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111Cd perturbed angular correlations
Journal of Physics and Chemistry of Solids 60 (1999) 749–757
Hyperfine interactions in the perovskites SrHfO3 and BaHfO3 observed with 111In/ 111Cd perturbed angular correlations P. de la Presa a, R.E. Alonso a, A. Ayala a,1, S. Habenicht b, V.V Krishnamurthy b,2, K.P. Lieb b,*, A. Lo´pez Garcı´a b,3, M. Neubauer b, M. Uhrmacher b a
Programa TENAES, Departamento de Fı´sica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina b II. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstr. 7-9, D-37073 Go¨ttingen, Germany Received 7 July 1998; accepted 1 December 1998
Abstract The electric field gradients in perovskite SrHfO3 and BaHfO3 powder samples were investigated by means of Perturbed Angular Correlation spectroscopy, using implanted 111In hyperfine probes. The measurements cover the temperature ranges from 22 to 893 K for SrHfO3 and from 293 to 875 K for BaHfO3, respectively. In both compounds, three quadrupole interactions were established. The largest fraction showing a pronounced dynamic interaction is assigned to 111In/ 111Cd probe atoms on substitutional Hf sites in both compounds. For SrHfO3, the temperature dependence of the dynamic interaction was associated to the Pnma $ Imma phase transition for SrHfO3 at about 700 K. We discuss the results in relation to those obtained for 111 In/ 111Cd-probes in PbZrO3 and BaTiO3, and for 181Hf/ 181Ta-probes in AHfO3 (A Ca, Sr, Ba). 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Perturbed angular correlation spectroscopy
1. Introduction The perovskite oxides ABO3, with a transition metal on their B-sites, display a wide variety of complex structural instabilities and electronic properties, depending on the combination of A and B cations [1,2]. Owing to their varied structure and composition, these materials have attracted intense interest in many applied and fundamental areas of solid-state science and advanced materials research. The main characteristic of these compounds is that small changes in the
* Corresponding author. Tel.: ⫹49-551-397631; fax: ⫹49-551394493. E-mail address: [email protected] (K.P. Lieb) 1 Present address: Departamento de Fı´sica, Universidade Federal do Ceara´, CP 6030, 60455-760 Fortaleza, CE Brazil. 2 Present address: Muon Science Laboratory, RIKEN, Wako Saitama 351-01, Japan. 3 Permanent address: Programa TENAES, Departamento de Fı´sica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina.
structural parameters introduce dramatic changes in the electrical properties. Then, if the appropriate nuclear probe is used, Perturbed Angular Correlation (PAC) spectroscopy is a suitable technique to study these changes because they depend on the electron density around the probe nucleus. In the past decades, SrHfO3 was reported as cubic [3] or pseudocubic [4] and non-systematic studies on structural phase transitions were performed till date. Guevara et al. [5] have recently carried out the first neutron diffraction experiment determining the orthorhombic structure (Pnma) at room temperature. Cuffini et al. [6] have measured, by X-ray diffraction, the temperature dependence of the lattice constant at higher temperatures, establishing two structural phase transitions. These experiments were inspired by the discrepancy between the results from Xray diffraction (XRD) and PAC experiments at RT: a cubic structure of the compound was deduced from XRD, while the observation of a well-defined static electric field gradient (EFG) [7] in the PAC measurements is not compatible with a perfect cubic perovskite lattice. Previous PAC studies on the electric quadrupole
0022-3697/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00344-8
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interactions of Ta-probe atoms in AHfO3 (A Ca, Sr or Ba) perovskites were carried out by several authors [7–9]. More recently, we have initiated a systematic investigation on the role of A-atoms in these materials. The PAC experiments on CaHfO3, SrHfO3 and BaHfO3 have allowed us to characterize the temperature dependence of the EFG tensors in temperature regions where phase transitions occur [10– 12]. In oxides, the EFGs are particularly sensitive to small local changes of the positions and/or charges of the ions in the immediate vicinity of the probe atoms. In the three hafnates studied with the 181Ta probe, welldefined or broadly distributed EFGs as well as relaxation phenomena were detected. The different behavior of the hyperfine interactions in the three compounds was interpreted in terms of the Hf–O bonds and correlated to the A–O bonds [13]. Contrary to what was expected in purely ionic compounds, the changes observed in the quadrupole parameters indicate that there exists an important contribution of the A cations to the electron density and lattice dynamics. In the present work, we address the question: What changes of the quadrupole interactions are introduced by a different impurity species sitting at the B-site? For this purpose, we used the hyperfine probe 111In/ 111Cd and performed PAC spectroscopy on polycrystalline SrHfO3 and BaHfO3 perovskites. For many binary and ternary oxides, this nuclear probe was used to elucidate charge transfer and defect reactions and to systematically measure the EFG as function of the oxygen coordination and ion valency. In this way, it should be possible to learn how nominally 2 ⫹ cations (5s-, 5p-states) on the B-site affect the electron density in these perovskites, keeping in mind that the quadrupole interaction reflects the coupling of these impurity states to mostly 2p-oxygen orbitals. 181
2. Experimental procedure and data analysis Both types of samples were prepared by solid-state reactions as described in Ref. [14], using stoichiometric mixtures of carbonates and oxides of the corresponding cations. Then the as-prepared perovskites were analyzed by X-ray diffraction, using the Rietveld procedure; the reported lattice constants in cubic BaHfO3 were reproduced [14]. Concerning SrHfO3, we refer to the recent neutron diffraction analysis by Guevara et al. [5]. For the 111In implantation, pills of 8 mm diameter and 0.5 mm thickness were prepared under a pressure of 0.5 kbar. The samples were implanted at RT using the Go¨ttingen ion implanter IONAS [15,16] at a total dose of approximately 10 12 radioactive 111In ions and an implantation energy of 400 keV. In the first and second implanted SrHfO3 samples, the annealing process was studied (described later). After a 4-h heating at 1673 K in air, no changes in the PAC spectra were observed. For that reason, all the other samples were annealed by this procedure.
