Alexandre I. Rykov

Abstract: Solid products of detonation of several explosives contain the nanodiamonds, whose net mass reaches up to the value of tens percents. Due to the high density of a diamond phase (3.5 g/cm3) these nanoparticles show a very good contrast in x-ray range even there is a background of other part of a carbon-based composite. This contrast gave us the opportunity of studying the dynamics of the nano-diamonds formation and expansion. In this work, the solutions for both direct and inverse problems of small-angle x-ray scattering (SAXS) by the exploding substance are examined. The solution of a direct problem is presented as the determination of the angular dependences of intensity of SAXS, and solution of the inverse problem consist of the reconstruction of distribution of particles on the size, followed by the recuperation of the space-time picture of the nanoparticle nucleation and growth, incineration, dispersal and disappearance. In an ideal, the distribution of particles on the size in various points of the spaces and at different moments of time must be determined from the time-resolved SAXS experiments. This is, however, an ill-posed problem. We turn to a number of assumptions, and consider their consequences. The experimental data were obtained in an explosion chamber on a beamline of the VEPP-3 storage ring. The distribution functions of the nanoparticles depending on one parameter (mean size) and two parameters (size and dispersion) are analyzed. The results allow to make a conjecture that the particles grow in size during several micro-seconds behind front of a detonation. �зве��но, ��о �о��аве �ве�д�� п�од�к�ов де�она�ии неко�о��� вз��в�а��� ве�е��в �оде�жа��� де�она�ионн�е наноалмаз�, ма��ова� дол� ко�о��� може� до��ига�� де���ков п�о�ен�ов. �лагода�� в��окой пло�но��и алмазной �аз� (3.5 г/�м3) нано�а��и�� облада�� �о�о�ей кон��а��но���� в �ен�генов�ком изл��ении на �оне о��ал�ной �а��и �гле�одного компози�а, ��о да�� возможно��� из��ени� динамики и� �о�ми�овани� и пе�еме�ени�. � �або�е �а��ма��ива���� ва�иан�� �е�ени� как п��мой �ак об�а�ной зада� мало�глового �енгенов�кого �а��е�ни� ан�амбл�ми �о�ми����и��� и �а�п�о���ан���и��� нано�а��и�. Ре�ением п��мой зада�и �вл�е��� оп�еделение �глов�� зави�имо��ей ин�ен�ивно��и �а��е�ни�, а �е�ением об�а�ной � во���ановление �е�п�еделени� �а��и� по �азме�� и п�о���ан��венно-в�еменной ка��ин� и� за�ождени� и �о��а, �а���едо�о�ени� и и��езновени�. � идеале, �а�п�еделение �а��и� по �азме�� в �азли�н�� �о�ка� п�о���ан��ва и в �азн�е момен�� в�емени н�жно извле�� из мало�глового ди��ак�ионного �к�пе�имен�а. �днако, об�а� неко��ек�но��� в �акой по��ановке об�а�ной зада�и �а��е�ни� за��авл�е� на� п�ибегн��� к ��д� п�едположений, ва�иан�� ко�о��� �а��ма��ива����. Эк�пе�имен�ал�н�е данн�е пол��ен� во вз��вной каме�е на накопи�еле �Э��-3. Р�д однопа�аме��и�е�ки� и дв��па�аме��и�е�ки� ��нк�ий �а�п�еделени� �а��и� по �азме�� п�ове������ на п�едме� �оо�ве���ви� �азли�н�м кад�ам �вол��ии ан�амблей нано�а��и�. Рез�л��а�� позвол��� �дела�� в�вод о �ом, ��о �о�� �а��и� п�одолжае��� в �е�ении не�кол�ки� мик�о�ек�нд за ��он�ом де�она�ии.

Abstract:

