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The TiPd2 compound studied by PAC with 181Ta and 111Cd probes
Journal of Alloys and Compounds 385 (2004) 53–58
The TiPd2 compound studied by PAC with 181Ta and 111Cd probes P. Wodniecki a,b,∗ , B. Wodniecka a , A. Kuli´nska a,b , M. Uhrmacher b , K.P. Lieb b a
b
IFJ PAN, Kraków, Poland II Physikalisches Institut, Universität Göttingen, D-37077 Göttingen, Germany
Received 6 April 2004; received in revised form 28 April 2004; accepted 28 April 2004
Abstract The intermetallic compound TiPd2 has been investigated with the perturbed angular correlation technique using radioactive 181 Hf/181 Ta and 111 In/111 Cd probes. The measurements confirmed an orthorhombic distortion of the C11b structure in the temperature range from 24 to 1023 K. However, the temperature dependence of the 111 Cd quadrupole parameters demonstrated that above 650 K the indium ions reach the lattice site of high symmetry characterized by an axially symmetric electric field gradient (EFG). The situation for the 181 Hf/181 Ta probes is more complex: we observed two electric field gradients, whose fractions interchange near 700 K, possibly due to 181 Hf probes switching their positions but still occupying a low-symmetry site. © 2004 Elsevier B.V. All rights reserved. Keywords: Transition metal compounds; Hyperfine interactions; PAC
1. Introduction Some uncertainties exist in the phase diagram of Pd–Ti, particularly for Pd-rich alloys, possibly because of the appearance of metastable precipitates. The TiPd2 compound occurs in two modifications in the composition range 33–35 at.% Ti. A thermal effect near 1280 ◦ C and a transformation of the microstructure demonstrated the existence of a polymorphic transformation of TiPd2 [1]. Krautwasser et al. performed a structure determination of both TiPd2 forms having related arrangements [2]. These authors found the orthorhombic structure of the low-temperature phase and the exact tetragonal C11b structure for the high-temperature modification. On the contrary, Okamoto reports that the TiPd2 phase, existing below 1553 K, has the tetragonal C11b structure while the high-temperature ␣TiPd2 phase adopts an orthorhombic distortion of the C11b arrangement [3]. The MoSi2 compound, which is the prototype of the simple tetragonal C11b structure (space group D17 4 h – I4/mmm), has six atoms in the unit cell. Both the Si (4(e) 4mm) and Mo (2(a) 4/mmm) sites have axial symmetry around the c-axis ∗
Corresponding author. E-mail address: [email protected] (P. Wodniecki).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.121
(see Fig. 1) [4] and the EFG tensor is thus diagonal at both sites of the structure. The orthorhombic distortion of the C11b lattice should cause a lowering of the site symmetries manifesting itself in an asymmetry of the EFG tensor. The EFG parameters of several MoSi2 -type Hf2 X and Zr2 X compounds were calculated recently [5] using the WIEN97 code [6,7] and compared with the experimental data. These results showed that the EFGs at the Hf site are very similar to the EFGs at the Ta impurity, but there is no direct correlation between the Zr and Ta EFGs, although Hf and Zr are chemically very similar. However, the axial symmetry of the EFGs measured with 181 Hf/181 Ta and 111 In/111 Cd probes were always observed to be in agreement with the site symmetries [8–11]. In the present paper we report on measurements of the electric field gradients (EFG) present in the TiPd2 lattice on two chemically different PAC probes, 181 Hf/181 Ta and 111 In/111 Cd, both being impurities in this matrix. Although Ti and Hf are located in the same group of the periodic table, their atomic volumes differ significantly. Also the In atoms are larger than Ti and Pd. Special attention must therefore be devoted to the sites occupied by these probes. For both impurities PAC proved to be a sensitive means for obtaining information on the probe surrounding, which yields changes of the hyperfine parameters.
