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An off-center ion near a Ba site in BaTiO3 as studied by EPR under uniaxial stress
PERGAMON
Solid State Communications 116 (2000) 133±136
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An off-center ion near a Ba site in BaTiO3 as studied by EPR under uniaxial stress S. Lenjer a, R. Scharfschwerdt a, Th.W. Kool b, O.F. Schirmer a,* b
a Fachbereich Physik, University of OsnabruÈck, D-49069 OsnabruÈck, Germany Laboratory for Physical Chemistry, University of Amsterdam, NL-1018 Amsterdam, Netherlands
Received 8 May 2000; received in revised form 10 July 2000; accepted 10 July 2000 by J. Kuhl
Abstract In BaTiO3 a paramagnetic defect is identi®ed by EPR which is attributed to a paramagnetic S 1=2 ion off-center near a Ba site. Twelve magnetically distinguishable sites contribute to the angular dependence pattern of the spectra, each of the sites being characterized by an orthorhombic g-tensor. The relative occupations of the sites change under uniaxial stress in a manner consistent with the model. The features of the center are highly characteristic for the indicated off-center position, as revealed by comparison with the previously identi®ed Ti 31 near a Sr 21 site in SrTiO 3. On the basis of the principal values of the g-tensor the paramagnetic ion can be identi®ed with Ni. It is discussed that Ni 11 is the most likely charge state. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ferroelectrics; C. Paint defects; E. Electron paramagnetic resonance PACS: 71.70.Ch; 76.30.Fc; 77.84.Dy
1. Introduction
2. Experimental results
This communication is part of an extended effort to gather information on the possible defect structures in BaTiO3 [1± 6]. If doped adequately, such crystals show high photorefractive sensitivities [7]. Since the photorefractive effect in inorganic compounds is usually triggered by the photoionisation of defects, it is advisable to have at hand a large stock of possible lattice perturbations for tailoring the properties of a material or for understanding its photorefractive properties [7]. EPR is a powerful tool in identifying impurities and intrinsic defects with unpaired electrons. Here we report the EPR identi®cation of a paramagnetic ion, substituting for Ba 21 and going off-center near such a site. The assignment is also based on the change of the relative line intensities under uniaxial stress; on this way a linear stress coupling coef®cient b 3:8 £ 10230 m3 could be determined. Several arguments support the identi®cation of the ion as Ni 11.
The defect treated here was detected by EPR in BaTiO3 crystals, which had been grown from melts containing 10 000 ppm Na and 1000 ppm Rh [8], after strong oxidation (heating at 9008C under 20 £ 105 Pa of pure oxygen for 2 h and fast quenching). The observations were made at n 9:11 GHz in the rhombohedral low temperature phase of BaTiO3 T , 180 K: The signals could be detected at temperatures between 4.2 K up to about 40 K. As determined from the simultaneous measurement of an EPR standard, the corresponding defect had a concentration of about 20 ppm. Crystals of the typical size 2 £ 2 £ 3 mm3 were used. We tend to attribute the center, characterized by a spin S 1=2 and an orthorhombic g-tensor, to Ni, apparently being present as a background impurity in the specimens. Fig. 1a shows the angular dependence of the g-values of this center with the magnetic ®eld B rotating, for example, in a (100) plane. Assuming that BaTiO3 can be considered as cubic, neglecting its slight low temperature rhombohedral distortion, the experimental data can be explained on the basis of a superposition of 12 symmetry related equivalent centers as indicated in Fig. 1b. Each of the branches in this ®gure is described by a Hamiltonian consisting of only a Zeeman effect with an orthorhombic
* Corresponding author. Fax: 149-541-969-2670. E-mail address: [email protected] (O.F. Schirmer).
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00307-0
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S. Lenjer et al. / Solid State Communications 116 (2000) 133±136
Fig. 1. Angular dependence of the EPR lines of the Ni center with B oriented in a (001) plane, T 15 K: At B k [100] three EPR lines, 1±3 are observed, typical for orthorhombic symmetry. The small splittings of the EPR lines with B in an arbitrary direction result from the slight tilting of the g-tensor principal axes. The lines in Fig. 1a are the calculated line positions, as based on the superimposed Zeeman effects of 12 equivalent centers as indicated in (b). There the orthorhombic g-tensors are symbolized by cuboids. A representative one (hatched) among the 12 equivalent tensors indicates that there is a slight tilting around the [001] axis by t 3:08: The tensors are grouped as indicated by the Roman numbers. Fig. 1a shows how they contribute to the resonances.
