Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects
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PROPERTIES OF CRYSTALS
Circular Dichroism Spectra of Langasite Family Crystals in the Range of Electronic Transitions of Structure Defects V. I. Burkova, E. P. Peredereœa, E. V. Fedotova, B. V. Mill’b, and Yu. V. Pisarevskiœc a
Moscow Institute of Physics and Technology, Institutskiœ per. 9, Dolgoprudnyi, Moscow oblast, 141700 Russia e-mail: [email protected] b Moscow State University, Moscow, Russia c Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskiœ pr. 59, Moscow, 119333 Russia Received December 3, 2007
Abstract—The absorption and circular dichroism spectra of langasite family crystals are studied. Wide bands in the range of 285–500 nm that are related to the structure lattice defects are found. For all the crystals, these bands are in approximately the same spectral region, have identical structures, and can be attributed to one type of defects formed by cation vacancies and excess oxygen atoms in the optically active positions. PACS numbers: 78.20.Ek DOI: 10.1134/S1063774508050192
INTRODUCTION About 140 compounds with the Ca3Ga2Ge4O14 structure (sp. gr. P321), referred to as the langasite family (La3Ga5SiO14), are presently known [1–3]. Single crystals of langasite family compounds are of great interest as piezoelectric materials for acoustoelectronic devices and sensors of physical quantities [1]. These crystals, activated by transition-metal ions, are promising for quantum electronics [4]. Optical characteristics of langasite family crystals were studied in [5–7], where the data on the transmission spectra, optical activity, linear dichroism, and refractive-index dispersion were presented. The crystals are transparent to ~7 µm and exhibit weak absorption bands in the range 250–550 nm. These bands are usually related to the electronic transitions between the states of defects of various types [8]. Since the crystals are optically active, it is natural to use circular dichroism (CD) spectroscopy for deriving information about the nature of their defect centers. The CD spectra of the Ca3Ga2Ge4O14, Sr3Ga2Ge4O14, and La3Ga5SiO14 crystals were studied in [9–11]. Wide peaks were found in the range of 270–450 µm, which were attributed to growth defects. The reflection spectra of La3Ga5SiO14 in the range of 3.5–30 eV (354–41 nm) were investigated in [12]. EXPERIMENTAL SAMPLES AND MEASUREMENT TECHNIQUES The crystals Ca3Ga2Ge4O14 (CGG) (1),1 Sr3Ga2Ge4O14 (SGG) (2), La3Ga5SiO14 (LGS) (3), 1
The numbering used in Figs. 1 and 2.
La3Ga5GeO14 (LGG) (4), La3Nb0.5Ga5.5O14 (LNG) (5), La3ía0.5Ga5.5O14 (LTG) (6), and La3íi0.5Ga5Si0.5O14 (LGS-Ti) (7) of the [001] orientation were grown by the Czochralski method in an O2–N2 atmosphere [1]. The crystals had weak yellow-green or reddish yellow color of various shades. The samples were cut normally to the optical axis c; their thickness varied from 0.07 to 0.33 mm. The transmission spectra were measured with Hitachi-330 and Lambda-900 spectrophotometers in the range 200–3200 nm. The CD spectra were recorded with a Mark-3S dichrograph (Jobin–Yvo
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