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<p>Isomorphic substitution of atoms in crystals, accompanied by solid solution formation, represents a fundamental phenomenon in natural and synthetic materials. This process follows two principal mechanisms: (1) isovalent substitution, where ions of equal charge replace each other, and (2) heterovalent substitution involving charge imbalance. The latter can be further categorized based on structural consequences: substitutions that preserve the unit cell stoichiometry versus those altering the total number of atoms/ions (<i>N</i>) per unit cell. The <i>N</i>-changing heterovalent substitutions are particularly significant in creating complex solid solutions across various inorganic systems. Beyond simple ionic replacements, more sophisticated structural transformations occur through “blocky isomorphism” – a process where extended structural fragments (blocks, chains, or layers) undergo collective substitution. This mechanism enables the formation of modular architectures, where the successful integration of different structural modules depends critically on their geometric compatibility at interfacial boundaries. This review systematically examines two key aspects of structural complexity in crystalline materials: (1) heteropolyhedral substitutions involving coordinated polyhedral units, and (2) “blocky isomorphism” phenomena. Through representative examples from mineral and synthetic systems, we analyze how these substitution mechanisms govern topological modifications in crystal structures, ultimately determining their physical properties and chemical behavior.</p>

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The crystal chemistry and topology of modular structures. II. Heteropolyhedral substitutions and “blocky isomorphism” in inorganic compounds

  • 期刊论文

  • 发表日期：

    2026年5月7日

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  • Sergey M. Aksenov

摘要

Isomorphic substitution of atoms in crystals, accompanied by solid solution formation, represents a fundamental phenomenon in natural and synthetic materials. This process follows two principal mechanisms: (1) isovalent substitution, where ions of equal charge replace each other, and (2) heterovalent substitution involving charge imbalance. The latter can be further categorized based on structural consequences: substitutions that preserve the unit cell stoichiometry versus those altering the total number of atoms/ions (N) per unit cell. The N-changing heterovalent substitutions are particularly significant in creating complex solid solutions across various inorganic systems. Beyond simple ionic replacements, more sophisticated structural transformations occur through “blocky isomorphism” – a process where extended structural fragments (blocks, chains, or layers) undergo collective substitution. This mechanism enables the formation of modular architectures, where the successful integration of different structural modules depends critically on their geometric compatibility at interfacial boundaries. This review systematically examines two key aspects of structural complexity in crystalline materials: (1) heteropolyhedral substitutions involving coordinated polyhedral units, and (2) “blocky isomorphism” phenomena. Through representative examples from mineral and synthetic systems, we analyze how these substitution mechanisms govern topological modifications in crystal structures, ultimately determining their physical properties and chemical behavior.