Abstract: s....... (TITLES, JOURNAL, AUTHORS and Abstracts): 1. AN ISING FERRIMAGNET WITH LAYERED AND CHAINED MAGNETIC SUBLATTICES: CA2FEMNO5 JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, VOLUME 320, ISSUE 6, MARCH 2008, PAGES 950-956 A.I. RYKOV, K. NOMURA, Y. UEDA, A.N. VASILIEV  Abstract Nakahara et al. [Mater. Lett. 30 (1997) 163�167] have refined their X-ray diffraction patterns of Ca2FeMnO5 with the brownmillerite structure, in which all the octahedral in-plane sites are occupied by Mn, and all the tetrahedral in-chain sites are occupied by Fe. Our Mössbauer study of Ca2FeMnO5 is in agreement with this cationic distribution. We determine the spin orientation with respect to the crystal axes and suggest the model of layered magnetic structure. From our Mössbauer spectra and from analysis of the crystal and magnetic structures in a number of brownmillerite-related oxides we suggest for Ca2FeMnO5 the mixed-spin and S=2 Ising system. The Fe3+ and Mn3+ moments order antiferromagnetically (AFM) both in-plane and in perpendicular direction and aligned up and down vertically in the out-of-plane direction. Although we cannot exclude a small tilt and a related small ferromagnetic component, basically, each of Fe and Mn spin subsystems generate individually a C-type sublattice and two these sublattices are superposed to give an unusual G-like type structure with two sorts of spins. Nuclear forward scattering (NFS) and nuclear inelasting scattering (NIS) techniques were used to characterize, respectively, the hyperfine interactions and phonon density of states (DOS) of 57Fe. Comparing the vibrational properties of several brownmillerites we were able to give an approximate DOS for tetrahedral and octahedral positions. 2. ROOM-TEMPERATURE MAGNETISM IN CHEMICALLY OXYGENATED CONDUCTING OXIDE SR2FECOO6 THIN SOLID FILMS, VOLUME 515, ISSUE 24, 15 OCTOBER 2007, PAGES 8645-8648 KIYOSHI NOMURA, ALEXANDRE I. RYKOV, ALEXANDER P. NEMUDRY, TAKAYA MITSUI  Abstract Using chemical oxidation technique, we prepared a new half-metal Sr2FeCoO6. Mössbauer study has presented a simultaneous test of stability of the oxygen content in this material. Caused by the oxygen leak, the Mössbauer spectra were progressively changed in the sample kept at room temperature. The spectra were measured within 3 months alternately with and without field of 4 kOe of a permanent magnet. The strong magnetizing effect of such a moderate field is conceptually well explained by nanostructuring, assisted by the unexpected effect of the superparamagnetic fraction permanently increasing with the �leak-time�. 3. XAFS AND CEMS STUDY OF DILUTE MAGNETO-OPTICAL SEMICONDUCTOR, FE DOPED TIO2 FILMS THIN SOLID FILMS, VOLUME 515, ISSUE 24, 15 OCTOBER 2007, PAGES 8649-8652 KIYOSHI NOMURA, HIROMI EBA, KENJI SAKURAI, ALEXANDRE RYKOV, TETSUYA HASEGAWA   Abstract TiO2 films doped with 6% Fe were prepared by pulsed laser deposition (PLD) under different oxygen pressures, and characterized by X-ray absorption fine spectra (XAFS) and conversion electron Mössbauer spectra (CEMS). The edge energy and spectrum profiles of Fe� and Ti K X-ray absorption showed only Fe3+ and Ti4+ states for rutile TiO2 films prepared under 10� 1 Torr, the metallic Fe and Ti4+ for rutile TiO2 films prepared in 10� 6 Torr, and the metallic Fe and the average valance of less than �4+� for Ti in TinO2n�x films prepared by the PLD under 10� 8 Torr. The metallic Fe clusters are also found in the TEM images of TinO2n�x film. Magnetic property of Fe doped TiO2 films prepared by PLD at high vacuum (10� 6 and 10� 8 Torr) is considered to originate mainly from the magnetic metal iron clusters. 4. THE VORTEX DEPINNING TRANSITION IN UNTWINED YBACUO USING COMPLEX IMPEDANCE MEASUREMENTS PHYSICA C: SUPERCONDUCTIVITY, VOLUMES 408-410, AUGUST 2004, PAGES 577-578 A. PAUTRAT, C. GOUPIL, CH. SIMON, B. ANDREWJEWSKI, A. I. RYKOV, S. TAJIMA   Abstract We present surface impedance measurement of the vortex linear response in a large untwined YBCO crystal. The depinning spectra obtained over a broad frequency range (100 Hz�30 MHz) are those of a surface pinned vortex lattice with a free flux flow resistivity (two modes response). The critical current in the �Campbell� like regime and the flux flow resistivity in the dissipative regime are extracted. Those two parameters are affected by the first order transition, showing that this transition may be related to the electronic state of vortices. 