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.     

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.     

Temperature dependence of the electric field gradient in HfF4.HF.2H2O

The hyperfine quadrupole interaction in HfF₄.HF.2H₂O was studied using the time differential perturbed angular correlation (TDPAC) technique. The electric field gradient (EFG) and the corresponding asymmetry parameter were measured in the temperature range from 9 to 350 K. The EFG is characterized by a rather strong increase with temperature, whereas the asymmetry parameter reaches its maximum value (η≈1) at aboutT=160 K. At 420 K, the complex is dehydrated and looses HF: The quadrupole parameters determined from subsequent TDPAC measurements are charateristic for HfF₄.     

Electric field gradient sign determination in low magnetic fields perpendicular to the gamma rays in57Fe Mössbauer spectroscopy

The determination, by Mössbauer⁵⁷Fe spectroscopy, of the sign and the asymmetry parameter of the electric field gradient (EFG), in a non-magnetic powder sample, needs the use of an external magnetic field. Usually a “parallel-magnetic-field-to-the-gamma-rays” configuration is employed. In the present work, theoretically generated spectra in both configurations, parallel an perpendicular, are compared. The result is that the spectra in the perpendicular configuration show always a larger asymmetry for the same fields intensitics. In consequence, the sign determination of the EFG, is possible with magnetic fields significantly smaller in the perpendicular case than in the parallel one. Furthermore, the practical consequences are important, because EFG studies with conventional electromagnets, allowing only fields lower than 25 kGauss are feasible. As an example. Mössbauer experimental results obtained at room temperature for natural pyrite (FeS₂) in perpendicular fields of 15 and 20 kGauss, created by a conventional electromagnet, are shown here.     

Hindered rotation and phase transition in sodium gallohydride according to the NMR data

Polycrystalline sodium gallohydrides with different hydrogen isotope compositions, NaGaH₄, NaGaH_2.56D_1.44, and NaGaD₄, were studied over the temperature range 100–375 K by the ²H, ²³Na, and ^69,71Ga NMR methods in magnetic fields of 1.4, 1.88, and 7.04 T. The data on spin-lattice deuteron relaxation T ₁(²H) were used to determine the activation energy (～35 kJ/mol) of anion rotational motions and quadrupole coupling constant (QCC) χ _Q ⁰ (²H) = 90 kHz for the rigid lattice. To within measurement errors, the activation energy and ²H QCC were independent of the isotope composition. The low-temperature ²H NMR line was a doublet with ²H QCC χ(²H) = 70 kHz. The difference between χ ₀ and χ(T) was interpreted in terms of the librational averaging of the electric field gradient (EFG) according to the Bayer equation. The frequency of librations at 113 K was estimated (31 cm^?1). The shape of the ^69,71Ga NMR line was indicative of second-order quadrupole interaction with a nonzero asymmetry parameter η of the EFG tensor. Temperature variations in the QCC and η parameter at gallium were considered using the distorted tetrahedron model. The η value changed from 0.65 to zero (axial EFG tensor symmetry characteristic of tetragonal lattices) over the temperature range 140–295 K and again increased to 0.3 at 300–365 K, which was evidence of a change in the orientation of the distorted tetrahedral anion in the lattice. Changes in the asymmetry parameter were in agreement with the phase transition in sodium gallohydride recorded as a heat capacity jump.     

Electric field gradients in PrBa2 Cu 3 O 7 : LSDA+U results and comparison with experiment

Electric-field gradient (EFG) and asymmetry parameter (η) at all oxygen and copper sites of nonsuperconducting PrBa₂Cu₃O₇ (Pr123) compound were calculated using the full-potential (linearized)-augmented-plane-wave plus local orbitals method. Exchange and correlation effects were treated by LSDA+U for Cu(3d) and Pr(4f) electrons. The effects of changing screened Coulomb parameters U_Pr, U_Cu1, and U_Cu2 on the results were individually studied. The calculated EFG of O2 site is close to the EFG of O3 site at variance with the experimental result. It was shown that by increasing superconducting holes in O2 and O3 sites the EFG at these sites increase and vice versa. The most famous theories which have been proposed to explain the suppression of superconductivity in perfect (without any mis-substitution or other defects) Pr123 compound are not consistent with the experimental EFG at O2 and O3 sites. By replacing one Pr atom at the Ba site on unit cell of Pr123 (Pr_Ba), it was shown that Pr_Ba mis-substitution reduces the superconducting holes in both O2 and O3 sites and could be responsible for the suppuration of superconductivity in Pr123 samples. It is very probable that the unusual behaviors of experimental EFG at O2 and O3 sites of Pr123 are related to oxygen defects which are produced with Pr_Ba mis-substitution.     

Mixed hyperfine interactions and defect structure in magnesiowüstite Fe−Mg−O

An analysis of the 4.2 K spectra of Fe_xO (x∼0.91) and (Fe_0.4Mg_0.6)_xO is presented in which it is considered that because of the large electric field gradient at Fe²⁺ defect sites, the spectra cannot be described by Lorentzian sextets. It is assumed that the magnetic hyperfine field vector is oriented at random in the coordinate system defined by the EFG main axis, and that the EFG coordinate system is also distributed randomly. The simplifying assumption of the asymmetry parameter η=0 allows an analytical formula to be used to describe the complex spectra. Distributions of both magnetic hyperfine field and quadrupole interaction were progressively refined resulting in reasonable fits to the spectra with the main features being well reproduced. The magnetic hyperfine field distribution is rather broad with several features present while distinct values were obtained in the distribution of quadrupole interactions. These latter values are considered to correspond to the defect configurations around the Fe²⁺ sites. The distribution of hyperfine fields is considered to reflect the varying strengths of superexchange due to the high defect concentration as well as the effects of magnetic relaxation.     

