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1.
X-band electron paramagnetic resonance (EPR) study of Cr 3+-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr 3+ ion-doped DTP for site I are: g x = 2.096 ± 0.002, g y = 2.167 ± 0.002, g z = 2.220 ± 0.002, D = (89 ± 2) × 10 ?4 cm ?1, E = (16 ± 2) × 10 ?4 cm ?1. EPR study indicates that Cr 3+ ion enters the host lattice substitutionally replacing K + ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters ( B = 521 cm ?1, C = 2,861 cm ?1), cubic crystal field splitting parameter (Dq = 1,851 cm ?1) and nephelauxetic parameters ( h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal. 相似文献
2.
Low-temperature luminescence spectra of stoichiometric Cr: LiNbO 3 and of congruent Cr, Mg: LiNbO 3 were studied. Cr 3+ impurity ions preferentially occupy Li + sites (Cr Li) in the LiNbO 3 crystal lattice, while Cr 3+ ions substituting for Nb 5+ ions (Cr Nb) occur in addition to Cr Li centers in codoped Cr, Mg: LiNbO 3 crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr 3+ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the 4 T 2 state resulting in crossing with the 2 E state, the replacement of the broad band 4 T 2 → 4 A 2 emission by a narrow R-line emission 2 E → 4 A 2 occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr Li 3+ and Cr Nb 3+ centers at pressures which correlated well with estimations based on the 4 T 2- 2 E energy gap (230 and 1160 cm ?1) and on the rate of their pressure-induced change (14.35 and 11.4 cm ?1/kbar, respectively). 相似文献
3.
Laser spectroscopy of Cr 3+ ions makes it possible to follow the crystallization process in a cordiente glass (52 SiO 2, 347 Al 2O 3, 12.5 MgO and 08 Cr 2O 3) Absorption and fluorescence are interpreted by structural considerations showing the variation of Cr 3+ environment during heat treatment Fluorescence line narrowing is performed at 4 2 K giving information on the detailed crystal structure in MgAl 2O 4 spinel microcrystallites formed during heat treatment The splitting of the 2E level is close to 70 cm ?1 and the ¦2 D¦ parameter ranges between 0 95 and 1 35 cm ?1 This wide distribution is associated with the well known disordered distribution of Mg 2+ and Al 3+ cations in MgAl 2O 4 spinels. 相似文献
4.
Attempts were made to grow CeO 2 and ThO 2 single crystals doped with transition metal ions. Only Fe 3+ and Mn 2+ could be detected by the EPR technique. The EPR spectrum of Fe 3+ in CeO 2 exhibits the well-known fine structure in cubic fields. The parameters are g=2.0044(1) and a=15.6(1)·10 ?4 cm ?1. The hyperfine constant A for 57Fe in hexahedral coordination was found to be 8.9(1)·10 ?4 cm ?1. The EPR spectrum of Mn 2+ in CeO 2 reveals two cubic Mn 2+ centers. The parameters for center 1 are g=1.9999(1) and A=86.9(1)·10 ?4 cm ?1 and for center 2 g=1.9984(1) and A=87.0(1)·10 ?4 cm ?1. Heating the Mn doped CeO 2 samples in hydrogen, the Mn 2+ centers transform from cubic into trigonal centers with approximate values of g=1.9988(2), A=84.5(6)·10 ?4 cm ?1 and D=203(1)·10 ?4 cm ?1. The two observed Mn 2+ centers in ThO 2 exhibit a priori axial symmetry with approximate values of g=2.0006(2), A=88.9(4)·10 ?4 cm ?1 and D=33(3)·10 ?4 cm ?1. 相似文献
5.
Studies involving the piezospectroscopy method have shown that the symmetry of the pair centers of Cr 3+-Cr 2+ ions in the KZnF 3 crystal is tetragonal. In this paper we develop a microscopic model of a pair center. We use the temperature dependence of
the integrated intensity of the absorption line to find the effective hopping integral for an e
g electron, t
σσ=205 ± 10 cm −1, and the polaron reduction factor, equal to 0.11. By analyzing the selection rules for exchange-induced electric dipole transitions
under double-exchange conditions we identify all the absorption lines of Cr 3+-Cr 2+ pairs.
Zh. éksp. Teor. Fiz. 114, 1421–1429 (October 1998) 相似文献
6.