g-tensor, H mB Bg B; with S 1=2: The g-tensor has the principal values gx 2:36 ^ 0:01; gy 2:16 ^ 0:01 and gz 2:24 ^ 0:01: It is symbolized by the hatched cuboid in Fig. 1b. The x-axis of the representative tensor approximately extends along a [100] direction, y approximately along [010] and z along [001], respectively (Fig. 1b). There is a slight tilting of the x-axis by t 3:0 ^ 0:18 towards a negative [010] direction; the y-axis is turned by the same amount towards the [100] direction. The total angular dependence (Fig. 1a) is reproduced by the superposition of the 12 g-tensors (Fig. 1b), resulting from the representative one by the symmetry operations of the cubic group. For B along one of the k100l type directions three EPR lines are observed, to which the three groups of centers-I, II and III (Fig. 1b)-contribute in the way indicated at the right of Fig. 1a. As shown in the EPR spectrum of Fig. 2, at 15 K the intensities of the resonances change under an externally
Fig. 2. Change of the EPR spectrum of Ni for B k [100] under the in¯uence of an externally applied uniaxial stress. The small satellite near line 3 is due to a different defect.
applied stress Pmax 0:08 GPa: Fig. 3 shows the variations of the intensity ratio of the different indicated signals under the in¯uence of this stress. As will be explained below, the changes are due to a rearrangement of the ions in such a way that the moveable off-center ion aligns along the stress direction. 3. Discussion The angular dependence pattern in Fig. 1a is highly speci®c for a small paramagnetic ion, replacing an A site ion in an ABO3 perovskite crystal. This was established by Schirmer and MuÈller [9], who identi®ed a topologically identical pattern for Ti 31 (d 1) ions in SrTiO3 sitting near a Sr 21 site off-center along one of the k100l type directions; the g-tensor axes are slightly tilted by t 2:58 in a way identical to that indicated in Fig. 1b. The tilting angle did not change by altering the temperature below the phase transition temperature Tc < 105 K: Because of the tilting this center shows orthorhombic symmetry in the low temperature phase of SrTiO3 [10,11]. The center could be produced by irradiation with fast neutrons [9] or by a strong reduction treatment of undoped SrTiO3 [12]. The off-center movement of Ti 31 was attributed to its small radius, being lower than that of the replaced Sr 21. Ti 31 is shifted along one of the cubic axes and is tilted away towards one of the 12 O 22. When applying a static electric ®eld it was observed that a Ti 31 off-center ion is aligned at 4.2 K along the ®eld direction. This alignment excludes the presence of a nearby defect or associated impurity ion as demonstrated in earlier static electric ®eld experiments also for the Cr 51 and V 41 impurity ions in SrTiO3 [13±15].
S. Lenjer et al. / Solid State Communications 116 (2000) 133±136
Fig. 3. Plot of in ln(I2/I1) versus external uniaxial stress P applied along a k100l type direction, T 15 K: The solid line represents the best ®t of Eq. (1) to the experimental points with a linear stress coupling coef®cient b 3:8 £ 10230 m3 : The curve in the ®gure is slightly shifted (by 20.23) to negative values because of a prealignment of the centers probably by internal strains.
Because the angular plot of the center treated here (Fig. 1a) is rather unusual and has the same structure as that of SrTiO3:Ti 31, we can attribute it to a similar microscopic model: A comparatively small paramagnetic ion sitting off-center near a Ba-site in BaTiO3. As discussed below, the ion is likely to be identi®ed with Ni 11. The measurements performed under uniaxial stress support this model: If applied along the [001] direction, such stress leads to the intensity changes shown in Fig. 2, observed at 15 K. The fact, that a corresponding reorientation occurs at all at such low temperatures again excludes the presence of a nearby defect or an associated impurity. It is seen that under the indicated stress the concentration of centers of type III decreases as compared with the sum of centers I and II. This means, on the other hand, that ions going off-center along the stress axis, i.e. centers of type I, are favored; their energy is lowered, and thus their concentration increased under applied stress. As seen from Fig. 1b, the concentration of centers II, likewise oriented perpendicular to the stress axis, is then alsoÐas the type III centersÐexpected to decrease. The sum of I and II, however, increases as compared to III, Fig. 2. The linewidths do not change and the positions do not shift under stress. Therefore, it is justi®ed to use only the heights of the EPR lines as measures for the center concentrations, as in the following