Electron paramagnetic resonance enhanced crystal field analysis for low point-group symmetry systems: C2v sites in Sm3+:CaF2/SrF2 (original) (raw)
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Journal of Materials Chemistry, 2012
Six compounds with formula Sr 2 Fe 1.9 M 0.1 O 5+y (M ¼ Mn, Cr, Co; y ¼ 0, 0.5) were synthesized in air and argon, exhibiting surprisingly different properties depending on the B-cation type in spite of the low (5%) doping level. All argon synthesized phases, y $ 0, have long range brownmillerite ordering of oxygen vacancies with Icmm symmetry as shown by neutron diffraction (ND). All show long-range G-type antiferromagnetic order with N eel temperatures, T N , from variable temperature ND of 649(3) K, 636(2)K and 668(5)K for Cr, Mn and Co-compounds, respectively, compared with Sr 2 Fe 2 O 5 , T N ¼ 693 K. Competing ferromagnetic interactions may be responsible for the anomalously low value in the M ¼ Mn case. The air synthesized phases with y $ 0.5 show surprising variation with M as investigated by X-ray, TOF and constant wavelength neutron diffractions. The M ¼ Co compound is isostructural with Sr 4 Fe 4 O 11 (Sr 2 Fe 2 O 5.5 ), Cmmm, while the M ¼ Cr phase is cubic, Pm-3m, and that for M ¼ Mn appears to be cubic but the reflections are systematically broadened in a manner which suggests a local Cmmm structure. NPDF studies show that the local structure of the Cr phase is better described in terms of a Cmmm ordering of oxygen vacancies with Fe-O coordination numbers of five and six. The M ¼ Co material shows C-type antiferromagnetic long-range magnetic order at 4 K as found for Sr 4 Fe 4 O 11 . T N $ 230 K is inferred from a ZFC-FC magnetic susceptibility divergence compared with T N ¼ 232 K for un-doped Sr 4 Fe 4 O 11 . The M ¼ Cr and Mn compounds show no long-range magnetic ordering down to 4 K, but the divergence of ZFC and FC susceptibility data indicative of spin glass-like transitions occur at 60Kand60 K and 60Kand45 K for Cr and Mn, respectively. ND shows both diffuse and sharp Bragg magnetic reflections at positions consistent with a Cmmm cell for the M ¼ Mn phase. For the M ¼ Cr material, a very weak magnetic Bragg peak indexed as (1/2 1/2 1/2), consistent with a G-type AF order, is found at 4 K. These results rule out a spin glass-like ground state for both materials.
Low-Frequency Optically Detected EPR of Sm3+ in Cs2NaYF6 Single Crystal
Applied Magnetic Resonance, 2011
The Sm 3? ion in the Cs 2 NaYF 6 single crystal was studied by optically detected electron paramagnetic resonance spectroscopy. Magnetic resonance signals were recorded by Faraday rotation at the frequency of 0.6-0.85 GHz and magnetic fields of about 0.14 T. The hyperfine parameters of 147 Sm 3? and 149 Sm 3? isotopes were determined.
5D0→7F0 transitions of Sm2+ in SrMgF4:Sm2+
2004
Temperature-dependent emission spectra of Sm 2+ -doped SrMgF 4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong 5 D 0 → 7 F 0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm −1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF 4 which will require new experiments.
First-principles calculations for the H center in SrF2 crystals
The ground state of H-center systems for the SrF 2 crystal is simulated with two different arrangements, which are oriented along either [100] or [111] axes. The calculations are based on hybrid Hartree-Fock and density functional theory exchange functionals by using Becke's three-parameter method combined with the nonlocal correlation functionals of Perdew and Wang. The energy difference between H centers with different orientations shows that the H center oriented in the [111] direction in alkaline earth fluorides is the most stable configuration. The geometric relaxations of the neighboring atoms surrounding the H centers are presented. The combination energy of an H center and the formation energy of the related F-H pair in both alkaline earth fluorides are discussed. We report also the electronic structure of the H center systems. The effective charges and spins of the substitutional and interstitial fluorine atoms show that the hole is located at the interstitial fluorine in the system with the [111] orientation of the H center. The band structures are illustrated. With the help of studying the total and partial density of states, the constituents of the defect bands are clarified.
Hyperfine Interactions, 2004
We have studied the isostructural series of double-perovskites Sr 3 Fe 2 MO 9 (M = Mo, Te, W, U) by Mössbauer spectrometry and AC susceptibility measurements. The hyperfine structure of Mössbauer spectra at room temperature is attributed to the presence of high spin state Fe 3+ ions sensing both static and fluctuating magnetic hyperfine fields with different relative areas that depend on M. The magnetically split signal-indistinguishable from the background in the Mo compound spectrum-increases with the Fe-site disorder in the sequence Mo < U < Te < W. The spectra at 77 K demonstrate that the W-perovskite sample is already magnetically fully ordered, while the other three cations suggest ordering temperatures that increase from Te to U to Mo. At 4.2 K all the spectra are completely magnetically split and display hyperfine fields that range from 49 up to 53 T. Coincident with the X-ray and neutron diffraction results, the hyperfine parameters are consistent with Fe atoms centered in oxygen octahedral units, coordinated to different numbers of M-centered octahedra. The AC susceptibility response is χ max ≈ 3.5 × 10 −5 emu/g.Oe for the Mo compound and increases for the W, Te and U compounds with values of χ max 1.6 × 10 −4 , 3.0 × 10 −4 , and 9.4 × 10 −3 emu/g.Oe, respectively. The out-of-phase component, χ , could only be detected for the U compound. Its frequency dependence displays a shift that denotes a spin-glass-like state arising from the chemical disorder.
