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Lattice vibrations of mercuric oxides
OI’TICS
LATTICE
VIBRATIONS
E. A. DECAMPS, Lahoratoivc
de 1’Etat
COMMUNICATlOhZ
OF
MERCURIC
1972
OXIDES
M. DURAND, Y. MARQUETON
Cristallin, GWX@E dc Physique Cristallinc, l:niz,ersit& dc Rcwws, R5 Rcnncs-Reaulieu,
Equipc associ6e Fmnce
au C.N.RS.no.
15,
and B. AYRAULT
Received
:31 July
1971
The different forms of mercuric oxide and notably, its “cinnabarn form were studled with the Raman laser and far-infrared techniques at low frequencies and different temperatures. The vibrations correspondmg to movements “en bloc” of the mercury and oxygen sub-lattices seem predominant.
1. INTRODUCTION Mercuric oxide, HgO, crystallizes under numerous forms. The first one, orthorhombic, presents all the exterior appearances of dimorphism. Two forms, respectively, red and yellow, are obtained by different chemical preparation methods. Polymorphism has not, however, been proved and this color difference is generally related to the size of the crystal grains, which are very small for yellow oxide, and larger for red oxide. The cell contains four molecules. the symmetry group being noted Pnma or DiE (fig.1 1. The second rhombohedral form, dark red. recently discovered by Laruelle [I] belongs to group P3I21 or D! with three molecules per cell. The only compound of the XM formula crystallizing in the same space group is mercuric sulphide, HgS, in the cinnabar form which has similar cell dimensions to HgO. The structure of this type of compound consists of helicoidal chains with six atoms per turn (fig. 2). The binary coordination indicates, a priori, a covalent bond between Hg and 0; in contradistinction the bond between the chains should be partially ionic. We have recorded the far-infrared spectra, between 400 and 4 cm-l at 300 and 90oK of three forms of oxide: yellow and red orthorhombic and dark red rhombohedral. The Raman spectra were obtained on a Coderg spectrophotometer with double monochromator, the light source being the 6328 A radiation of a Spectraphysics He-Ne laser of 70 mW. This study should be considered as 358
.Hy 00
Y= ‘/4 Y = li4
ii Hg Yz~~ ) 0 Yz3,.
Fig. 1.
preliminary until tests have been completed on monocrystalline HgO, a sample of which is being prepared.
2. THEORETICAL VIBRATIONS OF THE RHOMBOHEDRAL FORM The Di symmetry of the cinnabar group is identical to that of quartz and the factor group has already been studied [2]. It is isomorphic to group 3m(C 3”) of the equilateral triangle and possesses three irreducible representations AI, A2 and E. The symmetry of the phonons of large amplitude waves is determined by F. the factor group corresponding to atom displacements of the elementary cell. The three irreducible represen-
Volume 4, number
5
OPTICS COM~lUNICATIOKS
.January 1972
theoretically active modes in infrared are of types A2 and E, while the symmetrical Al vibrations do not interact with light in the first-order phenomena. The latter are visible in Raman scattering as well as the E modes, because of the absence of an inversion center. From the application of the new formulation ]3] of the Pauling electronegativity concept in terms of a dielectric definition of ionicity, it is possible to consider a correspondence between the cinnabar structure and the rock-salt structure. The six neighbors of each site define a distorted octahedral configuration.
3. THEORETICAL VIBRATIONS ORTHORHOMBIC FORM
16 for the If one accepts the D2h symmetry orthorhombic form, group theory according to the exclusion law, determines the active Raman and infrared modes, Alg, Jj2g, Blu, BgU. However, taking into consideration the results, a more sophisticated determination of structure by X-ray techniques seems necessary.
Fig.%.
tations F = 2Al + 4A2 + 6E must be related to the three acoustical phonons in the center of the zone corresponding to the translation of the crystal as a whole which would be I = 2AI + 3A2 + 5E, and would define ten optical frequency modes. The
rpo
0
OF THE
2po
4. EXPERIMENTAL
RESULTS
AND ATTRIBUTIONS
Figs. 3 and 4 give the experimental results. The predicted attributions are indicated here.
