EPR Of Mn2 Research Articles

Recombination luminescence and EPR of Mn doped Li2B4O7 single crystals

Manganese doped Li2B4O7 (LTB) crystals have been studied using luminescence and EPR spectroscopy, extending the investigations for the first time from the X band to the Q band for higher resolution. Mn2+ ions are shown to substitute dominantly at lithium sites charge compensated by a nearby lithium vacancy. Excitation spectra of the Mn2+ ion emission have been studied in the energy range of 2.5–20 eV. Weak excitation bands up to 5.5 eV are ascribed to specific transitions within the same 3d5 electronic configuration according to the Tanabe-Sugano diagram. A doublet of intense excitation bands in the 7–8 eV region is assigned to electronic transitions to the Mn2+6D term split by crystal field into two sub-bands. Thermostimulated luminescence (TSL) curves were studied in the temperature range 5–600 K. Based on the EPR studies and the comparison of TSL spectra of irradiated crystals with spectra of photostimulated luminescence, models of TSL processes are proposed. Emission and TSL processes are shown to be strongly influenced by local lattice relaxation and the mobility of Li vacancies participating in complexes with three different charge states of the Mn dopant.

Phase Transition in LiCsSO4 by EPR of Mn2+ Ions

Temperature EPR studies of Mn2 +-doped LiCsSO4 have been reported. The coexistence of resonance lines ascribed to the para- and ferro-phase has been found in the vicinity of the second order phase transition.

Paraelectric-Incommensurate Phase Transition in Rb2ZnCl4 Studied by EPR of Mn2+ Probe

The EPR spectra of Mn2+ probe have been investigated in Rb2ZnCl4 (RZC) crystals. Temperature changes of EPR line position and local line width in the range of paraelectric-incommensurate phase transition have been described on the basis of the simple “local” model. EPR data confirm non-classical character of RZC critical properties, corresponding to universality class of 3d XY Heizenberg model.

Ferroelastic Phase Transition in [P(CH 3 ) 4 ] 2 MnBr 4 by EPR of Mn 2+ Ions

This paper presents temperature changes of the EPR lines in [P(CH 3 ) 4 ] 2 MnBr 4 in the vicinity of the ferroelastic phase transition temperature T c =376.5K. Significant changes in the EPR spectrum have been observed in the temperature range of 370-380K. It is suggested that the changes observed above T c are caused by rotations of the MnBr 4 groups loosely situated in the crystal lattice, whereas changes below T c are a direct result of rebuilding of the ferroelastic domain structure.

Effect of the lactate precursor on the microstructure of Li 1+ xMn 2− xO 4 with 0≤ x≤0.33

Using EPR of Mn 4+, the microstructure of Li 1+ x Mn 2− x O 4 with 0≤ x≤0.33 obtained from lactate precursors and by a solid state reaction has been compared. It has been shown that the microstructure of ex-lactate Li 4Mn 5O 12 is similar to that of the sample obtained by a solid state reaction at 400°C: in addition to the statistically distributed Mn 4+ and Li +, there are regions where Li + ions tend to segregate. For 0≤ x≤0.1, thermal decomposition of Li–Mn lactate precursors at 450°C in air yields vacancy-rich spinels, in which the excess Mn 4+ and the corresponding metal vacancies are randomly distributed. For annealed at 750°C ex-lactate Li 1+ x Mn 2− x O 4, the spinel microstructure depends on the temperature regime of sample cooling and on the lithium content x. The procedure for the preparation of stoichiometric Li 1+ x Mn 2− x O 4 with 0≤ x≤0.1 and a homogeneous cation distribution from lactate precursors has been specified.

Spin-lattice relaxation of Co(II) in hexaaquocobalt(II) picrylsulphonate tetrahydrate: An estimate from EPR line width of the dopant, Mn(II)

Single crystal EPR of Mn(II)-doped [Co(H 2O) 6] (picsul) 2·4H 2O is studied in the temperature range of 123–300 K. The angular variation of the zero-field transitions, observed from a single magnetically distinct site, is successfully simulated with an asymmetric zero-field tensor with D=436×10 −4 cm −1 and E=79×10 −4 cm −1. Sharp EPR spectra observed for Mn(II), a paramagnetic dopant in the paramagnetic host lattice, are attributed to host-spin-lattice relaxation narrowing. Based on the theory of exchange narrowing of dipolar coupling, the T 1 of Co(II) has been estimated to be 0.54×10 −12 s at 300 K. From the temperature dependence of T 1, it is postulated that two-phonon Raman process is the dominant mechanism for spin-lattice relaxation above 250 K, and below 250 K a cross-relaxation process is evident.

