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Structural transitions of KNO3 and TlNO3 under high pressure
Journal of Molecular Structure, 247 (1991) 397-402 Elsevier Science Publishers B.V., Amsterdam
397
Structural transitions of KN03 and T1N03 under high pressure* Z.X. Shen” and W.F. Shermanb “Department of Physics, Imperial College, Prince Consort Road, London SW7 2BZ (UK) bDepartment of Physics, King’s College London, Strand, London WC2R 2LS (UK) (Received 4 March 1991)
High pressure (up to 50 kbar) Raman studies have been carried out for KNO, and T1N03 in the internal symmetric stretch mode, vi, region of the NO; ions. Previously unreported structural changes for both crystals were found at similar high pressures, as indicated by the changes of the weak bands on the shoulder of the main component. The pressure induced changes in the spectra suggest that TlNO, undergoes a second order phase transition, whereas KNO, shows changes in the “alternative structure”, with the main crystal structure unchanged.
INTRODUCTION
The isolated NO, ion has a plane structure, with the nitrogen atom at the centre and the three oxygen atoms forming an equilateral triangle. This makes it much easier for the ion to librate or rotate within its plane rather than perpendicular to it within a crystal lattice. Ionic nitrate crystals have a tendency to form rotationally disordered phases, particularly at higher temperatures, and to show first order structural phase transitions with temperature and pressure. (For general background and references see ref. 1.) These phase transitions are often related to the ordering (or disordering) of the rotational positions of the nitrate ions, but also often include a rearrangement of translational positions of the nitrate groups. Such transitions are usually accompanied by dramatic changes in the corresponding Raman (and infrared) spectra. It is however also possible that only subtle and continuous changes (for example, changes in the angular range of libration motion) occur in the crystal. The Raman spectra changes associated with this second order type of phase transition can be small, can extend over a large temperature range and are completely reversible (in contrast to first order phase transitions which show significant hysteresis ). *Dedicated to the memory of Professor George Wilkinson.
0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
398
In recent years, several authors [1,2,3] have reported “alternative structures” (AS) in nitrate crystals. The AS is a secondary structure coexisting with the main crystal structure. Although the AS has a somewhat higher potential than that of the main structure, it nevertheless has a non-zero equilibrium probability at temperatures above absolute zero. It exists as a small proportion in the crystal, so that the main structure retains its long range order. The AS is temperature sensitive; it is most pronounced at high temperature and tends to freeze out at low temperature. As a result, anomalous secondary bands not predicted on the basis of the main structure, appear in Raman and IR spectra. We have mainly used the symmetric stretch mode Y, of the NO, ions in our high pressure Raman studies of TlN03 and KNO,. The Raman spectrum of this mode is very intense and relatively simple, so any weak features will come out more strongly in this region and can be analysed more easily. EXPERIMENT
The crystals of TINOB and KNO, were grown by slowly evaporating aqueous solutions of the corresponding chemicals at room temperature. Raman spectra were recorded using a Spex Ramalog 5M spectrometer, with a Spectra Physics Ar+ ion laser operating on the 488 nm line acting as the excitation source. Backscattering geometry was used. The high pressure was generated by a diamond anvil cell. RESULTS AND DISCUSSION
TINO, The Raman spectra of the vi mode of the NO; ions under high pressures are shown in Fig. 1 at pressures of 0.001,10.8,27.0 and 43.6 kbar, respectively. Under ambient conditions, TINOB belongs to space group C&, with four molecules per unit cell, and no phase transition has been reported at room temperature in the pressure range O-40 kbar [ 41. Factor group analysis gives four Raman active bands for the v1 mode, Al, AZ, B1 and B2 (see p. 305 of ref. 1 for detailed analysis). The A, mode, which is the totally symmetric in-phase stretching of all four NO; ions in the unit cell, is expected to be by far the strongest Raman band. Three bands were observed at 0.001 kbar, at 1041.5,1039.7 and 1037.5 cm-‘. The band at 1041.5 cm-’ is the strongest and was assigned to the A, band. The other two much weaker bands situated at its low frequency shoulder are more difficult to assign with certainty. Balasubrahmanyan and Janz [ 5 ] observed a weak band, ~3 cm-l lower than the strong v1 band, but did not assign it. Brooker [ 31 assigned a weak band at 1037 cm-’ which is 4.8 cm-’ lower than the main component, to a band due to a thermally disordered alternative struc-
399
I
I
I
I 1040
1060
I
cm-1 Fig. 1. Raman spectra of the pI mode of TlNO, under high pressures.
