A mixed valence compound in the two dimensional MPS3 family: V0.78PS3 structure and physical properties

Mat. R e s . B u l l . , Vol. 20, p p . 1053-1062, 1985. P r i n t e d in t h e USA. 0025-5408/85 $3.00 + .00 C o p y r i g h t ( c ) 1985 P e r g a m o n P r e s s L t d .

A MIXED VALENCE COMPOUND IN THE TWO DIMENSIONAL MPS_ FAMILY STRUCTURE AND PHYSICAL P R O P E R T I E ~

: VO'78PS3

G. Ouvrard, R. Fr~our, R. Brec and J. Rouxel Laboratoire de Chimie des Solides, L.A. 279 2, rue de la Houssini~re - 44072 Nantes Cedex, (France)

(Received June 6, 1985: Communicated by P. Hagenmuller)

ABSTRACT The

non

crystals

stoichiometrie from

compound

a preparation

V

7 PS

O. 8 3 corresponding

has to

been the

obtained atomic

ratio

as

single V/P/S

=

1/1/3. It cristallizes with monoclinic symmetry, space groupo C2/m, with the unit c e l l parameters a = 5.867(i~ ~, b = 1 0 . 1 6 0 ( 2 ) A , c = 6.657(1) ~, B = 1 0 7 . 0 8 ( 2 ) ° , V : 379.3(1) Ao and Z = 4. The structure refinement was made down to a reliability factor value R = 3.3 % from 445 reflexions (I > 3 0 (I)) and 31 variables. The material has the same layer structure as FePS 3 with the occurrence of the thiophosphate anion (P2S6/-including a P 2 pair. In V 0 78PS3, the charge equilibrium implies the following developped formu]'a : VII34. V0.44III [] 0.22 pIV S -II. The phase is a semi-conductor with a small activation energy o3 0.24 eV, in accord with a vanadium mixed valence, and it presents, at low temperature, an antiferromagnetic order (T N = 62 K).

I. INTRODUCTION The MPS phases constitute a rather large family. Its members have been 3 reported for many metals in particular the first row transition metals (M = V, Mn, Fe, Co, Ni, Zn) (I-4). For these elements, preparation of the derivatives MnPB_, NiPS 3 and ZnPS_3 can be carried out without difficulty. The samples obtained are very homogeneous and stoichiometry is easily achieved from stoichiometric atomic proportions (5-8). In the case of FePS3, if the simple Fe/P/S = l/l/3 atomic proportion is used, a systematic excess sulfur remains in the synthesis tubes. It seems necessary to start with a large su]fur excess

1053

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G. OUVRARD, et al.

Vol. 20, No. 9

to achieve proper stoichiometry (5), at least on single crystals, although no systematic analysis has been made to definitively prove it. When trying to obtain COPS_ some difficulties arise (9). CoSp is initially formed and a 3 several month reaction is necessary to finally 9ield pure COPS_. Successive 3 grindings and firings could probably speed and improve the preparation process.

TABLE I

X Ray Powder Diffraction of ~.78PS3

dobs.

dcalc"

h k 1

6.375 5.086 3.184

6.364 5.081 3.182

001 020 002

66.5 16.8 20.6

2. 899 2.899 2.8O4

2011 130

35.5

2.901

2. 499 2.499 2.403

200 I l 3 [ 040 131 1 2O2 112

2.326 2 326

132 201

1.8513

202 I

2.803

2. 804 2. 541

2.542 2.503 2.403 2.326 1,8510

1.8512

1.6935

1.6929

1.6934 1.6424

1.6426

l.6366

1.6367 1.6006 1.5910 1.4953 1.4625 1.4020 1.2497 1.1622 1.1588

t I

I/Io*

1.6366 1,6364 I.6001 1.6000 1.5909 !.4950 .4949 .4948 1.4624 1.4622 1.4018 1.4020 1.2495 1 2494 1.1616 .1595 1595 1595

I!

