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Electric field gradient at 99Ru nuclei in the series of intermetallic compounds Ce1−xLaxRu2 synthesized at high pressure
Solid State Communications 135 (2005) 373–376 www.elsevier.com/locate/ssc
Electric field gradient at 99Ru nuclei in the series of intermetallic compounds Ce1KxLaxRu2 synthesized at high pressure A.V. Tsvyashchenkoa,*, G.K. Ryasnyb, B.A. Komissarovab, L.N. Fomichevaa, A.A. Sorokinb a
Vereshchagin High Pressure Physics Institute of Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russian Federation b Skobeltsyn Nuclear Physics Institute of Moscow State University, 119899 Moscow, Russian Federation Received 14 September 2004; accepted 13 May 2005 by L.V. Keldysh Available online 13 June 2005
Abstract Parameters of the electric quadrupole interaction for the first excited state (EZ89.7 keV) of 99Ru nuclei for a number of the cubic Laves phase compounds Ce1KxLaxRu2, synthesized at high pressure, were determined by the perturbed angular ggcorrelation method. Compounds were synthesized at 8 GPa. It was revealed that the decrease of the average valence of a rare earth ion, caused by the substitution of La for Ce, results in the monotonous decrease of the quadrupole frequency nQ from 43.3 MHz for CeRu2 to 33.1 MHz for LaRu2. q 2005 Elsevier Ltd. All rights reserved. PACS: 75.30.Mb; 71.29.Lp; 71.70.Jp Keywords: A. Intermetallic compounds; D. Electric quadrupole interaction; E. Perturbed angular correlations
Two decades ago, a joint team of researchers from the Tata Institute (Bombay, India) and Bonn University (Germany) published the results of the study of the electrical quadrupole interaction arising between the electrical field gradient in CeRu2, PrRu2 and NdRu2 compounds and an electric quadrupole moment of 99Ru nucleus which is in the crystallographic position of Ru with the 3m-point symmetry [1]. Measurements of the electric quadrupole interaction parameters for the cascades of g-transitions 528 and 89.7 keV, going through the 1st excited state 89.7 keV in 99 Ru, were performed by the perturbed angular correlation (PAC) method. The values of the quadrupole frequency nQ, equaled, respectively, to 42.1, 29.3 and 27.0 MHz, were
* Corresponding author. Tel.: C7 95 3340593; fax: C7 95 3340012. E-mail address: [email protected] (A.V. Tsvyashchenko).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.05.034
determined for the CeRu2, PrRu2 and NdRu2 compounds (nQZeQVzz/h, where Q—the nuclear quadrupole moment and Vzz—the main component of the electrical field gradient). The observed sufficient difference of the nQ values for the CeRu2, PrRu2 and NdRu2 compounds was connected [1] with the different values of valencies of cerium, which has suppositional valency of 4C in the CeRu2 compound, and of praseodymium and neodymium, which have the 3C valency in the PrRu2 and NdRu2 compounds. But it is well-established now on the basis of the X-ray absorption data (XANES) that the valency of cerium in the intermetallic compounds is an intermediate (or mixed) one [2], and changes from nZ3.3 [3] to nZ3.23 [4]. The latter value (nZ3.23) was obtained for CeRu2 samples, synthesized at high pressure [4]. The valency of cerium in the CeRu2 compound actually does not depend on the conditions of the preparation of this compound, though such characteristics as the magnetic susceptibility and the superconductivity transition
