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Raman scattering of NiS2
Solid State Communications, Vol. 23, pp. 847-852, 1977.
Pergamon Press.
Printed in Great Britain
RAMAN SCATTERING OF NiS2 T. Suzuki, K. Uchinokura, T. Sekine and E. Matsuura Institute of Physics, University of Tsukuba, Sakura-mura, Ibaraki, Japan 300-31
(Received 18 May 1977 by Y. Toyozawa) Raman scattering measurement of NiS2 is done and five optical phonon peaks are found. These peaks are assigned to As, El and 37"#phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS2 and MnS2. THE TRANSITION metal dichalcogenides of pyrite-type structure (T~,) have been extensively studied by various methods in recent years. These materials and their mixed crystals show electrical properties which vary from metallic conductivity to semiconductivity. NiS2 is a semiconductor like FeS2 and MnS2 but its energy gap is very narrow (about 0.03 eV). Especially NiS2 and its mixed crystals with other transition metal dichalcogenides (e.g. NiS2_xSex) exhibit Mott transitior, under the pressure or with the composition x. Raman scattering of pyrite-type compounds has been observed in FeS2 [1,2], MnSa [3,4], CoSe2 [5] and CuS% [5]. However two Te phonon peaks are not observed in FeS2 and the symmetries of the observed phonons are not yet determined in MnS2. Furthermore CoS2, NiS2, CuS=, ZnS2, Co~ _~,Fe~S2 and Coo.gNio. i Sa were investigated by Anastassakis and Perry [6] recently. But five phonon peaks are found only in ZnS2 and their symmetry assignments are speculative. Consequently the complete set of five Raman active optical phonon peaks of this type of crystal has not been observed. Recently the force constants were calculated from the infrared absorption and Raman scattering data of MnS2 [7, 8], FeS2 [8] and others [7]. Their results reveal the bonding nature of pyrite-type crystals considerably. Single crystal NiS2 of a pyrite.type compound was obtained by the chemical transport method [9]. The obtained crystal is a plate-like one which has a wide (111) plane. The surface of this crystal is fairly flat and clean so that we use the as.grown surface for measurements. Measurements of Raman scattering were performed at room temperature using the reflection method. An Ar ion laser beam was used with 5145 or 4880 A lines which were incident on a face of the sample nearly in Brewster's angle and were scattered and collected to the double monochromator SPEX 1401 with third monochromator. The symmetries of phonons were determined by polarization nature of scattered light. The incident beam is polarized by a Glan-Laser prism polarizer in the reflection plane and a Glan-
Thompson prism analyzer in front of the monochromator is set to pass a polarization component of scattered light parallel or perpendicular to the reflection plane. The Stokes Raman spectrum at room temperature with incident light of 4880 A~and resolution Aw = 14 cm -1 is shown in Fig. 1. Apparently there are three peaks at about 270,480 and 600 cm-l, but the peaks at 270 and 480 cm -I are both broad and asymmetrical. Measurement with higher resolution for these peaks was performed and the results are shown in Fig. 2. Clearly each peak consists of two peaks. Then we conclude that we fred all of the five peaks of Raman active optical phonons whose energies are 272,281,480,487 and 596 cm -1. Pyrite-type crystal structure belongs to the space group T~ and has five Raman active phonons A t, E e and 3Tg. These phonons have Raman scattering intensity matrices as shown below in the case of given directions (subscript shows the direction of the incident light in the sample) and polarizations of incident and scattered light. The notations used in the matrices are Loudon's [10]. For X = [I001, Y = [010] andZ = [OOl], fl 2
tl 2
l
4b ~
Ioao(Eg) =
I
4b ~ 4b2] ' d 2
d2
lo~o(ri ) =
da ;
d2 d2 forX= [0111, Y= [1001andZ= [01il, a2
Iloo(A#) =[
847
a2
], a2
848
RAMAN SCATTERING OF NiS2
Vol. 23, No. l ]
XIO 3
•.
NJS 2
~880A at Room Temp.
t/) I-Z
82
t..)
