- Email: [email protected]
Optical properties of some perovskite halides from 8 to 35 eV
Solid State Communications, Vol. 55, No. 2, pp. 157-158, 1985. Printed in Great Britain.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
OPTICAL PROPERTIES OF SOME PEROVSKITE HALIDES FROM 8 TO 35 eV G. Samoggia Dipartimento di Fisica "A. Volta", Pavia, Gruppo Nazionale di Struttura della Materia del CNR I, 27100 Pavia, Italy F. Parmigiani CISE S.p.A., P.O. Box 12081,20100 Milano, Italy and F. Leccabue Maspec Institute, CNR 43100 Parma, Italy
(Received 6 February 1985 by F. Bassani) The room temperature absorption, in the threshold region of single crystals of K2CoF4, Rb2CoFa, K2NiF4 was measured. Synchrotron radiation was used as continuum light source. The threshold is attributed to electron orbital promotion transitions 3dn"*3dn-l(4s, 4p) I of 3d metal ions. Crystals reflectivity was measured in the 8 - 3 5 eV region. It is shown that all the optical structures are associated only with transitions of the alkali and halide electrons. TRANSITION METAL COMPOUNDS A 2MF4 (A = K, Rb; M = Mn, Fe, Co, Ni, Cu, Zn) are ionic crystals with halide-perovskite layer structure that easily cleave along planes perpendicular to c-axis [1]. In the paramagnetic state, A2MF4 compounds crystallize in the D ~ tetragonal system. Many studies, mainly on the magnetic properties, were performed [ 1 - 4 ] . However, no systematic investigation on the optical and electronic properties was undertaken [5]. The present work describes the measurements of the threshold absorption and reflectance, R, in the XUV region up to 35eV, of the layer crystals K2CoF4, K2NiF4, Rb2CoF4 and of the perovskite RbNiF3. Synchrotron light of the PULS-Frascati facility was used as a continuum light source. The XUV light crossed a calibrated W mesh before impinging on the crystal. The ratio between the reflected light, and the mesh output gave the sample reflectance. Lack of mesh sensitivity below 12 eV forbade precise R measurements in the first exciton region. Isomorphous A2MF4 single crystals were grown by the flux method under argon/H2 atmosphere using a mixture of AF and 35 mol % AMF3 at a cooling rate of ~ l°Chr -1. AMF3 single crystals were prepared by the Bridgman method from the stoichiometric mixture or from AMF3 powders obtained from precipitation in aqueous solution. The mixtures were heated to about 1050°C and cooled at a rate of 2 - 3 ° C h r -1 , to about 650°C. The single crystals, obtamed, several millimeters in size, were then controlled by Lane back reflection technique.
All A 2MF4 and AMFa crystals exhibit faint absorption bands in the visible and near UV region associated with dipole forbidden d - d intra-atomic transitions. A sharp absorption increase, in the vacuum ultraviolet, is attributed either to parity allowed 3d-4p intra-atomic, charge transfer or band to band transitions. Figure 1 reports the absorption coefficient, measured at the room temperature of some A2MF4 crystals. It is becoming apparent that the strong absorption edge has a sharp increase which begins at "- 5.7 eV in K2CoF4 and Rb2CoF4 crystals and at ~ 6 , 3 in K2NiF4. A similar trend, i.e. a strong dependence of the energy threshold on the number of d electrons (d 7 for Co ++, d s Ni ++ ions), was observed also for some AMF3 crystals [ 6 - 7 ] . Unfortunately only two band calculations for AMF3 crystals are reported in literature: an LCAO for KNiF3 [7] and a second, more recent one, for KMgF3 [8]. No theoretical data exist for A 2MF4 crystals, therefore it is impossible to compare directly our experimental observation with theory. However it should be noticed that, in the atomic case, d levels shift upward about 3 eV when going from Ni 2+ (d s) to Mn 2+ (d s) and the numerical value of the shift is modified by the crystalline environment without altering the atomic trend [10, 11]. As a result, accordingly, it is reasonable to associate the absorption edge of A2MF4 and AMF3 crystals to 3d metal ion intra-atomic transitions 3d n -- (4s, 4p) 3d" - i. The RT reflectance spectra are reported in Fig. 2. We can group the spectra in two series: those belonging to potassium compounds and those to rubidium
1 ~'/
158
OPTICAL PROPERTIES OF SOME PEROVSKITE HALIDES
Vol. 55, No. 2 20
.---10 3
10 ._u
11/
c
I 5.5
I 5.7
f.S
I 6.1 "ha,(eV)
I 6.3
6.5
n
I~
1
{
Fig. 1. Optical absorption of Rb2CoF4, K2CoF4, K2NiF4 crystals in the threshold region.