Low-temperature luminescence spectra of stoichiometric Cr:LiNbO 3, congruent Cr:LiNbO 3 and congruent Cr,Mg:LiNbO 3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr 3+ impurity ions that preferentially occupy Li + sites (Cr Li) in the Cr:LiNbO 3 crystals. Low-field centers related to Cr 3+ substitution of Nb 5+ (Cr Nb) occur in addition to Cr Li in co-doped Cr,Mg:LiNbO 3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr 3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4
T
2 state, resulting in its crossing with the 2
E state of Cr 3+. This level-crossing effect was observed for the dominant Cr 3+
Li and Cr 3+
Nb centers at pressures that correlate well with estimations based on the 4
T
2- 2
Eenergy gap (230 cm -1 and 1160 cm -1) and on the rate of their pressure-induced change (14.35 and 11.4 cm -1/kbar, respectively). We also studied inhomogeneous broadeningof the 2
E? 4
A
2transitions at ambient pressure for the minor high-field “defect” Cr 3+
Li centers in congruent LiNbO 3. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis
of a microscopic hierarchic model for perturbed Cr 3+ ions in the LiNbO 3 lattice.
Received: 25 June 2001 / Published online: 2 November 2001 相似文献
7.
Interaction of an unpaired electron of the tetrahedral V 4+ ion in Ca 3In 2Ge 3O 12 garnet with the four nearest In nuclei gives rise to clearly resolved structure of EPR spectra. We report the observation and analysis of these spectra at three frequencies and at temperature 77 K. The temperature dependence between liquid helium and the room temperatures was also studied. Parameters of the spin Hamiltonian are g ∥=1·8735; g ⊥=1·9825; ∣ ¦A ∥|=150·4×10 ?4cm ?1; ¦ A ⊥¦ = 36·7 × 10 ?4 cm ?1. The supertransferred hyperfine interaction was found to be isotropic, absolute value of the corresponding parameter a is 22·1 × 10 ?4 cm ?1. 相似文献
8.
Cr 3+ and Nd 3+ non-equivalent crystal field sites were observed. By use of a high-resolution dye laser, 2E and 4A 2 splittings, zero-phonon 4A 2 → 2E, 2T 1, 4T 2 transitions and vibronic levels of 2E doublets of Cr 3+ non-equivalent centres were recorded. Energy transfers from Cr 3+ multisites to Nd 3+ were studied by site-selective spectroscopy and fluorescence decays. 相似文献
9.
Sensitization of the fluorescence of Er3+ in fluoride phosphate glass containing up to 20 mol% phosphates by codoping with Cr3+ and Yb3+ is shown. The low order of ligand field strength for Cr3+ (Dq/B=2.04) results in broad Cr3+ fluorescence overlapping the Yb3+ absorption. The electronic energy transfer efficiency approaches 100%. Deviations of donor decay from the Förster law are interpreted in terms of the inhomogeneously acceptor distribution. The electronic energy transfer efficiency of Yb3+ → Er3+ reaches a maximum value of 75% for glasses containing 20 mol% phosphates. The transfer is shown to be migrationally accelerated by means of GAF-LAF-FB theory. From Judd-Ofelt parameters a stimulated emission cross-section for the transition4I13/2 →4I15/2 of Er3+ of 6.2×10−20 cm2 is derived. The c.w. laser action of Er3+ by Cr3+ excitation and double-step energy transfer is shown. The output is tuned continuously from 1536 to 1596 nm. Flashlamp pumping is also shown. 相似文献
10.
Electron spin resonance has been investigated in zinc oxide single crystals containing vanadium. Several groups of ordinary and forbidden transitions can be observed. The experimental results are interpreted with the aid of the spin Hamiltonian, for which the following parameters were determined: g∥=1.945; ⊥=1.937; ¦ D¦=750×10 ?4 cm ?1, ¦ A¦=68 × 10 ?4 cm ?1; ¦ B ¦=93×10 ?4 cm ?1; ¦ A?P¦=65×10 ?4cm ?1. 相似文献
11.
