Solid solutions are crystalline phases consisting of at least two components in a freely variable composition within certain limits. Depending on the miscibility of the components, solid solutions are divided into two types, i.e., solid solutions of unlimited solubility of components and limited solubility of components [
1]. Solid solutions among inorganic substances are widely studied and well-known solid phases for which the structure and properties of the material are dependent on the component ratio. Such phases and the change of properties they provide are widely found in such material classes as metal alloys, minerals, ceramics, etc [
2,
3]. On the contrary, solid solutions formed between organic compounds are researched notably less often [
4]. However, the interest in the formation of solid solutions between organic solids has significantly increased during the last decade, which is clearly indicated by the increase in the number of scientific publications investigating this phenomenon, mainly by testing previously accepted and expressing new hypotheses in the field of crystal engineering [
5,
6]. In this study, the formation of solid solutions in binary systems formed by thioxanthone and xanthone was explored. In each of the studied systems, mixtures of substances with different component ratios were crystallized, and powder X-ray diffractometry (PXRD) and construction of phase diagrams from thermal analysis (DSC) data were used to determine the solubility of substances in each other. The investigation of the xanthone–thioxanthone binary system reveals the existence of two solid solutions, each formed on the basis of the parent structures of xanthone and thioxanthone, respectively. One of these solid solutions exhibits miscibility of both molecules within a broad composition range (>0-80 mol% of xanthone). In addition, the crystalline structure of the solid solution involving thioxanthone:xanthone (75:25 mol%) is also presented.
Author Contributions
Conceptualization, T.R.; methodology, T.R., A.B. and K.S.; software, T.R., A.B. and K.S.; validation, T.R.; formal analysis, K.S.; investigation, T.R. and K.S.; resources, T.R.; data curation, T.R. and K.S.; writing—original draft preparation, T.R., A.B. and K.S.; writing—review and editing, T.R., A.B. and K.S.; visualization, T.R., A.B. and K.S.; supervision, T.R.; project administration, A.B. and K.S.; funding acquisition, A.B. and K.S. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded through the Latvian Council of Science project, “Crystal Engineering of Pharmaceutical Multicomponent Phases for More Efficient Crystalline Phase Design” (Project No. lzp-2018/1-0312).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available upon request from the corresponding author.
Acknowledgments
K.S. acknowledges the European Social Fund project “Strengthening of the Capacity of Doctoral Studies at the University of Latvia within the Framework of the New Doctoral Model”, identification No. 8.2.2.0/20/I/006, and MikroTik Ltd. doctoral scholarship in the field of natural and medical sciences administrated by the University of Latvia Foundation.
Conflicts of Interest
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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