Dielectric Properties of Bi 12 SiO 20 Crystals Doped by Cr and Mn Ions

Abstract

Frequency-temperature characteristics of dielectric permittivity and loss factor of Bi 12 SiO 20 crystals doped by Cr and Mn ions have been carried out in audio frequency range in temperature interval 290 to 800 K. Dielectric anomalies of relaxation (Bi 12 SiO 20 :Cr) and resonance (Bi 12 SiO 20 :Mn) types and Cole-Cole dependences close to linear ones have been observed. The results are discussed on the basis of "screened hopping" model. Photorefractive crystals Bi 12 MO 20 (BMO, where M = Si, Ge, Ti) possess high dielectric permittivity (ε = 30 to 160 at T ≈ 300 K [1-4]) which is sensitive to concentration of intrinsic defects [2-4]. Particularly the increase of ε is supposed due to the increase of Bi 2 O 3 content [4].Near 4.2 K in BMO crystals anomalies of frequency-temperature characteristics ε(ν, T ) and tan δ(ν, T ), typical for "virtual" ferroelectrics are observed [4][5][6]. Anomalies are caused by relaxors with small activation energies E a ∼ = 0.05 eV [5]. It seems that the same relaxors in the form of the "dipole" impurities in Bi 12 GeO 20 (BGO) crystals are responsible for the relaxation anomalies of ε(ν, T ) and tan δ(ν, T ) in audio frequencies range at T = 20 to 40 K. It is noted correlation between the "strength of relaxation" (concentration of relaxors) and intensity of jellow coloration of BGO crystals [6].Discontinuities of ε(ν, T ) and tan δ(ν, T ) typical for the relaxation polarization also take place near the room temperature [7]. Mutually correlating anomalies of internal friction Q −1 (T ) and ε(T ) at high temperatures T = 500 to 700 K (ν = 10 3 Hz), which depend on type of impurity (Al or Cr) and content of oxygen vacancies are described. It is supposed that these 67

Photoluminescence of Bi12SiO20 crystals

“…One has to take into consideration intense temperature quenching of the PL of the type I [1] and temperature activation of the intrinsic photoconductivity with increasing temperature up to 300 K [3], on the one hand, and increase in the intensity of this PL and temperature quenching of the photoconductivity of BSO at Т ≤ 80 K (see Fig. 1c, curves 1 and 2), on the other hand.…”

“…Besides, the existence of deep levels, with the energy of optical activation Е а О ≈ 2.6-3.0 eV, has been evidenced by spectral studies of the photoconductivity and the optical absorption in BSO. These levels are responsible for electron generation, providing a high 'extrinsic' photoconductivity and a 'shoulder' of the absorption profile near the fundamental absorption edge in BSO [3][4][5]. Magneto-optical and PCE studies have revealed that the intra-centre transitions taking place in the Bi-O complexes also contribute to the absorption shoulder [4,6].…”

“…Magneto-optical and PCE studies have revealed that the intra-centre transitions taking place in the Bi-O complexes also contribute to the absorption shoulder [4,6]. On the other hand, a number of effects (e.g., thermal activation and extinction of the photoconductivity, and non-stability of the characteristics of optical holographic recording [3,7,8]) have been observed in the sillenites, of which understanding requires some information about recombination centres. In this case photoluminescence (PL) studies can be useful.…”