Crystal structure and incommensurate magnetic structrue in geometrically frustrated fcc lattice of double perovskite Hg₂MnTeO₆

Wei-Tin Chen^1,2,3*, En-Pei Liu^1,4, Chin-Wei Wang⁵, Angel Arevalo-Lopéz⁶, Chien-Lung Huang^7,2,8

¹Center for Condensed Matter Sciences (CCMS), National Taiwan University, Taipei, Taiwan
²Center of Atomic Initiative for New Materials (AI-Mat), National Taiwan University, Taipei, Taiwan
³Taiwan Consortium of Emergent Crystalline Materials (TCECM), National Science and Technology Council, Taipei, Taiwan
⁴Department of Physics, National Taiwan University, Taipei, Taiwan
⁵National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁶Unité de Catalyse et Chimie du Solide (UCCS), Université de Lille, Centrale Lille, France
⁷Department of Physics, National Cheng Kung University, Tainan, Taiwan
⁸Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan, Taiwan

* Presenter:Wei-Tin Chen, email:weitinchen@ntu.edu.tw

Ordered double perovskites (dPv) A₂BB’O₆ were extensively investigated due to the observed magneto-electrical properties, such as room temperature magnetoresistance in half-metallic Sr₂FeMoO₆ oxide.^[1] When replacing one of the B cations with non-magnetic cation, low temperatures spin glass states were reported possibly due to geometric spin frustrated B-sublattice in A₂MnBO₆ dPv (A = Ca and Sr, B = non-magnetic Sb, Ta),^[2] while antiferromagnetic behaviours were reported in A₂MnTeO₆ (A = Ca and Sr, B = non-magnetic Te).^[3] For Ba₂MnB'O₆ (B' = non-magnetic Te or W), although antiferromagnetic behaviours were observed with respectively T_N = 20 and 8 K, different spin arrangements were shown in these materials.^[4]

In order to further investigate the magnetic interaction in the geometrically frustrated fcc lattice of double perovskite systems, we have designed and successfully prepared novel mercury containing B-site ordered dPv Hg₂MnTeO₆ (HMTO), Ca₂MnTeO₆ (CMTO), and also (Hg/Ca)₂MnTeO₆ (HCMTO) solid solution with high-pressure high-temperature (HPHT) synthesis techniques. The obtained polycrystalline samples were characterized with ultrahigh resolution synchrotron XRD (TPS19A, NSRRC). The materials were evident to be single phases with space group R-3 and P2₁/n respectively for Hg and Ca dPv analogues, where transition metal Mn and non-magnetic Te cations are rock-salt ordered. Antiferromagnetic behaviours were observed for HMTO and CMTO from magnetic susceptibility measurements with T_N = 11 and 10 K, respectively. Neutron powder diffraction (WOMBAT and ECHIDNA, ANSTO) were carried out for HMTO and CMTO. Despite the difference in crystal structures, comparable antiferromagnetic structures were derived in both HMTO and CMTO below T_N. Surprisingly, a commensurate-to-incommensurate magnetic phase transition was observed in HMTO down to based temperature. The crystal structures, magnetic behaviours, and magnetic interaction will be discussed in this presentation.

[1] K. I. Kobayashi et. al. Nature, 395, 677 (1998)
[2] T. K. Mandal et. al. J. Solid State Chem., 181, 2325 (2008)
[3] L. O.-S. Martin et. al. Eur. J. Inorg. Chem., 1362 (2006)
[4] O. H. J. Mustonen et. al. Chem. Mater., 32, 7070 (2020)

Keywords: incommensurate magnetic structure, magnetic frustration, neutron diffraction, high pressure synthesis, symmetry-mode analysis