Dicalcium ruthenate
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| Identifiers | |
|---|---|
3D model (JSmol) |
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| Properties | |
| Ca2O4Ru | |
| Molar mass | 245.22 g·mol−1 |
| Related compounds | |
Other cations |
Distrontium ruthenate |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Dicalcium ruthenate, commonly referred to as calcium ruthenate with the chemical formula Ca2RuO4, is a stoichiometric oxide compound that hosts a multi-orbital (band) Mott insulating ground state as it exhibits strong coupling between lattice, spin and orbital degrees of freedom.[1] For this reason, Ca2RuO4 serves as an important "meeting-point" between conceptual developments[2][3] of strongly correlated multi-band physics and advanced experimental spectroscopies.[4][5] Its electronic structure and also orbital magnetism are therefore subjects of experimental and theoretical scrutiny. Ca2RuO4 belongs to the Ruddlesden–Popper family of layered perovskites (n = 1), consisting of RuO6 octahedral sheets separated by rock-salt CaO layers.[6]
Ca2RuO4 undergoes a first-order metal–insulator transition near 357 K that coincides with a structural change from a metallic long-c (L-Pbca) to an insulating short-c (S-Pbca) orthorhombic phase. The transition features an abrupt distortion of the RuO6 octahedra, where the in-plane Ru–O bonds lengthen and the apical bond shortens, producing a flattened octahedron with an enhanced tilt.[7] "Pbca" refers to an orthorhombic space group (No. 61).[8]
The crystal structure of Ca2RuO4 is a strongly distorted variant of the K2NiF4 structure, with substantially larger rotations and tilts of the RuO6 octahedra than in Sr2RuO4, driven by the smaller size of the Ca ions.[9] The RuO6 octahedra display cooperative tetragonal distortions that transform as the Γ1+ irreducible representation of the I4/mmm phase, along with pronounced rotations (QR, X2+) and tilts (QT, X3+). Together, these distortions lower the symmetry from the high-symmetry tetragonal I4/mmm structure to the orthorhombic Pbca phase observed at room temperature.[10]
The electronic states near the Fermi level in Ca2RuO4 are derived primarily from Ru–O antibonding bands with Ru t2g character (dxy, dxz, dyz), occupied by four electrons per Ru ion. Below approximately 340 K, the material undergoes a first-order transition from a high-temperature metallic phase to a low-temperature insulating phase without a change in crystal symmetry. Instead, the two phases differ in the degree of RuO6 octahedral distortion and in the relative occupancies of the Ru t2g orbitals. In the insulating phase, the lower-energy dxy orbital is fully occupied, while the higher-energy dxz and dyz orbitals are each half-filled. In contrast, in the metallic phase the three t2g orbitals have approximately equal occupancies of about 4/3 electrons per orbital.[11]
Negative thermal expansion has also been reported in conjunction with this c-axis compression.[12] The metal-insulator transition is sensitive to electrical current.[13][14] Ca2RuO4 exhibits a current-induced metal-insulator transition in which the application of sufficiently high current densities drives a reversible transition to a low-resistance state, accompanied by hysteretic resistive switching. In epitaxial thin films, this transition is highly stable and tunable, with the threshold current and transition temperature controlled by the applied current amplitude rather than Joule heating.[15]
Ca2RuO4 is an ordinary paramagnetic metal above the transition temperature. Below 110-115 K, it develops a long-range anti-ferromagnetic ordering.[16] The magnetic structure has B-centered magnetic order similar to La2NiO4.The easy axis for magnetization is parallel to the a or b axis in the Ru-O plane.[17] Spin direction aligns with the octahedral tilt axis, not with bond elongation, highlighting strong magneto-elastic and spin–orbit coupling.[7]