5. LOW-ENERGY EXCITATIONS IN BROWNMILLERITES AND RELATED OXIDES PHYSICA B: CONDENSED MATTER, VOLUME 350, ISSUE 4, 30 JULY 2004, PAGES 287-304 A. I. RYKOV, K. NOMURA, T. MITSUI, M. SETO   Abstract It is generally believed that the glasses and crystals differ very much from each other by their vibrational spectra. However, the crystalline systems with frozen disorder may exhibit the vibrational density of states (DOS) peaked in low-energy region quite resembling to the Bose peak typically observed in glasses. Common origin of these features in disordered crystalline and amorphous systems is discussed in the frameworks of two theoretical concepts: the soft potential and the broken ladder models. The experimental results are presented for large series of standard, oxygen deficient, layered perovskite-based systems and systems containing the metal-oxygen chains. They were studied with using the nuclear (57Fe) inelastic scattering of synchrotron radiation. The largest excess of vibrational density of states with respect to the Debye-like one was found in the brownmillerite CaSrFeCoO5 with the peak energy 7.5 meV. The peak in E�2-weighted DOS g(E)/E2 exceeds the Debye behavior (g(E)�E2) by 4 times at room temperature and grows at cooling even further. 6. COMMENSURABILITY OF HIGHER ORDER BETWEEN VORTEX AND CRYSTAL LATTICE IN RBA2CU3O7 (R=Y, ND) PHYSICA C: SUPERCONDUCTIVITY, VOLUME 404, ISSUES 1-4, 1 MAY 2004, PAGES 215-219 H. K?PFER, G. RAVIKUMAR, A. A. ZHUKOV, P. A. J. DE GROOT, C. MEINGAST, A. I. RYKOV, S. TAJIMA, TH. WOLF   Abstract Commensurate states, investigated by magnetization, are observed in very pure RBa2Cu3O7 (R=Y, Nd) single crystals if the intervortex distance ac becomes equal to an integer number k of the crystal lattice constant d (ac=kd). Incommensurate states at higher temperatures and fields develop, split up and become finally commensurate with k/2 or even with a smaller fraction k/n of the crystal lattice constant (nac=kd). These additional states of higher order (n>1) are related to a superstructure of the vortex lattice with respect to the crystal lattice. 7. MAGNETIC PENETRATION DEPTH AND REVERSIBLE MAGNETIZATION IN SINGLE CRYSTALS OF BI2SR2CACU2O8 GROWN BY TSFZ METHOD PHYSICA C: SUPERCONDUCTIVITY, VOLUMES 357-360, PART 1, SEPTEMBER 2001, PAGES 284-287 A. I. RYKOV, T. TAMEGAI Abstract We analyze the reversible magnetization in Bi2Sr2CaCu2O8+δ using several theoretical concepts, starting from the London model. The field dependence of the empirical �superfluid density� dM/d ln B is fitted with using the theory of circular cell. Additionally, we take into account the pinning mechanism associated with dislocation structures naturally existing in the crystals grown by travelling-solvent-floating-zone (TSFZ) technique. This allows us to explain: (i) the rapid variation of dM/d ln B at small fields; (ii) the reduction of thermal fluctuation term with respect to the theoretical prediction for decoupled pancakes. In the light of this reduction we derive the curves Hc1(T) and λ2(0)/λ2(T) from the reversible magnetization similar to the ones obtained by direct measurements of Hc1 by micro-Hall AC technique, and with microwave surface impedance, respectively. 