Lattice dynamics of MgSiO3 perovskite

A lattice dynamical study of the geophysically important mineral MgSiO₃ in its orthorhombic perovskite phase, with space group Pnma (D _2h ¹⁶ ) has been carried out using a rigid ion model, with the potential consisting of Coulombic and short-ranged interactions. With the help of program DISPR, the ionic charges and radii were optimized using the equilibrium conditions. The resulting potential model is employed to predict the elastic constants and the phonon dispersion relations. The computed long wavelength optic modes are in good agreement with the corresponding experimental Raman and infrared active bands. The phonon density of states has been obtained and is used to evaluate the specific heat, the mean square displacements and thermal parameters of atoms.     

Investigation of hyperfine interactions in GdCrO3 perovskite oxide using PAC spectroscopy

Perturbed angular correlation (PAC) measurements have been carried out in the antiferromagnetic GdCrO₃ perovskite oxide using ¹¹¹In (¹¹¹Cd) and ¹⁸¹Hf(¹⁸¹Ta) nuclear probes. The radioactive parent nuclei ¹¹¹In and ¹⁸¹Hf were introduced in the compound through a chemical process during sample preparation. The PAC measurements were carried out in the temperature range 20–300 K. Measurements with the ¹⁸¹Ta indicated a unique quadrupole interaction above 170 K and a combined electric quadrupole and magnetic dipole interactions below this temperature. The observed interaction was assigned to the probe nuclei substituting Cr sites. Measurements with ¹¹¹Cd showed two quadrupole interactions. Only one of the fractions however, showed a combined electric and magnetic interaction in the temperature rage 20–170 K which was assigned to ¹¹¹Cd probe substituting Cr site. The other fraction was attributed to the Gd site. The present results are compared with those of LaCrO₃ and NdCrO₃.     

Pressure-induced spin transitions of iron in MgSiO3 perovskite: a GGA+U study

First-principles calculations using the GGA+U method have been made to investigate the spin transitions of iron in MgSiO₃ perovskite up to 120 GPa, with Fe²⁺ and Fe³⁺ at the A and B sites involving different substitution mechanisms and various charge compensation configurations. Our results, unlike those from previous local density approximation and generalized gradient approximation calculations, are now consistent with available experimental data for the spin states of Fe³⁺ in perovskite. In particular, our calculations show that both Fe²⁺ and Fe³⁺ at the A site do not exhibit any spin transition at the lower mantle conditions. However, Fe³⁺ at the B site in both (Mg_0.9375Al_0.0625)(Si_0.9375Fe_0.0625)O₃ and (Mg_0.9375Fe_0.0625)(Si_0.9375Fe_0.0625)O₃ undergoes a high-spin to low-spin transition at ～ 35 GPa, and this spin-transition pressure is largely independent on substitution mechanisms and charge compensation configurations.     

Magnetic hyperfine interactions in GdAl3

We have studied the magnetic hyperfine interactions in GdAl₃ using¹⁵⁵Gd Mössbauer spectroscopy between the temperatures of 4K and 90K. Previous studies on GdAl₃ have shown that antiferromagnetic ordering occurs at 18K, and that a fit of the susceptibility to 1/(T-θ_p) yields a θ_p value of ?89K. The large ratio of θ_p to T_N is indicative of magnetic frustration between competing ferro-and antiferromagnetic interactions, which may be due to a combination of the oscillatory nature of the RKKY interaction and the geometry of the hexagonal lattice. Our studies show that the saturation magnetic hyperfine field at the Gd site is ?24.0 T, with the moments lying in the basal plane. The efg at the gadolinium site is 2.55(1)×10¹⁷V cm^?2 which is considerably larger than the value predicted by a point charge calculation. This difference may indicate that there is a conduction electron contribution. A helical magnetic structure has been calculated from RKKY theory.     