The PAC spectroscopy was performed using four NaI(Tl) detectors in standard 90⬚ geometry. The apparatus has a time resolution of about 3.5 ns for the 171–245 keV g – g cascade in 111Cd, involving the 5/2 ⫹ intermediate state (mean life t 122 ns, quadrupole moment Q 0.83 b [17]) and fed in the EC-decay of 111In. Depending on the 111 In activity left in the sample, each measurement took between 12 and 48 h. The samples were heated in-situ, either in high vacuum (⬇10 ⫺5 mbar) or in air. The 22 K data in SrHfO3 were taken by attaching the sample to the tip of a closed-cycle helium cryostat mounted in a high-vacuum chamber. From the coincidence counting rates N(u ,t) between detectors separated by the angle u 90⬚ or 180⬚, the experimental perturbation functions [18], R t 2
N 180⬚; t ⫺ N 90⬚; t ; N 180⬚; t ⫹ 2N 90⬚; t
were calculated and fitted to the expression X i R t A22 fi G22 t ⫹ C;
1
2
i
taking into account the finite time resolution of the PAC spectrometer. As usual, A22 denotes the anisotropy of the g – g correlation, C is a time-independent baseline shift, fi is the fraction of probes at the site i, and Gi22 t is the corresponding perturbation factor. Based on the recent measurements of 181Ta in SrHfO3, we have used an approximate expression for G22 t that is able to describe static and/or fluctuating quadrupole interactions [19] G22 t exp ⫺l2 t
3 X
s2n h cos vn ht exp ⫺ dvn t2 =2:
n0
3 This formula refers to a Gaussian EFG distribution having the mean quadrupole coupling constant vQ peQVzz =20h and relative width d , and the constant asymmetry parameter h Vxx ⫺ Vyy =Vzz ; Vzz being the largest component of the EFG tensor. The frequencies vn h and amplitudes s2n h depend on the magnitude and asymmetry of the EFG [18,20]. The relaxation factor exp ⫺l2 t describes fluctuations between the various EFGs within the distribution and, in this way, introduces another damping mechanism into the perturbation function. By setting either the width d or the relaxation rate l 2 to zero, a relaxation mechanism of a welldefined EFG according to the Abragam–Pound model (see [21]), or a static distribution of EFGs can be modeled. As in many oxides, the occurrence of intrinsic structural and electronic defects, phase transitions, and several possible impurity sites can introduce difficulties in the fitting of the measured PAC spectra. It was only through the measurements in several samples and a careful, iterative fitting procedure that consistent sets of EFG parameters could be deduced in the present analyses. In particular, we have verified that the two limiting cases of a purely static EFG
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distribution (either of Gaussian or Lorentzian type) or of a purely statistical nature (d 0) gave fits of less good quality, in particular in the time region between 150 and 350 ns (described later).
3. Results
Fig. 1. Three perturbation functions for 111Cd probes in SrHfO3, taken after 111In ion implantation into polycrystalline samples and 4 h annealings at 1673 K.
Fig. 1 shows the perturbation functions R(t) taken at 293, 693, and 893 K for SrHfO3; the deduced hyperfine parameters at these temperatures and at 22 K are listed in Table 1. At all temperatures, three fractions fi were required to describe the PAC data in SrHfO3. The largest fraction, f1 45%–70%, is characterized by a low quadrupole frequency of v Q1 3.4 Mrad/s and an asymmetry parameter of h 1 0.51 at RT. The evolution of the hyperfine parameters of this fraction is illustrated in Fig. 2. One notes a gradual decrease of v Q1 and increase of h 1 with the temperature. The parameters d 1 and l 2, which account for the damping of fraction 1, develop from a distribution of static EFGs at 22 K (d 1 26%, l 2 0) to a fluctuating regime at 793 K (d 1 0, l 2 8 MHz). At this point, we would like to emphasize again that the broad maximum seen in all PAC spectra in the time interval between 150 and 350 ns essentially determines the variation of the parameters d 1 and l 2 and does not allow one to set either of them to zero over the full temperature range studied. To illustrate this, in Fig. 3 two fits of a PAC
Table 1 Hyperfine interaction parameters in SrHfO3 and BaHfO3 powder samples measured with annealing at 1673 K T (K) SrHfO3 22
293
693
893
BaHfO3 293
375
875
111
Cd probes after
111
In-ion implantation and 4 h
Fraction i
fi (%)
v Q (Mrad/s)
h
d (%)
l 2 (Mhz)
1 2 3 1 2 3 1 2 3 1 2 3
60(4) 27(3) 13(2) 52(4) 36(4) 12(2) 65(4) 9(2) 26(4) 63(3) 17(3) 20(2)
3.8(2) 18(1) 37(1) 3.4(2) 18(1) 37(1) 2.4(2) 18(1) 38(1) 2.0(2) 17(1) 36(1)
0.48(5) 0.40(5) 0.19(2) 0.51(6) 0.22(2) 0.16(3) 0.85(15) 0.30(7) 0.08(3) 0.85(15) 0.25(7) 0
26(3) 19(6) 6(2) 13(1) 38(12) 6(3) 7(3) 15(2) 4(1) — 28(10) 4(1)
— — — 2.1(4) — — 8.0(8) — — 3.6(7) — —
1 2 3 1 2 3 1 2 3
64(2) 19(2) 17(2) 70(2) 10(1) 20(2) 76(3) 5(1) 19(3)
0 15(1) 37(1) 0 16(1) 39(1) 0 18(1) 33(2)
0 0.54(4) 0.09(3) 0 0.34(7) 0.09(3) 0 0 0
0 13(1) 8(1) 0 13(1) 5(1) 0 4(1) 24(3)
37(2) — — 24(2) — — 12(1) — —
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Fig. 2. Variation of the hyperfine parameters of the predominant fraction EFG1 in SrHfO3. The dotted line indicates the region of the phase transition.
spectrum measured at 653 K are shown: one of them was obtained by fitting Eq. (2) (middle) and the other one from a pure static Gaussian distribution fit (bottom). Clearly, the first fit is better than the second one, although there is no great difference in the total x 2 values. The small difference in x 2 is because of the fact that the statistical errors of the experimental points are larger at long coincidence times, just in the range where the differences between the two fitting functions become significant. In Fig. 4, three perturbation functions R(t) are displayed that show different stages of the annealing process of a SrHfO3 sample after 111In-ion implantation: at RT, the asimplanted sample presents a typical PAC spectrum of a highly disordered material. The strongly damped spectrum can be fitted using the hyperfine parameters v Q 33 Mrad/ s, h 0.6 and d 45%. At 900 K, two EFG components can be distinguished, characterized by a large fraction (⬇82%), which again has a broad distribution of quadrupole frequencies (v Q 13 Mrad/s, h 0.60, d ⬇ 63%), and the remaining 18% fraction having a narrower distribution (v Q 37 Mrad/s, h 0.10, d 13%). The latter is similar to that of fraction 3 in the fully annealed samples. In the last annealing step, the sample was heated at 1273 K in air during 4 h and the perturbation was then measured at
Fig. 3. Experimental PAC spectra for SrHfO3 at 653 K (top) and two different fits: the fit shown in the middle was obtained by Eq. (3) and the one shown at the bottom by using a static Gaussian distribution function. The corresponding hyperfine parameters of fraction 1 are also indicated.
900 K. Now, the three fractions already discussed before were found. A further 4-h annealing at 1673 K reduced the EFGs relative distribution widths, but did not change the v Q values. The PAC spectra for the BaHfO3 compound were measured at 293, 375, 425, 474, 526, 574, 775, and 875 K, after 4-h annealings of the samples in air at 1673 K had been performed. Fig. 5 illustrates the evolution of the perturbation functions, while Table 1 summarizes the deduced quadrupole parameters measured at the corresponding temperatures. Again, one discerns three EFG contributions whose fractions stay essentially constant over the full temperature range. The predominant EFG1, having a fraction of f1 65%–80%, is characterized by a cubic environment (v Q1 h 1 0), but shows a strong relaxation at 300– 530 K. As shown in Fig. 6, the relaxation rate l 2 drops from 37 to 13 MHz in this temperature range and then stays constant up to 875 K. The second EFG has, on the average, a much smaller fraction of f2 ⬇ 13% and is characterized by a frequency distribution centered at v Q2 ⬇ 17 Mrad/s and having the width d 2 ⬇ 13% and asymmetry parameter h 2 ⬇ 0.3. Finally, the third contribution to the perturbation functions
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Fig. 4. Evolution of the measured and fitted perturbations during the annealing process of SrHfO3. The spectra refer to the as-implanted state (top), to a 4 h annealing at 900 K (middle), and to a 4 h annealing at 1273 K followed by the PAC measurement at 900 K (bottom).