54
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
cording to the phase diagram [3]. The sample was annealed in an evacuated and sealed quartz tube at 1173 K for 5 days. The X-ray diffraction spectrum, taken after the annealing program (see Fig. 2), was compared with two available standards for the TiPd2 compound. It was found to be in full agreement with the 21-0609 JCPDS database standard. It should be, however, pointed out, that this standard does not agree with the X-ray diffraction pattern simulated for the correct C11b structure corresponding with the 21-0610 JCPDS database standard. According to the work of Krautwasser et al. [2], the TiPd2 compound has a distorted orthorhombic C11b structure when annealed at 1473 K (a = 3.41 Å, b = 3.07 Å, c = 8.56 Å) and after an additional 10-min annealing at 1573 K the TiPd2 sample reaches the ideal C11b structure (a = b = 3.24 Å, c = 8.48 Å). This statement is in agreement with the polymorphic transformation of the TiPd2 structure, reported by Eremenko and Shtepa [1] at ≈1553 K. Here it should be mentioned, that the latest edition of the Handbook of Phase Diagrams [3] wrongly states that the orthorhombic structure exists above 1553 K, while below this temperature the TiPd2 phase exhibits an exact MoSi2 arrangement. In order to introduce the 181 Hf radioactive probes in TiPd2 , some 15 mg of the sample, encapsulated in an evacuated quartz tube, were neutron-irradiated in the pile of the ´ MARIA reactor at Swierk, at a flux of about 1014 neu−2 −1 trons cm s . As the thermal neutrons penetrate the full sample, the PAC probe atoms generated with the reaction 180 Hf(n, ␥)181 Hf were distributed over the whole volume. After neutron irradiation the sample was annealed for 15 h at 1073 K. Doping TiPd2 with radioactive 111 In probes was done by irradiating a 0.5-mm thick slice of the material with some 1012 400-keV 111 In ions at room temperature, by means of the Göttingen ion implanter IONAS [12]. As
Fig. 1. Nearest neighborhood of 2(a) (4/mmm) and 4(e) (4mm) sites of the MoSi2 prototype of C11b I4/mmm crystal lattice (a = 3.197 Å, c = 7.871 Å, z = 0.333).
2. Experimental procedure
sample holder
The TiPd2 compound was prepared by multiple arc melting, under argon atmosphere, the proper masses of the high-purity components. In order to assure doping of the sample with 181 Hf/181 Ta probe atoms a small amount of natural hafnium was added during the melting procedure. The nominal composition of the Ti0.98 Hf0.02 Pd2 alloy obtained in that way was determined by the masses of components. A minor mass loss (ca. 0.5% of 280 mg) evidenced during arc melting was attributed to evaporation of the more volatile palladium. After such correction the palladium concentration in the sample amounted to 65.7(3) at.% and was in the homogeneity range 65–67 at.% of the TiPd2 phase ac-
Intensity
TiPd2
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
2θ Fig. 2. The XRD spectrum of TiPd2 sample (slice) compared with the 21-0609 JCPDS database standard for orthorhombic TiPd2 (the 21-0610 standard for C11b TiPd2 phase does not fit the measured pattern).
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58 Table 1 The QI parameters for
111 Cd
and
181 Ta
55
in TiPd2 compound
Probe
νQ (300 K) (MHz)
η (300 K)
|Vzz | (300 K) (1018 V cm−2 )
νQ (800 K) (MHz)
η (800 K)
b (10−5 K−3/2 )
111 Cd
91.5(1) 396(2)
0.39(1) 0.81(1)
4.70(5) 7.83(8)
82(1) 85(1)
0 0.49(1)
0.53(1)a 2.5(1)
181 Ta a
Below 750 K.
directly after implantation the PAC spectrum exhibited only a broad distribution of quadrupole frequencies, the sample was annealed for 15 h at 1123 K. This temperature allowed us to remove the irradiation defects and to diffuse the probe atoms to substitutional lattice sites. The PAC measurements were carried out in the temperature range from 20 to 1023 K with a standard fast-slow set-up equipped with four detectors. The analysis of all the experimental R(t) spectra was performed by applying the expression of the perturbation factor G2 (t) [13] valid for static electric hyperfine interactions:
All measured EFG parameters are summarized in Table 1. The deduced EFG components Vzz do not include the uncertainties of the quadrupole moments.