8. PRESSURE DEPENDENCE OF THE OXYGEN ORDERING PROCESS IN RBA2CU3OX PHYSICA C: SUPERCONDUCTIVITY, VOLUMES 341-348, PART 1, NOVEMBER 2000, PAGES 439-440 H. LEIBROCK, K. GRUBE, W. H. FIETZ, S. I. SCHLACHTER, K. -P. WEISS, A. I. RYKOV, S. TAJIMA, B. OBST, P. SCHWEISS, H. W?HL   Abstract In YBa2Cu3Ox (x<7.0), above 225 K a oxygen ordering in the CuO chains is visible as a glass-like transition in thermal-expansion measurements. The large contribution of this process to the thermal expansion and the Tc changes observed under application of external hydrostatic pressure point to a strong dependence of the oxygen ordering and its activation energy Ea on the unit-cell volume V. If, however, V is changed chemically by replacing Y with other rare earth ions, R, the values of Ea reported in the literature show only a slight dependence on V. We have measured the thermal expansion of a YBa2Cu3O6.94 single crystal under hydrostatic pressure up to 0.5 GPa. The activation energy at ambient pressure was determined to be 1.0±0.1 eV with a pressure dependence dEa/dp = 78 meV/GPa. The volume dependence dEa/dV = �48 meV/�3 obtained from these pressure experiments is clearly higher than dEa/dV = �5.4 meV/�3 from R-ion substitution. Thus we conclude that a non-uniform compression of interatomic distances within the unit cell has to be considered to explain the difference between dEa/dV(p) and dEa/v(R). The contribution, �αV, of the ordering process to the thermal-expansion coefficient, αV, increases with pressure, indicating that more and more oxygen atoms take part in the process. From an extrapolation to negative pressure via R-ion substitution we expect a vanishing (R-Nd) or inverted (R=La) pressure effect on oxygen ordering. 9. VORTEX PINNING IN UNTWINNED YBCO PHYSICA C: SUPERCONDUCTIVITY, VOLUMES 341-348, PART 2, NOVEMBER 2000, PAGES 1059-1060 A. PAUTRAT, C. GOUPIL, CH. SIMON, N. L?TKE-ENTRUP, B. PLA?AIS, P. MATHIEU AND, Y. SIMON, A. I. RYKOV AND, S. TAJIMA   Abstract Three different AC and DC experiments have been performed in untwinned YBCO to clarify the mechanism of vortex pinning. Pinning by surface irregularities is the only mechanism consistent with all experiments. This questions usual interpretation in terms of bulk collective pinning theories. In particular, the irreversibility line appears as the dissapearance of superficial critical current. 10. HIGH-FREQUENCY LINEAR AC RESPONSE OF A PINNED VORTEX LATTICE PHYSICA B: CONDENSED MATTER, VOLUMES 284-288, PART 1, JULY 2000, PAGES 719-720 N. L?TKE-ENTRUP, B. PLA?AIS, P. MATHIEU, Y. SIMON, A. PAUTRAT, C. GOUPIL, C. SIMON, A. RYKOV, S. TAJIMA   Abstract We report an extensive investigation of the complex penetration depth λAC for a transverse field in the mixed state for an untwinned YBaCuO crystal and conventional superconductors (PbIn, Nb, V). A broad frequency range (Ω/2�=1 kHz�10 MHz) is used to cover the crossover Ωp from pinned to viscous vortex line dynamics. Skin effect conditions are realized by using thick samples d1 mm allowing a direct comparison with theory. Bulk pinning theories rely on a local complex resistivity, which entails a relatively narrow pinning crossover λAC�(1�iΩ/Ωp)�1/2. Recent 2-mode-theories, which treat pinning as a boundary condition problem, predict a broad crossover λAC�(1+(�iΩ/Ωp)1/2)�1. Our results are fully consistent with the 2-modes electrodynamics and show the irrelevance of bulk pinning in homogeneous samples.   