.~,.:r,~.:;'...~,~,~"...c.- -.'r...¢ ,...-.,¢.,~.
50
"':.,
.
~ ..:"
I
I
200
400
I
L
600
800
FREQUENCY SHIFT ( c m - ' )
Fig.
I. The unpolarized Stokes Raman spectrum at room temperature with incident light of 4880 ~, and with resolution Aeo = 14cm -~. The peaks at 270 and 4 8 0 c m -~ consist of two peaks as shown by Fig. 2. Incident light in the sample is directed approximately to [ 1 ] 1 ].
I--
:D
-q)-
it
>. rr rr I-nr"
<.5. >I-U3
Z I.U I--
z
•
~_e
•
.•
*
~
.
o
•,
",
• •
,|
250
I
300
,l
450
",
•
i
500
FREQUENCY S H I F T ( c m -~ )
Fig. 2. The unpolarized spectrum with higher resolution ~
= 5.6 cm -~ at 270 and 480 cm -1
• I ! I•% I
Vol. 23, No. 11
RAMAN SCATTERING OF NiS2 b2
3b 2 ]
IIoo(Es) =
4b 2
,
3b 2
b2
Table 1. Raman scattering peaks o f FeS2, MnS2 and NiSa. Data o f FeS2 and MnS2 are quoted from references [2] and [3], respectively. The peaks o f MnS2 are not assigned Crystal
I2oo(Ti) =
d2
849
Mode
FeS2 (cm-1)
NiS2 MnS2 (cm-1) (cm-1)
Ag
385
480
224
Es
351
281
246
Tt Ts Ts
441
272 487 596
486 655 743
d2 ; d2
d2
and f o r X = [110], Y = [ l l i ] andZ= [112], Ini(As) =
f J a2
,
a2
lid(El) =
b2
2b 2
b 2]
2b 2
0
2b ~ ,
[ b2
Illi(Ts) = ~
2b 2
3d 2
d2
d2
4d 2
2d 2
d2
I
b2
2d2] d2 . 3d 2
Here, Y is chosen to be the direction of incident light in the sample. Note that the expression of reference [2] which corresponds to I11i(Er) is incorrect [11]. We observe (ZZ) or (ZX) component in the experiment. From these matrices we know that A s peak appears only in the component (ZZ) in all directions of incident light, and E s phonon peak always appears except in the component (ZX) with Y = [010]. Ts phonon peaks appear in the component (ZX) with Y = [010] and Y = [11 i], and in the component (ZZ) with Y = [100] and Y = [111]. The observed spectra are shown in Fig. 3. In Y ( Z X ) ? w i t h Y = [010] [Fig. 3(a)] there are two peaks at 272 and 487 cm-~, and they should be Ts phonons. On the other hand, in Y ( Z Z ) Y there remain two peaks at 480 and 281 cm -1 and they should be A s or E# phonons. In Fig. 3 (b) the peak at 480 cm-I disappears in Y ( Z X ) ? w i t h Y = [100], but the peak at 281 cm -1 remains. Therefore the former peak (480 cm -1) is identified as A s phonon and the latter (281 cm -1) as E s phonon. Four peaks out of five observed peaks are assigned to A i, E r and 2Ts and therefore we conclude that the remaining small peak of 596 cm-I is Ts phonon. The results are shown in Table 1 with data of FeS2 and MnS2. In Fig. 3(c) the observed intensity of Ts phonon peak at 272 cm-I is nearly equal to E t phonon peak in the component (ZZ). It means that the intensity of Eg * Note added in proof: after we had submitted this paper, we found that infrared absorption data of NiS2 had been obtained by LUTZ H.D. & WILL1CH P., Z. Anorg. Allg. Chem. 405,176 (1974).
