The electric field gradients for Hg at different lattice sites in Be

¹⁹⁹Tl ions were implanted into a Be single crystal. From time differential perturbed e^– angular correlation experiments with the 5/2^– state of¹⁹⁹Hg the effective electric field gradientV _zz ^eff =±15.2(1.8)×10¹⁷ V · cm^–2 was deduced for Hg on an octahedral interstitial site in Be. From a nuclear orientation experiment, the sign ofV _zz ^eff for^197mHg implanted in Be is determined. In time differential perturbed angular correlation experiments with the 5/2 state of¹⁹⁹Hg populated in the -decay of¹⁹⁹Au implanted into Be, the effective electric field gradient for Hg on a substitutional site in Be was determined asV _zz ^eff =–1.2(2)×10¹⁷V · cm^–2. The dependence of the site population of Tl implanted in Be on annealing was investigated by use of the channeling technique. In a comparison of the derived effective field gradients with the corresponding lattice field gradient contributions different electronic enhancement factors are derived for different sites of Hg in Be.     

Quadrupole interaction studies of Hg in Zn and lattice location of Hg and Au in Zn

¹⁹⁷Hg,¹⁹⁹Tl and¹⁹⁹Au ions were implanted into Zn single crystals. From time differential perturbed e^–- and - angular correlation experiments with the 5^–/2 states in¹⁹⁷Hg and¹⁹⁹Hg, respectively, the quadrupole interaction for Hg in Zn is reinvestigated. Only the data for the¹⁹⁹Au sample exhibit a single unique quadrupole precession pattern in agreement with the result of the lattice location studies by use of the channeling technique, which show Hg to occupy random lattice sites in Zn while Au is substitutional.From the measured interaction frequency, sign and magnitude of the effective electric field gradient for Hg on the substitutional site in Zn is determined asV _zz ^eff = +15.2(1.8)×10¹⁷V·cm^–2. By comparison of this value to the corresponding lattice field gradient contribution an electronic enhancement factor ofK(HgZn)=2.6 is deduced in disagreement with a previous derivation of this quantity, but which supports the proposal that the electronic field gradient depends linearly upon the valence of the impurity probe atom.     

TDPAC studies on181Hf implanted into diamond

¹⁸¹Hf ions were implanted at (900±10)K into (100) cleaved single crystal diamonds. After subsequent annealing residence site parameters were determined for the implanted ions employing the (133 keV)–(482 keV) cascade in¹⁸¹Ta in TDPAC technique. Fractions of 5% each were determined that experience axially symmetric electric field gradients (EFG) V_zz(1)=5.5×10¹⁷ V cm^–2 and V_zz(2)=9.6×10¹⁷ V cm^–2, respectively. The bulk of the ions are strongly disturbed by still higher electric field gradients.     

The magnetic hyperfine field and the electric field gradient at181Ta impurities in Gd metal

The magnetic hyperfine fieldH _hf and the electric field gradientV _zz at¹⁸¹Ta impurties in metallic Gd were determined by time differential perturbed angular correlation measurements with the 133 keV K-conversion electron 482 keV -cascade of¹⁸¹Ta. The sources for these measurements were prepared by implantation of radioactive¹⁸¹Hf ions into Gd. The results are: |H _hf(TaGd; 77 K)|=285(14)kG, and |V _zz(TaGd; 330 K)|=5.32(15)·10¹⁷V/cm². The value ofH _hf fits well into the systematics for 5d impurities in Gd and indicates a positive core polarisation contribution, which is expected if the conduction electrons of Gd have to a large extent d-character. The electric field gradients of the 5d impurities in Gd are not consistent with a proportionality between the ionic and the electronic contribution.     

The electric field gradient at188Os nuclei in Re metal

The electric quadrupole interaction of the first excited 2⁺ state of¹⁸⁸Os in hexagonal rhenium metal was investigated by means of the time-differential perturbed angular correlation technique. From the observed quadrupole frequencyV _Q=170(7) MHz, we deduce an electric field gradient value of |V_zz|=4.77(23)·10²¹V/m² for the system OsRe. The half-life of the 2⁺ state was measured to be 641(4)ps.     