EPR of Mn 2+-ion-doped single crystals of Mg[C 4H 3O 4] 2·6H 2O

Electron paramagnetic resonance of Mn 2+-ion-doped single crystals of magnesium bis(hydrogen maleate) hexahydrate, Mg[C 4H 3O 4] 2·6H 2O, has been studied at room and liquid nitrogen temperatures using X-band frequencies. Two differently oriented but otherwise identical complexes of Mn 2+ ions are found in these crystals. From the observed EPR spectra, spin-Hamiltonian parameters were calculated. It was found that the Mn 2+ ions substitute for Mg 2+ ions. The observed negative shift in g ∥-value of Mn 2+ at room temperature may be due to the polarization effects of the host Mg 2+ ions which produce a local static magnetic field at the Mn 2+sites. The observation of resolved Mn 2+ spectra at room temperature and its broadening on lowering the temperature in Mg(C 4H 3O 4) 2·6H 2O:Mn 2+ single crystals has been explained. The percentage of covalency in the MnO bond has been estimated to be 8%.

EPR of Mn2+-ion-doped single crystals of Mg[C4H3O4]2·6H2O

EPR of Mn2+-ion-doped single crystals of Mg[C4H3O4]2·6H2O

EPR studies of structural phase transition in Mn2+doped (NH4)4PuF8

Abstract EPR investigations of Mn2+:(NH4)4PuF8 are carried out in the 300–77 K temperature range, and information on structural changes is obtained using 19F superhyperfine structure (shfs) of the EPR of Mn2+ impurity at the Pu4+ site. EPR spectrum observed at room temperature (RT) exhibits a six-line hyperfine structure, corresponding to the ΔM x =1/2↔-1/2 transition with g=2.007±0.001 and A=99±2 G. The hyperfine components are superposed by multiplets of 19F shfs arising from interaction with neighbouring fluoride ions. The intensity distribution of shfs at RT suggests the presence of eight equivalent fluorines in the neighbourhood of Mn2+. The changes observed in the 19F shfs at 109 K have given direct evidence for the change in the immediate 19F coordination around Pu4+. It is suggested that these changes may be associated with a dynamic (square antiprism) ↔ static (dodecahedron) transition.

Single crystal EPR study of Mn(II)-doped cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II): Mn(II) in imidazole ligated low symmetry site

EPR of Mn(II)-doped single crystals of cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II) has been studied in the temperature range 300-90 K. Mn(II) replaces Cd(II) substitutionally giving rise to two magnetically inequivalent sites. The large magnitude and high asymmetry component of the D-tensor is in accordance with the low symmetry of the substitutional site. Due to the non-first order nature of the spectrum all the work had to be carried out at Q-band frequency. Forbidden transitions corresponding to Δ m I = ± 1 and ±2 are observed for a number of orientations and their positions have been calculated by perturbation theory taking into account the second order admixtures due to cross terms in D and hyperfine coupling constant A. The spin—Hamiltonian parameters at 300 K are A = −83.5, D = −790.0, E = −80.4 (all in units of 10 −4 cm −1) and g = 2.0014.

Electron paramagnetic resonance of Mn 2+-doped KCo xMg 1−xF 3 crystals

The X-band EPR of Mn 2+-doped in KCo x Mg 1− x F 3 single crystals has been studied at 70–400K. The resolved Mn 2+ spectra were recorded for x<0.5 and high temperatures. A narrowing of EPR linewidths is considered as an effect of the random modulation of the magnetic interactions between the Mn 2+ and Co 2+ ions due to the spin-lattice relaxation processes of the Co 2+ ions. The Co 2+ spin-lattice relaxation time τ host has been estimated in these perovskite crystals using a modified Mitsuma theory.

EPR of Mn2+‐Doped Tutton Salts. Role of Hydrogen Bonding

AbstractThe work deals with electron paramagnetic resonance of Mn2+‐doped zinc potassium selenate hexahydrate and zinc cobalt selenate hexahydrate Tutton salt single crystals. The different field parameters for the two hosts are evaluated from the observed z‐ and x‐axes spectra. The D parameters for the two hosts are compared with the corresponding reported values for hexahydrate. This parameter shows a systematic variation with the change in radii of the M′ ion in the series of salts ZnM′2(SeO4)2. 6H2O with M′ as K, Rb, and Cs. The case of M′ as NH4, however, does not show such a systematics. Similar behaviour is found for two other such series of Tutton salts, as well. Further it is found that D is, in general, negative for K, Rb, and Cs Tutton salts but is positive for NH4 Tutton salts. This unusual behavior of NH4 Tutton salts is attributed to two types of hydrogen bondings existing in such Tutton salts, in contrast to the only one type linking the H2O groups to the sulphate groups in all other Tutton salts. An attempt is also made to explain the unusual reported negative value of D in ammonium‐iron, ammonium‐cadmium, and ammonium‐nickel Tutton salts.

Phase transitions in K1-xLixTaO3

Abstract Mixed K1-xLi x TaO3 crystals representing the class of materials paraelectric at high and polar at low temperatures have been examined by EPR of Mn2+, Fe3+ and Cu2+ with particular regard to the “polar glass” state. The single crystal formula of K6Li4Ta10O30 is confirmed by the EPR data.

