Researchers from the California Institute of Technology, the National Synchrotron Light Source, the Geophysical Laboratory, and the Advanced Photon Source have discovered Invar behavior at high pressures in Pd-25 at.% Fe, a composition far from the Invar composition of Pd-75 at.% Fe. We, used density functional theory calculations, energy-dispersive x-ray diffraction, and synchrotron Mössbauer spectroscopy to investigate the mechanical and magnetic properties of Pd-25 at.% Fe through the Invar transition in the resistively heated diamond anvil cell. The synchrotron Mössbauer measurements revealed a collapse of the ⁵⁷Fe magnetic moment between 8.9 and 12.3 GPa at 300 K, coinciding with a transition in bulk modulus found by x-ray diffraction measurements. Heating the sample under a pressure of 7 GPa showed negligible thermal expansion from 300 to 523 K, demonstrating that Invar behavior can be induced by pressure in an alloy composition very different from those exhibiting Invar behavior at ambient pressure. The first-principles calculations show that pressure causes the electronic structure near the Fermi level in Pd-25 at.% Fe to become similar to that of classic thermal Invar alloys. By tuning the electronic structure, pressure should cause materials of many chemical compositions to exhibit Invar behavior. This work has been published in Physical Review Letters [M. L. Winterrose, et al., Phys. Rev. Lett., 102, 237202 (2009)].

Recently the ⁵⁷Fe phonon partial density of states (PDOS) in L1₂-ordered Pd₃Fe was studied at high pressures by nuclear resonant inelastic x-ray scattering (NRIXS) measurements and density functional theory (DFT) calculations. The NRIXS spectra showed that the stiffening of the ⁵⁷Fe PDOS with decreasing volume was slower from 12 to 24 GPa owing to the pressure-induced Invar transition in Pd₃Fe, with a change from a high-moment ferromagnetic (FM) state to a low-moment (LM) state observed by nuclear forward scattering. Force constants obtained from fitting to a Born–von Kármán model showed a relative softening of the first-nearest-neighbor (1NN) Fe-Pd longitudinal force constants at the magnetic transition. For the FM low-pressure state, the DFT calculations gave a PDOS and 1NN longitudinal force constants in good agreement with experiment, but discrepancies for the high-pressure LM state suggest the presence of short-range magnetic order. This work has been published in Phys. Rev. B 83, 134304 (2011)