi[PO4]F crystallizes with a monoclinic
pseudomerohedrally twinned structure, space group P21/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) Å,
β = 90° and Z = 4, and Na2Ni[PO4]F crystallizes with a monoclinic twinned structure, space group P21/c,
a = 13.4581(8), b = 5.1991(3), c = 13.6978(16) Å, β = 120.58(1)° and Z = 8. For x = 0.7 and 1, the
structures contain NiFO3chains made up of edge-sharing NiO4F2octahedra, whereas for x = 2 the chains
are formed of dimer units (face-sharing octahedra) sharing corners. These chains are interlinked by PO4
tetrahedra forming a 3D framework for x = 0.7 and different Ni[PO4]F layers for x = 1 and 2. A sodium/
lithium disorder over three atomic positions is observed in Li1.3Na0.7Ni[PO4]F structure, whereas the
alkali metal atoms are well ordered in between the layers in the LiNaNi[PO4]F and Na2Ni[PO4]F
structures, which makes both compounds of great interest as potential positive electrodes for sodium cells.
1. Introduction
The key requirements for a material to be successfully used as a
positive electrode in a rechargeable lithium battery are as
follows: the material has a stable structure during charge and dis-
charge, has a high ionic and electric conductivity, has a high
energy density and contains preferably low cost and environmen-
tally benign materials. Moreover, the safety of the system is
questionable for the large-scale batteries needed for hybrid elec-
tric vehicles (HEV).1
Based on these requirements, an improvement of the electro-
chemical properties and safety of the existing materials such the
ABO2 layered oxides (LiNi1/3Mn1/3Co1/3O2, LiNi1−x−yCox-
AlyO2) with α-NaFeO2-type structure is needed. Therefore,
different techniques have been used (cation and/or anion substi-
tution, coating, etc.) [ref. 2 and references therein]. In addition,
there is a need for exploring new systems in order to discover
new materials like in the case of polyanion (XO4y−; X = P, Si)-
based materials which have been extensively studied as positive
electrode materials in lithium ion batteries. The olivine LiFePO4
(theo. capacity = 170 mA h g−1) is a particularly promising can-
didate due to its advantages of being environmentally benign,
inexpensive and safe.3However, there is a need for a higher
energy density which could be achieved by increasing the
capacity and the discharge voltage.
Several researchers have tried to expand the capacity by using
materials such as Li2FeSiO4(theo. capacity = 331 mA h g−1)
which theoretically could extract two lithium atoms. Higher dis-
charge capacities than LiFePO4have been observed in these sili-
cates.4Few other researchers have focused their studies on the
A2MPO4F fluorophosphates. Indeed, Okada et al. have studied
Li2CoPO4F (theo. capacity = 287 mA h g−1) but concluded that
this compound has no advantage over LiCoPO4.5The same
group has determined the redox potential of 5.3 V against Li
metal for Li2NiPO4F compound, by using a sebaconitrile-based
electrolyte which is stable above 6 Vagainst Li/Li+.6It should be
noted that Li2CoPO4F and Li2NiPO4F are isostructural. The
Nazar and Tarascon research groups have studied Li2FePO4F
(theo. capacity = 292 mA h g−1). They demonstrated that the
intercalation of one Li atom into LiFePO4F (Tavorite structure) is
possible and a reversible and stable capacity of 145 mA h g−1
around a potential of 3 V is observed for this material.7,8Nazar
et al. have also synthesized Na2−xLixMPO4F by different ion
exchange methods starting from layered structure of Na2MPO4F
(M = Fe, Co, Mg). They concluded that these materials are
promising as positive electrode materials for Li-ion or Na-ion
energy storage devices.9They have also synthesized Na2NiPO4F
and suggested that it is isostructural to Na2FePO4F structure,10
which is wrong as demonstrated in this paper.
In this paper, we report the crystal structure study of the
Li2Ni[PO4]F–Na2Ni[PO4]F or Li2−xNaxNi[PO4]F system (0 ≤ x
≤ 2), which led to the discovery of two new layered structures
(for x = 1 and 2). The structural relationship between the differ-
ent layered compounds – LiNaNi[PO4]F, Na2Ni[PO4]F, Na2M
†Electronic supplementary information (ESI) available. CSD numbers
423847–423849. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt12375d
Research Institute for Ubiquitous Energy Devices, National Institute of
Advanced Industrial Science and Technology (AIST), Midorigaoka 1-8-
31, Ikeda, Osaka 563-8577, Japan. E-mail: shikano.masahiro@
aist.go.jp, benyahia.hamdi@voila.fr; Fax: +81-72-751-9609;
Tel: +81-72-751-7932
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[PO4]X (M = Mg, Fe, Co; X = F, OH), Ca3[SiO4]O, and Ca2Na
[SiO4]F – is discussed in detail.
2.Experimental section
2.1.Synthesis
Powder sample of Li1.3Na0.7Ni[PO4]F was prepared by direct
solid state reaction from a stoichiometric mixture of LiF, Li2CO3,
Na2CO3, (CH3COO)2Ni·4H2O, and (NH4)H2PO4. The mixture
was ground in an agate mortar, pelletized and heated at 350 °C
for 2 h and at 800 °C for 48 h with intermediate grinding.
LiNaNi[PO4]F was prepared following the reaction (1):
LiNi½PO4? þ NaF ¼ LiNaNi½PO4?F
The mixture was wet-ball milled in ethanol for 2 h in a planetary
ball-mill. The mixed slurry was dried, pelletized and fired at
700 °C for 12 h.
