analogy between the two cases is treated by Trifonov. 9 For the Mossbauer case, Visscher 10 has specifically calculated the absorption cross section for a monochromatic 7-ray beam incident on an absorber with a simple Debye spectrum of phonons. Strong absorption maxima are very clearly shown at energies corresponding to high-phonon probability densities near the cutoff frequency and at integral multiples of this frequency.We feel that the almost universal agreement found here between observed and theoretical phonon energies must be taken as very strong experimental evidence that individual normal lattice phonons of many modes near the Brillouin zone boundary may be excited with high probability, and that a large part of the broadening of at least the R 2 and N bands in the alkali halides is due to lattice interactions involving normal lattice phonons of appreciable wave vector. In support of these conclusions, we feel it is reasonable that excitation of 9 E. A. Trifonov, Dokl.
The paramagnetic resonance of two tetragonal nickel spectra in the cubic phase of SrTiC>3 has been investigated below the structural phase transition at 105°K. Both spectra show the same rotation angle measured from the tetragonal directions as the iron-oxygen vacancy complex. This angle is 0.90±0.05 o at 77°K and 1.25±0.10° at 4.2 Q K. One of the axial nickel spectra is stable at room temperature and has been reported before by Rubins and Low with ^n = 2.029±0.01, gj.=2.352rfc0.001 (gwgi), and can be assigned to a low-spin Ni 3+ ion next to an oxygen vacancy associated with two electrons, the unpaired spin being in an e-type, x 2 -y 2 orbital directed away from the near neutral vacancy. Observed titanium superhyperfine structure supports these assignments. The importance of this first microscopic evidence of two differently charged states of an oxygen-vacancy defect complex in high-dielectric-constant oxide materials is emphasized. The difficulty of observing by EPR pairs when the defects are singly charged is discussed.
EPR data for NiSiF6⋅6H2O at Ku band and Ka band between 77 and 298 °K were fitted to an isotropic g value of approximately 2.25 with ‖D‖ varying from (0.166±0.005) cm−1 at 77 °K to (0.51±0.01) cm−1 at 298 °K. The values g∥=2.26±0.01 and ‖D‖= (0.125±0.005) cm−1 were obtained from a Ku band measurement at 4.2 °K. The magnitude of the temperature gradient ∂D/∂T had a broad maximum of about 1.8×10−3 cm−1/ °K near 200 °K, decreasing to about 1.5×10−3 cm−1/ °K near room temperature. The spin–lattice relaxation rate was determined from linewidth measurements to vary as T2 above 200 °K. From an expression for the Raman process in the high temperature limit, the Debye Temperature ϑD was deduced to be approximately 107 °K.
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