Abstract

Luminescence spectroscopy is essentially sensitive to local environments. Consequently, it can be one of the most useful methods for elucidation of the short-range structure of the amorphous material. The luminescence studies of amorphous materials are not very common in literature. We report here on a comparative study of the luminescence spectra in amorphous insulating Ca₃Cr₂Si₃O₁₂ and its corresponding crystalline counterpart. The amorphous Ca₃Cr₂Si₃O₁₂ compound has been obtained by a fast cooling of the liquid solution. The amorphous state was confirmed by x-ray diffraction analysis. The chemical composition of the specimens was determined by electron-probe microanalyser. In the crystalline state this compound has garnet structure and exhibits long-range antiferromagnetic order with the Neel temperature of 12 K. According to our magnetic and EPR measurements no magnetic long-range order has been found in amorphous Ca₃Cr₂Si₃O₁₂. Below 20 K this compound has to be considered as insulating spin glass with large antiferromagnetic interactions. The luminescence spectra of both considered compounds have been measured at 1.7 K. The emission decreases strongly with increasing temperature and disappears above 10 K. In both cases the luminescence spectra consist of the sharp lines followed by a number of broad bands. The emission which is observed in garnet crystal is not intrinsic but results from ²E - to - ⁴A₂transitions of impurity-perturbed Cr³⁺ ions. The bound-exciton lines are not observed in luminescence spectrum. Several very sharp lines, found at energies lower than intrinsic ⁴A₂-to-²E absorption, are attributed to the magnon assisted transitions. The luminescence spectrum of amorphous Ca₃Cr₂Si₃O₁₂ is interpreted in terms of spin-wave-like excitations. The spin-wave-like excitations in spin-glass state have been predicted theoretically and confirmed experimentally in number of neutron scattering experiments. In the luminescence spectrum of amorphous Ca₃Cr₂Si₃O₁₂ the sharp line near 13240 cm^-1 is interpreted as zero-phonon-zero-magnon transition and strong line near 13025cm^-1 is attributed to magnon-assisted transition. Second broad band at 12840cm is identified as a vibronic transition. Analysis of the magnon-assisted transition yields value of the average nearest-neighbour exchange interaction between chromium ions of about 20cm^-1. This result is in agreement with our EPR data. In the case of antiferromagnetic garnet the value of the exchange interaction is estimated to be equal 0.6cm^-1. This surprising difference can be explained by considerable change in distance between magnetic ions in garnets after amorphization. Analysis of the emission line shape leads to the conclusion that the distribution of the crystal field acting on Cr ions in amorphous Ca₃Cr₂Si₃O₁₂ is narrow in contrary to that observed by Mössbauer spectroscopy in amorphous Y₃Fe₅O₁₂ [1].

© 1984 Optical Society of America