Dicalcium ruthenate

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Calcium Ruthenate
Dicalcium Ruthenate.png
Identifiers
3D model (JSmol)
  • InChI=1S/2Ca.O4Ru/c;;1-5(2,3)4/q2*+2;-4
  • Key: JWAHTWBBWFKEMH-UHFFFAOYSA-N
  • [Ca+2].[Ca+2].[O-][Ru]([O-])([O-])[O-]
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).

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]

Contents

Electronic, structural, and magnetic properties

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]

Epitaxial thin films

Distortions of the RuO6 octahedra are a key factor in determining the electronic ground state of Ca2RuO4. In bulk materials, hydrostatic and uniaxial pressure can modify these distortions and induce a transition to a metallic phase with low-temperature ferromagnetism but this would be incompatible with oxide electronics. In thin films, epitaxial strain provides an alternative mechanism for tuning the lattice structure, leading to a change in the electronic and magnetic properties. [18] It has been grown on various substrates using techniques like molecular beam epitaxy [19] and pulsed laser deposition. [20] Ca2RuO4 thin films grown on substrates including LaAlO3 (100), LaSrAlO4 (001), and NdCaAlO4 (110) are subject to distinct epitaxial strain states that significantly modify their lattice constants and RuO6 octahedral geometry. X-ray diffraction and reciprocal-space mapping indicate that films deposited on LaAlO3 and LaSrAlO4 remain fully coherent with the substrate, while those grown on NdCaAlO4 accommodate strain only partially within the film plane. Despite these differences, all films preserve the orthorhombic Pbca-type structure of bulk Ca2RuO4, exhibiting characteristic rotations and tilts of the RuO6 octahedra. [18]

Epitaxial strain strongly influences the transport properties of Ca2RuO4 thin films. Under compressive biaxial strain, particularly on LaAlO3 and LaSrAlO4 substrates, the films display metallic behavior over a wide temperature range, in contrast to the Mott-insulating ground state of bulk Ca2RuO4. Films on [LaAlO3 retain a metal–insulator transition near room temperature, though with a reduced resistivity jump and pronounced hysteresis, consistent with partial substrate clamping, while films on LaSrAlO4 remain metallic from low temperatures to at least 400 K. In comparison, films grown on NdCaAlO4 remain insulating over the full measured temperature range. [18]

Recent work on LaAlO3 substrate grown films show the emergence of a periodic nano-texture of the insulating and metallic phases akin to ferroelectric domains below the transition temperature as seen by synchrotron X-ray imaging and confirmed by cryo-TEM. [21] [22]

As Sr replaces Ca in Ca
2-xSr
xRuO
4, the strong RuO6 octahedral tilt and flattening are progressively reduced, which suppresses the Mott insulating S-Pbca phase and drives the system metallic. For x ≳ 0.15–0.2 the metal–insulator transition disappears, and long-range antiferromagnetism vanishes. At higher Sr content, only rotational distortions remain (I41/acd symmetry), producing a correlated metal. [7] Ca1.8Sr0.2RuO4 has been proposed as a candidate system for orbital selective Mott physics. [23] The bilayer compound Ca3Ru2O7 is metallic, but display a sequence of electronic transitions below 60 K. Finally, Sr2RuO4 hosts an unconventional superconducting state. [24]