I

133 3311 O6O 114

2.5 100.0 5.0 3.3 21.4 39.2 1.4

33 33 02 I 06 1 2O4 I 133 0O4 33 33 31

9.4

I

8.0 8.6 5.7 10.4

o62

4°! I 261

4.1

262 400 404 262 115

1

15.4

I

6.4

03 63 43 335

I

0.4 15.0

* Intensities calculated with Lazy Pulverix Programm (12).

Vol.

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9

MPS 3 FAMILY

1055

In 1969, Klingen (4) obtained hexagonal shaped black crystals from a synthesis carried out at about 450°C for two months from the atomic ratio V/P/S I/1/3. From the cell parameter determined by a X Ray analysis, and by comparison with the other MPS_ cell constants, he attributed the formula VPS 3 to the 3 crystals. Since the observed parameters and symmetry were quite similar to the expected values, and although no chemical analysis had been done, the above results were not questioned. However, considering the great stability of V III in a sulfur octahedral environment, a compound such as V ~ PS 3 would T III rather be formed instead of V I I PS , the more so since a phase z/~ such as in2/3 [] 1/3 PS3 is known

to exist

(i0).

3

From the above arguments, and in addition due to the difficulties encountered in synthesizing large amounts of pure "VPS3" (5), it was decided to determine the structure of the single crystals obtained according to Klingen's synthetic conditions and to analyse them. It is hence shown in this article that these crystals correspond actually to a non stoichiometric VO.78PS 3 phase. II. E X P E R I M E N T A L

Single crystals used in this study were gathered from a sample obtained from the pure elements taken in the atomic ratio V/P/S = 1/1/3 and heated according to the reported procedure of Klingen (4). Using V P S as a standard 2 13 (II), elemental analysis determined by microprobe analysis ~M1crosonde Ouest) yielded the following elements weight concentrations : % V = 23.8, % P = 18.1, % S = 58.1. These data are to be compared with the percentages calculated for the formula V 0 78PS3 : % V = 23.8, % P = 18.6, % S = 57.6. Both results are in very good agre%ment and, in any case, the experimental data are very far from the theoretical concentrations calculated for' a VPS_ stoichiometric formula (% V = 28.6, % P = 17.4, % S = 54.0). This compositlon is, in addition~ complety confirmed by the structure determination (see section III). Prior to diffractometer recording, the single crystal X Ray pattern quality was checked on Weissenberg films. Finally, a flat, thin, hexagonal crystal was chosen with dimensions, shape and faces as shown on figure I. 1015 reflexions were recorded in an independant quarter space. After Lorentz, polarization factor and absorption correction, then averaging, 445 reflexions with I > 3 a (I) were kept for the structure refinement. A monoclinic symmetry (C centered cell) was determined for the compound. Least squares parameter refinement was made from X Ray Guinier powder spectra ISi as standard and I CuKa_ = 1.54051 A) £ yielding }he cell constants o: a : 5.867(1) A, b = 10.160(2) A, c = 6.657(1) ~, ~ : 107.08(2) ~, V : 379.3(1) ~ and Z : 4 A V PS_ in' 0.78 dexed Guinier powder spectra is given on Table I.

FIG. i.

VO.78PS 3 single shape.

crystal

size and

1056

G. OUVRARD,

III S T R U C T U R E

et al.

Vol.

20, N o .