11. EVIDENCE FOR VORTEX PINNING BY SURFACE IRREGULARITIES IN UNTWINNED YBACUO CRYSTALS PHYSICA C: SUPERCONDUCTIVITY, VOLUME 332, ISSUES 1-4, MAY 2000, PAGES 61-65 CH. SIMON, A. PAUTRAT, C. GOUPIL, N. L?TKE-ENTRUP, B. PLA?AIS, Y. SIMON, P. MATHIEU, A. RYKOV, S. TAJIMA   Abstract A direct method of induction in a small pick up coil shows that in untwinned YBaCuO crystals, the subcritical current flows close to the surfaces of the sample. By studying the frequency dependence of the ac penetration depth, we have also shown that the two-modes through Mathieu�Simon (MS) model of pinning by surface irregularities is able to account for the observed spectrum. The Campbell model of weak bulk pinning can be completely ruled out. Above the first order transition line, the critical current is zero and the ac response is that of an ohmic system. This can be interpreted by the curvature of the vortices close to the surface. This shows the irrelevance of weak bulk pinning models for clean untwinned YBaCuO crystals that were often proposed as model systems for this case.   12. NEW PICTURE OF PHONONIC RAMAN SCATTERING IN YBA2CU3O7?X SINGLE CRYSTALS JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, VOLUME 59, ISSUES 10-12, 11 OCTOBER 1998, PAGES 1997-1999 M. F. LIMONOV, A. G. PANFILOV, A. I. RYKOV, S. TAJIMA, A. YAMANAKA   Abstract We present Raman scattering spectra in detwinned YBa2Cu3O7�x single crystals concerned with both the out-of-plane and in-plane anisotropies of Ag Raman lines in the normal and superconducting states. In overdoped crystals, we find pronounced superconductivity-induced x�y anisotropy in the softening and broadening of the 340 cm�1 line below Tc. It suggests the x�y anisotropy of the superconducting gap (�x��y), as expected in the case of the d+s wave gap, or a contribution of the chain superconductivity with a symmetry different from that for the plane. The doping dependence of this phonon anomaly indicates that the gap energy increases with reducing doping level. 13. OPTICAL STUDY OF PAIR-BREAKING EFFECT IN ZN-SUBSTITUTED YBA2CU3O7?? JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, VOLUME 59, ISSUES 10-12, 11 OCTOBER 1998, PAGES 2018-2020 N. L. WANG, S. TAJIMA, R. HAUFF, A. I. RYKOV, T. MIMURA   Abstract The Zn-doping effects on the a-, b- and c-axis optical conductivity spectra have been investigated for highly oxygenated YBa2Cu3O7�δ crystals. The absolute values of the diagonal components of penetration depth tensor were obtained. The measurement reveals that Zn substitution leads to very strong but anisotropic suppression of the condensate density along the three crystallographic axes.   14. MAGNETIZATION JUMPS IN LARGE CRYSTALS OF YBA2CU3O7?? WITH INHOMOGENEOUS VORTEX STATES PHYSICA C: SUPERCONDUCTIVITY, VOLUME 297, ISSUES 1-2, 1 MARCH 1998, PAGES 133-143 A. I. RYKOV   Abstract The magnetization jumps at the first order vortex transition were studied in several YBa2Cu3O7�δ single crystals subjected to different thermal treatments. It turns out that the heights of the magnetization jumps depend not only on the sample preparation but also on the measurement procedure. Using the technique of SQUID magnetometry the dependence of the jump heights on the length of sample movement in the gradiometer was established. The results are interpreted in terms of multipolar magnetic response from the inhomogeneous magnetic state obtained in the field cooling (FC) regime.   15. HIGH ENERGY SPIN EXCITATIONS OF A HIGH-TC SUPERCONDUCTOR YBA2CU3O6.6 PHYSICA B: CONDENSED MATTER, VOLUMES 241-243, DECEMBER 1997, PAGES 856-858 T. NISHIJIMA, M. ARAI, Y. ENDOH, S. M. BENNINGTON, S. TAJIMA, K. TOMIMOTO, A. I. RYKOV, N. KOSHIZUKA, Y. SHIOHARA   Abstract High energy inelastic neutron scattering measurements have been performed to study the magnetic excitation spectrum on a high quality crystal of YBa2Cu3O6.6 (Tc 64 K) by using pulsed neutron techniques. Magnetic excitations at the antiferromagnetic zone center have been clearly observed up to 150 meV for the first time. The magnetic resonance, which has been observed at 41 meV in the fully doped material, is observed at 35 meV.   