753
Fig. 6. Temperature variation of the fluctuation rate l 2 of the substitutional fraction: (a) plotted versus T; (b) plotted logarithmically versus 1/T to deduce the activation energy Ea.
is the well-defined, symmetric EFG3, having v Q3 ⬇ 38 Mrad/s and d 3 ⬇ 5%. Evidently, the hyperfine parameters of fractions 2 and 3 in BaHfO3 are very similar to those of the corresponding fractions in SrHfO3.
4. Discussion 4.1. Substitutional site
Fig. 5. Perturbation functions for various temperatures.
111
Cd probes in BaHfO3 taken at
One of the important goals of PAC experiments is to identify those parts of the hyperfine perturbation to the attributed to probe atoms on substitutional, defect-free lattice sites. In the case of 181Hf/ 118Ta probes in hafnium compounds, this task is simple, because the probes are usually introduced via thermal neutron capture, leaving them on their ‘‘natural’’ sites. In the case of ion-implanted 111 In, the identification can pose problems. Criteria of the symmetry and magnitude of the EFG were applied to identify – or at least to make plausible – the corresponding fraction(s) in the perturbation function. In ionic oxides, the estimation of the EFG with a point charge model (PCM) was very useful to determine the corresponding site of the probe, as long as the probe ions do not distort the local structure. In the following, we argue that fraction 1 should be attributed to the B-site in the hafnates. Table 2 lists the measured quadrupole parameters of the
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Table 2 Comparison of quadrupolar interaction measured with 181Ta and 111Cd probes in SrHfO3, PbZrO3 and BaTiO3 perovskites and in HfO2 and ZrO2 at RT Compound/structure
SrHfO3 orthor. PbZrO3 orthor. BaTiO3 tetrag. HfO2 monocl. ZrO2 monocl. Point charge model a
181
111
Ta
v Q (Mrad/s) 20.4(6) 82(1) 32.2(9) 123(1)) 113(1)
h 0.45(2) 0.84 0.15(5) 0.34(3) 0.33
The large error of the frequency ratio is owing to that of the
v Q (Mrad/s) 3.4(2) 14.6(8) 4.9(5) 17.4(3) 17.4(3) 111
two probe atoms in the perovskites SrHfO3, PbZrO3, and BaTiO3 in the orthorhombic or tetragonal phase. When comparing the v Q-values for 181Ta and 111Cd in SrHfO3, their ratio at RT is v Q(Ta)/v Q(Cd) 6.0(3), consistent with the ratio of 6.5(12) expected from the scaling of the quadrupole moments and Sternheimer factors of both probes [17,21]. Further, the asymmetry parameters for the two probes agree very well with each other (h ⬇ 0.5). For orthorhombic PbZrO3 and tetragonal BaTiO3 at RT, the measured quadrupole parameters give frequency ratios of v Q(Ta)/v Q(Cd) 6.6(8) and 5.6(3), respectively, and again the asymmetry parameters are in good agreement for both nuclear probes [8,22–24]. We finally mention the recent PAC study of Luthin et al. [25] in monoclinic HfO2 and ZrO2, in which the frequency ratios v Q(Ta)/v Q(Cd) 7.0(2) in HfO2 and 6.5(2) in ZrO2 were deduced. In the cubic phase of BaHfO3, present over the whole temperature range considered, the interpretation of fraction 1 as corresponding to substitutional sites is, of course, based on the vanishing EFG. Another strong argument for the substitution of 111In into the B-site is based on the findings in many binary and ternary oxides in which the 111In-probes, after ion implantation and thermal annealing, have substituted into sites that have octahedal oxygen coordinations similar to the structure of In2O3 [26]. The PCM was proved to be reasonably successful in reproducing EFGs for ionic compounds [27–29]. If the covalency increases, the calculated values of the quadrupolar frequency begins to deviate from the experimental ones [38]. In contrast, the values of the asymmetry parameter h often agree with the PCM predictions, i.e. whenever the cation–oxygen bond length exceeds ˚ [26–28]. The corresponding bond lengths dM – O ⬇ 2 A for SrHfO3 calculated from the atomic positions [5] are thus exceeding the dM⫺OI 2:13 A and dM⫺OII 2:05 A, critical value. In our case, PCM gives v Q 74.5 Mrad/s, which strongly deviates from the measured value, v Q1 3.4(2) Mrad/s. However, the calculated value, h 0.46, is in good agreement with the experimental one, h 1 0.51(6). Based on these arguments, we
v Q(Ta)/ v Q(Cd)
Cd
h 0.51(6) ⬇1 0.25(6) 0.60(3) 0.64(4)
6.0(3) 5.6(3) 6.6(8) 7.1(2) 6.5(2) 6.5(12) a
Cd quadrupole moment.
tentatively assign fraction 1 to 111Cd probes substituted on the B-site. Although we cannot explain the origin of the other two sites, we would like to remark that they have very similar EFGs and thermal behavior in both compounds, indicating that they are independent of the presence of A cations. It is also interesting to note that they appear in the as-implanted samples or after the 900 K heating (see Fig. 4), remaining at higher temperatures. 4.2. SrHfO3 SrHfO3 has a simple perovskite structure, which was reported as cubic or pseudocubic [3,4]. Recently, Cuffini et al. [6] have performed X-ray diffraction analyses of polycrystalline SrHfO3 samples as function of the temperature, from RT up to 1273 K. These authors have identified three phases between RT and 1273 K, two of them with orthorhombic structure (space groups Pmma and Imma) below 900 K and a cubic phase for T ⱖ 973 K (space group Pm3m). PAC spectra with 181Ta probes taken in the same temperature interval have confirmed these findings [12]: up to and including 593 K, an asymmetric EFG1 (h 1 0.5) with v Q1 decreasing linearly with temperature was identified, followed by an EFG with h 1 1 between 693 and 893 K. Around 1000 K, v Q1 vanishes in favor of a strongly damped cubic environment (l 2 ⬇ 70 MHz). The present PAC data, measured using 111In/ 111Cd probes up to 893 K, are consistent with these previous results insofar as the non-cubic structure persists up to the highest temperature: the quadrupole constant v Q1 decreases for increasing temperature from 3.4 Mrad/s at RT to 2.0 Mrad/s at 893 K. The asymmetry parameter h 1 first increases slowly for increasing temperature, but then rises to 0.85 around 650 K and remains at high values up to 893 K (see Fig. 2). The variation of the asymmetry parameter h 1 is a very good indicator of the slight rearrangement of oxygen octahedra within the orthorhombic phase. We want to point out that, as indicated in Fig. 2, the phase transition for SrHfO3, measured with 111Cd, is accompanied by a change in the fluctuation rate of the relaxation parameter, from a
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Fig. 7. Two different perspectives of two phases of SrHfO3. On the left, the Pnma structure at RT is shown; on the right side, the Imma structure above 700 K. The crystallographic b-axis is perpendicular to the plane of the paper (top) or parallel to the plane of the paper (bottom). The open circles represent Sr atoms, the filled circles are oxygen, and the Hf atoms are located in the centers of the octahedra.
