Reversible high-pressure phase transition in LaN

Abstract

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) Å, 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 fitting a third-order Birch-Murnaghan equation of state to the experimental p–V data. The corresponding extrapolated bulk modulus of HP-LaN is found to be B0 = 278(6) GPa and its pressure derivative B′ = 1.2(2). Both as-calculated bulk moduli are compared to the respective values obtained from an Eulerian strain versus normalized stress plot to be 143(2) GPa for ambient LaN and 293(7) GPa for HP-LaN. Compared to other binary nitrides such as δ-ZrN or δ-HfN having bulk moduli of 285 GPa and 306 GPa, respectively, the extrapolated bulk moduli of HP-LaN are in the same order of magnitude, ranking HP-LaN as a highly incompressible material.