With electron nuclear double resonance (ENDOR) the superhyperfine interactions of Fe3+ impurities with five shells of Li neighbours were measured in a congruent LiTaO3 crystal. From the analysis of the ENDOR spectra it is found that Fe3+ is on a substitutional Li+ site on the threefold symmetry axis of the crystal. From the shape of the ENDOR spectrum it is concluded that most Fe3+ impurities are on very perturbed sites.
The superhyperflne (SHF) inleraclions of Fe'+ impurities with five shells o f Li neighbours were measured in B nearly stoichiometric LiNbOj clystal by applying eleclron nuclear double resonance (ENWR). Sirnilarty lo what was found for Li'BO,, lhe Sile of Fe'+ in LiNbO, has a threefold ymmely. The analysis of the ENDOR speclra clearly s h m that Fe'+ is on a subslilulional ti+ site. lhe varialions of shape and intensily of WWR lines belonging to specific ligand nuclei when rotating lhe clyslal in lhe Static magnetic field are atlribuled to local disorder.
For LiNb03 crystals heavily doped with Mg and codoped with Cr a single isotropic ~~~l i n e w i t h g = 1.971 t 0.002andaminimallinewidthof 1.8 f 0.2 mThasbeenobserved. With ENDOR it could be shown unambiguously that the new defect is C3' substituting for Nb. The 53Cr hyperfine and quadrupole constants are A,, = 50.8 2 0.2 MHz, A, = 51.0 rt 0.2 MHz and P = 0.10 rt 0.05 MHz. The crystal field at the chromium site is smaller by more than an order of magnitude than for previously reported Cr3+ centres in LiNb03, indicating that those centres are not on the Nb site.
With ENDOR, Mn2+ centres in both stoichiometric and congruent LiNbO3 have been investigated. An analysis of four identified Li shells with 24 Li neighbours gives sufficient proof for the Mn2+ occupying a Li site. In stoichiometric LiNbO3 both neighbouring on-axis Nb nuclei were also identified and found to experience changed electric field gradients compared with remote Nb nuclei and NMR data in the literature. Signs of the parameters of the Mn2+ spin Hamiltonian (A/sub //, perpendicular to /<0, D>O, P>0) have been determined. Most centres probably contain a defect such as charge-compensating Li vacancy in the first Li shell. As inferred from line-broadening effects, additional defects, especially in congruent crystals, have a measurable influence on the relaxation of nuclei near to the Mn2+.
he aggregation of F centres and OH-molecules to FH(OH-) centres in KBr leads Io a very interesting optical and thermal bstabibty. The htruc~~ms of the bistable FH(OH-) centres were investigated by using optical and mnventional detection of electron nuclear double w n a n c e (ENWR). The anaiyis of the "WR spectra of both FH(OH-) mnfigurations b prsented. It b shown that in both tistable configurations the OH-molecule resides on a nat-nearest anion site (fourth shell). ?he bistable mnfigurations differ in the orienfation of the OH-dipole with respect to the F centre. The thermal bstabilily ir tentatively explained as being due to enmpy changes with temperature.
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