Huntite-family nominally-pure and activated/co-activated LnM₃(BO₃)₄ (Ln = La–Lu, Y; M = Al, Fe, Cr, Ga, Sc) compounds and their-based solid solutions are promising materials for lasers, nonlinear optics, spintronics, and photonics, which are characterized by multifunctional properties depending on a composition and crystal structure. The purpose of the work is to establish stability regions for the rare-earth orthoborates in crystallochemical coordinates (sizes of Ln and M ions) based on their real compositions and space symmetry depending on thermodynamic, kinetic, and crystallochemical factors. The use of diffraction structural techniques to study single crystals with a detailed analysis of diffraction patterns, refinement of crystallographic site occupancies (real composition), and determination of structure–composition correlations is the most efficient and effective option to achieve the purpose. This approach is applied and shown primarily for the rare-earth scandium borates having interesting structural features compared with the other orthoborates. Visualization of structures allowed to establish features of formation of phases with different compositions, to classify and systematize huntite-family compounds using crystallochemical concepts (structure and superstructure, ordering and disordering, isostructural and isotype compounds) and phenomena (isomorphism, morphotropism, polymorphism, polytypism). Particular attention is paid to methods and conditions for crystal growth, affecting a crystal real composition and symmetry. A critical analysis of literature data made it possible to formulate unsolved problems in materials science of rare-earth orthoborates, mainly scandium borates, which are distinguished by an ability to form internal and substitutional (Ln and Sc atoms), unlimited and limited solid solutions depending on the geometric factor.

Kinetic order-disorder phase transitions (space group space group $ I\bar 4 $ ↔ space group I4₁/a) have been considered for nonactivated and activated scheelite compounds (Na_0.5Gd_0.5)WO₄ (NGW), (Na_0.5Gd_0.5)MoO₄ (NGM), (Na_0.5La_0.5)WO₄(NLW), and (Na_0.5La_0.5)MoO₄ (NLM) synthesized by the Czochralski method and structural and growth sources of crystal dissymmetrization have been suggested. For NGW, it was shown that an increase in the difference between the content of Gd and Na in two positions of the structure with space group $ I\bar 4 $ and their ratio leads to an increase in the deviation from centrosymmetry. On the basis of available literature data and our results, it was demonstrated that the degree of order depends on the initial composition of the reaction mixture, crystal growth and cooling rates, activator concentrations, and postgrowth treatment conditions. The inconsistency between X-ray diffraction data and asynchronous second harmonic generation studies was explained by the possibility of formation of centrosymmetric superstructures and/or local ordering of atoms.

N-(Propargyl)indole-2-carbonitriles undergo DBU-catalyzed addition of CH-acids to nitriles, followed by cyclization to give 9-aminopyrido[1,2-a]indoles. The reaction proceeds through the initial formation of a push–pull enamine, alkyne–allene rearrangement and intramolecular nucleophilic cyclization. Nitromethane and malonate esters can be employed as the CH-acids. The resulting compounds were found to exhibit fluorescence properties.

The retropinacol-like rearrangement in 5-oxohomoadamantane series represents an efficient synthetic route to new polycyclic 2,3,4-tri- and 2,4-disubstituted derivatives of homoadamantane.