390
400
SQO
SpO
+cm-l
58 Infrared
LB0
HgO rhomb. 300
K
Raman
3 cm-’ Fig. 3. 359
Infrared
%S’ ortho. -300K
--aoK Raman
_1
332’5 4.00[:, s’oo
$oo )
9
The infrared spectrum between 4 and 400 cm-1 exhibits ‘one band only. It seems, by its position (180 cm-l I and its width to correspond to the group of cinnabar vibrations consisting of the longitudinal and transverse components of the A2(51 and E(4) modes \vert% the modes are numbered upwards from the lowest frequency observed. The A2(1 i alld A2(8) vibrations as welt as the degenerate modes do not appear. Fig. 5 shows a representation of the three A2 modes which should be respectiveiy a rotation of the helicoidal chain around the ternary axis (1) a purely axial movement (2) for a displacement 1’ of the oxygen atoms and a con~~eIlsatil1~ nlovei~lcn& 2; 12.4 of the mcrcury atoms. and finally, a rotation (31 of the two atom types in opposed senses. Case 1 corresponds to a zero frequency mode neglecting mutual interaction between chains. On the contrary, in case 2, each one of the sublattice Hg and 0 gives a rigid translation, the O-O and Hg-Hg distances being conserved during the movement. This cast corresponds to an optical mode vector in rock-salt. A first-order electrical moment corresponds in the rigid ionic model only to this movement. The Iowest frequency of vibration seen in the HgS spectrum does not appear here while the highest frequency vibration seems 360
cm-’
to be situated outside our domain of study. On the other hand the band at I80 cm-l. little affected by temperature, corresponds exactly to the product 128 \ 1.4 of the band frequency of HgS with the ratio of the square roots of the masses 0 and S. This vibration corresponds then in part, in harmonic approximation to the type 2 movement already described. Note that it seems to slightly split at room temperature. The spectrum extends to high frequencies with two bands at 560 and 573 cm-l. Some of observed lines are most likely ghost or Ne lines. It is to be noted that in the elementary theory of cinnabar of ‘Zallen et al. 12 1, the intensities of the Raman and infrared active vibrations arc coIil~lenlentary. This is not the case of HgO where the line at 180 cm-l is one of the most intense of the Raman spectrum which presents in addition some second-order lines. It seems then, that the real structure of rhombohedral HgO should be slightly different from that of cinnabar. 4.2. 1’1~ or11~ovhow~l~icj2vm Contrary to the preceding form, the infrared spectrum presents in its general aspect. a certain analogy with the one obtained for cinnabar at least for the yellow samples 141.
Volume
4, number
5
OPTICS
COMMUNICATIONS
This spectrum includes (fig. 4) three important bands at 60,147 and 232 cm-l which at liquidnitrogen temperature become 62, 147 and 235 cm. It seems that the last band corresponds to a group of different vibrations. Only the study of monocrystals would resolve this question. The Raman spectrum does not show any similarity with the mercuric sulphide spectrum. Two facts are to be noted: the existence of a vibration at 178 cm-1 which may correspond to the same movement of the Hg and 0 sublattices discussed for the rhombohedral form. Secondly, diminution and displacement of the band towards the high frequency zone from 326.5 to 332.5 cm-l when one passes from room temperature to liquidnitrogen temperature. The effect on the intensity is inverted for the weak line at 431 cm-I. Very high frequency bands are observed equally at 560,566 cm-l. No interpretation of these bands is at present possible. The red variety made of larger grains, presents an infra-red spectrum much less sharp than those obtained with the yellow form, and the Raman spectrum is slightly different.
January
1972
Fig. 5.
accepted. The orthorhombic HgO variety which has cell dimensions very close to the HgS one, gives a translational frequency for the Hg and 0 sub-lattices which does not differ significantly from the one given by the HgO rhombohedral variety.
ACKNOWLEDGEMENTS We thank Mr. E. S. Eccles of Smiths Industries, Cheltenham UK, for his kind assistance.
REFERENCES 5. CONCLUSION A great part of the observed vibrations has been accounted for. It seems that the rhombohedral HgO phase should be slightly different from the rhombohedral HgS (cinnabar) as it is currently
[1] P.Laruelle, Thsse, Paris (1960) (Masson Edit.). [Z] R. Zallen, G. Lucovsky, W. Taylor, A. Pinczuck and E. Burstein, Phys.Rev. Bl (1970) 4038. [3] J.C.Phillips and J.A.van Vechten, Phys.Rev. Letters 22 (1969) 705. [4] E.A.Decamps and A.Hadni, J.Chim.Phys.65 (1968) 1030.