evaluation. Quantitatively, the alignment under stress is treated in this way: Neglecting the small tiltings, there are six equivalent k100l type off-center pocket states. This equivalence is lifted by external stress, leading to twofold degeneracy for the pockets extending along the stress direction and fourfold for those perpendicular to the stress. For stress P along the [001] direction, the ®rst class contains centers I (Fig. 1b), the second one centers II and III. The energy difference between both situations is given by [16] DU bP; where b is the appropriate linear stress coupling coef®cient and P, the magnitude of the stress; for details see Ref. [2]. As a quantity which can easily be compared with experiment we use the relation I2 =I1 II 1 III =IIII ; indicating in an
135
obvious manner the concentrations of the off-center ion in the various pocket states. As seen from Fig. 1a, IIII is proportional to the intensity of line 1 in Fig. 2, (II 1 III) to the sum of lines 2 and 3, if B is applied along [100], perpendicular to the stress direction, [001]. Assuming that the pocket I has the lowest energy and concentration n1, the concentrations of centers II and III relative to it are n1 exp 2bP=kT: We thus expect: I2 11a 2bP a exp : 1 2a kT I1 This is con®rmed by Fig. 3: the stress dependence of the experimental ratios I2/I1 is well reproduced using b 3:8 £ 10230 m3 : This value is of the same order as that found for the Fe 21 ±O 2 hole center in SrTiO3 [17]. According to Eq. (1) ln(I2/I1) should cross the ordinate at zero in Fig. 3. The slight downward shift by about 20.2 can be attributed to a small prealignment due to internal strains. Next we want to address the identi®cation of the offcenter ion. Although the present defect appeared in crystals intentionally doped with Rh and Na, the discussed EPR spectra cannot be assigned to either of these ions. The spectra of EPR-active charge states of Rh in BaTiO3, Rh 41 and Rh 21, have been studied in detail previously [1], both showing features quite different from those found here. Rh spectra are especially distinguished by their hyper®ne splitting into two components, caused by interaction with the I 1=2 nuclear spin of the 100% abundant isotope 103Rh. The presence of Na is only revealed after illumination of the crystals, which leads to hole capture at an O22 ion next to Na 1 [2]. So the spectra reported here must be attributed to a background impurity. Its mean g-value, 2.25, is typical for the paramagnetic charge states of nickel, Ni 31, Ni 21 or Ni 11, if Ni 31 is considered in its strong crystal ®eld (low spin (LS)) case; the high spin (HS) con®guration, occuring in weak crystal ®elds, has a de®nite different signature, see below. Except for this case the above ions are notoriously dif®cult to distinguish, their mean g-values for distorted octahedral surroundings lying together rather closely in the range 2.17 to 2.29, see Table 1 in Ref. [18]. Only Cu 21 ions have similar values, but these are characterized by their typical hyper®ne splittings into four lines, caused by the 63,65Cu isotopes I 3=2; 100% abundant together. Therefore we tend to attribute the investigated paramagnetic ion to Ni. Also this element has an I 3=2 isotope, 61Ni, which however occurs only in 1.13% of all cases. Since each of the expected four hyper®ne structure lines has only an intensity of 0.3% of the central, I 0; line they are hard to detect with the available signal to noise ratio (Fig. 2). Among the conceivable Ni charge states, Ni 21 can be disregarded, because it has a S 1 ground state, in contrast to the identi®ed S 1=2: Ni 21 would furthermore lead to rather wide Dms ^1 transitions, not of the type observed. Also LS±Ni 31 can be discarded since Ni31 is known to occur in its HS state when sitting near an A site of an ABO3 oxide perovskite, as has been found for Ni 31 in KTaO3 by Hannon
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[19]. The axiality of this center can best be explained by assuming that it also goes off-center; it has the characteristic g-values gk 2:22 and g' 4:42: These result from an 4A2 orbital ground state, appropriate for the 3d 7 Ni 31 ion in the weak cubic crystal ®eld at the twelvefold coordinated K-site, to which an axial ®eld due to the off-center movement is superimposed. The fact that g' < 2gk is typical for a S 3=2 system, such as HS±Ni 31, with an axial splitting large compared to the microwave quantum. Since the resonances in the present case are caused-according to the arguments given so far-by a Ni ion near the A site ion, Ba, in BaTiO3, it should have similar g-values, if occurring as Ni 31. The present defect shows quite different values; thus Ni 31 can be excluded, and we tend to ascribe the