ture, but mentioned that correlation field components were also observed. It is fair to say that the band at 1039.7 cm-‘, which is only 1.8 cm-l from the main band and is well within the range of correlation field splitting, is one of the other three components (A,, B1 or B,). An argument is given below in an attempt to assign the other weak band that was observed. The frequency differences of this weak band from the main band are 4.0,4.2 and 4.2 cm-‘, at pressures 0.001, 10.8 and 27 kbar respectively. They are constant within the error of the experiment. This means that this weak component has the same pressure dependence as the main band, and it is therefore reasonable to suggest that the two bands belong to the same structure (i.e. C& ). IR transmission and reflection spectra (see Chapter 7 of ref 1) give the B1 and Bz modes both at x 1040 cm-‘. This indicates that the band at 1039.7 cm-’ is the B1 and/or B, component of the Y, mode, and the band at 1037.5 cm-’ is the IR inactive but Raman allowed A2 component. Later in this paper, a weak component at the low frequency shoulder of the main band of v1 in KNO, is assigned to a band of an alternative structure, not a component of the basic structure. In the KN03 case, the weak band has a different pressure dependence from that of the main band, and the frequency difference becomes significantly larger at high pressures. From Fig. 1 we can see that the weak band at 1037.5 cm-’ has disappeared by 43.6 kbar. It has apparently been replaced by a weak band at 1060.8 cm-‘,
I
I 20
I
I 40
KIM Fig. 2. Frequency shifts of Raman v1 modes of TINOB with pressure: 0, main band; + and n , weak correlation field components.
which is at the high frequency shoulder of the main band (Fig. 2). This indicates a phase transition between 43.6 and 27 kbar, and the new phase shall be referred to as phase IV. Raman bands in the lattice region also show some changes in the same pressure range, which supports the suggestion of a phase transition (see Chapter 7 of ref. 1 for details). There are possible frequency discontinuities against pressure for the v1 and or, modes, but they are barely larger than the experimental error. There are several facts to be noted: (1) the v4 bands of the NO; ions do not split further in the high pressure phase IV, (ii) the shapes of the Y, bands have no detectable changes in phases III (ambient phase) and IV, (iii) the Raman spectra in the lattice region do not show new bands in phase IV. These imply that there is no major crystal structural change involved in the III-IV phase transition. The structure of the high pressure phase probably still belongs to in the space group C&, with 2~4. There are 22 space groups (n=1,2...22) C& category, all of which can accommodate four molecules. KNO, Alternative structures (AS) have been found for many alkaline nitrate crystals at ambient pressure. We report here different AS preferred for KN03 under high pressures. Figure 3 shows the Raman spectra of the v1 mode of the NO, ions at 6.3, 17.2, 38.9and 46.2 kbar respectively. A well-established phase transition II-
401
IV occurs at 3.4 kbar [ 1,6]. We will concentrate on pressures above this II-IV transition. The strong band has A, symmetry and comes from the main structure. A weak band was observed at the low frequency shoulder of the main band below 17.2 kbar and at the high frequency shoulder above 38.9 kbar. The pressure dependencies of the Raman bands are plotted in Fig. 4. It is generally accepted that the weak band at ambient pressure (0.001 kbar) in phase II is due to an AS and its separation from the main band is about 2 cm- ’ [ 3 1. We believe that the similar phase IV band is also due to an AS, and the separation increased from 2.9 cm-’ at 6.3 kbar to 3.4 cm-’ at 17.2 kbar. This implies that the main band has a bigger pressure dependence than the weak one and they will not cross each other at higher pressures. The separation between the high frequency weak band and main band is 4.8 cm-l at 38.9 kbar and 5.2 cm-l at 46.2 kbar, so the weak band has a bigger pressure dependence in this case. Hence this weak band has a different cause from the one at pressures below 17.2 kbar. Apart from those discussed above, there are no other unexplained spectral changes (i.e. extra bands, frequency-shift discontinuities or anomalous band
I
I
I
1070
I 1050
CM-'
I
L.