16. RAMAN STUDY OF UNTWINNED YBA2CU3O7 SINGLE CRYSTALS: NEW EVIDENCE FOR THE X - Y ANISOTROPY OF THE SUPERCONDUCTING GAP PHYSICA C: SUPERCONDUCTIVITY, VOLUMES 282-287, PART 2, AUGUST 1997, PAGES 1029-1030 M. F. LIMONOV, A. I. RYKOV, S. TAJIMA   Abstract We present a new picture of Raman scattering in YBa2Cu3O7 single crystals which shows both the out-of-plane and the in-plane anisotropies of the electron-phonon interactions. The x-y anisotropy in the softening and the broadening for the 340 cm�1 mode strongly suggests the anisotropy of the superconducting gap amplitude �x � �y, which leads to the d+s symmetry of the gap wave function. 17. UNUSUAL TRANSPORT AND STRUCTURAL PROPERTIES OF MECHANICALLY TREATED POLYCRYSTALLINE SILVER IODIDE : PART III. STRUCTURAL STUDY SOLID STATE IONICS, VOLUME 96, ISSUES 3-4, APRIL 1997, PAGES 233-237 N. F. UVAROV, E. F. HAIRETDINOV, A. I. RYKOV, YU. T. PAVLYUKHIN   Abstract The effect of mechanical treatment on the crystal structure of AgI was studied by X-ray diffraction and EXAFS techniques. It was shown that even moderate mechanical treatment of AgI results in the appearance of stacking faults in a close-packed anion sublattice. After an intense mechanical treatment the random β-γ polytype structure is formed. The polytypes are built of domains separated by antiphase boundaries which are the most probable sources of cationic Frenkel defects governing the high ionic conductivity and anomalous dielectric properties of mechanically treated AgI. 18. MAGNETIC SUSCEPTIBILITY OF THE ORTHORHOMBIC 2D-ANTIFERROMAGNET YSR2CU2GAO7 PHYSICA C: SUPERCONDUCTIVITY, VOLUME 263, ISSUES 1-4, MAY 1996, PAGES 359-362 A. I. RYKOV, Y. UEDA, A. HAYASHI   Abstract The fact that the magnetic susceptibilities in the paramagnetic regime of the parent insulating materials La2CuO4 and YBa2Cu3O6 increase monotonically with temperature is well explainable in terms the square-lattice 2D Heisenberg model. It is predicted by the model that the position of Xmax occurs at a temperature close to the intraplanar exchange parameter J. We show that the Ga-based cuprate YSr2Cu2GaO7 is an orthorhombic antiferromagnet with TN = 387 K. Above TN, two broad maxima of magnetic susceptibility are observed. The first broad maximum is located at 520 K. The position of the second maximum lies beyond the measurable range ( > 800 K). Thus, the observed susceptibility behavior disagrees with that predicted by the isotropic Heisenberg model. Such a discrepancy is tentatively explained by a loss of the symmetry in the CuO2 network.   19. NMR AND MAGNETIC SUSCEPTIBILITY IN SUPERCONDUCTING AND ANTIFERROMAGNETIC GA-BASED CUPRATES Y1?XCAXSR2CU2GAO7 (0?X?0.3) PHYSICA C: SUPERCONDUCTIVITY, VOLUME 257, ISSUES 1-2, 20 JANUARY 1996, PAGES 86-98 ALEXANDRE I. RYKOV, YUTAKA UEDA, ATSUSHI GOTO, HIROSHI YASUOKA   Abstract Magnetic susceptibility and NMR/NQR measurements were performed on Y1�xCaxSr2Cu2GaO7 (x=0, 0.1, 0.2, 0.3). The single phase samples annealed at 600°C under oxygen pressure of 30 MPa are superconductors with Tc=35 K for x=0.2 and x=0.3. In spite of the presence of a small Curie-like term, we show that the spin susceptibility in the normal state increases with Ca doping and reaches the value �spin�0.9 cm3/Cu-mole, which is comparable to other superconducting cuprates. From the observation of Cu zero-field resonance (AFNR) and susceptibility data the parent compound is classified as 2D antiferromagnet (TN=387 K). The transition from antiferromagnetic insulator to superconductor occurs with increasing concentration of carriers, but extends over several tens percent of Ca. The superconductivity is significantly suppressed by increasing disorder within limits of solubility for Ca. The Ga NQR spectra are narrow in both antiferromagnetic and superconducting regimes, but heavily broadened in the intermediate spin-glass-like domain. From x=0 to x=0.3, the 63Cu quadrupole frequency increases from 24 to 28 MHz due to the charge transfer resulting in superconductivity. Other EFG parameters are not markedly changed from those given in YSr2Cu2GaO7 by Pieper [Physica C190(1992)261]. 