Hyperfine interaction of111Cd in antiferromagnetic cobalt oxide

The temperature dependence of the hyperfine field of substitutional¹¹¹Cd in antiferromagnetic CoO has been measured by means of the perturbed angular correlation technique. The Larmor frequency ω_L is found to obey a power law ω_L(t=ω₀ t ^β) up tot _max=0.4 wheret=1−T/T _N is the reduced temperature withT _N=291.2(3) K and β=0.393 (5) the critical exponent. The results are discussed and compared with PAC experiments on¹¹¹Cd in NiO and Ni and with results obtained by other methods.     

Nuclear quadrupole interaction at133Ba (EC)133Cs in Eu1Ba2Cu3O7−°

We studied the nuclear quadrupole interaction (NQI) at¹³³Ba(EC)¹³³Cs in Eu₁Ba₂Cu₃O_7–6 by time differential perturbed angular correlation between 11 K and 300 K. No anomalies in the NQI parameters around T_c were detected. All Ba sites appear to be identical and the precession frequency is ω=80(2) Mrad/s (assuming axial symmetry), independent of temperature.     

Study of the structure and electric quadrupole interaction in the intermetallic compound Hf2Fe

The electric quadrupole interactions at¹⁸¹Ta probe nuclei in a cubic Hf₂Fe lattice were studied by the TDPAC method. In addition, the crystalline structure study of the Hf₂Fe compound was performed. The results of the EQI measurement show the presence of two independent interactions, one at low frequency, characterized by ω _Q ⁽¹⁾ =33 Mrad/s and δ=30%, and the other at high frequency described by ω _Q ⁽²⁾ =207 ± 2 Mrad/s and δ=4%. Both interactions are found to be compatible with the crystalline structure established in this investigation. The large temperature dependence of the electric field gradient of the second interaction in the range from 78K to 1183K was determined. The change in the EFG follows the empiricalT ^3/2 -relation.     

Polymorphic Phase Transformation in In2La and CeIn2

Nuclear quadrupole interactions of ¹¹¹Cd probes in In₂La and CeIn₂ were measured using perturbed angular correlation of gamma rays (PAC). Near room temperature, a single non-axially symmetric quadrupole interaction was observed in each compound, with ω ₀ = eQV _zz /ℏ = 78.8(2) Mrad/s and η = 0.312(1) for In₂La at 11°C and ω ₀ = 80(1) Mrad/s and η = 0.29(2) for CeIn₂ at 34°C. The observed non-axial symmetry is consistent with the reported CeCu₂ structure of the phases. The non-axially symmetric interactions were replaced completely by axially symmetric interactions (η = 0) at 524°C, with ω ₀ = 101.0(1) Mrad/s in In₂La and ω ₀ = 96.9(4) Mrad/s in CeIn₂. The change in symmetry is attributed to a polymorphic phase transformation. Based on symmetry information from the quadrupole interaction and on chemical arguments, it is proposed that high-temperature phases of In₂La and CeIn₂ both have the AlB₂ (C32) structure.     

Radiation damage in γ-Mo4O11

We investigated the radiation damage induced by thermal neutron capture of⁹⁸Mo in γ-Mo₄O₁₁ by measuring the⁹⁹Mo(β⁻)⁹⁹Tc nuclear quadrupole interaction by time differential perturbed angular correlations. At room temperature, a fraction of about 20% of all activated Mo-atoms exhibits a large quadrupole interaction of ω₁=550 Mrad/s and an asymmetry parameters of η≊0.75, practically independent of the dose in the range from 7.9·10¹⁶ to 7.8·10¹⁸ n/cm². We attribute this signal to Mo-atoms with a near-neighbour oxygen vacancy which was created in a knock-on process following prompt de-excitation by gamma emission after the thermal neutron capture.     

The nuclear quadrupole interaction at 111Cd and 181Ta sites in anatase and rutile TiO2: A TDPAC study

The nuclear quadrupole interaction of the I=5/2 state of the nuclear probes ¹¹¹Cd and ¹⁸¹Ta in the anatase and rutile polymorphs of bulk TiO₂ was studied using the time differential perturbed angular correlation (TDPAC) method. The fast–slow coincidence setup is based on CAMAC electronics. For anatase, the asymmetry parameter of the electric field gradient was η=0.22(1) and a quadrupole interaction frequency of ω_Q=44.01(3) Mrad/s was obtained for ¹⁸¹Ta. For rutile, the respective values are η=0.56(1) and ω_Q=130.07(9) Mrad/s. The values for rutile match closely with the literature values. In case of the ¹¹¹Cd probe produced from the beta decay of ¹¹¹Ag, the quadrupole interaction frequency for anatase was negligibly small as indicated by an unperturbed angular correlation in anatase. On the other hand for rutile the quadrupole frequency is ω_Q=61.74(2) Mrad/s and the asymmetry parameter η=0.23(1) for the ¹¹¹Cd probe. The results are interpreted in terms of the surrounding atom positions in the lattice and the charge state of the probe nucleus.     