References

  1. ↑ Cuono, Giuseppe; Forte, Filomena; Romano, Alfonso; Noce, Canio (2025-02-03). "Emerging new phases in correlated Mott insulator Ca 2 RuO 4". Journal of Physics: Condensed Matter. 37 (5): 053002. arXiv: 2411.16472 . doi:10.1088/1361-648X/ad906d. ISSN   0953-8984.
  2. ↑ Gorelov, E.; Karolak, M.; Wehling, T. O.; Lechermann, F.; Lichtenstein, A. I.; Pavarini, E. (2010-06-01). "Nature of the Mott Transition in Ca2RuO4". Physical Review Letters. 104 (22) 226401. arXiv: 1001.4705 . doi: 10.1103/PhysRevLett.104.226401 . ISSN   0031-9007. PMID   20867184.
  3. ↑ Han, Qiang; Millis, Andrew (2018-08-08). "Lattice Energetics and Correlation-Driven Metal-Insulator Transitions: The Case of Ca2RuO4". Physical Review Letters. 121 (6) 067601. arXiv: 1801.06215 . doi: 10.1103/PhysRevLett.121.067601 . ISSN   0031-9007. PMID   30141680.
  4. ↑ Jain, A.; Krautloher, M.; Porras, J.; Ryu, G. H.; Chen, D. P.; et al. (2017-03-27). "Higgs mode and its decay in a two-dimensional antiferromagnet". Nature Physics. 13 (7). Springer Science and Business Media LLC: 633–637. arXiv: 1705.00222 . Bibcode:2017NatPh..13..633J. doi: 10.1038/nphys4077 . ISSN   1745-2473.
  5. ↑ Sutter, D.; Fatuzzo, C. G.; Moser, S.; Kim, M.; Fittipaldi, R.; et al. (2017-05-05). "Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4" (PDF). Nature Communications. 8 (1) 15176. Springer Science and Business Media LLC. doi: 10.1038/ncomms15176 . ISSN   2041-1723. PMC   5424259 . PMID   28474681.
  6. ↑ González, J. W.; León, A. M.; González-Fuentes, C.; Gallardo, R. A. (2025-02-20). "Altermagnetism in two-dimensional Ca2RuO4 perovskite" . Nanoscale. 17 (8): 4796–4807. doi:10.1039/D4NR04053H. ISSN   2040-3372.
  7. 1 2 3 Friedt, O.; Braden, M.; André, G.; Adelmann, P.; Nakatsuji, S.; Maeno, Y. (2001-04-12). "Structural and magnetic aspects of the metal-insulator transition in Ca2 − xSrxRuO4". Physical Review B. 63 (17). arXiv: cond-mat/0007218 . doi:10.1103/PhysRevB.63.174432. ISSN   0163-1829.
  8. ↑ "Space Group 61: Pbca; P b c a". img.chem.ucl.ac.uk. Retrieved 2025-12-08.
  9. ↑ Alexander, C. S.; Cao, G.; Dobrosavljevic, V.; McCall, S.; Crow, J. E.; Lochner, E.; Guertin, R. P. (1999-09-15). "Destruction of the Mott insulating ground state of Ca 2 RuO 4 by a structural transition". Physical Review B. 60 (12): R8422 –R8425. doi:10.1103/PhysRevB.60.R8422. ISSN   0163-1829.
  10. ↑ Das, Hena; Azuma, Masaki (2025-11-21). "Mechanistic insights into colossal negative thermal expansion in Ca2RuO4 mott insulator via computational modeling approaches". npj Computational Materials. 11 (1): 356. doi:10.1038/s41524-025-01839-4. ISSN   2057-3960.
  11. ↑ Georgescu, Alexandru B.; Millis, Andrew J. (2022-05-31). "Quantifying the role of the lattice in metal–insulator phase transitions". Communications Physics. 5 (1): 135. doi:10.1038/s42005-022-00909-z. ISSN   2399-3650.
  12. ↑ Takenaka, Koshi; Okamoto, Yoshihiko; Shinoda, Tsubasa; Katayama, Naoyuki; Sakai, Yuki (2017-01-10). "Colossal negative thermal expansion in reduced layered ruthenate" (PDF). Nature Communications. 8 (1) 14102. Bibcode:2017NatCo...814102T. doi: 10.1038/ncomms14102 . ISSN   2041-1723. PMC   5234094 . PMID   28071647.
  13. ↑ Curcio, Davide; Sanders, Charlotte E.; Chikina, Alla; Lund, Henriette E.; Bianchi, Marco; et al. (2023-10-16). "Current-driven insulator-to-metal transition without Mott breakdown in Ca 2 RuO 4". Physical Review B. 108 (16). arXiv: 2303.00662 . doi: 10.1103/PhysRevB.108.L161105 . ISSN   2469-9950.