20. CHARGE TRANSFER TO THE LOCAL SINGLET STATES AS A FUNCTION OF LI CONTENT IN LA2CU1?XLIXO4 AND LA1.85SR0.15CU1?XLIXO4 PHYSICA C: SUPERCONDUCTIVITY, VOLUME 247, ISSUES 3-4, 1 JUNE 1995, PAGES 327-339 ALEXANDRE I. RYKOV, HIROSHI YASUOKA, YUTAKA UEDA   Abstract Magnetic susceptibility (�) and NMR/NQR measurements were performed on La2Cu1�xLixO4 0�x�0.5) and La1.85Sr0.15Cu1�xLixO4 (0�x�0.425). The narrow 63Cu NQR line at νQ = 46.3 MHz (1.3 K) is detected in La2Cu0.5Li0.5O4. This frequency is higher than all 63Cu-NQR frequencies found so far in all the cuprates. Moreover, a further increase of νQ is observed in the solid solutions La2Cu1�xLixO4 with decreasing x. Corresponding electric field gradients (EFG's) are considered in terms of ionic and cluster models. The 3d hole density nx2�y2 � 1 and the radial average r�3 � 9.5 a.u. are derived from the EFG's, suggesting the d9 state of the formally trivalent copper. The antiferromagnetic ordering is rapidly suppressed as a function of x in the La2Cu1�xLixO4, and for x � 0.03 we find TN � 0. The evolution of � vs. x shows a gradual decrease of Cu spin susceptibility and its disappearance in the diamagnetic La2Cu0.5Li0.5O4, in which � = �6.7 � 10�5 cm3/mole is composed of Van Vleck and core terms. The electronic state is implied to be a local singlet (1A1g) of primarily d9 character. The quadrupole signature of the local singlet in the range 46.3 � 63νQ � 47.0 MHz is observed for x> 0.15 in both systems, La2Cu1�xLixO4 and La1.85Sr0.15Cu1�xLixO4. The differentiation of charge between local singlet and conductivity band states is discussed. 21. STRUCTURAL ASPECTS AND ANTIFERROMAGNETIC ORDERING IN THE "123" DERIVATIVE LNSR2CU2GA1-XFEXO7-? (LN = Y, HO) JOURNAL OF SOLID STATE CHEMISTRY, VOLUME 113, ISSUE 1, NOVEMBER 1994, PAGES 94-108 A. RYKOV, V. CAIGNAERT, G. VAN TENDELOO, J. M. GRENECHE, F. STUDER, N. NGUYEN, A. DUCOURET, P. BONVILLE, B. RAVEAU   Abstract The magnetic ordering in the orthorhombic phases Ln Sr2Cu2Ga1-xFexO7 (Ln = Y, Ho; 0.03 � x � 0.95) has been investigated by 57 Fe Mössbauer spectroscopy. Iron occupies the Ga site (Cu(1) site) for small doping rates (x � 0.03), but occupies the Cu(1) and Cu(2) sites for iron-rich compositions (0.5 � x � 0.95). For x = 0.9, the magnetic ordering takes place below TN � 420 K in both the Cu(1) and Cu(2) sites. The down concave curve of the temperature dependence of the hyperfine field in the Cu(1) site suggests a rather weak coupling between the Cu(1) and Cu(2) sites. The principal EFG axis VZZ is directed in both sites along the longest axis of the orthorhombic cell, i.e., perpendicular to the alternating sheets. The sign of VZZ is positive for the Cu(2) sites (�EQ = 0.6 mm/sec) and negative for the Cu(1) sites (�EQ = -1.9 mm/sec). In the distorted tetrahedral site, the EFG tensor is non-axial, however, its asymmetry is small (η < 0.4). The magnetic hyperfine field is perpendicular to the VZZ axis for both sites; i.e., H lies in the plane of the shortest axes. A structural analysis by electron microscopy reveals the presence of a large number of defects in the (Fe, Ga)O chain ordering as well as in the basic structure. The presence and the abundance of these defects is in agreement with the Mössbauer observations.   