Magnetic hyperfine interactions of 119Sn probe atoms in the binary perovskite CaCu3Mn4O12

The manganite CaCu₃Mn₄O₁₂ doped by ¹¹⁹Sn atoms (about 1 at % with respect to Mn atoms) is studied by Mössbauer spectroscopy. The introduction of diamagnetic tin atoms is found not to affect the structure of the manganite. Tetravalent tin atoms are shown to substitute for the isovalent manganese atoms that are located in an octahedral oxygen surrounding. The cluster method of molecular orbitals is used to calculate the contributions of Mn⁴⁺ and Cu²⁺ cations that belong to different structural sublattices to the hyperfine magnetic field at ¹¹⁹Sn nuclei (H _Sn = 105 kOe at T = 77 K). These partial contributions are analyzed, and the intrasublattice Mn⁴⁺-O-Mn⁴⁺ exchange interactions are found to play a significant role in the formation of the magnetic structure of the manganite.     

Orbital following and hyperfine interactions in CH3

In a recent paper, Beveridge and Miller conclude that for the calculation of isotropic hyperfine interactions in simple planar hydrocarbon radicals, a semi-empirical molecular orbital method has some advantages over a semi-empirical valence bond method. In the present work we examine this conclusion and find that the arguments which support it are questionable. Our own critical comparison of the two methods does not lead to a clear conclusion. In addition, we report a method for calculating the degree of orbital following in a spin-unrestricted Hartree-Fock molecular orbital wave function and give numerical results for CH₃ in its out-of-plane bending mode.     

Time-dependent hyperfine interactions in moderately diluted Ni(Hf)

The magnetic hyperfine interaction of¹⁸¹Ta in Ni(Hf) has been measured as a function of temperature by means of the time differential γγ angular correlation technique. While for very low concentrations of less than 10^?4 a pure static interaction is observed, which follows the bulk magnetization apart from small deviations, spectra obtained from a source with 1·10^?3 Hf content show a more complex behaviour. From the comparison of the experimental data with theoretical spectra, which result from various dynamical models, we conclude that for a Hf concentration of 1·10^?3 the impurity-impurity interaction causes a local breakdown of the ferromagnetic ordering near the Curie point. The arising paramagnetic domains may change their geometrical form and position so that a probe nucleus is subject to a time-dependent hyperfine interaction.     

Investigation of hyperfine interactions in RMO 3 (R = La, Nd; M = Cr, Fe) antiferromagnetic perovskite oxides using PAC spectroscopy

Because of its sensitivity to point defects atomically close to probe nuclei, it is reasonable to expect that perturbed angular correlation (PAC) spectroscopy has the potential to assist in the identification of diffusion mechanisms. It is of interest to characterize under what conditions diffusing defects are visible and how the defects affect relaxation in PAC spectra. This was investigated using a stochastic model to simulate PAC spectra for the special case of non-impurity PAC probes diffusing via a simple vacancy mechanism on the Cu-sublattice in Cu₃Au-structured compounds.     

Electrostatic hyperfine interactions in Y(Fe,Al)2

Room temperature⁵⁷Fe Mössbauer and X-ray diffraction measurements are reported for Y(Fe_xAl^1–x)₂, 0.1x 0.9. For x0.65 broadened and on the Al-rich side structurized Bragg-peaks are observed. The asymmetry of the quadrupole split Mössbauer spectra depends on x. Whereas two different slopes are obtaind for the concentration dependence of the mean quadrupole splitting on the Al- and the Fe-rich side, the isomer shift exhibits a smooth dependence on x, which can be correlated to the change in volume caused by the Al substitution.     

Electric quadrupole interactions in the CdTiO3 perovskite2

The TDPAC technique has been used to measure the EFG at the Cd site in the perovskite CdTiO₃. The experimental results obtained at 293 K are compared with ionic lattice sum calculations of the EFG. In addition measurements at 16 and 77 K show that no considerable modification of the EFG is observed at the Cd site at temperatures below and above the previously described phase transition at 50 K.     

Theory of hyperfine interactions in aqueous Mn3+-porphyrin system

The electronic structure and associated magnetic hyperfine properties of Mn³⁺-porphyrin with two ligand water molecules are studied using the Unrestricted Hartree-Fock procedure with an aim to understand the observed anomalous water proton relaxivity associated with Mn³⁺ (TPPS₄). Our results show that the electron spin distribution is strongly localized on the metal ion and the point dipole model seems to be a good approximation for the dipolar hyperfine interactions in this system. It appears that the anomalous relaxivity observed in the Mn³⁺ (TPPS₄) is not due to the strong delocalization of the spin distribution associated with the metal ion and may result from unusual dynamical effects.     

