75As N.Q.R. and 19F N.M.R. study of solid AsF3

The temperature dependence of the ⁷⁵As pure quadrupole resonance frequency in AsF₃ has been studied from 77 K to a temperature very close to the melting point. It is found that the effective electric field gradient at the As site does not shift much from the gas-phase value and that the temperature dependence of the ⁷⁵As N.Q.R. frequency seems to follow the Bayer-Kushida model. The occurrence of a line-width transition in the ¹⁹F N.M.R. spectrum between 200 and 150 K demonstrates the presence of low-frequency, large-amplitude molecular motions which are too slow to average out the ⁷⁵As quadrupole coupling.     

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.     

Quadrupole interaction studies of Hg in Sb

Time differential perturbed angular correlation and nuclear orientation studies of the electric quadrupole interaction for Hg in Sb have been performed. The effective field gradients |V _zz ^eff (HgSb)|=1.43(18)×10¹⁷V cm^–2 at room temperature andV _zz ^eff (HgSb)=+1.8(2) × 10¹⁷V cm^–2 below 0.05 K have been derived. These two values are no indication for an anomalous temperature dependence of the effective field gradient for Hg in Sb. The value of the electric field gradient fits well into the systematics for Hg in other hosts. It is shown that the electronic enhancements of the field gradients are correlated to the valence of the impurities and are rather insensitive to the host properties.On leave of absence from: University of Lisboa, Portugal     

Nuclear quadrupole interaction in the low dimensional hf-trichalcogenides

We report on Time Differential Perturbed Angular Correlation measurements of the nuclear quadrupole interaction of¹⁸¹Hf(β⁻)¹⁸¹ and its temperature dependence in the low dimensional Hf- trichalcogenides HfS₃, HfSe₃ and HfTe₃. In HfS₃ the temperature dependence of the precession frequency is a slowly decreasing function with a negative curvature, the deviation from axial symmetry of the electric field gradient is practically temperature independent. In HfSe₃ the NQI shows a peculiar behaviour; between 10K and 300K the quadrupole frequency and the asymmetry parameter increase, between 300K and 400K the frequency decreases while the deviation from axial symmetry increases, indicating a structural phase transition near room temperature. In HfTe₃ the quadrupole frequency is practically temperature independent below 600K and increases linearely at higher temperatures, the asymmetry parameter increases steadily in the temperature range from 10K to 600K and then decreases. There is a strong correlation between the strength and the asymmetry of the electric field gradient so that the system depends on one control parameter only.     

Electric quadrupole interaction of192Tl(8−) nuclei in hexagonal Tl and in tetragonal in metals

The TDPAD technique has been used to measure the electric quadrupole coupling constants for the¹⁹²Tl(8^–) isomer in hexagonal Tl at different temperatures and in tetragonal In at room temperature. The values measured weree ² qQ/h=24.0(15) MHz extrapolated toT=0K, and 33.1(16) MHz at 293K for the Tl and In hosts,respectively, The electric field gradient for Tl impurities in In was determined from systematics of isoelectric systems, yielding |Q[¹⁹²T1(8^–)]|=0.44(7)b.The almost perfect symmetry of the T1 crystalline structure makes this metal a sensitive probe to current models for electric field gradients in metals. The magnitude and temperature dependence of the field gradient in thallium metal are discussed within the framework of such models.     

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.     

Electric quadrupole interaction of210Po(6+) in Bi single crystal

The electric quadrupole coupling constant of the²¹⁰Po(6⁺) isomer in a Bi single crystal was measured as a function of the temperature using the time-differential perturbed angular correlations technique. The electric field gradient (EFG) for Po in Bi is found to be temperature independent in the range 77–477 K. Using a calculated value of the nuclear quadrupole moment, the magnitude of the EFG is derived, and in turn serves to re-examine the systematics of quadrupole moments of λh ₉ ^2/2 configurations in the lead region.     

