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

Abstract

Negative thermal expansion (NTE) is an intriguing physical phenomenon. Layered Ca2RuO4 exhibits giant NTE over a wide temperature range from 200 to 400 K, which makes it attractive for fundamental research and industrial applications. However, a clear physical understanding is lacking for the appearance of NTE over such a wide temperature range and the oxygen-content-dependent switch from NTE to positive thermal expansion (PTE). Herein, we present insights into the average crystal structure, local structure, and electronic and orbital states of Ca2RuO4. Surprisingly, a previously overlooked monoclinic distortion is identified by electron diffraction and synchrotron X-ray diffraction (SXRD). X-ray absorption fine structure (XAFS) and synchrotron X-ray pair distribution function (PDF) analyses show large local distortions in monoclinic Ca2RuO4. Moreover, local stress on Ru cations is confirmed by the existence of over-bonding states, which relaxes along with NTE. Theoretical calculations indicate that dxy orbital ordering and disordering in the monoclinic structure are the origins of NTE. Moreover, interstitial oxygen plays a critical role in stabilizing elongated RuO6 and locally breaks the dxy orbital ordering, facilitating the occurrence of PTE. This work elucidates the electronic and orbital states in NTE materials with defective lattices and provides a different route to designing unconventional NTE materials.

Chemical Diversity for Tailoring Negative Thermal Expansion

“…Besides, the chemical substitution of Ru with Sn in Ca 2 RuO 4 ceramic pellet could extend the NTE window up to a higher temperature, such as 700 K. 294 In order to explore the microscopic origin of the oxygen-content-dependent switch in Ca 2 RuO 4 , Hu et al provided a comprehensive physical scenario with insight from the average crystal structure, local structure, and orbital-related calculations. 295 Indeed, a monoclinic structural distortion can be identified in Ca 2 RuO 4 , which exhibits NTE behavior (Figure 80b). With the increase of the interstitial oxygen, the appearance of the orthorhombic structure in the oxidized Ca 2 RuO 4 becomes the stabilizer of the elongated RuO 6 , which breaks the orbital ordering of d xy and results in the PTE in Ca 2 RuO 4.05 .…”

“…And the lower inset shows the reversibility of linear thermal expansion of Ca 2 RuO 3.74 . (b) The temperature dependence of ΔV of reduced Ca 2 RuO 4 and oxidized Ca 2 RuO 4.05 . Reproduced with permission from refs and .…”

Interplay between Oxygen Octahedral Rotation and Deformation in the Acentric ARTiO4 Series toward Negative Thermal Expansion

“…Mechanisms of NTE identified in octahedral frameworks so far can roughly fall into five categories in terms of the origin: (1) rigid unit modes as in ScF 3 , (2) intermetallic charge transfer as in BiNiO 3 , (3) magnetovolume effect as in Mn 3 Cu 1– x Ge x N, (4) spontaneous polarization as in PbTiO 3 -based composites, and (5) orbital ordering as in Ca 2 RuO 4 . , The first mechanism involves lattice vibrations that alter the connectivity of the octahedral units, whereas the second and third ones are related to size-altering distortions. The fourth and fifth mechanisms lead to volumetric NTE by changing the shape of coordination octahedra.…”

Negative Thermal Expansion in Fluoroapatite Pb5(VO4)3F Enhanced by the Steric Effect of Pb2+

“…In recent decades, several mechanisms and materials for NTE have been discovered using electronic phase transitions that are accompanied by substantial volume shrinkage. The origins of the NTE properties vary from some physical properties, including complicated correlations among electron, spin, and phonon degrees of freedom, such as the magnetovolume effect in Mn 3 GaN, La­(Fe,Co,Si) 13 , Hf 1– x Ta x Fe 2 , and CuO, large NTE enhanced by a microstructural effect for the metal–insulator transition in Ca 2 RuO 4 -related materials, charge order in V 2 OPO 4 , and interatomic charge transfer in BiNiO 3 -based materials . Ashby-type plots proposed by Coates and Goodwin showed that phonon-mediated NTE is favorable over a wide range of operating temperatures, but its magnitude tends to be relatively small, while NTE due to electronic transitions exhibits a large thermal expansion coefficient, but its temperature range is limited .…”