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Flux-growth and characterization of LiFePO4 single crystals G. Lianga,*, J. Lib, R. Bensonc, K. Parkd
, D. Vakninb, and J. T. Markertd aDepartment of Physics, Sam Houston State University, Huntsville, Texas 77341, USA
bAmes Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA cRigaku Americas Corporation, 9009 New Trails Drive, The Woodlands, Texas 77381, USA
dDepartment of Physics, University of Texas at Austin, Austin, Texas 78712, USA
_____________________________________________________________________________
Abstract
Large size high quality LiFePO4 single crystals have been grown by flux growth technique
with LiCl as flux. The as-grown single crystals have volumes up to about 300 mm3(∼ 1.0 g).
Single-crystal x-ray diffraction (XRD) measurements at T = 293 K shows the crystals are
orthorhombic with space group Pnma (Z = 4). The lattice parameters obtained from the
refinement are: a = 10.3172 (11) Å, b = 6.0096(8) Å, c = 4.6775 (4) Å. The Fe-O and P-O bond
lengths were obtained. Powder XRD pattern of ground LiFePO4 single crystals shows that the
crystals are pure phase. Magnetic susceptibility, measured with applied field along the a-axis,
shows that the Fe ions are antiferromagnetically ordered at Neel temperature TN = 51 ± 2 K.
Above TN, the Fe ions are in the paramagnetic state with an effective moment μeff = 5.42 μB/Fe,
which is close to the μeff value of the Hund’s rule ground state of Fe2+ ions with orbital moment
quenched.
PACS: 61.10. Nz; 81.10.-h; 61.66.Fn; 75.50.Ee
Keywords: A2. Growth from high temperature solutions; A2. Flux method; A1. X-ray diffraction;
A1. Crystal structure; B1. Lithium iron phosphate.
_______________________________________________________________________________
*Corresponding author: Tel: +1-936-294-1608; fax: +1-936-294-1585.
E-mail address: [email protected] (G. Liang)
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1. Introduction
Lithium iron phosphate, LiFePO4, has been considered as one of the most promising
candidates for next generation rechargeable Li-ion batteries cathode material due to its high
theoretical specific capacity (∼170 mAh/g), high cycle life, low cost, high thermal stability, and
non-toxicity [1-6]. However, the intrinsically poor electronic conductivity in the range from 10-10
S/cm to ∼ 10-5 S/cm of LiFePO4 [3, 7, 8] limits the delivery of high specific capacity at high
discharge rates. At present, there is a controversy regarding whether the enhancement in the
electronic conductivity for cation-doped LiFePO4 is truly due to the substitution of Li+ by the
cations or due to the grain-boundary impurity network [3, 9-12]. The best way to resolve this
controversy is to synthesize pure phase and sizable (> 10 mm3, for example ) cation-doped
LiFePO4 single crystals for electronic conductivity studies, because such single crystals are free
of impurity grain-boundaries and thus that complicating factor can be ruled out. Also, the
anisotropy of the magnetic and electronic structure can be studied only by using high quality and
sizable single crystals. Thus, it appears very important to synthesize large-size high quality
LiFePO4 and cation-doped LiFePO4 single crystals for the study of the electronic conductivity
and other physical/chemical properties.
Currently, due to the unavailability of large size LiFePO4 single crystals, almost all of the
studies including electronic conductivity measurements were carried out on polycrystalline
LiFePO4-based materials synthesized by various methods [3, 13-19]. In the past, few results on
the growth of LiFePO4 single crystals were reported. For example, the hydrothermal growth [20,
21] has been reported, but the grown LiFePO4 single crystals were too small (with radius less
than 0.15 mm) to be used for certain physical property studies such as the measurements of four
probe electronic conductivity. Recently, growth of LiFePO4 crystals using an optical floating
zone [22] technique was reported. In the 1960s, Mercier et al. [23-26] reported the growth of
single crystals of LiMPO4 (M = Mn, Co, Ni, Fe) by a flux method, however, the size and quality
of the crystals were not reported. To our knowledge, there have been virtually no detailed reports
on the growth of sizable pure phase LiFePO4 single crystals using flux method. Very recently,
we have successfully grown LiFePO4 single crystals by a flux method for magnetic neutron
scattering studies from spin-waves [27]. In this paper, we report the details of the growth of
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sizable and high quality LiFePO4 single crystals by standard flux method and the results on the
single-crystal (SC) x-ray diffraction (XRD), powder XRD, and magnetic susceptibility.