KBAR
Fig. 3. High pressure F&man spectra of the Y, mode of KNO,. Fig. 4. Pressure dependences of the Raman u1 mode of KNOB, phase IV: 0, main band; A, alternative structure side band; *, phase II at the lowest pressure.
shapes), for the main Y, band or other Raman active bands (including lattice modes) between 6.3 and 46.2 kbar. Therefore there is no evidence of a phase transition for the main crystal structure. We propose (i) that the weak phase IV bands belong to alternative structures and (ii) that a different AS is preferred at high pressures to that which appears at low pressures. This is indicated by the disappearance of the low pressure low frequency side band and the appearance of the high pressure high frequency side band. Thus, the lowest potential of the system, corresponding to the main structure, remains the same throughout the pressure range 6-50 kbar. However the next lowest potential, corresponding to the alternative structure, has changed. The switch over of the alternative structure occurs at about 30 kbar. CONCLUSIONS
High pressure Raman studies have been performed for KNO, and TINOB. A new second order phase transition has been found for T1N03, and evidence has been found for different alternative structures at low and high pressure in phase IV KNO, in the pressure range 6-50 kbar. The phase transition III-IV of T1N03 occurs at pressures between 30-35 kbar, marked by the disappearance of one of the low frequency weak components and the appearance of another at the high frequency side of the main v1 band. The new high pressure phase is believed to still be one of the C& space groups with four molecules per unit cell. Some observed changes in other spectral regions are also associated with this phase transition. Different alternative structures were found for KN03 by variation of pressure. While AS at ambient pressure have been observed by many people, the pressure induced AS changes have never been previously reported in the literature. The study of the AS is important for the understanding of the stability of the main crystal structure.
REFERENCES 1 2 3 4 5 6
Z. Shen, Ph.D. Thesis, King’s College London (1989). S.V. Karpov and A.A. Shultin, Sov. Phys. Solid State, 17 (1976) 1915. M.H. Brooker, J. Chem. Phys., 68 (1978) 67. E. Rapopart and C.W.F.T. Pistorous, J. Chem. Phys., 44(4) (1966) 1514. K.Balasubrahmanyan and G.J. Janz, J. Chem. Phys., 57(10) (1971) 4084. J.A. Medina, Ph.D. Thesis, King’s College London (1982).
397
Structural transitions of KN03 and T1N03 under high pressure* Z.X. Shen” and W.F. Shermanb “Department of Physics, Imperial College, Prince Consort Road, London SW7 2BZ (UK) bDepartment of Physics, King’s College London, Strand, London WC2R 2LS (UK) (Received 4 March 1991)
High pressure (up to 50 kbar) Raman studies have been carried out for KNO, and T1N03 in the internal symmetric stretch mode, vi, region of the NO; ions. Previously unreported structural changes for both crystals were found at similar high pressures, as indicated by the changes of the weak bands on the shoulder of the main component. The pressure induced changes in the spectra suggest that TlNO, undergoes a second order phase transition, whereas KNO, shows changes in the “alternative structure”, with the main crystal structure unchanged.
INTRODUCTION
The isolated NO, ion has a plane structure, with the nitrogen atom at the centre and the three oxygen atoms forming an equilateral triangle. This makes it much easier for the ion to librate or rotate within its plane rather than perpendicular to it within a crystal lattice. Ionic nitrate crystals have a tendency to form rotationally disordered phases, particularly at higher temperatures, and to show first order structural phase transitions with temperature and pressure. (For general background and references see ref. 1.) These phase transitions are often related to the ordering (or disordering) of the rotational positions of the nitrate ions, but also often include a rearrangement of translational positions of the nitrate groups. Such transitions are usually accompanied by dramatic changes in the corresponding Raman (and infrared) spectra. It is however also possible that only subtle and continuous changes (for example, changes in the angular range of libration motion) occur in the crystal. The Raman spectra changes associated with this second order type of phase transition can be small, can extend over a large temperature range and are completely reversible (in contrast to first order phase transitions which show significant hysteresis ). *Dedicated to the memory of Professor George Wilkinson.
0022-2860/91/$03.50
0 1991-
Elsevier Science Publishers B.V.
