The angular overlap model applied to the calculation of nuclear quadrupole interactions

By the application of the angular overlap model to the calculation of nuclear quadrupole interactions (NQI), it is shown that it is possible to predict the NQI for the¹¹¹Cd nucleus in a cadmium complex with known coordination geometry. This fact makes it relevant to apply such calculations to nuclear quadrupole interaction data for¹¹¹Cd substituted zinc enzymes. It is demonstrated that with an approximate knowledge about the geometry and type of protein ligands from X-ray diffractions, it is possible to extract knowledge about type and geometry of nonprotein ligands in zinc enzymes, such as coordination of water, anions, or substrate molecules.     

Nuclear quadrupole interaction of111Cd on type-1 Cu-sites in blue copper proteins

The nuclear quadrupole interaction (NQI) of¹¹¹Cd substituted for Cu(II) on type-1 sites in blue copper proteins is characterized by high values of ₀ in the region of 300 Mrad/s, close to that for the catalytic zinc site in alcohol dehydrogenase. Type-1 Cu has usually two sulfur ligands and two nitrogen ligands and in some cases an oxygen ligand in either a distorted tetrahedral geometry or in a trigonal bipyramidal geometry. The near tetrahedral arrangement together with the ligand sphere containing the same number of sulfur ligands explains the value of ₀ in the blue copper proteins. The present work determined the partial NQI for methionine using the known structure of azurin. This value was then used in the angular overlap model to calculate the NQI for ascorbate oxidase the structure of which is also known and gave good agreement with experiment. NQI data for laccase and stellacyanin the structures of which are unknown, are also given.On leave from Technische Universität München, Germany.     

Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

Ferromagnetic Semiconductors Studied by Hyperfine Interaction of Nuclear Probes

The ferromagnetic spinels CdCr₂Se₄ and CuCr₂Se₄ were investigated by PAC (perturbed angular correlations) after implantation of the probes ¹¹¹In(¹¹¹Cd), ^111mCd, ¹¹¹Ag(¹¹¹Cd) and ⁷⁷Br(⁷⁷Se). The site occupation of different probes was determined by magnetic and electric hyperfine interactions. Theoretical calculations of hyperfine parameters by the WIEN97 code gave satisfactory efg (electric field gradients) but the magnetic hyperfine fields are inconsistent with the experiments. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electric-Field Gradients at Group-III Sites on GaAs and InAs (111)B Surfaces

We report a comparison of electric-field gradients (EFGs) measured at the group-III sites on epitaxially-grown surfaces of 2×2-reconstructed GaAs and InAs crystals. For this purpose, we used ¹¹¹In→¹¹¹Cd perturbed-angular-correlation (PAC) spectroscopy. Sharp spectral lines characterize the perturbation functions corresponding to both surfaces. On GaAs surface, we observe only one well-defined nuclear electric-quadrupole interaction (NQI); and on the InAs surface, we observe a primary and a secondary NQI. Very similar but experimentally-distinguishable values of the EFG-parameters characterize the primary interactions corresponding to the GaAs and InAs surfaces. Specifically, for the GaAs and InAs surfaces, ω _Q =28.0±0.2 Mrad s⁻¹ and η=0.43±0.02 and ω _Q =28.8±0.2 Mrad s⁻¹ and η=0.39±0.02, and the angles between EFG z-axis and the (111) direction are 65°±3° and β=53°±3°, respectively. These unexpected results indicate that the so-called lattice contribution to the EFG is not significant. Moreover, for the primary NQIs on both surfaces, the similar parameter values demonstrate that chemical differences between the ¹¹¹In probe and the indigenous Ga surface atoms cause no large quantitative effects. This information indicates that impurity probes and PAC spectroscopy can be used effectively to investigate III–V surfaces. This revised version was published online in September 2006 with corrections to the Cover Date.     

Temperature dependence of electric field gradient in TbCoO3

The temperature dependence of the electric field gradient (efg) in TbCoO₃ perovskite was measured by perturbed angular correlation (PAC) technique using ¹¹¹Cd and¹⁸¹Ta nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced into the compound through a chemical process during sample preparation. The electric quadrupole interactions at ¹¹¹Cd show two different sites, assigned to probe substituting Tb and Co atoms. The temperature dependence of quadrupole frequencies show sharp discontinuities which have been interpreted as thermally activated spin state transitions from low-spin ground state configuration to the intermediate-spin state and from intermediate-spin to high-spin state of Co^3?+? ion. For ¹⁸¹Ta only one interaction was observed, which was assigned to probe at Co site. Indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out. No magnetic order was observed till 77 K.     

Hyperfine interactions of111Cd impurities in Cr2O3

The electric and (antiferro-)magnetic hyperfine interaction of¹¹¹Cd in α-Cr₂O₃ after¹¹¹In implantation was studied via PAC. Two fractions with axially symmetric electric field gradients were observed having antishielding factors of V_zz/V_zz ^pc=79(12) and 106(16), respectively. From the line broadening of the Fourier components of the first fraction its supertransferred magnetic field was estimated as B_loc (20 K)≤0.41 T, about two orders of magnitude smaller than for substitutional¹¹¹Cd in NiO and CoO.     

