Physical properties and lithium intercalates of CrPS4

Solid State Communications, Vol. 28, pp.61—66.

0038—1098/78/1001—0061 $ 02.00/0

© Pergamon Press Ltd. 1978. Printed in Great Britain. PHYSICAL PROPERTIES AND LITHIUM INTERCALATES OF CrPS4 A. Louisy, G. Ouvrard, D.M. Schleich and R. Brec Laboratoire de Chimie Minérale A, U.E.R. de Chimie Faculté des Sciences de Nantes, 2 Chemin de la Houssimère 44072 Nantes Cédex, France (Received 10 April 1978 by E.F. Bertaut) Chromium thio-phosphate, CrPS4, has a layered structurebased on hexagonally close packed sulfur layers. Magnetic susceptibility measurements indicated an antiferromagnetic ordering perpendicular to the C axis and the presence of trivalent chromium in the paramagnetic region. Optical transmission spectra showed the fundamental absorption edge to occur at 2.4 eV. Using n-butyl lithium techniques, it was found that CrPS4 can readily accomodate lithium. This insertion is not followed 3~by magnetic any parammoment. Lithium NMRstructure measurements gave a rather eter change in the host nor reduction of thehigh Cr (0.31 eV) energy of activation for lithium movement and use of CrPS 4 as a cathode against a lithium anode showed a sharp drop in potential. 1. INTRODUCTION

aspects of the reduction of the host structure through lithium intercalation are discussed.

The chromium thio-phosphate CrPS4 structure was first determined by Diehi and Carpentier [1]. This structure has a monodlinic symmetry (C2, C~)and is characterized by puckered hexagonally close-packed sulfur layers with the AB—AB sequences. Chromium and phosphorus atoms are in distorted octahedral and tetrahedral interstices, respectively. This compound belongs to the class of layered compounds (Fig. 1), with the typical van der Waal’s gap between sulfur layers. In the field of our research about intercalation in the layered MPX3 phases [2, 3], we found it of interest to study lithium intercalation in CrPS4 in view of possible use in batteries. No physical measurements having been performed, we report here the magnetic and optical properties of CrPS4 and of some Li~CrPS4intercalated phases. Some OS

2. EXPERIMENTAL 2.1. Synthesis of CrPS4 CrPS4 was prepared by heating proper weights of high-purity (99 .99%) starting materials in evacuated silica tubes. A temperature gradient from 750 to 650°C for five days, followed by a slow cooling (40°C/day) allowed us to obtain large plate-like crystals. No transport agent was used. 2.2. Sample preparation of Li~CrPS4phases The intercalation of lithium was made either chemically according to the butyl-lithium technique described by Armand [4] or electro-chemically by slow galvano-