398
In recent years, several authors [1,2,3] have reported “alternative structures” (AS) in nitrate crystals. The AS is a secondary structure coexisting with the main crystal structure. Although the AS has a somewhat higher potential than that of the main structure, it nevertheless has a non-zero equilibrium probability at temperatures above absolute zero. It exists as a small proportion in the crystal, so that the main structure retains its long range order. The AS is temperature sensitive; it is most pronounced at high temperature and tends to freeze out at low temperature. As a result, anomalous secondary bands not predicted on the basis of the main structure, appear in Raman and IR spectra. We have mainly used the symmetric stretch mode Y, of the NO, ions in our high pressure Raman studies of TlN03 and KNO,. The Raman spectrum of this mode is very intense and relatively simple, so any weak features will come out more strongly in this region and can be analysed more easily. EXPERIMENT
The crystals of TINOB and KNO, were grown by slowly evaporating aqueous solutions of the corresponding chemicals at room temperature. Raman spectra were recorded using a Spex Ramalog 5M spectrometer, with a Spectra Physics Ar+ ion laser operating on the 488 nm line acting as the excitation source. Backscattering geometry was used. The high pressure was generated by a diamond anvil cell. RESULTS AND DISCUSSION
TINO, The Raman spectra of the vi mode of the NO; ions under high pressures are shown in Fig. 1 at pressures of 0.001,10.8,27.0 and 43.6 kbar, respectively. Under ambient conditions, TINOB belongs to space group C&, with four molecules per unit cell, and no phase transition has been reported at room temperature in the pressure range O-40 kbar [ 41. Factor group analysis gives four Raman active bands for the v1 mode, Al, AZ, B1 and B2 (see p. 305 of ref. 1 for detailed analysis). The A, mode, which is the totally symmetric in-phase stretching of all four NO; ions in the unit cell, is expected to be by far the strongest Raman band. Three bands were observed at 0.001 kbar, at 1041.5,1039.7 and 1037.5 cm-‘. The band at 1041.5 cm-’ is the strongest and was assigned to the A, band. The other two much weaker bands situated at its low frequency shoulder are more difficult to assign with certainty. Balasubrahmanyan and Janz [ 5 ] observed a weak band, ~3 cm-l lower than the strong v1 band, but did not assign it. Brooker [ 31 assigned a weak band at 1037 cm-’ which is 4.8 cm-’ lower than the main component, to a band due to a thermally disordered alternative struc-
399
I
I
I
I 1040
1060
I
cm-1 Fig. 1. Raman spectra of the pI mode of TlNO, under high pressures.
ture, but mentioned that correlation field components were also observed. It is fair to say that the band at 1039.7 cm-‘, which is only 1.8 cm-l from the main band and is well within the range of correlation field splitting, is one of the other three components (A,, B1 or B,). An argument is given below in an attempt to assign the other weak band that was observed. The frequency differences of this weak band from the main band are 4.0,4.2 and 4.2 cm-‘, at pressures 0.001, 10.8 and 27 kbar respectively. They are constant within the error of the experiment. This means that this weak component has the same pressure dependence as the main band, and it is therefore reasonable to suggest that the two bands belong to the same structure (i.e. C& ). IR transmission and reflection spectra (see Chapter 7 of ref 1) give the B1 and Bz modes both at x 1040 cm-‘. This indicates that the band at 1039.7 cm-’ is the B1 and/or B, component of the Y, mode, and the band at 1037.5 cm-’ is the IR inactive but Raman allowed A2 component. Later in this paper, a weak component at the low frequency shoulder of the main band of v1 in KNO, is assigned to a band of an alternative structure, not a component of the basic structure. In the KN03 case, the weak band has a different pressure dependence from that of the main band, and the frequency difference becomes significantly larger at high pressures. From Fig. 1 we can see that the weak band at 1037.5 cm-’ has disappeared by 43.6 kbar. It has apparently been replaced by a weak band at 1060.8 cm-‘,
I
I 20
I
I 40
KIM Fig. 2. Frequency shifts of Raman v1 modes of TINOB with pressure: 0, main band; + and n , weak correlation field components.
