Crystal structural, electric and magnetic studies on the misfit layer compounds “LnMS3” (Ln=rare-earth metal; M=Ti,V,Cr)

SOLID STATE

Solid State lonics 63-65 (1993) 696-701 North-Holland

IONICS Crystal structural, electric and magnetic studies on the misfit layer compounds "LnMS3" (Ln = rare-earth metal; M = Ti,V, Cr) N. Cho, S. Kikkawa ~, F. Kanamaru The Institute of Scientific and Industrial Research, Osaka University, Osaka 567, Japan

Y. Takeda, O. Yamamoto Department of Chemistry, Faculty of Engineering, Mie University, Tsu 514, Japan

H. Kido and T. Hoshikawa Osaka Municipal TechnicalResearch Institute, Morinomiya 1-6-50, Osaka 536, Japan

A series of misfit layer "LnMSf' were prepared where Ln = La,Ce,Pr,Nd,Sm,Gd and M = Ti,V,Cr. There was an upper limit of the ionic radius ratio R (M)/R (Ln) = 0.61 for the formation of the misfit layerstructure. Powder X-raydiffractiondata of"LaCrS3" was indexed with the monoclinic LaS sublanice (a= 5.942/~, b= 5.776/k, c= 11.078/~,r = 95.38° ), the pseudo-monoclinicCrS2 sublattice (a= 5.936/~, b= 3.449/~, c= 11.063/k, r = 94.78 ° ) and their supercell. The temperature dependence of the dimension of each subcell indicates that the interaction between LaS and CrS2 layers becomes stronger at lower temperature. The (hk0) precession photograph of the "LaVSfl' singlecrystal was also interpreted in terms of two component meshes, tetragonaland orthohexagonal subcells."LnVSf' with Ln=La,Ce,Pr,Nd were isostructural with "LaCrS3".Their transport properties were semiconductive and the activation energyaround room temperature decreased with a decrease of ionic radius of Ln. The semiconductive property changedto metallic at low temperature in a range from 150 to 200 K. In the case of Ln = Sm and Gd, the semiconductormetal transition was not observed down to 17 K.

I. Introduction Distorted rocksalt type LnS layers are alternately stacked with MS2 layers in the misfit layer compounds "LnMS3". The real composition o f " L n M S 3 " should be represented as (LnS)xMS2 (x ~ 1.2), where the value of x corresponds to a ratio in the unit cell lengths of the LnS and the MS2 sublattices along the misfit b-axis. (LnS)xMS2 is denoted hereafter as "LnMS3" in the present paper. Varieties of the layer stacking have been extensively studied in the case when the transition metal M = Nb and Ta, where M atoms are in slightly distorted trigonal prisms of sulfur [ 1 ]. The structure is very complicated because of the i n c o m m e n s u r a t e structure composed of LaS and MS2 layers, but much attention should be denoted to their properties which arise from the twoTo whom correspondenceshould be addressed.

dimensional character and interaction between LnS and MS2 layers. Most studies on the misfit layer compounds have been concerned with their crystal structure, but several studies have also been made on the properties of "LnMS3" with M = N b , T a . Superconductivity with Tc=2.43 K was reported in (LaS)I.~4NbS2 [2]. The electrical and magnetic properties of (LnS)xTaS2 [3] and (CeS)l.16NbS2 have been studied [ 4 ]. These properties were not directly related to the i n c o m m e n s u r a t e structure in those papers. The structure and properties of "LnMS3" with M = Ti, V and Cr have been scarcely investigated. It is known that some kinds of"LnMS3" (M:Ti,V, Cr) are isostructural with "LaCrS3" [ 5,6 ]. "LaCrS3" also has an incommensurate structure composed of two kinds of alternately stacked layers; pseudo-tetragonal two-atom-thick LaS layers with distorted NaC1 type and pseudo-orthohexagonal three-atom-thick CrS2 with Cdl2 type [ 7,8 ]. The pe-

0167-2738/93/$ 06.00 © 1993 ElsevierSciencePublishers B.V. All rights reserved.