  14. ↑ Suen, C. T.; Marković, I.; Zonno, M.; Heinsdorf, N.; Zhdanovich, S.; et al. (2024). "Electronic response of a Mott insulator at a current-induced insulator-to-metal transition". Nature Physics. 20 (11): 1757–1763. arXiv: 2308.05803 . Bibcode:2024NatPh..20.1757S. doi: 10.1038/s41567-024-02629-3 . ISSN   1745-2473.
  15. ↑ Tsurumaki-Fukuchi, Atsushi; Tsubaki, Keiji; Katase, Takayoshi; Kamiya, Toshio; Arita, Masashi; Takahashi, Yasuo (2020-06-24). "Stable and Tunable Current-Induced Phase Transition in Epitaxial Thin Films of Ca2RuO4". ACS Applied Materials & Interfaces. 12 (25): 28368–28374. doi:10.1021/acsami.0c05181. ISSN   1944-8244.
  16. ↑ Braden, M.; André, G.; Nakatsuji, S.; Maeno, Y. (1998-07-01). "Crystal and magnetic structure of Ca2RuO4: Magnetoelastic coupling and the metal-insulator transition" . Physical Review B. 58 (2): 847–861. doi:10.1103/PhysRevB.58.847. ISSN   0163-1829.
  17. ↑ Cao, G.; McCall, S.; Shepard, M.; Crow, J. E.; Guertin, R. P. (1997-08-01). "Magnetic and transport properties of single-crystal Ca 2 RuO 4: Relationship to superconducting Sr 2 RuO 4" . Physical Review B. 56 (6): R2916 –R2919. doi:10.1103/PhysRevB.56.R2916. ISSN   0163-1829.
  18. 1 2 3 Dietl, C.; Sinha, S. K.; Christiani, G.; Khaydukov, Y.; Keller, T.; Putzky, D.; Ibrahimkutty, S.; Wochner, P.; Logvenov, G.; van Aken, P. A.; Kim, B. J.; Keimer, B. (2018-01-15). "Tailoring the electronic properties of Ca2RuO4 via epitaxial strain". Applied Physics Letters. 112 (3). doi:10.1063/1.5007680. ISSN   0003-6951.
  19. ↑ Nobukane, Hiroyoshi; Hirose, Koki; Isono, Kakeru; Higashiizumi, Mizuki; Sakoda, Masahito; Takahashi, Korekiyo; Tanda, Satoshi (2025-09-29), Bose glass in Ca$_2$RuO$_4$ nanofilms, arXiv, doi:10.48550/arXiv.2509.24300, arXiv:2509.24300, retrieved 2025-12-14
  20. ↑ Wang, X.; Xin, Y.; Stampe, P. A.; Kennedy, R. J.; Zheng, J. P. (2004-12-20). "Epitaxial thin film growth of Ca2RuO4+δ by pulsed laser deposition". Applied Physics Letters. 85 (25): 6146–6148. doi:10.1063/1.1841451. ISSN   0003-6951.
  21. ↑ Shao, Ziming; Schnitzer, Noah; Ruf, Jacob; Gorobtsov, Oleg Yu.; Dai, Cheng; Goodge, Berit H.; Yang, Tiannan; Nair, Hari; Stoica, Vlad A.; Freeland, John W.; Ruff, Jacob P.; Chen, Long-Qing; Schlom, Darrell G.; Shen, Kyle M.; Kourkoutis, Lena F. (2023-07-11). "Real-space imaging of periodic nanotextures in thin films via phasing of diffraction data". Proceedings of the National Academy of Sciences. 120 (28) e2303312120. doi:10.1073/pnas.2303312120. PMC   10334741 . PMID   37410867.
  22. ↑ Gorobtsov, Oleg Yu.; Miao, Ludi; Shao, Ziming; Tan, Yueze; Schnitzer, Noah; Goodge, Berit Hansen; Ruf, Jacob; Weinstock, Daniel; Cherukara, Mathew; Holt, Martin Victor; Nair, Hari; Chen, Long-Qing; Kourkoutis, Lena Fitting; Schlom, Darrell G.; Shen, Kyle M. (2024). "Spontaneous Supercrystal Formation During a Strain-Engineered Metal–Insulator Transition". Advanced Materials. 36 (32) 2403873. doi:10.1002/adma.202403873. ISSN   1521-4095.
  23. ↑ Koga, Akihisa; Kawakami, Norio; Rice, T. M.; Sigrist, Manfred (2004-05-28). "Orbital-Selective Mott Transitions in the Degenerate Hubbard Model". Physical Review Letters. 92 (21) 216402. arXiv: cond-mat/0401223 . Bibcode:2004PhRvL..92u6402K. doi: 10.1103/PhysRevLett.92.216402 . ISSN   0031-9007. PMID   15245300.
  24. ↑ Mackenzie, Andrew Peter; Maeno, Yoshiteru (2003-05-01). "The superconductivity of Sr 2 RuO 4 and the physics of spin-triplet pairing". Reviews of Modern Physics. 75 (2): 657–712. doi:10.1103/RevModPhys.75.657. ISSN   0034-6861.
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