Na2Ni[PO4]F was prepared following the reaction (2):
ð1Þ
Na4Ni7½PO4?6þ Na3PO4þ 7NaF ¼ 7Na2Ni½PO4?F
After ball milling, the mixture was pelletized and fired at 600 °C
for 24 h in a platinum crucible under air. The progress of the
reactions was followed by powder X-ray diffraction and impuri-
ties have been observed in these samples. The crystals used for
the single crystal investigations were grown in a LiF/NaF flux.
100 mg of LiNaNi[PO4]F sample was thoroughly mixed in an
agate mortar with 900 mg of LiF/NaF. The mixture was placed
in a platinum tube and fired at 870 °C for 12 h and then cooled
to room temperature at a rate of 15 °C h−1. After washing the
mixture with distilled water, brown, dark yellow, yellow and
white single crystals of Li1.3Na0.7Ni[PO4]F, LiNaCo[PO4]F,
Na2Ni[PO4]F, and Na3PO4, respectively, were identified in the
sample using the combination of EDX and single crystal diffrac-
tion analyses.
ð2Þ
2.2. Electron microprobe analysis
Semiquantitative EDX analyses of different single crystals
including the ones investigated on the diffractometer were
carried out with a JSM-500LV (JEOL) scanning electron micro-
scope. The experimentally observed compositions were close to
the ideal ones: Li1.3Na0.7Ni[PO4]F, LiNaNi[PO4]F, and Na2Ni-
[PO4]F. Some crystals showed surface impurities of Na3PO4.
The resolution of the machine did not allow a reliable determi-
nation of the lithium and oxygen content, however the crystal
colours and the determined Na :Ni :P: F ratios enabled us to
distinguish between the different phases.
2.3.X-Ray diffraction
To check the purity of the Li2−xNaxNi[PO4]F (x = 0.7, 1, and 2)
powders, routine X-ray powder diffraction measurements were
performed (Fig. 1). The data were collected at room temperature
over the 2θ angle range 10° ≤ 2θ ≤ 80° with a step size of 0.01°
using a Rigaku diffractometer operating with Cu-Kα1,α2 radi-
ations. Full pattern matching refinements were performed with
the Jana2006 program package.11The backgrounds were esti-
mated by a Legendre function, and the peak shapes were
described by a pseudo-Voigt function. Evaluation of these data
revealed the refined cell parameters a = 10.744(1), b = 6.2170
(6), c = 11.147(1) Å, and V = 744.6(2) Å3for x = 0.7; a =
6.7766(8), b = 11.154(1), c = 5.0314(5) Å, β = 90° and V =
380.31(9) Å3for x = 1; and a = 13.4048(4), b = 5.1944(3), c =
13.6648(13) Å, β = 120.41(1)° and V = 820.60(15) Å3for x = 2,
in good agreement with the single crystal data listed in Table 1.
Single crystals of Li2−xNaxNi[PO4]F (x = 0.7, 1, and 2) suit-
able for X-ray diffraction were selected on the basis of the size
and the sharpness of the diffraction spots. The data collection
was carried out on a Rigaku R-AXIS RAPID diffractometer
using Mo-Kαradiation. Data processing and all refinements were
performed with the Jana2006 program package. A spherical-type
absorption correction was applied. For data collection details,
see Table 1.
Fig. 1
impurities is given along the top.
The XRPD patterns of the Li2−xNaxNi[PO4]F (x = 0.7, 1, and 2) samples obtained by solid state reaction route. A legend for the different
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3.Results and discussion
3.1.Structure refinement
The extinction conditions observed for Li1.3Na0.7Ni[PO4]F were
compatible with space groups Pnma and Pna21. The structure
was solved in the centro-symmetric space group Pnma. Most of
the atomic positions were located using the superflip program
implemented in the Jana2006 package. The use of difference-
Fourier synthesis allowed us to localize the remaining oxygen
atom positions. At this stage of the refinement the chemical
formula was LiNaNi[PO4]F. The refinement of the alkali metal
occupancies showed a strong decrease for Na1 and a slight
increase for Li2 and Li3 atomic positions, indicating a possible
Na/Li disorder over the three crystallographic sites. By introdu-
cing the Li1, Na2 and Na3 atoms and applying restrictions on
their occupancies, coordinates and displacement parameters, the
reliability factors decreased. However, the difference-Fourier
synthesis has shown a density residue close to Li3. This was
attributed to the final Na3 atomic position which is slightly
shifted from Li3. Only occupancy restrictions have been applied
to Li3 and Na3 (Table 2). With anisotropic displacement par-
ameters for all positions, the residual factors converged to the
values listed in Table 1 and the chemical formula became
Li1.3Na0.7Ni[PO4]F. Inspection of the data bases then readily
revealed the structural relationship with Li2Ni[PO4]F, Li2Co-
[PO4]F and Li2Fe[PO4]F.12–14
The cell parameters determined for LiNaNi[PO4]F suggested