361
LATTICE
VIBRATIONS
E. A. DECAMPS, Lahoratoivc
de 1’Etat
COMMUNICATlOhZ
OF
MERCURIC
1972
OXIDES
M. DURAND, Y. MARQUETON
Cristallin, GWX@E dc Physique Cristallinc, l:niz,ersit& dc Rcwws, R5 Rcnncs-Reaulieu,
Equipc associ6e Fmnce
au C.N.RS.no.
15,
and B. AYRAULT
Received
:31 July
1971
The different forms of mercuric oxide and notably, its “cinnabarn form were studled with the Raman laser and far-infrared techniques at low frequencies and different temperatures. The vibrations correspondmg to movements “en bloc” of the mercury and oxygen sub-lattices seem predominant.
1. INTRODUCTION Mercuric oxide, HgO, crystallizes under numerous forms. The first one, orthorhombic, presents all the exterior appearances of dimorphism. Two forms, respectively, red and yellow, are obtained by different chemical preparation methods. Polymorphism has not, however, been proved and this color difference is generally related to the size of the crystal grains, which are very small for yellow oxide, and larger for red oxide. The cell contains four molecules. the symmetry group being noted Pnma or DiE (fig.1 1. The second rhombohedral form, dark red. recently discovered by Laruelle [I] belongs to group P3I21 or D! with three molecules per cell. The only compound of the XM formula crystallizing in the same space group is mercuric sulphide, HgS, in the cinnabar form which has similar cell dimensions to HgO. The structure of this type of compound consists of helicoidal chains with six atoms per turn (fig. 2). The binary coordination indicates, a priori, a covalent bond between Hg and 0; in contradistinction the bond between the chains should be partially ionic. We have recorded the far-infrared spectra, between 400 and 4 cm-l at 300 and 90oK of three forms of oxide: yellow and red orthorhombic and dark red rhombohedral. The Raman spectra were obtained on a Coderg spectrophotometer with double monochromator, the light source being the 6328 A radiation of a Spectraphysics He-Ne laser of 70 mW. This study should be considered as 358
.Hy 00
Y= ‘/4 Y = li4
ii Hg Yz~~ ) 0 Yz3,.
Fig. 1.
preliminary until tests have been completed on monocrystalline HgO, a sample of which is being prepared.
2. THEORETICAL VIBRATIONS OF THE RHOMBOHEDRAL FORM The Di symmetry of the cinnabar group is identical to that of quartz and the factor group has already been studied [2]. It is isomorphic to group 3m(C 3”) of the equilateral triangle and possesses three irreducible representations AI, A2 and E. The symmetry of the phonons of large amplitude waves is determined by F. the factor group corresponding to atom displacements of the elementary cell. The three irreducible represen-
Volume 4, number
5
OPTICS COM~lUNICATIOKS
.January 1972
theoretically active modes in infrared are of types A2 and E, while the symmetrical Al vibrations do not interact with light in the first-order phenomena. The latter are visible in Raman scattering as well as the E modes, because of the absence of an inversion center. From the application of the new formulation ]3] of the Pauling electronegativity concept in terms of a dielectric definition of ionicity, it is possible to consider a correspondence between the cinnabar structure and the rock-salt structure. The six neighbors of each site define a distorted octahedral configuration.
3. THEORETICAL VIBRATIONS ORTHORHOMBIC FORM
16 for the If one accepts the D2h symmetry orthorhombic form, group theory according to the exclusion law, determines the active Raman and infrared modes, Alg, Jj2g, Blu, BgU. However, taking into consideration the results, a more sophisticated determination of structure by X-ray techniques seems necessary.
Fig.%.
tations F = 2Al + 4A2 + 6E must be related to the three acoustical phonons in the center of the zone corresponding to the translation of the crystal as a whole which would be I = 2AI + 3A2 + 5E, and would define ten optical frequency modes. The
rpo
0
OF THE
2po
4. EXPERIMENTAL
RESULTS
AND ATTRIBUTIONS
Figs. 3 and 4 give the experimental results. The predicted attributions are indicated here.