Hydrogen bonds in single crystals of para and meta-aminobenzonitrile as revealed by the Zeeman effect on the nuclear quadrupole resonance of 14N

The |e ² qQ/h| and η values for both the cyano and the amino-nitrogen of the title compounds are reported at room temperature as well as at 273 K and 77 K. From Zeeman effects on single crystals of both compounds the orientation of the principal axes of the electric field gradient (efg) on both nitrogen nuclei, as well as a number of data on the crystallographic and molecular structure, are obtained. A reliable interpretation is given of the experimental resonance frequencies, which are also safely assigned to the right nitrogen atoms. By studying the dependence of the resonance linewidth on the orientation, with respect to the efg principal axes, of the Zeeman field B ₀, the spatial distribution of the hydrogen nuclei around the ¹⁴N nuclei is determined. In this way hydrogen bonds are shown to exist in both single crystals. An analysis based on the Townes and Dailey theory allows an estimate to be made of the possible variation of the population of the cyano-nitrogen lone-pair orbital when involved in hydrogen bonds.     

Electric field gradients at the 111In site in the ferroelectric and paraelectric phases of LiTaO3

The temperature dependence of the electric-field gradient of ¹¹¹Cd in single crystalline LiTaO₃ was studied from room temperature to 1040 K in the ferroelectric and paraelectric phases. The data taken at room temperature show unambiguously the presence of two quadrupole interaction frequencies, ν_Q1=230 MHz and ν_Q2=242 MHz, with nonzero asymmetry parameters, while above the Curie temperature (T_C=878 K) the data are well described by a unique frequency. The electric field gradient shows a usual temperature dependence, increasing aproximately in a linear fashion until T_C and then decreasing faster. The initial increase is explained mostly by the lattice expansion, while above T_C it is necessary to consider Li and O displacements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Paramagnetic fluctuations in Yb3Ga5O12 investigated with 172Yb PAC probe

¹⁷²Yb PAC measurements were carried out on Yb₃Ga₅O₁₂ at temperatures ranging from 14 to 1100 K. The time dependent hyperfine interaction is clearly evident below room temperature. It is attributed to spin fluctuations of Yb ion in the ground state of the ²F_7/2 multiplet. Above 500 K, the temperature dependence of the electric field gradient is shown to result from a varying population of the crystal electric field levels. This revised version was published online in August 2006 with corrections to the Cover Date.     

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).     

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

The temperature dependence of the electric field gradient in the AgIn₂ intermetallic compound was measured using the TDPA correlation technique. From 80 K up to 415 K the electric field gradient follows a linear temperature dependence. Below 80 K a change in this behaviour is observed.Work supported in part by FINEP and CNPq.     

Study of combined magnetic and electric hyperfine interactions for cadmium in Dysprosium by time differential perturbed angular correlations

The combined magnetic and electric hyperfine interaction at the site of a¹¹¹Cd impurity in magnetically ordered Dysprosium has been investigated as a function of temperature by time differential perturbed angular correlation measurements. Three different phases have been found in metallic Dy with transition temperatures of 85 and 179 °K in agreement with the results of bulk material measurements. In the paramagnetic phase above 179 °K a pure electric quadrupole interaction has been observed. The various contributions to the electric fieldgradient are analyzed and it is shown, that the dominant contribution comes from the conduction electrons. In the ferromagnetic phase which extends from 0 to 85 °K the magnetic hyperfine field at the site of¹¹¹Cd has the same temperature dependence as the spontaneous magnetization. The value of the hyperfine field at 4.2 °K is ¦H _eff¦=(221 ± 4) kG. At 85 °K a transition to the antiferromagnetic phase of Dy occurs, which shows a hysteresis of the transition temperature. In the antiferromagnetic phase the temperature dependence of the hyperfine field deviates considerably from the magnetization curve. It is suggested that this deviation might be due to a temperature dependence of thes-f exchange interaction.     