center to Ni 11. Its spin is S 1=2; as identi®ed. The observation of Ni 11 on a Ti site in BaTiO3 [20] with the same mean g-value as in the present case is consistent with this assignment. The ground state of the Ni 11 ion presented in this paper apparently is stabilized by interaction with essentially the four O22 ions which Ni 11 approaches by its off-center movement along a k100l type direction. This is supported by the fact that the Ni 11 ions align towards those pockets lying along the stress axis. Under stress the distance between Ni 11 and the four O22 ions along the stress direction is shortened. Which orbital is lowest is dif®cult to predict without knowing the strength of the axial crystal ®eld. If it is weak, the orbital sequence is determined by the cubic potential at the dodecahedral Ba position. Its sign is the same as that of a tetrahedral crystal ®eld, leading to a T2type (xy, yz, zx) hole ground state. If, on the other hand, the axial ®eld is strong, an E-type orbital x2 2 y2 or 3z2 2 r 2 will be lowest, neglecting small admixtures compatible with the orthorhombic symmetry. This orthorhombicity of the center is probably caused by a slightly closer attachment of Ni to one of its O22 neighbors. Such a feature was established as the reason for the orthorhombicity in the analogous case of off-center Ti 31 in SrTiO3, where the low symmetry of the center depends on a slight inequivalence of the four O22 ions, driven by the structural phase transition of the material [9]. In the present case the orthorhombicity might be caused by the inequivalence of the four O22 ions due to the small trigonal distortion of BaTiO3 in its low-temperature rhombohedral phase. Such an inequivalence is absent in the case of KTaO3, which is cubic at all temperatures. This appears to be the reason why off-center Ni 31 in KTaO3 is axially symmetric. A similar situation, met in SrTiO3 above its 105 K phase transition temperature, supports this conclusion: There the EPR spectrum of off-center Ti 31 is axially symmetric, since SrTiO3 in this crystal phase is cubic. 4. Summary We have shown that the observed resonances are caused by a paramagnetic S 1=2 ion, off-center near a Ba site in
BaTiO3. The interpretation of the EPR data, especially those obtained under application of uniaxial stress, shows that the ion is shifted essentially along a k100l type direction and aligns along the stress axis. The attachment of the ion to one of its four surrounding O22 ions appears to be the reason for the orthorhombicity observed in the present case. A detailed discussion shows that the paramagnetic ion in question is likely to be Ni; all observations support to the assumption that the charge state is Ni 11. This conclusion is consistent with the properties of the off-center ions Ni 31 (low spin) in KTaO3 and Ti 31 in SrTiO3. Acknowledgements We thank Dr H. Hesse for the supply of the crystals and M. Meyer for supplementary measurements. The research reported here was supported by DFG, Sonderforschungsbereich 225. References [1] E. Possenriede, P. Jacobs, O.F. Schirmer, J. Phys.: Condens. Matter 4 (1992) 4719. [2] T. Varnhorst, O.F. Schirmer, H. KroÈse, R. Scharfschwerdt, T.W. Kool, Phys. Rev. B 53 (1996) 116. [3] R. Scharfschwerdt, A. Mazur, O.F. Schirmer, H. Hesse, S. Mendricks, Phys. Rev. B 54 (1996) 15 284. [4] O.F. Schirmer, H.-J. Reyher, M. WoÈhlecke, in: F. AgulloÂLoÂpez (Ed.), Insulating Materials for Optoelectronics, World Scienti®c, Singapore, 1995 (chap. 4). [5] E. Possenriede, H. KroÈse, T. Varnhorst, R. Scharfschwerdt, O.F. Schirmer, Ferroelectrics 151 (1994) 199. [6] S. KoÈhne, O.F. Schirmer, H. Hesse, T.W. Kool, V. Vikhnin, J. Superconductivity 12 (1999) 193. [7] P. GuÈnter, J.P. Huignard, Photorefractive materials and their applications, Topics Appl. Phys. 61 (1988) 62. [8] R. Scharfaschwerdt nad, O.F. Schirmer, H. Hesse, D. Rytz, Appl. Phys. B 68 (1999) 807. [9] O.F. Schirmer, K.A. MuÈller, Phys. Rev. B 7 (1973) 2986. [10] H. Unoki, T. Sakudo, J. Phys. Soc. Jpn 23 (1967) 546. [11] Th. Von Waldkirch, K.A. MuÈller, W. Berlinger, Phys. Rev. B 5 (1972) 4324. [12] P.P.J. Van Engelen, J.C.M. Henning, Phys. Lett. 25A (1967) 733. [13] H.J. de Jong, M. Glasbeek, Solid State Commun. 28 (1978) 683. [14] Th.W. Kool, M. Glasbeek, J. Phys.: Condens. Matter 3 (1991) 9747. [15] Th.W. Kool, H.J. de Jong, M. Glasbeek, J. Phys.: Condens. Matter 6 (1994) 1571. [16] L.D. Landau, E.M. Lifshitz, The Theory of Elasticity, Pergamon Press, Oxford, 1975, p. 12. [17] Th.W. Kool, M. Glasbeek, J. Phys.: Condens. Matter 5 (1993) 361. [18] K.A. MuÈller, W. Berlinger, R.S. Rubins, Phys. Rev. 186 (1969) 361. [19] D.M. Hannon, Phys. Rev. 164 (1967) 366. [20] S. Lenjer, PhD thesis, University of OsnabruÈck, 1999.