01/2

•Cr

0 2

~4

14

1I~ 34

01/2 34

~1/4

34

OOh 2 4

•1/4

34

1/4 01/2

4

02 1/4

3/4

0/2

01/2 14

~~‘03/4

1/4034

1/4O~/4

1/4

01/2 •34

0 •1/4

1/4

34

3/4

2

14

0

if

4

el4

01/2

/

/

3/4 /4

01/2

~

~

/ 0 V2

C

CrPS4

LAYERED

Fig. 1. 61

STRUCTURE

1/434

62

PHYSICAL PROPERTIES AND LITHIUM INTERCALATES OF CrPS4

static reduction of CrPS4. This reduction was performed in button generators with lithium perchlorate as the electrolyte in a solution of propylene carbonate. 2.3.X-ray analysis Samples were analysed by X-ray diffractometry (CuKa radiation). Debye—Scherrer chambers with a nominal diameter of 114.83 mm were used for air sensitive intercalated phases, a Siemens type F powder diffractometer otherwise. In this latter case, least square refinement of the lattice parameters were conducted on data obtained at slow scan speed (1/2°/mn),with silicon as the internal standard. 2.4.Chemical analysis Density checks were made on several large single crystals (~ 80mg) by a hydrostactic method using Cd4 as the liquid. Oxydation of CrPS4 in air in a microthermobalance (MTB. 10-8 Setarain) with a 250°C/hr heating program led, at 1000°C,to CrPO4 and was used as an additional verification of the purity of the starting material. Results were always found to be within 1% of the calculated values. After intercalation, lithium was analyzed in the filtered solution by flame spectrophotometry with a Unicam SP 1900 absorption spectrophotometer. 2.5. Magnetic measurements Magnetic susceptibility measurements on ground single crystals were carried out with a Faraday balance. Field strengths of 5.OkOe were used from 4.2 to 300K and of 7.4kOe from 75 to 300K respectively for CrPS4 and the Li~CrPS4intercalated compounds. Hygroscopic intercalates were handled in a dry glove box, and we used a small teflon boat sealed with a tightly screwed cap of the same material for our magnetic study of these phases. 2.6. Optical spectra recording Very thin CrPS4 crystals were easily obtained due to the splitting habit of this material. Several spectra with crystals of various thicknesses were recorded from 20,000 to 5,000 A. Sample thicknesses were determined from the interference fringes or/and from microscope measurements. The spectrometer was a Beckman DK-2A. 2.7. NMR analysis 7 NMR were performed at 20method). MHz on Li using ameasurements SXP Bruker spectrometer (pulsed The spin lattice relaxation times T 1 were measured by observing magnetization recovery vs time in a ir—ir/2 pulse sequence. At least seven data points were used to determine each T1 value. Temperatureswere taken with a platinum resistor placed near the r.f. sample coil. The ir/2 pulses used were approximately 4psec long.

Vol. 28, No. 1

Table 1 *

dobs

~

h kI

6.17 4.343 4.265 4.134

6.14 4.341 4.264 4.134

001 111 111 20 1

100 5 15 20

4.007 3.236 3.069 3.016 2.711

4.002 3.238 3.069 3.015 2.710

201 3 10 002 220 202

1 2 90 10 80

2.690 2.512 2.341 2.170 2.129 2.063 2.030 2.000

2.687 2.513 2.343 2.171 2.132 2.067 2.032 2.001

221 401 02 2 222 222 402 42 1 402

60 5 5 5 5 15 10 5

1.936 1.813 1.782

1.936 1.813 1.782

20 ~ 040 023

30 10 70

1.753

1.753 1.752 1.739 1.708

31 3 422 041 223

1.707 1.577 1.554 1.537 1.507 1.508 1.481 1.482 1.464 1.413 1.282

31 3 62 1 621 602 242 440 11 ~ 5 1 3 204 024 1 34

1.283

533

1.740 1.707 1.578 1.555 1.536 1.507 1.482 1.464 1.413 1.283 *

-

I/Jo

10 5 5 2 1 1

10 25 15

-

5

Estimated intensities. 3. RESULTS AND DISCUSSIONS

3.1. Physical properties of CrPS4 CrPS4is powder withlattice its corresponding indexing given inspectrum Table 1. The parameters are a = lO.863(3)A, b = 7.253(2)A, c = 6.142(1)A and (3 = 91.87 (2). They are in very good agreement with those reported by Carpentier [11. The density measured on single crystals was found to be dObS = 2.884 for a theoretical value of dth~ = 2.898. Optical absorption measurements were performed on four different samples perpendicular to the c axis