fast to a slow fluctuation regimen near 700 K. The relaxation processes for both 111Cd and 181Ta probes in SrHfO3 show similarities: l 2 increases for increasing temperature, reaching a maximum around 720 K for 111Cd (see Fig. 2) and around 1000 K for 181Ta [12]. It is interesting to note that both maxima are close to phase transition temperature in this compound, around 700 K for the Pmma $ Imma orthorhombic rearrangement and around 1000 K for the orthorhombic-to-cubic transition, respectively. A very similar evolution of phases was noted in a recent combined X-ray diffraction and 181Ta-PAC study on the SrRuO3 perovskite [6,30]. In conclusion, the X-ray data as well as the two PAC studies with 181Ta and 111Cd probes on SrHfO3 complement each other and confirm the existence of two non-cubic phases below 900 K. SrHfO3 is a perovskite whose distortion arises from the tilt of the oxygen octahedra in three orthogonal directions (for Pnma space group) with Hf atoms sitting in the center of the octahedra. Although the dielectrical constant has not been measured yet, SrHfO3 is not expected to be ferroelectric because of its structure. The phase transition at ⬇ 700 K involves the loss of the tilt in one direction and concomitantly no significant changes in the lattice parameters are observed. This result indicates that the phase transition is
related to very small and smooth changes in the structural parameters. To illustrate this, in Fig. 7 we show schematically the two orthorhombic phases of SrHfO3 from two different perspectives (figures from Ref. [6]). At this point it is interesting to compare this behavior to that of ferroelectric compounds such as BaTiO3, in which the phase transition from cubic-to-tetragonal is characterized by the central cation moving away from the central crystallographic site. The PAC experiments have shown that for ferroelectric and antiferroelectric compounds such as BaTiO3 and PbZrO3 the quadrupolar frequencies as well as the asymmetry parameters markedly change from one phase to another [23,24]. Moreover, it is possible to observe the hysteresis process close to the phase transitions [31]. In contrast, for SrHfO3, which we assume to be non-ferroelectric, the structural phase transitions are characterized by a bell-shaped variation of the relaxation parameter l 2. 4.3. BaHfO3 Dynamic effects in PAC measurements are visible as a loss of nuclear spin alignment with time. Relaxation processes were observed in many oxides and traced to several physical mechanisms. For 111In/ 111Cd impurities,
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charge or oxygen vacancy fluctuations as in the cases of Ce2O3 [32], Ga2O3 [33] and Cr2O3 [34], were discussed. Further, relaxation processes related to the delayed restoration of the electronic shells of the 111Cd probe atoms after the electron-capture radioactive decay of 111In, as best seen in In2O3 [35] and La2O3 [36]. Finally, phase transitions between different oxides also show strongly damped PAC spectra, as, for example, observed in the CuO $ Cu2O phase transition [37]. In these systems, only one or two electric field gradients are present, so that various fluctuation and relaxation scenarios can be differentiated using highly precise PAC measurements. There are certain similarities between the temperature variations of the fluctuation rate l 2 for BaHfO3: l 2 decreases with increasing temperature for both 111Cd (see Fig. 6) and 181Ta [8,11]. In Fig. 6, we have plotted ln l 2 versus the inverse temperature, inferring a thermally activated fluctuation process at T 300–600 K, l 2 / exp(⫺Ea/ kT). The resulting activation energy, Ea 47(4) meV, points to a fluctuation of the electronic environment of the probe atom. The origin of these dynamic effects, in particular at lower temperatures, is still not clear.
5. Conclusions The present PAC study of implanted 111In-probe ions in polycrystalline SrHfO3 and BaHfO3 complements and, to a large extent, confirms previous work on the hyperfine interactions in perovskite oxides of ABO3 structure. Based on the ratios of quadrupole coupling constants for 111Cd and 181Ta probes and on the well-established tendency of 111In to substitute into the center of oxygen octahedra and on PCM calculations, the largest EFG fraction f1 was assigned to the substitutional (defect-free) B-site. Consistent with the recent X-ray diffraction and 181Ta-PAC analyses on SrHfO3, this EFG1 shows a reversible variation of the asymmetry parameter from h 1 0.6 to h 1 ⬇ 0.85 at the Pmma $ Imma phase transition temperature. A similarly rapid variation of the quadrupole frequency v Q1 occurs at the orthorhombic-to-cubic phase transition temperature. Once again, PAC proves to be a very sensitive ‘‘phase meter’’ in oxides, caused by to the strong dependence of the EFG on the local oxygen coordination. In SrHfO3, the dynamic part of the perturbation function of fraction 1, was fitted with a model of exponential loss of spin alignment proportional to exp(⫺l 2t). The fluctuation rate l 2 shows a maximum value of l 2 8 MHz around the Pmma $ Imma phase transition: A similar bell-shaped variation of l 2 with temperature had been observed in 181 Ta-PAC on SrHfO3 to occur at the Imma $ cubic phase transition around 1000 K. In cubic BaHfO3, the rapid drop in alignment between RT and 600 K was fitted with a model of a thermally activated fluctuation process, having an activation energy Ea of some 50 meV. In both systems, two smaller fractions with f2 ⬇ f3 ⬇
10%–20 % were found, whose EFG parameters are very close to each other for both matrices. The origin of these fractions still needs to be explained. Although the systematics of EFG parameters for the substitutional B-site hyperfine probes 111Cd and 181Ta was complemented and the variation of these parameters along several phase transitions was followed in the present work, the transport and trapping of defects near the PAC probes and their influence on the perturbation functions are still unclear. This is not only a consequence of the fact that there is little information on defects (oxygen vacancies, charge transfers) in the two perovskites, but there may also arise differences depending on the method of preparation of the samples. PAC studies on single-crystal samples are therefore required to better elucidate this point. Such measurements may shed some light onto the so-far unexplained fluctuation mechanisms. For this purpose, PAC measurements with 111In probes in SrHfO3 single crystals were started.
Acknowledgements The authors are indebted to Dr. S.G. Marchetti for his help in the sample preparation and to D. Purschke and Dr. L. Ziegeler for their cooperation in the tracer-ion implantations with the IONAS implanter. This work was supported by CICPBA and CONICET, Argentina, as well as by Deutscher Akademischer Austauschdienst (DAAD) and by Deutsche Forschungsgemeinschaft (DFG), Bonn, Germany. K.P.L. and A.L.-G. wish to thank these organizations for supporting their mutual visits in La Plata and Go¨ttingen.
