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Infrared absorption due to pair modes of the NO2− ions doped in KI single crystals
Volume 95A, number 3,4
PHYSICS LETTERS
25 April 1983
INFRARED ABSORPTION DUE TO PAIR MODES OF THE NO~ IONS DOPED IN KI SINGLE CRYSTALS S.S. KHATRI and A.L. VERMA Department of Physics, North-Eastern Hill University, Shillong-793003, India
Received 25 October 1982
Several extra bands in the vicinity of the antisymmetric stretching vibration of the NO2 ion doped in KI single crystals containing ~ 1019 N O 2 ions per cm 3 have been observed in high resolution infrared studies at 1.7 K. These extra features are believed to arise from interactions between the induced dipole moments during v3 vibrations of the nearby nitrite ion pairs.
In this communication, we report our results of high resolution infrared studies on the NO~- :KI system containing high concentration of the NO 2- ion in KI single crystals. We have observed several temperature-independent and closely-spaced distinct bands near the antisymmetric stretching vibration v 3 of the NO~ i o n a t 1252.75 cm - 1 . At 1.7 K w i t h ~ l mm thick crystals containing ~ 1 0 1 8 - 1 0 1 9 NO~- ions per cm 3, the main/2 3 band becomes saturated and many additional features appear on the high and low frequency sides of the v 3 band with spacing ranging from ~ 0 . 2 - 1 . 2 c m - 1. The bands on the low-frequency side of v 3 are much stronger than those on high-frequency side and the relative intensities of these features between 1.7 and 10 K were found to be temperature independent. In order to check the concentration dependence, crystals containing varying concentrations of NO~ ions were used. The intensities of these components relative to the intensity of the 1)3 fundamental band at 1227 c m - 1 due to the 15NO~ ions in natural abundance in the same crystals were found to vary roughly quadratically with concentration. We could not resolve any additional fine structure due to tunneling associated with these satellites with the instrumental resolution of ~-~O.05 c m - 1 used for these measurements, although satellites were much broader. The fine structure is presumably not resolved due to interaction effects between the NO~impurities. 0 0 3 1 - 9 1 6 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 03.00 © 1983 North-Holland
The totally symmetric u 1 fundamental band is much weaker than the v 3 band and shows an asymmetric and very broad absorption band without any apparent fine structure. The absorption tail on the low-frequency side of the v 1 band extends up to ~15 c m - 1 from the centre of the band. Using high resolution, two broad features at ~1308.2 and 1307.3 c m - 1 can be resolved indicating formation of some sort of "band heads", Earlier studies on this NO~ :KI system by Narayanamurti et al. [ 1] reported closely spaced lines at 1245.0 1248.5, 1254.5, 1261.0 and 1265.0 c m - 1 near the u3 band at 1253 c m - 1 and they interpreted the appearance of the high frequency satellite at 1254.5 c m - 1 due to almost a free rotation of the ion around its b-axis. On the basis of roughly quadratic concentration dependence of the intensity of this shoulder and other features in this region from our studies, the explanation for the origin of these bands by Narayanamurti et al. [ 1] seems incorrect. They [ 1 ] also observed large half-width associated with the v 3 band in the NO~ :KI which showed a stronger temperature dependence than in the NO~- :KI system. This broadening is due to the tunneling of the off-centred NO~ ions in KI lattice along the twelve equivalent (110) equilibrium orientations which was not resolved in their studies. Using low concentrations (~1017 ions per cm 3) and thick crystals, we have observed the splitting of the v 3 band at 1.7 K into six prominent 191
Volume 95A, number 3,4
PHYSICS LETTERS