Hyperfine Interactions of 181Ta in Zr2Ni Observed Using PAC Spectroscopy

We have measured nuclear electric–quadrupole interactions (EQI) at ¹⁸¹Ta impurities substituted as Hf atoms into the Zr site in Zr₂Ni. Using perturbed-angular-correlation (PAC) spectroscopy, we measured the EQI over temperatures ranging from 10 to 1200 K. Over the entire range of temperature, the Zr₂Ni crystal has a bct Al₂Cu structure that includes a single Zr site. The crystal field symmetry surrounding this site is rather low, giving rise to a highly asymmetric electric-field gradient tensor. At 10 K, the EQI is characterized by an angular frequency ω₀=601(3) Mrad s⁻¹, and an asymmetry parameter η=0.835(2). At 1200 K, ω₀ decreases to 516(3) Mrad s⁻¹, and η also decreases to 0.790(4). Although weak, the temperature dependence of ω₀ is consistent with a (1−BT ^3/2) power law, in which B=6×10⁻⁶ K^−3/2. The EQI also manifests a very narrow linewidth. We observed no evidence either for magnetic ordering or for structural phase transitions in the temperature range covered by this experiment. Moreover, the sharpness of the EQI indicates that the samples as prepared are remarkably free of strain and defects. These results indicate that the Zr₂Ni structure does not promote the formation of defects and that the power-law dependence of ω₀ on T is insensitive to the asymmetric nature of the crystal. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine Interactions in CeT2Ge2 (T = Mn,Co) Heavy Fermions Compounds Measured by TDPAC

Time Differential Perturbed γ–γ Angular Correlation (TDPAC) technique was used to measure the magnetic hyperfine field at both Ge and Ce sites in CeMn₂Ge₂ and CeCo₂Ge₂ intermetallic compounds. The ¹¹¹In (¹¹¹Cd) probe nuclei was used to investigate the hyperfine interaction at Ge sites, while the ¹⁴⁰La (¹⁴⁰Ce) nuclei was used to measure the magnetic hyperfine field at Ce site. The present measurements cover the temperature ranges from 10–460 K for CeMn₂Ge₂ and 9–295 K for CeCo₂Ge₂, respectively. The result for ¹¹¹Cd probe showed two distinct electric quadrupole frequencies above magnetic transition temperatures, in both compounds and a combined interaction in the magnetic region. The temperature dependence of the magnetic hyperfine field at ¹¹¹Cd at Mn site for the CeMn₂Ge₂ compound showed a transition from ferromagnetic to antiferromagnetic phase around 320 K and from antiferromagnetic to paramagnetic phase at 420 K. While a small magnetic field was measured on ¹¹¹Cd at Co site, no magnetic field on ¹⁴⁰Ce site was observed in CeCo₂Ge₂. This revised version was published online in September 2006 with corrections to the Cover Date.     

Shape coexistence near the double-midshell nucleus 111Rh

The decay of ¹¹¹Ru obtained from fast on-line chemical and mass separation has been investigated by β-γ-t and γ-γ coincidence techniques. Earlier spin and parity assignments of ¹¹¹Rh levels based on extrapolations of level systematics are confirmed. In particular, the K=1/2 intruder band is supported by the hindrance of E2 transitions between deformed and spherical states and enhancement of intraband E2 transitions. The excitation energies of intruder band members in Rh isotopes show a minimum at ¹⁰⁹Rh₆₄, with two neutrons less than ¹¹¹Rh at the N=66 midshell. This trend, which differs from the one in the higher-Z neighbouring elements Ag and Cd with minima at N=66, follows the evolution of deformation observed in the lower-Z elements Ru and Mo. Received: 25 June 1997 / Revised version: 10 September 1997     

