Collapse of the charge disproportionation and covalency-driven insulator-metal transition in (formula presented) under pressure

P. Adler, U. Schwarz, K. Syassen, M. P. Pasternak

Research output: Contribution to journalArticlepeer-review

Abstract

The effect of pressure on electronic properties and crystal structure of (Formula presented) was studied up to 45 GPa. Experimental methods employed were (Formula presented) Mössbauer spectroscopy (MS), monochromatic synchrotron powder x-ray diffraction, optical reflectance between 0.6 and 4 eV, and electrical resistance measurements. Mössbauer spectra of the magnetically ordered as well as of the paramagnetic phase demonstrate that the charge disproportionation of (Formula presented) disappears at pressures between 15 and 21 GPa. The diffraction data show that the tetragonal Ruddlesden-Popper-type crystal structure (space group (Formula presented) is retained up to the highest pressure. The optical spectra reveal a continuous increase with pressure of the near-infrared oscillator strength, which indicates a pressure-driven transition from an insulating towards a metallic ground state. This is confirmed by the electrical resistance measurements which evidence a sluggish pressure-induced insulator-metal (IM) transition with a clear incipient metallic state at (Formula presented) GPa. The changes in electronic state are not associated with any detectable anomaly in the pressure dependence of lattice parameters. The high-pressure behavior of (Formula presented) is discussed in terms of a strengthening of the covalent (Formula presented) interactions under pressure. Within the impurity model for the electronic structure of transition metal compounds the IM transition in (Formula presented) can be attributed to the closure of a (Formula presented)-type energy gap. The ambient- and high-pressure properties of (Formula presented) and related (Formula presented) oxides are compared.

Original languageEnglish
Pages (from-to)4609-4617
Number of pages9
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume60
Issue number7
DOIs
StatePublished - 1999

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