components at 1252.41, 1252.57, 1252.75, 1252.83, 1252.97 and 1253.24 c m - 1 whose relative intensities are temperature dependent [2]. This structure has been interpreted [2] as arising from four different types o f tunneling motions possible for the NO~ ion displaced along the (110) direction in the KI lattice. From the calculated potential barriers using a doublewell harmonic oscillator model [3], the lowest librational modes are estimated to appear at ~-~42, ~81, ~ 8 8 and ~ 2 2 4 c m - 1 [2]. In light o f this discussion, the temperature-independent structure showing quadratic concentration dependence observed by us for higher concentrations of the NO 2 ions in KI cannot be attributed to librational or other type o f motions performed by isolated NO~ impurities in the KI lattice. We have interpreted this structure as arising due to pair modes and triplet clusters of the NO 2 ions involving coupling among the induced electric dipole moments during the v 3 vibration of the NO~- ions. The absorption tail e x t e n d ing to ~ 1 5 c m - 1 on the low-frequency side o f the v 1 mode and appearance of " b a n d heads" are also believed to arise due to similar type of interactions during the v 1 vibration. From uniaxial stress induced dischroism in vibronic spectra Evans and Fitchen [4]
25 April 1983
concluded that the symmetry axis of the NO 2 ion in KI is along the (110) direction and its O - O axis is along the (001) direction. Due to its large observed dipole moment of 0.97 debye [5] we have considered the off-centre position o f the N O ] ion in KI in our calculations. The transition dipole moment o f the ion during the v 3 vibration is along O - O axis; i.e. (001) direction in KI. Using a model of two coupled localized harmonic oscillators we have calculated [6] the frequencies of all the observed pair modes and modes due to triplet clusters of the NO~- ions in different orientations. As there are several possibilities for mutual orientations o f the symmetry axes of the NO 2 ions in nearest-neighbour pairs, second nearest-neighbour pairs, triplet clusters, open triplet clusters and closed triplet clusters, each type o f cluster is expected to have many different interaction energies giving rise to distinct bands and we have explained the observed bands arising due to most probable pairs based on energy and probability consideration as given in table 1. The coupling between the ions is provided by the interaction between the transition dipole moments during v 3 vibrations o f the NO~ ions. The calculated and observed frequencies are in good agreement and are given in table 1. We have obtained the magnitude
Table 1 Observed and calculated satellite bands due to nearby interacting induced dipoles during v3 vibration of the NO2 ions in KI on the low- and high-frequency side of the v3 band. All values are given in cm -1 . Low-frequency side
High-frequency side
Observed
Calculated
Type of cluster a)
Observed
Calculated
Type of cluster a)
1252.06 1251.78 1250.97 1250.56 1250.12
1252.09 1251.53 1251.00 1250.56 1250.10 1250.29 1249.08 1248.80 1247.97 1247.19
triplets nn pairs 2nd nn pairs nn pairs triplets triplets nn pairs open triptets open triplets closed triplets
1254.00 1255.06 1256.07 1257.16 1258.13 1258.97 1259.92
1253.94 1255.30 1256.11
closed triplets nn pairs open triplets
1258.17 1258.89 1259.93
open triplets triplets open triplets
1245.25
open triplets
1249.14 1248.76 1248.25 1247.23 1246.15 1245.25
a) Positions of the ions in the nearest neighbours (nn) paris are near (0, 0, 0) and (a/2, a/2, 0); in the 2nd nearest neighbours (2nd nn) pairs near (0, 0, 0) and (a, 0, 0); in the triplet clusters near (0, 0, 0), (a/2, a/2, 0) and (0, a, 0); in the open triplet clusters near (0, 0, 0), (a/2, a/2, 0) and (a, a, 0) and in the closed triplet clusters near (0, 0, 0), (a/2, a/2, O) and (0, a/2, a/2) where a is the lattice constant of KI. 192
Volume 95A, number 3,4
PHYSICS LETTERS
of the transition dipole moment to be kttrans = 0.248 debye. A detailed analysis is in progress and will be published elsewhere. One of the authors (ALV) is grateful to Professor Van der Elsken of the University of Amsterdam for providing experimental facilities.
25 April 1983
References [ 1] V. Narayanamurti, W.D. Seward and R.O. Pohl, Phys. Rev. 148 (1966) 481. [2] S.S. Khatri and A.L. Verma, J. Phys. C, to be published. [3] M. Gomez, S.P. Bowen and J.A. Krumhansl, Phys. Rev. 153 (1967) 1009. [41 A.R. Evans and D.B. Fitchen, Phys. Rev. B2 (1970) 1074. [5] H.S. Sack and M.C. Moriarty, Solid State Commun. 3 (1965) 93. [6] S.S. Khatri and A.L. Verma, to be published.