J
A
/~~
\\
Rb2c°F~~
compounds. All potassium spectra have a close resemblance to each other and consist of several peaks. Also, these spectra are partially similar to the KMFa and to the potassium fluoride spectra. The first exciton peak I I I I I lies at 9.7 in K2NiF4 and at 10.4eV in KF and should 10 15 20 25 30 lh w(eV) belong to transitions coming from the valence level, mostly 2p (F-) to conduction levels. The region 13Fig. 2. RT reflectance spectra of K2CoF4, K2NiF4, 19 eV mainly includes the transitions coming from the Rb2CoF, and RbCoF3 in the 10-35eV region. valence band to the higher conduction band. A broad structure at 14-15 eV is common to both perovskites Piacentini and F. Antonangeli for their continuous and their halides. The reflectivity in the range 19-21 eV is dominated assistance during the experimental runs. by excitations from the K ÷ (3p) core to 3pS4s and 3pS3d I levels of the same ion [6, 7, 12]. Several other REFERENCES peaks, observed in the region 22-30eV, appear in each 1. H. Burger, K. Strobel, R. Geick & W. Muller, of the K÷ compounds. The reflectivity cut off is at J. Phys. C9, 4213 (1976). about 30eV in K compounds but at lower energies 2. K. Strobel & R. Geick, J. Phys. C9, 4223 (1976). in Rb compounds ( " 27 eV). 3. J.P. Gosso, P. Moch, M. Qullichini, J-Y. Gesland & The rubidium compounds (Rb2CoF,, RbCoFa) J. Nonet J. Physique 40, 1067 (1979). have spectra which are very similar to each other and to 4. H. Sontag, F. Macco, K. Fendler, W.P. Lehmann & R. Weber, J. Phys. C15, L409 (1982). RbF, and display a set of well structured peaks in the 5. J.B. Parkinson, Proc. Phys. Soc. 2,2012(1969). 16-19 eV. region. These peaks, again interpreted on the 6. J.H. Beaumont, A.J. Bourdlllon & J. Bordes, J. basis of a localized excitation model, mainly correspond Phys. CIO, 333 (1977). to mp6--mp s nd excitations of the alkali ion splitted 7. H. Onuki, F. Sugawara, Y. Nishihara, M. Hirano, by the crystal field [12]. Y. Yamaguchi, A. Ejiri, H. Takahashi & H. Abe, Solid State Commun. 20, 35 (1976). Finally, in the spectral region up to 35eV. the 8. L.F. Mattheiss, Phys. Rev. B6, 4718 (1972). optical spectra of perovskite halides are dominated by 9. R.A. Heaton & C.C. Lin Phys. Rev. B25, 3538 transitions involving only the alkali and the halide ions, (1982). no appreciable transition from 3(/state of Ni or Co is 10. S. Antoci& L. Mihic Phys. Rev. B21,3383 (1980). found. This behavior is identical to that observed in 11. A.B. Kunz & G.T. Jurratt. Solid State Commun. perovskite crystals reflectance [8, 9]. 25, 9 (1978). 12. C. Satoko & S. Sugano, J. Phys. Soc. Japan 34, 701 (1973). Acknowledgements - We wish to thank Prof. M.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
OPTICAL PROPERTIES OF SOME PEROVSKITE HALIDES FROM 8 TO 35 eV G. Samoggia Dipartimento di Fisica "A. Volta", Pavia, Gruppo Nazionale di Struttura della Materia del CNR I, 27100 Pavia, Italy F. Parmigiani CISE S.p.A., P.O. Box 12081,20100 Milano, Italy and F. Leccabue Maspec Institute, CNR 43100 Parma, Italy