an orthorhombic symmetry, however the apparent systematic
absences were not consistent with any known orthorhombic
space group. Therefore, we solved first the structure in the tricli-
nic space group P1ˉ. Most of the atomic positions were located
using the sir2004 program.15After few refinement cycles fol-
lowed by difference-Fourier syntheses, the whole structure was
determined (18 atoms). Then, using the Platon suite of crystallo-
graphic programs,16we determined that a higher symmetry
exists with surprisingly a monoclinic space group P21/c instead
of an orthorhombic one. Starting from the triclinic structural
model and by changing the space group from P1ˉ to P21/c we
reduced the atomic positions by a factor of two. With isotropic
atomic displacement parameters (ADP), the residual factors con-
verged to R(F) = 0.178 and wR(F2) = 0.408 for 37 refined par-
ameters and 868 used reflections. With anisotropic displacement
parameters, the residual factors did not decrease, R(F) = 0.168
and wR(F2) = 0.392 for 82 refined parameters. The refinement of
the alkali metal occupancies did not show any significant devi-
ation from the standard values. At this stage of the refinement,
all the atoms are well behaved despite the high values of the
reliability factors and the goodness of fit (s = 8.3), which are a
clear hint for a symmetry problem, most likely twinning. There-
fore, the Platon suite program has been used in order to detect a
possible twinning of the structure. The two fold axis parallel to c
has been proposed to be a possible twinning element and conse-
quently we introduced the twin matrix (1ˉ0 0, 0 1ˉ0, 0 0 1). By
Table 1
Crystallographic data and structure refinements for Li2−xNaxNi[PO4]F (x = 0.7, 1, and 2) compounds
FormulaLi1.3Na0.7Ni[PO4]FLiNaNi[PO4]FNa2Ni[PO4]F
Crystal color
Crystal radius, mm
M/g mol−1
Crystal system
Space group
a/Å
b/Å
c/Å
β (°)
V/Å3
Z
Density calcd/g cm−3
Temperature/K
F(000)/e
Diffractometer
Monochromator
Radiation/Å
Scan mode
h k l range
θmin, θmax(°)
Linear absorption coeff./mm−1
Absorption correction
Tmin/Tmax
No. of reflections
No. of independent reflections
Reflections used
Rint
Refinement
No. of refined parameters
R factors R(F)/wR(F2) [I ≥ 0σ(I)]
g. o. f.
Weighting scheme
Twin ratio
Diff. Fourier residues/e−Å−3
Brown sphere
0.10
197.9
Orthorhombic
Pnma
10.7874(3)
6.2196(5)
11.1780(4)
Yellow sphere
0.05
202.6
Monoclinic (twinned)
P21/c
6.772 (4)
11.154(6)
5.021(3)
90.00(2)
379.3(4)
4
3.55
293(1)
392
Rigaku R-AXIS RAPID
Graphite
Mo-Kα, 0.71069
Multi-scan
±8, ±14, ±6
3.01, 27.47
5.57
Sphere
0.660/0.665
3622
868
868
0.032
F2
83
0.0228/0.0628
1.18
w = 1/(σ2(I) + 0.001296I2)
0.447(2)
−1.18/+1.20
Yellow sphere
0.05
218.6
Monoclinic (twinned)
P21/c
13.4581(8)
5.1991(3)
13.6978(16)
120.58(1)
825.14(15)
8
3.52
293(1)
848
Rigaku R-AXIS RAPID
Graphite
Mo-Kα, 0.71069
Multi-scan
±17, ±6, ±17
3.5, 27.4
5.23
Sphere
0.676/0.681
7599
1877
1877
0.052
F2
164
0.041/0.069
1.18
w = 1/(σ2(I) + 0.0004I2)
0.452 (2)
−0.43/+0.55
749.97(7)
8
3.50
293(1)
765
Rigaku R-AXIS RAPID
Graphite
Mo-Kα, 0.71069
Multi-scan
±13, ±8, ±14
3.65, 27.43
5.60
Sphere
0.4411/0.4509
6967
928
928
0.024
F2
105
0.0218/0.0522
1.13
w = 1/(σ2(I) + 0.0009I2)
−0.29/+0.29
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Page 4

incorporating this twinning option, the residuals immediately
dropped drastically to the values listed in Table 1. There are
three other possible twin matrixes (a consequence of the coset
decomposition) which can be used and which lead to the same
refinement result; 2[100], m[100], and m[001].
Na2Ni[PO4]F crystallizes with a monoclinic symmetry. The
structure was solved in the centro-symmetric space group P21/c.
Most of the atomic positions were located using the superflip
program implemented in the Jana2006 package. With anisotropic
atomic displacement parameters (ADP), the residual factors con-
verged to R(F) = 0.131 and wR(F2) = 0.302 for 163 refined par-
ameters and 1877 used reflexions (s = 5.21). By introducing the
twin matrix (1 0 1, 0 1ˉ0, 0 0 1ˉ) suggested by the Platon suite
program and corresponding to the twofold axis twinning element
[201], the reliability factors decreased significantly to R(F) =
0.041 and wR(F2) = 0.069 for 164 refined parameters (s = 1.18).
For x = 0.7, 1 and 2, the refined atomic positions and anisotropic
displacement parameters are given in Tables 2 and S1 of the
ESI,† respectively. Further details on the structure refinements
may be obtained from the Fachinformationszentrum Karlsruhe,
D-76344 Eggenstein-Leopoldshafen, by quoting the Registry
No. CSD-423849, CSD-423847, and CSD-423848 for x = 0.7,
1, and 2, respectively.
