Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates

We present a detailed investigation of the specific heat of Ca3（Ru1-xMx）2O7 （M = Ti, Fe, Mn） single crystals. Depending on the dopant and doping level, three distinct regions are present： a quasi- two-dimensional metallic state with antiferromagnetic （AFM） order formed by ferromagnetic bilayers （AFM-b）, a Mott insulating state with G-type AFM order （G-AFM）, and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mort transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T2, in the localized region. The C ∝ T2 term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.

Structure and chemical valence study of Sr_(n+1)Ru_nO_(3n+1)(n= 1, 2, ∞) series

Effect of structure parameter n and its coupling with the connection mode among RuO6 octahedra of Srn＋1RUnO3n＋1 （n = 1, 2, ∞） are investigated. The gradually enhanced rotation and tilting effect with increasing n are observed in Srn＋1RUnO3n＋1. Besides, the chemical valence of Ru is not changed, while the one of Sr gradually varies with increasing n, which highlights the great contribution of connection mode to the chemical environment. Our results show a strong n dependence on the connection mode between octahedra in Srn＋1RUnO3n＋1 （n = 1, 2, ∞）.

CaCo_1-xRu_xO_y: Role of Ru/Co Ratio on Its Transport Properties

Calcium cobaltites, especially Ca3Co4O9 with a misfit layered structure, are promising thermoelectric materials due to their suitability for high temperature applications and low densities. The existence of low spin-state electronic configurations for both and species is one of the key parameter to explain the large thermopower values. Ruthenium oxide, with a layered structure, exhibits strong electron-electron correlation and the extended nature of their 4d electrons enhances orbital overlapping which is expected to influence the transport characteristics of CaCo1-xRuxOy (CCR) samples, by affecting the spin state of the 3d Co ions. The effect on thermopower and electrical resistivity due to partial substitution of Co by Ru ions, up to 0.33 moles, from 300 to 600 K was investigated. A sharp decline in resistivity and in thermopower was observed until a transition ion ratio (TIR), (Ru/(Ru + Co)), of 0.5 is reached, beyond which both the properties became less sensitive to TIR. These variations in the transport properties are explained by the presence of 4d Ru in close proximity to the Co, which could influence the spin and oxidation state of Co ions. The Co rich and Ru rich samples exhibit very distinct microstructures and phase assemblages.

Role of A-sites in pyrochlore lanthanide ruthenate for electrocatalysis of oxygen evolution reaction

Developing highly stable and efficient catalysts for oxygen evolution reaction(OER)is extremely important to sustainable energy conversion and storage,but improved efficiency is largely hindered by sluggish reaction kinetics.Dense and bimetal ruthenates have emerged as one of the promising substitutes to replace single-metal ruthenium or iridium oxides,but the fundamental understanding the role of A-site cations is still blurring.Herein,a family of lanthanides(Ln=all the lanthanides except Pm)are applied to synthesize pyrochlore lanthanide ruthenates(Ln_(2)Ru_(2)O_(7)),and only Ln_(2)Ru_(2)O_(7)(Ln=Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,or Lu)with pure phase can be obtained by the ambient-pressure calcination.Compared with the perovskite ruthenates(SrRuO_(3))and rutile RuO_(2),the[RuO_(6)]units in these Ln_(2)Ru_(2)O_(7) present the largely distorted configurations and different energy level splitting to prevent the excessive Ru oxidation and dissolution,which leads the primary improvement in the electrocatalytic OER performance.In the similar crystalline field split states,the charge transfer between[RuO6]units and Ln^(3+)cations also affect catalytic activities,even in the Ln_(2)Ru_(2)O_(7) surface reconstruction during the OER process.Consequently,Tb_(2)Ru_(2)O_(7) showed the highest OER performance among all the prepared Ln_(2)Ru_(2)O_(7) with similar morphologies and crystallization.This systematic work gives fundamental cognition to rational design of high-performance OER electrocatalysts in proper water electrolysis technologies.

Pressure-Induced Intermetallic Charge Transfer and Semiconductor- Metal Transition in Two-Dimensional AgRuO_(3)

The intricate correlation between charge degrees of freedom and physical properties is a fascinating area of research in solid state chemistry and condensed matter physics.Herein,we report on the pressureinduced successive charge transfer and accompanied resistive evolution in honeycomb layered ruthenate AgRuO_(3).Structural revisiting and spectroscopic analyses affirm the ilmenite type R-3 structure with mixed valence cations as Ag^(+1/+2)Ru^(+4/+5)O_(3) at ambient pressure.In-situ pressure-and temperature-dependent resistance variation reveals a successive insulatormetal-insulator transition upon pressing,accompanied by unprecedented charge transfer between Ag and Ru under applied pressure,and a further structural phase transition in the insulator region at higher pressure.These phenomena are also corroborated by in-situ pressure-dependent Raman spectra,synchrotron X-ray diffraction,bond valence sums,and electronic structure calculations,emphasizing the dominated rare Ag2+,and near zero thermal expansion in the ab-plane in the metallic zone mostly due to the Jahn-Teller effect of d9-Ag2+.The multiple electronic instabilities in AgRuO_(3) may offer new possibilities toward novel and unconventionally physical and chemical behaviors in strongly correlated honeycomb lattices.

Optical spectroscopy study of Ca_3(Ru_(0.91)Mn_(0.09))_2O_7 single crystal in high magnetic fields

The magneto-optical spectrum, with magnetic fields up to 42 T, of double layered ruthenates Ca_3(Ru_(0.91)Mn_(0.09))_2O_7(CRMO) single crystal is studied. Both the temperature and magnetic field induced insulator-to-metal transitions(IMTs) are observed via magneto-optical properties of the crystal. The critical magnetic field(H//c) of IMT for CRMO is found to be as large as 35 T at 5 K. The fine structure of optical spectra identified the antiferromagnetic/ferro-orbital-ordering configurations of Ru 4d orbitals at low temperatures. Meanwhile, the configuration of orbital polarization of such double-layer CRMO single crystal is discussed. These results suggest that the orbital degree of freedom plays an important role in the field induced IMT of multi-orbital system.