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Unusual phase transition mechanism in rare-earth ethylsulfates
Solid State Communications, Printed in Great Britain.
Vo1.42,No.5,
pp.347-351,
1982.
0033-1098/82/170347-05$03.00/O Pergamon Press Ltd.
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES V.A.VOLOSHIN Physico-Technical Institute, Academy of Sciences of the Ukrainian SSR, 340048 Donetsk, U S S R (Received 20 June 1981 by E.A. Kaner) A hypothesis is put forward that at pressure 30 kbar f-orbitals begin to participate in generation of a chemical bond between a rare-earth ion and oxygen. This phenomenon is presumed to be responsible for spectral changes observed in praseodymium ethylsulfate at its compression.
Rare-earth ethylsulfates have been thoroughly investigated for dozens of years and have actually become model materials for testing many theoretical developments, in particular, the crystal field theory /I/. Methods more sensitive than the conventional optical spectroscopy reveal a pair interaation between rareearth ions /2/. As the temperature decreases to 0.1 K, magnetic ordering occurs /3/. There is, of course, the interaction between optical phonons and 4f-electrons too. (The analogous interaction was studied in Nd(OH)3 /4h Manifestation of all these phenomena is, however, much weaker than the transition intensity within the f-orbital. The optical spectrum of these compounds, therefore, is entirely determined by en isolated f-configuration /5;6/. In other words a number of states, for example of 4f2-configuration, is 91. This is the maximum number which can be revealed by an analysis of the spectrum of praseodymium ethylsulfate subjected to both electric end magnetic fields. From this point of view praseodymium ethylsulfate is convenient for
the study of pressure effect on the state of f-electrons. It was shown /7/ that up to 25 kbax the 4f2-configuration remains isolated and the following changes in its energy state take place. Due to the "wave funotion expansion" the Coulomb repulsion between f-electrons decreases (from F2 = 310 cm-' at normal pressure to 309.1 cm-1 at high pressure). Within the experimental errors the spin-orbital interaction (6 =759 cm-'). This is the same fact seems to be due to the summation of "nuclear" and "electron"contributions to this value since the changes of these contributions with pressure are of opposite signs. As for the crystalline field parameter Bg changes most of all (from 25.3 cm-' to 30.9 cm-'). The absorption spectrum of praseodymium ethylsulfate (or lanthanum ethylsulfate with praseodymium impurity) recorded at pressures not higher than 25 kbar can be interpreted without difficulty (see Fig. 1). Thus, isolation of the f-configuration remains valid up to 25 kbar. Further pressure increase, however, gives unexpected results /8/. In the interval from 26 to 32 kbar the spect347
348
UNUSUAL PHASE TRANSITION MECHANISM IN HARE-EARTH ETHYLSULFATES
0 k bar 22w
-
22426
-
Vol. 42, No. 5
22 kbar
2123i ?I 282
46958 (6162wi4
-
-
46922 l6626 46663
(0 o-
36 0
Fig. 1. Scheme of the 4f2-configuration levels of praseodymium ethyl-sulfate. T=78K, P=O and 22 kbar.
Fig. 2. Spectrum of lanthanum ethylsulfate with praseodymium impurity in the region 3H,-3P,.T=4.2K; P=O; 16; 32 kbar.
rum changes markedly: the number of
lines increases so that the "high pressure phase" spectrum can no longer be interpreted in terms of the isolated f-configuration. For example, in addition to further shift, intensity of a single line 3H4(z2)-3Po(0) starts to fall off sharply end next to it a quickly growing doublet line appears. And by 33 kbar instead of the previous single line two low-intensity lines end one high-intensity doublet line appear (see Fig. 2). At heating of a sample compressed to 33 kber a reverse process takes place. Redistribution of intensities begins approximately at 130 K and at about I60 K only one line remains. There are also less significant changes in the spectrum. Certain lines seem to split, but the gap between the components is small. At the slow heating of a sample whose axis is normal to the light propagation one can observe intensity redistribution of these components. The main conclusion is indubitable at low pressure the spectrum is determined by the isolated f-configuration
whereas at higher pressure this isolation no longer exists. To imagine the interactions responsible for disturbance of the isolation one must consider the structures of these single crystals /9/ (Fig. 3).