Hyperfine Interactions in the Sr0.88Ba0.12HfO3 Compound

The temperature dependence of the hyperfine parameters in a powder sample of the Sr_0.88Ba_0.12HfO₃ compound has been investigated for the first time using perturbed angular correlation spectroscopy. The time spectra were measured as a function of the temperature from 293 to 1273 K. Activated ¹⁸¹Ta nuclei were used as hyperfine probes at the Hf sites. The analysis of the time spectra indicates the presence of two different surroundings for the Ta probes. The most populated site (f ₁≈75% at laboratory temperature) was fitted with the usual static quadrupole hyperfine interactions found in most perovskite-type compounds. The resulting quadrupole frequency is ω _Q ≈24 Mrad/s at this temperature. This frequency continuously decreases to nearly 9 Mrad/s at 1273 K. Its line width temperature dependence displays three regions and the changes observed probably correspond to phase transitions. The other site has a large and temperature independent quadrupole frequency ω _Q ≈120 Mrad/s that reversibly transforms into the first at high temperatures and probably originates from some defect in the structure. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine interactions in 111Cd-doped lutetium sesquioxide

We report here first Perturbed Angular Correlation (PAC) results of the electric field gradient (EFG) characterisation at ¹¹¹Cd impurities located at both non-equivalent cation sites of the bixbyite structure of Lutetium sesquioxide, between room temperature (RT) and 1273 K. The comparison with results coming from a systematic ¹¹¹Cd PAC study in bixbyites and with point-charge model (PCM) predictions shows the presence of a trapped defect at RT in the neighbourhood of the asymmetric cation site, which is completely removed at T > 623 K. The anomalous EFG temperature dependence in Lu₂O₃ can be described in the frame of a “two-state” model with fluctuating interactions, which enables the experimental determination of the acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electronic Relaxation in Indium Oxide Films Studied with Perturbed Angular Correlations

The electronic relaxation due to the ¹¹¹In → ¹¹¹Cd electron capture process was studied by measuring perturbed γγ-angular correlations after ¹¹¹In-ion implantation into indium oxide films of various compositions and thicknesses and deposited on different substrates. X-ray diffraction, Rutherford backscattering and secondary ion mass spectrometry were used to characterize the microstructure and composition of the samples. The film thickness and type of substrate were found to have much less influence on the after effect than the deposition parameters and impurities.     

Structural phase transition of Rh2O3 studied via PAC

The structural phase transition from α-Rh₂O₃ to β-Rh₂O₃ has been studied via the PAC method on dilute¹¹¹Cd impurity atoms implanted into α-Rh₂O₃. The measured antishielding factor β=112 (2) in the α-phase is similar to that found for¹¹¹Cd in other oxides with corundum structure (Al₂O₃. Cr₂O₃, Fe₂O₃). Oxygen with different crystallographic matrices show a pronounced correlation of the electric field gradient with the NN oxygen coordination.     

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.     

The martensitic phase transition in NiTi

The technique of perturbed angular correlation of gamma rays (PAC) was applied to study the martensitic phase transition of a shape-memory-effect alloy Ni_49.9Ti_50.1 doped with¹¹¹In/Cd probe atoms. Spectra measured above and below the martensite starting temperature,M _s 340 K, exhibit quadrupole interactions of probes at cubic and noncubic lattice sites, consistent with the respective crystal structures. Unlike spectra measured belowM _s those measured above exhibit a large frequency distribution attributed to lattice displacement waves observed in diffraction studies. Analysis tentatively suggests that the waves are dynamic and not static. BelowM _s, at 290 K, a static quadrupole interaction was observed with coupling frequency ₁=34 Mrad/s and asymmetry parameter =0.18, increasing to ₁=40.5 Mrad/s and =0.34 at 77 K.     

Hyperfine interactions at 181Hf(→181Ta) impurities implanted in Er2O3 and Gd2O3: structural and electronic dependence of the EFG in bixbyite sesquioxides

The time-differential-perturbed γ-γ angular-correlation technique (TDPAC) with ion-implanted ¹⁸¹Hf tracers has been applied to study the hyperfine interactions of ¹⁸¹Ta impurities in the cubic bixbyite structure of Er ₂ O ₃ and Gd ₂ O ₃ . The TDPAC experiments were performed in air in the temperature range 300-1073 K (in the case of Er ₂ O ₃ ) and 300-1173 K (in the case of Gd ₂ O ₃ ). Three electrical-quadrupole interactions were found in each oxide in the whole studied temperature range. Two of them were attributed to the electric-field gradients (EFG) acting on ¹⁸¹Ta probes substitutionally located on the two nonequivalent free-of-defects cation sites of the bixbyite structure. The EFG results are compared with predictions of the point-charge model and discussed together with previous results obtained with the probes ¹¹¹Cd and ¹⁸¹Ta in other isomorphous sesquioxides. The temperature dependence of the hyperfine parameters for both oxides is also discussed in terms of dilatometric expansion data. Received 29 December 2000 and Received in final form 8 March 2001     

