Pressure dependence of the electric field gradient in Sb and Sb1−x M x (M=ln,Zn, Ge,Pb, Cd,Sn) at the site of111Cd

Using the¹¹¹Cd-TDPAC (time differential perturbated angular correlation) method, the pressure dependence of the electric field gradient (EFG) in Sb and Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn) was investigated. The application of a phenomenological ansatz for the parametrisation of the pressure and temperature dependence of the EFG made it possible to combine temperature data gained in former studies [1], [2] with the pressure dependent data presented in this paper. The resulting pressure dependence of –2±0.2 MHz/kbar is shown to be independent of concentration and element of admixture. Results for the volume and explicit temperature dependence agree with existing information on the mixed system Sb_1–x M _x (M=ln, Zn, Ge, Pb, Cd, Sn); the investigation of the EFG in Sb_1–x–y M _x Pb _y showed that the resulting EFG may be interpreted as the weighted sum of the individual contributions of the two metals.This paper is dedicated to Prof. Dr. W. Kreische on the occasion of his 60th aniversary on 02.02.1995     

The electric quadrupole interaction of 111Cd in ZrZn2 and Zn in the samples prepared at 8 GPa

The perturbed angular correlation (PAC) measurements with the ¹¹¹In-¹¹¹Cd nuclear probe embedded into the lattice of the cubic (C15) Laves compound ZrZn₂ showed that ¹¹¹Cd nuclei experienced an axially symmetric electric quadrupole interaction with a frequency ν _Q  = 132.4 MHz at room temperature. The samples were synthesized and doped with the probe at a pressure 8 GPa. The temperature dependence of ν _Q was shown to be linear: ν _Q (T) = 147(1 − 0.033 T) MHz. Since the value of ν _Q is very close to that known for ¹¹¹Cd in the lattice of Zn, we have checked if it could be assigned to residual Zn metal in the sample. For the Zn sample melted and doped with ¹¹¹In at 8 GPa we have obtained ν _Q  = 117.3 MHz at 300 K and 127 MHz at 80 K – both values considerably lower than that for ¹¹¹In doped Zn samples prepared at an ambient pressure. These data, and the fact that ν _Q (T) in Zn is known to follow the T ^3/2 law, allow to attribute the ν _Q value quoted above to ¹¹¹Cd nuclei at the substitutional sites with tetrahedral symmetry in the Zn sublattice of ZrZn₂.     

Temperature and pressure dependence of magnetic and electric hyperfine interactions in metallic Samarium

The magnetic and electric hyperfine interaction at¹¹¹Cd impurities in Samarium has been investigated by TDPAC measurements. The quadrupole frequency is _Q=20.0(2) MHz at 290 K and has a linear temperature dependence with the same slope (dln_Q/dT)_290K=–7.3(2) 10^–4 K^–1 in the rhombohedral and the hcp phase. The pressure dependence up to 7 kbar is (dln _Q/dT)=+8.7(1.4) 10^–3 kbar^–1. The magnetic hyperfine field of¹¹¹Cd in Sm is H_hf=242(6) kG at 4.2 K. Its temperature dependence confirms the existence of 2 different magnetic phases in Sm. The crystal field parameters B ₂ ⁰ and B ₄ ⁰ have been estimated from a comparison of H_hf(T) with molecular field models. The TDPAC spectra in the magnetic phases suggest that the impurities preferentially occupy the hexagonal Sm sites.     

Vacancy trapping at111In in tungsten

TDPAC measurements were carried out in tungsten foils and single crystals using implanted¹¹¹In as a probe nucleus. After damaging the bcc tungsten lattice by heavy ion implantation, electron- and proton-irradiation the trapping of three different defects in the temperature range 293–1,000 K was observed. They were characterized by quadrupole interaction frequencies of v _Q ¹ = 142(2) MHz, v _Q ² = 181(5) MHz and v _Q ³ = 263(5) MHz. The largest fraction of In nuclei experiencing electric field gradients corresponding to v _Q ¹ and v _Q ² was reached at app. 630 K whereas for v _Q ³ this maximum appeared at app. 850 K. The defect configurations producing the different electric field gradients could be identified as a monovacancy trapped in a nearest neighbour 111 position (v _Q ¹ ), a double vacancy consisting of two adjacent vacancies in the nearest neighbour shell of the probe atom (v _Q ² ) and possibly a larger cluster (v _Q ³ ).     

TDPAC measurement of quadrupole interaction at111Cd in the nearly heavy fermion system USn2.9

Quadrupole interaction at¹¹¹Cd probe has been studied in the nearly heavy fermion system USn_3– using the TDPAC method. We observe a large quadrupole interaction frequency withv _Q =89.9(10) MHz at 300 K which is much higher than the earlier reported result. The observed frequency shows a linear temperature dependence in the range 25–300 K.     

