0020-1685/05/4102- © 2005 Pleiades Publishing, Inc. 0152 Inorganic Materials, Vol. 41, No. 2, 2005, pp. 152–155. Translated from Neorganicheskie Materialy, Vol. 41, No. 2, 2005, pp. 197–200. Original Russian Text Copyright © 2005 by Milenov, Veleva, Petrova, Gospodinov, Skorikov, Egorysheva, Kargin, Vasil’ev. INTRODUCTION Photorefractive Bi 12 SiO 20 (BSO) crystals (sillenite structure, sp. gr. I23) find wide application in optical information recording, storage, and processing systems [1, 2]. The key to optimizing the performance of BSO in various photorefractive applications lies in identify- ing the native and impurity defects determining the dis- tribution of energy levels in the band gap of BSO. Such information can be gained from ac conductivity data. The conductivity of undoped BSO crystals has been measured at frequencies from 1 kHz to 100 MHz and temperatures from 290 to 600 K [3–6]. The conduction in BSO in this temperature range was shown to be due to hopping between localized states. Doping with Al, P, Fe, Cr, Mn, and Ni has a significant effect on the conductivity of BSO and the associated activation energy [7, 8]. In this paper, we report the first ac conductivity mea- surements on BSO crystals doped with Re, Os, Ru, and Rh. EXPERIMENTAL BSO single crystals were grown by the Czochralski process in the [001] direction in air from platinum cru- cibles, using a stoichiometric growth charge [9–11]. The starting materials used were extrapure-grade Bi 2 O 3 and SiO 2 . The dopants were introduced into the growth charge in the form of oxides. The grown crystals were 30–45 mm in diameter and 70–100 mm in length. The dopant concentration was determined by atomic absorption spectrophotometry (Perkin-Elmer Model 3030): Os 1.76 × 10 20 , Re 1.80 × 10 20 , Rh 1.63 × 10 20 , and Ru 3.23 × 10 20 cm –3 . In conductivity measurements, we used polished sin- gle-crystal plates measuring 12 × 12 × 1 (±0.25) mm, cut normal to the [001] direction. Au electrodes were deposited by thermal evaporation. The electrode diam- eter was 10 ± 0.5 mm. The temperature (294–600 K) was maintained with a stability of ± 0.1 K by a digital controller. Conductivity was measured in the frequency range 10 4 to 10 7 Hz using a Hewlett-Packard 4275 bridge. The relative uncertainty in conductivity was 3% over the entire frequency range studied. RESULTS AND DISCUSSION The real part of 10-kHz conductivity, σ = σ 1 + iσ 2 , of our samples shows Arrhenius behavior (Fig. 1), charac- teristic of thermally activated processes, σ 1 = A exp(–E a /(kT)), (1) where A is the preexponential factor for the real part of conductivity and E a is the activation energy of conduc- tion. Doping was found to increase σ 1 by an order of magnitude. The conductivity curves can be divided into two distinct portions, 450–600 and 290–380 K. The activation energy of conduction in the crystals studied is = 0.86–1.08 eV in the high-temperature portion and = 0.07–0.13 eV in the low-temperature portion (table). The present results differ insignificantly from the data reported earlier for undoped BSO [3, 7, 8], which leads us to conclude that the conduction in the doped crystals is due to hopping transport. In the range 290–380 K, our data attest to hopping between local- ized states near the Fermi level (conductivity is a weak function of temperature). At higher temperatures, the E a ' E a '' Electrical Conductivity of Bi 12 SiO 20 Single Crystals Doped with Os, Re, Ru, and Rh T. I. Milenov*, M. N. Veleva*, D. P. Petrova**, M. M. Gospodinov*, V. M. Skorikov***, A. V. Egorysheva***, Yu. F. Kargin***, and A. Ya. Vasil’ev*** * Institute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko ch. 72, Sofia, 1784 Bulgaria ** Southwest University, Ivan Mikhailov str. 66, Blagoevgrad, 2700 Bulgaria *** Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia e-mail: anna_egorysheva@rambler.ru Received October 5, 2004 Abstract—Bi 12 SiO 20 crystals doped with Os, Re, Ru, and Rh were grown by the Czochralski technique, and their conductivity (real and imaginary parts) was measured as a function of temperature and frequency. The results are interpreted as evidence that the charge transport in the crystals is due to hopping along chains of localized states.