3.2.Crystal structure
Li1.3Na0.7Ni[PO4]F is isostructural with Li2Ni[PO4]F.12The
structure consists of edge-sharing chains of NiF2O4octahedra
running along the b axis (Fig. 2a and 2b). The NiFO3infinite
Table 2
Atom positions and isotopic displacement parameters (Å2) for Li2−xNaxNi[PO4]F compounds (x = 0.7, 1, and 2)
AtomOccupancy Wyck.xyzUeq(Å2)
Li1.3Na0.7Ni[PO4]F
Na1/Li1
Na2/Li2
Na3
Li3
Ni1
Ni2
P1
P2
O1
O2
O3
O4
O5
O6
F1
F2
0.622(5)/0.378(5)
0.092(7)/0.908(7)
0.0816
0.9184
1
1
1
1
1
1
1
1
1
1
1
1
8d
4c
4c
4c
4a
4b
4c
4c
8d
4c
4c
4c
4c
8d
4c
4c
0.22329(11)
0.2737(3)
0.5362(15)
0.4524(5)
0
0
0.03923(7)
0.24559(6)
0.18230(12)
0.25675(20)
0.18101(18)
0.37867(17)
0.48882(16)
0.01156(12)
0.12472(14)
0.44811(14)
0.00773(18)
1/4
1/4
1/4
0
0
1/4
1/4
0.0464(2)
1/4
1/4
1/4
1/4
0.5469(2)
1/4
1/4
0.34482(11)
0.5833(3)
0.2260(12)
0.2295(4)
0
1/2
0.74398(5)
0.07747(6)
0.03007(12)
0.21231(19)
0.74345(16)
0.02255(16)
0.62568(15)
0.31888(11)
0.47454(12)
0.39205(12)
0.0190(4)
0.0171(12)
0.0157(15)
0.0157(15)
0.00713(13)
0.00728(13)
0.00775(19)
0.00766(18)
0.0141(4)
0.0231(7)
0.0167(6)
0.0135(5)
0.0095(5)
0.0128(4)
0.0109(4)
0.0131(4)
LiNaNi[PO4]F
Li
Na
Ni
P
O1
O2
O3
O4
F
1
1
1
1
1
1
1
1
1
4e
4e
4e
4e
4e
4e
4e
4e
4e
0.7463(8)
0.5099(2)
0.98037(7)
0.24213(13)
0.4539(3)
0.1029(4)
0.1937(4)
0.2160(4)
0.7904(3)
0.0823(4)
0.16529(11)
0.17231(3)
0.41794(6)
0.12388(19)
0.32913(19)
0.07442(18)
0.95895(18)
0.21364(17)
0.2344(15)
0.7480(3)
0.76617(9)
0.7742(2)
0.2372(5)
0.6364(5)
0.5745(5)
0.1445(4)
0.4652(3)
0.0062(15)
0.0149(5)
0.00562(13)
0.0049(2)
0.0118(6)
0.0100(7)
0.0086(7)
0.0085(6)
0.0102(5)
Na2Ni[PO4]F
Na1
Na2
Na3
Na4
Ni1
Ni2
P1
P2
O1
O2
O3
O4
O5
O6
O7
O8
F1
F2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
4e
0.0812(5)
0.8296(5)
0.4189(5)
0.6700(5)
0.17655(13)
0.32311(13)
0.9126(2)
0.4117(2)
0.3941(3)
0.8924(3)
0.3452(3)
0.4589(7)
0.1303(6)
0.3689(6)
0.9584(7)
0.8461(3)
0.7527(5)
0.2516(5)
0.7704(5)
0.7387(5)
0.2579(5)
0.2602(5)
0.28356(13)
0.72328(13)
0.2015(4)
0.2919(3)
0.5852(7)
0.5922(7)
0.3511(7)
0.7686(7)
0.7172(8)
0.2165(9)
0.7622(6)
0.3422(6)
0.0043(8)
0.0047(9)
0.4118(5)
0.9194(4)
0.5824(5)
0.8385(4)
0.35096(13)
0.42485(13)
0.8230(2)
0.8385(2)
0.8460(3)
0.2984(3)
0.3862(3)
0.0884(7)
0.0964(7)
0.7188(7)
0.6233(7)
0.7081(3)
1.0019(3)
0.4704(3)
0.016(2)
0.015(2)
0.017(2)
0.0138(19)
0.0086(4)
0.0080(4)
0.0077(8)
0.0071(7)
0.0138(18)
0.0133(18)
0.0113(17)
0.012(3)
0.014(2)
0.014(2)
0.011(3)
0.0124(17)
0.0115(15)
0.0126(14)
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chains are cross connected by the PO4tetrahedra giving rise to
channels and cavities in which the sodium and the lithium atoms
are located, respectively (Fig. 2a). Interatomic distances and
bond valence sums (BVS)17,18are listed in Table 3.
In the Ni1F2O4octahedra, the Ni1–X (X = O, and F) distances
range from 2.014 to 2.097 Å with an average value of 2.052 Å,
whereas in the Ni2F2O4octahedra the Ni2–X distances range
from 2.045 to 2.073 Å with an average distance of 2.056 Å. This
is very similar to the Ni2+environments in Li2Ni[PO4]F, in
which the distances range from 2.013 to 2.082 Å and from 2.011
to 2.081 with average distances of 2.044 Å and 2.054 Å, respect-
ively. The BVS of 1.956 and 1.934 are in very good agreement
with the expected value of +2 for Ni2+. The P1O4tetrahedron is
quite regular, however P2O4is slightly distorted with one short
P–O distance of 1.511 Å. The average P–O distances of 1.543
and 1.533 Å for P1 and P2, respectively, are consistent with the
value of 1.55 Å estimated from the effective ionic radii of the
four-coordinated P5+and O2−.19The BVS of 4.89 and 5.02 are
in agreement with the expected value of +5 for P5+.
The sodium atoms are mainly located in the tunnels. The Na1/
Li1+ions are bonded to four O2−and one F−atoms with an
average Na1/Li1–X distance of 2.271 Å. They form a distorted
square pyramid similar to the Li1 coordination in the Li2Ni[PO4]F.
