Negative thermal expansion of Ca₂RuO₄ with oxygen vacancies*

Abstract

Oxygen vacancies have a profound effect on the magnetic, electronic, and transport properties of transition metal oxides but little is known about their effect on thermal expansion. Herein we report the effect of oxygen defects on the structure formation and thermal expansion properties of the layered perovskite Ca2RuO4 (CRO). It is shown that the CRO containing excess oxygen crystallizes in a metallic L-CRO phase without structure transition from 100 K to 500 K and displays a normal thermal expansion behavior, whereas those with oxygen vacancies adopt at room temperature an insulating S-CRO phase and exhibit an enormous negative thermal expansion (NTE) from 100 K to about 360 K, from where they undergo a structure transition to a high temperature metallic L-CRO phase. Compared to the L-CRO containing excess oxygen, the S-CRO structure has increasingly large orthorhombic strain and distinctive in-plane distortion upon cooling. The in-plane distortion of the RuO6 octahedra reaches a maximum across 260 K and then relaxes monotonically, providing a structure evidence for the appearance of an antiferromagnetic orbital ordering in the paramagnetic phase and the Ag phonon mode suppression and phase flip across the same temperature found recently. Both the L- and S-CRO display an antiferromagnetic ordering at about 150–110 K, with ferromagnetic ordering components at lower temperature. The NTE in S-CRO is a result of a complex interplay among the spin, orbital, and lattice.

Improving the Thermal Expansion and Capacitance Properties of MoO3 by Introducing Oxygen Vacancies

“…7 Nevertheless, a few studies about the effects of oxygen vacancies on the thermal expansion property have been reported. 4,7,8 Because the figure of merit of thermal shock resistance of a material is inversely proportional to the coefficients of thermal expansion (CTE), 9−11 reducing or tailoring the CTE is of potential importance for a variety of device applications.…”

“…Oxygen vacancies have a profound effect on the electric, magnetic, and optical properties of transition-metal oxide materials and play a vital role in metal-insulating transition, , magnetoresistance, absorption, energy storage of supercapacitors, , and even in the structure phase formation . Nevertheless, a few studies about the effects of oxygen vacancies on the thermal expansion property have been reported. ,, Because the figure of merit of thermal shock resistance of a material is inversely proportional to the coefficients of thermal expansion (CTE), − reducing or tailoring the CTE is of potential importance for a variety of device applications.…”

Origin and Absence of Giant Negative Thermal Expansion in Reduced and Oxidized Ca2RuO4

“…The existence of interstitial oxygen ( O i ) seems to play a critical role in the absence of NTE. The excess oxygen was discussed in the previous study, but its effect on octahedral deformation and orbital ordering still needs to be resolved. To investigate the effect of interstitial oxygens, theoretical calculations were performed on different geometries, including the experimental structure of O_CRO at 100 K (denoted as the initial structure, Figure a), the fully relaxed structure (Figure b), and the oxygen-insertion structure (Figure c).…”

“…The existence of interstitial oxygen (O i ) seems to play a critical role in the absence of NTE. The excess oxygen was discussed in the previous study, 45 but its effect on octahedral deformation and orbital ordering still needs to be resolved.…”

Understanding Negative Thermal Expansion of Zn2GeO4 through Local Structure and Vibrational Dynamics

“…Since the discovery of isotropic NTE of ZrW 2 O 8 in a wide temperature range (0.3–1050 K), much effort has been devoted to understanding the mechanisms of NTE and discovering novel NTE materials. , A full understanding of NTE mechanisms is still challenging since the NTE behavior can be driven by different mechanisms in different categories of materials, , such as low-frequency phonon in framework metal oxides, − cyanides , and fluorides; , magneto volume effects in manganese nitrides , and alloys; , spontaneous polarization in PbTiO 3 -based perovskites; or charge transfer phenomena in perovskite oxides BiNiO 3 . Interesting is the case of Ca 2 RuO 4 , where NTE occurs only in the samples with oxygen vacancies, , and a complex interplay between orbital, spin, and lattice degrees of freedom takes place.…”