Fig. 3. Main structural units of rareearth ethylsulfate. Black points: Pr(H20)a. White points: groups C2H5S04 (two oxygen atoms from the SO,, group lying outside the rareearth ion plane are not shown).
There ere two molecules of Pr(C H SO )*YH 0 in a unit cell end 25 43 2
Vol. 42, No. 5
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
the intimate environment of anion is constituted by 9 water moleoulea (two "small" triangles above and below the central ion which lies in the "lerge" triangle plane). Sawwiae arranged C2H5SO4 groups lie much farther than water moleoules end interaot with the latter8 via the hydrogen bonds. (Fig. 3 does not show two oxygen ions lying outside the rare-earth ion plene). We are unaware of eny calculations of molecular orbitals for Pr(H20)g-oomplex, but we took part in the oalculations of a similar oomplex of EuO8 /IO/. In this uase it is assumed that the 2s- and 2p- atomic orbitals of oxygen and the 6s-, 6p-, 5d-atomio orbital8 of a rare-earth ion pertioipate in formation of molecular orbitals. In the calculations the distanoe between the central ion and the ligand (R) as well as the symmetry of possible figures were varied. As an example the data for a dodecahedron are presented in the following table.
349
assume with a sufficientodegree of probability that at REl.6 A the f-orbital will begin to take part in the formation of a chemiaal bond. It should be noted that the real wave functions are somewhat expanded /ll/ as compared to those calculated by the HartreeFoolsmethod (Fig.4 is taken from /12/). Thus for the f-orbital of praseodymium ion this nexpansion" (from @-') = 0.92 to (r*) = 1.38) is approximately equal to the ratio of the Slater integrals calculated by the Hartree-Fock method (F2=486) end the value obtained by way of comparison with the experiment (I?2= 313) /13/: 486 Ti7z&z+
1.5.
Therefore ono may suppose that in fact inclusion of the f-orbitals into a chemical bond begins at R c
1.6 x 1.5 f 2.4 f.
ORBITAIS FOR DODECAREDRON EuO8
TAE%E:POPULATION
R, % 6s 6~ 5d 2s 2P
2.8 0.400 0.478 0.812 1.964 4.325
OF ATOMIC
2.4 0.287 0.247 1.314 1.963 4.303
2.0 0.162 0.026
1.794 1.966 4.286
This table as well as the corresponding data for a cube, y antiprism, etc. show that at R> 2.8 A in formation of a chemical bond the 6~' and 6p-electrons prevail. At Rz2.4 A the d-eleotrons begin to expel the p-electrons due to rigid enezgy-symmetry conditions and at R=2.0 A the chemical bond is almost entirely determined by the d- and s-electrons. Though these calculations are not yet extended to the f-orbital, one can
i
2
3
I
5
6
R ((IA)
Fig. 4.
Radial distribution of the rare-earth ion electron density calculated by the Hartree-Fock method12.