Electric field gradient in LiMnFeF6 compounds. polarizable point charge calculations and Mössbauer data

The polarizable point charge model is used to explain Mössbauer data on the electric field gradient (EFG) at the iron site in α- and β-LiMnFeF₆. Lattice summations are performed in direct space. The fluorine polarizability which fits the data is in reasonable agreement with the expected value $\text{(α}{}_{\mathrm{F}}\text{～ 1.2}\text{A}\text{?}{}^3\text{)}$ The dipolar contribution is found to be important. In both cases the main EFG component is positive. In β-LiMnFeF₆, according to the structure, the EFG tensor is axial with respect to the c axis; in α-LiMnFeF₆, the asymmetry parameter η is large (?0.5) and the OZ axis is along the a axis.     

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb₆X₁₂]²⁺ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable B_o field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of ⁹⁵Mo nuclei in structurally related compounds containing [Mo₆Br₈]⁴⁺ cluster cores. Experimentally determined δ_iso(⁹³Nb) values are in the range from 2400 to 3000 ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (V_zz and δ₃₃) coinciding with the molecular four-fold axis of the [Nb₆X₁₂]²⁺ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (V_zz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (V_yy) and the largest CS principal axis (δ₁₁) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by ^79/81Br NQR and ³⁵Cl solid-state NMR measurements.     

Crystal-chemistry aspects and electric-field-gradient (EFG) dispersion in Ca-gallogermanate-type structures studied by Mössbauer spectroscopy

The crystal field disorder in some trigonal germanates of the type X_3-yLn_yFe_2+yGe_4-yO₁₄ (X = Ba, Sr; Ln = La, Nd; y = 0, 1) is studied by ⁵⁷Fe M?ssbauer effect. The dispersion of the electric field gradient (EFG) at the octahedral sites of these compounds is investigated. A correlation of the experimental and calculated EFG data with some crystal-chemistry aspects is presented. Received 3 February 1999     

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.     

Impurity induced EFG at111Cd in the fcc silver lattice

Electric field gradients (EFG) due to Cu, Zn and Ga impurities in silver were studied by the perturbed angular correlation technique using the probe atom¹¹¹In/¹¹¹Cd. A large deviation from the axial symmetry was observed for the EFG at a nearest neighbour site to Zn and Ga impurities. The molecular cluster method calculations can reproduce the magnitude of the EFG, however, does not account for the asymmetry parameter.     

On the Mössbauer site assignment in YBa2(Cu1−xFex)3Oy

The assignment of the four⁵⁷Fe Mössbauer sites which are observed in Fe-substituted YBa₂Cu₃O_y is presented. We present a systematic study of highly-controlled samples with low Fe-concentration over the whole oxygen concentration range. Two sites are found to dominate in the limit of high oxidation (y→7), indentified as Fe⁴⁺ on the two crystallographic Cu sites: Cu(1) and Cu(2). These are replaced at lower oxidation (y→6) by two others, which we identify with Fe³⁺ on these same two sites. Consistency is found with the sign of the electric field gradient (EFG), the asymmetry parameter η and the isomer shiftI _S for this model. The magnetic structure in the limit y→6 shows changes in the antiferromagnetic (AF) structure dependent on iron concentration. Mössbauer and spin-echo NMR results show a change in the Cu(2) AF layer sequence at low temperatures, without any magnetic moments on the Cu(1) chain sites.     

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.     

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.     

Determination of the asymmetry parameter of EFG tensor from moments of NQR nutation spectra in powders

A novel approach to determination of the asymmetry parameter of the EFG tensor from zero-field nutation NQR spectra of the spinI = 3/2 nuclei in powder samples is reported. The proposed theoretical treatment uses lineshape analysis of the nutation NQR spectra by the method of line moments. The analytical formulas for the lineshape of the powder nutation spectrum are given. It is shown that the asymmetry parameter can be determined from the second moment 〈ω²〉 and the frequency of only one singularity ω₂ of the nutation spectrum. It is also shown that the asymmetry parameter can be determined from the second and fourth spectrum moments alone. The method is successfully demonstrated for the simulated nutation NQR spectra of the spinI = 3/2 nuclei in powder samples.     

The signature of the nuclear quadrupole interaction in continuous phase transitions

A correlation between the largest component of the electric field gradient (EFG) tensor, V_zz, and its asymmetry parameter η is discovered in a series of compounds which exhibt phase transitions. It is shown that all the EFG tensor components depend linearly on a single control parameter. A convenient way to unravel this correlation from V_zz, η-data is presented.     

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.     

Local structure analysis of smoky and colorless topaz using single crystal Al NMR

The angular dependence of the ²⁷Al NMR spectrum was measured for single crystals of smoky and colorless topaz, Al₂SiO₄(F,OH)₂. Smoky topaz was obtained by irradiating high energy neutrons to colorless topaz. The quadrupole coupling constant e²Qq/h and the asymmetry parameter η were obtained from the analysis of the angular dependences of quadrupole splitting of the ²⁷Al NMR spectrum. The local structures around the aluminum atoms in smoky and colorless topaz were discussed from the magnitude and the direction of the electric field gradient. The directions of principal axes of the EFG tensor of ²⁷Al were close to the directions of Al-O and Al-F bonds. The difference in the bond lengths between Al(1)-F(1) and Al(1)-F(2) was found to affect the x and y components of the EFG tensor.     

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.