Effects of chemical composition on the structural stability, elastic, vibrational, and electronic properties of Cs₂NaLnX₆ (Ln = La…Lu, X = F, Cl, Br, I) elpasolites

Elpasolite crystals are very important materials, both from the applied and fundamental points of view. Those elpasolites, which contain rare earth ions with a high atomic number Z, are very much suitable for the low-cost high-performance gamma-ray detection, applications in medicine, food industry, nuclear energy production, processing, and detection of nuclear proliferation. The thermal and structural stabilities are important parameters required for detecting applications, because the performance conditions for such devices are usually very harsh. Since it is widely believed that elpasolites may have even better detection properties, the lack of systematic studies on the elpasolites and thus the unavailability of reliable data on their physical properties and trends in their variation caused by chemical composition considerably hinders search for more efficient new materials. Therefore, to fill in this gap and provide with all essential information about a large number of elpasolites crystals, for the first time, the structural stability, elastic, vibrational, and electronic properties of 60 cubic elpasolite Cs₂NaLnX₆ (Ln = La, …, Lu, X = F, Cl, Br, I) crystals were consistently calculated in the framework of the same computational approach based on the density functional theory (DFT). Variation of all calculated parameters (such as the lattice constants, elastic constants, Debye temperature, normal vibrational modes frequencies, Mulliken effective charges, bond populations, and band gaps) across the considered groups of crystals was analyzed and several trends, which are important for the search and preparation of new stable materials with improved performance, were identified.