N. Cho et al. / Misfit layer compounds "LnMS f" (Ln = rare-earth metak M = Ti, V,Cr)

riodicity of the two subcells fit each other along the a- and c-axes, but not along the b-axis. The real composition was estimated, therefore, to be La72Cr6oS192 = 60 [ (LaS) 1.2Cr52 ] [ 9 ]. The physical properties of "LnMS3" have not yet been well investigated. Rather preliminary studies have been reported so far. Low electrical resistivities of 10 -4 and 10 -5 flcm were reported on "LaTiS3" at 298 and 4.2 K, respectively [5]. Low resistivity and small positive Seebeck coefficient were observed in "LnVS3" [6]. "LaVS3" showed a metal-like temperature dependence of resistivity but neither "NdVS3" nor "GdVS3" showed this behavior [ 6 ]. "LnCrS3" was insulating and exhibited Curie-Weiss paramagnetism. There has been no report on the relation between the physical properties and the incommensurate structure of "LnMS3" where M=Ti,V,Cr. Charge transfer between the LnS and MS2 layers is also interesting as well as the structural aspect of the lattice mismatch between the layers. We could take X-ray precession photographs of "LaVS3". The reciprocal lattice was explained considering two kinds of subcells. The powder X-ray pattern of "LnMS3" also could be fully indexed with the combination of two subcells. In the present study, we investigated a formation region of"LaCrS3" type crystal structure in the combinations of various kinds of Ln and M. Low-temperature X-ray diffraction was taken to study the temperature dependence of the cell dimension of each subcell in "LaCrS3". The electrical and magnetic properties were also measured on a series of "LnMS3" with the "LaCrS3" type structure.

2. Experimental Rare-earth sesquisulfides were prepared by reaction of granular rare-earth metals with sulfur in evacuated quartz ampoules at 873 K for one week. Commercially available Ln2S3 were used when Ln = La, Sm and Y (Nacalai Tesque, Inc.). "LnMS3" (Ln=La,Ce,Pr,Nd,Sm,Gd, M = T i , V , C r ) was prepared by heating mixtures of transition-metal powder, Ln2S3, and an excess amount of sulfur in evacuated quartz ampoules at 1273 K for one week. After this process was repeated twice, the products were ground and pressed into pellets in an inert atmosphere. The pellets were then sintered in evacuated

697

quartz ampoules at 1273 K for 3 days. Crystals of "LaMS3" (M:Ti,V,Cr) were grown using LaCI3 or I2 as mineralizer. Single crystals with size of 0.3 × 0.3 × 1 m m 3 were obtained as black platelet with metallic luster. Using this single crystal, each sublattice of LaS and MS2 was determined by X-ray diffraction using precession camera. Powder X-ray diffractometry was carried out using a monochromatized Cu Kct ( 1.5405 ,~) radiation. Low-temperature powder X-ray diffraction was carried out to investigate the interaction between LaS and CrSz layers in a temperature range of 77 to 300 K. The electrical resistivities of"LnTiS3" and "LnVS3" were measured by the four-probe method on sintered pellets in the temperature range of 17 to 300 K. The magnetic susceptibility of "LnVS3" and "LnCrS3" was measured using a Faraday balance in a temperature range between 77 K and room temperature.

