Raman Spectra of Nd∶La3Ga5SiO14and Nd∶La3Ga5.5Ta0.5O14 Crystals

摘要

Single crystals of Nd:La_3Ga_5SiO_(14)(NLGS) and Nd:La_3Ga_(5.5)Ta_(0.5)O_(14)(NLGT) were grown by Czochralski method. Both crystals exhibit a langasite structure(space group D23(P321) with Z = 1), whose general formula is A_3BC_2D_3O_(14), where A, B, C and D represent the dodecahedral, octahedral and tetrahedral positions, respectively. The adding of low content of Nd does not change the crystal structure. Although they are isostructural, the distribution of atoms among the symmetric sites is different. While NLGT exhibits a perfect orderly structure, NLGS exhibits an intrinsically disordered structure due to the random distribution of Ga and Si atoms at the C3(2) symmetry sites. To understand what changes occur in the phonon distribution when Ta replaces Si, we recorded and analyzed room temperature polarized Raman spectra. We observe 8A1+18E modes for NLGS and 10A1+13E for NLGT crystals. When Si is substituted by Ta, we observe that the wavenumber of some modes related to vibrations involving Ga and O atoms decreases. This agrees with the fact that the Ga–O bonds are shorter in NLGS than in NLGT.