Effect Of Oxygen Vacancy On The Structure And Thermal Expansion Properties Of Ca₂RuO_4+?

In recent years,the negative thermal expansion?NTE?of materials has received considerable attention due not only to its curious physics and chemistry behind but also to its potential implications for a variety of applications to meet rapid advances in modern technology.In this work,we focus on the thermal expansion of Ca₂Ru O₄?CRO?by oxygen vacancy in the layered compound CRO.According to the studies on structural analyses,we have achieved new insight to the NTE mechanism;NTE property is controlled by controlling the existence of oxygen vacancy,and it is revealed that the influence of oxygen vacancy on the electrical and magnetic properties of materials.This work provides not only valuable information on effect of the types of oxygen defects on the structural formation,evolution and correlated changes in physical properties of CRO,but also a stage for future experimental and theoretical investigations to settle the debating on the nature of the Mott metal-insulator transition based on renewed models taking the oxygen vacancies into account.The main contents of this paper are as follows:?1?The samples with different concentration of oxygen vacancy were prepared by solid state reaction method,and the coefficient of thermal expansion?CTE?of CRO could be adjusted from positive to negative.We have studied the crystal cell structure of CRO by synchrotr on X-ray diffraction?SXRD?,X-ray diffraction,and X-ray photoelectr on spectroscopy?XPS?.The experimental results show that oxygen vacancy affects two types of CRO structure,one is S-Pbca?c?11.94??with a short c axis,the other is L-Pbca?c?12.35??with a long c axis,thus affecting the evolution of CRO structure,thermal expansion and electrical transport.Through the structural refinement of the variable temperature XRD data,it is found that only the S-Pbca structure at 150-350 K shows the NTE property,and at 350-500 K,it changes into the high temperature L-Pbca phase,and the NTE property disappears.The obvious feature of the transition from low-temperature S-Pbca to high-temperature L-Pbca is that the lattice constant of c axis increases obviously near 350 K,which is due to the release of orthogonal strain.The excess oxygen in the interstitials will oversaturate the Ca coordination and lead to a charge transfer towards the Ru-O planes which requires oxidation of Ru higher than 4+as evidenced by the XPS analysis.It consequently results in smaller Ru-O?1?and larger Ru-?O2?equilibrium distances and a relaxation of the strain implied by the misfit of smaller radius of Ca in the structure.At the same time,it also modulates the orbital polarization and the interlayer van der Waals interactions.?2?A_gmode oscillations result in x/y-anisotropic octahedral distortions.An increase in bond angle may facilitate superexchange interaction and favor the antiferromagnetic orbital ordering.It is the oxygen content that governs the crystallization of CRO either in the S-Pbca or the L-Pbca phase.When T=250 K,both the oxygen vacancy sample?R-CRO?and the peroxide sample?O-CRO?had the largest structural distortion.Electrical properties experiments show thatthe presence of oxygen vacancies regionalizes electron spin in Ru 4d,hindering electron transport and thus increasing the resistivity.Moreover,The presence of oxygen vacancies reduces Ru⁴⁺partially to Ru³⁺which makes the Ru4d orbital become half full and changes the overlapping and hybridization degree between Ru 4d and O2p orbitals,leading to the weakening of the coupling strength between Ru^m+?m=3,4?and O2p spin orbitals.The negative thermal expansion of CRO is caused by the changes of bond length and bond angle and magnetic phase transition in Ru O₆octahedron.