peak should be larger than that of Tz peak by 1.5 times in the component (ZX) according to the intensity matrices. But the observed E t peak is slightly smaller than Ti peak. This discrepancy is interpreted as follows. The laser beam is incident on the sample with Brewster's angle, so the incident beam is not strictly directed to [1 l l ] crystal axis in the sample. If the angle of this discrepancy from [111] axis is about ten degrees the intensity o f E t peak in the component (ZX) should be merely 1/3 of that in the component (ZZ). On the other hand the intensity of Tg peak in the component (ZX) should be somewhat larger than that of Ts peak in the component (ZZ). Therefore we conclude that this discrepancy is caused by the situation of our experiment, and the assignement of peaks written before does not contradict the experimental results shown in Fig. 3 (c). Lauwers and Herman [8] calculated force fields of FeS2 and MnS2. According to their results the energy of Ts phonon peaks which have not been found in FeS2 are close to that ofA s phonon. Furthermore they pointed out that higher frequency peaks of MnS2 of reference [3] are not first-order optical phonon peaks. Considering these, the present results can be compared with the results of FeS2 and MnS2 and would make the bonding nature of pyrite-type crystal clearer. For instance the energies ofA s phonon which depend on the bonding of S - S are 385 cm-l, 486 cm-I and 480 cm -1 in FeS2, MnS2 and NiS2, respectively. Therefore S-S bondings of MnS2 and NiS2 have almost the same strength. On the other hand the bonding in FeS2 is fairly weaker than in NiS2. S - S bonding lengths of FeS2, MnS2 and NiS2 are 2.14, 2.09 and 2.07 A, respectively, and the above results correspond to these length qualitatively. To our knowledge infrared absorption data of NiS2 have not been given,* and so bonding nature is not known in detail. But we can say from the Table 1 that bonding nature of NiS2 has more similarity to that of MnSz than FeS2. Nevertheless the energies except for A s phonon in NiS2 are different from those of MnS: considerably, and these results should be investigated furthermore.
850
RAMAN SCATTERING OF NiS2
Vol. 23, No. l I
,:~'. '.~"
~.
OQ
,ll
o •
"~-. J
..--, -.
.1_ 7-
114.
•
• e
|
N N
~ ' .
v
W O
~
I
II
X
II
~
m
ID •
O •
A
o
II
N
(SIlNi3 AI::IVI::IIlS~) AIISN':IINI
•
•
•
852
RAMAN SCATTERING OF NiS2
Vol. 23, No. 11
REFERENCES 1.
USHIODA S., Solid State Commun. ! 1,307 (1972).
2.
MACFARLANE R.M., USHIODA S. & BLAZEY K.W., Solid State Commun. 14, 851 (1974).
3.
VERBLE J.L. & HUMPHREY F.H., Solid State Commun. IS, 1693 (1974).
4.
EYSEL H., SIEBERT H. & AGIORGITIS G., Z. Naturf. 24b, 932 (1969).
5.
ANASTASSAKIS E., Solid State Commun. 13, 1297 (1973),
6.
ANASTASSAKIS E. & PERRY C.H., Proc. Int. Conf. on Light Scattering in Solids, Campinas, Brazil (1975).
7.
LUTZ H.D., WILLICH P. & HAEUSELER H., Z. Naturf 31a, 847 (1976).
8.
LAUWERS H.A. & HERMAN M.A.,J. Phys. Chem. Solids37,831 (1976).
9.
BOUCHARD R.J., J. Crystal Growth 2, 40 (1968).
10.
LOUDONR.,Adv. Phys. 13,423(1964);Adv. Phys. 14,621 (1965).
11.
Expression in reference [2] was obtained by incorrectly using inverse transformation matrix, and therefore corresponds to the intensity matrix ~ the case of chosen axes X = (1/x/2, 1/x/3, 1/x/6), Y = (-- 1/x/2, 1/,4'3, 1/x/6) and Z = (0, -- 1/x/3, x/2/x/3). Macfarlane et al. seem to have used as a starting Raman tensor the expression ofWALLIS R.F. & MARADUDIN A,A., Phys. Rev. B3, 2063 (1971). As a natural consequence, we come to the same intensity matrix as Illi(E,) in the present paper by the correct transformation from Wallis and Maradudin's expression.
