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.     

Thermal evolution of the electric field gradient at 181 Ta in αHfNi

The perturbed angular correlation method has been employed to study the temperature dependence of the ¹⁸¹Ta hyperfine interaction parameters in the polycrystalline intermetallic compound αHfNi. At ambient temperature the frequency of the electric quadrupole interaction was ω _Q = 26.0(2) Mrad/s and the asymmetry parameter η = 0.22(1). The magnitude of the observed electric field gradient decreases with increasing temperature from 78 to 900 K. The calculations were done using the augmented plane wave plus local orbitals method as implemented in the WIEN2k code, using the generalized gradient approximation. In addition, a supercell calculation with Ta impurity located at the hafnium site was performed. The obtained result is in a good agreement with the experiment.     

PAC study on111Cd in antiferromagnetic CuO

400 keV¹¹¹In⁺ ions were implanted into CuO powder. After annealing at 570 K, more than 50% of the implants were found on substitutional sites in the monoclinic CuO lattice. PAC-spectra taken below the Néel temperatureT _N≈230 K revealed a broadening of the quadrupole spectrum and additional satellite frequencies caused by combined electric and magnetic hyperfine interaction. Assuming that the orientation of the electric field gradient is given by the point charge model and taking the 〈010〉 direction of the supertransferred fieldB _tr in CuO from neutron diffraction, we find a Larmor frequency of ω_L≈40 MHz corresponding toB _tr≈2.7 T at 60 K. Supported in part by BMFT under contract FK213N54930.     

Magnetic and quadrupole interaction studies at 111Cd in GdMn2 and TbMn2

The temperature dependence of the electric quadrupole interaction and the magnetic hyperfine field at ¹¹¹Cd probe sites in RMn₂ (R=Gd, Tb) has been studied by TDPAC method. In the paramagnetic region the quadrupole interaction frequency ν_Q in both compounds varies linearly with temperature. Below the Neel temperature, we find an abrupt decrease in the magnitude of ν_Q which is consistent with an expansion of the unit cell. As an important feature, the data near T_N shows the coexistence of localized and itinerant magnetism of Mn atoms. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

TDPAC characterization of tin oxides using181Ta

In connection with a general study of the evolution of tin-oxygen thin films, we report here on the hyperfine interactions of¹⁸¹Ta substitutionally replacting tin in the isolated phases SnO and SnO₂. For this purpose, pure SnO pressed powder and a thin SnO₂ film were implanted with¹⁸¹Hf. In both cases, unique quadrupole frequencies were found after thermal annealing treatments. The results indicate that the following hyperfine parameters: υ _Q = 740.6(2.1) MHz, η=0.07(2) and υ _Q = 971.5(1.9) MHz, η=0.72(1) characterize¹⁸¹Ta in SnO and SnO₂, respectively.     

Investigation of Hyperfine Interactions in GdNiIn Compound

Perturbed gamma–gamma angular correlation technique was used to measure the hyperfine interactions in the compound GdNiIn using the ¹¹¹InCd → and ¹⁴⁰La¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. A unique quadrupole frequency with asymmetry parameter η = 0.78 was observed for ¹¹¹Cd probe at In sites for the measurements above Curie temperature. Below T _C , the spectra for ¹¹¹Cd show combined magnetic dipole and electric quadrupole interaction. Below 85 K, a unique magnetic interaction is observed at ¹⁴⁰Ce. A linear relationship between the saturated magnetic hyperfine field and the magnetic transition temperature was observed for both probes, indicating that the main contribution to the mhf comes from the conduction electron polarization.     