REFINEMENT

Considering the set of the cell parameters and the conditions limiting possible reflexions (hkl with h+k = 2n), V^ _^PS~ was assumed to have the same •/ ~ structure as FePS~ (C2/m space group) (4~. ~ e f z n e m e n t was carrled out using J the same atomic position e.g. V(4g), P(4i), S.(4i) and S~(8d). With anisotroi . L . . pic thermal factor and full occupancy of the vanadium position, the relzabzlity factor set at R = 7.6 To, the vanadium equivalent thermal factor remaining high. The R value dropped to 3.7 To upon freeing of the occupancy ratio, the vanadium content corresponding then to V 0 760(2)PS3. At that point, the structure could be considered as completely s o l v e d , w i t h a vanadium concentration close to that of the microprobe analysis. However, a Fourier difference map showed two significant, if rather small, peaks at (0,0,0) and (0.057, 0.333, 0.I 7 O) owith densities of i.i and 0.63 e.~ -~ , the following peak being found at O-3e.A-3.Normally, a much smoother density decrease is recorded on final Fourier difference data. Such lightly occupied positions having been found in o-- 3 NiPS~ and CoPS 3 (13) (with however a higher density of about 3.5 e.A ), it was ~ecided to introduce them in the refinement and to attribute them respectively to vanadium (noted V') and phosphorus (noted P'). Because of the very small amount of each species involved, a low occupancy ratio waso taken and only the isotropic thermal factor was considered and fixed at 0.7 A 2, a value estimated reasonable for both atoms. Of the added atoms, only the occupancy ratio of V' (TV,) was refined, that of P' (Tp,) being set equal to TV,/4 to get the same multiplicity x Tvalue. Tp was then freed. With these conditions, s%ructure refinement yielded a final reliability factor of R = 3.3 % from the 445 reflexions and with 31 variable~, the Fourier difference being then utterly featureless (D = 0.3(1) e.~-J).The secondary extinction coefficient was 7(2).10 -7 . Table ~ % a t h e r s the positional parameters and temperature factors. Figure 2 represents a projection of the structure of VO.78PS 3 along the b axis.

TABLE I I

(a)

Positional Parameters and Their Estimated Standard Deviations

Atom

X

B(.~) 2

Y

I M~ttiplicit~ I x T

V

0.000

P

0.0572(3) 0.7460(3) 0.2504(2) 0.000

S1 $2 Vie p'*

0.057

0.3328(2) 0.000 0.000 0.1680(1) 0.000 0,333

0.000

0.1697(3) 0,2464(3) 0.24?2(2) 0.000 0.170

0.84(2) 0.77(2) 0.98(2)

1.o1(1) 0.7 0.7

0,377(2) 0.485(4) O. 50 1.O 0.010(i) 0.01

0.75 0.97 1 1 0.04 0.01

Starred atoms : not all the p a r a m e t e r s refined (see text). Anisotropically refined atoms are &iven in the form of the isotropzc equivalent thermal parameter defined as : (4/3) * [A2*B(I.I) + B2*B(2.2) + C2"B(3.3) + AB(COS GAMMA)*B(I.2) + AC(COS BETA)*B(I.3) + BC(COS ALPHA)*B(2.3)]

9

Vol. 20, No, 9

MPS 3 FAMILY

TABLE Refined

Name V P $I

$2

e(,.,) 0.0062(3) O.OOSO(3) 0.0057(3) 0.0078(2)

Temperature

8(2.2) 0.00211(9) 0.0021(1) 0.0029(:) 0.00221(6)

1057

II (b)

Factor

Expressions

B(3.3)

]

o.oos6(2)

I I I

I

B(1.2)

o

I

I

- Beta's

B(2.3)

B(1.3)

I I I o.oo~4(q

0.0052(3) 1 0 I 0.0033(5) 0.0072(3) I 0 I 0.0071(4) 0.0078(2) I-0.0023(3) 1 0.00&5(3)

I

o o o -0.0009(3)

I

The form of the anisotropic thermal parameter is : EXP [-(B(1.1)*H2 + B(2.2)*K2 + B(3.3)*L2 + B(I.2)*HK

+ B(1.3)*HL

+ B(2.3)*KL)].

a

i°o

F IG. 2.

VO.78PS 3 structure be followed

projection along the b axis. Arrows indicate the track by lithium ions from a "Oh" site to another through "Td" voids.

IV. STRUCTURE

DESCRIPTION

to

AND DISCUSSION

The structure determination results show V 0 78PS3 to belong to the MPS 3 structural CdCI_z type. The main difference lie% in the vanadium vacancies w i t h i n the layers. Let us consider first the meaning of the extra vanadium and phosphorus atoms added in the final refinement. V' position in (0,0,0) corresponds to ~ h e center of the octahedra (see figure 2) normally fully occupied by the phosphorus pairs. Introduction of V' had then to be correlatively accompanied by the refinement of T _p, this being expected to decrease It is exactly what is observed (see table II (a)) with T V , , setting at 0.04 and Tp lowered from 1 to 0.97 . The P' position (0.057, 0.333, 0.170) corresponds to the octahedral vanadium sites. Since these are not fully occupied, the phosphorus atoms can be easily located in the voids left by vanadium.