The real distance between the praseodymium ion and the oxygen ion in ethylsulfate is 2.47 i /9/. On the basis of elastic properties of ethylsulfates /14/ and data on pressure effects on crystal field parameters one may suppose that at 30 kbar this distance is
r
350
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
(O.l-O.lz)A smaller, i.e. equals 2.35 A. Comparison of these data allows us to assume that one and the same phenomenon is responsible for all the changes in the spectrum. This phenomenon is inclusion of the f-electrons into the chemical bond. This must lead to spontaneous decrease of a distance between the rare-earth ion end the ligends, and consequently to further increase of the overlapping integrals with the participation of the f-electrons. The properties of substances formed by the chemical bond of this kind must change at least twice during compression. The first change is due to the transition from the chemical bond determined by the s- end p-electrons to that determined by the d-electrons, and the second change occurs when in the formation of a chemical bond the f-electrons participate. Like e.nydissociation of a chemical these two transitions are equilibrium processes characterized by a certain constant. And it is due to this fact that the "high pressure phase" spectrum is observed over a sufficiently wide temperature range. As opposed to en ordinary chemical bond here the complex is not destroyed at heating but the chemical returns to its initial state. Thus the main effect is that the f-orbital is no longer isolated. The main effect must be accompanied by the secondary ones and first of all by the manifestation of electron-phonon interaction. In particular, wha all other necessary conditions are satisfied,the inclusion of the f-orbital9 into a chemical bond must be accompanied by a strong manifestation of the Jahn-Teller effect. Let us recollect here the idea persistently repeated in /15/: "‘r7e should exclude from our consideration, however, orbitally degenerate
Vol. 42, No. 5
electronic states in which the degenerate electrons do not contribute appreciably to the molecular binding and are not perturbed therefore by nuclear displacements. Such is the case for the inner degenerate electronic shells of the paramagnetic rare-earth ionic salts". It is highly probable that the hydrogen bond vibrations superimpose on the f-configuration spectrum. The reconstruction of the hydrogen bond system which may arise due to compression must influence somehow the spectrum too. In the ethylsulfate unit cell there are two praseodymium ions. These two ions interact with each other due to hydrogen bonds via the water molecules and the oxygen ions from the SO4-group. This interaction is observed in the "high pressure phase" spectrum as the "Davydov splitting" and in the case of the odd number of electrons in the f-orbital it leads to the lifting of tbz Kremers degeneration. Both these effects should be much weaker than the JahnTeller effect but in the narrow-band spectrum caused by the f-f-transitions they must be noticeable. And finally due to the sharp enhancement of the 4f-orbital-ligand coupling an indirect exchange will increase and the temperature of the transition in the magnetically ordered state will grow. Strictly speaking the spectrum observed is no longer the 4f2-configuration one. And the greater is the contribution of the Qf-orbitala to the chemical bond, the weaker effects will be observed in tb recorded spectrum. As a result, this spectrum can hardly be regarded as the f-configuration one perturbed by various interactions. The spectrum lines will continue to broaden and intensify; the spectrum will begin to "smear".
vol. 42, No. 5
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
The phenomenon described wa8 observed on praseodymium and neodymium
351
ethylsulfate, but it surely spreads over wider class of substances.
REFERENCES
1. R.J.Elliott, K.W.H.Stevens, Proc. Roy. Sot. (London) m, 387,(1953). 2. J.M.Baker, Phys.Rev.A, a, 1341, (1964). 3. K.N.R.Taylor, M.J.Darby, Physics of Rare Earth Solids, Chapman and Hall Ltd., London 1972. 4. K.Ahrens, H.Gerlinger, H.Lichtblau, G.Sohaaok, C.Abstreiter, S.MroceKowski, J.Phys.C:Solid St.Phys.2, 4545, (1980). 5. J.B. Gruber, J.Chem.Phys. 38, 946,
(1963). 6. E.Y.Wong, O.M. Stefsudd, D.R.John-
ston, J.Chem.Phya. 2,
786, (1963).
7. V.A.Voloshin. The Effect of Pressu-
re on the Spectrum of Rare Earths (in Russian), Naukova Dwnka, Kiev, 1979. 8. V.A.Voloshin, L.A.Ivchenko, JETP LETTERS 32, 100, (1980).
9. J.Albertsson, J.Elding, Acta Cry&. a, 1460 (1977). 10. J.Neru&s, V.Lazauskaa, V.Volo?&naa. Lietuvos fizikos rinkinys, 21 3, (1981) (in Russian). 11. C.KJbrgensen, Mat.Fys.Medd.Den. Vid.Selsk. 2, No. 22, (1956). 12. P.Anisimov, R.Dagys, A.Sargautis, V,Tutlis. Lietuvoe fizikos rinki nys, 18, 449, (1978) (in Russian). 13. A.Savukinas, K.Eriksonas, N.Kulagin. V International Conference on High Pressure end Technology. Program and Abstracts. USSR, Moscow, Nauka, 1975, 84 (in Russian). 14. I.M.Krygin, S.N.Lukin, G.N.Neilo, A.D.Prokhorov. High Pressure Physics and Technology, No.2, 85, (1980) (in Russian). 15. H.A.Jahn, E.Teller, Proc.Roy.Soc. E, 220 (1937).
