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Pressure-induced luminescence of N-isopropylcarbazole single crystal
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
8 August 1986
P~SS~~U~D LU~C~~ OF N-ISOPROP~CA~AZOLE SINGLE CRYSTAL * J. KALINOWSKI, J. GODLEWSKI, 2. DREGER and P. MONDALSKI Chair ofMolecular Physics, Technical university of Ga’ahsk,80-952 Gdahsk, Poland Received 20 March 1986; in final form 19 May 1986
The spontaneous emission of light from single crystals of Nisopropylcarbazole under changing hydrostatic pressure (pressure-induced luminescence) has been observed for the first time. The luminescence consists of bursts of light around characteristic pressures at which pressure-induced structural transitions are believed to take place, This new phenomenon is interpreted ln terms of the electrical discbarge in the ambient gas involving high electric fields produced by spontaneous polarization at fresh microcrystal faces emerging during crystal cracking on structural transformations.
Crystalline N-i~propy~~~b~ole
NIPC
I
n3C/:\cn H
3
is known to be pyroelectric [I] and to show strong triboluminescence (TBL) [2] as well as pyroelectric luminescence (PEL) [3,4], The latter is due to the heating- or cooling-induced change in the polarization of this polar molecular crystal. As can be inferred from the temperature dependence of the thermal expansion coefficient and pyroelectric coefficient (11, the secondary pyroelectric coef~cient accounts for the major polar~ation change. Therefore, one would expect a polarization change to appear as a result of hydrostatic pressure exerted upon the crystal. The resulting electric field could lead to electrical discharge in the surrounding medium or in the crystal itself, the effect to be seen as pressure-induced luminescence (PIL). In order to check this supposition we have observed several NIPC single crystals under changing hydrostatic pressure by using a variable pressure cell fitted with a photomultiplier tube. Single crystals of NIPC were prepared as described elsewhere 141. Crys* Work supported in part by the Polish Academy of Sciences under program CPBP 01.12.
tals, typically -1 X 3 X 5 mm3, were mounted in a metal foil holder with only minimum contact between crystal and substrate. The pressure system consisted of a Unipress optical pressure cell (made of beryllium bronze) with two, sapphire windows of aperture 0.4 cm, and a multistage 150: 1 gas pressure intensifier driven by a motor pump. Helium was used for the pressure gas inside the cell, The cell was connected to the pressure chamber of the intensifier by a flexible capillary tube 2 m long and 0.3 mm inner diameter (see refs. [5,6]). The light emission behaviour of the crystals was observed under varying pressure in the range 105-IO9 Pa, Pressure could be increased at a rate ~3 MPa/s and decreased at a rate ~2 MPafs. The time constant of the luminescence recording circuit was ~1 s. A typical evolution of PIL of a NIPC crystal measured under changing pressure is shown in fig. 1. The luminescence varies gradually with pressure and time, being observed only over a small pressure range and peaking at a particular pressure pin = 0.6 GPa (when pressure increases) and p& = 0.44 GPa (when pressure decreases). The pressure positions of these peaks are reproducible from crystal to crystal and do not depend on the “history” of the crystal (reproducibility between pressure runs; see f@. 1). In contrast, the positions of several luminescence maxima at the end of decompression are not reproducible and appear only with a suf~ciently drastic alteration in the rate of pressure change.
480
0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
8 August 1986
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
/ 123454
12
3
4
5
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7
t tminl -
0.7 z
F C e s
0.5;
z
0.35: g
A z
ff 0.1
/ 1
1
23456
234567
IfminIFig. 1, Time evolution of pressure (right-hand side scale) and corresponding PIL (left-hand side scale) of a NIPC single crystal. Note a drop in the intensity,of the reproducible maxima of the PIL in the second pressure run.
These observations suggest that the luminescence may result from dielectric bre~down. The breakdown field can be produced either by a change in the spontaneous polarization of the samples with varying hydrostatic pressure or by fresh microcrystal faces appearing during crystal cracking; the latter being the result of structural transformations at particular pressures. The polarization in a pyroelectric crystal at equilibrium is compensated by free charges acquired from the surrounding medium. Compressing or decompressing induces a polarization until the charges once again reach equilibrium. The change in the polarization (APi), resulting from a uniform pressure (Au/) is given by APi = dijA@j
(i, j = 1,2,3),
(1)
where dii are piezoelectric coef~cients. Using a simple static method we have measured d3j
for NIPC crystals and found dsl = -5.8 X lo-l2 C/N, ds2 = 14.5 X lo-l2 C/N and ds3 = 6 X IO-l2 C/N. From eq. (1) with the above values of d3i, the change in the electric field (Mp3 =Ep3 - EO, EO = 0) for the 0.6 GPa change in the pressure is Ep3 = APS/eoe = 3 X lo8 V/m.