panel). A fit to these data requires a single EFG with a non-zero asymmetry parameter. The relatively large distribution width of the quadrupole frequencies around the centre value νQ = 396(2) MHz indicates that the unique site of the impurity probe atoms in the lattice is not well defined. The PAC measurements at different temperatures proved to have a very strong temperature dependence of this frequency: the νQ value decreases from 430 MHz at 20 K to 200 MHz at 750 K. Similar strong temperature dependences were already observed for two other palladium compounds with MoSi2 structure - Hf2 Pd and Zr2 Pd [8]. In the temperature range 650–750 K the observed PAC pattern undergoes a transformation and above this temperature has to be fitted with a different set of the quadrupole interaction parameters. The left lower panel of Fig. 3 illustrates this perturbation factor measured at 973 K. Along the temperature range of this transformation both sets of the hyperfine parameters have to be applied to describe the PAC spectra. The evolution of the corresponding Fourier transforms with temperature is presented in right panel of Fig. 3. Fig. 4 illustrates the temperature dependences of the quadrupole frequencies and probe fractions and Fig. 5 the width δ of the Lorentzian νQi distribution versus temperature for 111 Cd and 181 Ta probes in the TiPd2 sample. Below 740 K the temperature dependence of νQ follows the T3/2 -law found by Christiansen et al. [16]. The variation of νQ above 700 K is much weaker. The parameter δ for 181 Ta probes is much greater than for 111 Cd and it rises with temperature below the transformation temperature while decreases above this temperature range. Finally, Fig. 6 illustrates the temperature evolution of the measured asymmetry parameters η for both hyperfine probes. For 181 Ta, the η-value decreases with increasing temperature from 0.87 at 30 K up to 0.43 at 723 K, where the transformation of the PAC pattern occurs. These results indicate that the sites of the 181 Hf/181 Ta impurity probes are not well defined. Around 650–750 K the 181 Hf probes either change their position in the lattice or a so far unknown polymorphic phase transition of the TiPd2 lattice occurs. In order to differentiate between these two possibilities, PAC measurements with another impurity probe were carried out.
3. Results and discussion
3.2. PAC measurements on
G2 (t) =
k 3 fi s2n (ηi ) cos(gn (ηi )νQi t) exp(−gn (ηi ) δi t). i=1
n=0
(1) The least squares fits of the perturbation factor to the experimental data yielded the fractions fi of probes exposed to different EFGs characterized by the quadrupole frequencies νQi and asymmetry parameters ηi . A broadening of the EFGi was described by the width δi of the Lorentzian νQi distribution. The observed fractions fi indicate the population of non-equivalent probe sites in the sample. Since the sample showed evidence of the non-random orientation of the crystallites the PAC data were fitted with free s2n parameters. The EFG-values were calculated from the measured quadrupole frequencies νQ according to: Vzz =
hνQ . eQ
(2)
In these calculations the quadrupole moment value Q = 0.83(13)b for 111 Cd [14] and Q = 2.36(5)b for 181 Ta [15] were adopted. The measured temperature variations of the quadrupole frequencies were fitted using the T3/2 -dependence [16]: Vzz (T) = Vzz (0)[1 − bT3/2 ].
3.1. PAC measurements on
(3)
181 Hf/181 Ta
probes
A 181 Ta PAC spectrum taken at room temperature for the annealed TiPd2 sample is presented in Fig. 3 (lower left
111 In/111 Cd
probes
PAC measurement executed at room temperature after implantation of 111 In probes into TiPd2 slices (the same material as used in the case of the 181 Hf experiments) and after annealing resulted in the perturbation factor presented at the
56
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
Fig. 3. Representative PAC spectra for 181 Hf/181 Ta probes in TiPd2 sample taken at room temperature and at 973 K (left) and the Fourier transforms of the perturbation factors measured in the 30–973 K temperature range (right).
upper left side) is characterized by an axially symmetric EFG corresponding to νQ = 81(1) MHz, δ = 1% and η = 0. The right hand side of Fig. 7 shows the Fourier transforms of the spectra taken at the indicated temperatures from 24 K up to 923 K. The quadrupole frequency decreases gradually over the whole temperature range (Fig. 8) following again a T3/2 -dependence (as in case of 181 Hf/181 Ta probes). The corresponding asymmetry parameter decreases with increasing temperature from η ≈ 0.5 at 24 K down to zero above 750 K, as shown in Fig. 6.
4. Discussion and summary Fig. 4. Temperature dependence of the quadrupole frequencies νQ and fractions f of 111 Cd and 181 Ta probes in TiPd2 sample.
left side of Fig. 7 (lower panel). It is clearly seen, that we are dealing with a unique, asymmetric EFG, which is much better defined (δ ≈ 1.6%) than in the case of 181 Ta probes. Further measurements at elevated temperatures exhibited a decrease of the quadrupole frequency and asymmetry parameter. The PAC spectrum of 111 Cd taken at 923 K (Fig. 7,
The TiPd2 sample showed evidence of nonrandom orientation of crystallites. The hyperfine interaction parameters fitted with Eq. (1) (with free s2n parameters) and Eq. (3) are collected in Table 1 for both 111 Cd and 181 Ta PAC probes. As it was mentioned before, both PAC probes used in the present experiments have to be treated as dilute impurities in the TiPd2 compound, assuming that interstitial sites are unlikely and probe-defect configurations involving radiation
Fig. 5. The width δ of the Lorentzian νQi distribution vs. temperature for 111 Cd and 181 Ta probes in TiPd sample. 2
Fig. 6. Temperature dependence of the asymmetry parameters η measured for 111 In/111 Cd and 181 Hf/181 Ta probes in TiPd2 sample.
