An anti CuO₂-type metal hydride square net structure in Ln₂M₂As₂H_x (Ln = La or Sm, M = Ti, V, Cr, or Mn)

Using a high pressure technique and the strong donating nature of H^-, a new series of tetragonal La₂Fe₂Se₂O₃-type layered mixed-anion arsenides, Ln₂M₂As₂H_x, was synthesized (Ln=La or Sm, M=Ti, V, Cr, or Mn; x∼3). In these compounds, an unusual M₂H square net, which has anti CuO₂ square net structures accompanying two As^3- ions, is sandwiched by (LaH)₂ fluorite layers. Notably, strong metal-metal bonding with a distance of 2.80 Aa was confirmed in La₂Ti₂As₂H_2.3, which has metallic properties. In fact, these compounds are situated near the boundary between salt-like ionic hydrides and transition-metal hydrides with metallic characters.