The Crystal Structure of Ni4Mo

A nickel-molybdenum alloy containing about 20 atom percent of molybdenum exists above 900°C as a cubic close-packed structure. If this alloy is quenched and then held at temperatures below 840°C, an ordering reaction occurs which results in a slight tetragonal distortion of the cubic structure. This ordered structure requires for its description a tetragonal cell with 2.5 times the volume of the distorted face-centered cubic cell. The completely ordered structure can be described analytically as follows: space group: C4h5—I4/m; dimensions of unit cell: a0=5.720A, c0=3.564A, c/a=0.6231; atomic positions: 2 Mo in 2(a), 8 Ni in 8(h), x=0.200. y=0.400.