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
Fig. 7. PAC spectra taken for temperature range.
111 In/111 Cd
57
probes at 293 and 923 K (left) and the Fourier transforms of the perturbation factors measured in the 24–923 K
defects have been destroyed during the annealing process. Even under these constraints, predictions of the possible crystallographic site occupied by the probes are very tentative. The PAC spectra are characterized by a single EFG in the case of 111 In impurities and most probably by two EFGs in the case of 181 Hf impurities. As a first important result we note that the smooth temperature dependence of the quadrupole frequency and asymmetry parameter observed for the 111 Cd probes rule out any sudden structural transformation of either the TiPd2 matrix itself or the 111 In site in the 24–923 K temperature range. Additional differential scanning calorimeter measurements gave also no evidence of any phase transition in this temperature range. A further conclusion can be drawn from the fact that at least below 650 K, the EFGs for both hyperfine probes are characterized by non-zero asymmetry parameters. This finding indicates that from 20 K to about 650 K the TiPd2 phase has not the ideal MoSi2 structure. The continuous decrease of the η-parameter for 111 Cd reaching η ≈ 0 at about 650 K indicates that the lattice structure is “improving” with temperature and above 750 K the axial symmetry of sites occupied by the 111 Cd probes is achieved.
The results for the 181 Ta probes are more difficult to interpret. Clearly, one or two asymmetric EFGs pertain over the full temperature range. The rapid changes of the hyperfine interaction parameters νQ and η and the EFG fractions in the narrow temperature range 650–750 K suggests that the 181 Ta probes probably switch their position, but the symmetry of their environment remains still low. As the asymmetry parameter for 181 Ta is decreasing rapidly with temperature, parallel to the η-value for 111 Cd (Fig. 6), one may presume that both impurities occupy identical sites below 650 K. Summarizing our results obtained with both dilute PAC probes, the crystal structure of TiPd2 clearly does not correspond to that of a perfect MoSi2 -type of structure, at least at temperatures below about 1023 K. This finding is in agreement with the results of earlier studies [1,2]. Our findings indicate, however, that above 750 K the surrounding of sites occupied by the 111 Cd probe atoms is axially symmetric as in the exact MoSi2 structure. In the temperature interval where the EFG of the 111 Cd probes becomes axially symmetric, the 181 Ta probes switch probably their position, but exhibit again a large asymmetry parameter, corroborating the orthorhombic distortion of the C11b structure reported by Krautwasser et al. [2]. Unfortunately, due to technical reasons it was not possible to extend our PAC measurements towards higher temperatures exceeding 1280 K, where the TiPd2 phase should reach the ideal C11b structure and therefore only axially symmetric EFGs should be observed.
References Fig. 8. Temperature dependence of the quadrupole frequency νQ for 111 Cd in TiPd2 compound.
[1] V.N. Eremenko, T.D. Shtepa, Sov. Powder Metall. Met. Ceram. 3 (1972) 228. [2] P. Krautwasser, S. Bahn, S. Schubert, Z. Metallk. 59 (1968) 724.
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P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
[3] H. Okamoto, Phase Diagrams for Binary Alloys, ASM, Material Park, Ohio, 2000. [4] P. Villars, L.D. Calvert, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, ASM, Materials Park, Ohio, 1991. [5] L.A. Terrazos, H.M. Petrilli, M. Marszałek, H. Saitovitch, P.R.J. da Silva, P. Blaha, K. Schwarz, Solid State Comm. 121 (2002) 525. [6] P. Blaha, K. Schwarz, J. Luitz, Wien97-Code, K. Schwarz, TU Wien, 1999, ISBN 3-9501031-0-4. [7] P. Blaha, K. Schwarz, W. Faber, J. Luitz, Hyp. Interact. 126 (2000) 389. [8] B. Wodniecka, M. Marszałek, P. Wodniecki, H. Saitovitch, P.R.J. da Silva, A.Z. Hrynkiewicz, J. Alloys Comp. 219 (1995) 132. [9] P. Wodniecki, B. Wodniecka, M. Marszałek, A.Z. Hrynkiewicz, Z. Naturforsch. 51a (1996) 437.