(2)
Here e,, stands for the vacuum permittivity and e m 3.5 is taken as the dielectric constant of either the crystal or the ambient gas *. Such a high electric field cannot, however, appear on the crystal in a reproducible way when the field is increasing continu* Note that under the pressures applied (-0.6 GPa and higher) the gas density approaches the density in solids. However, below 0.6 GPa, E for the gas is usually smaller than 3.5 so that Eps in the gas exceeds Epa in the crystal causing the probably of gas breakdown to be greater than that for the crystal itself. 481
Volume 128, number 5,6
CHEMICAL PHYSICS LETTERS
ously because lower fields (E < lo* V/m), preceding it, would lead to an earlier electrical breakdown of the gas surrounding the crystal or of the crystal itself. The rn~~urn breakdown field =lO* V/m would then be reproduced every =200 MPa and the light emission would consist of a regular sequence of at least three bursts of light before the pressure has reached the value mO.6 GPa. This is not found experimentally and, therefore, we believe that the field generated in the course of the polarization change is compensated by increasing crystal conductivity and can never reach the value required for the breakdown under present conditions. The time evolution of the observed lightning is, on the other hand, compatible with the second mechanism for the creation of the electric field, i.e. with the sudden emergence of the spontaneous polarization (P) through creation of fresh (electrically uncompensated) facets of microc~st~s produced as a result of cracking during structural phase transitions. The spontaneous polarization field is given by (3) and with the molecular dipole moment of NIPC & w 1O-2g C m its value is mlOg V/m. The integral on the right-hand side of (3) runs over the total volume (v) of the crystal. A field of the above value greatly exceeds the breakdown field of the compressed gas as well as the crystal itself. Therefore, we conclude that sharp luminescence maxima at pin = 0.6 GPa and pde = 0.44 GPa are due to a correlation between breakdown and crystal cracking at pressureinduced structural phase transfo~ations. This explanation provides - for the first time - evidence for pressure-induced phase transitions in NIPC crystals. The emission peak corresponding to the phase transformation occurs at lower pressure @&.) when pressure is decreasing. This fact suggests that the phase transition might be of the order-disorder type, with the one-particle molecular potential described by a function with a double asymmetric well. The intense emission appearing at i 40 K when the crystal is heated or cooled at or below atmospheric pressure [ 1,3,4], which is assigned to the temperature-induced phase transition, does not necessarily mean that we observe two identical structural transfo~ations. Although we believe that in this case (similar to those 482
8 August 1986
for pyrene (73 and tetracene [8]) they are identical, further work on construction of the phase diagram is required in order to give a reliable answer to this question. Also, it is not possible at present to answer conclusively whether the breakdown takes place in the ambient gas or in the crystal itself, However, the lack of black carbon traces on the sample after severa pressure runs suggests that breakdown occurs in the ambient gas. Another feature observed in the present experiment is one or a sequence of a few light pulses appearing when the decompression rate is suddenly accelerated (see fig, 1). Possible explanations are that an uncompensated electric field still exists within the crystalline sample and a sudden decrease in the pressure is sufficient for the dielectric breakdown of the ambient gas, or that microcrystallites produced during the phase transition undergo rel~ation-educed mutual friction leading to tribolum~escence. The effect can also be caused by interaction of a gas stream leaving the cell with the crystal; this is called turboluminescence 191. Due to the fact that the decompression light pulses disappear when decompression proceeds slowly, it cannot be decided which of the above mechanisms is prevailing. The most important result of the present investigation is the reproducible appearance of light maxima at the phase transition pressures. This effect can be used as a relatively precise method for determination of the phase diagram of various polar crystals and the results on NIPC will be presented in a forthcoming paper. References [ 11 R. Nowak and R. Poprawski, Ferroelectrics Letters 1 (1984) 175. [2] R. Nowak, A. Krajewska and M. Samoo,Chem. Phys. Letters 94 (1983) 270. [3] Z. Dreger, J. Kalinowski, R.Nowak and J. Sworakowski, Mat. Sci. (Wroclhw) 10 (1984) 67, [4] Z. Dreger, J. Kalinowski,R. Nowak and J. Sworakowski, Chem. Phys. Letters 116 (1985) 192. [S] J. Kalinowski and R. Jankowiak, Chem. Phys. Letters 53 (1978) 56. [6] R. Jankowiak and J, Kahnowski, Mol. Cryst. Liquid Cryst. 48 (1978) 187. [7f W. Jones and M.D. Cohen, Mol. Cry& Liquid Cryst. Letters 41 (1977) 103. [SI J. Kalinowski, R. Jankowiak and H. Bilssler, J. Luminescence 22 (1981) 397. I91 D.M. Hanson, J.S. Pate1 and M.C. Nelson, Mat. Sci. ~ro~w) 10 (1984) 459.
