1. Could you explain the significance of your article to the non-specialist? 

In this contribution we predict existence and selected properties of - as yet unknown - ternary hydrides of late lanthanide metals, Yb and Tm, thus preceding possible experimental studies in this area. Importantly, we claim that moderate-T_C superconductivity could be achieved in related materials by chemical doping and/or applying external pressure. Note that attempts of explicit design of entirely novel families of superconductors are immensely rare in the literature.

2. What has motivated you to conduct this work? 

We were largely driven by poor performance of the existing hydride superconductors, by strong hydride-fluoride analogy, and obviously by existence of related ternary fluoride phases. Intrigued by 'an inverse isotope effect' seen for PdH_x, we have applied here a simplistic description of hydride superconductors based on a 'two-parabolas mixed-valence model'; this in turn allowed us to anticipate that two factors may drastically improve the T_C values: i) an increased separation of the metal centers and/or ii) larger contraction of metal valence orbitals. Both means seem counter-intuitive at the first sight, and they are related to the unique nature of a very mobile hydride ligand.

3. Where do you see this work developing in the future? 

We hope that theoretical findings of our work will be verified in the future by experiment. Indeed, high-pressure synthesis of novel Cs-Ln-H phases is now attempted in Dag's Noréus laboratory in Stockholm. We are excited to learn when our compounds see the daylight and what properties they might have! Confirmation of theoretical predictions is dear to heart of every theoretical chemist...

4. Are there any particular challenges facing future research in this area? 

Theory predicts that new phases will be stable only under 7-20 GPa of external pressure. Necessity to apply 200,000 atm certainly rules out any practical large-scale applications. It is therefore crucial to know if the compounds studied would be quenchable, i.e. if they could survive decompression to 1 atm without simultaneous decomposition. Strict isolation from oxygen and water is another difficult requirement.