Solid State Communications 116 (2000) 133±136
www.elsevier.com/locate/ssc
An off-center ion near a Ba site in BaTiO3 as studied by EPR under uniaxial stress S. Lenjer a, R. Scharfschwerdt a, Th.W. Kool b, O.F. Schirmer a,* b
a Fachbereich Physik, University of OsnabruÈck, D-49069 OsnabruÈck, Germany Laboratory for Physical Chemistry, University of Amsterdam, NL-1018 Amsterdam, Netherlands
Received 8 May 2000; received in revised form 10 July 2000; accepted 10 July 2000 by J. Kuhl
Abstract In BaTiO3 a paramagnetic defect is identi®ed by EPR which is attributed to a paramagnetic S 1=2 ion off-center near a Ba site. Twelve magnetically distinguishable sites contribute to the angular dependence pattern of the spectra, each of the sites being characterized by an orthorhombic g-tensor. The relative occupations of the sites change under uniaxial stress in a manner consistent with the model. The features of the center are highly characteristic for the indicated off-center position, as revealed by comparison with the previously identi®ed Ti 31 near a Sr 21 site in SrTiO 3. On the basis of the principal values of the g-tensor the paramagnetic ion can be identi®ed with Ni. It is discussed that Ni 11 is the most likely charge state. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Ferroelectrics; C. Paint defects; E. Electron paramagnetic resonance PACS: 71.70.Ch; 76.30.Fc; 77.84.Dy
1. Introduction
2. Experimental results
This communication is part of an extended effort to gather information on the possible defect structures in BaTiO3 [1± 6]. If doped adequately, such crystals show high photorefractive sensitivities [7]. Since the photorefractive effect in inorganic compounds is usually triggered by the photoionisation of defects, it is advisable to have at hand a large stock of possible lattice perturbations for tailoring the properties of a material or for understanding its photorefractive properties [7]. EPR is a powerful tool in identifying impurities and intrinsic defects with unpaired electrons. Here we report the EPR identi®cation of a paramagnetic ion, substituting for Ba 21 and going off-center near such a site. The assignment is also based on the change of the relative line intensities under uniaxial stress; on this way a linear stress coupling coef®cient b 3:8 £ 10230 m3 could be determined. Several arguments support the identi®cation of the ion as Ni 11.
The defect treated here was detected by EPR in BaTiO3 crystals, which had been grown from melts containing 10 000 ppm Na and 1000 ppm Rh [8], after strong oxidation (heating at 9008C under 20 £ 105 Pa of pure oxygen for 2 h and fast quenching). The observations were made at n 9:11 GHz in the rhombohedral low temperature phase of BaTiO3 T , 180 K: The signals could be detected at temperatures between 4.2 K up to about 40 K. As determined from the simultaneous measurement of an EPR standard, the corresponding defect had a concentration of about 20 ppm. Crystals of the typical size 2 £ 2 £ 3 mm3 were used. We tend to attribute the center, characterized by a spin S 1=2 and an orthorhombic g-tensor, to Ni, apparently being present as a background impurity in the specimens. Fig. 1a shows the angular dependence of the g-values of this center with the magnetic ®eld B rotating, for example, in a (100) plane. Assuming that BaTiO3 can be considered as cubic, neglecting its slight low temperature rhombohedral distortion, the experimental data can be explained on the basis of a superposition of 12 symmetry related equivalent centers as indicated in Fig. 1b. Each of the branches in this ®gure is described by a Hamiltonian consisting of only a Zeeman effect with an orthorhombic
* Corresponding author. Fax: 149-541-969-2670. E-mail address: [email protected] (O.F. Schirmer).
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00307-0
134
S. Lenjer et al. / Solid State Communications 116 (2000) 133±136
Fig. 1. Angular dependence of the EPR lines of the Ni center with B oriented in a (001) plane, T 15 K: At B k [100] three EPR lines, 1±3 are observed, typical for orthorhombic symmetry. The small splittings of the EPR lines with B in an arbitrary direction result from the slight tilting of the g-tensor principal axes. The lines in Fig. 1a are the calculated line positions, as based on the superimposed Zeeman effects of 12 equivalent centers as indicated in (b). There the orthorhombic g-tensors are symbolized by cuboids. A representative one (hatched) among the 12 equivalent tensors indicates that there is a slight tilting around the [001] axis by t 3:08: The tensors are grouped as indicated by the Roman numbers. Fig. 1a shows how they contribute to the resonances.