The BVS calculation by using the five shortest Na1/Li1–X con-
tacts gives 1.075, in good agreement with the expected value of
+1. The coordination polyhedra of the lithium ions are not well
defined. If one consider only the shortest Li–X distances, both
Li2 and Li3 would be tetrahedrally coordinated. However, the
BVS calculations have shown that both Li2 and Li3 are strongly
underbonded. When the coordination sphere of Li2 and Li3 are
increased to five, the BVS become 0.948 and 0.976, respectively
(Table 3). Thus, the Li2 and Li3 atoms are best considered to be
4 + 1 coordinated. These lithium polyhedra are slightly different
from those in Li2Ni[PO4]F (Fig. 3). Indeed, for x = 0, the Li2–
O5 distance is less stretched (2.168 compared to 2.368 Å) and
the Li3 atom is 4 + 2 instead of 4 + 1-coordinated.
LiNaNi[PO4]F compound crystallizes with a layered structure.
It is a monoclinic pseudomerohedrally twinned structure which
consists of Ni[PO4]F layers with the lithium and sodium atoms
well-ordered in the interlayer space (Fig. 4a). The nickel atoms
are octahedrally coordinated to four oxygen and two fluorine
atoms. These octahedra share edges (via 1F and 1O) and form
infinite chains running along the c axis. The PO4 tetrahedra
connect these chains to form Ni[PO4]F layers (Fig. 4b), in
between which are located the lithium and sodium atoms. The
interatomic distances Na–X, Ni–X and P–O are listed in Table 3.
In LiNaNi[PO4]F, the NiO4F2octahedra are regular in shape
with Ni–X distances ranging from 2.028 to 2.067 Å and with an
average value of 2.043 Å. The environment of Ni in LiNaNi-
[PO4]F is similar to the one in Li2Ni[PO4]F and Li1.3Na0.7Ni-
[PO4]F although the structures are different. The PO4tetrahedra
are also quite regular, with P–O distances ranging from 1.519 to
1.546 Å with an average value of 1.532, only slightly lower than
the value of 1.55 Å estimated from the effective ionic radii of
the four-coordinated P5+and O2−.19The sodium atoms are co-
ordinated to five oxygen and two fluorine atoms (Fig. 5a). The
Na–X distances range from 2.379 to 2.633 Å with an average
value of 2.493 Å. This sodium environment is similar to sodium
polyhedra in Na2Co[PO4]F (Fig. 5c). The lithium atoms are 4 +
2 coordinated (Fig. 5b). Indeed, if only the four short distances
are used for BVS calculations, the lithium atoms become under-
bonded. The BVS of 0.989, 1.028, 1.994, and 5.038 are in very
good agreement with the expected values of +1, +1, +2, and +5
for Li+, Na+, Ni2+, and P5+, respectively.
Na2Ni[PO4]F crystallizes with a new type of structure which
consists of Ni[PO4]F layers with the sodium atoms located in the
interlayer space (Fig. 6a). Both nickel atoms are octahedrally
coordinated to four oxygen and two fluorine atoms. The
Ni1O4F2 and Ni2O4F2 octahedra share a face and form
Ni2O5F2.5 dimers units (Fig. 6b). These units are connected
together by sharing corners (via F) to form ribbons running par-
allel to the b axis (Fig. 6b). The PO4tetrahedra connect these
ribbons to form Ni[PO4]F layers in-between which are located
the sodium atoms (Fig. 6a). The interatomic distances Na–X,
Ni–X and P–O are listed in the Table 3. The Ni1O4F2 and
Ni2O4F2octahedra are regular with an average Ni–X distance of
2.055 and 2.058 Å, respectively. Such a six-coordinated Ni2+
environment occurs also in the Li2Ni[PO4]F and Li1.3Na0.7Ni-
[PO4]F structures. The P1O4 tetrahedron is regular in shape,
whereas P2O4is distorted with P1–O distances ranging from
Fig. 2
view along [100] without the phosphorus and alkali metal atoms for
clarity reasons (b).
Perspective view of the Li1.3Na0.7Ni[PO4]F structure (a), and
Fig. 3
(a), and Li2Ni[PO4]F structures (b).
