Mechanism of anisotropic thermal expansion phenomena in Ca₂RuO₄

Thermal strains in functional materials can seriously impede their device applications. Negative thermal expansion (NTE), a rare and interesting phenomenon observed in certain materials which contract upon heating, can be effectively harnessed to this end. Layered ruthenate Ca₂RuO₄ is one such material exhibiting giant NTE within a 200 K temperature window with a value of α_L ~ -115 X 10^-6 /K of the linear coefficient of thermal expansion¹. Our recent observations also lead to the possibility of tuning the thermal expansion of this system from a negative to a positive value by varying its oxygen content and by chemical substitutions^1,2. Ca₂RuO₄ undergoes structural phase transition as a function of temperature with the metal-insulator transition below 357 K. Also formation of orbital and antiferromagnetic ordered states was observed below 260 K and 110 K, respectively. Here, I will elaborate upon the couplings between various order parameters and their impact on the thermal expansion properties of this system. I will discuss the electronic and phononic contributions to the thermal expansion properties of Ca₂RuO₄ and related systems. Our findings will expectedly contribute to the development of methods of systematic control of the thermal expansion properties of Ca₂RuO₄ and to the search for new NTE materials within layered K₂NiF₄ lattice framework.



[1] K. Takenaka et al., Nat. Commun. 8, 14102 (2017).

[2] L. Hu et al., Chem. Mater. 33 (19), 7665-7674 (2021).