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temperature are different for the CeRu2 samples, prepared at different external pressures [4]. It is shown on the basis of the band structure calculations for CeRu2 that there is a broad 4f band in this compound, which ensures 30% contribution into the density of states at the Fermi level [5,6,7]. This high degree of 4f–4d-electron hybridization in cerium intermetallic compounds was demonstrated in a number of photoelectronic spectroscopy experiments [8]. The considerably itinerant behavior of the cerium 4f-electrons in CeRu2 compound, emerging due to the strong hybridization with the conduction electrons of the Ru 4d-band, was also confirmed by the results of the X-ray photoemission studies [9]. Thus, the interpretation of the observed values of quadrupole frequency nQ, suggesting the integer-valued valency of the Ce ion, is not correct. Therefore, one must interpret the observed difference of the quadrupole frequencies by taking into account the itinerant character of Ce 4f-electrons. One should expect the change of the hybridization degree and, correspondingly, the change of density of states of 4f-electrons, when substituting La for Ce in a number of the Ce1KxLaxRu2 compounds. This is the consequence of the overall decrease of the 4f-electron concentration, i.e. of the decrease of the average valency of a rare-earth ion (because, when substituting La for Ce, the valency of Ce remains constant [10]), and, to a lesser degree, the result of the increase of distance between the rare-earth ions, which can lead to the reduction of the overlapping of 4f-electron wave functions. All this leads to the change in the charge density and correspondingly to the change of the electrical field gradient (EFG) on the 99Ru nucleus for a number of the Ce1KxLaxRu2 compounds. It must also be noted that in Ref. [4] and [11] we have studied the charge state of Ru ions for a number of the Ce1Kx LaxRu2 compounds, synthesized at the 8 GPa pressure, by using the TDPAC method on 111Cd impurity nuclei, whose concentration in compounds did not exceed 0.1 at.%. The study has revealed that there are two charge states of Ru in the CeRu2 compound, synthesized at high pressure. The first state is the one with the completely occupied 4f-band (RuC), to which the 150 MHz quadrupole frequency nQ corresponds, and the second one with the partially occupied 4f-band (RuCC), to which the 220 MHz quadrupole frequency nQ corresponds. Similar mixed charge state of the nickel ions (NiC and NiCC) was observed earlier in the GdNi2 cubic Laves phase by 181Ta-TDPAC method [12]. It was shown in Ref. [11], that the change of the 4f-electron concentration, occurring at the replacement of Ce by La for a number of the Ce1Kx LaxRu2 compounds, does not lead to the smooth change of the quadrupole frequency values, but leads to a sharp depression of one of the frequencies, namely, the frequency nQZ150 MHz, which corresponds to an univalent state of ruthenium (RuC). It points to the fact that the 4f-electrons do not contribute to the electrical field gradient which
influences the 111Cd impurity nuclei. Thus, it was found in Ref. [11] that only 4d- and 5d-electrons provide primary contribution to the electrical field gradient (EFG) for the 111 Cd nuclei. It was shown that it is possible, using different nuclei as probes, to study the band structure peculiarities, emerging as the result of the contributions of the electrons with the quantum numbers lZ2 and 3, that is, it makes possible the study of the individual contribution of d or f-electrons into EFG. In this work, as in Ref. [11], for the same Ce1KxLaxRu2 compounds synthesized at 8 GPa (where xZ0, 0.1, 0.2, 0.35, 0.5, 0.8 and 1.0), the 99Ru nuclei were used instead of the 111Cd nucleus as a probe with the purpose of investigating the Ce 4f-electron contribution into the electrical field gradient. The 99Rh (T1/2Z16 days) parent activity was received according to the 99Ru (p,n) 99Rh reaction by the irradiation of 99Ru (enriched up to 95%) by protons with the energy of 7 MeV. A few milligrams of the irradiated Ru, containing the 99Rh nuclei, were introduced into the stoichiometric mixture of Ce, La and Ru of the corresponding composition for the preparation of samples in the high pressure chamber, according to the