193
PHYSICS LETTERS
25 April 1983
INFRARED ABSORPTION DUE TO PAIR MODES OF THE NO~ IONS DOPED IN KI SINGLE CRYSTALS S.S. KHATRI and A.L. VERMA Department of Physics, North-Eastern Hill University, Shillong-793003, India
Received 25 October 1982
Several extra bands in the vicinity of the antisymmetric stretching vibration of the NO2 ion doped in KI single crystals containing ~ 1019 N O 2 ions per cm 3 have been observed in high resolution infrared studies at 1.7 K. These extra features are believed to arise from interactions between the induced dipole moments during v3 vibrations of the nearby nitrite ion pairs.
In this communication, we report our results of high resolution infrared studies on the NO~- :KI system containing high concentration of the NO 2- ion in KI single crystals. We have observed several temperature-independent and closely-spaced distinct bands near the antisymmetric stretching vibration v 3 of the NO~ i o n a t 1252.75 cm - 1 . At 1.7 K w i t h ~ l mm thick crystals containing ~ 1 0 1 8 - 1 0 1 9 NO~- ions per cm 3, the main/2 3 band becomes saturated and many additional features appear on the high and low frequency sides of the v 3 band with spacing ranging from ~ 0 . 2 - 1 . 2 c m - 1. The bands on the low-frequency side of v 3 are much stronger than those on high-frequency side and the relative intensities of these features between 1.7 and 10 K were found to be temperature independent. In order to check the concentration dependence, crystals containing varying concentrations of NO~ ions were used. The intensities of these components relative to the intensity of the 1)3 fundamental band at 1227 c m - 1 due to the 15NO~ ions in natural abundance in the same crystals were found to vary roughly quadratically with concentration. We could not resolve any additional fine structure due to tunneling associated with these satellites with the instrumental resolution of ~-~O.05 c m - 1 used for these measurements, although satellites were much broader. The fine structure is presumably not resolved due to interaction effects between the NO~impurities. 0 0 3 1 - 9 1 6 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 03.00 © 1983 North-Holland
The totally symmetric u 1 fundamental band is much weaker than the v 3 band and shows an asymmetric and very broad absorption band without any apparent fine structure. The absorption tail on the low-frequency side of the v 1 band extends up to ~15 c m - 1 from the centre of the band. Using high resolution, two broad features at ~1308.2 and 1307.3 c m - 1 can be resolved indicating formation of some sort of "band heads", Earlier studies on this NO~ :KI system by Narayanamurti et al. [ 1] reported closely spaced lines at 1245.0 1248.5, 1254.5, 1261.0 and 1265.0 c m - 1 near the u3 band at 1253 c m - 1 and they interpreted the appearance of the high frequency satellite at 1254.5 c m - 1 due to almost a free rotation of the ion around its b-axis. On the basis of roughly quadratic concentration dependence of the intensity of this shoulder and other features in this region from our studies, the explanation for the origin of these bands by Narayanamurti et al. [ 1] seems incorrect. They [ 1 ] also observed large half-width associated with the v 3 band in the NO~ :KI which showed a stronger temperature dependence than in the NO~- :KI system. This broadening is due to the tunneling of the off-centred NO~ ions in KI lattice along the twelve equivalent (110) equilibrium orientations which was not resolved in their studies. Using low concentrations (~1017 ions per cm 3) and thick crystals, we have observed the splitting of the v 3 band at 1.7 K into six prominent 191
Volume 95A, number 3,4
PHYSICS LETTERS