Ferrimagnetism and hyperfine interactions in RFe5Al7(R = rare earth)

Magnetization and Mössbauer (⁵⁷Fe, ¹⁵⁵Gd) studies of RFe₅Al₇(R = Y, Sm to Lu, ThMn₁₂ crystal structure) in magnetic fields up to 50 kOe and temperatures 4.1 to 500 K have been performed. The Mössbauer studies yield the distribution of the iron ions among the various inequivalent crystallographic and magnetic sites, the hyperfine fields and their temperature dependence. The magnetization curves display a great variety of unusual magnetic phenomena. Among those; strong anisotropy, magnetic and thermal hysteresis (H_c = 24 kOe for DyFe₅Al₇ at 4.1 K), negative magnetization at low temperatures when the sample is cooled in a magnetic field (even in 1 Oe), compensation points, maxima points and time-dependent magnetization. Most of the phenomena observed can be explained in terms of a spin structure previously suggested for the RFe₆Al₆ compounds, composed of 4 magnetic sublattices. The rare earth moments lie antiparallel to the iron moments in the (j) site and to the ferromagnetic component of a canted antiferromagnetic structure of iron in the (f) site. Iron in the (i) site is nonmagnetic.     

The hyperfine field of sodium in iron

The hyperfine field of NaFe has been measured using Low Temperature Nuclear Orientation of cold implanted²⁴Na. The combination of both the γ-and β-anisotropies yields information on the implantation fraction of Na in Fe. Furthermore, an, upper limit for the relaxation time of Na in Fe could be deduced.     

The hyperfine field of oxygen in iron

From an on-line nuclear orientation measurement on¹⁵O implanted in iron, the hyperfine field of oxygen in iron was deduced as +12.2(16) T. This experiment, together with two measurements on¹⁷F, which was implanted in the same foil, also yielded information on the relaxation and implantation behaviour of light impurities when implanted at low dose and at low temperature.     

The hyperfine field of krypton in iron

The hyperfine field of krypton nuclei at lattice sites in iron has been determined using nuclear orientation. From the anisotropy of the electrons emitted in the decay of⁸⁵Kr implanted in iron, a hyperfine field value of +31.2(8) T was deduced for a substitutional fraction of approximately 37%. Also, a relaxation time of the order of 900 see was observed at temperatures between 7 and 11 mK.     

Pac study of the hyperfine interactions in154Gd3+ in rare-earth-garnets

The attenuation of the anisotropy of the γ-γ angular correlation for a rare-earth ion in magnetic iron and non-magnetic aluminum rare-earth garnets is governed by the static hyperfine electric field gradient (EFG) produced by the surrounding ions and by the static and fluctuating hyperfine magnetic fields produced by the 4f electrons of the ion. The latter effect depends upon the correlation time (τ_c) of the 4f electrons which, in turn, is determined by the interaction of the ionic spin with lattice vibrations and other magnetic ions. In an attempt to determine the significance of spin-spin relaxation on τ_c, perturbed angular correlation (PAC) measurements were performed on the S-state ion¹⁵⁴Gd³⁺ because spin-lattice interactions were expected to be small. The time-integral attenuation coefficients of the 1274/123 keV γ-γ cascade in¹⁵⁴Gd were measured in the temperature region 4.2–650 K for¹⁵⁴Gd³⁺ incorporated in GdAlG, GdIG, YAlG, and YIG. Rotation measurements were also made on the same cascade in the iron garnets at room temperature with an applied magnetic field in the range 0–15 kg. Employing independent measurements and calculated estimates of the static hyperfine magnetic field and EFG, the observed data could be interpreted in terms of reasonable values of τ_c. The correlation times were found to range from about 0.13 ns in YIG to about 1.30 ns in YAlG with GdAlG having a value of approximately 0.71 ns. In the case of GdIG, a self-consistent analysis of the data required a value of the lattice EFG which was larger than that estimated from the nearest neighbor point-ion model. The correlation time in GdIG was then found to be the same as in YIG.