The absorption spectra of ruby with various Cr concentrations have been studied in the wavelength region between 50 μ and 1 mm at liquid helium and nitrogen temperatures. At liquid helium temperatures two strong absorptions have been found at 37.7 and 106 cm ?1 and weak absorptions at 18.8, 30.7, 33.2 and 43.0 cm ?1. These lines depend on the Cr concentration and vanish at N 2 temperatures. The two strong absorptions have also been observed in Al 2O 3 crystals doped with Ti or V. Sapphire did not show any of these absorptions. The origin of the two strong absorptions in Al 2O 3 crystals doped with Ti, V or Cr are discussed by consideration of magnetic dipole transitions of exchange coupled Cr 3+ - pairs, the crystal field splitting of Ti 3+ impurities and impurity — induced lattice modes. The weak absorption lines, on the other hand, may be assigned to transitions between the exchange levels of the second-nearest and fourth-nearest Cr 3+ pairs if one makes use of the data obtained from the EPR and fluorescence spectra. For the second-nearest Cr 3+ pairs group theory of the exchange interaction predicts transitions with ΔS=2, whereas for the fourth-nearest neighbours no restriction for ΔS exists. 相似文献
12.
Chromium-containing NASICON-related phosphates of the type Na (1+x)Cr xM (2−x)P 3O 12) (M = Ti, Hf, Zr) have been synthesised by solid state reaction and structurally characterised by Rietveld refinement of the powder X-ray diffraction data. Materials of composition A (1+x)/2Cr xZr (2−x)P 3O 12 (A = Cd, Ca, Sr), have also been prepared and characterised. The crystal structure of Na (1+x)Cr xM (2−x)P 3O 12 corresponds to R-3c symmetry for x values ranging from 0.15 to 2.00, whereas compounds of composition A (1+x)/2Cr xZr (2−x)P 3O 12 corresponding to R-3c are obtained when x ≤ 1.00 for Sr 2+ and Ca 2+, and x ≤ 1.50 for Cd 2+. The polarizing effect of the two different metal ions A and M on the phosphorus atom and the P–O bond was studied by both 31P MAS NMR and infrared spectroscopy and shows that the electron density on the phosphorus, and thus the strength of the P–O bonds, are affected by both the interstitial (A) and the structural (M) metal ions. 相似文献
13.
We report for the first time on tunable room temperature cw laser action of a transition metal laser material. In Cr 3+:GdScGa-garnet crystals almost the total fluorescence is channelled into the broad band 4 T 2→ 4 A 2 transition. The fluorescence lifetime is τ=120 μs. Laser pumping in the blue-green and yellow-red spectral range yields pump thresholds around 250 mW and slope efficiencies up to 11%. The wavelength of the free running Cr 3+ laser is centered at 777 nm. 相似文献
14.
Studies of the photoluminescence spectra of Cr 3+ ions in KMgF 3 crystals co-doped with Cr 3+ and Ni 2+ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the 2
E4
A
2 transition and broadband luminescences associated with the 4
T
24
A
2 transitions. Cr 3+ ions substituting without local charge compensation on the octahedral Mg 2+ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband 4
T
24
A
2 emission, which results from the thermal occupancy of an excited 4
T
2 state just above the 2
E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr 3+ ions having Mg 2+ or K + vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr 3+–K + vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R
1 and R
2 lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr 3+ ions having a nearest neighbour Mg 2+ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the 4
T
2 level lies below 2
E and the photoluminescence spectrum at low temperature takes the form of a polarized broad 4
T
24
A
2 band with peak at 760 nm and radiative decaytime of 54 s. 相似文献
15.
ABSTRACT Single-crystal and powder EPR studies of VO 2+-doped lithium hydroxylammonium sulphate (LiNH 3OHSO 4) were carried out at room temperature. The results indicate the presence of two magnetically inequivalent VO 2+ sites. The VO 2+ ion takes up a substitutional position in the host lattice. The angular variation of EPR spectra in three mutually perpendicular planes were used to determine the spin Hamiltonian parameters, and the values obtained were the following: For Site 1, g x = 2.0249 ± 0.0002, g y = 1.9698 ± 0.0002, g z = 1.9552 ± 0.0002, A x = (51 ± 2) × 10 ?4 cm ?1, A y = (93 ± 2) × 10 ?4 cm ?1, and A z = (165 ± 2) × 10 ?4 cm ?1; and for Site 2, g x = 2.0267 ± 0.0002, g y = 1.9743 ± 0.0002, g z = 1.9213 ± 0.0002, A x = (40 ± 2) × 10 ?4 cm ?1, A y = (80 ± 2) × 10 ?4 cm ?1, and A z = (155 ± 2) × 10 ?4 cm ?1. The optical absorption spectrum recorded at room temperature shows four bands. From the optical and EPR data, various molecular coefficients are evaluated, and the nature of bonding in the crystal is discussed. 相似文献
16.
