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1.
27Al and 55Mn nuclear magnetic resonance shift, K, and 27Al spin lattice relaxation time, T
1, have been measured for the six-dimensional face-centered icosahedral quasicrystals, Al 75-x
Pd 15Mn 10+x
with x=0, 2 and7. The Al 75Pd 15Mn 10 quasicrystal exhibits a temperature independent Knight shift K and( T
1
T) –1=0.022±0.002 (K s) –1 in a temperature range from room temperature to 5 K because there exist no Mn atoms with local magnetic moment. The replacement of Al with Mn drastically decreases the 27Al K, the 55Mn K and T
1 of 27Al, andthe 27Al K becomes negative. There is an additional contribution to the spin lattice relaxation time independent of temperature. This is considered to be due to the presence of a localized magnetic moment in the replaced Mn atoms. 相似文献
2.
The spin-Hamiltonian parameters ( g factor g //, g ⊥ and hyperfine structure constants A //, A ⊥) for Er 3+ ion at the trigonal Al 3+ site of AlN crystal are calculated by diagonalising the 52 × 52 energy matrix. The matrix are related to the ground mutiplet 4I 15/2 and the first to third excited multiplets 4I 13/2, 4I 11/2 and 4I 9/2 for 4f 11 ions in trigonal crystal field under an external magnetic field. The crystal-field parameters used in the matrix are obtained from the superposition model and the local lattice relaxation due to the substitution of Er 3+ for Al 3+ is considered. The calculated spin-Hamiltonian parameters are in reasonable agreement with the experimental values and the signs of hyperfine structure constants are suggested. The results are discussed. 相似文献
3.
Specific features of the properties of Ga–P lattice vibrations have been investigated using the percolation model of a mixed Ga1 – xAl
x
P crystal (alloy) with zero lattice mismatch between binary components of the alloy. In contrast to other two-mode alloy systems, in Ga1 – xAl
x
P a percolation splitting of δ ~ 13 cm–1 is observed for the low-frequency mode of GaP-like vibrations. An additional GaP mode (one of the percolation doublet components) split from the fundamental mode is observed for the GaP-rich alloy, which coincides in frequency with the gap corresponding to the zero density of one-phonon states of the GaP crystal. The vibrational spectrum of impurity Al in the GaP crystal has been calculated using the theory of crystal lattice dynamics. Upon substitution of lighter Al for the Ga atom, the calculated spectrum includes, along with the local mode, a singularity near the gap with the zero density of phonon states of the GaP crystal, which coincides with the mode observed experimentally at a frequency of 378 cm–1 in the Ga1 – xAl
x
P (x < 0.4) alloy. 相似文献
4.
The temperature dependences of nuclear magnetic resonance and magic angle spinning nuclear magnetic resonance spectra of 27Al nuclei in natrolite (Na 2Al 2Si 3O 10· 2H 2O) have been studied. The influence of water molecules and sodium ions mobility on the shape of the 27Al NMR spectrum and framework dynamics have been discussed The temperature dependences of the spin–lattice relaxation times T 1 of 27Al nuclei in natrolite have also been studied. It has been shown that the spin–lattice relaxation of the 27Al is governed by the electric quadrupole interaction with the crystal electric field gradients modulated by translational motion of H 2O molecules in the natrolite pores. The dipolar interactions with paramagnetic impurities become significant as a relaxation mechanism of the 27Al nuclei only at low temperatures (<270 K). 相似文献
5.
Spin–lattice relaxation of 3Не in contact with the ordered Al 2O 3 fiber aerogel has been studied at the temperature of 1.6 K in fields of 0.1–0.5 T by the pulsed nuclear magnetic resonance (NMR) method. An additional mechanism of the relaxation of 3Не in aerogels is found and it is shown that this relaxation mechanism is not associated with the adsorbed layer. A hypothesis about the influence of intrinsic paramagnetic centers on the relaxation of gaseous 3Не is proposed. 相似文献
6.