Pergamon Press.
Printed in Great Britain
RAMAN SCATTERING OF NiS2 T. Suzuki, K. Uchinokura, T. Sekine and E. Matsuura Institute of Physics, University of Tsukuba, Sakura-mura, Ibaraki, Japan 300-31
(Received 18 May 1977 by Y. Toyozawa) Raman scattering measurement of NiS2 is done and five optical phonon peaks are found. These peaks are assigned to As, El and 37"#phonons which are all of the Raman active optical phonons in pyrite-type crystal. The results are compared with those of FeS2 and MnS2. THE TRANSITION metal dichalcogenides of pyrite-type structure (T~,) have been extensively studied by various methods in recent years. These materials and their mixed crystals show electrical properties which vary from metallic conductivity to semiconductivity. NiS2 is a semiconductor like FeS2 and MnS2 but its energy gap is very narrow (about 0.03 eV). Especially NiS2 and its mixed crystals with other transition metal dichalcogenides (e.g. NiS2_xSex) exhibit Mott transitior, under the pressure or with the composition x. Raman scattering of pyrite-type compounds has been observed in FeS2 [1,2], MnSa [3,4], CoSe2 [5] and CuS% [5]. However two Te phonon peaks are not observed in FeS2 and the symmetries of the observed phonons are not yet determined in MnS2. Furthermore CoS2, NiS2, CuS=, ZnS2, Co~ _~,Fe~S2 and Coo.gNio. i Sa were investigated by Anastassakis and Perry [6] recently. But five phonon peaks are found only in ZnS2 and their symmetry assignments are speculative. Consequently the complete set of five Raman active optical phonon peaks of this type of crystal has not been observed. Recently the force constants were calculated from the infrared absorption and Raman scattering data of MnS2 [7, 8], FeS2 [8] and others [7]. Their results reveal the bonding nature of pyrite-type crystals considerably. Single crystal NiS2 of a pyrite.type compound was obtained by the chemical transport method [9]. The obtained crystal is a plate-like one which has a wide (111) plane. The surface of this crystal is fairly flat and clean so that we use the as.grown surface for measurements. Measurements of Raman scattering were performed at room temperature using the reflection method. An Ar ion laser beam was used with 5145 or 4880 A lines which were incident on a face of the sample nearly in Brewster's angle and were scattered and collected to the double monochromator SPEX 1401 with third monochromator. The symmetries of phonons were determined by polarization nature of scattered light. The incident beam is polarized by a Glan-Laser prism polarizer in the reflection plane and a Glan-
Thompson prism analyzer in front of the monochromator is set to pass a polarization component of scattered light parallel or perpendicular to the reflection plane. The Stokes Raman spectrum at room temperature with incident light of 4880 A~and resolution Aw = 14 cm -1 is shown in Fig. 1. Apparently there are three peaks at about 270,480 and 600 cm-l, but the peaks at 270 and 480 cm -I are both broad and asymmetrical. Measurement with higher resolution for these peaks was performed and the results are shown in Fig. 2. Clearly each peak consists of two peaks. Then we conclude that we fred all of the five peaks of Raman active optical phonons whose energies are 272,281,480,487 and 596 cm -1. Pyrite-type crystal structure belongs to the space group T~ and has five Raman active phonons A t, E e and 3Tg. These phonons have Raman scattering intensity matrices as shown below in the case of given directions (subscript shows the direction of the incident light in the sample) and polarizations of incident and scattered light. The notations used in the matrices are Loudon's [10]. For X = [I001, Y = [010] andZ = [OOl], fl 2
tl 2
l
4b ~
Ioao(Eg) =
I
4b ~ 4b2] ' d 2
d2
lo~o(ri ) =
da ;
d2 d2 forX= [0111, Y= [1001andZ= [01il, a2
Iloo(A#) =[
847
a2
], a2
848
RAMAN SCATTERING OF NiS2
Vol. 23, No. l ]
XIO 3
•.
NJS 2
~880A at Room Temp.
t/) I-Z
82
t..)
