Solubility Limit and Microstructure of Rare-Earth Elements in (La_1–xR_x)₃Ga₅SiO₁₄ (R = Gd–Ho) Single Crystals and Ceramic Solid Solutions

(La_1–xR_x)₃Ga₅SiO₁₄ single crystals and ceramic solid solutions where R = Gd–Ho and 0 ≤ х ≤ 0.4 (as-batch) were characterized by X-ray powder diffraction (XRD) and analytical electron microscopy in order to determine the solubility limits of lanthanides in the langasite structure. Langasite-based solid solutions are the dominant phase up to the highest concentrations, but at x ≥ 0.15 for holmium and x ≥ 0.2 for terbium, garnet R₃Ga₅O₁₂ and La₂SiO₅-type impurity phases start to precipitate. Langasite single crystals have homogeneous structure where Tb, Dy, or Ho substitutes for La up to х = 0.05, and where Gd does up to х = 0.2. At х > 0.1, however, inclusions of impurity phases with the above structures appear in (La_1–xTb_x)₃Ga₅SiO₁₄ crystals. The magnetization curves of (La_1–xR_x)₃Ga₅SiO₁₄ (R = Ho and Tb) crystals measured at 1.85–2 K exhibit strong magnetocrystalline anisotropy, where the magnetic moment per R³⁺ ion is roughly the same for all of the heavy lanthanide concentrations studied. The temperature-and-frequency dependent dielectric constant and dielectric loss tangent of (La_1–xHo_x)₃Ga₅SiO₁₄ (x ≤ 0.2) and (La_1–xTb_x)₃Ga₅SiO₁₄ (x ≤ 0.3) ceramic samples were studied in the range T = 77–700 K at frequencies f from 1 kHz to 1 MHz. Debye-type relaxation with an activation energy of about 2 eV was detected.