22. QUADRUPOLE INTERACTIONS AND VIBRATIONAL ANISOTROPY OF TETRAHEDRAL FE(III) IN THE "123" DERIVATIVE LNSR2CU2GA1-XFEXO7 (LN = Y, HO) JOURNAL OF SOLID STATE CHEMISTRY, VOLUME 109, ISSUE 2, APRIL 1994, PAGES 295-306 A. RYKOV, V. CAIGNAERT, B. RAVEAU   Abstract The layered copper oxides Ln Sr2Cu2Ga1-xFexO7 (Ln = Y, Ho) have been synthesized for 0 � x � 1. An orthorhombic structure (S.G. Ima2, a � 22.9 �, b � 5.5 �, c � 5.4 �) forms in the oxides prepared in Ar atmosphere for x rising up to 0.95. The transition from orthorhombic to tetragonal "123" structure (S.G. P4/mmm, a123 � 3.84 �, c123 � 11.4 �) in the range of 0.6 < x < 0.95 is induced by heating in an oxygen flow. The Mössbauer study reveals that iron exhibits the predominant tetrahedral coordination in the orthorhombic phase, but the pyramidal coordinations in the oxygenated tetragonal LnSr2Cu2FeO7.5. The characteristic Mössbauer quadrupole doublet associated with the tetrahedral "Ga" site turned out to be asymmetric suggesting an anisotropic thermal vibration of iron in this site. The anisotropy of recoil-free fraction is consistent with a large distortion of the tetrahedral site, where the EFG is parallel to the a direction of the unit cell. The negative sign of Vzz and the large value of quadrupole splitting (|�EQ| � 1.93 mm/sec) suggest a similarity between the geometry of FeO4 tetrahedra in both oxides Ln Sr2Cu2Ga1-xFexO7 and Ln Ba2Cu3-xFexO-y.   23. THE COMPLEX DISTRIBUTION OF IRON IN THE (Y, CA)BA2(CU, FE)3O6+Y CUPRATE: A M?SSBAUER STUDY PHYSICA C: SUPERCONDUCTIVITY, VOLUME 205, ISSUES 1-2, 15 JANUARY 1993, PAGES 63-77 A. RYKOV, V. CAIGNAERT, N. NGUYEN, A. MAIGNAN, E. SUARD, B. RAVEAU   Abstract Mössbauer studies with 57Fe in Y1�zCazBa2Cu3�xFexO6+y for x=0.09 and 0.24, z=0.09 and 0.24, and y between 0 and 1, allowed, for fixed z and x values, different distributions of iron on the copper sites to be observed according to thermal treatment under different gaseous atmospheres (Ar, air, oxygen). Three types of coordination - a tetrahedral and two pyramidal - are proposed for Fe on Cu(1) sites; one of the pyramidal Cu(1) sites is compatible with the existence of small iron clusters (Fe2O9, Fe3O13, etc.). The accomodation of tetrahedral coordination by a fraction of iron atoms induces the transition from orthorhombic to tetragonal symmetry with increasing x. The migration of iron from these sites into the Cu(2) layer restores the orthorhombic symmetry. An increase in the occupancy of Fe on the Cu(2) site leads to a sharp detrimental effect on the critical temperature Tc; this confirms the predominant role of the Cu(2) sites in superconductivity for the �123�-structure.   24. STRUCTURE OF AMORPHOUS FERRITES OF LANTHANUM, DYSPROSIUM, AND BISMUTH REACTIVITY OF SOLIDS, VOLUME 7, ISSUE 1, APRIL 1989, PAGES 29-42 A. I. RYKOV, YU. T. PAVLUKHIN, N. I. SYROTINA, V. V. BOLDYREV   Abstract Mechanical activation of the lanthanum, dysprosium, and bismuth morphotropic series of ferrites with perovskite structure leads to the formation of amorphous materials. Mössbauer spectra indicate lowering of the Fe3+ coordination number on amorphization. X-ray diffraction reveals the changes in the short-range order of the arrangement of the heavy cations with respect to the starting crystalline samples, as well as structural differences between the amorphous ferrites. The structure of the amorphous LaFeO3 can be described as quasicrystalline with trigonal-prismatic La3+ configuration. Structural differences between the amorphous samples are due to distortions in the perovskite cells of the original crystal namely: an ionic misfit in DyFeO3, and covalency of the chemical bonds in BiFeO3.