.~,.:r,~.:;'...~,~,~"...c.- -.'r...¢ ,...-.,¢.,~.
50
"':.,
.
~ ..:"
I
I
200
400
I
L
600
800
FREQUENCY SHIFT ( c m - ' )
Fig.
I. The unpolarized Stokes Raman spectrum at room temperature with incident light of 4880 ~, and with resolution Aeo = 14cm -~. The peaks at 270 and 4 8 0 c m -~ consist of two peaks as shown by Fig. 2. Incident light in the sample is directed approximately to [ 1 ] 1 ].
I--
:D
-q)-
it
>. rr rr I-nr"
<.5. >I-U3
Z I.U I--
z
•
~_e
•
.•
*
~
.
o
•,
",
• •
,|
250
I
300
,l
450
",
•
i
500
FREQUENCY S H I F T ( c m -~ )
Fig. 2. The unpolarized spectrum with higher resolution ~
= 5.6 cm -~ at 270 and 480 cm -1
• I ! I•% I
Vol. 23, No. 11
RAMAN SCATTERING OF NiS2 b2
3b 2 ]
IIoo(Es) =
4b 2
,
3b 2
b2
Table 1. Raman scattering peaks o f FeS2, MnS2 and NiSa. Data o f FeS2 and MnS2 are quoted from references [2] and [3], respectively. The peaks o f MnS2 are not assigned Crystal
I2oo(Ti) =
d2
849
Mode
FeS2 (cm-1)
NiS2 MnS2 (cm-1) (cm-1)
Ag
385
480
224
Es
351
281
246
Tt Ts Ts
441
272 487 596
486 655 743
d2 ; d2
d2
and f o r X = [110], Y = [ l l i ] andZ= [112], Ini(As) =
f J a2
,
a2
lid(El) =
b2
2b 2
b 2]
2b 2
0
2b ~ ,
[ b2
Illi(Ts) = ~
2b 2
3d 2
d2
d2
4d 2
2d 2
d2
I
b2
2d2] d2 . 3d 2
Here, Y is chosen to be the direction of incident light in the sample. Note that the expression of reference [2] which corresponds to I11i(Er) is incorrect [11]. We observe (ZZ) or (ZX) component in the experiment. From these matrices we know that A s peak appears only in the component (ZZ) in all directions of incident light, and E s phonon peak always appears except in the component (ZX) with Y = [010]. Ts phonon peaks appear in the component (ZX) with Y = [010] and Y = [11 i], and in the component (ZZ) with Y = [100] and Y = [111]. The observed spectra are shown in Fig. 3. In Y ( Z X ) ? w i t h Y = [010] [Fig. 3(a)] there are two peaks at 272 and 487 cm-~, and they should be Ts phonons. On the other hand, in Y ( Z Z ) Y there remain two peaks at 480 and 281 cm -1 and they should be A s or E# phonons. In Fig. 3 (b) the peak at 480 cm-I disappears in Y ( Z X ) ? w i t h Y = [100], but the peak at 281 cm -1 remains. Therefore the former peak (480 cm -1) is identified as A s phonon and the latter (281 cm -1) as E s phonon. Four peaks out of five observed peaks are assigned to A i, E r and 2Ts and therefore we conclude that the remaining small peak of 596 cm-I is Ts phonon. The results are shown in Table 1 with data of FeS2 and MnS2. In Fig. 3(c) the observed intensity of Ts phonon peak at 272 cm-I is nearly equal to E t phonon peak in the component (ZZ). It means that the intensity of Eg * Note added in proof: after we had submitted this paper, we found that infrared absorption data of NiS2 had been obtained by LUTZ H.D. & WILL1CH P., Z. Anorg. Allg. Chem. 405,176 (1974).