Vol. 28, No. 1

PHYSICAL PROPERTIES AND LIThIUM INTERCALATES OF CrPS4

63

4cn~ oC 1O

CrPS 4 op
2

1.2

1.4

1~6

i.e

20

22

2~4

2.6

eV

Fig. 2. 5cm,7x 105cm,3x 10~cmand (t=9x 10 4 x 10~cm) in the spectral range from 0.6 to 2.8 eV. Comparison of absorption coefficients calculated from these various samples showed that in the region of intense optical absorption, the reflectance is minimal with respect to the absorption and may be ignored. Figure 2 shows a curve ofa (absorption coefficient) vs energy (eV). The first absorption begins at 1.4eV and is substantially weaker than the second (a = i0~cm~as opposed to 104 cm —i). This first absorption may be 3 . attnbuted to a d—d type transition ford chromium. In3 atransition strong field octahedral one expects a 3d metal cation toenvironment, exhibit a maximum of 3 broad spin allowed transitions from a 4A 4A2g ~ 4T 4A 2~ground 4T state (i.e. ~* 7’2g, 15(F), 2~+ 1~(P).The additional two spin allowed transitions would, in this case, be obscured by the fundamental charge transfer or band gap transition which occurs at approximately 2.4eV. Magnetic susceptibility measurements were performed on a single crystal which was placed in a silica bucket with the crystal c axis perpendicular and then parallel to dH/dz.No field dependent impurities were detectable in the sample. Figure 3 shows a plot of x vs temperature and indicates an antiferromagnetic interaction perpendicular to the c axis with a Neel point of 36 K. In the paramagnetic region one observes isotropic magnetic behaviour with a Curie constant which is temperature dependent. Least squares fits of the data to the formula x = Xo + CIT + 0 show a consistent decrease in C from 2.4 over the region from 77 to 180 K to 1.91 from 180 to 300 K. The low temperature fits show a high degree of non-linearity and are undoubtedly in error due to the effect of the antiferromagnetism. The .

CrPS 4 Susceptibility

emu/mole

________________________________

020

. *

0.15

I ~_b~OC

.

.

~OC

*

0.05

~

.4.

__________________________________

0.00 0

50

100

Fi 3 g.

150

200

250

300

Temperature (K)

high temperature region, which fits the equation x = —90 x 10-6 + [1.9/(T— 55)1 is in good agreement with the presence of trivalent chromium (S = 3/2 and C =3~in 1.87 spin onlyenvironment d 3). Since the ground state of an for octahedral is 4A Cr 2~,one would expect the orbital contribution to the moment to be

64

PHYSICAL PROPERTIES AND LITHIUM INTERCALATES OF CrPS4

Vol. 28, No. 1

Table 2 Octahedral sites

4(c) 2(b) 2(b)

Tetrahedral sites

X

Y

Z

Potential

0.24 0.50 0.0

0.26 0.26 0.26

0.52 0.50 0.50

—0.38 0.38 0.86 — —

strongly quenched and thus an isotropic spin only moment. 3.2. Lithium intercalation of CrPS4 In the layered structure of CrPS4 (Fig. 1), distorted octahedral and tetrahedral sites in the van der Waal’s gap are available for alkali metal intercalation. The distances between the center of the octahedrons and the surrounding sulfur atoms span from 2.53 to 2.78 A and suit well the Li—S distances (2.74 A) [5] in an octahedral environment. Tetrahedral sites are much smaller with the corresponding distances within a 2.18—2.22 A range. Potential site calculations based on a simple electrostatic model of the structure show all these sites to be favorable to lithium ion intercalation (Table 2). These factors led us to test CrPS4 as cathode in a button generator.* Figure 4 illustrates potential vs Li concentration in Li~CrPS4under various discharge conditions. It appears clearly that CrPS4 proved to be a poor cathode as voltage drops drastically, even for a current 2. On the diagram, a corndensity as as 2OpAcm parison cansmall be made with the discharge curve of a superior cathode material (NiPS 2). 3 under 170 pA cm The physical studies we made on the lithium intercalates were carried out on compounds obtained through chemical intercalation using butyl-lithium. As for the UXMPX3, intercalated species [2], one observed no change in the X-ray powder spectra of the Li~CrPS4 phases, although for high lithium concentration (approximately 3 lithiums per mole of CrPS4) the intensity of the lines decreases. No by-products due to a possible destruction of the CrPS4 host structure could be found on the X-ray diagrams. Without parameter evolution, it is not possible to determine the limit of intercalation as was done on other intercalated series [6—8]. After reaction, lithium analysis of the filtered solution indicates the amount of unreacted lithium. However, one must be very careful in concluding a total lithium intercalation, as some decomposition of the butyllithium may occur during the intercalation process. For *

This study was carried out at “Compagnie Générale d’Electricité”, Laboratoire de Marcoussis by A. Le Mehaute.

