Samples of the Nd5−xTbxMo3O16+δ series were obtained by solid-state synthesis from metal oxides at 1050°C. The formation of solid solutions based on cubic and monoclinic phases and a two-phase region between them was observed in the Nd5−xTbxMo3O16+δ compositions. Increasing the terbium content in the system leads to a decrease in the unit cell parameters of the cubic and monoclinic phases within their homogeneity regions, which confirms the formation of solid solutions. It has been established that the terbium oxidation state in the Nd5Mo3O16+δ crystal structure is +3. The predominant placement of terbium atoms in the 8c position is observed by crystal structure refinement and confirmed by the results of the atomistic simulation. The introduction of terbium into the crystal structure of neodymium molybdate leads to a decrease in the Ln1–O1 and Ln2–O2 interatomic distances. The atomistic simulation was performed by the GULP program using the fit potential of the terbium ion. Terbium molybdate with Tb5Mo3O16+δ composition is a subtraction solid solution based on Tb2MoO6. Increasing the unit cell parameters of Tb5Mo3O16+δ monoclinic phase compared to Tb2MoO6 was confirmed by structure refinement and atomistic simulation.

— Isomorphous bismuth substitution for neodymium in the Nd 5 Mo 3 O 16 + δ compound has been studied by X-ray diffraction. The crystal structure of a single-phase solid solution with the composition Nd 4.95 Bi 0.05 Mo 3 O 16 + δ has been refined by the Rietveld method, and the electrical conductivity of the modified molybdate has been measured in the temperature range 300–700°C.

This paper presents the investigation of the heterovalent substitution of cadmium for lanthanum in the La2-xCdxMoO6-x/2 system. The samples were synthesized by the solid state reaction method at 1000°C. The samples were characterized by X-ray powder diffraction with Rietveld refinements, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy methods. The study results revealed that cadmium incorporation in the lanthanum molybdate leads to the transformation of the tetragonal structure of La2MoO6 to a cubic fluorite-like one. The content of the cubic phase reaches 94% in the Lа1.4Cd0.6MoO5.7 sample. The unit cell parameter of fluorite-like-phase decreases with cadmium content rising. The preferred location of cadmium ions in the cubic structure was established by the Rietveld refinement method. The heterovalent substitution cadmium for lanthanide in tetragonal La2MoO6 molybdate leads to the cubic fluorite phase stabilization in a similar way as it occurs in the process of reduction.

The isomorphous substitution of smaller RE elements (Ln = Nd, Eu, Gd, Ho, Tm, and Yb) for lanthanum in the apatite silicate solid solutions La(9-x)Ln(x)(SiO4)(6)O-1.5 was studied by X-ray powder diffraction and the Rietveld structure refinement, scanning electron microscopy and energy-dispersive X-ray microanalysis. Single-phase samples were prepared by solid-state synthesis at a moderate temperature of 1200 degrees C using an amorphous SiO2 nanopowder (10-40 nm) as a reactant. As the atomic number of Ln increases, the complete solubility, 0 <= x <= 9, found in the systems with Ln = Nd, Eu, Gd, and Ho changes to a limited one for Ln 1/4 Tm (0 <= x < 1.5) and Yb (0 <= x < 1). The distribution of La and smaller Ln over two structurally independent cationic sites is close to statistical. In both cationic polyhedra, Ln(1)O-9 and Ln(2)O-7, the bond lengths Ln - O decrease with x, except the longest bonds Ln(1) - O(3) and Ln(2) - O(1) which increase slightly. The experimental results on the substitution limits agree with the values of the mixing energy, and critical temperature of miscibility calculated in the approximation of a regular solid solution. (C) 2017 Elsevier Masson SAS. All rights reserved.

Sodium holmium double silicates with the apatite structure and compositions NaHo 9 (SiO 4 ) 6 O 2 and Na 2 Ho 8 (SiO 4 ) 6 O have been prepared by a sol-gel process and characterized by X-ray diffraction. The crystal structure of NaHo 9 (SiO 4 ) 6 O 2 has been refined, and the electrical conductivity of both compounds has been measured.

Двойные силикаты натрия и гольмия со структурой апатита состава Na2Ho8(SiO4)6O и NaHo9(SiO4)6O2 синтезированы золь–гель-методом. Соединения изучены методом рентгенофазового анализа, уточнена кристаллическая структура NaHo9(SiO4)6O2, измерены электрофизические свойства.

Sodium neodymium silicate NaNd 9 (SiO 4 ) 6 O 2 with the apatite structure has been synthesized and studied by X-ray diffraction and scanning electron microscopy. No sodium oxide sublimation occurred during ceramic synthesis by the selected method. The Rietveld method shows that sodium atoms in the structure are ordered (in the 4 f position). The O(4) oxygen atoms, which are not involved in silicate ions, are positioned in the centers of triangles formed by Nd(2) atoms. Rather long Nd-Nd distances (3.940 Å) indicate a possible existence of luminescent properties.

Bioactive powders of silicon substituted hydroxyapatites containing 1.4–4.3 wt% Si were synthesized by precipitation from aqueous solutions of calcium nitrate and ammonium (sodium) hydrophospate. Tetraethoxysilane and sodium silicate in basic media were used as a source of silicon. Using scanning electron microscopy, surface morphology of the obtained products was studied. It has been found that procedure using sodium silicate allows obtaining of materials with 100–200 nm particle size. Procedure that utilizes tetraethoxysilane leads to 40–100 nm particles. Chemical, X–ray phase analysis and infrared spectroscopy of samples annealed at 900 °C indicate the formation of monophase crystalline products possessing apatite structure.