2. Experimental details
LiFePO4 single crystals were grown by a standard flux growing technique from
stoichiometric mixture of high purity FeCl2 (99.999% Aldrich) and Li3PO4 (99.999% Aldrich),
carried out in an Ar atmosphere. LiCl was used as the flux during the following chemical
reaction: FeCl2 +Li3PO4 +LiCl = LiFePO4 + 3LiCl. To obtain large-size single crystals, the
molar ratio between the LiFePO4 and LiCl was kept at a value close to 1:3. The growth was
performed in sealed platinum crucibles. Small holes of about 50 μm diameter were made on the
crucibles to release the high vapor pressure of LiCl. The mixture was pre-melted at 800 °C and
then heated at 890 °C for 5 hours (h), soaked at 890 °C for 5 h, slowly cooled down to 710 °C at
a rate of 0.7 °C/h, and then further cooled to 650 °C at a rate of 1.5 °C/h. The furnace was turned
off at 650 °C and naturally cooled to room temperature. The crystals were extracted from the
mixture by dissolving the extra LiCl by water at room temperature. This protocol is similar to the
one used for LiNiPO4 SC growth [28]
The SC XRD data were measured at T = 293 K using a Rigaku SPIDER x-ray diffractometer
with Mo Kα radiation (λ = 0.7107 Å) to a resolution corresponding to sinθmax/λ = 0.6486 Å-1.
The data refinement was done using program SHELXL [29]. Powder XRD of ground single
crystals at room temperature was measured on a Rigaku Geigerflex diffractometer using Cu Kα
radiation. The intensity data were accumulated at 0.02° step and a scanning rate of 5 seconds per
step. The data was analyzed by software package Jade 6.1 provided by the Material Data Inc.
The temperature dependent magnetic susceptibility measurements were carried out on a
commercial superconducting quantum interference device (SQUID) magnetometer (model
MPMS, Quantum Design) in the temperature range 5-300 K and at a field of 1 kOe.
3. Results and discussion
The as-grown single crystals have volumes up to about 300 mm3 and mass up to 1.0 g, with
average dimensions 4 mm × 4 mm × 6 mm ≈ 100 mm3. Fig. 1 shows some of the as-grown
crystals with volume between 100 mm3 and 200 mm3. Most of the as-grown crystals are
irregular in shape and dark-greenish in color. The single crystal sample used for SC XRD
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measurement was a small piece (about 0.06 mm3) which was cut from a bigger rectangular
crystal used for the magnetization measurement (see below). Shown in Fig. 2 is the SC XRD
pattern measured with the x-ray along the a-axis of the crystal. The measurements of 1330
reflections gave 350 unique reflections with Rint = 0.032 and I > 2σ (I). The refinement method
used is the full-matrix least-squares on F2, with the goodness-of-fit on F2 to be 1.100. The
refinement result indicates that the crystal has orthorhombic crystal structure with space group
Pnma (No. 62) and Z = 4, and yields lattice parameters: a = 10.3172 (11) Å, b = 6.0096(8) Å, c =
4.6775 (4) Å. The obtained atomic coordinates for Li, Fe, P, and O are listed in Table I.
Our result is consistent with the earlier single crystal XRD results reported by Streltsov et al.
[20], i.e., the cations occupy three different positions: an octahedral (Fe) site, a octahedral (Li)
site, and a tetrahedral (P) site. Fig. 3 is a general view of the structure which contains the FeO6
octahedra (in orange) and PO4-3 tetrahedra (in yellow). Each FeO6 octahedron is connected to
four other FeO6 octahedra by corner-sharing in the b-axis (or [010]) and c-axis ([001]) directions,
and connected to four PO4-3 tetrahedra in a-axis ( or [100]) direction via edge- and corner-
sharing. The Li ions are located at the centers of highly distorted LiO6 octahedra. Table II
summarizes the values of the Fe-O and P-O bond lengths and Table III lists all of the bond
angles formed between any two O-Fe bonds or two O-P bounds. The different values of the Fe-O
bond-lengths and the deviations of the bond-angles from 90° clearly indicate that the FeO6
octahedra are distorted. The maximum difference between the bond lengths is 0.181 Å within an
average bond length of 2.154 Å.
Figure 4 presents powder XRD pattern of a ground single crystal in the 2θ range of 15° ≤ 2θ
≤ 65°. The Kα2 lines have been removed from the pattern. All the reflections in the pattern can
be indexed with the orthorhombic structure of space group Pnma. No impurity trace or inclusion
is observed, indicating the single crystals synthesized by our flux method consist of a single
phase. The least square refinement was performed over the 2θ range 15° ≤ 2θ ≤ 65° with
intensity weight, yielding the following values of the lattice parameters: a = 10.3167 (12) Å, b =
5.9980(10) Å, c = 4.6905 (6) Å. For the least square fit, the estimated standard deviation (ESD)
is 0.014°, the average of Δ(2θ) is 0.012°, and the Smith-Snyder figure-of-merit is F(24) = 34.6
(58). Compared with the values determined from the SC XRD data above, the value of the
parameter a is the same within the ESD range, but the values of parameters b and c have a very
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small difference. These values of lattice parameters are almost identical to those powder XRD
results reported in the literature [4, 22, 30].
