In the last few years metal hydrides have attracted a lot of attention due to their unusual properties that make them an interesting object for both basic studies and applications. According to theoretical predictions, in some cases experimentally proved, these materials could exhibit high temperature superconductivity due to the presence of high-frequency vibrations of hydrogen atoms in metallic lattice. A principal requirement for such phenomenon is high hydrogen concentration, which can be achieved by using diamond anvil cell for the generation of high hydrogen pressure combined with laser heating. In this way a series of new unexpected polyhydrides was synthesized and nearly room temperature superconductivity was reported in the lanthanum hydride with hydrogen to metal atomic ratio of H/ La = 10 [1,2].

Until recently, no hydrides of transition metals in groups IV-XII were known with composition H/Me > 2. In present study the reaction of ZrH₂ and HfH₂ with molecular hydrogen was studied by in-situ X-ray diffraction in a DAC at room temperature at high pressures up to 39 GPa. The formation of novel zirconium and hafnium hydrides was observed at 8.2(5) and 12.4(5) GPa respectively. These new hydrides had the cI16-type structure of metal lattice, identical to that of Th₄H₁₅, and the compositions of new hydrides were estimated to be Zr₄H₁₅ and Hf₄H₁₅ on the basis of their lattice volumes. The decomposition of these hydrides back into ZrH₂ and HfH₂ during decompression was observed at 3.5(3) and 3(1) GPa respectively. A fit of the V (P) dependencies with a Murnaghan equation of state, assuming pressure derivative of bulk moduli to be 4 gave us the values of zero-pressure volumes 32.4(4) ų per Zr atom and 31.3(4) ų per Hf atom, and the bulk moduli of 160(10) and 145(10) GPa respectively for Zr₄H₁₅ and Hf₄H₁₅. Search of a possible superconductivity in these new hydrides is in progress.

1. Somayazulu, Maddury; Ahart, Muhtar; Mishra, Ajay K.; et al, Physical Review Letters 122(2) 027001 2019
2. A. P. Drozdov, P. P. Kong, V. S. Minkov, S. P. Besedin, M. A. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. B. Prakapenka, E. Greenberg, D. A. Knyazev, M. Tkacz, M. I. Eremets, arXiv:18101561 [cond-mat.supr-con]