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G. OUVRARD, e t a ] .

Vol. 20, No. 9

Comparing the results with and without the introduction of V' and P', the obtained formula, with their estimated standard deviations, are respectively P S ~ n a n d V 774(6) PO 99(I)$3" Obviously, only the second formula Vo'760(@)de£ermine~'from the microprobe analysis and justifies, if necesoverlaps that sary, the adding of V' and P' The occurrence of these positions may be seen as having several possible origins. There may be a small disorder between vanadium and phosphorus distribution within each layer, or a slight crystal twinning. It is also possible that some faults take place within the single crystal in the stacking sequences of the atomic planes. Such a phenomenon is observed for NiPS and CoPS but not in the cases of MnPS3, FePS_3 and CdPS 3 (13). It must be n%ticed tha~ no detectable amount of cation is found in the Van der Waals'gap of the structure. Remarkably it is the case of all the MPS 3 structures currently studied (13). Table III gathers the main interatomic distances encountered in the structure.

V. PHYSICAL PROPERTIES 10-1 [ Log O"

i o -2

10 ~ I-

\'. QQO 800 °g 0

103/T

I0

3

4

5

6

7

8

9

10

F IG. 3.

Logarithm of conductivity versus reciprocal temperature obtained from VO.78PS 3 single crystal.

11

The DC electrical conductivity of V 7 PS was measured on a s~ng~e ~rystal by the Van der Pauwmethod, the electric contacts being made on the edges of the hexagonal sample. On figure 3 is drawn the logarithm of conductivity versus reciprocal temperature. The activation energy for conductivity defined by 0 = ~ e x p (-E/2kT) is E = 0,24 eV, whereas room temperature resistivity is p = 7 R cm. These values are to be compared with that obtained for other M P S ' s MnPS and 3 " 3 NiPS present very high roora 3 temperature resistivity of about l ( ~ c m and do not appear to be semi-conductors as far as their electrical resistivity are concerned. The activation energy calculated for NiPS and CoPS were found 3 3 respectively equal to 1.5 eV and 1.05 eV (14,9). V 0 78PS3's low E and G values a~r'e to be attributed to an electronic hopping process between V II and V III in the phase. In effect, the structure determination contains, like in the other MPS3 compounds, (P2S6)4-

Vol. 20, No. 9

MPS3 FAMILY

1059

anionic groups. The charge balance in VO.78PS 3 must thus corresponds to the formulation • V II VIII [] PS 3. With room temperature conductivity of ~l 0.34 0.44 0.22 about I0 -I ~ cm -I, VO.78PS 3 can hence be considered as a mixed valence compound belonging P. Day (15).

to

class

II according

TABLE

Main Interatomic

to the classification

of M. Robin

and

III

Distances

(in A)

I I

P-P

P-S 1

P-S 2

V-V

V-S 1

V-S 2

2.030(2)

2.035(2)

3.382(2)

2.504(2)

2.502(2)

I I I 2.160(3)

I There exists a possibility to check this ionic distribution by using the known ability of the M P S ' s to intercalate ions or molecules within their Van der W a a l s ' g a p ( 1 6 - 2 1 ) 2 In effect, since it has been recently shown that the o x y d i z i n g centers are the metal d orbitals (22), a potential change should occur upon i n t e r c a l a t i o n - r e d u c t i o n by lithium, following complete reduction III II of V to V . Using an experimental teflon battery described elsewhere (23), and the electrochemical chain Ni/Li/IM LiCIO. in dioxolane/V 0 78PS3/Ni a thermodynamic discharge curve has been obtained ~Fig.4). A 2 0 ~ A.cm -2 current density was chosen, the time of discharge (i hour) being followed by a four hour relaxation. The amount of V 0 78PS3 used in the positive holder was of the order of 20 mg, mixed with 20 ~ of carbon black. The potential of the electrode was measured after relaxation, against a lithium electrode reference.

