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1.
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   

2.
Room temperature steady and time resolved emission spectra of LiIn1−xTmx(WO4)2 (where thulium concentration is 0, 0.5, 1, 5 and 10 at%) blue phosphors, under UV excitation energy have been investigated. The concentration quenching effect on the blue emission, due to the (WO4)−2 groups and 1G43H6 emission transition of Tm3+ were studied. Two energy transfer mechanisms are shown. The first takes place between excited (WO4)−2 groups and the 1G4 energy level of Tm3+, and is mainly analyzed by phonon-assisted energy transfer. The second mechanism is due to an energy transfer from the excited Tm3+ ions to the surrounding ground state Tm3+ ions. The non-exponential decay curves of the 1G4 level observed for higher concentrations are analyzed by the Inokuti–Hirayama model. We think that the quenching effect between Tm3+ ions is mainly linked to the dipole–dipole interactions.  相似文献   

3.
With the help of group theory analysis of absorption spectra of the transition 3H63F3 of Tm3+ ion in TmAl3(BO3)4 crystal, measured at several temperatures from 1.8 till 293 K, it has been shown, that the local symmetry of the Tm3+ environment is C3 and it decreases to C1 at a low temperature. Effective selection rules and polarizations of lines at high enough temperatures (when the line-width is larger than the splitting in C3 symmetry) have been obtained.  相似文献   

4.
Vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12 have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb2Co2V3O12) and 12.666 Å (NaPb2Ni2V3O12), respectively. The magnetic susceptibility of NaPb2Ni2V3O12 shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μeff of NaPb2Ni2V3O12 is 3.14 μB due to Ni2+ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb2Co2V3O12, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb2Co2V3O12.  相似文献   

5.
Two Ce3+-doped scintillator crystals, LSO (Lu2SiO5:Ce) and LPS (Lu2Si2O7:Ce), are studied by EPR spectroscopy. The analysis indicates that Ce3+ substitutes for Lu3+ ion in a C2-symmetry site for LPS and in two C1-symmetry sites for LSO, with a preference for the largest one, with 6+1 oxygen neighbors. Angular dependence of the EPR spectrum shows that the electronic ground state of Ce3+ is different in these two matrices. It is mainly composed of |MJ|=5/2 state in LPS and |MJ|=3/2 state in LSO. The temperature dependence of the linewidth shows a noticeably long spin lattice relaxation time, especially in LPS, which is the result of a stronger crystal field in LPS than in LSO.  相似文献   

6.
A novel layered hydrotalcite-like material, Co7(H2O)2(OH)12(C2H4S2O6), has been prepared hydrothermally and the structure determined using single crystal X-ray diffraction (a=6.2752(19) Å, b=8.361(3) Å, c=9.642(3) Å, α=96.613(5)°, β=98.230(5)°, γ=100.673(5)°, R1=0.0551). The structure consists of brucite-like sheets where 1/6 of the octahedral sites are replaced by two tetrahedrally coordinated Co(II) above and below the plane of the layer. Ethanedisulfonate anions occupy the space between layers and provide charge balance for the positively charged layers. The compound is ferrimagnetic, with a Curie temperature of 33 K, Curie-Weiss θ of −31 K, and a coercive field of 881 Oe at 5 K.  相似文献   

7.
Crystal structure of the 4-methylpyridinium tetrachloroantimonate(III), [4-CH3C5H4NH][SbCl4], has been determined at 240 K by X-ray diffraction as monoclinic, space group, P21/n, Z=8. Differential scanning calorimetry and dilatometric studies indicate the presence of two reversible phase transitions of first order type, at 335/339 and 233/289 K (cooling/heating) with ΔS=0.68 and 2.2 J mol−1 K−1, respectively. Crystal dynamics is discussed on the basis of the temperature dependence of the 1H NMR spin-lattice relaxation time T1 and infrared spectroscopic studies. The low temperature phase transition at 233 K of an order-disorder type is interpreted in terms of a change in the motional state of the 4-methylpyridinium cations. The phase transition at 335 K, probably of a displacive type, is characterised by a complex mechanism involving the dynamics of both the cationic and anionic sublattice. The 1H NMR studies show that the low temperature phase III is characterised only by the dynamics of the CH3 groups.  相似文献   

8.
The vibrational spectra of Eu[Co(CN)6]·4H2O and luminescence spectra of Eu3+ in this compound, using 355 nm excitation at temperatures down to 10 K, have been assigned. A clear distinction is made between the n=5 and 4 members of the Ln[M(CN)6nH2O series from the vibrational spectra. The electronic spectra show prominent vibronic structures, particularly for the 5D07F2 sideband. A resonance occurs between the transitions 5D07F1(III) and 5D07F0+ν(Eu−N). A crystal field analysis of the derived energy data set is presented for Eu3+ in eight coordination geometry.  相似文献   

