Reversible high-pressure phase transition in LaN

In situ high-pressure X-ray powder diffraction experiments on LaN up to 60.1 GPa at ambient temperature in a diamond-anvil cell revealed a reversible, first-order structural phase transition starting at ∼22.8 GPa and completed at ∼26.5 GPa from the ambient cubic phase (Fm3¯m, no. 225) to a tetragonal high-pressure phase (P4/nmm, no. 19, a = 4.1060(6), c = 3.0446(6) A, Z = 2, wRp = 0.011), which has not been claimed in theoretical predictions. HP-LaN is isotypic with a high-pressure polymorph of BaO, which crystallizes in a tetragonally distorted CsCl-type structure. The phase transition is accompanied by a volume collapse of about 11% which corresponds well with the reported data on HP-BaO. A linear extrapolation of the c/a ratio of the tetragonally distorted CsCl-type sub-cell reaches a value c/a = 1 of cubic CsCl-type HP-LaN at 91(12) GPa. In addition, the compressibility of LaN was investigated and resulted in a bulk modulus for the ambient pressure phase of B0 = 135(3) GPa and B′ = 5.0(5) after fittin...