Surrounding of the sodium and lithium atoms in Li1.3Na0.7Ni[PO4]F
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Table 3
brackets
Interatomic distances (in Å) and bond valences (BV) for Li2−xNaxNi[PO4]F compounds (x = 0.7, 1, and 2). Average distances are given in
Distance BV DistanceBV
Li1.3Na0.7Ni[PO4]F
Na1/Li1–O2
Na1/Li1–O3
Na1/Li1–O6
Na1/Li1–O1
Na1/Li1–F1
<Na1/Li1–X>
2.1436(19)
2.2179(17)
2.3272(18)
2.3320(18)
2.346(16)
2.27335
0.401/0.161
0.328/0.132
0.244/0.098
0.240/0.097
0.165/0.093
1.378/0.581
1.07515 a
0.241
0.171
0.207
0.088
0.9485 a
0.291
0.221
0.172
0.071
0.9765 a
Ni1–O1 (×2)
Ni1–F2 (×2)
Ni1–O5 (×2)
<Ni1–X>
Ni2–O4 (×2)
Ni2–O6 (×2)
Ni2–F1 (×2)
<Ni2–X>
2.0144(13)
2.0438(9)
2.0966(11)
2.0516
2.0455(12)
2.0477(13)
2.0733(10)
2.0555
0.378
0.298
0.302
1.9566
0.347
0.345
0.275
1.9346 a
Li2–O1 (×2)
Li2–F1
Li2–O3
Li2–O5
<Li2–X>
Li3–F2
Li3–O6 (×2)
Li3–O2
Li3–O4
<Li3–X>
1.994(2)
2.015(4)
2.051(4)
2.369(4)
2.084
1.818(5)
2.027(3)
2.120(6)
2.446(5)
2.087
P1–O3
P1–O6 (×2)
P1–O5
<P–O>
P2–O2
P2–O1 (×2)
P2–O4
<P–O>
1.528(2)
1.5441(15)
1.5540(18)
1.5425
1.511(2)
1.5312(14)
1.5603(19)
1.5334
1.272
1.218
1.186
4.8944 a
1.332
1.261
1.166
5.0204 a
LiNaNi[PO4]F
Li–F
Li–O4
Li–O1
Li–O3
Li–O2
Li–F
<Li–X>
Ni–O4
Ni–O2
Ni–F
Ni–O2
Ni–O3
Ni–F
<Ni–X>
1.892(6)
1.974(8)
2.034(6)
2.035(6)
2.656(6)
2.664(6)
1.9844
2.028(3)
2.036(3)
2.038(2)
2.043(3)
2.051(3)
2.067(2)
2.0438
0.237
0.253
0.215
0.215
0.040
0.029
0.9896 a
0.364
0.356
0.303
0.349
0.342
0.280
1.9946 a
Na–O1
Na–O4
Na–F
Na–O3
Na–O1
Na–F
Na–O1
<Na–X>
2.383(4)
2.379(3)
2.433(3)
2.524(3)
2.528(4)
2.574(3)
2.633(4)
2.4937
0.209
0.211
0.130
0.142
0.141
0.089
0.106
1.0287 a
P–O1
P–O4
P–O2
P–O3
<P–O>
1.519(3)
1.530(3)
1.532(3)
1.546(3)
1.5317
1.303
1.265
1.258
1.212
5.0384 a
Na2Ni[PO4]F
Na1–O5
Na1–F2
Na1–F1
Na1–O8
Na1–O2
Na1–O7
Na1–O7
<Na1–X>
2.267(11)
2.349(8)
2.371(7)
2.372(9)
2.390(6)
2.481(5)
2.815(5)
2.3716
2.4357
2.296(13)
2.364(7)
2.365(8)
2.375(7)
2.393(8)
2.512(5)
2.783(5)
2.3846
2.4417
2.026(4)
2.029(4)
2.032(11)
2.069(4)
2.081(4)
2.096(5)
2.0555
1.535(9)
1.543(11)
1.543(4)
1.557(4)
1.5445
0.285
0.163
0.153
0.215
0.205
0.160
0.065
1.1816 a
1.2467 a
0.264
0.220
0.156
0.152
0.203
0.147
0.071
1.1426 a
1.2137 a
0.366
0.310
0.360
0.326
0.270
0.303
1.9356 a
1.248
1.221
1.221
1.176
4.8664 a
Na2–F1
Na2–O6
Na2–O2
Na2–O7
Na2–O5
Na2–F2
Na2–O5
<Na2–X>
2.334(9)
2.363(8)
2.380(9)
2.424(9)
2.465(6)
2.575(10)
2.909(6)
2.42356
2.49287
2.342(6)
2.365(10)
2.396(7)
2.409(14)
2.470(6)
2.603(6)
2.909(6)
2.4306
2.4997
2.016(6)
2.030(5)
2.048(7)
2.067(4)
2.094(7)
2.098(3)
2.0588
1.488(9)
1.536(8)
1.542(6)
1.555(4)
1.5302
0.169
0.220
0.210
0.187
0.167
0.088
0.050
1.0416 a
1.0917 a
0.166
0.219
0.201
0.194
0.165
0.082
0.050
1.0276 a
1.0777 a
0.321
0.362
0.345
0.328
0.260
0.301
1.9176 a
1.417
1.245
1.225
1.182
5.0694 a
Na3–O6
Na3–O1
Na3–F1
Na3–F2
Na3–O3
Na3–O4
Na3–O4
<Na3–X>
Na4–F1
Na4–O5
Na4–O1
Na4–O4
Na4–O6
Na4–F2
Na4–O6
<Na4–X>
Ni1–O2
Ni1–F2
Ni1–O7
Ni1–O8
Ni1–F1
Ni1–O3
<Ni1–X>
P1–O7
P1–O5
P1–O8
P1–O2
<P1–O>
Ni2–F2
Ni2–O1
Ni2–O4
Ni2–O3
Ni2–F1
Ni2–O8
<Ni2–X>
P2–O6
P2–O4
P2–O3
P2–O1
<P2–O>
aBond valence sum, BV = e(r0–r)/bwith the following parameters: b = 0.37, r0(NaI–O) = 1.803, r0(NaI–F) = 1.677, r0(NiII–O) = 1.654, r0(NiII–F) =
1.596 and r0(PV–O) = 1.617 Å.17,18The BVS of anions are given in Fig. S2.†
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1.535 to 1.557 Å and P2–O ranging from 1.488 to 1.555 Å. The
average P–O distances of 1.5445 and 1.5302 Å are in agreement
with the value of 1.55 Å estimated from the effective ionic radii
of the four-coordinated P5+and O2−. All the sodium atoms are
considered six coordinated, although an additional stretched Na–
O distance exists around each sodium atom (Fig. 7). The Na–X
distances cover a large range; Na–Xmin distances range from
2.267 to 2.342 Å; Na–Xmaxrange from 2.481 to 2.603 Å; and
the average Na–X distances range from 2.371 to 2.435 Å (CN =
6). The results of the (BVS) calculations (Table 3) confirmed the
expected charge balance NaI2NiIIPVO−II4F−I.