Figure 5 shows the temperature dependent magnetic susceptibility, χ(T) and inverse
magnetic susceptibility, χ-1(T) curves. The χ(T) was measured on the crystal (dimensions 0.7
mm × 1.6 mm × 2.6 mm, mass 10.4 mg) from which the SC XRD sample was taken. A magnetic
field of 1 kOe was applied along the a-axis ([100] direction) of the crystal. The χ(T) curve in
Fig. 5 indicates that LiFePO4 is antiferromagnetically (AFM) ordered at Neel temperature TN =
51 ± 2 K, where TN is defined as the temperature at the cusp of the χ(T) curve. This TN value is
very close to the values (≈ 50 ± 2 K) reported in the literature [31-33]. Below TN, the magnetic
susceptibility decreases with the decrease of temperature and stays almost constant below 30 K,
showing a typical behavior of the perpendicular susceptibility (with the field perpendicular to the
easy axis, here the b-axis) for AFM single crystals [34, 35]. Above TN, the Fe ions are
paramagnetic, as can be seen clearly from the linear dependence of the χ-1 on the temperature T,
shown in the inset of Fig. 5. Using the Curie-Weiss law χ(T) = C/(T -θ) with the Curie constant
C = NAg2μB2S(S+1)/3kB [35], the χ-1(T) data can be well fitted to χ-1= (T - θ )/C in the range of
60 K ≤ T ≤ 300 K (solid line in the inset of Fig. 5) with C = 3.667 ± 0.018 emu•K/mole and a
Curie temperature θ = -90.9 K ± 1.1 K. The negative value of θ is also an indication of the
antiferromagnetism. The effective magnetic moment obtained by μeff = (8C)1/2 is thus 5.42 ±
0.01 μB per Fe ion. This μeff value is slightly greater than the “spin-only” (i.e., with orbital
angular momentum L fully quenched by crystal field (CF)) moment 4.90 μB for the high spin
state (S = 2) of Fe2+ (d6) ion [34] and substantially smaller than the free ion value of 6.71 μB
calculated from the total angular momentum J = L + S. This result indicates that the Fe ions in
the crystal are divalent and their orbital moments are substantially quenched by CF. The value of
μeff observed here is in excellent agreement with those observed in other compounds containing
Fe2+ ions, such as FeO (5.33 μB), FeF2 (5.59 μB), FeCl2 (5.38 μB), FeS (5.24 μB), KFeCl3 (5.50
μB), and BaLa2FeS5 (5.41 μB) [36-40]. The values of the -θ (≈ 91 K) and μeff (5.42 μB)
measured for the LiFePO4 samples in this study are considerably smaller than the values reported
by Creer et al. (-θ = 129 K and μeff = 5.65 μB) [41] and Arcon et al. (-θ = 115 K and μeff =
5.85 μB) [32] , but almost identical to the values (-θ = 88 K and μeff = 5.45 μB) reported by
Santoro et al. [31].
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4. Conclusions
LiFePO4 single crystals have been successfully grown by standard flux growth technique in
Pt crucibles using LiCl as the flux. The crystals have volumes of up to 300 mm3 with an average
volume of about 100 mm3, which are large enough for all the physical property measurements
including four probe electronic conductivity measurements. Single-crystal XRD measurements
showed that the crystals have orthorhombic crystal structure with space group Pnma (Z = 4). The
bond lengths between Fe and O in the FeO6 octahedra and between P and O in the PO4-3
tetrahedra were obtained. The high quality of the crystals is supported by the powder XRD
measurement on ground LiFePO4 single crystals, which shows that the crystals are pure in phase.
The magnetic susceptibility measurements indicate that the Fe ions in the crystal are
antiferromagnetically ordered at a Neel temperature TN ≈ 51 ± 2 K, above which the system is
paramagnetic with effective moments of the Fe2+ ions close to the value for the orbital-moment-
quenched Hund’s rule ground state.
Acknowledgements
The work at Sam Houston State University (SHSU) is supported by a grant from the SHSU
EGR program and by an award from the Research Corporation. The work at University of Texas
at Austin is supported by the Welch Foundation under Grant No. F-1191 and by the National
Science Foundation under Grant No. DMR-0605828. The work at Ames Laboratory is supported
by the Department of Energy, Office of Basic Energy Sciences under contract number W-7405-
Eng-82.
