hydrides are conventionally labeled with Greek letters, and α is reserved for the metal. The known ZrHx phases are γ (x = 1), δ (x = 1.5–1.65) and ε (x = 1.75–2). Fractional x values often correspond to mixtures, so the compositions with x = 0.8–1.5 usually contain a mixture of α, γ and δ phases, and δ and ε phases coexist for x = 1.65–1.75. As a function of increasing x, the transition between δ-Zr and ε-Zr is observed as a gradual distortion of the face-centered cubic δ (fluorite-type) to face-centered tetragonal ε lattice. This distortion is accompanied by a rapid decrease in Vickers hardness, which is constant at 260 HV for x < 1.6, linearly decreases to 160 HV for 1.6 < x < 1.75 and stabilizes at about 160 HV for 1.75 < x < 2.0. This hardness decrease is accompanied by the decrease in magnetic susceptibility. The mass density behaves differently with the increasing hydrogen content: it decreases linearly from 6.52 to 5.66 g/cm3 for x = 0–1.6 and changes little for x = 1.6–2.0. Preparation and chemical properties Zirconium hydrides form upon interaction of the metal with hydrogen gas. Whereas this reaction occurs even at room temperature, homogeneous bulk hydrogenation is usually achieved by annealing at temperatures of 400–600 °C for a period between several hours and a few weeks. At room temperature, zirconium hydrides quickly oxidize in air, and even in high vacuum. The formed nanometer-thin layer of oxide stops further oxygen diffusion into the material, and thus the change in composition due to oxidation can usually be neglected. However, the oxidation proceeds deeper into the bulk with increasing temperature. The hydrogen is anionic due to the electronegativity difference between Zr and H. When prepared as thin films, the crystal structure can be improved and surface oxidation minimized. Zirconium hydrides are soluble in hydrofluoric acid or alcohol; they react violently with water, acids, oxidizers or halogenated compounds. Applications Formation of zirconium hydrides is an important factor in the operation of several types of nuclear reactors, such as boiling water reactors Fukushima I and II, which suffered from a series of explosions caused by the 2011 Tōhoku earthquake and tsunami. Their uranium fuel pellets are enclosed in metal rods made from Zircaloy – an alloy of typically about 98.25% zirconium with 1.5% tin and minor amounts of other metals. Zircaloy is used because of its