The Role of Structural and Compositional Heterogeneities in the Insulator-to-Metal Transition in Hole-Doped APd₃O₄ (A = Ca, Sr)

Abstract

The cubic semiconducting compounds APd 3 O 4 (A = Ca, Sr) can be hole-doped by Na substitution on the A site and driven towards more conducting states. This process has been followed here by a number of experimental techniques in order to understand the evolution of electronic properties. While an insulator-to-metal transition is observed in Ca 1−x Na x Pd 3 O 4 for x ≥ 0.15, bulk metallic behavior is not observed for Sr 1−x Na x Pd 3 O 4 up to x = 0.20. Given the very similar crystal and (calculated) electronic structures of the two materials, the distinct behavior is a matter of interest.We present evidence of local disorder in the A = Sr materials through the analysis of the neutron pair distribution function which is potentially at the heart of the distinct behavior. Solid-state 23 Na nuclear magnetic resonance studies additionally suggest a percolative insulator-to-metal transition mechanism wherein presumably small regions with a signal resembling metallic NaPd 3 O 4 form almost immediately upon Na substitution, and this signal grows monotonically with substitution. Some signatures of increased local disorder and a propensity for Na clustering are seen in the A = Sr compounds.

Electron–phonon coupling in APd3O4: A = Ca, Sr, and Sr0.85Li0.15

“…It is GGA+U calculations which produces small gap for U ~ 4.5 eV (See Figure S2 in supplementary material). Other calculations based on hybrid functionals [23] also produce similar gaps in these systems confirming the role of electron correlation. Due to the small gap in these systems thermally-and/or photo-excited carriers can couple to the phonon degree of freedoms exhibiting the electron-phonon coupling with varying strength due to the temperature and laser power variations [42,43].…”

“…Hence, it is very important to understand phonons and their correlation with electronic degrees of freedom in these systems. APd3O4 (A = Ca and Sr) systems may also be suitable for thermoelectric applications such as switching and sensing devices due to their narrow bandgap [17,21,23].…”

Electronic structure of ternary palladates and effect of hole doping: A valence band photoemission spectroscopic study

“…The x-ray diffraction patterns collected at room temperature exhibit single phase samples without any signature of impurity. Cubic lattice constants are found to be 5.74 Å, 5.83 Å and 5.82 Å for CaPd 3 O 4 , SrPd 3 O 4 and Sr 0.85 Li 0.15 Pd 3 O 4 respectively which are in well agreement with earlier reports [8,9,12]. Temperature dependent x-ray diffraction and Raman spectroscopic experiments were also performed and will be reported elsewhere [13].…”

“…Narrow band gap semiconducting ternary palladates have attracted significant attention in recent past due to observation of electron/hole doped IMT [8,9] and for the possibility of being an excitonic insulator [10]. A recent theoretical investigation suggests Dirac semimetallic ground state having three sets of Weyl nodes in the vicinity of Fermi edge in these palladium oxides [11].…”

Fermi-level Dirac crossings in 4d and 5d cubic metal oxides: NaPd3O4Teicher1, Lamontagne2, Schoop3 et al. 2019Phys. Rev. BSelf Cite80110View full textAdd to dashboardCite

“…The compounds (Ca/Sr)M 3 O 4 have previously been suggested to be topological semimetals, but these compounds are actually insulating both at appropriate levels of electronic structure theory as well as in experimental studies. 18,32 We present here a detailed electronic structure description of the two compounds NaPd 3 O 4 and NaPt 3 O 4 using a combination of plane-wave pseudopotential and planewave local orbital density functional theory calculations in tandem with tight-binding models to identify Dirac crossings near the Fermi level that appear to form a nodal cube, not unlike the nodal-sphere semimetallic state proposed previously by Wang et al 11 Spin-orbit coupling fragments this nodal cubic state into 14 protected Dirac crossings in both the Pd and Pt compounds. This work points to the potential of complex oxides to enrich the domain of Dirac and related quantum materials and supports the view that there may be a role for previously ignored features in the electronic structure playing a role in the catalytic properties of compounds of the platinum group metals.…”