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1.
Magnesium aluminate (MgAl2O4) doped with trivalent chromium (Cr3+) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr3+ ions. The luminescence of Cr3+-activated MgAl2O4 exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper 2Eg4A2g ground state of Cr3+ ions. By correlating EPR and optical data the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr3+ ions with the ligands and the Cr3+ ions are in distorted octrahedral environment.  相似文献   

2.
ZnAl2O4:Mn green light emitting powder phosphors have been prepared by urea combustion technique involving furnace temperatures about 500 °C in a short time (<5 min). The prepared powders were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectrometry and the surface area measurements by a Brunauer-Emmet-Teller (BET) adsorption isotherms. The EPR spectrum exhibits a resonance signal at g≈2.0, which shows a six-line hyperfine structure (hfs). From the EPR spectra the spin-Hamiltonian parameters have been evaluated at room temperature as well as at 110 K. EPR and photoluminescence (PL) studies revealed that manganese ions were present in divalent state and the site symmetry around Mn2+ ions is distorted tetrahedral. The spin concentration (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed. The green emission at 511 nm in ZnAl2O4:Mn phosphor is assigned to a transition from the upper 4T16A1 ground state of Mn2+ ions.  相似文献   

3.
The magnetic property of double doped manganite Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 with a fixed ratio of Mn3+:Mn4+=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr3+ for Mn3+ ions causes the increase of magnetization and the rise of Tc. As the Cr3+ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn3+ and Cr3+ ions causes a magnetic upturn, namely, the second FM phase transition.  相似文献   

4.
The magnetic behavior of the FeInxCr2−xSe4 system (with x=0.0, 0.2 and 0.4) has been investigated by magnetic and Mössbauer spectroscopy. Hyperfine parameters indicate that iron is in the Fe2+ oxidation state, with a minor (∼9%) Fe3+ fraction, located at different layers in the structure. Low-field magnetization curves as a function of temperature showed that the antiferromagnetic (AFM) order temperature is TN=208(2) K for FeCr2Se4 and decreases to 174(3) K for FeIn0.4Cr1.6Se4. The effective magnetic moment μeff decreases with increasing In contents, and shows agreement with the expected values from the contribution of Fe2+ (5D) and Cr3+ (4F) electronic states. A second, low-temperature transition is observed at TG∼13 K, which has been assigned to the onset of a glassy state.  相似文献   

5.
Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr3+ ions doped in (30−x) (NaPO3)6+30PbO+40B2O3+xCr2O3 (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr2O3 in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr3+ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr3+ ions in octahedral symmetry. This band has been assigned to the transition 4T2g (F)→4A2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.  相似文献   

6.
Quasi-two-dimensional organic metals β″-(BEDT-TTF)4NH4[M(C2O4)3] · DMF containing the oxalate complexes of Cr3+ or Fe3+ ions between the conducting organic layers of the BEDT-TTF molecules are studied by EPR spectroscopy, and the contributions of these metallic complexes, conduction electrons, and non-equilibrium lattice defects to the magnetic susceptibility are determined. An analysis of the temperature dependence of the magnetic susceptibility and the EPR line shape has revealed partial localization of conduction electrons at T < 20 K in the crystals with Cr3+ ions. The size of the localization region is close to the size of an individual BEDT-TTF molecule. The localization effect weakens as nonequilibrium defects disappear during long-term storage at room temperature. The localization of conduction electrons is found to be accompanied by the appearance of weak antiferromagnetic interaction between the Cr3+ ions at T < 20 K, which disappears when Fe3+ ions substitute for Cr3+ ions.  相似文献   

7.
Magnetization and susceptibility were investigated as a function of temperature and magnetic field in polycrystalline Mn[Cr0.5Ga1.5]S4 spinel. The dc susceptibility measurements at 919 Oe showed a disordered ferrimagnetic behaviour with a Curie-Weiss temperature θCW=−55 K and an effective magnetic moment of 5.96 μB close to the spin-only value of 6.52 μB for Cr3+ and Mn2+ ions in the 3d3 and 3d5 configurations, respectively. The magnetization measured at 100 Oe revealed the multiple magnetic transitions with a sharp maximum at the Néel temperature TN=3.9 K, a minimum at the Yafet-Kittel temperature TYK=5 K, a broad maximum at the freezing temperature Tf=7.9 K, and an inflection point at the Curie temperature TC=48 K indicating a transition to paramagnetic phase. A large splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations at a temperature smaller than TC suggests the presence of spin-glass-like behaviour. This behaviour is considered in a framework of competing interactions between the antiferromagnetic ordering of the A(Mn) sublattice and the ferromagnetic ordering of the B(Cr) sublattice.  相似文献   