method described in Ref. [4]. In the electron capture decay of 99Rh to 99Ru, the first exited state 89.7 KeV of 99Ru is most intensely occupied by g-ray cascades, namely 528–89.7 and 353–89.7 KeV. Both the cascades are characterized by almost the same coefficients of the unperturbed angular correlation with the average value AwK0.17. So, to increase the efficiency of counting, both cascades were registered by the starting detector of the spectrometer of delayed coincidences. The 99Ru exited state with the 89.7 keV energy has the T1/2Z20.5 ns half-life period, the IZ3/2 spin and the 0.23 b quadrupole moment. The time differential perturbed angular correlation (TDPAC) measurements were performed by using an automatic scintillation spectrometer with the movable detector. The time resolution in the energy range of the 353–89.7 and 528–89.7 KeV g-ray cascades in 99Ru was about 1.8 ns. All measurements were carried out at room temperature and normal pressure. The time dependence of the angular anisotropy was obtained by the usual combination of the delayed coincidence spectra, measured for the 90 and 1808 angles between detectors: RðtÞ Z 2½Nðt; 1808Þ K Nðt; 1808Þ C 2Nðt; 908Þ Angular anisotropy spectra were traced for all samples in the time range up to 200 ns, namely, for almost ten half-life periods of 99Ru with EZ89.7 keV and could be approximated with the help of the least square method by the expression RðtÞ Z A2 Q2 ½cðtÞ C 0:2a C 0:8a cos u0 t where Q2z0.7 is the correction for the detector angular
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resolution, c(t) takes into account possible contribution of irregular positions of the testing nucleus in the lattice; in this case c(0)CaZ1, and precession frequencies u0ZpnQ for the nucleus spin IZ3/2. Angular anisotropy spectra for the CeRu2, Ce0.65La0.35Ru2 and LaRu2 compounds are shown at Fig. 1. The precession amplitudes show that about 80% of the 99Rh host atoms replace the Ru atoms, located in the crystallographic position with the trigonal symmetry. The value of constant background in the R(t) spectra corresponds to z20% of the 99 Ru nuclei, on which the EFG is practically equal to zero. It is possible that this background results from the substitution of the 99Rh atoms for La and Ce atoms, located in the positions with the cubic symmetry. The value of nQZ43.3 MHz for CeRu2 is in good agreement with the data in Ref. [1], obtained for the samples prepared at the normal pressure. On the other hand, we did not observe any traces of frequency, weakly indicating to the presence of the 85 MHz component cited in Ref. [1]. The substitution of La for Ce leads to a monotonic decrease of the quadrupole frequency up to 33.1(2) MHz for LaRu2, as it is seen from Fig. 2. It must be also noted that no attenuation of the precession for the samples with significant replacement of Ce by La (for example, for the Ce0.5La0.5Ru2 samples) was observed, although such attenuation could
Fig. 1. Examples of the anisotropy time spectra for (Ce1KxLax)Ru2 at xZ0, 0.35 and 1.0. A steep drop of points at small decays is due to the contribution of prompt coincidences of the annihilation quanta. In the case of LaRu2 this contribution was diminished by selecting only the 353 keV g-rays in the start channel. This spectrum was measured with the doubled number of channels per nanosecond and with the BaF2 crystal in the stop channel which significantly narrowed the initial drop of the anisotropy.
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Fig. 2. The quadrupole frequencies for 99Ru in (Ce1KxLax)Ru2 versus the relative concentration parameter x. The solid curve shows the result of the fit of the experimental points by a second order polynomial. The dashed line was obtained by the linear extrapolation of the data for Ce-rich samples. The points for PrRu2 and NdRu2 are taken from Ref. [1].
