High-pressure dependent X-ray absorption of SmNiO3 at the Ni K- and Sm-L3 edges

NESTOR E. MASSA; ALINE Y. RAMOS; HELIO C. N. TOLENTINO; NARCIZO M. SOUZA-NETO; JAIRO FONSECA, JR.; JOSE A. ALONSO

Campinas

Workshop; 2015 IUCr High-Pressure Workshop; 2015

International Union of CRYSTALLOGRAPHY

SmNiO3 is a member of the family of undoped compounds RNiO3 (R=Rare Earth) in which a sharp temperature driven insulator to metal phase transition takes place. The transition from high temperature orthorhombic metallic to an insulating charge disproportionate (2Ni3+ → Ni3+δ+Ni3−δ) monoclinic distorted phase is consequence of the Ni-O bond length increase and a simultaneous decrease of the Ni-O-Ni angle. SmNiO3, TIM = 400 K, also has an intermediate paramagnetic insulating phase, and orders magnetically below TN~205 K The magnetic ground state consist of an unusual antiferromagnetic structure, with k=(1/2,0,1/2). Pressure P causes increments in the Ni-O-Ni angle and simultaneous diminution of the Ni-O distances inducing the interplay of bandwidth W, charge energy transfer Δ, and Coulomb repulsion energy U. An increment in W and thus W/U, eventually closes the insulating gap yielding a reduction of TIM. This negative slope, -TMI/dP, is shared by all RNiO3 (R=Rare Earth≠ La.). Here we report XANES and EXAFS measurements of SmNiO3 from 20 K to 600 K and up to 38 GPa at the Ni K- and Sm L3- edges. They were performed at Laboratorio Nacional de Luz Sincrotron, Campinas, Brazil, in the DXAS energy dispersive beamline. Increasing pressure induces Ni-O-Ni angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group (metallic LaNiO3). A multiple component pre-edge tail accounts for 1s transitions to 3d-4p states while a post-edge shoulder raises distinctively smoother due to the reduction of electron-phonon interactions as Ni 3d and O 2p orbitals overlap. Pressure dependent room temperature Ni white line peak energies show an abrupt 2.4 GPa valence discontinuity understood as due to non-equivalent Ni sites with Ni3+δ + Ni3-δ charge disproportionate in the monoclinic distortion turning at TMI into Ni3+ of the perovskite distorted orthorhombic Pbnm metal oxide phase. At 20 K, still distinctive, this turns smoother due to localization. Octahedral bond contraction at 300 K and 20 K show breaks in its monotonic increase at the different structural changes up to 38 GPa. We also found that Sm L3-edge does not show distinctive behaviors either at 300 K or 20 K up about 35 GPa but bonds contraction undergoes a critical distortion at intermediate pressures. Our findings suggest that the white line pressure dependent anomaly may be used as a reliable alternative for delineating pressure phase diagrams in RNiO3 (R=Rare Earth).