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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: phy_gnl@shsu.edu (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