Quadrupole interactions at57Fe in titanium metal

The electric field gradient (EFG) at⁵⁷Fe in titanium is measured using Mössbauer spectroscopy in the temperature range 80°–380°K. The value of EFG obtained at room temperature is 0.53(4)×10¹⁷ V/cm². The value of EFG obtained is compared with the conduction electron charge shift model.     

The internal electric fieldgradient in antiferroelectric PbZrO3 studied by perturbed angular correlations

The internal electric fieldgradient (EFG) at the Zr-site in antiferroelectric polycristalline PbZrO₃ has been investigated as a function of temperature by time differential angular correlation measurements. For the application of this technique PbZrO₃ was doped with small amounts of radioactive¹⁸¹Hf. In the antiferroelectric phase of PbZrO₃ the EFG decreases continously with increasing temperature and exhibits a sharp discontinuity at 230 °C, the Curie point of the compound. In the cubic phase a small remaining EFG is observed which is probably caused by lattice imperfections in the source. The antiferroelectric distortion of the lattice decreases slightly when the temperature approaches the Curie point, as shown by the temperature dependence of the asymmetry parameter of the EFG. The measured EFG at room temperature is compared with the result of a lattice sum calculation.     

Systematics of electric field gradients in metals

The intensive study of electric field gradients (EFG) at the site of atomic nuclei in noncubic metals has revealed several systematic trends, e.g. in the relation of the total EFG to the so called lattice EFG as well as in its variation with temperature. Numerous investigations have been carried out in order to test these systematics. The results will be reviewed and compared to the known trends. Progress in theoretical calculations of the EFG in pure and impurity — host systems will be discussed and compared to the latest available experimental data. Recent measurements of the EFG at metal surfaces and new calculations of the EFG at host sites in impure cubic metals may contribute to the understanding of the EFG in metals.     

TDPAC measurements on phase transition and hydrogen up-take in C15 laves phases

Intermetallic compounds with C15 structure are studied by TDPAC in the search for changes in the electric field gradient (EFG) on¹⁸¹Ta probes due to martensitic phase transition and hydrogen absorption. The residual low EFG of the cubic phase of HfV₂, ZrV₂ and Hf₅Zr₅V₂ is distinctly increased in the low temperature modification. Hydrogen does not affect the distribution of probe sites, but influences the features of the martensitic transition. In TaV₂ the sites with low EFG are split into an unperturbed and a strongly perturbed fraction.     

Pressure dependence of the electric field gradient in the AgIn2 intermetallic compound

The pressure dependence of the electric field gradient (EFG) in the AgIn₂ intermetallic compound was measured from zero up to 35 kbar using the time differential perturbed angular correlation technique in¹¹¹Cd. The unit cell volume and thec/a ratio variations with pressure were measured up to 80 kbar. The temperature dependence of the cell parameters was also measured, in a range varying from 300 K up to 458 K. The relationship of these results showed that the contribution of the lattice thermal expansion to the EFG variations is about 1/3, a small but not negligible part. The estimated EFG volumetric dependence is at variance with the systematic results found in pure metals. Work supported in part by FINEP and CNPq (Brasil).     

Nuclear acoustic resonance of V51 in a single crystal of V3Si above and below the martensitic transformation

The EFG and the Knight shift anisotropy of V⁵¹ in a single crystal of V³Si have been measured both above and below the martensitic transformation temperature T_m, using the nuclear acoustic resonance technique. Deviations of the axial symmetry of the EFG tensor have been found below T_m. The orientation of the tetragonal c axis in the crystal is also given.     

Isotope ratio of Cl NQR spin-lattice relaxation times in 1D hydrogen-bonding system of tetramethylpyrazine-chloranilic acid at high temperatures

The temperature dependences of spin-lattice relaxation time T ₁ of ³⁵Cl and ³⁷Cl NQR were studied for the co-crystal of tetramethylpyrazine (TMP) with chloranilic acid (H₂ca), TMP-H₂ca, in which one-dimensional hydrogen bonding is formed by alternate arrangement of TMP and H₂ca. The isotope ratio ³⁷Cl T ₁ / ³⁵Cl T ₁ was determined to be 1.0 ± 0.1 above ca. 290 K where a steep decrease of spin-lattice relaxation time T ₁ with increasing temperature was observed. In this temperature range it is suggested that the relaxation is originated from the slow fluctuation of electric field gradient (EFG). Beside EFG fluctuation due to the external-charge-density fluctuation, the small angle reorientation of the quantization axis triggered by a proton transfer motion between N...H-O and N-H...O hydrogen bonding states is proposed.     