4(c) 4(c) 4(c) 4(c)

X

Y

Z

Potential

0.17 0.57 0.33 0.43

0.52 0.49 0.50 0.03

0.35 0.35 0.65 0.65

—0.46 0.46 0.46 0.46 — — —

E (VoLt)

3

05 Lithium con~en~

~ (°)

2 OtA/c m 21 CrPS4 225,~~~.A/cm~ 1 mA/cm2 1*) 170~~A/c m2 NiPS 3 (x)

---

Fig. 4. some elevated lithium compositions, we have observed the formation of a highly reactive decomposition product. This grey decomposition phase has been observed in the preparation of compositions with lithium content greater than 3 moles of U per mole of CrPS4. Figure 5 shows plots of 1 lx vs temperature for Li~CrPS4(0
Vol. 28, No. 1

1/

PHYSICAL PROPERTIES AND LITHIUM INTERCALATES OF CrPS4 1moIe~ emu X

65

100

80

60

(+) Cr PS

40

(°)Li 0.5CrPS4 (.) Li1.0 CrPS4 (A) Li2.0 CrPS4

20

~o

0

50

100

150

200

250

300

Fig. 5. T1~(~1)

1o~

4. CONCLUSION —

Li0~75CrPS4 —

10

215

30

35

~

-~

Fig. 6. U~NMR was performed for several Li~CrPS4phases (0
We have shown that layered CrPS4 is antiferromagnetic and can readily intercalate to form Li~CrPS4compounds. No parameter expansions have been observed and this makes it difficult to determine the phase limit. The occupancy of all of the octahedral holes in the van der Waals gap would lead to U2CrPS4. Additional octahedral and tetrahedral voids exist in this structure which could account for a higher lithium concentration. However, these octahedral sites are potentially unfavorable to lithium occupation and the tetrahedral sites appear too small to accept a lithium ion without a strong distortion of the structure. Magnetic susceptibifity measurements have shown 3~(d3) to Cr2~(d4). of the that intercalation is notAssuming followed an by ionization the reduction of Cr lithium atoms present in the U~CrPS 4structure, one is left with a most interesting dilemma as to the location of these electrons. Further physical now under way, 31 NMR, and optical andstudies conductivity measurenamely p ments on intercalated single crystals, should help us understand the reduction process of intercalation. Initial non thermodynamic electrochemical discharges have indicated that this compound has less promise than MPX3 phases as high energy cathode. The high energy of activation (0.31 eV) determined by NMR measurements may help explain the large decrease in voltage under rapid cathodic discharge.

66

PHYSICAL PROPERTIES AND UTHIUM INTERCALATES OF CrPS4

Acknowledgements

—

Vol. 28, No. 1.

One of the authors, D.M. Schleich, was partially supported by NSF grant No. 39737. REFERENCES

1.

DIEHL R. & CARPENTIER D.,Acta Cryst. B33, 1399 (1977).

2. 3. 4.

LE MEHAUTE A., OUVRARD G., BREC R. & ROUXEL J.,Mat. Res. Bull. 12, 12, 1191(1977). PERCHE P. & LE MEHAUTE A., Power Sources No. 7, Proceeding of 11th mt. Symposium, Brighton, Sept (1977). ARMAND M., Fast Ion Transport in Solids, (Edited by VAN GOOL W.). North Holland, Amsterdam (1973).

5.

SHANNON R.D.,Acta Cryst. A32, 751(1976).

6.

OMLOO W.P. & JELLINEK F.,J. Less Common metals 20, (2), 121 (1970).

7. BREC R., RITSMA J., OUVRARD G. & ROUXEL J., Inorg. Chem. 16, (3), 660 (1977). 8.

BREC R., OUVRARD G., RITSMA J. & ROUXEL J., Rev. Ohm. Mm. 13, 348 (1976).

9.

ABRAGAM A.,Principles ofNuclear Magnetism. Oxford University Press, London (1961).

10.

BERTHIER C., CHABRE Y., MINIER M. & TRICHET L., SFP Congress, Poitiers (1977).