have appeared at the accidental distribution of ions with the different charges, for example, of the La ions with the 3C valency and Ce ions with the 3.23C valency. This can be viewed as an additional indication to the fact that the differences of the values of the EFG are determined only by the differences of the 4f-electron contributions for the compounds with the different relative concentration of Ce and La. The data for the LaRu2 are obtained for the first time. Let us note that the increase of the quadrupole frequency (nQ) for the LaRu2, PrRu2 and NdRu2 compounds, where the valency is estimated as equal to 3C, with the decrease of the lattice parameter at the transition from the LaRu2 to the NdRu2 compound (Fig. 2), has not been observed. At the same time in Ref. [13] the increase of quadrupole frequency with the decrease of the lattice constant was observed for a number of the RIn3 and RSn3 compounds, studied on the 111 Cd nuclei. Thus, the 30% difference of quadrupole frequencies for the LaRu2 and CeRu2 compounds and the differences of the same frequencies for LaRu2 and PrRu2 compounds are not connected with the differences of their crystal lattice parameters, but with the different concentrations of their 4f-electrons making additional contribution to the electrical field gradient. Therefore, at the substitution of La for Ce, when the average valency for the rare-earth atom changed linearly, the dependence of the observed quadrupole frequency on the concentration diverges from the linear one (Fig. 2) in such a way that the frequency value for LaRu2 exceeds the frequency value for PrRu2, and this is indicative of the presence of insignificant concentration of the 4f-electrons in LaRu2 compound. It should be also noted that the quadrupole frequency nQ
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reduces by two times at the fall of the rare-earth valency from 3C to 2C in the RIn3 and RSn3 compounds, for example, for Tm3CIn3 nQZ86.7 MHz, and for Yb2CIn3 nQZ38.7 MHz. So, it is not surprising that the increase of the Ce valency for the CeRu2 compound by about 1/4 in relation to the trivalent Pr and Nd for the PrRu2 and NdRu2 compounds leads to almost similar increase of the quadrupole frequency (nQ) for CeRu2 compound. Thus, analyzing the results of the present and previous works [4,11], we can assess partial contributions of the electrical field gradient of the different electrons with the quantum numbers lS2 in the Ru sites of the cubic Laves phase of CeRu2. The Ce 4f-electron contribution can be assessed as the difference of the electric field gradients [Vzz(99Ru: CeRu2)KVzz(99Ru: PrRu2)], measured for the 99 Ru nuclei, and equal to approximately 0.25!1018 V/cm2. The 4d-electron contribution can be considered equal to approximately the electrical field gradient difference on the 111 Cd nuclei, located in the Ru sites in CeRu2 compound for electron phase, in which Ru has unoccupied 4d band (that is, RuCC) and for electron phase in which 4d Ru band is fully occupied (i.e. RuC). That is, this difference [Vzz(RuCC: CeRu2)KVzz(RuC: CeRu2)] is equal to 0.38!1018 V/cm2. It follows from here that 4f electrons makes approximately 40% contribution into the total electrical field gradient of the itinerant f-, d-electrons at the Ru site. Also, it follows from the band calculations [5–7] that the Ce 4f-electrons give approximately similar contribution into the density of states at the Fermi level. From the results obtained in this work, it is possible to draw the following conclusions: – at the substitution of La for Ce for a number of Ce1Kx La x Ru2 compounds, the continuous decrease of the value of electric field gradient on the 99Ru nuclei with the continuous decrease of the 4f-electron concentration is observed; – it is revealed that there occurs an insignificant filling by electrons of La 4f-band for LaRu2 compound (although it is usually considered to be empty); – it is shown that the use of 99Ru and 111Cd testing nuclei for the determination of the electrical field gradients on the Ru sites by the PAC method enables us to estimate partial contributions into the EFG both from Ce 4f- and Ru 4d-electrons, the
ratio of which correlates with the corresponding contributions of electrons to the density of states at the Fermi level.
Acknowledgements Authors are thankful to the student A. Uzhbekova for her help in the performing of measurements by the PAC method. The work is performed with the support of the Russian Foundation for Fundamental Research (grant No. 04-02-16061 and grant No. 02-02-17506).