g-tensor, H mB Bg B; with S 1=2: The g-tensor has the principal values gx 2:36 ^ 0:01; gy 2:16 ^ 0:01 and gz 2:24 ^ 0:01: It is symbolized by the hatched cuboid in Fig. 1b. The x-axis of the representative tensor approximately extends along a [100] direction, y approximately along [010] and z along [001], respectively (Fig. 1b). There is a slight tilting of the x-axis by t 3:0 ^ 0:18 towards a negative [010] direction; the y-axis is turned by the same amount towards the [100] direction. The total angular dependence (Fig. 1a) is reproduced by the superposition of the 12 g-tensors (Fig. 1b), resulting from the representative one by the symmetry operations of the cubic group. For B along one of the k100l type directions three EPR lines are observed, to which the three groups of centers-I, II and III (Fig. 1b)-contribute in the way indicated at the right of Fig. 1a. As shown in the EPR spectrum of Fig. 2, at 15 K the intensities of the resonances change under an externally
Fig. 2. Change of the EPR spectrum of Ni for B k [100] under the in¯uence of an externally applied uniaxial stress. The small satellite near line 3 is due to a different defect.
applied stress Pmax 0:08 GPa: Fig. 3 shows the variations of the intensity ratio of the different indicated signals under the in¯uence of this stress. As will be explained below, the changes are due to a rearrangement of the ions in such a way that the moveable off-center ion aligns along the stress direction. 3. Discussion The angular dependence pattern in Fig. 1a is highly speci®c for a small paramagnetic ion, replacing an A site ion in an ABO3 perovskite crystal. This was established by Schirmer and MuÈller [9], who identi®ed a topologically identical pattern for Ti 31 (d 1) ions in SrTiO3 sitting near a Sr 21 site off-center along one of the k100l type directions; the g-tensor axes are slightly tilted by t 2:58 in a way identical to that indicated in Fig. 1b. The tilting angle did not change by altering the temperature below the phase transition temperature Tc < 105 K: Because of the tilting this center shows orthorhombic symmetry in the low temperature phase of SrTiO3 [10,11]. The center could be produced by irradiation with fast neutrons [9] or by a strong reduction treatment of undoped SrTiO3 [12]. The off-center movement of Ti 31 was attributed to its small radius, being lower than that of the replaced Sr 21. Ti 31 is shifted along one of the cubic axes and is tilted away towards one of the 12 O 22. When applying a static electric ®eld it was observed that a Ti 31 off-center ion is aligned at 4.2 K along the ®eld direction. This alignment excludes the presence of a nearby defect or associated impurity ion as demonstrated in earlier static electric ®eld experiments also for the Cr 51 and V 41 impurity ions in SrTiO3 [13±15].
S. Lenjer et al. / Solid State Communications 116 (2000) 133±136
Fig. 3. Plot of in ln(I2/I1) versus external uniaxial stress P applied along a k100l type direction, T 15 K: The solid line represents the best ®t of Eq. (1) to the experimental points with a linear stress coupling coef®cient b 3:8 £ 10230 m3 : The curve in the ®gure is slightly shifted (by 20.23) to negative values because of a prealignment of the centers probably by internal strains.
Because the angular plot of the center treated here (Fig. 1a) is rather unusual and has the same structure as that of SrTiO3:Ti 31, we can attribute it to a similar microscopic model: A comparatively small paramagnetic ion sitting off-center near a Ba-site in BaTiO3. As discussed below, the ion is likely to be identi®ed with Ni 11. The measurements performed under uniaxial stress support this model: If applied along the [001] direction, such stress leads to the intensity changes shown in Fig. 2, observed at 15 K. The fact, that a corresponding reorientation occurs at all at such low temperatures again excludes the presence of a nearby defect or an associated impurity. It is seen that under the indicated stress the concentration of centers of type III decreases as compared with the sum of centers I and II. This means, on the other hand, that ions going off-center along the stress axis, i.e. centers of type I, are favored; their energy is lowered, and thus their concentration increased under applied stress. As seen from Fig. 1b, the concentration of centers II, likewise oriented perpendicular to the stress axis, is then alsoÐas the type III centersÐexpected to decrease. The sum of I and II, however, increases as compared to III, Fig. 2. The linewidths do not change and the positions do not shift under stress. Therefore, it is justi®ed to use only the heights of the EPR lines as measures for the center concentrations, as in the following evaluation. Quantitatively, the