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Hyperfine interactions in the perovskites SrHfO3 and BaHfO3 observed with 111In/ 111Cd perturbed angular correlations P. de la Presa a, R.E. Alonso a, A. Ayala a,1, S. Habenicht b, V.V Krishnamurthy b,2, K.P. Lieb b,*, A. Lo´pez Garcı´a b,3, M. Neubauer b, M. Uhrmacher b a
Programa TENAES, Departamento de Fı´sica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina b II. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstr. 7-9, D-37073 Go¨ttingen, Germany Received 7 July 1998; accepted 1 December 1998
Abstract The electric field gradients in perovskite SrHfO3 and BaHfO3 powder samples were investigated by means of Perturbed Angular Correlation spectroscopy, using implanted 111In hyperfine probes. The measurements cover the temperature ranges from 22 to 893 K for SrHfO3 and from 293 to 875 K for BaHfO3, respectively. In both compounds, three quadrupole interactions were established. The largest fraction showing a pronounced dynamic interaction is assigned to 111In/ 111Cd probe atoms on substitutional Hf sites in both compounds. For SrHfO3, the temperature dependence of the dynamic interaction was associated to the Pnma $ Imma phase transition for SrHfO3 at about 700 K. We discuss the results in relation to those obtained for 111 In/ 111Cd-probes in PbZrO3 and BaTiO3, and for 181Hf/ 181Ta-probes in AHfO3 (A Ca, Sr, Ba). 䉷 1999 Elsevier Science Ltd. All rights reserved. Keywords: Perturbed angular correlation spectroscopy
1. Introduction The perovskite oxides ABO3, with a transition metal on their B-sites, display a wide variety of complex structural instabilities and electronic properties, depending on the combination of A and B cations [1,2]. Owing to their varied structure and composition, these materials have attracted intense interest in many applied and fundamental areas of solid-state science and advanced materials research. The main characteristic of these compounds is that small changes in the
* Corresponding author. Tel.: ⫹49-551-397631; fax: ⫹49-551394493. E-mail address: [email protected] (K.P. Lieb) 1 Present address: Departamento de Fı´sica, Universidade Federal do Ceara´, CP 6030, 60455-760 Fortaleza, CE Brazil. 2 Present address: Muon Science Laboratory, RIKEN, Wako Saitama 351-01, Japan. 3 Permanent address: Programa TENAES, Departamento de Fı´sica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina.
structural parameters introduce dramatic changes in the electrical properties. Then, if the appropriate nuclear probe is used, Perturbed Angular Correlation (PAC) spectroscopy is a suitable technique to study these changes because they depend on the electron density around the probe nucleus. In the past decades, SrHfO3 was reported as cubic [3] or pseudocubic [4] and non-systematic studies on structural phase transitions were performed till date. Guevara et al. [5] have recently carried out the first neutron diffraction experiment determining the orthorhombic structure (Pnma) at room temperature. Cuffini et al. [6] have measured, by X-ray diffraction, the temperature dependence of the lattice constant at higher temperatures, establishing two structural phase transitions. These experiments were inspired by the discrepancy between the results from Xray diffraction (XRD) and PAC experiments at RT: a cubic structure of the compound was deduced from XRD, while the observation of a well-defined static electric field gradient (EFG) [7] in the PAC measurements is not compatible with a perfect cubic perovskite lattice. Previous PAC studies on the electric quadrupole
0022-3697/99/$ - see front matter 䉷 1999 Elsevier Science Ltd. All rights reserved. PII: S0022-369 7(98)00344-8
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interactions of Ta-probe atoms in AHfO3 (A Ca, Sr or Ba) perovskites were carried out by several authors [7–9]. More recently, we have initiated a systematic investigation on the role of A-atoms in these materials. The PAC experiments on CaHfO3, SrHfO3 and BaHfO3 have allowed us to characterize the temperature dependence of the EFG tensors in temperature regions where phase transitions occur [10– 12]. In oxides, the EFGs are particularly sensitive to small local changes of the positions and/or charges of the ions in the immediate vicinity of the probe atoms. In the three hafnates studied with the 181Ta probe, welldefined or broadly distributed EFGs as well as relaxation phenomena were detected. The different behavior of the hyperfine interactions in the three compounds was interpreted in terms of the Hf–O bonds and correlated to the A–O bonds [13]. Contrary to what was expected in purely ionic compounds, the changes observed in the quadrupole parameters indicate that there exists an important contribution of the A cations to the electron density and lattice dynamics. In the present work, we address the question: What changes of the quadrupole interactions are introduced by a different impurity species sitting at the B-site? For this purpose, we used the hyperfine probe 111In/ 111Cd and performed PAC spectroscopy on polycrystalline SrHfO3 and BaHfO3 perovskites. For many binary and ternary oxides, this nuclear probe was used to elucidate charge transfer and defect reactions and to systematically measure the EFG as function of the oxygen coordination and ion valency. In this way, it should be possible to learn how nominally 2 ⫹ cations (5s-, 5p-states) on the B-site affect the electron density in these perovskites, keeping in mind that the quadrupole interaction reflects the coupling of these impurity states to mostly 2p-oxygen orbitals. 181
2. Experimental procedure and data analysis Both types of samples were prepared by solid-state reactions as described in Ref. [14], using stoichiometric mixtures of carbonates and oxides of the corresponding cations. Then the as-prepared perovskites were analyzed by X-ray diffraction, using the Rietveld procedure; the reported lattice constants in cubic BaHfO3 were reproduced [14]. Concerning SrHfO3, we refer to the recent neutron diffraction analysis by Guevara et al. [5]. For the 111In implantation, pills of 8 mm diameter and 0.5 mm thickness were prepared under a pressure of 0.5 kbar. The samples were implanted at RT using the Go¨ttingen ion implanter IONAS [15,16] at a total dose of approximately 10 12 radioactive 111In ions and an implantation energy of 400 keV. In the first and second implanted SrHfO3 samples, the annealing process was studied (described later). After a 4-h heating at 1673 K in air, no changes in the PAC spectra were observed. For that reason, all the other samples were annealed by this procedure.