Orientation of the oxygen-induced non-axially symmetric electric field gradient at181Ta in Ta

The interaction of dilute¹⁸¹Ta in Ta with interstitial oxygen has been investigated via the γ-γ TDPAC-technique applied to the 482 keV state in¹⁸¹Ta. The trapping of Oxygen leading to a quadrupole interaction frequency ν_Q=580(5) MHz with an asymmetry parameter of η-0.37 (1) has been observed after melting the radioactive parent isotope¹⁸¹Hf with Ta. The temperature dependence of the quadrupole interaction frequency between 17 K and 293 K was found to be very weak whereas η varied by about 10%. In a single crystal experiment the orientation of the three principal axes of the electric field gradient leading to ν_Q was determined. The V_zz-axis points in <110>, the V_yy-axis in <100> and the V_xx-axis in <110> direction. These results can be understood in the charge cloud model, suggested by Wrede et al. /1/ for a similar situation found in the HfNb system.     

Hf3Al2 and Zr3Al2 Isostructural Aluminides Studied by PAC with 181Ta and 111Cd Probes

The electric quadrupole hyperfine interactions for ¹⁸¹Hf/¹⁸¹Ta and for ¹¹¹In/¹¹¹Cd probes in polycrystalline Zr₃Al₂ and Hf₃Al₂ compounds were measured in the temperature range 24–1100 K. The hyperfine quadrupole interaction parameters were determined after different doping techniques and heat treatments of the samples.¹⁸¹Hf/¹⁸¹Ta was found to substitute the Hf/Zr sites and the ¹¹¹In/¹¹¹Cd impurities appear to substitute both the 8(j) Al sites and the three nonequivalent Hf/Zr sites.     

Electric field gradient at the Hf site and the phase transition in HfV2

The electric quadrupole interaction at the Hf site in the cubic HfV₂ compound, which shows lattice instability when cooled below 120 K, has been studied using the 482 keV (5/2⁺) state in¹⁸¹Ta. The time differential perturbed angular correlation (TDPAC) studies involving (133+136)-482 keV - cascade at room temperature show that only a fraction of¹⁸¹Ta nuclei see a cubic symmetry while the remaining experience a weak, randomly oriented, electric quadrupole interaction described by a Gaussian frequency distribution with _Q=6.1±1.0 Mrad/sec and the relative width =0.5. There is a dramatic change in TDPAC pattern indicating a phase transition when the compound is cooled down to 77 K. At this temperature the quadrupole interaction frequency is found to be _Q=40.6±3.0 Mrad/sec with =0.35. The experimentally observed EFG both at the Hf and the V sites are compared to the theoretical estimates based on point charge model.     

Stacking fault defects in hep cobalt studied by PAC

A study was made of lattice defects in hep Co metal using the technique of perturbed γ-γ angular correlations. Lattice sites of¹¹¹Cd impurities were characterized by their nuclear magnetic dipole and electric quadrupole interaction frequencies, respectively ω_L and ω_O. The signal for defect-free substitutional Cd impurities was found to have collinear interactions at 77K, with ω_L=438 Mrad/s and ω_O=7 Mrad/s. Analysis of measurements after heavy deformation at 100 K and after annealings up to 700 K gave little evidence of thermal trapping of point defects. However, in all spectral measured after deformation, a highly stable defect site with ω_L=418 and ω_O Mrad/s, and showing collinear interactions, was also observed. It is attributed to a¹¹¹Cd probe in a stacking fault (SF). Assuming a random distribution of proble atoms and SF's, a SF density of 20% was estimated.     

Shape-driving effect of the proton 1/2[541] band in 171Ta

High-spin states in ¹⁷¹Ta were populated through the heavy-ion fusion-evaporation reaction ¹⁵⁷Gd (¹⁹F, 5n)¹⁷¹Ta at 105MeV beam energy. Lifetimes of the levels of the πh_9/21/2[541] band have been measured by using the Doppler shift attenuation method. The transition quadrupole moments ( Q_t) and the quadrupole deformation (β₂) have been extracted. Both β₂ and Q_t values decrease slightly with increasing rotational frequency. The average β₂ value of 0.26 is 18% larger than that of the πh_11/2 9/2[514] band. Total Routhian Surface calculations have been performed with the non-axial deformed Woods-Saxon potential and the predicted values of the quadrupole deformation β₂ are in good agreement with that deduced from our lifetime measurement. The shape-driving effect is discussed.     