which is at the high frequency shoulder of the main band (Fig. 2). This indicates a phase transition between 43.6 and 27 kbar, and the new phase shall be referred to as phase IV. Raman bands in the lattice region also show some changes in the same pressure range, which supports the suggestion of a phase transition (see Chapter 7 of ref. 1 for details). There are possible frequency discontinuities against pressure for the v1 and or, modes, but they are barely larger than the experimental error. There are several facts to be noted: (1) the v4 bands of the NO; ions do not split further in the high pressure phase IV, (ii) the shapes of the Y, bands have no detectable changes in phases III (ambient phase) and IV, (iii) the Raman spectra in the lattice region do not show new bands in phase IV. These imply that there is no major crystal structural change involved in the III-IV phase transition. The structure of the high pressure phase probably still belongs to in the space group C&, with 2~4. There are 22 space groups (n=1,2...22) C& category, all of which can accommodate four molecules. KNO, Alternative structures (AS) have been found for many alkaline nitrate crystals at ambient pressure. We report here different AS preferred for KN03 under high pressures. Figure 3 shows the Raman spectra of the v1 mode of the NO, ions at 6.3, 17.2, 38.9and 46.2 kbar respectively. A well-established phase transition II-
401
IV occurs at 3.4 kbar [ 1,6]. We will concentrate on pressures above this II-IV transition. The strong band has A, symmetry and comes from the main structure. A weak band was observed at the low frequency shoulder of the main band below 17.2 kbar and at the high frequency shoulder above 38.9 kbar. The pressure dependencies of the Raman bands are plotted in Fig. 4. It is generally accepted that the weak band at ambient pressure (0.001 kbar) in phase II is due to an AS and its separation from the main band is about 2 cm- ’ [ 3 1. We believe that the similar phase IV band is also due to an AS, and the separation increased from 2.9 cm-’ at 6.3 kbar to 3.4 cm-’ at 17.2 kbar. This implies that the main band has a bigger pressure dependence than the weak one and they will not cross each other at higher pressures. The separation between the high frequency weak band and main band is 4.8 cm-l at 38.9 kbar and 5.2 cm-l at 46.2 kbar, so the weak band has a bigger pressure dependence in this case. Hence this weak band has a different cause from the one at pressures below 17.2 kbar. Apart from those discussed above, there are no other unexplained spectral changes (i.e. extra bands, frequency-shift discontinuities or anomalous band
I
I
I
1070
I 1050
CM-'
I
L.
KBAR
Fig. 3. High pressure F&man spectra of the Y, mode of KNO,. Fig. 4. Pressure dependences of the Raman u1 mode of KNOB, phase IV: 0, main band; A, alternative structure side band; *, phase II at the lowest pressure.
shapes), for the main Y, band or other Raman active bands (including lattice modes) between 6.3 and 46.2 kbar. Therefore there is no evidence of a phase transition for the main crystal structure. We propose (i) that the weak phase IV bands belong to alternative structures and (ii) that a different AS is preferred at high pressures to that which appears at low pressures. This is indicated by the disappearance of the low pressure low frequency side band and the appearance of the high pressure high frequency side band. Thus, the lowest potential of the system, corresponding to the main structure, remains the same throughout the pressure range 6-50 kbar. However the next lowest potential, corresponding to the alternative structure, has changed. The switch over of the alternative structure occurs at about 30 kbar. CONCLUSIONS
High pressure Raman studies have been performed for KNO, and TINOB. A new second order phase transition has been found for T1N03, and evidence has been found for different alternative structures at low and high pressure in phase IV KNO, in the pressure range 6-50 kbar. The phase transition III-IV of T1N03 occurs at pressures between 30-35 kbar, marked by the disappearance of one of the low frequency weak components and the appearance of another at the high frequency side of the main v1 band. The new high pressure phase is believed to still be one of the C& space groups with four molecules per unit cell. Some observed changes in other spectral regions are also associated with this phase transition. Different alternative structures were found for KN03 by variation of pressure. While AS at ambient pressure have been observed by many people, the pressure induced AS changes have never been previously reported in the literature. The study of the AS is important for the understanding of the stability of the main crystal structure.
REFERENCES 1 2 3 4 5 6
Z. Shen, Ph.D. Thesis, King’s College London (1989). S.V. Karpov and A.A. Shultin, Sov. Phys. Solid State, 17 (1976) 1915. M.H. Brooker, J. Chem. Phys., 68 (1978) 67. E. Rapopart and C.W.F.T. Pistorous, J. Chem. Phys., 44(4) (1966) 1514. K.Balasubrahmanyan and G.J. Janz, J. Chem. Phys., 57(10) (1971) 4084. J.A. Medina, Ph.D. Thesis, King’s College London (1982).