E(V) 3

2,5

o,y 0,44 2

1,5

0,5

1 • dans LixVo,78PS 3

FIG.4.

Q u a s i - e q u i l i b r i u m discharge curve E(volt) = f(x) (x in Li V PSi) . .x 0.78 ~. obtained from the eiectrochemica± chain Ni/Li/1M LiCIO 4 in Dioxolane/ Vo. 78PS3/Ni •

Three breaks in the curve E(volt) = f(x in Lix VO.78PS 3) can be observed, one at around x = 0.22 and the two other, smaller, at about x = 0.44 and x = 0.67. Considering the developped formula of the phase, V01134 V I I I 4 D O 2 2 PS3, the first curve accident" can b'e attributed to the filling of the 0.22 empty sites of the structure slabs. It can be seen on figure 2 that the track from one octahedral site to another through a tetrahedral void is the same from a void situated in the Van der Waal's gap to the next one in the gap or in the the slab. The absence of some vanadium in their octahedral sites may also create a potential site change in the Van der Waal's gap responsible of the first break, the lithium cation remaining in the octahedra of that same gap. The break at x = 0.44 is clearly a t t r i b u t a ble to the reduction of the 0.44 vanadium III leading to the formulation . L~.I V II PS . Thls result allows O. 4 4 o. 78 3 thus to support the charge balance in

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Vol. 20, No. 9

this non stoichiometric compound. Without further physical informations, it is difficult to attribute the origin of the last accident on the curve at x = 0.67. All the magnetic MPS order antiferromagnetieally at low temperature (5), (24-27). Some deviation o~ this type of behaviour is expected, because of the magnetic_l unbalance in non stoichiometric mixed valence VO.78PS 3. Molar X and X variations versus temperature of the phase are presented on figure 5. Although not following a high temperature Curie variation, the behaviour of V PS can be related to that of an antiferromagnetic phase, with an orde0.78 _ 3 rlng ~empera~ure T. of 62 K. Below that temperature, X remains almost constant. Field change~ did not, within error, change the X value, excluding any ferrimagnetic behaviour that might have been expected considering the magnetic moment distribution. 2( ~

,

,

X.lo'(,m.~,o0e'l) 15000

1500

@•

• gee

•

o o O

100C

10000

+

5000

50G $ ÷÷

•

÷.+.+

0

• i

!

100

200

|00

T(K) FIG. 5.

Direct and reciprocal magnetic of VO.78PS 3 powder sample.

susceptibility

variation

versus

temperature

V. C O N C L U S I O N

Systematic structural determinations and analysis of the MPS 3 phases currently in progress (13) have shown, in the case of vanadium derlvative, that the reported VPS~ material actually corresponds to the non stoichiometric phase V 0 78PS3 . T~is vanadium deflclency is clearly demonstrated not only from the stru6tural determination but also from microprobe single crystal analysis. Some vanadium-phosphorus disorder corresponding to a very small intercationic substitution within the slabs was detected without being identified. The mixed valence natur~ of the compound implied by the occurence, in the structure, of the (PoS6)~-L anionic species and the vanadium vacancy is supported by physical studies. Low activation energies and antiferromagnetism are two of the characteristics of such phases. In addition, electrochemical data have shown to be a convenient way of quantitatively analyse the two oxydation state ratio.

Vol. 20, No. 9

MPS 3 FAMILY

1061

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2.

W. Klingen, (1968).

3.

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4.

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5.

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6.

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7.

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8.

R. Nitsche and P. Wild, Mat. Res. Bull. 2, 419 (1970).

9.

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I0.

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ii.

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12.

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13.

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14.

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17.

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18.

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R.

G.

Eulenberger

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and P.

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G.

Day,

R.

and

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H.

Hahn,

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Ouvrard,

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and J. Rouxel,

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20.

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21.

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22.

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23.

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24.

G. Le Flem, R. Brec, G. Ouvrard, Chem. Solids 43, 455 ([982).

Inorg.

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V o l . 20, N o .

25.

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26.

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27.

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2719

9