References [1] S.H. Devare, H.G. Devare, S. Scholz, L. Freise, M. Forker, Hyperfine Interact. 15/16 (1983) 629. [2] D. Wohlleben, J. Rohler, J. Appl. Phys. 55 (1984) 1904. [3] J. Chaboy, J. Garcia, A. Marcelli, J. Magn. Magn. Mater. 104– 107 (1992) 661. [4] A.V. Tsvyashchenko, L.N. Fomicheva, A.A. Sorokin, G.K. Ryasny, B.A. Komissarova, L.G. Shpinkova, K.V. Klimentiev, A.V. Kuznetsov, A.P. Menushenkov, V.N. Trofimov, A.E. Primenko, R. Kortes, Phys. Rev. B 65 (2002) 174513. [5] A. Yanase, J. Phys. F: Met. Phys. 16 (1986) 1501. [6] M. Higuchi, A. Hasegava, J. Phys. Soc. Jpn. 65 (1996) 1302. [7] S. Tanaka, H. Harima, A. Yanase, J. Phys. Soc. Jpn. 67 (1998) 1342. [8] J.C. Fuggle, F.U. Hillebrecht, Z. Zolnierek, R. Laesser, Ch. Freiburg, O. Gunnarsson, K. Schoenhammer, Phys. Rev. B 27 (1983) 7330. [9] S.H. Yang, H. Kumigashira, T. Yokoya, A. Chainani, T. Takahashi, H. Takeya, K. Kadowaki, Phys. Rev. B (1996) R11946. [10] S. Chudinov, M. Brando, A. Marcelli, M. Battisti, Physica B 244 (1998) 154. [11] A.A. Sorokin, G.K. Ryasny, B.A. Komissarova, L.G. Shpinkova, A.V. Tsvyashchenko, L.N. Fomicheva, A.S. Golubeva, Solid State Commun. 126 (2003) 181. [12] A.V. Tsvyashchenko, L.N. Fomicheva, E.N. Shirani, A.A. Sorokin, G.K. Ryasny, B.A. Komissarova, L.G. Shpinkova, Z.Z. Akselrod, O.I. Kochetov, V.N. Trofimov, Phys. Rev. B 55 (1997) 6377. [13] G.P. Schwarts, D.A. Shirley, Hyperfine Interact. 3 (1977) 67– 76.
Electric field gradient at 99Ru nuclei in the series of intermetallic compounds Ce1KxLaxRu2 synthesized at high pressure A.V. Tsvyashchenkoa,*, G.K. Ryasnyb, B.A. Komissarovab, L.N. Fomichevaa, A.A. Sorokinb a
Vereshchagin High Pressure Physics Institute of Russian Academy of Sciences, 142190 Troitsk, Moscow region, Russian Federation b Skobeltsyn Nuclear Physics Institute of Moscow State University, 119899 Moscow, Russian Federation Received 14 September 2004; accepted 13 May 2005 by L.V. Keldysh Available online 13 June 2005
Abstract Parameters of the electric quadrupole interaction for the first excited state (EZ89.7 keV) of 99Ru nuclei for a number of the cubic Laves phase compounds Ce1KxLaxRu2, synthesized at high pressure, were determined by the perturbed angular ggcorrelation method. Compounds were synthesized at 8 GPa. It was revealed that the decrease of the average valence of a rare earth ion, caused by the substitution of La for Ce, results in the monotonous decrease of the quadrupole frequency nQ from 43.3 MHz for CeRu2 to 33.1 MHz for LaRu2. q 2005 Elsevier Ltd. All rights reserved. PACS: 75.30.Mb; 71.29.Lp; 71.70.Jp Keywords: A. Intermetallic compounds; D. Electric quadrupole interaction; E. Perturbed angular correlations
Two decades ago, a joint team of researchers from the Tata Institute (Bombay, India) and Bonn University (Germany) published the results of the study of the electrical quadrupole interaction arising between the electrical field gradient in CeRu2, PrRu2 and NdRu2 compounds and an electric quadrupole moment of 99Ru nucleus which is in the crystallographic position of Ru with the 3m-point symmetry [1]. Measurements of the electric quadrupole interaction parameters for the cascades of g-transitions 528 and 89.7 keV, going through the 1st excited state 89.7 keV in 99 Ru, were performed by the perturbed angular correlation (PAC) method. The values of the quadrupole frequency nQ, equaled, respectively, to 42.1, 29.3 and 27.0 MHz, were
* Corresponding author. Tel.: C7 95 3340593; fax: C7 95 3340012. E-mail address: [email protected] (A.V. Tsvyashchenko).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.05.034
determined for the CeRu2, PrRu2 and NdRu2 compounds (nQZeQVzz/h, where Q—the nuclear quadrupole moment and Vzz—the main component of the electrical field gradient). The observed sufficient difference of the nQ values for the CeRu2, PrRu2 and NdRu2 compounds was connected [1] with the different values of valencies of cerium, which has suppositional valency of 4C in the CeRu2 compound, and of praseodymium and neodymium, which have the 3C valency in the PrRu2 and NdRu2 compounds. But it is well-established now on the basis of the X-ray absorption data (XANES) that the valency of cerium in the intermetallic compounds is an intermediate (or mixed) one [2], and changes from nZ3.3 [3] to nZ3.23 [4]. The latter value (nZ3.23) was obtained for CeRu2 samples, synthesized at high pressure [4]. The valency of cerium in the CeRu2 compound actually does not depend on the conditions of the preparation of this compound, though such characteristics as the magnetic susceptibility and the superconductivity transition
