The color of langatate crystals and its relationship with composition and optical properties

Abstract

Langatate crystals grown by the Czochralski method in 〈0001〉 and 〈01$ \bar 1 $1〉 directions both without subsequent annealing (from colorless to orange crystals) and with vacuum (colorless and pale‐green crystals) and air (orange crystal) annealing have been investigated by neutron diffraction, X‐ray diffraction and optical spectroscopy. The main types of point defects were compared with absorption bands: oxygen vacancies ‐ VO•• (0.35‐0.36 µm), the vacancies in the lanthanum position ‐ VLa‴ (0.29‐0.30 µm); the absorption band at 0.48‐0.49 µm is typical for the colored samples. The relationship between the X color coordinate (red component) and the oxygen contents has been obtained. The green color of the sample annealed in vacuum is due to the presence of Ta+3 ions in the La(Ta3+,Ga)O3 phase that has the structural and geometric agreement with langatate structure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Color Origins in Langatate Crystals

“…For LGT CK, a shift of the intrinsic absorption edge to a higher wavelength of 270 nm is observed. The absorption band at 280 nm in the spectra of CK and CI6 was recorded by authors of [5] [16] and attributed to "La vacancies" VLa′′′. The weak and wide absorption band at around 330-340 nm observed in the transmission spectra of CK was attributed [5] [16] to oxygen vacancies VO •• .…”

“…LGT crystals are commonly grown by Czochralski method using an Iridium crucible [5] [6] [10] [15]. Kimura et al [15] reported that LGT crystals contain iridium ions, which are responsible to the crystal color.…”

“…G.M. Kuz'micheva et al [5] reported that in oxygen-free growth atmosphere of langasite family, a dissociation of the melt and evaporation of Gallium suboxide are observed, which results in deficiency of Oxygen and Gallium. For that, LGT CI3 and CI6 have been grown in N2 atmosphere from Gallium rich charge composition to compensate the loss of gallium.…”

“…Indeed, point defects, such as impurities and oxygen vacancies significantly affect the physicochemical properties (optical, electrical, piezoelectric…) and induce the crystal color. Langatate crystals (LGT), commonly grown by the Czochralski method from a ternary melt of La2O3-Ga2O3 and Ta2O5 using an iridium crucible, differ from colorless to yellow-orange or pale-green [4] [5] depending on growth and post-growth treatment conditions. LGT crystals without subsequent annealing differs from colorless to yellow-orange crystals, depending on growth atmosphere.…”

“…After vacuum annealing, the LGT samples become colorless and orange colored after air annealing [13] [4] [5]. The authors [7] [14] have suggested that the oxygen present in the growth and annealing atmospheres is responsible for the crystal color linked by the authors [11] [5] [4] to the presence of oxygen vacancies in the structure.…”