8.
The sublattice magnetization of the quadratic-layer basal-plane antiferromagnet K2FeF4 has been studied by use of Mössbauer spectroscopy. A distribution of Néel temperatures with a width ~3 K is found at a mean TN = 67.2±0.3 K. For 0.3 < T/TN < 0.99 the sublattice magnetization is described by a power law with critical exponent β = 0.17±0.01.  相似文献   

9.
On the basis of the 120×120 complete energy matrices for a d3 configuration ion in a trigonal ligand field, for Cr3+ ions doped in MgTiO3 and LiTaO3, the local structures and EPR g factors of the octahedral (CrO6)9− clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg2+, Ta5+) ions are obviously different, after Cr3+ replacing the M, the local lattice structures around the Cr3+ ions are quite similar and close to those of the Cr2O3. This may be ascribed to the fact that the octahedral Cr3+ center in MgTiO3:Cr3+ and LiTaO3:Cr3+ systems and that in Cr2O3 exhibit similar octahedral (CrO6)9− clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr3+ ions doped in MgTiO3 and LiTaO3.  相似文献   

10.
This work investigates the origin of novel visible photoluminescence (PL) bands observed in the spinel MgAl2O4:Co2+. Besides the well-known fourfold-coordinated Co2+(Td) PL at 670 nm [N.V. Kuleshov, V.P. Mikhailov, V.G. Scherbitsky, P.V. Prokoshin and K.V. Yumashev, J. Lumin. 55 (1993) 265.], a rich structured PL band at 686 nm was also observed that we associate with uncontrolled impurities of sixfold coordinated Cr3+(Oh) by time-resolved spectroscopy and lifetime measurements and their variation with temperature. We also show that the lifetime of the Co2+(Td) emission at 670 nm varies from τ=6.7 μs to 780 ns on passing from T=10 to 290 K. This unexpected behaviour for Td systems is related to the excited-state crossover (4T12E), making the emission band to transform from a narrow-like emission from 2E at low temperature to a broad structureless band from 4T1 at room temperature.  相似文献   

11.
Antiferromagnetic phase transition in two vanadium garnets AgCa2Co2V3O12 and AgCa2Ni2V3O12 has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature TN=6.39 K for AgCa2Co2V3O12 and 7.21 K for AgCa2Ni2V3O12, respectively. The magnetic susceptibilities exhibit broad maximum, and these TN correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χmax). The magnetic entropy changes from zero to 20 K per mol Co2+ and Ni2+ ions are 5.31 J K−1 mol-Co2+-ion−1 and 6.85 J K−1 mol-Ni2+-ion−1, indicating S=1/2 for Co2+ ion and S=1 for Ni2+ ion. The magnetic susceptibility of AgCa2Ni2V3O12 shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μeff=3.23 μB Ni2+-ion−1 and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa2Co2V3O12. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state 2E(t26e) and the first excited state is spin quartet state 4T1(t25e2) which locates extremely close to the ground state. The low spin state S=1/2 for Co2+ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.  相似文献   

12.
CeAgAs2, an HfCuSi2 like layered pnictide, orders antiferromagnetically at TN=6.2(1) K. The ordering process was monitored in neutron diffraction experiments in the temperature range 10 K≥T≥3.5 K. At T=4 K the lattice parameters are a=5.7438(1) Å, b=5.7696(1) Å and c=21.0067(2) Å. The diffraction pattern of the antiferromagnetic phase with a propagation vector k=[0,0,0] point towards ferromagnetically ordered moments in Ce layers stacked along [001], the individual layers are coupled antiferromagnetically with a +− −+type sequence. The alignment of moments within the Ce layers cannot be determined reliably from the experimental data so that two different structure models are discussed. The proposed metamagnetic transition was confirmed by diffraction experiments applying an external magnetic field at T<TN. In the interval 4 K≤T≤6 K, a relatively small field of μ0H≈0.3 T suffices to fully suppress the antiferromagnetic ordering. The effect is completely reversible yet subject to hysteresis: After switching off the external field at any T<TN the magnetic reflections gain their original intensity within several 10 min indicating the restoring of the antiferromagnetic phase.  相似文献   

13.
Anion-deficient substituted ferrites Ca2Fe2 − x N x O5 (N = Sc3+, Al3+) and mixed manganite CaMn7O12 have been investigated by 119Sn and 57Fe probe M?ssbauer spectroscopy. The mechanism of charge compensation for heterovalent impurity Sn4+ ions in the structure of the ferrite Ca2Fe2O5 has been established. The presence of nonequivalent crystallographic positions of manganese cations, caused by their charge ordering in the structure of the manganite CaMn7O12, is shown. Magnetic ordering of Mn3+ and Mn4+ cations in the octahedral sublattice of CaMn7O12 at T < T M2 ≈ 90 K is established. Original Russian Text ? A.V. Sobolev, I.A. Presnyakov, K.V. Pokholok, V.S. Rusakov, T.V. Gubaidulina, A.V. Baranov, G. Demazeau, 2007, published in Izvestiya Rossiiskoi Akademii Nauk Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1347–1354.  相似文献   

