Dissymmetrization of crystals: Theory and experiment

The distribution of Cu2+ impurities in ZnSeO4·6H2O and Na2Cd(SO4)2·2H2O crystals was investigated by electron paramagnetic resonance. The tangential selectivity by sectors in pyramid growth was revealed. Dissymmetrization, i.e., decrease of point group symmetry, is explained by tangential selectivity occupying impurities and formation defects. An analysis by group theory is presented to explain dissymmetrization of crystals. The connection between the distribution of point defects in a crystal and the growth process is discussed. © 1980 Springer-Verlag

Electron paramagnetic study of fe3+ and gd3+ in na2zn(S04)24h20

The epr of Fe3- and Gd3- in single crystals of Na2Zn(S04)24H20 was studied at liquid nitrogen temperature using a Q band spectrometer. For each ion, two spectra superimposed in the (010) plane and along b were observed. The spectra were described by the most general spin Hamiltonian, the constants of which were obtained for both ions. Analysing the pseudo-symmetry of the fourth-order term of the spin Hamiltonian, it is found that Fe3- only modifies the orientation of the local structure slightly and that the charge defect is probably balanced by a Na- vacancy. It is shown that the local structure around Gd3- is very different from the one around Fe3-. © 1986 The Institute of Physics

Local environment of Gd3+ in MgF2

The electron paramagnetic resonance (EPR) of Gd3+ in MgF2 reveals that Gd3+ has two different environments in the lattice. One of them has D2h symmetry, the EPR spectrum is characterized by a large zero field splitting [B2 0= 968.10-4 cm-1; B2 2= 357.10-4 cm-1] and the fourth order term of the spin Hamiltonian is axial; it is assumed that one Gd3+ substitutes two Mg2+. The other Gd3+ center has only monoclinic symmetry. From the analysis of the fourth order term of the spin Hamiltonian of the corresponding spectrum it is seen that one Gd3+ substitutes one Mg2+ and that this substitution produces a large local distortion of the lattice. © 1990 Springer-Verlag

EPR of Gd3+ in Na2Cd(SO4) 22H2O, comparison with previous results obtained for Fe3+

An EPR experiment on Gd3+ in a single crystal of Na 2Cd(SO4)22H2O (CdK) was carried out at Q band frequency and nitrogen temperature. Two spectra related by symmetry were observed. All spin Hamiltonian constants have been calculated. The pseudosymmetries of the fourth-order term of the spin Hamiltonian are compared to those of Fe3+. It is observed that the substitution of Gd 3+ for Cd2+ induced a very important local distortion of the host lattice, which is discussed. © 1985 American Institute of Physics

Study of the environment of Fe3+ in Na2Cd(SO 4)2 2H2O by EPR

An EPR experiment on Fe3+ in a single crystal of Na 2Cd(SO4)2 2H2O was carried out at Q band frequency and nitrogen temperature. The angular dependencies of the spectra were studied in the three crystallographic planes. In order to have a very precise orientation of the crystal, the experiment was done with a twinned crystal (superposition of two spectra in all three planes). The zero-field splitting of Fe3+ is of the same order of magnitude as the quantum hv, so that many ΔM=2 transitions were observed. All spin-Hamiltonian parameters (20) were calculated using observed ΔM=1 and ΔM=2 transitions for 30 different orientations of the magnetic field. The pseudosymmetries of the fourth-order term of the spin Hamiltonian of Fe 3+ are compared to those we calculated using the superposition model. It is seen that Fe3+ is substituted for Cd2+ and that the orientation of the ocatahedral environment of Fe3+ does not differ much in orientation from that of Cd2+. Differences between the distortions of the Cd-O6 and Fe-O6 octahedra are discussed. © 1985 American Institute of Physics

EPR probes in sedimentary rocks: The features of Mn2- and free radicals distribution in the Permian formation in Tatarstan

EPR data (i.e. the distribution of Mn2+ in carbonate rocks and of free radicals in terrigenous rocks) may be used to characterize sedimentary beds, especially if they are unfossilferous. The study of the distribution of Mn2+ that substitutes Mg2+ and Ca2+ in sedimentary dolomites suggests that the distribution is affected by their genesis. © 1991 Springer

Polarized triplet production by circularly polarized photons

A process of the pair production by a circularly polarized photon in the field of unpolarized atomic electron has been considered in the Weizaecker-Williams approximation. The degree of longitudinal polarization of positron and electron has been calculated. An exclusive cross-section as well as a spectral distribution are obtained. We estimate the accuracy of our calculations at the level of a few percent. We show the identity of the positron polarization for considered process and for process of pair production in the screened Coulomb field of nucleus.Comment: 9 pages, 3 picture

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe3+ ions in tetrahedral and octahedral positions of Ge4+ in the crystal Li2Ge7O15 were studied using EPR. Fe3+ substitutes for Ge4+ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sum S(B4) of the B4 tensor of the spin Hamiltonian of Fe3+. The irreducible tensor products {V4⊗ V4}4 and {V4⊗V4}2 the V4 tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to the B4 and B2 tensors of the spin Hamiltonian of Fe3+, respectively. A comparison of the fourth-rank tensors B4 of the spin Hamiltonian and {V4⊗V4}4 of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO4] tetrahedra with the [Ge(1)O6] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe3+, Cr3+ and Cu2+ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of the M→M+1 and the M+2 →M+3 transitions of Fe3+ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature. © 1996 Springer

Derivation of the conditions for equivalent positions in crystals: The dissymmetrization of barite by electron spin resonance spectra

The conditions for equivalent positions on the (hkl) face of growing crystal are derived using sym- metry elements of the space group. It is shown by the example of the sp. gr.D2h 16 that the conditions of equiv- alent position formation coincide with conditions of the reflection of diffracted beams by crystal. It is estab- lished that electron spin resonance (ESR) centers in barite, SO4 -(I) and SO4 -(II), with only two conjugate spectra with equal intensity out of four, and SO4 -(III), with a different intensity of conjugate spectra KaM = 2, are localized into the growth pyramid of the (001) face with a [010] step. SO2 -,SO3 -, and (IV) centers, having an identical intensity of the conjugate ESR spectra with KαM = 2, are localized into the growth pyramid of the (210) face with a growth step [001]. © Pleiades Publishing, Inc., 2012

Group - Theoretical representation of holohedral forms of crystals by conjugated simple forms. Intergrowth of crystals

The schemes for division of holohedral simple forms of crystals into conjugated simple forms are derived by decomposition of the symmetry group of the primitive sublattice into double cosets. The number of equivalently oriented simple forms in the intergrowth of crystals, whose primitive space sublattices are parallel to one another, is equal to the number of holohedral permutational conjugated simple forms, which has the value 992 for all the 32 symmetry classes. © 2000 MAIK "Nauka/Interperiodica"