Concurrent Pathways in the Phase Transitions of Alloys and Oxides: Towards an Unified Vision of Inorganic Solids

The study of phase transitions is usually restricted to two to three transformations. Examples of such transitions include the CaF2 --> PbCl2 --> Ni2In in alloys, the NaCl --> CrB --> CsCl or the well documented transformation olivine --> spinel of the oxides A(2)XO(4). These transitions, traditionally regarded as partial processes, have prevented the construction of wider structure maps. One of the scarce examples of these maps was reported by Leger and Haines (Eur J Solid State Inorg Chem 34:785-796, 1997) concerning the phase transitions of AX(2) compounds (dihalides and dioxides), where increasing the coordination number of the A atom is linked to the pressure increase. The structural information, collected in these maps, is always of interest because it limits the number of possible transition paths which may relate a structure-type into another. However, a careful analysis of the partial phase transitions undergone by different compounds, at high temperature and high pressure, reveals that the partial transitions are not isolated processes but they overlap, forming a long, rational pathway that connects all the structures in a coherent manner. Alloys and their related oxides show a similar trend along their concurrent pathways which complement each other. In this work, the analysis is restricted to the AX(2) alloys and their corresponding oxides AX(2)O(4), and the results demonstrate that there exists a unifying principle that can be inferred through the simultaneous analysis of all the phase transitions involved in the concurrent structural journeys carried out by both types of compounds. The AX(2) alloys begin the walk in the fluorite-type structure, ending in the MoSi2-type structure. In the case of the oxides AX(2)O(4), their cation arrays follow a concurrent pathway that, starting at the filled fluorite-type structure, ends in the final Sr2PbO4-type structure. These structural journeys also allows for the discovery of several missing links (structure types) which fit into the general sequence and help one understand the whole transitions pathway as a rational process, which takes place simultaneously in the alloys as well as in the cation arrays of the oxides. Very recent works show that alkali metals (Na and K) also join the walk. The extended Zintl-Klemm concept (EZKC) and the concept that relates of oxidation-pressure-temperature effects provide a basis for understanding the observed transitions.