[10] P. Wodniecki, B. Wodniecka, A. Kuli´nska, Phys. Status Solidi B 236 (2003) 565. [11] L.A. Mendoza-Zelis, L.C. Damonte, A.G. Bibiloni, J. Desimoni, A.R. Lopez-Garcia, Phys. Rev. B 34 (1986) 2982. [12] M. Uhrmacher, M. Neubauer, W. Bolse, L. Ziegeler, K.P. Lieb, Nucl. Instrum. Methods B 139 (1998) 306. [13] H. Frauenfelder, R.M. Steffen in: K. Karlsson, E. Matthias, K. Siegbahn (Eds.), Perturbed Angular Correlations, North Holland, Amsterdam, 1963. [14] T. Butz, A. Lerf, Phys. Lett. A 97 (1983) 217. [15] P. Herzog, K. Freitag, M. Reuschenbach, H. Walitzki, Z. Phys. A 294 (1980) 13. [16] J. Christiansen, P. Heubes, R. Keitel, W. Klinger, W. Loefler, W. Sandner, W. Witthuhn, Z. Phys. B 24 (1976) 177.
The TiPd2 compound studied by PAC with 181Ta and 111Cd probes P. Wodniecki a,b,∗ , B. Wodniecka a , A. Kuli´nska a,b , M. Uhrmacher b , K.P. Lieb b a
b
IFJ PAN, Kraków, Poland II Physikalisches Institut, Universität Göttingen, D-37077 Göttingen, Germany
Received 6 April 2004; received in revised form 28 April 2004; accepted 28 April 2004
Abstract The intermetallic compound TiPd2 has been investigated with the perturbed angular correlation technique using radioactive 181 Hf/181 Ta and 111 In/111 Cd probes. The measurements confirmed an orthorhombic distortion of the C11b structure in the temperature range from 24 to 1023 K. However, the temperature dependence of the 111 Cd quadrupole parameters demonstrated that above 650 K the indium ions reach the lattice site of high symmetry characterized by an axially symmetric electric field gradient (EFG). The situation for the 181 Hf/181 Ta probes is more complex: we observed two electric field gradients, whose fractions interchange near 700 K, possibly due to 181 Hf probes switching their positions but still occupying a low-symmetry site. © 2004 Elsevier B.V. All rights reserved. Keywords: Transition metal compounds; Hyperfine interactions; PAC
1. Introduction Some uncertainties exist in the phase diagram of Pd–Ti, particularly for Pd-rich alloys, possibly because of the appearance of metastable precipitates. The TiPd2 compound occurs in two modifications in the composition range 33–35 at.% Ti. A thermal effect near 1280 ◦ C and a transformation of the microstructure demonstrated the existence of a polymorphic transformation of TiPd2 [1]. Krautwasser et al. performed a structure determination of both TiPd2 forms having related arrangements [2]. These authors found the orthorhombic structure of the low-temperature phase and the exact tetragonal C11b structure for the high-temperature modification. On the contrary, Okamoto reports that the TiPd2 phase, existing below 1553 K, has the tetragonal C11b structure while the high-temperature ␣TiPd2 phase adopts an orthorhombic distortion of the C11b arrangement [3]. The MoSi2 compound, which is the prototype of the simple tetragonal C11b structure (space group D17 4 h – I4/mmm), has six atoms in the unit cell. Both the Si (4(e) 4mm) and Mo (2(a) 4/mmm) sites have axial symmetry around the c-axis ∗
Corresponding author. E-mail address: [email protected] (P. Wodniecki).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.121
(see Fig. 1) [4] and the EFG tensor is thus diagonal at both sites of the structure. The orthorhombic distortion of the C11b lattice should cause a lowering of the site symmetries manifesting itself in an asymmetry of the EFG tensor. The EFG parameters of several MoSi2 -type Hf2 X and Zr2 X compounds were calculated recently [5] using the WIEN97 code [6,7] and compared with the experimental data. These results showed that the EFGs at the Hf site are very similar to the EFGs at the Ta impurity, but there is no direct correlation between the Zr and Ta EFGs, although Hf and Zr are chemically very similar. However, the axial symmetry of the EFGs measured with 181 Hf/181 Ta and 111 In/111 Cd probes were always observed to be in agreement with the site symmetries [8–11]. In the present paper we report on measurements of the electric field gradients (EFG) present in the TiPd2 lattice on two chemically different PAC probes, 181 Hf/181 Ta and 111 In/111 Cd, both being impurities in this matrix. Although Ti and Hf are located in the same group of the periodic table, their atomic volumes differ significantly. Also the In atoms are larger than Ti and Pd. Special attention must therefore be devoted to the sites occupied by these probes. For both impurities PAC proved to be a sensitive means for obtaining information on the probe surrounding, which yields changes of the hyperfine parameters.