Magnetic hyperfine fields at Gd and in sites in GdPdIn compound

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the hyperfine interactions in the intermetallic compound GdPdIn using ¹¹¹In→ ¹¹¹Cd and ¹⁴⁰La→ ¹⁴⁰Ce probe nuclei at the In and Gd sites, respectively. The PAC results for ¹¹¹Cd show two well-defined electric quadrupole frequencies above T _C assigned to probes occupying Gd and In sites, with ~50% of site occupation each. The fraction corresponding to In sites increases with temperature reaching 95% around 500 K. Below T _C the measurements for ¹¹¹Cd probe showed combined electric quadrupole plus magnetic dipole interaction with sharp increase in the magnetic field below around 80 K. A pure magnetic interaction with lower hyperfine field values was observed at the Gd sites occupied by ¹⁴⁰Ce below 100 K.     

Perturbed Angular Correlation of the stretched cascade in the decay of 180mHf using a digital spectrometer

We report on the measurement of the nuclear quadrupole interaction (NQI) at Hf sites using the nuclear probe ^180mHf in HfF₄·HF·2H₂O at 300 K by exploiting all possible start quanta in the stretched cascade with a digital Time Differential Perturbed Angular Correlation (TDPAC) spectrometer. With conventional spectrometers, multiple prompt start signals would paralyze the router. The gain in coincidence rate is about a factor of 5 compared to a conventional spectrometer using a single start only. With multiple starts ^180mHf is a promising new isomeric nuclear probe in TDPAC experiments. As an additional feature we implemented the possibility to measure up to four cascades simultaneously in order to save data collection time or to measure isobaric contaminations like ^111mCd and ¹¹¹In.     

Pressure and temperature dependence of the electric field gradient (EFG) in CdS

Using the time differential perturbed angular correlation technique (TDPAC), the electric hyperfine interaction of¹¹¹Cd in the II-VI-semiconductor CdS was investigated. The results of the temperature and pressure dependence of the electric field gradient (EFG) are discussed. The binding energyE _b for¹¹¹In at a Cd lattice site and a Cd vacancy (¹¹¹In_Cd–V_Cd–pair) could be estimated to 340 meV±80 meV by means of an Arrhenius-Plot. The disappearance of the 79 MHz and 73 MHz frequencies under a pressure of about 30 kbar could be due to their vacancy character or to the beginning of the phase transition to rocksalt structure.     

Sign of the electric field gradient at111Cd in Zr and Sb

The Β-γ TDPAC technique was applied to¹¹¹Ag implanted in Zr and Sb metal single crystals in order to determine sign and magnitude of the quadrupole interaction at the site of¹¹¹Cd in these metals. An analysis of the data taken at 293K yielded Ν_Q=+15.4(6) MHz for¹¹¹Cd in Zr and Ν_Q=?107.5(20) MHz for¹¹¹Cd in Sb. From these values electric field gradients of +7.3(8)×10¹⁶ V/cm² and ?5.56(62)×10¹⁷ V/cm² for Cd in Zr and Sb are derived respectively.     

Semiconductors with structurally determined vacancies – PAC studies

Ternary semiconductors of type □A^IIB ₂ ^III C ₄ ^VI with an ordered array of vacancies were investigated by PAC. Thereby the six probes ¹¹¹Cd, ¹¹⁷Cd(¹¹⁷In), ¹¹¹In(¹¹¹Cd), ¹¹¹Ag(¹¹¹Cd), ⁷⁷Br(⁷⁷Se) and ⁷⁷Kr(⁷⁷Br) were applied. The positions of the different probes are determined and the corresponding electric field gradients by the WIEN 95 code calculated. By ¹¹¹Ag(¹¹¹Cd)-probes the vacancies could be substituted. The electronic charge density distributions are discussed. At elevated temperatures the substances show order–disorder transitions. As compared to the X-ray diffraction patterns the beginning of disorder is observed by PAC at distinctly lower temperatures. This revised version was published online in August 2006 with corrections to the Cover Date.     

PAC study of CdFe2O4 and ZnFe2O4

The time-differential perturbed angular correlation technique has been used to study the temperature dependence of the electric-field gradient at ¹¹¹Cd nuclei in the spinels CdFe₂O₄ and ZnFe₂O₄. The samples were doped with radioactive ¹¹¹In tracers which decay to ¹¹¹Cd by electron capture. For both compounds the results of the measurements show the presence of two electric quadrupole interactions. One is well defined and corresponds to a fairly symmetrical electric field gradient. The other is broadly distributed and corresponds to electric field gradients with lower symmetry. The fractions of probes associated with these interactions are temperature dependent. The interactions are analysed taking into account the possible sites in the spinel structure for In occupation. The results are compared with those obtained with other spinels. The electric field gradient associated with the octahedral site is compared with the predictions of point charge model and with Mössbauer spectroscopy results at iron site.