390
400
SQO
SpO
+cm-l
58 Infrared
LB0
HgO rhomb. 300
K
Raman
3 cm-’ Fig. 3. 359
Infrared
%S’ ortho. -300K
--aoK Raman
_1
332’5 4.00[:, s’oo
$oo )
9
The infrared spectrum between 4 and 400 cm-1 exhibits ‘one band only. It seems, by its position (180 cm-l I and its width to correspond to the group of cinnabar vibrations consisting of the longitudinal and transverse components of the A2(51 and E(4) modes \vert% the modes are numbered upwards from the lowest frequency observed. The A2(1 i alld A2(8) vibrations as welt as the degenerate modes do not appear. Fig. 5 shows a representation of the three A2 modes which should be respectiveiy a rotation of the helicoidal chain around the ternary axis (1) a purely axial movement (2) for a displacement 1’ of the oxygen atoms and a con~~eIlsatil1~ nlovei~lcn& 2; 12.4 of the mcrcury atoms. and finally, a rotation (31 of the two atom types in opposed senses. Case 1 corresponds to a zero frequency mode neglecting mutual interaction between chains. On the contrary, in case 2, each one of the sublattice Hg and 0 gives a rigid translation, the O-O and Hg-Hg distances being conserved during the movement. This cast corresponds to an optical mode vector in rock-salt. A first-order electrical moment corresponds in the rigid ionic model only to this movement. The Iowest frequency of vibration seen in the HgS spectrum does not appear here while the highest frequency vibration seems 360
cm-’
to be situated outside our domain of study. On the other hand the band at I80 cm-l. little affected by temperature, corresponds exactly to the product 128 \ 1.4 of the band frequency of HgS with the ratio of the square roots of the masses 0 and S. This vibration corresponds then in part, in harmonic approximation to the type 2 movement already described. Note that it seems to slightly split at room temperature. The spectrum extends to high frequencies with two bands at 560 and 573 cm-l. Some of observed lines are most likely ghost or Ne lines. It is to be noted that in the elementary theory of cinnabar of ‘Zallen et al. 12 1, the intensities of the Raman and infrared active vibrations arc coIil~lenlentary. This is not the case of HgO where the line at 180 cm-l is one of the most intense of the Raman spectrum which presents in addition some second-order lines. It seems then, that the real structure of rhombohedral HgO should be slightly different from that of cinnabar. 4.2. 1’1~ or11~ovhow~l~icj2vm Contrary to the preceding form, the infrared spectrum presents in its general aspect. a certain analogy with the one obtained for cinnabar at least for the yellow samples 141.
Volume
4, number
5
OPTICS
COMMUNICATIONS
This spectrum includes (fig. 4) three important bands at 60,147 and 232 cm-l which at liquidnitrogen temperature become 62, 147 and 235 cm. It seems that the last band corresponds to a group of different vibrations. Only the study of monocrystals would resolve this question. The Raman spectrum does not show any similarity with the mercuric sulphide spectrum. Two facts are to be noted: the existence of a vibration at 178 cm-1 which may correspond to the same movement of the Hg and 0 sublattices discussed for the rhombohedral form. Secondly, diminution and displacement of the band towards the high frequency zone from 326.5 to 332.5 cm-l when one passes from room temperature to liquidnitrogen temperature. The effect on the intensity is inverted for the weak line at 431 cm-I. Very high frequency bands are observed equally at 560,566 cm-l. No interpretation of these bands is at present possible. The red variety made of larger grains, presents an infra-red spectrum much less sharp than those obtained with the yellow form, and the Raman spectrum is slightly different.
January
1972
Fig. 5.
accepted. The orthorhombic HgO variety which has cell dimensions very close to the HgS one, gives a translational frequency for the Hg and 0 sub-lattices which does not differ significantly from the one given by the HgO rhombohedral variety.
ACKNOWLEDGEMENTS We thank Mr. E. S. Eccles of Smiths Industries, Cheltenham UK, for his kind assistance.
REFERENCES 5. CONCLUSION A great part of the observed vibrations has been accounted for. It seems that the rhombohedral HgO phase should be slightly different from the rhombohedral HgS (cinnabar) as it is currently
[1] P.Laruelle, Thsse, Paris (1960) (Masson Edit.). [Z] R. Zallen, G. Lucovsky, W. Taylor, A. Pinczuck and E. Burstein, Phys.Rev. Bl (1970) 4038. [3] J.C.Phillips and J.A.van Vechten, Phys.Rev. Letters 22 (1969) 705. [4] E.A.Decamps and A.Hadni, J.Chim.Phys.65 (1968) 1030.
361