Influence of the temperature and the ionisation potential on the hyperfine interaction of 111Cd ions in gaseous radioactive indium‐halides

The hyperfine interaction (HFI) of ¹¹¹Cd in different indium‐halides was studied by perturbed angular correlation (PAC). The ¹¹¹Cd ion arises from a ¹¹¹In by electron capture decay (EC) and joined Auger cascade. Coulomb fragmentation of the indium‐halide molecule produces the free excited ¹¹¹Cd ion. The nucleus decays via a γ–γ‐cascade which is perturbed by the HFI. Collisions of the ¹¹¹Cd ion with indium‐halide molecules can strongly change the perturbation. The perturbation was measured by the attenuation coefficient G₂₂(∞) and G₂₂(t). The molecular densities varied between 10¹⁴ and 10²⁰ cm⁻³. The present article shows the temperature dependence of the indium(III)‐halides and a fit for an empirical basic approach. The investigation of the InBr shows the independence of the attenuation coefficient from the temperature by indium(I)‐halides in contrast to the dependence of the indium(III)‐halides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Point defects in NiAl near the equiatomic composition

Perturbed γ-γ angular correlation spectroscopy was applied to study the structure and properties of point defects near¹¹¹In probes in quenched and annealed NiAl samples with five compositions in the range c_Ni=48–52%. Quadrupole interaction signals are analysed in terms of near-neighbor Ni-vacancy and nextnear-neighbor Ni antisite atom defects. Large mono- and divacancy concentrations were observed in all samples after quenching. For c_Ni<50%, the vacancy defects could not be annealed out (structural defects). For C_Ni>50%, large quenched-in vacancy concentrations annealed out near 500°C. For c_Ni=50%, most vacancies annealed out near 500°C, but about 35% of the¹¹¹In probes retained a trapped monovacancy. This is attributed to a very low value of the formation enthalpy of a vacancy next to the In probe, estimated to be 0.16(2) eV. A lesser annelaing stage detected at 300°C is tentatively attributed to diffusion of Al-vacancies.     

The electric quadrupole interaction of 111Cd in ZrZn2 and Zn in the samples prepared at 8 GPa

The perturbed angular correlation (PAC) measurements with the ¹¹¹In-¹¹¹Cd nuclear probe embedded into the lattice of the cubic (C15) Laves compound ZrZn₂ showed that ¹¹¹Cd nuclei experienced an axially symmetric electric quadrupole interaction with a frequency ν _Q  = 132.4 MHz at room temperature. The samples were synthesized and doped with the probe at a pressure 8 GPa. The temperature dependence of ν _Q was shown to be linear: ν _Q (T) = 147(1 − 0.033 T) MHz. Since the value of ν _Q is very close to that known for ¹¹¹Cd in the lattice of Zn, we have checked if it could be assigned to residual Zn metal in the sample. For the Zn sample melted and doped with ¹¹¹In at 8 GPa we have obtained ν _Q  = 117.3 MHz at 300 K and 127 MHz at 80 K – both values considerably lower than that for ¹¹¹In doped Zn samples prepared at an ambient pressure. These data, and the fact that ν _Q (T) in Zn is known to follow the T ^3/2 law, allow to attribute the ν _Q value quoted above to ¹¹¹Cd nuclei at the substitutional sites with tetrahedral symmetry in the Zn sublattice of ZrZn₂.     

Structural lattice defects in silicon observed at111in by perturbed angular correlation

Probing of structural defects in silicon by the perturbed γγ angular correlation (PAC) technique is demonstrated between 77 K and 1300 K. The behaviour of radioactive¹¹¹ In probe atoms implanted at 295 K, is monitored during isochronal annealing in n-type, p-type and intrinsic Si. Trapping of defects, produced by the¹¹¹In implantation itself or by postirradiation is studied in P-doped crystals (10¹⁶/cm³-10¹⁷/cm³).