3.3.
with Na2M[PO4]F structure (M = Mg, Fe, and Co)
Structural relationship of Li2−xNaxNi[PO4]F (x = 1, and 2)
The Li2−xNaxNi[PO4]F structures (x = 1 and 2) are closely
related to the orthorhombic Na2M[PO4]F-type (M = Fe, Co, and
Mg). Indeed, the projection views of the crystal structures of
these compounds along the M[PO4]F layers do not show any sig-
nificant difference (Fig. 8a–8c), expect in their unit cell par-
ameters. In the three cases, the structures are built up of M[PO4]F
layers separated by the alkali metals ions. The main difference
resides in the connection between the MO4F2octahedra forming
the MFO3chains. In order to emphasize the features of each
structure, projection views perpendicular to the M[PO4]F layers
are depicted on Fig. 8d–8f. In Na2M[PO4]F-type structure (M =
Fig. 4
plane (a) and projection view of the Ni[PO4]F layer on the (100)
plane (b).
Projection view of the structure of LiNaNi[PO4]F on the (001)
Fig. 5
structure (b), and of the sodium atoms in Na2Co[PO4]F structure (c).
Surrounding of the sodium (a), and lithium atoms in LiNaNi[PO4]F
Fig. 6
plane (a) and perpendicular to the Ni[PO4]F layer plane, without the
alkali metal atoms for clarity reasons (b).
Projection view of the structure of Na2Ni[PO4]F on the (100)
Fig. 7
The stretched Na–O distance is highlighted in red.
Surrounding of the sodium atoms in Na2Ni[PO4]F structure.
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Mg, Fe, Co, and Ni), the MO4F2octahedra share faces and form
dimer units. These units are connected together by sharing
corners (via F) to form infinite chains running along the a or b
axis. In Na2Co[PO4]F, all the dimer units point in the same direc-
tion (Fig. 8g), however in Na2Ni[PO4]F-type structure, the dimer
units point in two different directions (Fig. 8i). On the contrary
to Na2M[PO4]F (M = Mg, Fe, Co, and Ni), in LiNaNi[PO4]F
structure the octahedra share edges and form bioctahedral units
with two possible orientation directions depending on the units
chosen (Fig. 8h). This observation, concerning the orientation of
the dimer units, enabled us to propose possible structural tran-
sition mechanisms to transform LiNaNi[PO4]F to Na2Ni[PO4]F,
Na2Co[PO4]F and even to a new theoretical-type of structure.
In LiNaNi[PO4]F structure, the NiFO3 infinite chains are
formed of edge-sharing octahedra, however for our convenience,
these chains are described in this section as edge-sharing dimer
units formed of edge-sharing octahedra (Fig. 9a). When the
oxygen atoms indicated by a double-headed arrow shift to each
other due to the replacement of Li by Na atoms, the edge-
sharing octahedra 1 and 2 are transformed to face-sharing octa-
hedral (Fig. 9b and 9d) and the edge-sharing octahedra 2 and 1
are transformed to corner-sharing octahedra. This leads to the
called 12–12 chain (Fig. 9b). The same mechanism could be
applied to the 2–1 dimer units. This leads to the called 21–21
chain (Fig. 9c). When only mechanism 1 is applied to all the
NiFO3infinite chains, an alternation of only one type of chain is
observed and that corresponds to Na2Co[PO4]F-type structure
(Fig. 10a). However, when mechanism 1 is applied to two suc-
cessive NiFO3chains and mechanism 2 to the following two
chains, an alternation of two 12–12 and two 21–21 chains is
Fig. 8
alkali metal atoms for clarity reasons (d), (e), and (f); and view of the different orientations of the dimer units forming the MO3F infinite chains (g),
(h), and (i) for Na2Co[PO4]F, LiNaNi[PO4]F, and Na2Ni[PO4]F compounds, respectively.
Projection view of the structures along the M[PO4]F (M = Co, and Ni) layers (a), (b), and (c); perpendicular to the layers and without the
Fig. 9
edge-sharing octahedra to the infinite chains of dimer units (formed of face-sharing octahedra) sharing corners, called (12–12 chain) (b) or (21–21
chain) (c). These transformations are due to the tilting of the octahedral sharing edges when the lithium atoms are replace by sodium atoms (d). This
tilt occurs in order to accommodate realistic Na–O distances.
