Isovalent and aliovalent cation substitutions in the anion sublattice of whitlockite-type ferroelectrics Ca9RE(VO4)7 with RE =Y and Yb

A solid state route was used for the preparation of whitlockite-type solid solutions Ca9Y(NbO4)x(VO4)7–x and Ca9+0.5xYb (GeO4)x(VO4)7–х in both powder and ceramic forms. The complex of methods, including synchrotron powder diffraction were applied for the characterization of the first-time obtained compounds. The limits of the anionic substitution were found up to х = 0.28 (Nb) and 0.35 (Ge). The synthesized solid solutions show ferroelectric properties. The Curie temperatures Tc were determined and gradually rise with x. The increase of Tc correlates with the enlargement of tetrahedral units upon Nb5+ or Ge4+ substitutions for V5+ cations. The intensity of optical second harmonic generation (SHG) induced by a Nd:YAG laser depends non-monotonically on the grain size in powder Ca9Y(NbO4)x(VO4)7–x samples, marking the length of the phase matching lc = 15 μm for x = 0.28 in comparison with lc = 9.14 μm in Ca9Y(VO4)7. Manifold increase in the SHG intensity in both solid solutions is partly related to the appearance of more polarizable Nb5+ and Ge4+ cations in the E1O4 tetrahedra on the 3-fold axis, resulting in an enlarged birefringence of the materials. The photoluminescence study of the Ca9+0.5xYb(GeO4)x(VO4)7–х reveals the same environment of the Yb3+ ions all alone the solid solution. The intensity of the luminescence rises with germanium substitution for vanadium about 2 times.