X-band electron paramagnetic resonance (EPR) studies are carried out on Fe 3+ ions doped in ammonium dihydrogen phosphate (ADP) single crystals at room temperature. The crystal field and spin Hamiltonian parameters are evaluated from the resonance lines obtained at different angular rotations. The obtained values of spin Hamiltonian and zero-field parameters of the Fe 3+ ion in ADP are: g = 1.994 ± 0.002, | D| = (220 ± 5) × 10 ?4 cm ?1 and a = (640 ± 5) × 10 ?4 cm ?1. On the basis of EPR data, the site symmetry of the Fe 3+ ion in the crystal is discussed. The Fe 3+ ion enters the lattice substitutionally replacing the NH 4 + sites. The optical absorption of the crystal is also studied at room temperature in the wavelength range of 195–925 nm. The energy values of different orbital levels are calculated. The observed bands are assigned as transitions from the 6 A 1g ( S) ground state to various excited quartet levels of the Fe 3+ ion in a cubic crystalline field. From the observed band positions, Racah interelectronic repulsion parameters ( B and C), cubic crystal field splitting parameter ( D q ) and Trees correction are calculated. There values are: B = 970, C = 1,923, D q = 1,380 cm ?1 and α = 90 cm ?1, respectively. On the basis of EPR and optical data, the nature of bonding in the crystal is discussed. The zero-field splitting (ZFS) parameters are also determined theoretically using B kq parameters estimated from the superposition model. The values of ZFS parameters thus obtained are | D| = (213 ± 5) × 10 ?4 cm ?1 and | E| = (21 ± 5) × 10 ?4 cm ?1. 相似文献
17.
Optical absorption and ESR spectra of Bi 12GeO 20 doped with Cr were measured before and after illumination with visible light. It was found that Cr 4+ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr 4+ center are Dq = 820 cm ?1 and B = 429cm ?1. The photochromic effect is explained in terms of a Cr 5+?Cr 4+ charge transfer process. 相似文献
18.
The spin-spin interaction of Dy 3+ ions in a KY(WO 4) 2 single crystal is investigated by electron paramagnetic resonance (EPR) spectroscopy at a temperature of 4.2 K and a frequency of 9.2 GHz. The EPR spectra of ion pairs located in different coordination shells are analyzed. It is revealed that the considerable contribution to the spin-spin interaction of the nearest neighbor ion pair nn is made not only by the magnetic dipole-dipole interaction but also by the isotropic exchange interaction with the parameter I nn = (+601 ± 17) × 10 ?4cm ?1. The exchange interaction in pairs of more widely spaced ions is substantially weaker: I 5n = (?38 ± 3) × 10 ?4cm ?1 and I 9n = (+18 ± 4) × 10 ?4cm ?1. For the other ion pairs, the magnetic dipole-dipole interaction dominates. It is found that the EPR spectra of single ions and ion pairs exhibit a superhyperfine structure associated with tungsten nuclei. 相似文献
19.
The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr 3+ ion Δ s = 66.7 GHz and between spin sublevels of the Cr 3+-Cr 3+ dimer Δ d1 = 71.5 GHz and Δ d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr 3+ ions with an Mg 2+ vacancy between them replacing three Mg 2+ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J z = 0.47 GHz and J t = 0.79 GHz. 相似文献
20.
Electron paramagnetic resonance has been observed for Dy 3+ in ZnSe and Tm 3+ in CdS. For Dy 3+ doped ZnSe, an isotropic spectrum was observed having well resolved hyperfine lines due to 161Dy and 163Dy. The g value obtained was 6.577 ± 0.002 and 161A was 191 ± 1 × 10 4 cm ?1 and 163A was 265 ± 3 × 10 ?4 cm ?1. The agreement of the observed g value to g(τ 6) = 6.667 and the point-charge calculations suggest that Dy 3+ occupies an interstitial (II) site with no local charge compensation. For Tm 3+ doped in hexagonal CdS, an axial spectrum consistent with g⊥ = 0, g∥ = 10.722 ± 0.007, D = 2.57 GHz (crystal field splitting) and 169A = 1207 ± 5 × 10 ?4cm ?1. The large g∥ value indicates that the Tm ion exits in the trivalent state. This is in reasonable agreement with previous reports of the non-Kramer's state of Tm 3+. 相似文献
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