14.
Both powder and single-crystal X-ray investigations show that Cr1 + xNb3?xSe10 which belongs to the FeNb3Se structure type, exists only in a narrow range of stoichiometry close to x=0.70. In contrast to the Fe analog, Cr1.70Nb2.30Se10 undergoes no disticnt metal-insulator transition; the resistivity and the thermoelectric power remain of the same order as the temperature is lowered from 300 K to 4.2 K. The thermoelectric powers S of Fe1 + xNb3?xSe10 (0.25<x<0.40) and FeVNb2Se10, however, follow such a temperature T dependence as S∝ 1T even at room temperature. On the basis of light-binding band calculations, these observations are interpreted in terms of the contribution of Cr or Fe d-orbitals to electron conduction.  相似文献   

15.
This paper reports on the luminescence and electron paramagnetic resonance (EPR) investigations on MgSrAl10O17:Mn2+ green-emitting phosphor. Single-phase MgSrAl10O17 was successfully synthesized by the one-step solution combustion route without the need for post-annealing at a higher temperature. Crystallization of the powder was confirmed by X-ray diffraction. The luminescence of Mn2+- activated MgSrAl10O17 shows a strong green-emission peak around 515 nm due to the 4T16A1 transition of Mn2+ ions under the excitation (453 nm). The EPR spectra of Mn2+ ions exhibit a sextet hyperfine structure centered at g ≈1.995. The Mn2+ ion occupies Mg sites which are in tetrahedral symmetry. The magnitude of the hyperfine splitting (A) indicates that Mn2+ is in a moderately ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed.  相似文献   

16.
The solid solution behavior of the Ni(Fe1−nCrn)2O4 spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe2O3 and Cr2O3 reactants, mixed to give NiCrFeO4, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, Tcs). A solvus exists below Tcs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.  相似文献   

17.
采用高温固相法合成了Cd3Al2Ge3O12:Cr3+多晶材料,利用X射线衍射对其结构进行了分析,通过Cr3+的室温吸收光谱、室温和77K发射光谱分别对其光谱特性和晶场参数进行了分析和计算.结果表明:在450 nm的蓝光激发下,Cd3Al2Ge3O12:Cr3+室温发 关键词: 3Al2Ge3O12:Cr3+')" href="#">Cd3Al2Ge3O12:Cr3+ 荧光光谱 晶场参数 可调谐激光  相似文献   

18.
The (PO4)3? units in a CsH2PO4 (CDP) crystal were replaced in a small fraction of sites by (CrO4)3? groups and the EPR of the Cr5+ center was investigated. Splitting of the EPR line appears at T1c=245 K, 91 K higher that the ferroelectric transition temperature Tc=154 K. The electronic wave function of Cr5+ (3d1) is identified as dx2?y2. The dx2?y2 function couples with the near protons and the reorientation of this unit in the two possible configurations occurs in the paraelectric phase and breaks the symmetry far above Tc. The observed correlation time 10?9 sec and associated activation energy ΔU=0.215 eV are discussed.  相似文献   

19.
Hg2Os2O7, which has the cubic pyrochlore structure, remains metallic down to the liquid helium temperature unlike its isostructural counterpart Cd2Os2O7, which shows metal-insulator transition at 226 K. Magnetization and heat capacity data for Hg2Os2O7 are presented. The magnetic anomaly at TN=88 K shares many characteristics in common with the metal-insulator transition in Cd2Os2O7, though Hg2Os2O7 remains metallic below TN. The heat capacity Cp shows no or very little change in the magnetic entropy around TN, supporting the view that there is no long-range ordering of localized spins. The measured value of electronic heat-capacity coefficient γ=21 mJ K−2mol−1 is comparable to the value obtained from band-structure calculation on Cd2Os2O7, suggesting that mass-enhancement is small in Hg2Os2O7. There is a pronounced peak in Cp/T3 at 13.1 K, which corresponds to a peak in the phonon density of states at 40 cm−1.  相似文献   

20.
It is shown that a static electric field affects the intensity ratio of the EPR lines of Cr5+ (d1) in SrTiO3 at 77K. The experimental results can be accounted for quantitatively by considering for the 2T2g electronic ground state of Cr5+ a strong Jahn-Teller interaction with localized ρg mode. The degeneracy of the resulting vibronic ground state is lifted appreciably by a macroscopic strain which, as a result of the large electrostrictive coupling in SrTiO3, is induced at moderate electric field strengths.  相似文献   

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