The study of the interaction of indium with tellurium in silicon

The perturbed γ–γ angular correlation method has been employed to study indium-impurity pairs in silicon, consisting of the probe atom (¹¹¹In/¹¹¹Cd) and several group-VI donors. Such pairs can be identified via the interaction between the quadrupole moments of the probe nucleus and the electric field gradient (EFG) associated with the formed defect complex. A new quadrupole interaction frequency (QIF) of ν_Q=444(1) MHz (η=0) is measured at T=293 K in Te implanted silicon after annealing the sample above 700 K. The complex is attributed to the In-Te pair along 〈100〉-crystal axis in silicon. In addition, the temperature dependence of the QIF characterizing the pair has also been studied. The implantations of S and Se could not lead to the formation of observable complexes despite similar treatment of all samples. PACS 61.72.-y; 76.80.+y; 61.72.Cc     

Nuclear orientation of 109Cd and 115mCd in Cd

The quadrupole interaction frequencies of ¹⁰⁹Cd$\text{Cd}$ and ^115mCd$\text{Cd}$ have been measured by low-temperature nuclear orientation as +110(4) MHz and -102(12) MHz, respectively. The electric field gradient of Cd$\text{Cd}$ and the quadrupole moment of the $\text{5}\text{2}$⁺ 245 keV level of ¹¹¹Cd are deduced.     

Pressure dependence of the electric field gradient at Cd impurities in antimony

The electric field gradient at¹¹¹Cd in Antimony was studied at hydrostatic pressures up to 7 kbar at 150 K, 293 K and 473 K. The logarithmic pressure derivatives, dlnq/dP, were found to be –67, –37 and –26 Mbar^–1 respectively. The isotropic volume dependence derived from these values was found to be positive and largely responsible for the anomalous temperature dependence, previously observed.     

Temperature dependence studies of the EFG at111Cd probes located in some intermetallic compounds

The temperature dependences of the electric field gradient (EFG) at¹¹¹Cd probes in nickel-indium and indium-tellurium intermetallic compounds have been studied by means of the time differential perturbed angular correlation of gamma rays (TDPAC). The room temperature interaction frequencies (e ² Qq/h) in each case were determined as In₂Te₅ (136 MHz), Ni₃In (13.8 MHz), NiIn (252 and 337 MHz) (two sites), and Ni₂In₃ (162 and 266 MHz) (two sites). The TDPAC pattern of¹¹¹Cd in NiIn did not agree with the previous assignment of the crystal structure. A general discussion of the results related to other temperature dependence studies of the EFG is presented and a general trend is discerned.Supported in part by the US Energy and Research Development Administration.     

PAC-experiments in electron-doped HTSC: Nd2−x CexCuO4

The perturbed angular correlation (PAC) method was applied to study the electron-doped HTSC Nd_2–x Ce_xCuO₄ (NC_xCO) forx=0, 0.06, 0.1 and 0.15.¹¹¹In was implanted at 400 keV into sintered bulk samples. After annealing the radiation damage at 1223 K in air, PAC spectra were taken at measuring temperatures between 15 and 823 K. Point charge model (pcm) calculations support the assumption that¹¹¹In replaces a Nd-ion. The width of the main frequency depends on the amountx of Ce-doping. The magnitude of the observed electric field gradient (efg) can be explained if the¹¹¹In probe catches simultaneously an oxygen vacancy at the O(2)-site.This work was supported by the BMFT, FKZ 13 N54930.     

Cavity formation at indium impurities in bcc metals

The defect formation in the bcc metals W and Mo above annealing stage III and the influence of rare gases on this process were investigated by means of the perturbed angular correlation technique using¹¹¹In as radioactive probe. In both metals a relatively high electric field gradient (EFG) could be observed at the indium site, characterized by the quadrupole interaction frequencies υ_Q=263 MHz, ν=0 and υ_Q=220 MHz, ν≈0.15 for W and Mo, respectively. The observations are assigned to the growth of threedimensional vacancy clusters at the probe atoms with the indium atoms situated in the inner surface of this cavities, thus experiencing the corresponding surface EFG.     

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.     

Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

Investigation of Hyperfine Interactions in CeIn3 byTDPAC

Magnetic hyperfine fields (mhf) at ¹¹¹Cd and ¹⁴⁰Ce nuclei, dilutely substituting the In and Ce sites, respectively, have been measured in the intermetallic compound CeIn₃ using perturbed angular correlation technique. A pure electric quadrupole interaction with an axially symmetric electric field gradient was observed at ¹¹¹In(EC)¹¹¹Cd probe nuclei at room temperature while a combined magnetic dipole and electric quadrupole interaction is observed below 10K. Below the ordering temperature, only a magnetic interaction is observed at ¹⁴⁰La(^–)¹⁴⁰Ce probe. The values of mhf measured experimentally as a function of temperature are discussed in terms of critical behavior.     