The PAC spectroscopy was performed using four NaI(Tl) detectors in standard 90⬚ geometry. The apparatus has a time resolution of about 3.5 ns for the 171–245 keV g – g cascade in 111Cd, involving the 5/2 ⫹ intermediate state (mean life t 122 ns, quadrupole moment Q 0.83 b [17]) and fed in the EC-decay of 111In. Depending on the 111 In activity left in the sample, each measurement took between 12 and 48 h. The samples were heated in-situ, either in high vacuum (⬇10 ⫺5 mbar) or in air. The 22 K data in SrHfO3 were taken by attaching the sample to the tip of a closed-cycle helium cryostat mounted in a high-vacuum chamber. From the coincidence counting rates N(u ,t) between detectors separated by the angle u 90⬚ or 180⬚, the experimental perturbation functions [18], R t 2
N 180⬚; t ⫺ N 90⬚; t ; N 180⬚; t ⫹ 2N 90⬚; t
were calculated and fitted to the expression X i R t A22 fi G22 t ⫹ C;
1
2
i
taking into account the finite time resolution of the PAC spectrometer. As usual, A22 denotes the anisotropy of the g – g correlation, C is a time-independent baseline shift, fi is the fraction of probes at the site i, and Gi22 t is the corresponding perturbation factor. Based on the recent measurements of 181Ta in SrHfO3, we have used an approximate expression for G22 t that is able to describe static and/or fluctuating quadrupole interactions [19] G22 t exp ⫺l2 t
3 X
s2n h cos vn ht exp ⫺ dvn t2 =2:
n0
3 This formula refers to a Gaussian EFG distribution having the mean quadrupole coupling constant vQ peQVzz =20h and relative width d , and the constant asymmetry parameter h Vxx ⫺ Vyy =Vzz ; Vzz being the largest component of the EFG tensor. The frequencies vn h and amplitudes s2n h depend on the magnitude and asymmetry of the EFG [18,20]. The relaxation factor exp ⫺l2 t describes fluctuations between the various EFGs within the distribution and, in this way, introduces another damping mechanism into the perturbation function. By setting either the width d or the relaxation rate l 2 to zero, a relaxation mechanism of a welldefined EFG according to the Abragam–Pound model (see [21]), or a static distribution of EFGs can be modeled. As in many oxides, the occurrence of intrinsic structural and electronic defects, phase transitions, and several possible impurity sites can introduce difficulties in the fitting of the measured PAC spectra. It was only through the measurements in several samples and a careful, iterative fitting procedure that consistent sets of EFG parameters could be deduced in the present analyses. In particular, we have verified that the two limiting cases of a purely static EFG
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distribution (either of Gaussian or Lorentzian type) or of a purely statistical nature (d 0) gave fits of less good quality, in particular in the time region between 150 and 350 ns (described later).
3. Results
Fig. 1. Three perturbation functions for 111Cd probes in SrHfO3, taken after 111In ion implantation into polycrystalline samples and 4 h annealings at 1673 K.
Fig. 1 shows the perturbation functions R(t) taken at 293, 693, and 893 K for SrHfO3; the deduced hyperfine parameters at these temperatures and at 22 K are listed in Table 1. At all temperatures, three fractions fi were required to describe the PAC data in SrHfO3. The largest fraction, f1 45%–70%, is characterized by a low quadrupole frequency of v Q1 3.4 Mrad/s and an asymmetry parameter of h 1 0.51 at RT. The evolution of the hyperfine parameters of this fraction is illustrated in Fig. 2. One notes a gradual decrease of v Q1 and increase of h 1 with the temperature. The parameters d 1 and l 2, which account for the damping of fraction 1, develop from a distribution of static EFGs at 22 K (d 1 26%, l 2 0) to a fluctuating regime at 793 K (d 1 0, l 2 8 MHz). At this point, we would like to emphasize again that the broad maximum seen in all PAC spectra in the time interval between 150 and 350 ns essentially determines the variation of the parameters d 1 and l 2 and does not allow one to set either of them to zero over the full temperature range studied. To illustrate this, in Fig. 3 two fits of a PAC
Table 1 Hyperfine interaction parameters in SrHfO3 and BaHfO3 powder samples measured with annealing at 1673 K T (K) SrHfO3 22
293
693
893
BaHfO3 293
375
875
111
Cd probes after
111
In-ion implantation and 4 h
Fraction i
fi (%)
v Q (Mrad/s)
h
d (%)
l 2 (Mhz)
1 2 3 1 2 3 1 2 3 1 2 3
60(4) 27(3) 13(2) 52(4) 36(4) 12(2) 65(4) 9(2) 26(4) 63(3) 17(3) 20(2)
3.8(2) 18(1) 37(1) 3.4(2) 18(1) 37(1) 2.4(2) 18(1) 38(1) 2.0(2) 17(1) 36(1)
0.48(5) 0.40(5) 0.19(2) 0.51(6) 0.22(2) 0.16(3) 0.85(15) 0.30(7) 0.08(3) 0.85(15) 0.25(7) 0
26(3) 19(6) 6(2) 13(1) 38(12) 6(3) 7(3) 15(2) 4(1) — 28(10) 4(1)
— — — 2.1(4) — — 8.0(8) — — 3.6(7) — —
1 2 3 1 2 3 1 2 3
64(2) 19(2) 17(2) 70(2) 10(1) 20(2) 76(3) 5(1) 19(3)
0 15(1) 37(1) 0 16(1) 39(1) 0 18(1) 33(2)
0 0.54(4) 0.09(3) 0 0.34(7) 0.09(3) 0 0 0
0 13(1) 8(1) 0 13(1) 5(1) 0 4(1) 24(3)
37(2) — — 24(2) — — 12(1) — —
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Fig. 2. Variation of the hyperfine parameters of the predominant fraction EFG1 in SrHfO3. The dotted line indicates the region of the phase transition.
spectrum measured at 653 K are shown: one of them was obtained by fitting Eq. (2) (middle) and the other one from a pure static Gaussian distribution fit (bottom). Clearly, the first fit is better than the second one, although there is no great difference in the total x 2 values. The small difference in x 2 is because of the fact that the statistical errors of the experimental points are larger at long coincidence times, just in the range where the differences between the two fitting functions become significant. In Fig. 4, three perturbation functions R(t) are displayed that show different stages of the annealing process of a SrHfO3 sample after 111In-ion implantation: at RT, the asimplanted sample presents a typical PAC spectrum of a highly disordered material. The strongly damped spectrum can be fitted using the hyperfine parameters v Q 33 Mrad/ s, h 0.6 and d 45%. At 900 K, two EFG components can be distinguished, characterized by a large fraction (⬇82%), which again has a broad distribution of quadrupole frequencies (v Q 13 Mrad/s, h 0.60, d ⬇ 63%), and the remaining 18% fraction having a narrower distribution (v Q 37 Mrad/s, h 0.10, d 13%). The latter is similar to that of fraction 3 in the fully annealed samples. In the last annealing step, the sample was heated at 1273 K in air during 4 h and the perturbation was then measured at
Fig. 3. Experimental PAC spectra for SrHfO3 at 653 K (top) and two different fits: the fit shown in the middle was obtained by Eq. (3) and the one shown at the bottom by using a static Gaussian distribution function. The corresponding hyperfine parameters of fraction 1 are also indicated.
900 K. Now, the three fractions already discussed before were found. A further 4-h annealing at 1673 K reduced the EFGs relative distribution widths, but did not change the v Q values. The PAC spectra for the BaHfO3 compound were measured at 293, 375, 425, 474, 526, 574, 775, and 875 K, after 4-h annealings of the samples in air at 1673 K had been performed. Fig. 5 illustrates the evolution of the perturbation functions, while Table 1 summarizes the deduced quadrupole parameters measured at the corresponding temperatures. Again, one discerns three EFG contributions whose fractions stay essentially constant over the full temperature range. The predominant EFG1, having a fraction of f1 65%–80%, is characterized by a cubic environment (v Q1 h 1 0), but shows a strong relaxation at 300– 530 K. As shown in Fig. 6, the relaxation rate l 2 drops from 37 to 13 MHz in this temperature range and then stays constant up to 875 K. The second EFG has, on the average, a much smaller fraction of f2 ⬇ 13% and is characterized by a frequency distribution centered at v Q2 ⬇ 17 Mrad/s and having the width d 2 ⬇ 13% and asymmetry parameter h 2 ⬇ 0.3. Finally, the third contribution to the perturbation functions
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Fig. 4. Evolution of the measured and fitted perturbations during the annealing process of SrHfO3. The spectra refer to the as-implanted state (top), to a 4 h annealing at 900 K (middle), and to a 4 h annealing at 1273 K followed by the PAC measurement at 900 K (bottom).