Study of highT c superconductivity in Y1Ba2Cu3O7−x by PAC

HighT _c superconductivity in the YBaCuO superconductor has been investigated through the quadrupole interaction of the probe nuclei⁹⁹Tc. The quadrupole interactions were measured by the TDPAC method from 77 to 296 K. The probe nuclei⁹⁹Tc were introduced into the YBaCuO superconducting specimen by diffusion. The derived electric quadrupole interaction parameters show that the probe nuclei are subject to a unique EFG interaction and occupy a substitutional lattice site in the YBaCuO superconductor. A strong EFG of 10¹⁹ V/cm² was observed. The temperature dependence of the EFG exhibits a linear decrease with temperature increase. Anomalies of both EFG and ν were found in the superconducting transition temperature region. The role of the oxygen vacancies in the Cu−O chains is discussed.     

Mössbauer Studies on Nanocrystalline Diol Capped γ-Fe2O3

Nanocrystalline indium (nano-In) was prepared with different particle sizes by electrochemical deposition. The temperature dependence of the local electric field gradient (EFG) of nano-In was investigated in a temperature range of 20–300 K using the probe ¹¹¹In for perturbed γγ angular correlation (PAC) experiments. The temperature dependence of the EFG of nano-In can be described by a (1−BT ^3/2) dependence as in bulk-In. It is shown that the low temperature value of the EFG and the proportionality constant B vary systematically with particle size.     

Magnetic resonance tensors in Uracil: Calculation of 13C, 15N, 17O NMR chemical shifts, 17O and 14N electric field gradients and measurement of 13C and 15N chemical shifts

The experimental ¹³C NMR chemical shift components of uracil in the solid state are reported for the first time (to our knowledge), as well as newer data for the ¹⁵N nuclei. These experimental values are supported by extensive calculated data of the ¹³C, ¹⁵N and ¹⁷O chemical shielding and ¹⁷O and ¹⁴N electric field gradient (EFG) tensors. In the crystal, uracil forms a number of strong and weak hydrogen bonds, and the effect of these on the ¹³C and ¹⁵N chemical shift tensors is studied. This powerful combination of the structural methods and theoretical calculations gives a very detailed view of the strong and weak hydrogen bond formation by this molecule. Good calculated results for the optimized cluster in most cases (except for the EFG values of the ¹⁴N3 and ¹⁷O4 nuclei) certify the accuracy of our optimized coordinates for the hydrogen nuclei. Our reported RMSD values for the calculated chemical shielding and EFG tensors are smaller than those reported previously. In the optimized cluster the 6-311+G** basis set is the optimal one in the chemical shielding and EFG calculations, except for the EFG calculations of the oxygen nuclei, in which the 6-31+G** basis set is the optimal one. The optimal method for the chemical shielding and EFG calculations of the oxygen and nitrogen nuclei is the PW91PW91 method, while for the chemical shielding calculations of the ¹³C nuclei the B3LYP method gives the best results.     

Hydrogen diffusion in titanium at room temperature

The diffusion of hydrogen in technically pure VT1-00 titanium is studied at room temperature under conditions of hydrogen penetration from a glow discharge plasma. Hydrogen penetration occurs under conditions of hydride phase growth. The diffusion coefficients of hydrogen in titanium at room temperature are estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 43–47, March, 1982.     

Electric quadrupole interaction of181Ta in Pd and Zn

A strong electric field gradient (EFG) has been observed at¹⁸¹ Ta in Pd and Zn metals using DPAC technique. The value of the EFG in Pd is found to be 12.5(8)×10¹⁷ V/cm² with an asymmetry parameter () of 0.33(2). The value of the EFG in Zn has the same value as in the Pd metal, but the asymmetry parameter has the value 0.36(3). The variation of EFG in Pd with temperature has shown a weak dependence. The results are interpreted in terms of oxygen trapping at Hf atoms during the sample preparation.     

Thio sol-gel synthesis of titanium disulfide thin films and nanoparticles using titanium(IV) alkoxide precursors

Titanium tetraisopropoxide reacts with hydrogen sulfide in butylamine solvent at room temperature to form an amorphous titanium alkoxy-sulfide which can be converted to crystalline titanium disulfide by heat treatment in a flowing hydrogen sulfide gas stream. The reaction has been studied using infrared and Raman spectroscopy, gas chromatography-mass spectrometry, X-ray diffractometry and energy-dispersive X-ray analysis measurements. Based on these studies, it is shown that a partially thiolysed alkoxide precursor forms through the replacement of a limited number of alkoxy groups by hydrosulfide moieties. This alkoxy-hydrosulfide is believed to form following a thiolysis-condensation mechanism similar to the hydrolysis-condensation process that occurs during the corresponding oxide sol-gel reaction. The alkoxy-hydrosulfide species can then be completely thiolysed at 800 °C in a stream of hydrogen sulfide to yield pure, hexagonal titanium disulfide in either film or particulate form.     