Physical Review B, 1997
Three different sites of the Cr 3ϩ ions in the fluoride perovskite KMgF 3 have been identified by absorption, selective optical excitation, and time-resolved emission spectroscopy of the 4 T 2 ↔ 4 A 2 transition. Highpressure measurements showing the crossover from low-crystal field to high-crystal field, allows us to situate the Dq/B values of the different chromium sites clearly below 2.3. The different spin-orbit components associated with the zero-phonon lines of the 4 T 2 ↔ 4 A 2 transition of each type of site are clearly shown on the optical spectra and identified by group-theory analysis. The decay profiles of the 4 T 2 level are exponential and the lifetimes at 15 K are 903, 919, and 473 s for the cubic, quadratic, and trigonal sites, respectively. The trigonal center which presents the lowest energy levels was peculiarly studied. Emission and excitation spectra are compared; their evolution versus temperature is followed and explained by the thermal population of the different spin-orbit sublevels. The phonon sideband of the trigonal site is compared with our previous lattice dynamic studies of the pure compound. The different peaks of the emission broadband are described in terms of phonons of the matrix and normal modes of the ͓CrF 6 ͔ 3Ϫ complex. ͓S0163-1829͑97͒03430-9͔
transitions of Sm2+ in SrMgF4:Sm2+
Journal of Alloys and Compounds, 2004
Temperature-dependent emission spectra of Sm 2+ -doped SrMgF 4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong 5 D 0 → 7 F 0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm −1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF 4 which will require new experiments.
Electronic structure of cubic ScF$_3$ from first-principles calculations
arXiv (Cornell University), 2012
The first-principles calculations have been performed to investigate the ground state properties of cubic scandium trifluoride (ScF3) perovskite. Using modified hybrid exchange-correlation functionals within the density functional theory (DFT) we have comprehensively compared the electronic properties of ScF3 obtained by means of the linear combination of atomic orbitals (LCAO) and projector augmented-waves (PAW) methods. Both methods allowed us to reproduce the lattice constant experimentally observed in cubic ScF3 at low temperatures and predict its electronic structure in good agreement with known experimental valence-band photoelectron and F 1s X-ray absorption spectra.
Electronic Structure and Core Spectroscopy of Scandium Fluoride Polymorphs
The microscopic knowledge of the structural, energetic, and electronic properties of scandium fluoride is still incomplete, despite the relevance of this material as an intermediate for the manufacturing of Al-Sc alloys. In a work based on first-principles calculations and x-ray spectroscopy, we assess the stability and the electronic structure of six computationally predicted ScF 3 polymorphs, two of which correspond to experimentally resolved single-crystal phases. In the theoretical analysis based on densityfunctional theory (DFT), we identify similarities among the polymorphs based on their formation energies, charge-density distribution, and electronic properties (band gaps and density of states). We find striking analogies between the results obtained for the low-and high-temperature phases of the material, indirectly confirming that the transition occurring between them mainly consists of a rigid rotation of the lattice. With this knowledge, we examine the x-ray absorption spectra from the Sc and F Kedge contrasting first-principles results obtained from the solution of the Bethe-Salpeter equation on top of all-electron DFT with high-energy-resolution fluorescence detection measurements. The analysis of the computational results sheds light on the electronic origin of the absorption maxima and provides information on the prominent excitonic effects that characterize all spectra. Comparison with measurements confirms that the sample is mainly composed of the high-and low-temperature polymorphs of ScF 3. However, some fine details in the experimental results suggest that the probed powder sample may contain defects and/or residual traces of metastable polymorphs.
Physical chemistry chemical physics : PCCP, 2011
Cluster and spin dynamics of a Sc 3 N@C 80 (CF 3 ) 2 derivative are studied by DFT in different charge states, from À3 to +1. For the neutral Sc 3 N@C 80 (CF 3 ) 2 , static DFT computations of many cluster conformers as well as Born-Oppenheimer molecular dynamics (BOMD) show that addition of two CF 3 groups to Sc 3 N@C 80 significantly changes dynamics of the Sc 3 N cluster: instead of free rotation as in Sc 3 N@C 80 , the cluster in Sc 3 N@C 80 (CF 3 ) 2 exhibits only hindered motions. Similar cluster dynamics is found in the mono-and trianions of Sc 3 N@C 80 (CF 3 ) 2 , while free rotation of the cluster is found in the cation. In the radical species, motions of the cluster dramatically change spin-density distribution. Spin populations of the metal atoms and the carbon cage are followed along the BOMD trajectories to reveal the details of the spin-flow. Sc ESR hyperfine coupling constants integrated over BOMD trajectories are found to be substantially different from the results of static DFT computations, which emphasizes that cluster dynamics should be taken into account for reliable predictions of spectroscopic properties.