alignment under stress is treated in this way: Neglecting the small tiltings, there are six equivalent k100l type off-center pocket states. This equivalence is lifted by external stress, leading to twofold degeneracy for the pockets extending along the stress direction and fourfold for those perpendicular to the stress. For stress P along the [001] direction, the ®rst class contains centers I (Fig. 1b), the second one centers II and III. The energy difference between both situations is given by [16] DU bP; where b is the appropriate linear stress coupling coef®cient and P, the magnitude of the stress; for details see Ref. [2]. As a quantity which can easily be compared with experiment we use the relation I2 =I1 II 1 III =IIII ; indicating in an
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obvious manner the concentrations of the off-center ion in the various pocket states. As seen from Fig. 1a, IIII is proportional to the intensity of line 1 in Fig. 2, (II 1 III) to the sum of lines 2 and 3, if B is applied along [100], perpendicular to the stress direction, [001]. Assuming that the pocket I has the lowest energy and concentration n1, the concentrations of centers II and III relative to it are n1 exp 2bP=kT: We thus expect: I2 11a 2bP a exp : 1 2a kT I1 This is con®rmed by Fig. 3: the stress dependence of the experimental ratios I2/I1 is well reproduced using b 3:8 £ 10230 m3 : This value is of the same order as that found for the Fe 21 ±O 2 hole center in SrTiO3 [17]. According to Eq. (1) ln(I2/I1) should cross the ordinate at zero in Fig. 3. The slight downward shift by about 20.2 can be attributed to a small prealignment due to internal strains. Next we want to address the identi®cation of the offcenter ion. Although the present defect appeared in crystals intentionally doped with Rh and Na, the discussed EPR spectra cannot be assigned to either of these ions. The spectra of EPR-active charge states of Rh in BaTiO3, Rh 41 and Rh 21, have been studied in detail previously [1], both showing features quite different from those found here. Rh spectra are especially distinguished by their hyper®ne splitting into two components, caused by interaction with the I 1=2 nuclear spin of the 100% abundant isotope 103Rh. The presence of Na is only revealed after illumination of the crystals, which leads to hole capture at an O22 ion next to Na 1 [2]. So the spectra reported here must be attributed to a background impurity. Its mean g-value, 2.25, is typical for the paramagnetic charge states of nickel, Ni 31, Ni 21 or Ni 11, if Ni 31 is considered in its strong crystal ®eld (low spin (LS)) case; the high spin (HS) con®guration, occuring in weak crystal ®elds, has a de®nite different signature, see below. Except for this case the above ions are notoriously dif®cult to distinguish, their mean g-values for distorted octahedral surroundings lying together rather closely in the range 2.17 to 2.29, see Table 1 in Ref. [18]. Only Cu 21 ions have similar values, but these are characterized by their typical hyper®ne splittings into four lines, caused by the 63,65Cu isotopes I 3=2; 100% abundant together. Therefore we tend to attribute the investigated paramagnetic ion to Ni. Also this element has an I 3=2 isotope, 61Ni, which however occurs only in 1.13% of all cases. Since each of the expected four hyper®ne structure lines has only an intensity of 0.3% of the central, I 0; line they are hard to detect with the available signal to noise ratio (Fig. 2). Among the conceivable Ni charge states, Ni 21 can be disregarded, because it has a S 1 ground state, in contrast to the identi®ed S 1=2: Ni 21 would furthermore lead to rather wide Dms ^1 transitions, not of the type observed. Also LS±Ni 31 can be discarded since Ni31 is known to occur in its HS state when sitting near an A site of an ABO3 oxide perovskite, as has been found for Ni 31 in KTaO3 by Hannon
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[19]. The axiality of this center can best be explained by assuming that it also goes off-center; it has the characteristic g-values gk 2:22 and g' 4:42: These result from an 4A2 orbital ground state, appropriate for the 3d 7 Ni 31 ion in the weak cubic crystal ®eld at the twelvefold coordinated K-site, to which an axial ®eld due to the off-center movement is superimposed. The fact that g' < 2gk is typical for a S 3=2 system, such as HS±Ni 31, with an axial splitting large compared to the microwave quantum. Since the resonances in the present case are caused-according to the arguments given so far-by a Ni ion near the A site ion, Ba, in BaTiO3, it should have similar g-values, if occurring as Ni 31. The present defect shows quite different values; thus Ni 31 can be excluded, and we tend