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A classification of discrete compounds in binary chiral systems of single and different substances (homo- and heteromolecular compounds) is presented. It considers both chemical and crystallographic characteristics of such compounds. Using amino acids as chiral model systems, features of crystal structures of equimolar and non-equimolar heteromolecular compounds are reviewed. In this connection, the concept of homo- and heteromolecular dimers in compounds formed by amino acids is introduced and analyzed. As a result, a correlation between the molecular dimer type and the side chain structure (linear or branched) and the conformation (extended or folded) of the relevant compounds' molecules is derived. Two non-equimolar discrete heterocompounds discovered in the chiral systems L-valine–L-isoleucine and L-valine–L-leucine are discussed using the concepts proposed. The previously studied first system features a 2 : 1 (Val : Ile) compound. A newly investigated second system is found to form a 3 : 1 (Val : Leu) compound, and its crystal structure is described and evaluated.

The thermal behavior of discrete phases formed in the respective systems of malic acid enantiomers and L-enantiomers of the amino acids valine and isoleucine was studied using the temperature-resolved PXRD method. In the (S)-malic acid–(R)-malic acid system, thermal deformations in crystal structures of stable compounds (enantiomer S, racemates RSI and RSII, and non-equimolar compound S3R) and polymorph transformations of metastable compounds (racemate RSIII and non-equimolar compound 3S1R) were examined. In the L-valine–L-isoleucine system, thermal deformations in crystal structures of stable compounds L-Val and L-Ile and non-equimolar compound V2I were investigated. Thermal deformation analysis included plotting the temperature dependence of the unit cell parameters and volume, calculating thermal deformation tensors, plotting figures of thermal expansion coefficients (CTE), and estimating the extent of thermal deformation anisotropy. In all the cases studied, the maximal thermal expansion was observed in the direction of the weakest hydrogen (malic acid) or van der Waals (valine and isoleucine) intermolecular bonds, i.e. in the directions closest to that perpendicular to the dimer molecule chains (malic acid) or to molecular layers (valine and isoleucine). The strongest anisotropy of thermal deformations in monoclinic crystals was observed in the ac plane, in which the symmetrically unfixed angle β can vary.

This work provides further step in considering artificial diamond crystals doped with beta-emitter 14C as chemically durable and environmentally friendly material to be applied as a core of nuclear electric photo-voltaic batteries. In order to investigate features of diamond radioluminescence the metallic radioactive technetium 99Tc and electron beam treatment have been used as external sources of beta-radiation simulating diamond self-glowing from admixture of 14C. Three types of commercially produced artificial diamonds have been applied: Ib, IIa and IIb. Intensive radioluminescence and cathodoluminescence have been observed for two diamond types: IIa and IIb, which can be suggested for further doping by 14C. It had been demonstrated that simulation of diamond self-glowing from 14C using 99Tc and electron beam were insufficiently different due to technical aspects of the experiment, not the difference in the excitation energy. Cathodoluminescence technique can be used in the future for the study of diamonds when optimizing it spectral properties.

Since knowledge on the occurrence of non-equimolar discrete compounds in binary systems containing chiral molecules is very limited, this study reviews and systematizes the current state of investigating such systems and summarizes the results on two example systems studied in detail by the authors. In particular, the identification and verification of the non-equimolar discrete compounds compared to other discrete solid phases occurring in the two systems are discussed by presenting the results of related SCXRD, PXRD, TRPXRD, DSC, IR, and HSM studies. The (S)-malic acid–(R)-malic acid system has been found to contain non-equimolar 1 : 3 and 3 : 1 stable (S3R and SR3) and metastable (3S1R and 1S3R) discrete compounds, along with the equimolar compounds RSI and RSII (known monoclinic modifications) and the recently discovered RSIII modification. Polymorphic transformations of the discrete phases are debated, and the crystal structure of the stable compound S3R is identified (S. G. P1). The L-valine–L-isoleucine system has been stated to contain a non-equimolar 2 : 1 discrete compound, V2I, that could independently be proven by the ternary solubility diagram in water and its crystal structure solved (S. G. C2). The results obtained are discussed in conjunction with the findings reported in the literature. In order to systematize the variety of terms used for the description of discrete phases in binary chiral systems of organic substances, a systematization of equimolar and non-equimolar compounds based on chemical and crystallographic characteristics is proposed.