753
Fig. 6. Temperature variation of the fluctuation rate l 2 of the substitutional fraction: (a) plotted versus T; (b) plotted logarithmically versus 1/T to deduce the activation energy Ea.
is the well-defined, symmetric EFG3, having v Q3 ⬇ 38 Mrad/s and d 3 ⬇ 5%. Evidently, the hyperfine parameters of fractions 2 and 3 in BaHfO3 are very similar to those of the corresponding fractions in SrHfO3.
4. Discussion 4.1. Substitutional site
Fig. 5. Perturbation functions for various temperatures.
111
Cd probes in BaHfO3 taken at
One of the important goals of PAC experiments is to identify those parts of the hyperfine perturbation to the attributed to probe atoms on substitutional, defect-free lattice sites. In the case of 181Hf/ 118Ta probes in hafnium compounds, this task is simple, because the probes are usually introduced via thermal neutron capture, leaving them on their ‘‘natural’’ sites. In the case of ion-implanted 111 In, the identification can pose problems. Criteria of the symmetry and magnitude of the EFG were applied to identify – or at least to make plausible – the corresponding fraction(s) in the perturbation function. In ionic oxides, the estimation of the EFG with a point charge model (PCM) was very useful to determine the corresponding site of the probe, as long as the probe ions do not distort the local structure. In the following, we argue that fraction 1 should be attributed to the B-site in the hafnates. Table 2 lists the measured quadrupole parameters of the
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Table 2 Comparison of quadrupolar interaction measured with 181Ta and 111Cd probes in SrHfO3, PbZrO3 and BaTiO3 perovskites and in HfO2 and ZrO2 at RT Compound/structure
SrHfO3 orthor. PbZrO3 orthor. BaTiO3 tetrag. HfO2 monocl. ZrO2 monocl. Point charge model a
181
111
Ta
v Q (Mrad/s) 20.4(6) 82(1) 32.2(9) 123(1)) 113(1)
h 0.45(2) 0.84 0.15(5) 0.34(3) 0.33
The large error of the frequency ratio is owing to that of the
v Q (Mrad/s) 3.4(2) 14.6(8) 4.9(5) 17.4(3) 17.4(3) 111
two probe atoms in the perovskites SrHfO3, PbZrO3, and BaTiO3 in the orthorhombic or tetragonal phase. When comparing the v Q-values for 181Ta and 111Cd in SrHfO3, their ratio at RT is v Q(Ta)/v Q(Cd) 6.0(3), consistent with the ratio of 6.5(12) expected from the scaling of the quadrupole moments and Sternheimer factors of both probes [17,21]. Further, the asymmetry parameters for the two probes agree very well with each other (h ⬇ 0.5). For orthorhombic PbZrO3 and tetragonal BaTiO3 at RT, the measured quadrupole parameters give frequency ratios of v Q(Ta)/v Q(Cd) 6.6(8) and 5.6(3), respectively, and again the asymmetry parameters are in good agreement for both nuclear probes [8,22–24]. We finally mention the recent PAC study of Luthin et al. [25] in monoclinic HfO2 and ZrO2, in which the frequency ratios v Q(Ta)/v Q(Cd) 7.0(2) in HfO2 and 6.5(2) in ZrO2 were deduced. In the cubic phase of BaHfO3, present over the whole temperature range considered, the interpretation of fraction 1 as corresponding to substitutional sites is, of course, based on the vanishing EFG. Another strong argument for the substitution of 111In into the B-site is based on the findings in many binary and ternary oxides in which the 111In-probes, after ion implantation and thermal annealing, have substituted into sites that have octahedal oxygen coordinations similar to the structure of In2O3 [26]. The PCM was proved to be reasonably successful in reproducing EFGs for ionic compounds [27–29]. If the covalency increases, the calculated values of the quadrupolar frequency begins to deviate from the experimental ones [38]. In contrast, the values of the asymmetry parameter h often agree with the PCM predictions, i.e. whenever the cation–oxygen bond length exceeds ˚ [26–28]. The corresponding bond lengths dM – O ⬇ 2 A for SrHfO3 calculated from the atomic positions [5] are thus exceeding the dM⫺OI 2:13 A and dM⫺OII 2:05 A, critical value. In our case, PCM gives v Q 74.5 Mrad/s, which strongly deviates from the measured value, v Q1 3.4(2) Mrad/s. However, the calculated value, h 0.46, is in good agreement with the experimental one, h 1 0.51(6). Based on these arguments, we
v Q(Ta)/ v Q(Cd)
Cd
h 0.51(6) ⬇1 0.25(6) 0.60(3) 0.64(4)
6.0(3) 5.6(3) 6.6(8) 7.1(2) 6.5(2) 6.5(12) a
Cd quadrupole moment.
tentatively assign fraction 1 to 111Cd probes substituted on the B-site. Although we cannot explain the origin of the other two sites, we would like to remark that they have very similar EFGs and thermal behavior in both compounds, indicating that they are independent of the presence of A cations. It is also interesting to note that they appear in the as-implanted samples or after the 900 K heating (see Fig. 4), remaining at higher temperatures. 4.2. SrHfO3 SrHfO3 has a simple perovskite structure, which was reported as cubic or pseudocubic [3,4]. Recently, Cuffini et al. [6] have performed X-ray diffraction analyses of polycrystalline SrHfO3 samples as function of the temperature, from RT up to 1273 K. These authors have identified three phases between RT and 1273 K, two of them with orthorhombic structure (space groups Pmma and Imma) below 900 K and a cubic phase for T ⱖ 973 K (space group Pm3m). PAC spectra with 181Ta probes taken in the same temperature interval have confirmed these findings [12]: up to and including 593 K, an asymmetric EFG1 (h 1 0.5) with v Q1 decreasing linearly with temperature was identified, followed by an EFG with h 1 1 between 693 and 893 K. Around 1000 K, v Q1 vanishes in favor of a strongly damped cubic environment (l 2 ⬇ 70 MHz). The present PAC data, measured using 111In/ 111Cd probes up to 893 K, are consistent with these previous results insofar as the non-cubic structure persists up to the highest temperature: the quadrupole constant v Q1 decreases for increasing temperature from 3.4 Mrad/s at RT to 2.0 Mrad/s at 893 K. The asymmetry parameter h 1 first increases slowly for increasing temperature, but then rises to 0.85 around 650 K and remains at high values up to 893 K (see Fig. 2). The variation of the asymmetry parameter h 1 is a very good indicator of the slight rearrangement of oxygen octahedra within the orthorhombic phase. We want to point out that, as indicated in Fig. 2, the phase transition for SrHfO3, measured with 111Cd, is accompanied by a change in the fluctuation rate of the relaxation parameter, from a
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Fig. 7. Two different perspectives of two phases of SrHfO3. On the left, the Pnma structure at RT is shown; on the right side, the Imma structure above 700 K. The crystallographic b-axis is perpendicular to the plane of the paper (top) or parallel to the plane of the paper (bottom). The open circles represent Sr atoms, the filled circles are oxygen, and the Hf atoms are located in the centers of the octahedra.
