Inorganic Chemistry, Vol.58, No.14, 9016-9027, 2019
High-Pressure Study of the Elpasolite Perovskite La2NiMnO6
Here we report a high-pressure investigation into the structural and magnetic properties of the double perovskite La2NiMnO6 using neutron scattering over a temperature range of 4.2-300 K at ambient pressure and over a temperature range of 120-1177 K up to a maximum pressure of 6.6 GPa. X-ray diffraction was also used up to a maximum pressure of 64 GPa, over a temperature range of 300-720 K. The sample was found to exist in a mixed rhombohedral/monoclinic symmetry at ambient conditions, the balance of which was found to be strongly temperature- and pressure-dependent. Alternating current magnetometry and X-ray absorption near-edge structure measurements were made at ambient pressure to characterize the sample, suggesting that the transition metal sites exist in a mixed Ni3+/Mn3+ and Ni2+/Mn4+ state at ambient temperature and pressure. Analysis of the magnetic properties of the sample shows that the Curie temperature can be enhanced by similar to 12 K with 2 GPa applied pressure, but it is highly stable at pressures beyond this. We report a pressure volume temperature equation of state for this material over this combined temperature and pressure range, with an ambient temperature bulk modulus of similar to 179(8) GPa. The previously reported transition from monoclinic to rhombohedral symmetry upon heating to 700 K is seen to be encouraged with applied pressure, transforming fully by similar to 1.5 GPa. Raman spectroscopy data were collected up to similar to 8 GPa and show no clear changes or discontinuities over the reported phase transition to rhombohedral symmetry or any indication of further changes over the range considered. The ambient-pressure Griineisen parameter gamma(th) was determined to be gamma(th) = 2.6 with a Debye temperature of 677 K. The individual modal parameters gamma(j) at ambient temperature were also determined from the high-pressure Raman data.