to ascribe the center to Ni 11. Its spin is S 1=2; as identi®ed. The observation of Ni 11 on a Ti site in BaTiO3 [20] with the same mean g-value as in the present case is consistent with this assignment. The ground state of the Ni 11 ion presented in this paper apparently is stabilized by interaction with essentially the four O22 ions which Ni 11 approaches by its off-center movement along a k100l type direction. This is supported by the fact that the Ni 11 ions align towards those pockets lying along the stress axis. Under stress the distance between Ni 11 and the four O22 ions along the stress direction is shortened. Which orbital is lowest is dif®cult to predict without knowing the strength of the axial crystal ®eld. If it is weak, the orbital sequence is determined by the cubic potential at the dodecahedral Ba position. Its sign is the same as that of a tetrahedral crystal ®eld, leading to a T2type (xy, yz, zx) hole ground state. If, on the other hand, the axial ®eld is strong, an E-type orbital x2 2 y2 or 3z2 2 r 2 will be lowest, neglecting small admixtures compatible with the orthorhombic symmetry. This orthorhombicity of the center is probably caused by a slightly closer attachment of Ni to one of its O22 neighbors. Such a feature was established as the reason for the orthorhombicity in the analogous case of off-center Ti 31 in SrTiO3, where the low symmetry of the center depends on a slight inequivalence of the four O22 ions, driven by the structural phase transition of the material [9]. In the present case the orthorhombicity might be caused by the inequivalence of the four O22 ions due to the small trigonal distortion of BaTiO3 in its low-temperature rhombohedral phase. Such an inequivalence is absent in the case of KTaO3, which is cubic at all temperatures. This appears to be the reason why off-center Ni 31 in KTaO3 is axially symmetric. A similar situation, met in SrTiO3 above its 105 K phase transition temperature, supports this conclusion: There the EPR spectrum of off-center Ti 31 is axially symmetric, since SrTiO3 in this crystal phase is cubic. 4. Summary We have shown that the observed resonances are caused by a paramagnetic S 1=2 ion, off-center near a Ba site in
BaTiO3. The interpretation of the EPR data, especially those obtained under application of uniaxial stress, shows that the ion is shifted essentially along a k100l type direction and aligns along the stress axis. The attachment of the ion to one of its four surrounding O22 ions appears to be the reason for the orthorhombicity observed in the present case. A detailed discussion shows that the paramagnetic ion in question is likely to be Ni; all observations support to the assumption that the charge state is Ni 11. This conclusion is consistent with the properties of the off-center ions Ni 31 (low spin) in KTaO3 and Ti 31 in SrTiO3. Acknowledgements We thank Dr H. Hesse for the supply of the crystals and M. Meyer for supplementary measurements. The research reported here was supported by DFG, Sonderforschungsbereich 225. References [1] E. Possenriede, P. Jacobs, O.F. Schirmer, J. Phys.: Condens. Matter 4 (1992) 4719. [2] T. Varnhorst, O.F. Schirmer, H. KroÈse, R. Scharfschwerdt, T.W. Kool, Phys. Rev. B 53 (1996) 116. [3] R. Scharfschwerdt, A. Mazur, O.F. Schirmer, H. Hesse, S. Mendricks, Phys. Rev. B 54 (1996) 15 284. [4] O.F. Schirmer, H.-J. Reyher, M. WoÈhlecke, in: F. AgulloÂLoÂpez (Ed.), Insulating Materials for Optoelectronics, World Scienti®c, Singapore, 1995 (chap. 4). [5] E. Possenriede, H. KroÈse, T. Varnhorst, R. Scharfschwerdt, O.F. Schirmer, Ferroelectrics 151 (1994) 199. [6] S. KoÈhne, O.F. Schirmer, H. Hesse, T.W. Kool, V. Vikhnin, J. Superconductivity 12 (1999) 193. [7] P. GuÈnter, J.P. Huignard, Photorefractive materials and their applications, Topics Appl. Phys. 61 (1988) 62. [8] R. Scharfaschwerdt nad, O.F. Schirmer, H. Hesse, D. Rytz, Appl. Phys. B 68 (1999) 807. [9] O.F. Schirmer, K.A. MuÈller, Phys. Rev. B 7 (1973) 2986. [10] H. Unoki, T. Sakudo, J. Phys. Soc. Jpn 23 (1967) 546. [11] Th. Von Waldkirch, K.A. MuÈller, W. Berlinger, Phys. Rev. B 5 (1972) 4324. [12] P.P.J. Van Engelen, J.C.M. Henning, Phys. Lett. 25A (1967) 733. [13] H.J. de Jong, M. Glasbeek, Solid State Commun. 28 (1978) 683. [14] Th.W. Kool, M. Glasbeek, J. Phys.: Condens. Matter 3 (1991) 9747. [15] Th.W. Kool, H.J. de Jong, M. Glasbeek, J. Phys.: Condens. Matter 6 (1994) 1571. [16] L.D. Landau, E.M. Lifshitz, The Theory of Elasticity, Pergamon Press, Oxford, 1975, p. 12. [17] Th.W. Kool, M. Glasbeek, J. Phys.: Condens. Matter 5 (1993) 361. [18] K.A. MuÈller, W. Berlinger, R.S. Rubins, Phys. Rev. 186 (1969) 361. [19] D.M. Hannon, Phys. Rev. 164 (1967) 366. [20] S. Lenjer, PhD thesis, University of OsnabruÈck, 1999.