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temperature are different for the CeRu2 samples, prepared at different external pressures [4]. It is shown on the basis of the band structure calculations for CeRu2 that there is a broad 4f band in this compound, which ensures 30% contribution into the density of states at the Fermi level [5,6,7]. This high degree of 4f–4d-electron hybridization in cerium intermetallic compounds was demonstrated in a number of photoelectronic spectroscopy experiments [8]. The considerably itinerant behavior of the cerium 4f-electrons in CeRu2 compound, emerging due to the strong hybridization with the conduction electrons of the Ru 4d-band, was also confirmed by the results of the X-ray photoemission studies [9]. Thus, the interpretation of the observed values of quadrupole frequency nQ, suggesting the integer-valued valency of the Ce ion, is not correct. Therefore, one must interpret the observed difference of the quadrupole frequencies by taking into account the itinerant character of Ce 4f-electrons. One should expect the change of the hybridization degree and, correspondingly, the change of density of states of 4f-electrons, when substituting La for Ce in a number of the Ce1KxLaxRu2 compounds. This is the consequence of the overall decrease of the 4f-electron concentration, i.e. of the decrease of the average valency of a rare-earth ion (because, when substituting La for Ce, the valency of Ce remains constant [10]), and, to a lesser degree, the result of the increase of distance between the rare-earth ions, which can lead to the reduction of the overlapping of 4f-electron wave functions. All this leads to the change in the charge density and correspondingly to the change of the electrical field gradient (EFG) on the 99Ru nucleus for a number of the Ce1KxLaxRu2 compounds. It must also be noted that in Ref. [4] and [11] we have studied the charge state of Ru ions for a number of the Ce1Kx LaxRu2 compounds, synthesized at the 8 GPa pressure, by using the TDPAC method on 111Cd impurity nuclei, whose concentration in compounds did not exceed 0.1 at.%. The study has revealed that there are two charge states of Ru in the CeRu2 compound, synthesized at high pressure. The first state is the one with the completely occupied 4f-band (RuC), to which the 150 MHz quadrupole frequency nQ corresponds, and the second one with the partially occupied 4f-band (RuCC), to which the 220 MHz quadrupole frequency nQ corresponds. Similar mixed charge state of the nickel ions (NiC and NiCC) was observed earlier in the GdNi2 cubic Laves phase by 181Ta-TDPAC method [12]. It was shown in Ref. [11], that the change of the 4f-electron concentration, occurring at the replacement of Ce by La for a number of the Ce1Kx LaxRu2 compounds, does not lead to the smooth change of the quadrupole frequency values, but leads to a sharp depression of one of the frequencies, namely, the frequency nQZ150 MHz, which corresponds to an univalent state of ruthenium (RuC). It points to the fact that the 4f-electrons do not contribute to the electrical field gradient which