The present review generalizes results of a comprehensive experimental study (by means of PXRD, SCXRD, TRPXRD, DSC, IR) of six chiral systems. These are enantiomeric systems of a single substance: (1) the system with a eutectic point (S- and R-E3ClMA), (2) the system forming a binary compound (l- and d-phenylglycine), and (3) the system forming equimolar and non-equimolar discrete compounds (S- and R-malic acid). Also these are diastereomeric systems and a system composed of enantiomers of different substances: (4) the diastereomeric system with continuous solid solutions (l- and l-allo-threonine), and (5) the diastereomeric system with a eutectic point (d- and l-allo-threonine), and (6) the system of enantiomers of different substances (l-valine and l-isoleucine) which contains a non-equimolar discrete compound. Crystal structures of discrete equimolar and non-equimolar homomolecular and heteromolecular compounds found in the systems under discussion are described. Also, polymorph diversity and the limits of solid solutions in such binary systems are discussed.

Two model compounds—valine C5H11NO2 (l-Val) and isoleucine C6H13NO2 (l-Ile)—were chosen to study isomorphism of the compounds formed by enantiomeric molecules having similar shapes, but differing in the number of chiral centers. The X-ray powder diffraction method was applied to investigate samples with various ratios of valine and isoleucine crystallized from aqueous solutions by cooling and isothermal methods. Limited solid state miscibility between valine and isoleucine was established. A non-equimolar discrete compound with the molecular ratio Val:Ile = 2:1 (V2I) has been discovered, and its crystal structure was determined. Accordingly, the corresponding regions of solid solutions and two-phase areas in-between were distinguished. Solubility data in water support the phase behavior in the system.

Luminescent properties of yttrium-aluminum garnet activated by different amount of europium (YAG:Eu3+) single crystals were studied by local cathodoluminescence (CL) technique. Samples were grown by flux method. CL spectra were obtained in the range from 400 to 800 nm with spectral resolution of 0.1 nm. The rise and decay kinetics of various CL bands were investigated. The concentration dependencies of CL intensity and decay times of transitions were plotted. The concentration quenching for 5D1-7F2 Eu3+ transition was observed at about 3.5 at.% of Eu by Y, and for 5D2-7F1 and 5D3-7F3 transitions – at about 2.5–3 at.% of Eu by Y. For transition 5D0-7F1 no concentration quenching was observed. The high-energy excitation capture efficiency by different energy levels of Eu3+ in YAG was estimated based on the dependence of the CL rise rate of these levels on the electron beam current density. It was found that 5D1 energy level excitation capture efficiency is about three times higher than that of the 5D0, 5D2 and 5D3 energy levels.

X-Ray diffractometry was employed to investigate the possibility of formation of non-equimolar discrete compounds in the chiral system of malic acid S- and R-enantiomers. Samples studied were mixtures of single enantiomers or an enantiomer and the various polymorphs of the racemic compound. From samples prepared via recrystallization of the melt and grinding of initial reactants, it is demonstrated that the malic acid enantiomers can form stable stoichiometric compounds in S:R ratios of 3:1 and 1:3. Compared to the known racemic and pure enantiomer phases, the corresponding compounds S3R/SR3 are characterized by a proper powder diffraction pattern and melting point.

Enantiomers (R(+) and S(−)), RS-racemate (double compound) and (R+S)-conglomerate (mechanical mixture of enantiomers) of malic acid C4H6O5 have been investigated by means of X-ray diffractometry and high temperature X-ray diffraction method. The RS-racemate was found to be able to form three polymorphic modifications, which we denominated as M1 (monoclinic, space group P21/c), M2 (monoclinic, space group Cc), and Tc (triclinic, space group P-1), the latter modification having been unknown before. Modification Tc was also described, and its Xray diffraction characteristics, including interplanar spacings d, hkl indices, unit cell parameters, were defined. In addition, X-ray diffraction characteristics for both reported earlier M1 and M2 monoclinic polymorphic modifications were measured with higher accuracy. The ability of RS-racemate to form one of the above three modifications (M1, M2, and Tc) or their mixtures containing various proportions and combinations of the components (M1+M2, M1+Tc, or M2 +Tc) was found to depend on the type of crystallization medium (a melt, aqueous medium, ethanol or acetone solution), crystallization rate (from 2–3 minutes to 4 months), and crystallization temperature. Heating S-enantiomer and M1 RS-racemate up to their respective melting points (100 °C and 124 °C, correspondingly) only made them undergo thermal deformations, while heating (R+S)-conglomerate in the temperature range of 96–110 °C resulted in its homogenization to form M2 RS-racemate, which, near the melting point (118 °C), namely, in the range of 112–116 °C, was transformed into M1 RS-racemate.