Raman spectroscopy, an ideal tool for studying the physical properties and applications of metal-organic frameworks (MOFs).

Metal-organic frameworks (MOFs) are a unique family of materials constructed by coordinating metal ions or clusters to bridging organic ligands. Many of these materials are well known for their intricate structures, and exceptional gas adsorption properties, and have potential applications in the separation of alkanes, catalysis, energy storage, surface-enhanced Raman spectroscopy (SERS) based detections, and diagnostics. In situ or in operando Raman spectroscopic studies provide real-time information about the different processes and associated structural changes in MOFs. In the last few decades, there has been phenomenal growth in the publications on MOFs containing insights from Raman spectroscopy. Such studies have helped the research community in identifying the adsorption sites, defect sites, structural or spin transitions, reaction centers, intermediates, etc. In this review, we present the current research status of Raman spectroscopy in probing the structure, guest adsorption, catalytic activity, and reaction mechanisms of MOFs, and their application in energy storage and SERS detection. We highlight the advancements in the Raman spectroscopy technique that have facilitated in situ studies in atmosphere as well as various chemical environments. We briefly discuss the relevance of computational studies in understanding phonon modes and predicting the stability of MOFs. Although this review is particularly focussed on works related to Raman spectroscopy of MOFs, we do discuss infrared studies on MOFs, where such results or analyses are missing from the Raman studies. These discussions have been provided with the intent to develop similar analysis techniques or methods in Raman spectroscopy research.

Minerals to Functional Materials: Characterization of Structural Phase Transitions and Raman Analysis of a Superionic Phase in Na6Co(SO4)4.

In search of promising Na+ ion conductors, we have detected a superionic phase in a Vantoffite mineral, Na6Co(SO4)4, at 570 °C, thus enhancing the use of minerals to produce futuristic solid state electrolytes. Na6Co(SO4)4 crystallizes concomitantly to produce di- and tetrahydrate forms from an aqueous solution. Both the crystal forms belong to a triclinic system, space group P1. The mineral transforms to a dehydrated phase as established by in situ single crystal X-ray diffraction at 217 °C and is shown to be isostructural with its Mn analogue. Even though thermal analysis indicates a single structural phase transition at 450 °C, the features associated with in situ powder X-ray diffraction as well as in situ Raman spectroscopy signify a second phase transition ≈540 °C and the behavior of ionic conductivity leads to a superionic phase (σ ≈ 10-2 S/cm at 570 °C). These observations are significant for the development and understanding of mineral based solid electrolytes.