Static electric quadrupole interaction of Ta- and Hf-ions in barium and lead titanate

The static electric quadrupole interaction of¹⁸¹Ta and¹⁷⁸Hf in polycrystalline barium and lead titanate at the site of titanium has been measured using time differential PAC and the Mössbauer effect. The electric field gradients (EFG) at room temperature at the¹⁸¹Ta nucleus are ¦V _zz¦=(3.6±0.2)·10¹⁷V/cm² in BaTiO₃ and ¦V _zz¦=(14.6±0.6)·10¹⁷ V/cm² in PbTiO₃. The temperature dependence of the quadrupole interaction has been studied giving the following EFG values: ¦V _zz¦=(2.4±0.2)·10¹⁷ V/cm² in the monoclinic and ¦V _zz¦=(1.1±0.3)·10¹⁷ V/cm² in the rhomboedral phase of BaTiO₃, and ¦V _zz¦=(15.7±0.6)·10¹⁷ V/cm² for¹⁸¹Ta/PbTiO ₃ at 77 °K. The EFG of¹⁷⁸Hf in PbTiO₃ has been derived from a Mössbauer effect experiment to beV _zz=+(10.7±0.5)·10¹⁷ V/cm². The results are compared with EFG's calculated in a point charge model and with experimental EFG's measured at⁴⁴Sc and⁵⁷Fe in the same titanates by other authors. Contributions of covalent bonds to the effective EFG's in perovskit crystals are discussed.     

Electric quadrupole nuclear orientation of182Ta as graphite intercalation compound TaCl5

Nuclear orientation of¹⁸²Ta achieved by electric quadrupole interaction in TaCl₅ molecules intercalated in oriented graphite crystals has been observed. Assuming the electric field gradient V_zz along the c-axis and proper intercalation yields V_zz=+0.32 (4)×10¹⁸V/cm². Other possibilities and sources of reduction of the nuclear orientation effect are considered.     

Electric quadrupole interactions in Ru-Sc alloys

In TDPAC studies of the electric quadrupole interaction in Ru_xSc_1–x alloys two different electric fieldgradients (EFG) have been observed at the site of⁹⁹Ru: V_zz(I)=12.6·10¹⁷ V/cm² and V_zz(II)=18.9·10₁₇ V/cm². The corresponding relative fractions f(I) and f(II), respectively, vary with the Ru concentration x. For low concentrations x<0.01 most nuclei (f(I)0.8) experience the smaller EFG V_zz(I). At x=0.01, however, the fraction f(I) goes abruptly to zero and V_zz(II) becomes dominant. In view of these results the previous interpretation of V_zz(II) as the EFG of dilute Ru on substitutional Sc sites can no longer be maintained. The Ru-Sc configurations producing these EFG's have not yet been identified. In the intermetallic compound Ru₂Sc the interaction is completely different, in RuSc₃, however, similar values have been observed.     

Magnetic hyperfine field and electric field gradient for111Cd at the Kr/Fe interface

Polycrystalline iron foils implanted to high Kr doses were doped with¹¹¹In and the magnetic hyperfine field as well as the electric field gradient measured. From the present TDPAC experiments we observed a substantial fraction of probes in a “defect” site, with its hyperfine parameters Δ|B_hf|=6.9% and V_zz=1.12 10¹⁷V/cm², in close similarity with those expected for the Kr/Fe interface.     

The quadrupole interaction at181Ta in Zn

The quadrupole interaction (QI) in hexagonal close packed zinc lattice was measured using the 482 keV, 10.6 ns probe state of¹⁸¹Ta employing the time-differential perturbed angular correlation technique. The electric field gradient (EFG) at¹⁸¹Ta in Zn was derived from the measured quadrupole interaction frequency at room temperature asV _zz =12.202×10¹⁷ V/cm². The quadrupole interaction measured at various temperatures displayed normal temperature dependence similar to that seen by this probe in many non-cubic hosts.     

The effect of temperature and concentration on the electric field gradient in binary indium alloys

The concentration and temperature dependence of the quadrupole hyperfine interaction of¹¹¹Cd in InTl (hcp),InPb (fct),InTl (fct) andInCd (fct and fcc) alloys were studied using the perturbed angular correlation technique. The change in the observed quadrupole interaction frequency with concentration can be described by a linear dependence on the axial ratioc/a in all cases. In the alloys with identical crystal structures the strength of thec/a dependence is independent of the solute, in contrast to the strength of the concentration dependence. In all cases where no phase transition occurs, the change in the electric field gradient with temperature follows the empirical relationV _zz (T)=V _zz(0) · (1–B·T ^3/2), where the coefficientB depends on the lattice structure, on the solute-solvent combination and on the concentration. The phase transitions ofInCd alloys at 293 K could clearly be seen as discontinuities in the temperature curves. A similar series of discontinuities observed around 116 K suggests the existence of a cubic low temperature phase.     

