Temperature Dependence of the EPR Spectra of Cr³⁺ and Cr⁵⁺ Ions in CaV₂O₆ and BaV₂O₆ Crystals

Abstract

V. S. GRUNIN et al.: Temperature Dependence of the EPR Spectra of Cr3+ and Cr5+ 765 phys. stat. sol. (b) 98, 765 (1980) An investigation is made of EPR spectra of CaV,O, and BaV20, single crystals doped with chromium. It is found that the point symmetry of the paramagnetic Cr3+ and C P ions depends on temperature. It follows from t,he symmetry of the Cr5+ spectrum in BaV,O, and its superhyperfine structure a t T = 77 K that the Cr5+ ion is displaced to one of two neighbouring V atoms. The observed change of the impurity ion symmetry cannot be explained both by the off-centre ion theory and the Jahn-Teller effect. HCCJIeEOBaHbI CneKTpbI 3nP MOHOKpUCTaJIJIOB CaV,O, U BaV,O, C EpUMeCbIO XpOMa. 3KCEepUMeHT nOKa3aJI, 9 T O TOge9HaH CUMMeTpUX IlapaMarHUTHbIX IlOHOB Cr3r U Cr6+ MeHHeTCH C TeMIlepaTypO6. H3 CHMMeTPUH CneKTpa UOHOB Cr5' B BaV,O, U ero CylIep-CBepXTOHHOfi CTpYKTYPbI npU T = 77 K CJleEyeT, ' XTO 3TOT MOH CMeUeH K OAHOMY M3 EByX COCeHHUX aTOMOB BaHaEUH. H~~J I K I @~M O~ kl3MeHeHHe CMMMeTpUK npUMeCHbIX UOHOB He MOIHeT 6bITb 06' bHCHeHO B PaMKaX MOAenU HeUeHTpanbHOCTU UJIU 3@@eKTa HHa-Tennepa.s = 112, g, = 1.983 k 0.001, A , = (2.24 & 0.02) X J , A, = (1.67 +_ 0.02) X J , A , = (0.4 & 0.2) x J , A; = (1.21 & 0.02) x J , A , = (7.04 k 0.02) X lopz6 J, J . g, = 1.901 & 0.001 . A; = (9.92 5 0.02) X The 2 and z axes of gand A-tensors for two non-equivalent centres are in the (001) plane and form the angles k0.262 rad with the [010] and [loo] axes, respectively, the y-axis being the corninon one and coincides with the [OOl] axis of crystal. The 2' and z' axes of the SHFS form the angle +0.350 rad with the z and z axes, 9' ) I [OOl] is common. 2.2 Temperuture dependence of the spectra 2.2.1 Cr3+ When the temperature was decreased below about 270 K every line of the Cr3+ ion spectrum in CaV,O, was doubled. The distance of the pairs increases when lowering temperature to about 200 I< and is constant for further decrease of temperature.

Oxidation State and Local Structure of Chromium Ions in LaOCl

“…Cr 5+ is a S = 1/2 system; therefore, there are no effects related to ZFS, and the EPR signal is not as complicated as for Cr 3+ . A notable feature occasionally observed in the spectra of Cr 5+ is HF structure caused by an interaction with the nearest cations [69][70][71][72]. We can observe a similar effect for the 2 mol% sample; however, due to the high Cr concentration, it is partially obscured.…”

Impurities and Charge Compensation in Some Molybdenium Oxides

“…There is a weak spectrum of Cr5+ ions in undoped crystals [ 5 ] . The intensity of this spectrum increases sharply by doping with chromium.…”

“…The hole is localized on the nonbonding x-orbital of the oxygen which is perpendicular to the crystal plane of this oxygen and two or three cations of the structure. Then the g-tensor components are (5) where fl(x) and f2(x) are functions of x = A/Ei (A is the spin-orbit constant, E$ the energy of the i-th level of the ground state) and therefore depend on the mutual positions of the oxygen and cations on the crystal plane. In practice the 9,-value (or g, on other designation) is almost always bigger than go = 2.0023.…”

Impurities and electron localization in V2O5 crystals

“…The electron density distribution has been explained by the impurities creating t7he extra Coulomb fields localizing the electron on one or more vanadium atoms [2,5,8], by the oxygen vacancies [l], and by dislocation shearing which gives rise to V-V pairs with one unpaired electron [9, 101. The conductivity of the crystals has been usually attributed to these electrons [3, 10 to 121. As has been shown by the systematic study of V,O, crystals there are many different EPR centres in these crystals caused by impurities.…”