54
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
cording to the phase diagram [3]. The sample was annealed in an evacuated and sealed quartz tube at 1173 K for 5 days. The X-ray diffraction spectrum, taken after the annealing program (see Fig. 2), was compared with two available standards for the TiPd2 compound. It was found to be in full agreement with the 21-0609 JCPDS database standard. It should be, however, pointed out, that this standard does not agree with the X-ray diffraction pattern simulated for the correct C11b structure corresponding with the 21-0610 JCPDS database standard. According to the work of Krautwasser et al. [2], the TiPd2 compound has a distorted orthorhombic C11b structure when annealed at 1473 K (a = 3.41 Å, b = 3.07 Å, c = 8.56 Å) and after an additional 10-min annealing at 1573 K the TiPd2 sample reaches the ideal C11b structure (a = b = 3.24 Å, c = 8.48 Å). This statement is in agreement with the polymorphic transformation of the TiPd2 structure, reported by Eremenko and Shtepa [1] at ≈1553 K. Here it should be mentioned, that the latest edition of the Handbook of Phase Diagrams [3] wrongly states that the orthorhombic structure exists above 1553 K, while below this temperature the TiPd2 phase exhibits an exact MoSi2 arrangement. In order to introduce the 181 Hf radioactive probes in TiPd2 , some 15 mg of the sample, encapsulated in an evacuated quartz tube, were neutron-irradiated in the pile of the ´ MARIA reactor at Swierk, at a flux of about 1014 neu−2 −1 trons cm s . As the thermal neutrons penetrate the full sample, the PAC probe atoms generated with the reaction 180 Hf(n, ␥)181 Hf were distributed over the whole volume. After neutron irradiation the sample was annealed for 15 h at 1073 K. Doping TiPd2 with radioactive 111 In probes was done by irradiating a 0.5-mm thick slice of the material with some 1012 400-keV 111 In ions at room temperature, by means of the Göttingen ion implanter IONAS [12]. As
Fig. 1. Nearest neighborhood of 2(a) (4/mmm) and 4(e) (4mm) sites of the MoSi2 prototype of C11b I4/mmm crystal lattice (a = 3.197 Å, c = 7.871 Å, z = 0.333).
2. Experimental procedure
sample holder
The TiPd2 compound was prepared by multiple arc melting, under argon atmosphere, the proper masses of the high-purity components. In order to assure doping of the sample with 181 Hf/181 Ta probe atoms a small amount of natural hafnium was added during the melting procedure. The nominal composition of the Ti0.98 Hf0.02 Pd2 alloy obtained in that way was determined by the masses of components. A minor mass loss (ca. 0.5% of 280 mg) evidenced during arc melting was attributed to evaporation of the more volatile palladium. After such correction the palladium concentration in the sample amounted to 65.7(3) at.% and was in the homogeneity range 65–67 at.% of the TiPd2 phase ac-
Intensity
TiPd2
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
2θ Fig. 2. The XRD spectrum of TiPd2 sample (slice) compared with the 21-0609 JCPDS database standard for orthorhombic TiPd2 (the 21-0610 standard for C11b TiPd2 phase does not fit the measured pattern).
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58 Table 1 The QI parameters for
111 Cd
and
181 Ta
55
in TiPd2 compound
Probe
νQ (300 K) (MHz)
η (300 K)
|Vzz | (300 K) (1018 V cm−2 )
νQ (800 K) (MHz)
η (800 K)
b (10−5 K−3/2 )
111 Cd
91.5(1) 396(2)
0.39(1) 0.81(1)
4.70(5) 7.83(8)
82(1) 85(1)
0 0.49(1)
0.53(1)a 2.5(1)
181 Ta a
Below 750 K.