Equilibrium point defects in TiAl studied by PAC

Site fractions of dilute¹¹¹In/¹¹¹Cd probe atoms in different lattice locations in quenched, equi-atomic TiAl are measured using perturbed angular correlations of gamma rays (PAC). The vacancy-free substitutional site fraction, observed via its nuclear quadrupole interaction, is observed to decrease with increasing quenching temperature, reflecting an increase in the thermal defect concentration. Assuming that the thermal defect is the Schottky defect, the law of mass action, and a binding energy of 0.2 eV between vacancies and the In probe, the experimental temperature dependence is analysed to yield a formation enthalpy of 4.7(4) eV and entropy of 25(4)k _B for the Schottky defect. The vacancy concentration on one sublattice is given in terms of these parameters by the expression [v]=exp(S _F/2k _B) exp(–E _F/2k _B T), and is found, for example, to have the value 1.4% at 1350°C, a large value.     

Temperature dependence of a lattice defect induced electric fieldgradient at181Ta in Pt

By implantation of¹⁸¹Hf ions radiation damage is produced in platinum. Subsequent annealing of the samples leads to the trapping of a unique defect at the probe atoms. The temperature dependence of the resulting electric field gradient eq(T) is measured by use of the TDPAC-method. Assuming a relation eq(T)=eq(0)·(1–B·T ^3/2) one findsB=(1.9±0.7)×10^–6 K^–3/2. This low value is of the same magnitude as the corresponding values for¹¹¹Cd in various locally distorted cubic metals.     

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.     

Mössbauer effect and electron spin resonance study of CdxMg1−xFe2O4 system

This work deals with the application of Mossbauer Effect (ME) in studying the crystal electric field and the cation distribution among tetrahedral (A) and the octahedral (B) sites of the spinal structure in the ferrite system Cd_x Mg_1–x Fe₂O₄ (x=0, 0.2,...1). The electron spin resonance technique (ESR) was also applied for studying this ferrite system. It was possible to characterize the ESR spectra of ferrite through the combination with the ME spectra.The ESR spectra of magnesium ferrite showed two resonance positions of Fe³⁺ ions and indicated that a strong exchange interaction is dominant in the pure Mg-ferrite. For high Cd concentration ferrites only single resonance line was observed. These results could be interpretted on the basis of the ME results where it indicated that Cd²⁺ ions prefer tetrahedral positions, forcing the Fe³⁺ ions from these positions to join those in the octahedral sites. The complete site occupation with different types of cations was successfully achieved from the ME spectra. The values of the quadrupole splitting indicated that for each ferrite in the system there exists an electric field gradient surrounding the Fe³⁺ ions in each of the octahedral and tetrahedral sites. The increase in the Mg-concentration increases the symmetry of the electric field at these sites.     

Hyperfine interactions in 111Cd-doped lutetium sesquioxide

We report here first Perturbed Angular Correlation (PAC) results of the electric field gradient (EFG) characterisation at ¹¹¹Cd impurities located at both non-equivalent cation sites of the bixbyite structure of Lutetium sesquioxide, between room temperature (RT) and 1273 K. The comparison with results coming from a systematic ¹¹¹Cd PAC study in bixbyites and with point-charge model (PCM) predictions shows the presence of a trapped defect at RT in the neighbourhood of the asymmetric cation site, which is completely removed at T > 623 K. The anomalous EFG temperature dependence in Lu₂O₃ can be described in the frame of a “two-state” model with fluctuating interactions, which enables the experimental determination of the acceptor energy level introduced by the Cd impurity in the band-gap of the semiconductor and the estimation of the oxygen vacancy density in the sample. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hyperfine Fields at the Cd Site in La0.67Cd0.25MnO3 CMR Manganites

Although Cd and Ca ions have the same valence and cation size, their incorporation into vacancy-doped La manganites induce different properties. While the incorporation of Ca leads to high Tc up to 250 K and induces a metallic-like behaviour, the incorporation of Cd severely reduces Tc and promotes insulator-like behaviour. In this work, the Cd hyperfine fields have been measured with the Perturbed Angular Correlations (PAC) technique after implantation and annealing of ^111m Cd in La–Cd–MnO₃ samples. The PAC results are compared with measurements of the resistivity and magnetization performed on the same samples. The mixed La and Mn site Cd occupancy is suggested as a possibility to explain the properties of the La–Cd–MnO₃ system.