3. Results and discussion Powder X-ray diffraction data of"LaCrS3" agreed with the previous results [ 6 ]. This could also be indexed on the basis of both monoclinic LnS and triclinic MS2 as described in the previous paper [8]. LaS layers are alternately stacked with CrS2 layers along the c-axis. The a-axis is common for both LaS and CrS2 sublattices. The cell dimension along the incommensurate b-axis corresponds to three times the b parameter in the LaS subcell and five times that in the CrS2 subcell. The real composition was estimated to be (LaS) ~.2CrS2 from the crystal structure and EPMA analysis [8]. The present lattice dimensions in table 1 were the same as the previous values. The lattice constant ofNaCl-type LaS is 5.815/~ [6]. The value is longer than 5.776/k of the b parameter of the LaS subcell in "LaCrS3" and shorter than 5.936 of the a-axis as presented in fig. 1. Both the subcells were distorted from the ideal lattices of cubic LaS and ortho-hexagonal CrS2, respectively, due to interaction between the LaS and CrS2 layers. The ratio of each dimension of the subcells along the b-axis, b ( L a S ) / b ( C r S 2 ) , is 1.686. This value is close to 5/ 3. Low-temperature X-ray diffractometry was carried out on "LaCrS3". The temperature dependence of the lattice parameters was measured on both subcells. The a-axis, the common axis, expanded with

698

N. Cho et al. / Misfit layer compounds "'LnM 3 (Ln = rare-earth metaL M = Ti, I~,Cr) c '

'

'

I

. . . . . . . . .

l

. . . . . . . . .

[

17.4

-i'ii

a

[] []

[ ] : 3bL~s

[]

a

'

E]

[] hl[:Ollllll fill s II l';I 1 (! ; ;tb ( I ,IIS ) > S[; ( r~]~,2 )

'

11:Sb~r~ a

bl r.~)

17.3 11

be

qt,)

C~ i

Fig. 1. A view of the unit cell of"LnMS3" (a), and comparisons of the cell dimensions between LnS with ideal NaCI-type structure (dotted line ) and the LnS sublattice in "LnMS3" (solid line) (b) and between MS2 with ideal CdI2-type structure (dotted line ) and the MS2 sublattice in "LnMS3" (solid line) (c). Table 1 Unit cell dimensions of"LaCrS3" and its subcells, LaS and CrS2 Compound a (~)

b (~)

a (deg)

fl (deg)

7

"LaCrS3" LaS CrS2

17.158 11,034 90.00 5.776 11.078 90.00 3.449 11.063 89.99

94.76 95.38 94.78

90.00 90.00 89.89

5.964 5.942 5.936

c (/k)

[]

O:

6.0 O

0

0

O

O

o3

.

i l l l l l l l j , , , I , , , , ,

. . . .

200

i00

(deg)

decreasing temperature down to 170 K and then shrank in the temperature range from 170 to 77 K as shown in fig. 2. The b parameters of the LaS and CrS2 sublattices shrank and expanded, respectively, with decreasing temperature down to 170 K and remained constant below 170 K. The discrepancy between 3b(LaS) and 5b(CrS2) decreased with lowering temperature. These results suggest that the interaction between LaS and CrS2 layers becomes stronger at lower temperature. "LaVS3" and "LaTiS3" were reported to be isostructural with "LaCrS3" [9 ]. Powder X-ray diffraction patterns of the present "LaVS3" were similar to that of"LaCrS3". We could obtain single crystals of "LaVS3" using LaC13 as mineralizer. Its precession photograph in the ab plane could be interpreted in terms of the two component meshes: a pseudo-tetragonal lattice, T, and a pseudo-orthohexagonal lattice, O, as shown in fig. 3. Similar results have been obtained using electron diffraction on misfit layer compounds [ 9,10 ]. The dimensions of both subcells were evaluated to be a(common)---5.9 A,

a-axis

I,,

300

T/K Fig. 2. Temperature dependence of the sublattice dimensions in "LaCrS3". Open circle, a (common); open square, 3b ( LaS ); filled square, 5b (CrS2).

T

Oo

•

•

200

.

