Irradiation‐induced change of valence of Cr³⁺ ions doped in alkali sulphates

Abstract

It is shown by rneaiis of investigation of both optical absorption spectra and Roentgen Kal-lines of chroiiiiuni doped in LiKSO,, LiNaSO,, and Li,SO,. H,O crystals, t h a t Xirradiation results in change of the impurity charge state in a following way : Cr3+ + h+Cr4, Cr4+ + h + Cr5+.Earlier we have studied the EYR and optical absorption spectra of LizSO,. H,O, LiKSO, and LiNaSO, crystals doped with Cr3+ (ALYBAKOV et al. 1979(ALYBAKOV et al. , 1982 and the change of spectra of chromium-doped lithium and lithium-potassium sulphates under the X-irradiation (ALYBAKOV et al. 1980(ALYBAKOV et al. , 1983. It was shown, that X-irradiation results in weakening of Cr3+ ion absorption bands; at the same time a new band appears at about 350 nm in Li,SO, : Cr and about 355 nni in LiKSO, : Cr. A new group of lines appears also in a highfield side of EPR spectrum of the irradiated crystal; spectrum is described by spin-Hamiltonian with 8 = 1/2. This spectrum is absent in irradiated undoped crystals, therefore its appearance was supposed to be connected with a chromium valence change. The spectruin is stable a t a room temperature, consequently, it may not be due t o either divalent or tetravalent chromium. We supposed, that Cr3+ converts in Cr+ by means of a successive capture of two electrons, Cr+ being in a low-spin state (X = l / 2 ) in the assumption of a strong crystalline field.A subsequent investigation has shown, that the intensity of a irradiation-induced absorption band in anhydrous sulphates increases when heating and becomes maximal after the annealing a t 200 'C; at LNT this band has a clearly pronounced fine structure (Fig. 1). The separation between the fine structure components is -830 cm-' in LiKSO, : Cr spectrum and -806 cm-l in LiNaSO, : Cr spectrum. It is difficult t o explain the appearance of a fine structure, if one connects the irradiation-induced absorption band with Cr+. On the other hand, absorption bands in the region of 350 t o 360nm with a fine structure were observed earlier in the crystals, containing Cr5+ ions in the oxygen environment (BAKKS et al.; SIMO et al.); the fine structure was interpreted, as electronic-vibrational one due t o interaction with a v,-mode of chromate ion.I n principle, during the irradiation chroniium ions may trap either electrons or holes. To elucidate a question about a valent state of chromium in the irradiated crystals of alkali sulphates, we have studied Roentgen K,>-lines of chromium in these crystals. Such an investigation is a direct way of studying of a valence change, as the displacement of K,,-lime maximum is completely caused by the change of the number of valent electrons. Figure 2 shows Kal-lines of chroinium in different valent states. The positions of chromium lines in standard samples were used t o construct the dependence of a line