peak should be larger than that of Tz peak by 1.5 times in the component (ZX) according to the intensity matrices. But the observed E t peak is slightly smaller than Ti peak. This discrepancy is interpreted as follows. The laser beam is incident on the sample with Brewster's angle, so the incident beam is not strictly directed to [1 l l ] crystal axis in the sample. If the angle of this discrepancy from [111] axis is about ten degrees the intensity o f E t peak in the component (ZX) should be merely 1/3 of that in the component (ZZ). On the other hand the intensity of Tg peak in the component (ZX) should be somewhat larger than that of Ts peak in the component (ZZ). Therefore we conclude that this discrepancy is caused by the situation of our experiment, and the assignement of peaks written before does not contradict the experimental results shown in Fig. 3 (c). Lauwers and Herman [8] calculated force fields of FeS2 and MnS2. According to their results the energy of Ts phonon peaks which have not been found in FeS2 are close to that ofA s phonon. Furthermore they pointed out that higher frequency peaks of MnS2 of reference [3] are not first-order optical phonon peaks. Considering these, the present results can be compared with the results of FeS2 and MnS2 and would make the bonding nature of pyrite-type crystal clearer. For instance the energies ofA s phonon which depend on the bonding of S - S are 385 cm-l, 486 cm-I and 480 cm -1 in FeS2, MnS2 and NiS2, respectively. Therefore S-S bondings of MnS2 and NiS2 have almost the same strength. On the other hand the bonding in FeS2 is fairly weaker than in NiS2. S - S bonding lengths of FeS2, MnS2 and NiS2 are 2.14, 2.09 and 2.07 A, respectively, and the above results correspond to these length qualitatively. To our knowledge infrared absorption data of NiS2 have not been given,* and so bonding nature is not known in detail. But we can say from the Table 1 that bonding nature of NiS2 has more similarity to that of MnSz than FeS2. Nevertheless the energies except for A s phonon in NiS2 are different from those of MnS: considerably, and these results should be investigated furthermore.
850
RAMAN SCATTERING OF NiS2
Vol. 23, No. l I
,:~'. '.~"
~.
OQ
,ll
o •
"~-. J
..--, -.
.1_ 7-
114.
•
• e
|
N N
~ ' .
v
W O
~
I
II
X
II
~
m
ID •
O •
A
o
II
N
(SIlNi3 AI::IVI::IIlS~) AIISN':IINI
•
•
•
852
RAMAN SCATTERING OF NiS2
Vol. 23, No. 11
REFERENCES 1.
USHIODA S., Solid State Commun. ! 1,307 (1972).
2.
MACFARLANE R.M., USHIODA S. & BLAZEY K.W., Solid State Commun. 14, 851 (1974).
3.
VERBLE J.L. & HUMPHREY F.H., Solid State Commun. IS, 1693 (1974).
4.
EYSEL H., SIEBERT H. & AGIORGITIS G., Z. Naturf. 24b, 932 (1969).
5.
ANASTASSAKIS E., Solid State Commun. 13, 1297 (1973),
6.
ANASTASSAKIS E. & PERRY C.H., Proc. Int. Conf. on Light Scattering in Solids, Campinas, Brazil (1975).
7.
LUTZ H.D., WILLICH P. & HAEUSELER H., Z. Naturf 31a, 847 (1976).
8.
LAUWERS H.A. & HERMAN M.A.,J. Phys. Chem. Solids37,831 (1976).
9.
BOUCHARD R.J., J. Crystal Growth 2, 40 (1968).
10.
LOUDONR.,Adv. Phys. 13,423(1964);Adv. Phys. 14,621 (1965).
11.
Expression in reference [2] was obtained by incorrectly using inverse transformation matrix, and therefore corresponds to the intensity matrix ~ the case of chosen axes X = (1/x/2, 1/x/3, 1/x/6), Y = (-- 1/x/2, 1/,4'3, 1/x/6) and Z = (0, -- 1/x/3, x/2/x/3). Macfarlane et al. seem to have used as a starting Raman tensor the expression ofWALLIS R.F. & MARADUDIN A,A., Phys. Rev. B3, 2063 (1971). As a natural consequence, we come to the same intensity matrix as Illi(E,) in the present paper by the correct transformation from Wallis and Maradudin's expression.