Crystals Based on Alkali and Alkaline Earth Metal Halides: An Overview

Abstract

Single crystals belonging to the A3MX5 ⋅ 2H2O family (where A = alkali metal,
M = alkaline earth metal, and X = halogen) possess exceptionally interesting optical
and electrical properties. However, their structurally incommensurate nature has
historically impeded precise X-ray crystal structure refinements. Introducing
intentional non-stoichiometry into such incommensurate structures offers a promising
pathway toward engineering novel ionic-conducting single crystals tailored for solid-
state batteries and next-generation energy storage applications.
In this work, single crystals of non-stoichiometric sodium calcium chloride hydrate,
Na3CaCl5 ⋅ 2H2O, were successfully grown from aqueous solutions of sodium
chloride and calcium chloride via the slow (free) evaporation method. The harvested
crystals (dimensions up to 6 × 4 × 2 mm3) were subjected to comprehensive
chemical, structural, thermal, mechanical, and electrical characterizations. Atomic
absorption spectroscopy (AAS) and X-ray diffraction (XRD) analyses confirmed the
metal ion ratio (Na and Ca) and verified the incommensurate structural nature of the
grown network. FTIR spectroscopy and thermogravimetric analysis (TGA)
confirmed the presence of lattice water of crystallization. The refined chemical
formula, formula weight, and experimental density were determined to be
Na3.665Ca0.335Cl4.335 ⋅ 0.153H2O, 254.12 g/mol, and 2.275 g/cm3, respectively.
Microhardness measurements verified high mechanical stability under applied loads.
Variable-temperature DC conductivity measurements revealed significant ionic
conduction in the non-stoichiometric lattice. This study demonstrates that controlled
non-stoichiometry in complex alkali–alkaline earth halides can successfully
transform traditional dielectric insulators into functional ionic conductors suitable for
solid-state energy devices.