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

Tailoring size, shape, and connectivity of oxygen coordination octahedra in perovskite-related oxides is known to play a key role in engineering their material properties, and furthermore, the interplay among the different types of oxygen octahedral distortions can open up new strategies for structure-driven control of functionalities. Here, we report that in layered perovskites AgRTiO4 (R: rare earth), the biaxial negative thermal expansion (NTE) arises from the interplay between the structural distortions that alter the octahedral connectivity and shape, which is a fundamentally different mechanism from those in the conventional NTE materials. AgRTiO4 was previously identified as having an orthorhombic (Pbcm) structure, but our experimental and theoretical study reveals that this compound adopts an acentric tetragonal (P4̅21m) structure due to (Φ00)(0Φ0)-type TiO6 octahedral rotations, as in the previously reported Na and K analogs, NaRTiO4 and KRTiO4. Thorough structural analysis reveals that the competition of the octahedral rotations with octahedral deformations drives the biaxial NTE; the decrease in the rotation amplitude caused by heating results in octahedral deformations, i.e., an out-of-plane elongation and an in-plane compression of TiO6 octahedra, leading to shrinkage of the lattice parameters a and b (a = b). The Ag+-R3+ layered ordering produces a built-in electric field compelling Ti4+ to off center, which is the source for the otherwise unfavorable coexistence of octahedral rotations and deformations. Despite the competition between the two octahedral distortions being predicted to be active in other members of the ARTiO4 series (A = Na, K, and Rb) as well, we do not observe experimentally the biaxial NTE for the Na members. Detailed analysis of calculated electronic structures highlights the essential role played by Ag–O–Ti covalent bonding in enhancing the octahedral deformation of AgRTiO4, which is directly responsible for the biaxial NTE. The present study provides an important example of functional properties that emerge from the coupling among distinct distortions of octahedral frameworks.