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.     

Temperature dependence of the electric field gradient in HfF4.HF.2H2O

The hyperfine quadrupole interaction in HfF₄.HF.2H₂O was studied using the time differential perturbed angular correlation (TDPAC) technique. The electric field gradient (EFG) and the corresponding asymmetry parameter were measured in the temperature range from 9 to 350 K. The EFG is characterized by a rather strong increase with temperature, whereas the asymmetry parameter reaches its maximum value (η≈1) at aboutT=160 K. At 420 K, the complex is dehydrated and looses HF: The quadrupole parameters determined from subsequent TDPAC measurements are charateristic for HfF₄.     

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.     

Temperature dependence of the electric quadrupole interaction at scandium impurities in zirconium and hafnium

The PAC probe⁴⁴Sc was employed to study the magnitude and temperature dependence of the electric quadrupole interaction at the site of the transition metal Sc in the hexagonal host lattices Zr and Hf. In Zr ∼ 100% of the probe atoms were situated on regular lattice sites and a quadrupole interaction frequency of υ_Q=7.8(1)MHz was measured at room temperature. For Hf the fraction of the probe atoms on regular lattice sites was smaller and a damping of the interaction pattern (υ_Q=9.3(5)MHz at 293 K) could not be excluded. The temperature dependence of the electric field gradient in both host lattices was found to be weak.     

Frequency Spectra of Quantum Beats in Nuclear Forward Scattering of 57Fe: The Mössbauer Spectroscopy with Superior Energy Resolution

Frequency spectra of quantum beats (QB) in nuclear forward scattering (NFS) are analysed and compared to Mössbauer spectra. Lineshape, number of lines, sensitivity to minor sites, and other specific properties of the frequency spectra are discussed. The most characteristic case of combined magnetic and quadrupole interactions is considered in detail for ⁵⁷Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of QB, six of which show the same energy separation as the six lines in Mössbauer spectra. Two other lines (called 2′ and 3′) are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction E _Q appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the 6th (largest frequency) lines. Each of the lines 3 and 5 generates a doublet split by 2E _Q, and the lines 2 and 4 generate triplets. In QB frequency spectra (QBFS) of thin absorbers of GdFeO₃ we demonstrate the enhanced spectral resolution compared to Mössbauer spectra. Small particle size in an antiferromagnet (Fe₂O₃) was found to affect the QBFS via enhancement of the intensity around zero-frequencies. An asymmetric hyperfine field distribution mixes up into the hybridization with dynamical beats, which enlarges the frequencies of the low-lying QBFS lines and makes their shifts relatively large compared to the shift of the highest-frequency line.     

Hyperfine interactions in cubic perovskites

ABO₃ perovskites display several physical properties determined by the characteristics of A and B cations. These compounds have cubic structure at high temperature. Lower symmetry cells that are distorted cubes are found at low temperature. Defects modify the properties of these compounds. Under standard conditions oxygen vacancies are produced. Cation substitution also alters the characteristics of perovskites. These materials have been studied by Perturbed Angular Correlation (PAC) spectroscopy and other hyperfine techniques. In this way abundant information is available to determine charge distributions close to probes. In the cubic phase perturbations were detected that are produced by the interaction of probes with defects. To show up these effects we analyze the quadrupole interaction at ¹⁸¹Ta in several compounds: ABO₃ with A=Ca, Sr and Ba, BaTi_1−x Hf _x O₃ and PbZr_1−x Ti _x O₃ for 0⩽x⩽1. Three different quadrupole interactions were found and are interpreted in terms of distinct probe-oxygen vacancy configurations. This revised version was published online in August 2006 with corrections to the Cover Date.