fast to a slow fluctuation regimen near 700 K. The relaxation processes for both 111Cd and 181Ta probes in SrHfO3 show similarities: l 2 increases for increasing temperature, reaching a maximum around 720 K for 111Cd (see Fig. 2) and around 1000 K for 181Ta [12]. It is interesting to note that both maxima are close to phase transition temperature in this compound, around 700 K for the Pmma $ Imma orthorhombic rearrangement and around 1000 K for the orthorhombic-to-cubic transition, respectively. A very similar evolution of phases was noted in a recent combined X-ray diffraction and 181Ta-PAC study on the SrRuO3 perovskite [6,30]. In conclusion, the X-ray data as well as the two PAC studies with 181Ta and 111Cd probes on SrHfO3 complement each other and confirm the existence of two non-cubic phases below 900 K. SrHfO3 is a perovskite whose distortion arises from the tilt of the oxygen octahedra in three orthogonal directions (for Pnma space group) with Hf atoms sitting in the center of the octahedra. Although the dielectrical constant has not been measured yet, SrHfO3 is not expected to be ferroelectric because of its structure. The phase transition at ⬇ 700 K involves the loss of the tilt in one direction and concomitantly no significant changes in the lattice parameters are observed. This result indicates that the phase transition is
related to very small and smooth changes in the structural parameters. To illustrate this, in Fig. 7 we show schematically the two orthorhombic phases of SrHfO3 from two different perspectives (figures from Ref. [6]). At this point it is interesting to compare this behavior to that of ferroelectric compounds such as BaTiO3, in which the phase transition from cubic-to-tetragonal is characterized by the central cation moving away from the central crystallographic site. The PAC experiments have shown that for ferroelectric and antiferroelectric compounds such as BaTiO3 and PbZrO3 the quadrupolar frequencies as well as the asymmetry parameters markedly change from one phase to another [23,24]. Moreover, it is possible to observe the hysteresis process close to the phase transitions [31]. In contrast, for SrHfO3, which we assume to be non-ferroelectric, the structural phase transitions are characterized by a bell-shaped variation of the relaxation parameter l 2. 4.3. BaHfO3 Dynamic effects in PAC measurements are visible as a loss of nuclear spin alignment with time. Relaxation processes were observed in many oxides and traced to several physical mechanisms. For 111In/ 111Cd impurities,
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charge or oxygen vacancy fluctuations as in the cases of Ce2O3 [32], Ga2O3 [33] and Cr2O3 [34], were discussed. Further, relaxation processes related to the delayed restoration of the electronic shells of the 111Cd probe atoms after the electron-capture radioactive decay of 111In, as best seen in In2O3 [35] and La2O3 [36]. Finally, phase transitions between different oxides also show strongly damped PAC spectra, as, for example, observed in the CuO $ Cu2O phase transition [37]. In these systems, only one or two electric field gradients are present, so that various fluctuation and relaxation scenarios can be differentiated using highly precise PAC measurements. There are certain similarities between the temperature variations of the fluctuation rate l 2 for BaHfO3: l 2 decreases with increasing temperature for both 111Cd (see Fig. 6) and 181Ta [8,11]. In Fig. 6, we have plotted ln l 2 versus the inverse temperature, inferring a thermally activated fluctuation process at T 300–600 K, l 2 / exp(⫺Ea/ kT). The resulting activation energy, Ea 47(4) meV, points to a fluctuation of the electronic environment of the probe atom. The origin of these dynamic effects, in particular at lower temperatures, is still not clear.
5. Conclusions The present PAC study of implanted 111In-probe ions in polycrystalline SrHfO3 and BaHfO3 complements and, to a large extent, confirms previous work on the hyperfine interactions in perovskite oxides of ABO3 structure. Based on the ratios of quadrupole coupling constants for 111Cd and 181Ta probes and on the well-established tendency of 111In to substitute into the center of oxygen octahedra and on PCM calculations, the largest EFG fraction f1 was assigned to the substitutional (defect-free) B-site. Consistent with the recent X-ray diffraction and 181Ta-PAC analyses on SrHfO3, this EFG1 shows a reversible variation of the asymmetry parameter from h 1 0.6 to h 1 ⬇ 0.85 at the Pmma $ Imma phase transition temperature. A similarly rapid variation of the quadrupole frequency v Q1 occurs at the orthorhombic-to-cubic phase transition temperature. Once again, PAC proves to be a very sensitive ‘‘phase meter’’ in oxides, caused by to the strong dependence of the EFG on the local oxygen coordination. In SrHfO3, the dynamic part of the perturbation function of fraction 1, was fitted with a model of exponential loss of spin alignment proportional to exp(⫺l 2t). The fluctuation rate l 2 shows a maximum value of l 2 8 MHz around the Pmma $ Imma phase transition: A similar bell-shaped variation of l 2 with temperature had been observed in 181 Ta-PAC on SrHfO3 to occur at the Imma $ cubic phase transition around 1000 K. In cubic BaHfO3, the rapid drop in alignment between RT and 600 K was fitted with a model of a thermally activated fluctuation process, having an activation energy Ea of some 50 meV. In both systems, two smaller fractions with f2 ⬇ f3 ⬇
10%–20 % were found, whose EFG parameters are very close to each other for both matrices. The origin of these fractions still needs to be explained. Although the systematics of EFG parameters for the substitutional B-site hyperfine probes 111Cd and 181Ta was complemented and the variation of these parameters along several phase transitions was followed in the present work, the transport and trapping of defects near the PAC probes and their influence on the perturbation functions are still unclear. This is not only a consequence of the fact that there is little information on defects (oxygen vacancies, charge transfers) in the two perovskites, but there may also arise differences depending on the method of preparation of the samples. PAC studies on single-crystal samples are therefore required to better elucidate this point. Such measurements may shed some light onto the so-far unexplained fluctuation mechanisms. For this purpose, PAC measurements with 111In probes in SrHfO3 single crystals were started.
Acknowledgements The authors are indebted to Dr. S.G. Marchetti for his help in the sample preparation and to D. Purschke and Dr. L. Ziegeler for their cooperation in the tracer-ion implantations with the IONAS implanter. This work was supported by CICPBA and CONICET, Argentina, as well as by Deutscher Akademischer Austauschdienst (DAAD) and by Deutsche Forschungsgemeinschaft (DFG), Bonn, Germany. K.P.L. and A.L.-G. wish to thank these organizations for supporting their mutual visits in La Plata and Go¨ttingen.
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