(Received 6 February 1985 by F. Bassani) The room temperature absorption, in the threshold region of single crystals of K2CoF4, Rb2CoFa, K2NiF4 was measured. Synchrotron radiation was used as continuum light source. The threshold is attributed to electron orbital promotion transitions 3dn"*3dn-l(4s, 4p) I of 3d metal ions. Crystals reflectivity was measured in the 8 - 3 5 eV region. It is shown that all the optical structures are associated only with transitions of the alkali and halide electrons. TRANSITION METAL COMPOUNDS A 2MF4 (A = K, Rb; M = Mn, Fe, Co, Ni, Cu, Zn) are ionic crystals with halide-perovskite layer structure that easily cleave along planes perpendicular to c-axis [1]. In the paramagnetic state, A2MF4 compounds crystallize in the D ~ tetragonal system. Many studies, mainly on the magnetic properties, were performed [ 1 - 4 ] . However, no systematic investigation on the optical and electronic properties was undertaken [5]. The present work describes the measurements of the threshold absorption and reflectance, R, in the XUV region up to 35eV, of the layer crystals K2CoF4, K2NiF4, Rb2CoF4 and of the perovskite RbNiF3. Synchrotron light of the PULS-Frascati facility was used as a continuum light source. The XUV light crossed a calibrated W mesh before impinging on the crystal. The ratio between the reflected light, and the mesh output gave the sample reflectance. Lack of mesh sensitivity below 12 eV forbade precise R measurements in the first exciton region. Isomorphous A2MF4 single crystals were grown by the flux method under argon/H2 atmosphere using a mixture of AF and 35 mol % AMF3 at a cooling rate of ~ l°Chr -1. AMF3 single crystals were prepared by the Bridgman method from the stoichiometric mixture or from AMF3 powders obtained from precipitation in aqueous solution. The mixtures were heated to about 1050°C and cooled at a rate of 2 - 3 ° C h r -1 , to about 650°C. The single crystals, obtamed, several millimeters in size, were then controlled by Lane back reflection technique.
All A 2MF4 and AMFa crystals exhibit faint absorption bands in the visible and near UV region associated with dipole forbidden d - d intra-atomic transitions. A sharp absorption increase, in the vacuum ultraviolet, is attributed either to parity allowed 3d-4p intra-atomic, charge transfer or band to band transitions. Figure 1 reports the absorption coefficient, measured at the room temperature of some A2MF4 crystals. It is becoming apparent that the strong absorption edge has a sharp increase which begins at "- 5.7 eV in K2CoF4 and Rb2CoF4 crystals and at ~ 6 , 3 in K2NiF4. A similar trend, i.e. a strong dependence of the energy threshold on the number of d electrons (d 7 for Co ++, d s Ni ++ ions), was observed also for some AMF3 crystals [ 6 - 7 ] . Unfortunately only two band calculations for AMF3 crystals are reported in literature: an LCAO for KNiF3 [7] and a second, more recent one, for KMgF3 [8]. No theoretical data exist for A 2MF4 crystals, therefore it is impossible to compare directly our experimental observation with theory. However it should be noticed that, in the atomic case, d levels shift upward about 3 eV when going from Ni 2+ (d s) to Mn 2+ (d s) and the numerical value of the shift is modified by the crystalline environment without altering the atomic trend [10, 11]. As a result, accordingly, it is reasonable to associate the absorption edge of A2MF4 and AMF3 crystals to 3d metal ion intra-atomic transitions 3d n -- (4s, 4p) 3d" - i. The RT reflectance spectra are reported in Fig. 2. We can group the spectra in two series: those belonging to potassium compounds and those to rubidium
1 ~'/
158
OPTICAL PROPERTIES OF SOME PEROVSKITE HALIDES
Vol. 55, No. 2 20
.---10 3
10 ._u
11/
c
I 5.5
I 5.7
f.S
I 6.1 "ha,(eV)
I 6.3
6.5
n
I~
1
{
Fig. 1. Optical absorption of Rb2CoF4, K2CoF4, K2NiF4 crystals in the threshold region.