Investigation of the hyperfine interaction of the transition element impurity193Ir in gadolinium

The combined magnetic and electric hyperfine interaction of dilute¹⁹³Ir impurities in ferromagnetic Gd has been investigated by means of the Mössbauer effect. The magnetic hyperfine field of¹⁹³Ir in Gd at 4.2 K is: |H _hf(Ir:Gd)|=624(6) kG.The electric fieldgradient at the site of Ir in Gd is:V _zz (Ir:Gd)=+19.5(5.0) × 10¹⁷ V/cm².The fieldgradient is axially asymmetric with an asymmetry parameter of 0.53(2)1.     

Structural phase transitions in Pd0.8 Si0.2 and Pd0.75 Si0.20 Ag0.05 alloys observed by pac

The phase transition in the alloys Pd_0.8 Si_0.2 and Pd_0.75 Si_0.20 Ag_0.05 have been investigated through the quadrupole interaction of¹¹¹Cd impurities. The quadrupole interactions were measured by means of the TDPAC technique from room temperature up to about 870 K. The variation of the quadrupole interaction with temperature in the alloy PdSiAg shows aT ^3/2 dependence below and above 629 K, with coefficientsB=5.43(25)·10⁻⁵ K^−3/2 andB=3.70(15)·10⁻⁵ K^−3/2, respectively. This demonstrates that the alloy undergoes a phase transition around 629 K. The existence of two electric field gradients observed in the alloy PdSi,V _zz (1)=3.47(54)·10¹⁷ V/cm² andV _zz (2)=2.29(36)·10¹⁷ V/cm², indicates that there are two different¹¹¹Cd sites. The corresponding fractionsf ₁ andf ₂ strongly depend on temperature. Below 520 K, most¹¹¹Cd nuclei are subject to the higher EFGV _zz (1) (f ₁≈70%), whereas above 520 Kf ₁ falls rapidly to zero andV _zz (2) becomes dominant. The temperature dependences of thef ₁ andf ₂ reveal a picture of the phase transition between the two crystal structures.     

DPAC study of radiation damage in178W recoil implanted silicon

The annealing behaviour of radiation damage in¹⁷⁸W recoil implanted n-type Si is studied from 295 to 641 K by the differential perturbed angular correlation method (DPAC), using¹⁷⁸Hf as probe nuclei for the first time. Preliminary results suggest that oxygen-vacancy (O-V) pairs are observed, which give rise to a quadrupole interaction characterized by |V _zz|=5.41×10¹⁸ V/cm² (v _Q=2550 MHz). The probe nuclei also experience an electric field gradient (EFG) due to distant defects.     

The electric field gradient at iodine impurities in zinc and cadmium

The electric quadrupole interaction at iodine implanted in zinc and cadmium single crystals is investigated, using the - perturbed angular correlation technique. The results show that the implanted¹²⁹Te and¹³²Te isotopes occupy two well-defined sites, identified by the electric field gradient parameters. One fraction of the impurities is substitutionally implanted, witheq _zz=–1.49(9)10¹⁸ and –1.58(15)10¹⁸V/cm², in Zn and Cd respectively. The other fraction is identified as an impurity-divacancy configuration with |eq _zz |=3.81(8)10¹⁸V/cm²; =0.72(4) for Zn and |eq _zz |=3.55(11)10¹⁸V/cm²; =0.53(13) for Cd. The electronic enhancement in the electric fieldgradient at the substitutional site equalsK=–0.66 and –0.56 respectively and follows the empirical trend ofK versus the impurity valence for the s-p elements in Zinc.     

Hyperfine interaction of ZrO2 — Tetragonal phase

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beV _zz =(17.5±0.4)·10¹⁷ Vcm⁻² and axially symmetric.     