Vo1.42,No.5,
pp.347-351,
1982.
0033-1098/82/170347-05$03.00/O Pergamon Press Ltd.
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES V.A.VOLOSHIN Physico-Technical Institute, Academy of Sciences of the Ukrainian SSR, 340048 Donetsk, U S S R (Received 20 June 1981 by E.A. Kaner) A hypothesis is put forward that at pressure 30 kbar f-orbitals begin to participate in generation of a chemical bond between a rare-earth ion and oxygen. This phenomenon is presumed to be responsible for spectral changes observed in praseodymium ethylsulfate at its compression.
Rare-earth ethylsulfates have been thoroughly investigated for dozens of years and have actually become model materials for testing many theoretical developments, in particular, the crystal field theory /I/. Methods more sensitive than the conventional optical spectroscopy reveal a pair interaation between rareearth ions /2/. As the temperature decreases to 0.1 K, magnetic ordering occurs /3/. There is, of course, the interaction between optical phonons and 4f-electrons too. (The analogous interaction was studied in Nd(OH)3 /4h Manifestation of all these phenomena is, however, much weaker than the transition intensity within the f-orbital. The optical spectrum of these compounds, therefore, is entirely determined by en isolated f-configuration /5;6/. In other words a number of states, for example of 4f2-configuration, is 91. This is the maximum number which can be revealed by an analysis of the spectrum of praseodymium ethylsulfate subjected to both electric end magnetic fields. From this point of view praseodymium ethylsulfate is convenient for
the study of pressure effect on the state of f-electrons. It was shown /7/ that up to 25 kbax the 4f2-configuration remains isolated and the following changes in its energy state take place. Due to the "wave funotion expansion" the Coulomb repulsion between f-electrons decreases (from F2 = 310 cm-' at normal pressure to 309.1 cm-1 at high pressure). Within the experimental errors the spin-orbital interaction (6 =759 cm-'). This is the same fact seems to be due to the summation of "nuclear" and "electron"contributions to this value since the changes of these contributions with pressure are of opposite signs. As for the crystalline field parameter Bg changes most of all (from 25.3 cm-' to 30.9 cm-'). The absorption spectrum of praseodymium ethylsulfate (or lanthanum ethylsulfate with praseodymium impurity) recorded at pressures not higher than 25 kbar can be interpreted without difficulty (see Fig. 1). Thus, isolation of the f-configuration remains valid up to 25 kbar. Further pressure increase, however, gives unexpected results /8/. In the interval from 26 to 32 kbar the spect347
348
UNUSUAL PHASE TRANSITION MECHANISM IN HARE-EARTH ETHYLSULFATES
0 k bar 22w
-
22426
-
Vol. 42, No. 5
22 kbar
2123i ?I 282
46958 (6162wi4
-
-
46922 l6626 46663
(0 o-
36 0
Fig. 1. Scheme of the 4f2-configuration levels of praseodymium ethyl-sulfate. T=78K, P=O and 22 kbar.
Fig. 2. Spectrum of lanthanum ethylsulfate with praseodymium impurity in the region 3H,-3P,.T=4.2K; P=O; 16; 32 kbar.
rum changes markedly: the number of
lines increases so that the "high pressure phase" spectrum can no longer be interpreted in terms of the isolated f-configuration. For example, in addition to further shift, intensity of a single line 3H4(z2)-3Po(0) starts to fall off sharply end next to it a quickly growing doublet line appears. And by 33 kbar instead of the previous single line two low-intensity lines end one high-intensity doublet line appear (see Fig. 2). At heating of a sample compressed to 33 kber a reverse process takes place. Redistribution of intensities begins approximately at 130 K and at about I60 K only one line remains. There are also less significant changes in the spectrum. Certain lines seem to split, but the gap between the components is small. At the slow heating of a sample whose axis is normal to the light propagation one can observe intensity redistribution of these components. The main conclusion is indubitable at low pressure the spectrum is determined by the isolated f-configuration
whereas at higher pressure this isolation no longer exists. To imagine the interactions responsible for disturbance of the isolation one must consider the structures of these single crystals /9/ (Fig. 3).