     

Nuclear orientation of111m Cd in Zn and Be and the quadrupole moment of the 245keV state

The 48.7 m^111m Cd activity was implanted in Zn and Be single crystals which were soldered to the cold finger of a dilution refrigerator and kept below 0.2 K during implantation. Subsequent nuclear orientation experiments allowed the determination of the quadrupole interaction frequencyv _Q of the 11/2^? isomeric state of¹¹¹Cd in Zn and Be as ?139 (15) MHz and +43(16) MHz respectively. With these results we derive the quadrupole moment of the 5/2⁺ 245 keV level of¹¹¹Cd including sign asQ = +0.83(13) b and the sign of the electric field gradient for Cd in Be. The half-life of^111m Cd was redetermined as 48.67 (6) m.     

Measurement of Quadrupole Interactions in LaMO3 (M = Cr,Fe, Co) Perovskites by TDPAC

The perturbed angular correlation (PAC) technique has been used to study the electric field gradient (EFG) in LaCoO₃ perovskite. The results are compared with those for LaCrO₃, LaFeO₃ measured earlier. The PAC probe, ¹¹¹In → ¹¹¹Cd, was introduced in the oxide lattice by means of chemical reaction during sample preparation. In the present work, the temperature dependence of the electric quadrupole interaction parameters, for LaCoO₃ was investigated. The resulting systematics of EFG at ¹¹¹Cd, in La(Cr,Fe,Co)O₃ perovskites, reveals a linear dependence with temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

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.     

Hyperfine interactions at R and In sites in RNiIn (R = Gd, Tb, Dy, Ho) compounds measured by perturbed angular correlation spectroscopy

Perturbed gamma–gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (mhf) in RNiIn (R = Gd, Dy, Tb, Ho) intermetallic compounds using the ¹¹¹In→¹¹¹Cd and ¹⁴⁰La→¹⁴⁰Ce probe nuclei. The PAC spectra for ¹¹¹Cd measured above magnetic transition temperature show a major fraction with a well defined quadrupole interaction for all compounds except GdNiIn where a single frequency was observed. PAC measurements below T _C showed a combined electric quadrupole plus magnetic dipole interaction for ¹¹¹Cd probe at In sites, and a pure magnetic interaction for ¹⁴⁰Ce at R sites. The temperature dependence of mhf measured with ¹⁴⁰Ce at R sites shows that the values of fields drop to zero at temperatures around the expected T _C for each compound. However, in the measurements with ¹¹¹Cd at In sites, the mhf values become zero at temperatures which are smaller than T _C . The difference between the temperatures at which mhf is zero for ¹⁴⁰Ce and ¹¹¹Cd probes correlates with T _C . For each compound this difference decreases with T _C . The results are discussed in terms of the RKKY model for magnetic interactions and the existence of two magnetic systems, with distinct exchange interaction energies due to different types of atomic layers in these compounds.     

Hyperfine interactions of 111Cd in TiO2 (Rutile) studied by perturbed angular correlations

The time-differential perturbed angular correlation for ¹¹¹Cd nuclei has been measured after ¹¹¹In implantation into polycrystalline TiO₂. The observed perturbation functions are characterized by a well-defined electric field gradient with the quadrupole coupling constant ν_Q = 105(1) MHz and the asymmetry parameter η = 0.18(1). We attribute these hyperfine parameters to ¹¹¹Cd on the (distorted) substitutional cation site in rutile. The PAC results are compared with those in SnO₂ rutile as well as with X-ray diffraction, RBS channeling experiments and point charge model calculations including relaxation of the probe atom surrounding.     

Electric quadrupole interaction at 111Cd and 119Sn in isostructural NiIn and CoSn compounds

The perturbed angular correlation technique and Mössbauer spectroscopy were applied to study the electric field gradient on ¹¹¹Cd and ¹¹⁹Sn probe atoms in isostructural NiIn and CoSn compounds. The ¹¹¹Cd PAC measurements performed in the temperature range 80--1100 K demonstrated the existence of two axially symmetric EFG's in each of the investigated compounds, related to the 2(d) and 1(a) probe sites in the B35 structure. A 1(a)-site preference for Cd probes in CoSn compound was observed. The temperature dependence of the quadrupole frequencies for ¹¹¹Cd in both compounds, interpreted in terms of the empirical model proposed by Christiansen et al.[1], follows a T^3/2 relation with different slope parameters for each of the observed frequencies. These results are combined with the data from the Mössbauer experiment. The ¹¹⁹Sn Mössbauer spectra taken at liquid nitrogen and at room temperatures showed two quadrupole split doublets with the intensity ratio 2:1 for CoSn and 8:1 for NiIn_0.99Sn_0.01sample, giving an evidence of 2(d)-site preference for tin atoms in NiIn. The EFG values measured on ¹¹⁹Sn are 2.5 to 4 times larger than those on ¹¹¹Cd nuclei, while the ratio of the respective Sternheimer antishielding factors is equal to 0.77. Within the limits of errors no differences were observed in the magnitude and temperature dependence of Debye--Waller factors for 2(d) and 1(a) ¹¹⁹Sn positions in CoSn and NiIn lattices.