influences the 111Cd impurity nuclei. Thus, it was found in Ref. [11] that only 4d- and 5d-electrons provide primary contribution to the electrical field gradient (EFG) for the 111 Cd nuclei. It was shown that it is possible, using different nuclei as probes, to study the band structure peculiarities, emerging as the result of the contributions of the electrons with the quantum numbers lZ2 and 3, that is, it makes possible the study of the individual contribution of d or f-electrons into EFG. In this work, as in Ref. [11], for the same Ce1KxLaxRu2 compounds synthesized at 8 GPa (where xZ0, 0.1, 0.2, 0.35, 0.5, 0.8 and 1.0), the 99Ru nuclei were used instead of the 111Cd nucleus as a probe with the purpose of investigating the Ce 4f-electron contribution into the electrical field gradient. The 99Rh (T1/2Z16 days) parent activity was received according to the 99Ru (p,n) 99Rh reaction by the irradiation of 99Ru (enriched up to 95%) by protons with the energy of 7 MeV. A few milligrams of the irradiated Ru, containing the 99Rh nuclei, were introduced into the stoichiometric mixture of Ce, La and Ru of the corresponding composition for the preparation of samples in the high pressure chamber, according to the method described in Ref. [4]. In the electron capture decay of 99Rh to 99Ru, the first exited state 89.7 KeV of 99Ru is most intensely occupied by g-ray cascades, namely 528–89.7 and 353–89.7 KeV. Both the cascades are characterized by almost the same coefficients of the unperturbed angular correlation with the average value AwK0.17. So, to increase the efficiency of counting, both cascades were registered by the starting detector of the spectrometer of delayed coincidences. The 99Ru exited state with the 89.7 keV energy has the T1/2Z20.5 ns half-life period, the IZ3/2 spin and the 0.23 b quadrupole moment. The time differential perturbed angular correlation (TDPAC) measurements were performed by using an automatic scintillation spectrometer with the movable detector. The time resolution in the energy range of the 353–89.7 and 528–89.7 KeV g-ray cascades in 99Ru was about 1.8 ns. All measurements were carried out at room temperature and normal pressure. The time dependence of the angular anisotropy was obtained by the usual combination of the delayed coincidence spectra, measured for the 90 and 1808 angles between detectors: RðtÞ Z 2½Nðt; 1808Þ K Nðt; 1808Þ C 2Nðt; 908Þ Angular anisotropy spectra were traced for all samples in the time range up to 200 ns, namely, for almost ten half-life periods of 99Ru with EZ89.7 keV and could be approximated with the help of the least square method by the expression RðtÞ Z A2 Q2 ½cðtÞ C 0:2a C 0:8a cos u0 t where Q2z0.7 is the correction for the detector angular
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resolution, c(t) takes into account possible contribution of irregular positions of the testing nucleus in the lattice; in this case c(0)CaZ1, and precession frequencies u0ZpnQ for the nucleus spin IZ3/2. Angular anisotropy spectra for the CeRu2, Ce0.65La0.35Ru2 and LaRu2 compounds are shown at Fig. 1. The precession amplitudes show that about 80% of the 99Rh host atoms replace the Ru atoms, located in the crystallographic position with the trigonal symmetry. The value of constant background in the R(t) spectra corresponds to z20% of the 99 Ru nuclei, on which the EFG is practically equal to zero. It is possible that this background results from the substitution of the 99Rh atoms for La and Ce atoms, located in the positions with the cubic symmetry. The value of nQZ43.3 MHz for CeRu2 is in good agreement with the data in Ref. [1], obtained for the samples prepared at the normal pressure. On the other hand, we did not observe any traces of frequency, weakly indicating to the presence of the 85 MHz component cited in Ref. [1]. The substitution of La for Ce leads to a monotonic decrease of the quadrupole frequency up to 33.1(2) MHz for LaRu2, as it is seen from Fig. 2. It must be also noted that no attenuation of the precession for the samples with significant replacement of Ce by La (for example, for the Ce0.5La0.5Ru2 samples) was observed, although such attenuation could
Fig. 1. Examples of the anisotropy time spectra for (Ce1KxLax)Ru2 at xZ0, 0.35 and 1.0. A steep drop of points at small decays is due to the contribution of prompt coincidences of the annihilation quanta. In the case of LaRu2 this contribution was diminished by selecting only the 353 keV g-rays in the start channel. This spectrum was measured with the doubled number of channels per nanosecond and with the BaF2 crystal in the stop channel which significantly narrowed the initial drop of the anisotropy.