Fig. 3. Main structural units of rareearth ethylsulfate. Black points: Pr(H20)a. White points: groups C2H5S04 (two oxygen atoms from the SO,, group lying outside the rareearth ion plane are not shown).
There ere two molecules of Pr(C H SO )*YH 0 in a unit cell end 25 43 2
Vol. 42, No. 5
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
the intimate environment of anion is constituted by 9 water moleoulea (two "small" triangles above and below the central ion which lies in the "lerge" triangle plane). Sawwiae arranged C2H5SO4 groups lie much farther than water moleoules end interaot with the latter8 via the hydrogen bonds. (Fig. 3 does not show two oxygen ions lying outside the rare-earth ion plene). We are unaware of eny calculations of molecular orbitals for Pr(H20)g-oomplex, but we took part in the oalculations of a similar oomplex of EuO8 /IO/. In this uase it is assumed that the 2s- and 2p- atomic orbitals of oxygen and the 6s-, 6p-, 5d-atomio orbital8 of a rare-earth ion pertioipate in formation of molecular orbitals. In the calculations the distanoe between the central ion and the ligand (R) as well as the symmetry of possible figures were varied. As an example the data for a dodecahedron are presented in the following table.
349
assume with a sufficientodegree of probability that at REl.6 A the f-orbital will begin to take part in the formation of a chemiaal bond. It should be noted that the real wave functions are somewhat expanded /ll/ as compared to those calculated by the HartreeFoolsmethod (Fig.4 is taken from /12/). Thus for the f-orbital of praseodymium ion this nexpansion" (from @-') = 0.92 to (r*) = 1.38) is approximately equal to the ratio of the Slater integrals calculated by the Hartree-Fock method (F2=486) end the value obtained by way of comparison with the experiment (I?2= 313) /13/: 486 Ti7z&z+
1.5.
Therefore ono may suppose that in fact inclusion of the f-orbitals into a chemical bond begins at R c
1.6 x 1.5 f 2.4 f.
ORBITAIS FOR DODECAREDRON EuO8
TAE%E:POPULATION
R, % 6s 6~ 5d 2s 2P
2.8 0.400 0.478 0.812 1.964 4.325
OF ATOMIC
2.4 0.287 0.247 1.314 1.963 4.303
2.0 0.162 0.026
1.794 1.966 4.286
This table as well as the corresponding data for a cube, y antiprism, etc. show that at R> 2.8 A in formation of a chemical bond the 6~' and 6p-electrons prevail. At Rz2.4 A the d-eleotrons begin to expel the p-electrons due to rigid enezgy-symmetry conditions and at R=2.0 A the chemical bond is almost entirely determined by the d- and s-electrons. Though these calculations are not yet extended to the f-orbital, one can
i
2
3
I
5
6
R ((IA)
Fig. 4.
Radial distribution of the rare-earth ion electron density calculated by the Hartree-Fock method12.
The real distance between the praseodymium ion and the oxygen ion in ethylsulfate is 2.47 i /9/. On the basis of elastic properties of ethylsulfates /14/ and data on pressure effects on crystal field parameters one may suppose that at 30 kbar this distance is
r
350
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
(O.l-O.lz)A smaller, i.e. equals 2.35 A. Comparison of these data allows us to assume that one and the same phenomenon is responsible for all the changes in the spectrum. This phenomenon is inclusion of the f-electrons into the chemical bond. This must lead to spontaneous decrease of a distance between the rare-earth ion end the ligends, and consequently to further increase of the overlapping integrals with the participation of the f-electrons. The properties of substances formed by the chemical bond of this kind must change at least twice during compression. The first change is due to the transition from the chemical bond determined by the s- end p-electrons to that determined by the d-electrons, and the second change occurs when in the formation of a chemical bond the f-electrons participate. Like e.nydissociation of a chemical these two transitions are equilibrium processes characterized by a certain constant. And it