Projection view of the Ni[PO4]F layer in LiNaNi[PO4]F compound (a), and transformation mechanisms from the infinite NiO3F chains of
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obtained and that corresponds to Na2Ni[PO4]F-type structure
(Fig. 10b). Furthermore, when mechanism 1 and 2 are applied to
successive NiFO3chains, then one 12–12 chain alternates with
one 21–21 chain and a new theoretical type of structure is
created (Fig. 10c). For the layered A2M[PO4]X compounds (A =
Li, and Na; M = Mg, Fe, and Co, Ni; X = F, and OH), the struc-
ture-type depends on the choice of the dimer orientations and on
the alternating chain-types, which themselves depend strongly
on the size of the transition metal ions and the sodium/lithium
ratio. In order to emphasize this influence, a common super-cell
to the three layered structure-types is built. Therefore, three
matrixes (0 0 1ˉ, 2ˉ0 1, 0 1 0), (2 0 0, 0 2 0, 0 0 1), and (0 1 0, 0
0 2, 1 0 0) have been used to transform the monoclinic Na2Ni-
[PO4]F, the monoclinic LiNaNi[PO4]F, and the orthorhombic
Na2M[PO4]X-sub-cells (M = Mg, Fe, and Co; X = F, and OH),
respectively, to the orthorhombic super-cells as depicted in
Fig. S1 of the ESI.† The evolution of the super-cell volume as a
function of the transition metal radius is given in Fig. 11. The
structural change is reflected by a discontinuity of this cell
volume evolution between r = 0.69 and 0.72. The decrease of
the cell volume in all the r range is in good agreement with the
replacement of Fe2+by Co2+, Mg2+or Ni2+and Na+by Li+, the
Shannon radius of Fe2+, Co2+, Mg2+and Ni2+being equal to
0.78, 0.745, 0.72, and 0.69 Å (CN = 6), respectively. The super-
cell volumes of Na2Ni[PO4]F and LiNaNi[PO4]F are 3.13% and
11.61% smaller than Na2Fe[PO4]F, respectively.
3.4.
Ca2Na[SiO4]F and Ca2Ca[SiO4]O structures
Structural relationship of LiNaNi[PO4]F with
The careful inspection of databases revealed a strong relationship
between LiNaNi[PO4]F (P21/c, a = 6.772(4), b = 11.154(6), c =
5.021(3) Å, β = 90°), Ca2Na[SiO4]F (Pnma, a = 5.335, b =
7.144, c = 12.438 Å),20and Ca2Ca[SiO4]O (P63/mmc, a = b =
7.099, c = 5.687 Å)21structures, although a clear difference in
the symmetry of these compounds exists. Fig. 12 emphasizes the
similarity between the three layered structures. The difference in
the symmetry is mainly due to the difference of the chemical
compositions. In Ca2Ca[SiO4]O, the three calcium atoms occupy
only one atomic position; 6h (0.1568, 0.3137, 1/4). Therefore,
Fig. 10
[PO4]F with an alternation of two 12–12 with two 21–21 chains; and (c) in the new-theoretical-type with an alternation of 12–12 with 21–21 chains.
Projection view of the M[PO4]F (M = Co, and Ni) layers: (a) in the compound Na2Co[PO4]F with only one type of chain; (b) in Na2Ni-
Fig. 11
radius for the layered A2M[PO4]X structures (A = Li, Na; M = Mg, Fe,
Co, Ni; X = F, OH).
Evolution of the super-cell volume with the transition metal
Fig. 12
Ca2Na[SiO4]F, and LiNaNi[PO4]F compounds.
View along the layers of the crystal structure of Ca3[SiO4]O,
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they are symmetrically equivalent. When one replaces 1/3 of the
calcium by sodium and 1/5 of the oxygen by fluorine atoms, the
Ca2Na[SiO4]F is obtained and a decrease in the symmetry is
observed. Since Na and Ca are chemically different, the 6h
atomic position (P63/mmc) has been split to 4c and 8d (Pnma) in
order to make the Na and Ca non-equivalent. This is the origin
of the symmetry lowering from P63/mmc to Pnma. When the
two atoms at the 8d (Pnma) atomic positions become chemically
different, a further splitting of the 8d position is expected and a
decrease in the symmetry should be observed. Indeed, when 2Ca
and Na has been replaced by Na, Ni, and Li, the 8d (Pnma) is
split to 2 × 4e (P21/c) atomic positions. The details of these
group–subgroup transformations are depicted in Fig. 13.
4.Conclusions
The Li2−xNaxNi[PO4]F system (0 ≤ x ≤ 2) has been reinvesti-
gated using solid state reaction route. The single crystals of the
compositions x = 0.7, 1, and 2 have been grown using salt flux
method. Crystal structure analysis has shown that a complete
solid solution, with the Li2Ni[PO4]F type structure, exists in the
composition 0 ≤ x ≤ 0.7 range. At x = 1 a first structural tran-
sition from a 3D to a layered structure has been observed.
During this transformation, a strong rearrangement of the differ-
ent atoms’ positions occurred, except for the nickel atoms which
remained approximately at the same positions. Indeed, in the x =
0 and 1 structures, infinite chains of edge-sharing octahedra
exist. The replacement of all the Li by Na atoms does not
influence the layered character of the structure, however the
infinite chains of octahedra sharing edges are transformed to
infinite chains of dimer unites (formed of face sharing octahedra)
sharing corners. The structural relationship between the different
layered A2M[PO4]X compounds (A = Li, Na; M = Mg, Fe, Co,
Ni; X = F, OH) have been discussed and phase transition mech-
anisms have been proposed. The structural changes depend
strongly on the size of the transition metal and the Li/Na ratio.
Na2Ni[PO4]F and the intermediate phase LiNaNi[PO4]F are
expected to be of great interest as positive electrodes for sodium
cells.
Acknowledgements
This work was carried out in the Li-EAD Project of the New
Energy and Industrial Technology Development Organization
(NEDO) in Japan. We are grateful to Professor Qiang Xu and
Mr. Peizhou Li for collecting the single crystal data.
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Fig. 13
Ca2Na[SiO4]F, and LiNaNi[PO4]F compounds are highlighted by the same color.
Theoretical group–subgroup transformation scheme for the structure of Ca2Ca[SiO4]O. The similarities with the experimental structures of
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