components at 1252.41, 1252.57, 1252.75, 1252.83, 1252.97 and 1253.24 c m - 1 whose relative intensities are temperature dependent [2]. This structure has been interpreted [2] as arising from four different types o f tunneling motions possible for the NO~ ion displaced along the (110) direction in the KI lattice. From the calculated potential barriers using a doublewell harmonic oscillator model [3], the lowest librational modes are estimated to appear at ~-~42, ~81, ~ 8 8 and ~ 2 2 4 c m - 1 [2]. In light o f this discussion, the temperature-independent structure showing quadratic concentration dependence observed by us for higher concentrations of the NO 2 ions in KI cannot be attributed to librational or other type o f motions performed by isolated NO~ impurities in the KI lattice. We have interpreted this structure as arising due to pair modes and triplet clusters of the NO 2 ions involving coupling among the induced electric dipole moments during the v 3 vibration of the NO~- ions. The absorption tail e x t e n d ing to ~ 1 5 c m - 1 on the low-frequency side o f the v 1 mode and appearance of " b a n d heads" are also believed to arise due to similar type of interactions during the v 1 vibration. From uniaxial stress induced dischroism in vibronic spectra Evans and Fitchen [4]
25 April 1983
concluded that the symmetry axis of the NO 2 ion in KI is along the (110) direction and its O - O axis is along the (001) direction. Due to its large observed dipole moment of 0.97 debye [5] we have considered the off-centre position o f the N O ] ion in KI in our calculations. The transition dipole moment o f the ion during the v 3 vibration is along O - O axis; i.e. (001) direction in KI. Using a model of two coupled localized harmonic oscillators we have calculated [6] the frequencies of all the observed pair modes and modes due to triplet clusters of the NO~- ions in different orientations. As there are several possibilities for mutual orientations o f the symmetry axes of the NO 2 ions in nearest-neighbour pairs, second nearest-neighbour pairs, triplet clusters, open triplet clusters and closed triplet clusters, each type o f cluster is expected to have many different interaction energies giving rise to distinct bands and we have explained the observed bands arising due to most probable pairs based on energy and probability consideration as given in table 1. The coupling between the ions is provided by the interaction between the transition dipole moments during v 3 vibrations o f the NO~ ions. The calculated and observed frequencies are in good agreement and are given in table 1. We have obtained the magnitude
Table 1 Observed and calculated satellite bands due to nearby interacting induced dipoles during v3 vibration of the NO2 ions in KI on the low- and high-frequency side of the v3 band. All values are given in cm -1 . Low-frequency side
High-frequency side
Observed
Calculated
Type of cluster a)
Observed
Calculated
Type of cluster a)
1252.06 1251.78 1250.97 1250.56 1250.12
1252.09 1251.53 1251.00 1250.56 1250.10 1250.29 1249.08 1248.80 1247.97 1247.19
triplets nn pairs 2nd nn pairs nn pairs triplets triplets nn pairs open triptets open triplets closed triplets
1254.00 1255.06 1256.07 1257.16 1258.13 1258.97 1259.92
1253.94 1255.30 1256.11
closed triplets nn pairs open triplets
1258.17 1258.89 1259.93
open triplets triplets open triplets
1245.25
open triplets
1249.14 1248.76 1248.25 1247.23 1246.15 1245.25
a) Positions of the ions in the nearest neighbours (nn) paris are near (0, 0, 0) and (a/2, a/2, 0); in the 2nd nearest neighbours (2nd nn) pairs near (0, 0, 0) and (a, 0, 0); in the triplet clusters near (0, 0, 0), (a/2, a/2, 0) and (0, a, 0); in the open triplet clusters near (0, 0, 0), (a/2, a/2, 0) and (a, a, 0) and in the closed triplet clusters near (0, 0, 0), (a/2, a/2, O) and (0, a/2, a/2) where a is the lattice constant of KI. 192
Volume 95A, number 3,4
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of the transition dipole moment to be kttrans = 0.248 debye. A detailed analysis is in progress and will be published elsewhere. One of the authors (ALV) is grateful to Professor Van der Elsken of the University of Amsterdam for providing experimental facilities.
25 April 1983
References [ 1] V. Narayanamurti, W.D. Seward and R.O. Pohl, Phys. Rev. 148 (1966) 481. [2] S.S. Khatri and A.L. Verma, J. Phys. C, to be published. [3] M. Gomez, S.P. Bowen and J.A. Krumhansl, Phys. Rev. 153 (1967) 1009. [41 A.R. Evans and D.B. Fitchen, Phys. Rev. B2 (1970) 1074. [5] H.S. Sack and M.C. Moriarty, Solid State Commun. 3 (1965) 93. [6] S.S. Khatri and A.L. Verma, to be published.
193