Hysteresis and γ-recoil effects in the superconducting compounds HfxZr1−xV2

The anisotropy of the 133–182 keV γ-γ cascade in¹⁸¹Ta measured as a function of temperature between 70K and 298K shows lattice instability, the phenomenon of hysteresis in the vicinity of the lattice transformation temperature (T_L) and a temperature dependent electric field gradient (EFG) at the cubic Hf sites in the superconducting cubic Laves phase (C15) compounds Hf_1?xZr_xV₂ (x=0.0, 0.25 and 0.75). These observations are qualitatively understood by assuming the presence of γ-recoil effects and microdomains fluctuating between the high temperature (T>T_L) and the low temperature (T < T_L) phases of these compounds.     

The properties of the (H2O)6 water cluster that depend on its density during temperature transitions

Water clusters (H₂O)₆ are simulated by the Monte Carlo method with the Metropolis function at various temperatures (T ₁ = 273 K, T ₂ = 298 K, and T′₁= 373 K) and densities (ρ₁ = 0.9998 g/cm³, ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system. It is established that the number of retained most probable configuration types at ρ₁ = 0.9998 g/cm³ during temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K is smaller than at ρ₃ = 0.00059 g/cm³. This result was acquired on the background of the following invariable parameters of the system with the same temperature transitions for each of three values of density: (i) the average number of retained most probable configuration types, (ii) the average fraction of weight coefficients of the most probable configuration types, and (iii) the average potential energy. The configuration type that was retained among the most probable configuration types of the system for all values of density (ρ₁ = 0.9998 g/cm², ρ₂ = 0.9167 g/cm³, and ρ₃ = 0.00059 g/cm³) of the system for temperature transitions from T ₁ = 273 K to T ₂ = 298 K and from T′₁ = 373 K to T ₂ = 298 K was also revealed.     

Hyperfine magnetic fields in Fe−Co alloys and their tempera ture dependences

We obtained⁵⁷Fe hyperfine field parameters from Fe₁−x-Co _x alloys (0≤x≤0.6) from 77 K to 900 K. We first discuss the origin of the low temperature hyperfine fields in terms of the 3d and 4s electrons at⁵⁷Fe atoms. The⁵⁷Fe hyperfine magnetic field (hmf) of Fe-Co alloys depends more weakly on temperature than the hmf of pure Fe. This temperature dependence occurs because the alignment of the magnetic moments at both the Fe atoms and at the Co atoms depend on temperature in the same way as the bulk magnetization of Fe-Co alloys.     

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.     

The quadrupole interaction of100Rh in various intermetallic compounds of palladium

The nuclear quadrupole interaction of the 75 keV excited state of¹⁰⁰Rh in the ordered intermetallic compounds PdHg, PdPb₂ PdSb and PdTe was measured. Using an estimate for the nuclear quadrupole moment of the 75 keV state in¹⁰⁰Rh and point ion lattice sums for the lattice electric field gradient (EFG) at the Rh site, the electronic contributionV _zz ^el to the total EFG was derived.V _zz ^el was found to be correlated with the average valence electron concentration in the compounds studied. In the case of PdPb₂, the temperature dependence of the quadrupole interaction was also studied and found not to follow theT ^3/2 rule which is observed in virtually all pure noncubic metals.Work partially supported by U.S. Department of Energy.     

The109Ag Knight shift in the palladium-silver alloy system

The¹⁰⁹Ag Knight shiftK in the PdAg system has been observed at 298 K. The temperature dependence of the shift is presented in conjunction with the data of Narath at 4 K. The shift is found to be temperature dependent for silver concentrations less than 60 at. %. An analysis ofK in terms of thes andd electron contributions shows that thes electron density of states at the Fermi energy is in agreement with a commons band uniformly filled at the Pd and Ag sites and thatK _d is linearly related to the susceptibility of the PdAg system with both the temperature and the alloy concentration as implicit parameters.     

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.