9.
Intense blue upconversion emission at 480 nm has been obtained at room temperature in Tm3+-Nd3+ co-doped Ta2O5 channel waveguides fabricated on a Si substrate, when the sample is excited with an infrared laser at 793 nm. The upconversion mechanism is based on the radiative relaxation of the Nd3+ ions (4F3/2 → 4I11/2) at about 1064 nm followed by the absorption of the emitted photons by Tm3+ ions in the 3H4 excited state. A coefficient of energy transfer rate as high as 3 × 10−16 cm3/s has been deduced using a rate equation analysis, which is the highest reported for Tm-Nd co-doped systems. The confinement of the 1064 nm emitted radiation in the waveguide structure is the main reason of the high energy transfer probability between Nd3+ and Tm3+ ions.  相似文献   

10.
New triethylammonium salts: [(C2H5)3NH]SbCl6 (TCA) and [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl (TCAT) have been synthesized. The compounds crystallise in monoclinic symmetry: space groups P21/n and P21/c, for TCA at 293 K and TCAT at 100 K, respectively. The crystal structure of [(C2H5)3NH]SbCl6 consists of discrete ionic pairs—triethylammonium cations and hexachloroantimonate anions—linked via the bifurcated N-H?Cl hydrogen bonds. The crystal structure of [(C2H5)3NH]SbCl6·1/2[(C2H5)3NH]Cl is composed of three symmetrically independent triethylammonium cations, chlorine anion and two symmetrically independent hexachloroantimonate anions. TCA undergoes a structural phase transition at 336 K (on heating) into the orthorhombic C222 space group, whereas TCAT reveals a structural phase transition at 332 K. The phase transitions are of the first order type. TCA shows a ferroelastic domain structure below 336 K. Differential scanning calorimetry, dilatometric, dielectric dispersion and Raman scattering measurements have been used to study the phase transition mechanisms in these triethylammonium salts.  相似文献   

11.
The magnetic properties and the lattice constants of Tm6Mn23 were determined before and after hydrogen absorption. The compound Tm6Mn23 is ferrimagnetic with an ordering temperature Tc = 404 K. After hydrogen absorption the magnetization is strongly reduced and does not point to the occurrence of magnetic ordering. The strongly increasing hyperfine splitting observed by means of 169Tm Mössbauer spectroscopy in the hydride for temperature below 60 K, however, shows that the Tm sublattice becomes magnetically ordered after H2 absorption as well. In uncharged and charged Tm6Mn23 the values of the Tm moments are close to the free ion values.  相似文献   

12.
Er-Tm-codoped Al2O3 thin films with different Tm to Er concentration ratios were synthesized by cosputtering from separated Er, Tm, Si, and Al2O3 targets. The temperature dependence of photoluminescence (PL) spectra was studied. A flat and broad emission band was achieved in the 1.4-1.7 μm and the observed 1470, 1533 and 1800 nm emission bands were attributed to the transitions of Tm3+: 3H4 → 3F4, Er3+: 4I13/2 → 4I15/2 and Tm3+: 3F4 → 3H6, respectively. The temperature dependence is rather complicated. With increasing measuring temperature, the peak intensity related to Er3+ ions increases by a factor of five, while the Tm3+ PL intensity at 1800 nm decreases by one order of magnitude. This phenomenon is attributed to a complicated energy transfer (ET) processes involving both Er3+ and Tm3+ and increase of phonon-assisted ET rate with temperature as well. It should be helpful to fully understand ET processes between Er and Tm and achieve flat and broad emission band at different operating temperatures.  相似文献   

13.
The study of the optical properties of a LiLuF4 crystal doped with Tm3+ yielded the discovery of a strong temperature dependence of the Tm-Tm diffusion coefficient. Spectroscopic characteristics have been investigated as a function of the sample temperature, with particular regard to the luminescence decay following pulsed excitation. An appreciable excitation of the lifetime of the 3F4 manifold is observed over the temperature range 8.9-298 K. The Judd-Ofelt calculations point out a radiative lifetime considerably longer than the experimental one. These facts suggest a theoretical interpretation based on the presence of impurities that quench the manifold and on a temperature-dependent energy migration between Tm3+ ions. A one-parameter best fit of the experimental measurements strongly confirms this hypothesis. Weak OH ion concentration is detected by means of IR and UV spectra, thus supporting the theoretical interpretation.  相似文献   

14.
The Ho3+/Yb3+ and Tm3+/Yb3+ doped P2O5-MgO2-Sb2O3-MnO2-AgO glasses were prepared by high temperature melting method. Under a 975 nm laser diode (LD) excitation, the single red and single blue upconversion (UC) emissions were observed in Ho3+/Yb3+ and Tm3+/Yb3+ doped samples, respectively. By studying the spontaneous radiative and multiphonon relaxation probabilities, we find that the multiphonon relaxation probability of 5I6 (Ho3+) state is very large (1.39 × 106 s− 1), which is helpful to the population of 5I7 state. The multiphonon relaxation probability of 3H5 and 3F2,3 (Tm3+) is also very large, which results in lots of population in 3F4 and 3H4 states. The results are that the red UC emission of Ho3+ and the blue UC emission of Tm3+ are stronger.  相似文献   