is due to this fact that the "high pressure phase" spectrum is observed over a sufficiently wide temperature range. As opposed to en ordinary chemical bond here the complex is not destroyed at heating but the chemical returns to its initial state. Thus the main effect is that the f-orbital is no longer isolated. The main effect must be accompanied by the secondary ones and first of all by the manifestation of electron-phonon interaction. In particular, wha all other necessary conditions are satisfied,the inclusion of the f-orbital9 into a chemical bond must be accompanied by a strong manifestation of the Jahn-Teller effect. Let us recollect here the idea persistently repeated in /15/: "‘r7e should exclude from our consideration, however, orbitally degenerate
Vol. 42, No. 5
electronic states in which the degenerate electrons do not contribute appreciably to the molecular binding and are not perturbed therefore by nuclear displacements. Such is the case for the inner degenerate electronic shells of the paramagnetic rare-earth ionic salts". It is highly probable that the hydrogen bond vibrations superimpose on the f-configuration spectrum. The reconstruction of the hydrogen bond system which may arise due to compression must influence somehow the spectrum too. In the ethylsulfate unit cell there are two praseodymium ions. These two ions interact with each other due to hydrogen bonds via the water molecules and the oxygen ions from the SO4-group. This interaction is observed in the "high pressure phase" spectrum as the "Davydov splitting" and in the case of the odd number of electrons in the f-orbital it leads to the lifting of tbz Kremers degeneration. Both these effects should be much weaker than the JahnTeller effect but in the narrow-band spectrum caused by the f-f-transitions they must be noticeable. And finally due to the sharp enhancement of the 4f-orbital-ligand coupling an indirect exchange will increase and the temperature of the transition in the magnetically ordered state will grow. Strictly speaking the spectrum observed is no longer the 4f2-configuration one. And the greater is the contribution of the Qf-orbitala to the chemical bond, the weaker effects will be observed in tb recorded spectrum. As a result, this spectrum can hardly be regarded as the f-configuration one perturbed by various interactions. The spectrum lines will continue to broaden and intensify; the spectrum will begin to "smear".
vol. 42, No. 5
UNUSUAL PHASE TRANSITION MECHANISM IN RARE-EARTH ETHYLSULFATES
The phenomenon described wa8 observed on praseodymium and neodymium
351
ethylsulfate, but it surely spreads over wider class of substances.
REFERENCES
1. R.J.Elliott, K.W.H.Stevens, Proc. Roy. Sot. (London) m, 387,(1953). 2. J.M.Baker, Phys.Rev.A, a, 1341, (1964). 3. K.N.R.Taylor, M.J.Darby, Physics of Rare Earth Solids, Chapman and Hall Ltd., London 1972. 4. K.Ahrens, H.Gerlinger, H.Lichtblau, G.Sohaaok, C.Abstreiter, S.MroceKowski, J.Phys.C:Solid St.Phys.2, 4545, (1980). 5. J.B. Gruber, J.Chem.Phys. 38, 946,
(1963). 6. E.Y.Wong, O.M. Stefsudd, D.R.John-
ston, J.Chem.Phya. 2,
786, (1963).
7. V.A.Voloshin. The Effect of Pressu-
re on the Spectrum of Rare Earths (in Russian), Naukova Dwnka, Kiev, 1979. 8. V.A.Voloshin, L.A.Ivchenko, JETP LETTERS 32, 100, (1980).
9. J.Albertsson, J.Elding, Acta Cry&. a, 1460 (1977). 10. J.Neru&s, V.Lazauskaa, V.Volo?&naa. Lietuvos fizikos rinkinys, 21 3, (1981) (in Russian). 11. C.KJbrgensen, Mat.Fys.Medd.Den. Vid.Selsk. 2, No. 22, (1956). 12. P.Anisimov, R.Dagys, A.Sargautis, V,Tutlis. Lietuvoe fizikos rinki nys, 18, 449, (1978) (in Russian). 13. A.Savukinas, K.Eriksonas, N.Kulagin. V International Conference on High Pressure end Technology. Program and Abstracts. USSR, Moscow, Nauka, 1975, 84 (in Russian). 14. I.M.Krygin, S.N.Lukin, G.N.Neilo, A.D.Prokhorov. High Pressure Physics and Technology, No.2, 85, (1980) (in Russian). 15. H.A.Jahn, E.Teller, Proc.Roy.Soc. E, 220 (1937).