Volume 128, number 5,6
8 August 1986
P~SS~~U~D LU~C~~ OF N-ISOPROP~CA~AZOLE SINGLE CRYSTAL * J. KALINOWSKI, J. GODLEWSKI, 2. DREGER and P. MONDALSKI Chair ofMolecular Physics, Technical university of Ga’ahsk,80-952 Gdahsk, Poland Received 20 March 1986; in final form 19 May 1986
The spontaneous emission of light from single crystals of Nisopropylcarbazole under changing hydrostatic pressure (pressure-induced luminescence) has been observed for the first time. The luminescence consists of bursts of light around characteristic pressures at which pressure-induced structural transitions are believed to take place, This new phenomenon is interpreted ln terms of the electrical discbarge in the ambient gas involving high electric fields produced by spontaneous polarization at fresh microcrystal faces emerging during crystal cracking on structural transformations.
Crystalline N-i~propy~~~b~ole
NIPC
I
n3C/:\cn H
3
is known to be pyroelectric [I] and to show strong triboluminescence (TBL) [2] as well as pyroelectric luminescence (PEL) [3,4], The latter is due to the heating- or cooling-induced change in the polarization of this polar molecular crystal. As can be inferred from the temperature dependence of the thermal expansion coefficient and pyroelectric coefficient (11, the secondary pyroelectric coef~cient accounts for the major polar~ation change. Therefore, one would expect a polarization change to appear as a result of hydrostatic pressure exerted upon the crystal. The resulting electric field could lead to electrical discharge in the surrounding medium or in the crystal itself, the effect to be seen as pressure-induced luminescence (PIL). In order to check this supposition we have observed several NIPC single crystals under changing hydrostatic pressure by using a variable pressure cell fitted with a photomultiplier tube. Single crystals of NIPC were prepared as described elsewhere 141. Crys* Work supported in part by the Polish Academy of Sciences under program CPBP 01.12.
tals, typically -1 X 3 X 5 mm3, were mounted in a metal foil holder with only minimum contact between crystal and substrate. The pressure system consisted of a Unipress optical pressure cell (made of beryllium bronze) with two, sapphire windows of aperture 0.4 cm, and a multistage 150: 1 gas pressure intensifier driven by a motor pump. Helium was used for the pressure gas inside the cell, The cell was connected to the pressure chamber of the intensifier by a flexible capillary tube 2 m long and 0.3 mm inner diameter (see refs. [5,6]). The light emission behaviour of the crystals was observed under varying pressure in the range 105-IO9 Pa, Pressure could be increased at a rate ~3 MPa/s and decreased at a rate ~2 MPafs. The time constant of the luminescence recording circuit was ~1 s. A typical evolution of PIL of a NIPC crystal measured under changing pressure is shown in fig. 1. The luminescence varies gradually with pressure and time, being observed only over a small pressure range and peaking at a particular pressure pin = 0.6 GPa (when pressure increases) and p& = 0.44 GPa (when pressure decreases). The pressure positions of these peaks are reproducible from crystal to crystal and do not depend on the “history” of the crystal (reproducibility between pressure runs; see f@. 1). In contrast, the positions of several luminescence maxima at the end of decompression are not reproducible and appear only with a suf~ciently drastic alteration in the rate of pressure change.
480
0 009-2614/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
8 August 1986
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
/ 123454
12
3
4
5
6
7
t tminl -
0.7 z
F C e s
0.5;
z
0.35: g
A z
ff 0.1
/ 1
1
23456
234567
IfminIFig. 1, Time evolution of pressure (right-hand side scale) and corresponding PIL (left-hand side scale) of a NIPC single crystal. Note a drop in the intensity,of the reproducible maxima of the PIL in the second pressure run.
These observations suggest that the luminescence may result from dielectric bre~down. The breakdown field can be produced either by a change in the spontaneous polarization of the samples with varying hydrostatic pressure or by fresh microcrystal faces appearing during crystal cracking; the latter being the result of structural transformations at particular pressures. The polarization in a pyroelectric crystal at equilibrium is compensated by free charges acquired from the surrounding medium. Compressing or decompressing induces a polarization until the charges once again reach equilibrium. The change in the polarization (APi), resulting from a uniform pressure (Au/) is given by APi = dijA@j
(i, j = 1,2,3),
(1)
where dii are piezoelectric coef~cients. Using a simple static method we have measured d3j
for NIPC crystals and found dsl = -5.8 X lo-l2 C/N, ds2 = 14.5 X lo-l2 C/N and ds3 = 6 X IO-l2 C/N. From eq. (1) with the above values of d3i, the change in the electric field (Mp3 =Ep3 - EO, EO = 0) for the 0.6 GPa change in the pressure is Ep3 = APS/eoe = 3 X lo8 V/m.