Emission Mössbauer spectroscopy on 57Fe fed by 57Mn ions implanted in the metal oxides ZnO, MgO and Al 2O 3 has been performed. The implanted ions occupy different lattice sites and charge states. A magnetic part of the spectra in each oxide can be assigned to Fe 3?+? ions in a paramagnetic state with unusually long relaxation time observable to temperatures up to several hundreds Kelvin. Earlier expectations that the magnetic spectra could correspond to an ordered magnetic state could not be confirmed. A clear decision for paramagnetism and against an ordered magnetic state was achieved by applying a strong magnetic field of 0.6 Tesla. The relaxation times deduced were compared to spin–lattice relaxation times from electron paramagnetic resonance (EPR). 相似文献
7.
The spin-lattice relaxation times T
1 in Ba 1−x
K xBiO 3 ( x=0.3, 0.4, 0.5) were measured in the normal temperature range (20–300 K).A substantial contribution to the spin-lattice relaxation
rate from dynamic local distortions of the crystal lattice near potassium atoms is found. The activation energy of this process
increases with decreasing potassium concentration, and the frequency of lattice excitations decreases. The nature of the low-frequency
lattice dynamics is discussed.
Pis’ma Zh. éksp. Teor. Fiz. 70, No. 5, 344–349 (10 September 1999) 相似文献
8.
The crystallographic structure and magnetic properties of La(Fell.4Alz.6)C0.02 are studied by magnetic measurernent and powder neutron diffraction with temperature and applied magnetic field. Rietveld refinement shows that La(Fe11.4Al1.6)C0.02 crystallizes into the cubic NaZn13-type with two different Fe sites: FeI (8b) and FeII (96i), and that A1 atoms preferentially occupy the FeII site. A ferromagnetic state can he induced at a medial temperature of 39 K-139 K by an external magnetic field of 0.7 T, and a large lattice is correspondingly found at 100 K and 0.7 T. In all other conditions, La(Fe11.4Al1.6)C0.02 has no net magnetization in the paramagnetic (T 〉 TN = 182 K) or antifer- romagnetic states, and thus keeps its small lattice. Analysis of the Fe Fe bond length indicates that the ferromagnetic state prefers longer Fe-Fe distances. 相似文献
9.
The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al 2O 3(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of , was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 10 9–10 10 cm −2. The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a=0.354 nm and c=0.569 nm, using Al 2O 3 as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved. 相似文献
10.
The thermodynamic properties, spin–lattice relaxation times, T1, and spin–spin relaxation times, T2, of the 27Al, 87Rb, and 133Cs nuclei in MAl(SO 4) 2·12H 2O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H 2O with increases in temperature. From our results for T1 and T2, we conclude that the discontinuities near Td in the T1 curves of the two crystals correspond to structural changes. In both crystals, below Td the water molecules surrounding the Al 3+ and M + nuclei form distorted octahedra, whereas above Td the water molecules around the Al 3+ and M + nuclei form regular octahedra and the environment of the Al 3+ and M + nuclei has cubic symmetry. Further, the T1 for the 27Al and 87Rb nuclei in RbAl(SO 4) 2·12H 2O below Td were found to increase with increasing temperature, whereas the T1 for the 27Al and 133Cs nuclei in CsAl(SO 4) 2·12H 2O were found to decrease. It is possible that this difference is due to the different characteristics of α- and β-type crystals. 相似文献
11.
Our studies involve measuring spin-lattice relaxation times for Nd 3+ ions in yttrium-aluminum garnets over the temperature range 4–50 K at 9.25 and 36.4 GHz for different orientations of the
external magnetic field in relation to the crystallographic axes. The temperature dependence of the relaxation rate is described
by T
1
−1
= AT
n+ b exp(−Δ/ kT), where n varies from sample to sample, with n=1 for “perfect” samples (i.e., with the longest relaxation times). Here Δ is approximately 130 cm −1, which is the energy of the excited Kramers doublet of the neodymium ion closest to the ground state, and this makes it possible
to interpret the second term in T
1
−1
as the contribution of two-stage relaxation proceeding through the intermediate level Δ. A strong field dependence of these
processes has been discovered: when the frequency was increased fourfold, the relaxation rate increased by a factor of 10.
The effect is a specific manifestation of the degeneracy of the excited level, breaking of the symmetry of the crystalline
field due to lattice defects, and the prevalence of deformations of a certain type in the spin-lattice interaction.
Zh. éksp. Teor. Fiz. 111, 332–343 (January 1997) 相似文献
12.