Synthesis and Mössbauer Study of Maghemite Nanowire Arrays

An analytical formula for the distance dependence of the electric field gradient produced by a Gaussian charge density distribution n(r) is derived. This charge density is displaced by z ₀ along the z-axis. The system has cylindrical symmetry; hence it suffices to calculate V _zz(0). It turns out that V _zz(0) is always smaller than the value with the total charge shrunk into a point. For distances larger than about four times the Gaussian width σ the expression approaches the point charge value. For z₀ → 0, i.e. a spherically symmetric charge distribution around the origin, V _zz(0) vanishes quadratically, as required by symmetry. A slab-wise calculation in cylindrical coordinates is presented which shows the contribution to V _zz(0) for infinitesimally thin slabs as a function of distance from the origin. This analytical formula allows for a fast computation of electric field gradients from a given charge density distribution for Gaussian expansions of Slater-type orbitals. An example for a hydrogen atom will be given.     

Electric field gradients at 181Ta probe in ZrNi: Results from perturbed angular correlation and first-principles calculations

Results of temperature dependent perturbed angular correlation (PAC) measurements in the equiatomic ZrNi alloy have been reported for the first time using ¹⁸¹Hf probe. At room temperature, values of quadrupole frequency and asymmetry parameter for the major component (~80%) are found to be ω_Q=26.8(4) Mrad/s, and η=0.413(7). The resulting electric field gradient comes out to be V_zz=2.99 ×10¹⁷ V/cm² and this corresponds to the probe nuclei occupying the regular substitutional Zr sites. In ZrNi system, no magnetic interaction is observed down to 77 K indicating absence of any magnetism in this material. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies on an inactive but similarly prepared sample confirm the dominant presence of the orthorhombic ZrNi phase in the sample. A complementary density functional theory (DFT) calculation results in V_zz=−2.35×10¹⁷ V/cm², η=0.46 at the ¹⁸¹Ta probe impurity site and zero magnetic moment on each atomic site, in close agreement with the experimental results. Furthermore, it is found that electric field gradient for the regular component follows a T^3/2 temperature dependence between 77 and 353 K, beyond which it varies linearly with temperature.     

A contrastive study on the influences of radial and three-dimensional satellite gravity gradiometry on the accuracy of the Earth's gravitational field recovery

The accuracy of the Earth’s gravitational field measured from the gravity field and steady-state ocean circulation explorer(GOCE),up to 250 degrees,influenced by the radial gravity gradient V zz and three-dimensional gravity gradient V ij from the satellite gravity gradiometry(SGG) are contrastively demonstrated based on the analytical error model and numerical simulation,respectively.Firstly,the new analytical error model of the cumulative geoid height,influenced by the radial gravity gradient V zz and three-dimensional gravity gradient V ij are established,respectively.In 250 degrees,the GOCE cumulative geoid height error measured by the radial gravity gradient V zz is about 2 1/2 times higher than that measured by the three-dimensional gravity gradient V ij.Secondly,the Earth’s gravitational field from GOCE completely up to 250 degrees is recovered using the radial gravity gradient V zz and three-dimensional gravity gradient V ij by numerical simulation,respectively.The study results show that when the measurement error of the gravity gradient is 3×10 12 /s 2,the cumulative geoid height errors using the radial gravity gradient V zz and three-dimensional gravity gradient V ij are 12.319 cm and 9.295 cm at 250 degrees,respectively.The accuracy of the cumulative geoid height using the three-dimensional gravity gradient V ij is improved by 30%-40% on average compared with that using the radial gravity gradient V zz in 250 degrees.Finally,by mutual verification of the analytical error model and numerical simulation,the orders of magnitude from the accuracies of the Earth’s gravitational field recovery make no substantial differences based on the radial and three-dimensional gravity gradients,respectively.Therefore,it is feasible to develop in advance a radial cold-atom interferometric gradiometer with a measurement accuracy of 10 13 /s 2-10 15 /s 2 for precisely producing the next-generation GOCE Follow-On Earth gravity field model with a high spatial resolution.     

High Temperature Diffusion of 6Li Adsorbed on a Ru(001) Single Crystal Surface as Seen by Pulse NMR

Diffusion measurements on lithium atoms adsorbed on a ruthenium single crystal were performed in the high temperature regime (1100–1200 K). Pulsed NMR techniques were utilized to produce and observe the decay of magnetization patterns from which the diffusion coefficient was extracted. The observed temperature dependence could be described by D = (10 ± 7) cm²/s · exp (−(0.46 ± 0.07) eV/kT). The extremely high diffusion coefficient and prefactor are understood by a gas like adsorbate behavior. The electric field gradient has been measured with ⁷Li: V _zz = −5.0 ± 0.1 10¹⁵ V/cm² with an inhomogeneity of less then 1% as judged by the width of the satellite transitions.