Host spin-lattice relaxation time and EPR studies of Mn2+ in single crystals of Co(HC4H2O4)2 · 4H2O

EPR of Mn2+ doped in Co(HC4H2O4)2 · 4H2O single crystals has been studied at room temperature and at liquid nitrogen temperature using X-band frequencies. The spin-Hamiltonian parameters were caculated. Mn2+ has been found to substitute for Co2+. Observation of resolved Mn2+ spectrum and broadening of the resonance lines on decreasing the temperature have been explained on the basis of host spin-lattice relaxation narrowing. The T1 of Co2+ has been estimated to be ≈8.3 x 10-12S at 300K.

EPR of Mn2+] in KHC2O4 single crystals

Abstract The EPR of Mn2+] in KHC2O4 single crystals has been studied in X-band at ∼ 295 K. The spectrum consists of a set of thirty lines corresponding to a single type of Mn2+] environment. Forbidden hyperfine transitions ▵m = ±1 were observed. The spin-Hamiltonian parameters are evaluated. Mn2+] appears to enter the lattice at interstitial sites.

EPR of Mn 2+ in [Co(H 2O) 6](ClO 4) 2 single crystals: Estimation of Co 2+ spin-lattice relaxation time

EPR of Mn 2+ doped cobalt perchlorate hexahydrate [Co(H 2O) 6](ClO 4) 2 was studied in the temperature range from 77 K to 400 K, at X-band (≅ 9.4 GHz). The observation of resolved Mn 2+ spectra at room temperature was interpreted in terms of a narrowing mechanism called “host spin-lattice relaxation”. In this mechanism, the fast spin-lattice relaxation of Co 2+ leads to a random modulation and appreciable averaging of the spin-spin interactions between the Mn 2+ and Co 2+ ions, which causes the narrowing of the Mn 2+ linewidths. The effective spin-relaxation was found to vary as time T l ~ T − n , where: n = 1.5 for 190 < T < 243 K and n = 1.8 for 243 < T < 400 K.

Aragonite→calcite transformation studied by EPR of Mn 2+ ions

The irreversible transformation aragonite→calcite has been studied both at different fixed heating rates (5, 10, 15 and 20 K/min) and at different fixed temperatures. Apparent progression rates of the transformation were observed above 685 K. At 730 K the transformation became sudden and violent. Time developments of the transformation at fixed temperatures have been discussed in terms of Avrami-Lichti's approach to transitions involving nucleation processes.

Estimation of T1 for Co 2+ ions from temperature variation of the EPR linewidths for Mn 2+ in Na 2Co(SO 4) 2 · 4H 2O single crystals

EPR of Mn 2+ doped in Na 2Co(SO 4) 2 · 4H 2O single crystals has been studied at various temperatures from 290 K to 90 K using an X-band spectrometer. The observation of resolved Mn 2+ spectra at 290 K in the host has been interpreted in terms of a random modulation of the interaction between the Mn 2+ and the Co 2+ ions by the rapid spin-lattice relaxation of Co 2+ ions. It is found that the effective spin-lattice relaxation time T 1 at 290 K is 1.66 × 10 −12 s for B‖ z axis.

Mn 2+ EPR study of phase transitions in dicalcium barium propionate Ca 2Ba(C 2H 5COO) 6

The phase transitions undergone by Ca 2Ba(C 2H 5COO) 6 single crystal have been studied via the X-band EPR of Mn 2+ in the temperature range 77–360 K. Two transitions have been detected at 267 K and at 207 K. The former is established to be a first order, while the latter a second-order phase transition, as deduced from the behaviour of the overall splitting of EPR lines and that of the EPR linewidth, as functions of temperature. The room temperature Mn 2+ spin Hamiltonian parameters are evaluated using a rigorous least-squares-fitting procedure. The angular variation of EPR spectra are studied in a number of planes to establish uniquely the orientations of the magnetic Z axes of the various Mn 2+ complexes in the unit cell. The angular variations of EPR spectra at various temperatures indicate that below 267 K the crystal structure is tetragonal.

Mn 2+ EPR study of phase transitions in dicalcium strontium propionate Ca 2Sr(C 2H 5COO) 6

X-band EPR of Mn 2+-doped single crystal of Ca 2Sr(C 2H 5COO) 6 has been used to study the phase transitions experienced by this crystal. Two phase transitions, a second-order transition at 278 ± 5 K and a first-order phase transition at 103 ± 10 K, have been deduced from the behaviour of overall splitting of lines and that of EPR linewidth, as functions of temperature. The angular variations of Mn 2+ EPR spectra have been studied in order to establish the orientations of the magnetic Z axes of the various Mn 2+ complexes in the unit cell. Further, from the behaviour of overall splitting of lines for the magnetic field orientations along the Z and X magnetic axes, as a function of temperature, a hypothetical phase transition to cubic symmetry has been deduced to occur at about 783 K. The room-temperature spin-Hamiltonian parameters are evaluated, using a rigorous least-squares fitting procedure. It is concluded, from the angular variation of the spectra, that the crystal structure is tetragonal below 278 K. Linewidth behaviour is studied to discuss the ordering of the ions surrounding Mn 2+.