A Mössbauer investigation of the quadrupole splitting in β-FeOOH

High resolution ⁵⁷Fe Mössbauer effect measurements at room temperature show that the spectra of pure, synthetic β-FeOOH in its paramagnetic state consist of a superposition of two discrete doublets, due to the existence of two different electric field gradients (EFG) which are proven to be related to the presence of halogen ions in this compound.     

Hyperfine spectroscopic study of the metal-insulator transition in V2O3

The metal-insulator (M-I) transition in vanadium sesquioxide V₂O₃ has been investigated by time differential perturbed angular correlation measurements of the electric fieldgradient (EFG) and the magnetic hyperfine field at dilute¹¹¹Cd impurities. The EFG undergoes a first-order change at the M-I transition at T_t=160 K, but does not reflect the high temperature resistivity anomaly. The increase of the EFG with temperature in the metallic phase can be attributed to thermal variations of the oxygen sublattice. The temperature dependence of the magnetic hyperfine field in the insulating phase follows a Brioullin function with a saturation value of H_hf(O)=15 KOe and an extrapolated Neel temperature, which, depending on the impurity concentration, varies between 188 and 230 K.     

Electric field gradients in Hf2Fe

Perturbed Angular Correlations (PAC) technique was applied to measure the electric field gradients (EFG) at ¹⁸¹Ta sites in Hf₂Fe. The compound has the cubic structure of the Ti₂Ni prototype with two non‐equivalent crystallographic sites for Hf atoms. EFGs for the two sites were measured as a function of the temperature. In addition, one more EFG was observed, which was assigned to the presence of defects in the lattice. The ratio of the measured EFG at the regular lattice sites has been used to assign charges to the ions through a simple calculation using the point charge model and it is shown that the lattice symmetry can explain the very different EFG at both sites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is ¹⁷²Yb. The temperature dependence of the hyperfine fields for ¹⁷²Lu(¹⁷²Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.     

The electric field gradient and its temperature dependence for indium in beryllium

The electric field gradient (EFG) for¹¹⁷InBe has been determined by a TDPAC measurement to be eq_tot=1.83(1) 10¹⁷V/cm² at room temperature. The sources were prepared by ion — implantation of¹¹⁷Cd in a Be single crystal. The EFG was found to decrease considerably with increasing temperature. The results are compared to those of other impurities in Be.     

Temperature dependence of the electric field gradient of204Pb in hexagonal Tl

The temperature dependence of the nuclear quadrupole interaction of²⁰⁴Pb(4⁺) isomer in hexagonal Tl host has been investigated by the TDPAD technique. It is the first reported EFG which does not respect the T^3/2 law fors-p metals. The chosen approach is the independent treatment of the ionic contribution and of the electronic contribution in the approximation of the additivity of the two contributions, avoiding any assumption of a direct temperature dependence for the total EFG.     

Mössbauer study of the magnetic and electric hyperfine interaction of dilute57Fe impurities in rare earth metal hosts

The hyperfine interaction of dilute⁵⁷Fe in the rare earth (RE)metals Gd to Lu was investigated by Mössbauer measurements with⁵⁷Co doped RE sources. In all hosts well split, 2-lines spectra were observed at room temperature, with slight asymmetries of the line intensities in some cases. The quadrupole splitting eQV_zz/2 increases from 0.29 mm/sec for Gd to 0.50 mm/sec for Tb, and decreases by less than 10 % between Tb and Lu. Only about 10 % of the corresponding electric fieldgradient (EFG) can be accounted for by the ionic EFG on a substitutional RE site. The temperature dependence of the EFG was measured in the case of Tb. No variation within 3 percent was found between 300 K and 700 K. Measurements of the magnetic hyperfine interaction at low temperatures were carried out in Tb. The saturation field of⁵⁷Fe in this host is H_hf(FeTb;4.2 K)=25(2) KOe. The temperature dependence of the magnetic hyperfine field does not follow the host magnetization (T_c=220K) but vanishes at about 80 K. Similar anomalies of H_hf(T) have previously been observed for other transition element impurities in the RE ferromagnets.