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Figure Captions:
Figure 1. Samples of as-grown LiFePO4 single crystals with volume ranging from 100 mm3 to
200 mm3.
Figure 2. Single-crystal XRD pattern measured with the x-ray beam along the a-axis of the unit
cell of the LiFePO4 single crystal.
Figure 3. The structure of the orthorhombic LiFePO4 showing the positions of the atoms. The
orange-yellow octehedra represent FeO6 and the yellow tetrahedral represent PO43-. The arrows
at the Fe-sites represent the spin moments.
Figure 4. Powder XRD pattern of the powder of ground LiFePO4 single crystal, taken at room
temperature and in the 2θ range of 15° ≤ 2θ ≤ 65°
Figure 5. Temperature dependent dc magnetic susceptibility measured in a field of 1 kOe. The
inset shows the inverse magnetic susceptibility. The solid line in the inset represent the linear fit
χ-1= (T - θ )/C according to the Curie-Weiss law.
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Tables: Table I. Atomic coordinates of LiFePO4 single crystal. =========================================== atom x y z Fe(1) 0.28198(4) 0.2500 0.47503(10) P(2) 0.40520(6) 0.7500 0.41808(17) O(3) 0.33456(12) 0.5464(2) 0.2847(3) O(4) 0.54280(18) 0.7500 0.2942(5) O(5) 0.40310(18) 0.7500 0.7428(4) Li(6) 0.5000 0.5000 0.0000 =========================================== Table II. Bond lengths (Å) =================================================================== atom---atom distance atom---atom distance
Fe Octahedron: Fe(1)---O(3) 2.0641(15) Fe(1)---O(3)1) 2.2451(15) Fe(1)---O(3)2) 2.0641(15) Fe(1)---O(3)3) 2.2451(15) Fe(1)---O(4)4) 2.106(2) Fe(1)---O(5)5) 2.1968(19) P Tetrahedron: P(2)---O(3) 1.5545(15) P(2)---O(3)6) 1.5545(15) P(2)---O(4) 1.533(2) P(2)---O(5) 1.519(2) =================================================================== Symmetry Operators: (1) -X+1/2,Y+1/2-1,Z+1/2 (2) X,-Y+1/2,Z (3) -X+1/2,-Y+1,Z+1/2 (4) -X+1,-Y+1,-Z+1 (5) -X+1/2,Y+1/2-1,Z+1/2-1 (6) X,-Y+1/2+1,Z Table III. Bond angles (°) =========================================================== Atom-atom-atom angle(°) atom-atom-atom angle (°)
O(3)-Fe(1)-O(3)1) 152.78(5) O(3)-Fe(1)-O(3)2) 119.34(6) O(3)-Fe(1)-O(3)3) 87.06(5) O(3)-Fe(1)-O(4)4) 89.74(4) O(3)-Fe(1)-O(5)5) 90.87(4) O(3)1)-Fe(1)-O(3)2) 87.06(5) O(3)1)-Fe(1)-O(3)3) 66.03(5) O(3)1)-Fe(1)-O(4)4) 97.41(6) O(3)1)-Fe(1)-O(5)5) 81.58(5) O(3)2)-Fe(1)-O(3)3) 152.78(5) O(3)2)-Fe(1)-O(4)4) 89.74(4) O(3)2)-Fe(1)-O(5)5) 90.87(4) O(3)3)-Fe(1)-O(4)4) 97.41(6) O(3)3)-Fe(1)-O(5)5) 81.58(5) O(4)4)-Fe(1)-O(5)5) 178.79(8) O(3)-P(2)-O(3)6) 103.79(8) O(3)-P(2)-O(4) 106.40(7) O(3)-P(2)-O(5) 113.23(7) O(3)6)-P(2)-O(4) 106.40(7) O(3)6)-P(2)-O(5) 113.23(7) O(4)-P(2)-O(5) 113.03(11) Fe(1)-O(3)-Fe(1)7) 127.43(6) Fe(1)-O(3)-P(2) 129.01(9) Fe(1)7)-O(3)-P(2) 94.66(7) Fe(1)4)-O(4)-P(2) 126.94(13) Fe(1)8)-O(5)-P(2) 120.45(11) ============================================================ Symmetry Operators: (1) -X+1/2,Y+1/2-1,Z+1/2 (2) X,-Y+1/2,Z (3) -X+1/2,-Y+1,Z+1/2 (4) -X+1,-Y+1,-Z+1 (5) -X+1/2,Y+1/2-1,Z+1/2-1 (6) X,-Y+1/2+1,Z (7) -X+1/2,Y+1/2,Z+1/2-1 (8) -X+1/2,Y+1/2,Z+1/2
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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