•

•.

a*

Fig. 3. Illustration of the (hkO) reciprocal lattice of"LaVS3'" single crystal.

b ( L a S ) = 5 . 6 7 A in the T cell and b ( V S 2 ) = 3 , 3 2 A in the O cell. The powder X-ray diffraction data could be fully indexed on the basis of these subcells and their superlattice as in the case of "LaCrS3". Preparation of the misfit layer "LnMS3" was investigated

N. Cho et al. / Misfit layer compounds "LnMS3"" (Ln = rare-earth metal; M = Ti, K Cr)

changing Ln = La,Ce,Pr,Nd,Sm,Gd,Y and M=Ti,V, Cr. Formation of the misfit layer compound was confirmed by powder X-ray diffraction for all kinds of Ln in the case of M = Cr as shown in table 2. "LnVS3" with the same structure type was also formed except for the case of Ln = Y. Titanium with the largest ionic radius among the three metals did not form the "LaCrS3" type structure in the case of Ln = Sm, Gd and Y. Formation of the misfit layer compound seems to be limited by the degree of misfit between LnS and MS2 layers, and the upper limit of the ionic radius ratio, R ( M ) / R (Ln), was around 0.61 in the present study. The lattice constant of NaCl-type LnS decreases with an increase of atomic number of Ln ions. Figure 4 shows the variation of the lattice parameter of each sublattice in "LnVS3" in comparison with the lattice constant of NaCl-type LnS. All lattice parameters of subcells decrease with a decrease of ionic radius of Ln ions. There is observed a significant difference in the grade of variation between the b and the a parameters of the LnS sublattice. The b (LnS) parameter decreases with the same gradient as that of NaCl-type LnS except for Ln = Sm and Gd, but the a parameter with obviously easier gradient. This difference is explained on the basis of the characteristic structure of "LnVS3" as follows. The a parameter is in common for both LnS and VS2 sublattices, but both the sublattices are incommensurate along the b-axis, i.e. a discrepancy in the length between 3b(LnS) and 5b(VS/) is allowed. The variation of the a parameter of the LnS sublattice is, therefore, strongly restricted with the a parameter of the VS2 sublattice whose ideal value is larger than the a parameter of NaCl-type LnS. The

699

upper limit of R ( M ) / R (Ln) mentioned above is also related to the difference in dimension along the a-axis between LnS and MS2. Takahashi et al. reported that single crystals of "LnCrS3" with L n = Y , G d , H o and Er had an isostructural monoclinic structure with a similar monoclinic "LaCrS3" structure but with a different set of lattice parameters [13]. Both "SmVS3" and "GdVS3" have basically similar crystal structure to that of"LaCrS3", but may have a different type of incommensurate structure. The temperature dependence of the electrical resistivity of "LnVS3" is shown in fig. 5. The compounds with Ln=La,Ce,Pr,Nd showed a marked transition from semiconductor to metallic conductor in a temperature range from 150 to 200 K. Electrical conduction is certainly disturbed with a discrepancy between the periodicities of the LnS and VS2 sublattices, 3b(LnS) and 5b(VS2), as well as the distortion of the VS2 layer which is a main pathway. It is considered that the observed metallic behavior is related to the lowering of the discrepancy at low temperature. While the semiconductor-metal transition was not observed in the compounds with L n = S m and Gd. The type of lattice matching between LnS and VS2 layers may be different in the latter case from the others as described above. Magnetic susceptibilities of "LnVS3" obeyed the Curie-Weiss law in a temperature range from 77 to 300 K and were in a range 10 - 3 t o 10 - 5 emu/g. The observed values are in fairly good agreement with the calculated ones for "Ln3+V3+S32-'' except for "SmVS3". Both Ln and V are almost in the trivalent state, while Sm seems to be almost in the divalent

Table 2 Formation diagram of misfit layer "LnMS3". Circles ( O ) and crosses ( X ) represent, respectively, that the misfit layer compound is formed or not. Values in parentheses are ionic radii for trivalent six-coordinated transition metal and eight-coordinated rare earth metal [ 11 ]. The ionic radius ratios, R ( M ) / R ( L n ) , are in square brackets La3+ (1.160)

Ce3+ (1.143)

pr3+ (1.126)

Nd3+ (1.109)

Sm3+ (1.079)

Gd3+ (1.053)

y3+ (1.019)

TP +

0

0

0

0

X

X

X

(0.670) V 3+ (0.640) Cr 3+

[0.578]