(2)
Here e,, stands for the vacuum permittivity and e m 3.5 is taken as the dielectric constant of either the crystal or the ambient gas *. Such a high electric field cannot, however, appear on the crystal in a reproducible way when the field is increasing continu* Note that under the pressures applied (-0.6 GPa and higher) the gas density approaches the density in solids. However, below 0.6 GPa, E for the gas is usually smaller than 3.5 so that Eps in the gas exceeds Epa in the crystal causing the probably of gas breakdown to be greater than that for the crystal itself. 481
Volume 128, number 5,6
CHEMICAL PHYSICS LETTERS
ously because lower fields (E < lo* V/m), preceding it, would lead to an earlier electrical breakdown of the gas surrounding the crystal or of the crystal itself. The rn~~urn breakdown field =lO* V/m would then be reproduced every =200 MPa and the light emission would consist of a regular sequence of at least three bursts of light before the pressure has reached the value mO.6 GPa. This is not found experimentally and, therefore, we believe that the field generated in the course of the polarization change is compensated by increasing crystal conductivity and can never reach the value required for the breakdown under present conditions. The time evolution of the observed lightning is, on the other hand, compatible with the second mechanism for the creation of the electric field, i.e. with the sudden emergence of the spontaneous polarization (P) through creation of fresh (electrically uncompensated) facets of microc~st~s produced as a result of cracking during structural phase transitions. The spontaneous polarization field is given by (3) and with the molecular dipole moment of NIPC & w 1O-2g C m its value is mlOg V/m. The integral on the right-hand side of (3) runs over the total volume (v) of the crystal. A field of the above value greatly exceeds the breakdown field of the compressed gas as well as the crystal itself. Therefore, we conclude that sharp luminescence maxima at pin = 0.6 GPa and pde = 0.44 GPa are due to a correlation between breakdown and crystal cracking at pressureinduced structural phase transfo~ations. This explanation provides - for the first time - evidence for pressure-induced phase transitions in NIPC crystals. The emission peak corresponding to the phase transformation occurs at lower pressure @&.) when pressure is decreasing. This fact suggests that the phase transition might be of the order-disorder type, with the one-particle molecular potential described by a function with a double asymmetric well. The intense emission appearing at i 40 K when the crystal is heated or cooled at or below atmospheric pressure [ 1,3,4], which is assigned to the temperature-induced phase transition, does not necessarily mean that we observe two identical structural transfo~ations. Although we believe that in this case (similar to those 482
8 August 1986
for pyrene (73 and tetracene [8]) they are identical, further work on construction of the phase diagram is required in order to give a reliable answer to this question. Also, it is not possible at present to answer conclusively whether the breakdown takes place in the ambient gas or in the crystal itself, However, the lack of black carbon traces on the sample after severa pressure runs suggests that breakdown occurs in the ambient gas. Another feature observed in the present experiment is one or a sequence of a few light pulses appearing when the decompression rate is suddenly accelerated (see fig, 1). Possible explanations are that an uncompensated electric field still exists within the crystalline sample and a sudden decrease in the pressure is sufficient for the dielectric breakdown of the ambient gas, or that microcrystallites produced during the phase transition undergo rel~ation-educed mutual friction leading to tribolum~escence. The effect can also be caused by interaction of a gas stream leaving the cell with the crystal; this is called turboluminescence 191. Due to the fact that the decompression light pulses disappear when decompression proceeds slowly, it cannot be decided which of the above mechanisms is prevailing. The most important result of the present investigation is the reproducible appearance of light maxima at the phase transition pressures. This effect can be used as a relatively precise method for determination of the phase diagram of various polar crystals and the results on NIPC will be presented in a forthcoming paper. References [ 11 R. Nowak and R. Poprawski, Ferroelectrics Letters 1 (1984) 175. [2] R. Nowak, A. Krajewska and M. Samoo,Chem. Phys. Letters 94 (1983) 270. [3] Z. Dreger, J. Kalinowski, R.Nowak and J. Sworakowski, Mat. Sci. (Wroclhw) 10 (1984) 67, [4] Z. Dreger, J. Kalinowski,R. Nowak and J. Sworakowski, Chem. Phys. Letters 116 (1985) 192. [S] J. Kalinowski and R. Jankowiak, Chem. Phys. Letters 53 (1978) 56. [6] R. Jankowiak and J, Kahnowski, Mol. Cryst. Liquid Cryst. 48 (1978) 187. [7f W. Jones and M.D. Cohen, Mol. Cry& Liquid Cryst. Letters 41 (1977) 103. [SI J. Kalinowski, R. Jankowiak and H. Bilssler, J. Luminescence 22 (1981) 397. I91 D.M. Hanson, J.S. Pate1 and M.C. Nelson, Mat. Sci. ~ro~w) 10 (1984) 459.