Low-field microwave response and electron paramagnetic resonance identification of valence states of manganese including octahedral Mn5+ in YAlO3 and CaYAlO4

Abstract

Zero-field signal in the electron paramagnetic resonance spectrum of Mn +2 in silicate glassesMicrowave response near zero magnetic field has been observed in YAlO 3 and CaYAlO 4 crystals dilutely doped with Mn in concentration ranging from 0.005 to 0.2 at. %. The response is due to magnetic-field-dependent nonresonant microwave losses, which co-exist with normal electron paramagnetic resonance ͑EPR͒ absorption due to Mn 2ϩ , Mn 4ϩ , and Mn 5ϩ ions in YAlO 3 and CaYAlO 4 . The valence states of manganese have been identified using a dual mode EPR cavity that allowed spectrum recording with polarization of the magnetic vector B 1 of the microwave perpendicular or parallel to the static magnetic field B 0 . The Mn 2ϩ ion, electron spin Sϭ 5 2 , and Mn 4ϩ , Sϭ 3 2 have been identified using normal B 1 ЌB 0 polarization, while Mn 5ϩ , Sϭ1, in unusual octahedral position, has been identified using B 1 ʈ B 0 polarization. The low-field response did not depend on mutual orientation of the vectors B 1 and B 0 and had the opposite phase with respect to the paramagnetic absorption. This shows that Mn-doped YAlO 3 and CaYAlO 4 exhibit magnetically induced microwave absorption, which has a minimum at zero magnetic field and increases with the applied magnetic field. The response in Mn-doped YAlO 3 increases with the introduction of Ca 2ϩ ions. The existence of manganese in different valence states, particularly metastable Mn 5ϩ in octahedral position, and the effect of calcium co-doping suggest the formation of relatively free charge carriers that can migrate in the vicinity of paramagnetic manganese ions. The spin-dependent mechanism of the charge migration is discussed to explain magnetoinduced microwave losses.

The dominance of paramagnetic loss in microwave dielectric ceramics at cryogenic temperatures

“…As the temperature cools down from 300 K to 4.2 K, the microwave loss decreases at the beginning for all doping levels, presumably from a decrease in the phonon population and a very small (most likely an insignificant change) reduction in band conduction or hopping transport. [6][7][8][9][10][11][12] As the temperature cools down further, however, a significant increase of loss is observed in Ni doped samples, and the loss scales with Ni-doping concentration. The "turning point" (the temperature where loss tangent starts to increase) in the loss tangent curves varies with doping levels.…”

“…Optimized materials are insulating with minimal free carrier conduction. 6,7 The loss that is present in such materials has most often been attributed to anharmonic-phonons. [8][9][10][11][12] Impurity defects, 13 extended defects, 14 and surface states in pores 15 have been experimentally correlated to an increase in the loss tangent, and this has been attributed to the production of softer phonon modes with increased anharmonicity.…”

Charge Separation and Surface Reconstruction: A Mn2+ Doping Study

“…These results suggest that the majority of photogenerated electrons (Ti(III)) are formed either via fast hole trapping at Mn substitutional sites or via direct excitation of Mn dopant atoms according to the reaction Note that Mn(III) (S ) 2) formed during excitation is an ion with an even number of electrons and is usually not observed using conventional X-band EPR due to commonly large zero field splitting and/or fast relaxation times. 18 Participation of Mn in light excitation suggests strong coupling of electronic states of Mn dopant atoms and TiO 2 lattice constituents. To confirm that these Mn atoms are incorporated in the TiO 2 lattice, we have investigated the samples in which Mn 2+ ions (6.3 × 10 -4 M, pH ) 3) are deliberately adsorbed on TiO 2 nanoparticles prepared with the same method.…”

Luminescence study of transition metal ions in natural magmatic and metamorphic yellow sapphires

“…In fact, no potential candidate (including Mn) is reliably evidenced above the detection limit of EDXRF and optical absorption. On the other hand, the confirmation of the occurrence of Mn species comes from EPR spectra that show weak resonances (inset of figure 4) characterized by hyperfine structures composed by sextets of lines spaced by about 9 mT, as expected from paramagnetic hyperfine interactions of the electronic configuration of Mn 2+ ions with the I=5/2 nuclear spin of the 100% abundant 55 Mn [14].…”