We report results of the atomic and electronic structures of Al 7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al 7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al 7C - cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al 7C - chemically inert. These results have further led us to the finding of a new neutral magic cluster Al 7N which has the same number of valence electrons as in Al 7C - and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al 7N) 2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
13.
Anelastic relaxation and 139LaNQR relaxation rates in La 2–xSr xCuO 4 for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering
processes. It is argued how the phase diagram of La 2–xSr xCuO 4 at the boundary between the antiferromagnetic (AF) and the spin-glass phase ( x
= 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the 139LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation
rates around 150 K and 80 K respectively in La 1.98Sr 0.02CuO 4, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding
to motion of charged stripes.
Received 18 May 2000 and Received in final form 11 July 2000 相似文献
14.
The interaction of high-power (10 11 W/cm 2) soft (1–3 keV) x-rays with inorganic oxides (Al 2O 3 and SiO 2) is studied. It is found that when the wavelength of the x rays is comparable to the lattice constant of the crystal, besides
generation of a high concentration of hot electrons and holes, there is broadening of the 2 p O 2−-subband in the upper valence band owing to the local action of the strong x-ray field on regular oxygen sites. As a consequence,
depending on the intensity of the x-ray pulses, a broadening of the fast (<1 ns) core-valence x-ray luminescence spectra is
observed.
Fiz. Tverd. Tela (St. Petersburg) 39, 286–289 (February 1997) 相似文献
15.
The influence of thin titanium layers on the growth of
silver clusters on -
Al
2
O
3
(0001) is investigated. We
demonstrate through in situ
RHEED measurements that titanium can relax stress in a growth
mode where two lattice parameters show up simultaneously. Above
a certain thickness, the lattice parameter closest to the bulk
value of titanium dominates. Depending on the amount of stress
in the titanium layer, silver films can either develop 3D
textures or grow in epitaxy and form 2D like films. 相似文献
16.
The physical properties of Al 1?x Cr x K(SO 4) 2·12H 2O ( x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for 27Al. On the basis of the physical properties of pure AlK(SO 4) 2·12H 2O, the effects of partially replacing Al 3+ with Cr 3+ ions were examined. Molecular motion changed with the concentration of Cr 3+ ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T 1ρ changed with the concentration of paramagnetic ions. 相似文献
17.
Crystals of lutetium gadolinium garnet solid solutions (Lu 1 − x
Gd
x
)Al 5O 12 (0 ≤ x ≤ 0.6) doped with Ce 3+ and Pr 3+ ions have been prepared by the horizontal directional crystallization method, and their optical and luminescence properties
have been investigated. It has been established that the introduction of gadolinium into the lutetium garnet lattice leads
to a decrease in the antisite luminescence (Lu Al centers) in the UV spectral range and to sensitization of the Ce 3+ ion luminescence. By contrast, the presence of gadolinium results in the quenching of the Pr 3+ luminescence due to the nonradiative excitation transfer from Pr 3+ ions to Gd 3+ ions. 相似文献
18.
Titanium-vanadium-chromium alloys are promising materials for hydrogen storage. They can absorb up to 3.8 wt % of hydrogen
with a variable (depending on the composition) temperature of hydrogen release in a convenient range. This paper reports on
the results of investigations of the TiV 0.80Cr 1.20H 5.29 hydride by continuous-wave (cw) and pulsed 1H nuclear magnetic resonance spectroscopy. It has been revealed that the hydrogen atoms occupy tetrahedral positions of the
face-centered cubic lattice. A model that takes into account the exchange between two states of hydrogen, i.e., mobile hydrogen
and hydrogen bound to the lattice, has been proposed for interpreting the temperature dependences of the relaxation times
T
1 and T
2 of 1H nuclei. The assumption that the exchange occurs in these alloys has made it possible, in particular, to explain the strong
difference between the relaxation times T
1 and T
2 in the high-temperature range. 相似文献
19.
Abstract The temperature dependence of four spectroscopic parameters reveals the existence of an amazing local relaxation in the Cu Cl 4 (NH 3) 2 2- centre placed in the NH 4 Cl lattice below T C = 242.5 K. Although the lattice parameter a decreases it is shown here that the Cu 2+ - Cl ? distance experiences however a significant increase. 相似文献
20.
The electronic energy structure of the defect system of c-BN x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster
version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic
structure of nonstoichiometric boron nitride c-BN 0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation.
Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997) 相似文献
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