Abstract
Lanthanide orthotantalates LnTaO4 (Ln = La, Nd, Dy and Lu) were synthesized by solid-state reactions at 1300 °C leading to well crystallized single-phase materials. XRD and infrared spectroscopic investigations showed that the samples exhibited three different monoclinic crystal structures depending on the lanthanide ion: P21/c (for La), I2/a (for Nd) and P2/a (for Dy and Lu). For LaTaO4, 21 polar modes could be depicted from the unpolarized infrared reflectivity spectrum, while group theory tools foreseen 33 infrared-active modes – the absent mode are likely hidden by accidental degeneracy. The smaller lanthanides (Nd, Dy and Lu) exhibited all the 15 predicted infrared-active bands, in perfect agreement with group-theory calculations, despite the mixing of polarization symmetry due to the polycrystalline nature of the samples. The intrinsic (infrared) dielectric properties were determined for all samples indicating that these orthotantalate ceramics could be candidates for microwave (MW) circuitry applications. LaTaO4 ceramics exhibited the best MW dielectric response among the investigated LnTaO4 (ɛr = 21.2 and estimated Qu × f ≈ 77 THz), followed by DyTaO4 (ɛr = 19.9 and Qu × f ≈ 75 THz), NdTaO4 (ɛr = 18.7 and Qu × f ≈ 55 THz), and LuTaO4 (ɛr = 16.2 and Qu × f ≈ 60 THz).
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