J
A
/~~
\\
Rb2c°F~~
compounds. All potassium spectra have a close resemblance to each other and consist of several peaks. Also, these spectra are partially similar to the KMFa and to the potassium fluoride spectra. The first exciton peak I I I I I lies at 9.7 in K2NiF4 and at 10.4eV in KF and should 10 15 20 25 30 lh w(eV) belong to transitions coming from the valence level, mostly 2p (F-) to conduction levels. The region 13Fig. 2. RT reflectance spectra of K2CoF4, K2NiF4, 19 eV mainly includes the transitions coming from the Rb2CoF, and RbCoF3 in the 10-35eV region. valence band to the higher conduction band. A broad structure at 14-15 eV is common to both perovskites Piacentini and F. Antonangeli for their continuous and their halides. The reflectivity in the range 19-21 eV is dominated assistance during the experimental runs. by excitations from the K ÷ (3p) core to 3pS4s and 3pS3d I levels of the same ion [6, 7, 12]. Several other REFERENCES peaks, observed in the region 22-30eV, appear in each 1. H. Burger, K. Strobel, R. Geick & W. Muller, of the K÷ compounds. The reflectivity cut off is at J. Phys. C9, 4213 (1976). about 30eV in K compounds but at lower energies 2. K. Strobel & R. Geick, J. Phys. C9, 4223 (1976). in Rb compounds ( " 27 eV). 3. J.P. Gosso, P. Moch, M. Qullichini, J-Y. Gesland & The rubidium compounds (Rb2CoF,, RbCoFa) J. Nonet J. Physique 40, 1067 (1979). have spectra which are very similar to each other and to 4. H. Sontag, F. Macco, K. Fendler, W.P. Lehmann & R. Weber, J. Phys. C15, L409 (1982). RbF, and display a set of well structured peaks in the 5. J.B. Parkinson, Proc. Phys. Soc. 2,2012(1969). 16-19 eV. region. These peaks, again interpreted on the 6. J.H. Beaumont, A.J. Bourdlllon & J. Bordes, J. basis of a localized excitation model, mainly correspond Phys. CIO, 333 (1977). to mp6--mp s nd excitations of the alkali ion splitted 7. H. Onuki, F. Sugawara, Y. Nishihara, M. Hirano, by the crystal field [12]. Y. Yamaguchi, A. Ejiri, H. Takahashi & H. Abe, Solid State Commun. 20, 35 (1976). Finally, in the spectral region up to 35eV. the 8. L.F. Mattheiss, Phys. Rev. B6, 4718 (1972). optical spectra of perovskite halides are dominated by 9. R.A. Heaton & C.C. Lin Phys. Rev. B25, 3538 transitions involving only the alkali and the halide ions, (1982). no appreciable transition from 3(/state of Ni or Co is 10. S. Antoci& L. Mihic Phys. Rev. B21,3383 (1980). found. This behavior is identical to that observed in 11. A.B. Kunz & G.T. Jurratt. Solid State Commun. perovskite crystals reflectance [8, 9]. 25, 9 (1978). 12. C. Satoko & S. Sugano, J. Phys. Soc. Japan 34, 701 (1973). Acknowledgements - We wish to thank Prof. M.