directly after implantation the PAC spectrum exhibited only a broad distribution of quadrupole frequencies, the sample was annealed for 15 h at 1123 K. This temperature allowed us to remove the irradiation defects and to diffuse the probe atoms to substitutional lattice sites. The PAC measurements were carried out in the temperature range from 20 to 1023 K with a standard fast-slow set-up equipped with four detectors. The analysis of all the experimental R(t) spectra was performed by applying the expression of the perturbation factor G2 (t) [13] valid for static electric hyperfine interactions:
All measured EFG parameters are summarized in Table 1. The deduced EFG components Vzz do not include the uncertainties of the quadrupole moments.
panel). A fit to these data requires a single EFG with a non-zero asymmetry parameter. The relatively large distribution width of the quadrupole frequencies around the centre value νQ = 396(2) MHz indicates that the unique site of the impurity probe atoms in the lattice is not well defined. The PAC measurements at different temperatures proved to have a very strong temperature dependence of this frequency: the νQ value decreases from 430 MHz at 20 K to 200 MHz at 750 K. Similar strong temperature dependences were already observed for two other palladium compounds with MoSi2 structure - Hf2 Pd and Zr2 Pd [8]. In the temperature range 650–750 K the observed PAC pattern undergoes a transformation and above this temperature has to be fitted with a different set of the quadrupole interaction parameters. The left lower panel of Fig. 3 illustrates this perturbation factor measured at 973 K. Along the temperature range of this transformation both sets of the hyperfine parameters have to be applied to describe the PAC spectra. The evolution of the corresponding Fourier transforms with temperature is presented in right panel of Fig. 3. Fig. 4 illustrates the temperature dependences of the quadrupole frequencies and probe fractions and Fig. 5 the width δ of the Lorentzian νQi distribution versus temperature for 111 Cd and 181 Ta probes in the TiPd2 sample. Below 740 K the temperature dependence of νQ follows the T3/2 -law found by Christiansen et al. [16]. The variation of νQ above 700 K is much weaker. The parameter δ for 181 Ta probes is much greater than for 111 Cd and it rises with temperature below the transformation temperature while decreases above this temperature range. Finally, Fig. 6 illustrates the temperature evolution of the measured asymmetry parameters η for both hyperfine probes. For 181 Ta, the η-value decreases with increasing temperature from 0.87 at 30 K up to 0.43 at 723 K, where the transformation of the PAC pattern occurs. These results indicate that the sites of the 181 Hf/181 Ta impurity probes are not well defined. Around 650–750 K the 181 Hf probes either change their position in the lattice or a so far unknown polymorphic phase transition of the TiPd2 lattice occurs. In order to differentiate between these two possibilities, PAC measurements with another impurity probe were carried out.
3. Results and discussion
3.2. PAC measurements on
G2 (t) =
k 3 fi s2n (ηi ) cos(gn (ηi )νQi t) exp(−gn (ηi ) δi t). i=1
n=0
(1) The least squares fits of the perturbation factor to the experimental data yielded the fractions fi of probes exposed to different EFGs characterized by the quadrupole frequencies νQi and asymmetry parameters ηi . A broadening of the EFGi was described by the width δi of the Lorentzian νQi distribution. The observed fractions fi indicate the population of non-equivalent probe sites in the sample. Since the sample showed evidence of the non-random orientation of the crystallites the PAC data were fitted with free s2n parameters. The EFG-values were calculated from the measured quadrupole frequencies νQ according to: Vzz =
hνQ . eQ
(2)
In these calculations the quadrupole moment value Q = 0.83(13)b for 111 Cd [14] and Q = 2.36(5)b for 181 Ta [15] were adopted. The measured temperature variations of the quadrupole frequencies were fitted using the T3/2 -dependence [16]: Vzz (T) = Vzz (0)[1 − bT3/2 ].
3.1. PAC measurements on
(3)
181 Hf/181 Ta
probes
A 181 Ta PAC spectrum taken at room temperature for the annealed TiPd2 sample is presented in Fig. 3 (lower left
111 In/111 Cd
probes
PAC measurement executed at room temperature after implantation of 111 In probes into TiPd2 slices (the same material as used in the case of the 181 Hf experiments) and after annealing resulted in the perturbation factor presented at the
56
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Fig. 3. Representative PAC spectra for 181 Hf/181 Ta probes in TiPd2 sample taken at room temperature and at 973 K (left) and the Fourier transforms of the perturbation factors measured in the 30–973 K temperature range (right).
upper left side) is characterized by an axially symmetric EFG corresponding to νQ = 81(1) MHz, δ = 1% and η = 0. The right hand side of Fig. 7 shows the Fourier transforms of the spectra taken at the indicated temperatures from 24 K up to 923 K. The quadrupole frequency decreases gradually over the whole temperature range (Fig. 8) following again a T3/2 -dependence (as in case of 181 Hf/181 Ta probes). The corresponding asymmetry parameter decreases with increasing temperature from η ≈ 0.5 at 24 K down to zero above 750 K, as shown in Fig. 6.