[0.586]

[0.595]

[0.6041

[0.621]

[0.6361

[0.658]

0

0

0

0

0

0

X

[0.5521

[0.5601

[0.5681

[0.5771

[0.5931

[0.6081

[0.628]

0

0

0

0

0

0

0

(0.615)

[0.5301

[0.5381

10.5461

[0.5551

[0.570]

[0.584l

[0.604]

700

N. Cho et al. / Misfit layer compounds "LnM 3 (Ln = rare-earth metal; M = Ti, V, Cr)

Table 3 Effective magnetic moments (/tB) of"LnVS3" La

Ce

2.68 2.83

/~obs ,u~jc

3.10 4.02 3.80 4.56 (Ce3+Ve3+S3) (pr3+V3+V3)

(ta3+V3+53)

Gd

11.4

I

Sm I

I

Pr

Nd Pr

I

I

j

I

I

Nd

Sm

3.81 1.87 8.54 4.59 2.95 8.43 (Nd3+V3+S3) (Sm3+V3+S3) (Gd3+V3+S3) 1.92 1.73 (Nd2+V4+S3) (Sm2+V4+S3)

Ce La

I

1

I

100K 50K 33K 25K 20K 17K

I

1t

11.2 11.0

Gd

c(common)

•

J I/~'

.2-..,----

10. 5.8 o<~

\

"~--~= 5.6 v

b

5, 3"I

\

nS

I

z~

0

3. t~

I

i. 04

I

I

I. 07

I

I

I

i. I0

I

I

I

I. 13

I

I

I

I

20 i 03KIT

40

0

Fig. 5. Logarithmic elcctricalresistivityagainst I/Tfor "LnVS3".

ll 16

Fig. 4. Cell dimension of each subcell in "LnVS3". The open circles represent the lattice parameters of LnS with NaC1 structure [121.

state in "SmVS3". The magnetic susceptibility was also measured on a series o f "LnCrS3". The chrom i u m was also almost in trivalent state and anomalous behavior was observed a r o u n d 110 K. Details o f the t e m p e r a t u r e dependence o f the susceptibility will be discussed elsewhere. In summary, a series o f the misfit layer compounds "LnMS3" were prepared. There was an upper limit o f the ionic radius ratio R ( M ) / R ( L n ) =0.61 for the formation o f the misfit layer

structure. The discrepancy in periodicity o f both LaS and CrS2 sublattices along the b-axis ( 3 b ( L a S ) and 5 b ( C r S 2 ) ) decreased with decreasing t e m p e r a t u r e and was almost eliminated a r o u n d 170 K. "'LnVS3" exhibited semiconductive behavior in a t e m p e r a t u r e range from about 200 K to RT. A semiconductor to metal transition was observed in a temperature range from 150 to 200 K in "LnVS3" with L n = L a , C e , P r and Nd.

Acknowledgements One author ( N . C ) expresses his thanks to Mr. T. Y a m a m o t o o f Osaka University for his technical assistance in the electrical conductivity measurements,

N. Cho et al. / Misfit layer compounds "LnMS~'" (Ln = rare-earth metal; M = Ti, V, Cr)

a n d to Mr. T. T a n a k a o f M A C at O s a k a U n i v e r s i t y for assistance in t a k i n g p r e c e s s i o n p h o t o g r a p h s . T h e a u t h o r s also express t h e i r t h a n k s to S a n t o k u M e t a l Inc. for t h e i r k i n d n e s s in s u p p l y i n g the rare-earth metals. T h i s r e s e a r c h was partly s u p p o r t e d by a G r a n t - i n - A i d for G e n e r a l Scientific R e s e a r c h f r o m the M i n i s t r y o f E d u c a t i o n , Science a n d C u l t u r e o f J a p a n a n d a grant f r o m the research p r o g r a m on " C r e a t i o n o f N e w F u n c t i o n a l M a t e r i a l s by N a n o Synthetic Method" of ISIR, Osaka University.

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