375
Fig. 2. The quadrupole frequencies for 99Ru in (Ce1KxLax)Ru2 versus the relative concentration parameter x. The solid curve shows the result of the fit of the experimental points by a second order polynomial. The dashed line was obtained by the linear extrapolation of the data for Ce-rich samples. The points for PrRu2 and NdRu2 are taken from Ref. [1].
have appeared at the accidental distribution of ions with the different charges, for example, of the La ions with the 3C valency and Ce ions with the 3.23C valency. This can be viewed as an additional indication to the fact that the differences of the values of the EFG are determined only by the differences of the 4f-electron contributions for the compounds with the different relative concentration of Ce and La. The data for the LaRu2 are obtained for the first time. Let us note that the increase of the quadrupole frequency (nQ) for the LaRu2, PrRu2 and NdRu2 compounds, where the valency is estimated as equal to 3C, with the decrease of the lattice parameter at the transition from the LaRu2 to the NdRu2 compound (Fig. 2), has not been observed. At the same time in Ref. [13] the increase of quadrupole frequency with the decrease of the lattice constant was observed for a number of the RIn3 and RSn3 compounds, studied on the 111 Cd nuclei. Thus, the 30% difference of quadrupole frequencies for the LaRu2 and CeRu2 compounds and the differences of the same frequencies for LaRu2 and PrRu2 compounds are not connected with the differences of their crystal lattice parameters, but with the different concentrations of their 4f-electrons making additional contribution to the electrical field gradient. Therefore, at the substitution of La for Ce, when the average valency for the rare-earth atom changed linearly, the dependence of the observed quadrupole frequency on the concentration diverges from the linear one (Fig. 2) in such a way that the frequency value for LaRu2 exceeds the frequency value for PrRu2, and this is indicative of the presence of insignificant concentration of the 4f-electrons in LaRu2 compound. It should be also noted that the quadrupole frequency nQ
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reduces by two times at the fall of the rare-earth valency from 3C to 2C in the RIn3 and RSn3 compounds, for example, for Tm3CIn3 nQZ86.7 MHz, and for Yb2CIn3 nQZ38.7 MHz. So, it is not surprising that the increase of the Ce valency for the CeRu2 compound by about 1/4 in relation to the trivalent Pr and Nd for the PrRu2 and NdRu2 compounds leads to almost similar increase of the quadrupole frequency (nQ) for CeRu2 compound. Thus, analyzing the results of the present and previous works [4,11], we can assess partial contributions of the electrical field gradient of the different electrons with the quantum numbers lS2 in the Ru sites of the cubic Laves phase of CeRu2. The Ce 4f-electron contribution can be assessed as the difference of the electric field gradients [Vzz(99Ru: CeRu2)KVzz(99Ru: PrRu2)], measured for the 99 Ru nuclei, and equal to approximately 0.25!1018 V/cm2. The 4d-electron contribution can be considered equal to approximately the electrical field gradient difference on the 111 Cd nuclei, located in the Ru sites in CeRu2 compound for electron phase, in which Ru has unoccupied 4d band (that is, RuCC) and for electron phase in which 4d Ru band is fully occupied (i.e. RuC). That is, this difference [Vzz(RuCC: CeRu2)KVzz(RuC: CeRu2)] is equal to 0.38!1018 V/cm2. It follows from here that 4f electrons makes approximately 40% contribution into the total electrical field gradient of the itinerant f-, d-electrons at the Ru site. Also, it follows from the band calculations [5–7] that the Ce 4f-electrons give approximately similar contribution into the density of states at the Fermi level. From the results obtained in this work, it is possible to draw the following conclusions: – at the substitution of La for Ce for a number of Ce1Kx La x Ru2 compounds, the continuous decrease of the value of electric field gradient on the 99Ru nuclei with the continuous decrease of the 4f-electron concentration is observed; – it is revealed that there occurs an insignificant filling by electrons of La 4f-band for LaRu2 compound (although it is usually considered to be empty); – it is shown that the use of 99Ru and 111Cd testing nuclei for the determination of the electrical field gradients on the Ru sites by the PAC method enables us to estimate partial contributions into the EFG both from Ce 4f- and Ru 4d-electrons, the
ratio of which correlates with the corresponding contributions of electrons to the density of states at the Fermi level.
Acknowledgements Authors are thankful to the student A. Uzhbekova for her help in the performing of measurements by the PAC method. The work is performed with the support of the Russian Foundation for Fundamental Research (grant No. 04-02-16061 and grant No. 02-02-17506).
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