4. Discussion and summary Fig. 4. Temperature dependence of the quadrupole frequencies νQ and fractions f of 111 Cd and 181 Ta probes in TiPd2 sample.
left side of Fig. 7 (lower panel). It is clearly seen, that we are dealing with a unique, asymmetric EFG, which is much better defined (δ ≈ 1.6%) than in the case of 181 Ta probes. Further measurements at elevated temperatures exhibited a decrease of the quadrupole frequency and asymmetry parameter. The PAC spectrum of 111 Cd taken at 923 K (Fig. 7,
The TiPd2 sample showed evidence of nonrandom orientation of crystallites. The hyperfine interaction parameters fitted with Eq. (1) (with free s2n parameters) and Eq. (3) are collected in Table 1 for both 111 Cd and 181 Ta PAC probes. As it was mentioned before, both PAC probes used in the present experiments have to be treated as dilute impurities in the TiPd2 compound, assuming that interstitial sites are unlikely and probe-defect configurations involving radiation
Fig. 5. The width δ of the Lorentzian νQi distribution vs. temperature for 111 Cd and 181 Ta probes in TiPd sample. 2
Fig. 6. Temperature dependence of the asymmetry parameters η measured for 111 In/111 Cd and 181 Hf/181 Ta probes in TiPd2 sample.
P. Wodniecki et al. / Journal of Alloys and Compounds 385 (2004) 53–58
Fig. 7. PAC spectra taken for temperature range.
111 In/111 Cd
57
probes at 293 and 923 K (left) and the Fourier transforms of the perturbation factors measured in the 24–923 K
defects have been destroyed during the annealing process. Even under these constraints, predictions of the possible crystallographic site occupied by the probes are very tentative. The PAC spectra are characterized by a single EFG in the case of 111 In impurities and most probably by two EFGs in the case of 181 Hf impurities. As a first important result we note that the smooth temperature dependence of the quadrupole frequency and asymmetry parameter observed for the 111 Cd probes rule out any sudden structural transformation of either the TiPd2 matrix itself or the 111 In site in the 24–923 K temperature range. Additional differential scanning calorimeter measurements gave also no evidence of any phase transition in this temperature range. A further conclusion can be drawn from the fact that at least below 650 K, the EFGs for both hyperfine probes are characterized by non-zero asymmetry parameters. This finding indicates that from 20 K to about 650 K the TiPd2 phase has not the ideal MoSi2 structure. The continuous decrease of the η-parameter for 111 Cd reaching η ≈ 0 at about 650 K indicates that the lattice structure is “improving” with temperature and above 750 K the axial symmetry of sites occupied by the 111 Cd probes is achieved.
The results for the 181 Ta probes are more difficult to interpret. Clearly, one or two asymmetric EFGs pertain over the full temperature range. The rapid changes of the hyperfine interaction parameters νQ and η and the EFG fractions in the narrow temperature range 650–750 K suggests that the 181 Ta probes probably switch their position, but the symmetry of their environment remains still low. As the asymmetry parameter for 181 Ta is decreasing rapidly with temperature, parallel to the η-value for 111 Cd (Fig. 6), one may presume that both impurities occupy identical sites below 650 K. Summarizing our results obtained with both dilute PAC probes, the crystal structure of TiPd2 clearly does not correspond to that of a perfect MoSi2 -type of structure, at least at temperatures below about 1023 K. This finding is in agreement with the results of earlier studies [1,2]. Our findings indicate, however, that above 750 K the surrounding of sites occupied by the 111 Cd probe atoms is axially symmetric as in the exact MoSi2 structure. In the temperature interval where the EFG of the 111 Cd probes becomes axially symmetric, the 181 Ta probes switch probably their position, but exhibit again a large asymmetry parameter, corroborating the orthorhombic distortion of the C11b structure reported by Krautwasser et al. [2]. Unfortunately, due to technical reasons it was not possible to extend our PAC measurements towards higher temperatures exceeding 1280 K, where the TiPd2 phase should reach the ideal C11b structure and therefore only axially symmetric EFGs should be observed.
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