15.
High resistivity single crystals of AgGaSe2 were grown by the horizontal Bridgman technique. The near band edge photoconductivity of the grown crystal at room temperature was found to be up to 2×104 times higher than the dark conductivity, under the illumination of 10−3 W/cm2. The photoconductivity spectrum consists primarily of three peaks, which are attributed to the transitions from Γ7(A), Γ6(B) and Γ7(C) states of valence band to the conduction band Γ6. The crystal field splitting and the spin-orbit splitting were determined from these peak energy positions of the photoconductivity spectrum.  相似文献   

16.
The ground state of the solid solution of the two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3 has been investigated by 1H NMR. The existence of a magnetic ordering in the sample with the Cl-content x=0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below TN=13.5 K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state.  相似文献   

17.
Double-layered manganite La1.4Ca1.6Mn2O7 has been synthesized using the solid-state reaction method. It had a metal-to-insulator transition at temperature TM1≈127 K. The temperature dependence of ac susceptibility showed a broad ferromagnetic transition. The two-dimensional (2D)-ferromagnetic ordering temperature (TC2) was observed as ≈245 K. The temperature dependence of its low-field magnetoresistance has been studied. The low-field magnetoresistance of double-layered manganite, in the temperature regions between TM1 and TC2, has been found to follow 1/T5. The observed behaviour of temperature dependence of resistivity and low-field magnetoresistance has been explained in terms of two-phase model where ferromagnetic domains exist in the matrix of paramagnetic regions in which spin-dependent tunneling of charge carriers occurs between the ferromagnetic correlated regions. Based on the two-phase model, the dimension of these ferromagnetic domains inside the paramagnetic matrix has been estimated as ∼12 Å.  相似文献   

18.
Cu(im)6 complexes in Zn(im)6Cl2·4H2O exhibit a strong Jahn-Teller effect which is static below 100 K and the complex in localized in the two low-energy potential wells. We have reinvestigated electron paramagnetic resonance (EPR) spectra in the temperature range 4.2-300 K and determined the deformation directions produced by the Jahn-Teller effect, energy difference 11 cm−1 between the wells and energy 300 cm−1 of the third potential well. The electron spin relaxation was measured by electron spin echo (ESE) method in the temperature range of 4.2-45 K for single crystal and powder samples. The spin-lattice relaxation is dominated by a local mode of vibration with energy 11 cm−1 at low temperatures. We suppose that this mode is due to reorientations (jumps) of the Cu(im)6 complex between the two lowest energy potential wells. At intermediate temperatures (15-35 K), the T1 relaxation is determined by the two-phonon Raman processes in acoustic phonon spectrum with Debye temperature ΘD=167 K, whereas at higher temperatures the relaxation is governed by the optical phonon of energy 266 cm−1. The ESE dephasing is produced by an instantaneous diffusion below 15 K with the temperature-independent phase memory time , then it grows exponentially with temperature with an activation energy of 97 cm−1. This is the energy of the first excited vibronic level. The thermal population of this level leads to a transition from anisotropic to isotropic EPR spectrum observed around 90 K. FT-ESE gives ESEEM spectrum dominated by quadrupole peaks from non-coordinating 14N atom of the imidazole rings and the peak from double quantum transition νdq. We show that the amplitude of the νdq transition can be used to determine the number of non-coordinating nitrogen atoms.  相似文献   

19.
The pyroelectric coefficient p3 in 3La(IO3)3.HIO3.7H2O has an average value 2.0×10-5 Cm-2 in the temperature range 152 to 240 K. The resistivity decreases from 1012 to 1010 ohm-cm between 258 and 338 K. At 298 K, the piezoelectric coefficient d33  19×10-12CN-1. Positive polarity is generated on (001) by increasing temperature or tensile stress. A displacement toward (001) by La3+ or H3O+ ions of 1×10-4 Å per K or 106Nm-2, or rotation of the water molecule or iodate ion dipoles by about 5 arc minutes per K or 106Nm-2, produces the observed polarity.  相似文献   

20.
In the present paper, phosphors with the composition Y3−x−yAl5O12:Bi3+x, Dy3+y were synthesized with solid state reactions. The luminescence properties of Bi3+ and Dy3+ in Y3Al5O12(YAG) and the energy transfer from Bi3+ to Dy3+ were investigated in detail. Bi3+ in YAG emits one broad band peaking at 304 nm which can be ascribed to the transition from excited states 3P0, 1 to ground state 1S0. Dy3+ in YAG emits two groups of peaks around 484 and 583 nm, respectively, which can be ascribed to the transitions from excited state 4F9/2 to ground states 6H15/2 and 6H13/2. The co-doping of Bi3+ enhances the luminescent intensity of Dy3+ by ∼7 times because Bi3+ can transfer the absorbed energy to Dy3+ efficiently. The mechanism of energy transfer was also discussed.  相似文献   

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