Date of Award

2026-08-01

Degree Name

Master of Science

Department

Physics

Advisor(s)

Harikrishnan Nair

Abstract

Na2Ni2TeO6 is a layered honeycomb antiferromagnet in which Ni2+ ions form magnetic honeycomb planesseparated by sodium layers. In this work, four compositionally complex oxides derived from Na2Ni2TeO6 were synthesized to test whether the NNTO-type structure remains stable when 50% of the Ni sublattice is replaced by mixtures of Cu, Zn, Fe, and Co. Powder X-ray diffraction and neutron powder diffraction show that all four samples retain the hexagonal P 63/mcm average structure of the parent NNTO phase, demonstrating that the layered honeycomb framework is stable against substantial multiple-cation substitution. A small CoFe2O4 secondary phase is present in all four samples and was included in the diffraction refinements. X-ray photoelectron spec- troscopy performed on CCO I supports predominantly divalent transition-metal ions, consistent with the nominal Na2X2TeO6 charge-balance model. Magnetic susceptibility measurements show that all four CCO samples retain predominantly antiferro-magnetic interactions but have lower transition temperatures than parent NNTO. The zero-field-cooled and field-cooled susceptibility data show magnetic irreversibility and broad high-temperature features, suggesting strong spin correlations above the ordering temperature, possibly associated with antiferromagnetic clusters or short-range correlated regions. Specific heat confirms magnetic anomalies in all four samples and shows reduced magnetic entropy compared with the theoretical spin-only values. Low-temperature neutron diffraction reveals magnetic scattering whose intensity increases with Co concentration. Overall, these results show that the NNTO structure is robust to 50% multiple-cation substitution, while the magnetic ordering is suppressed and redistributed by chemical disorder on the honeycomb transition-metal sublattice.

Language

en

Provenance

Received from ProQuest

File Size

86 p.

File Format

application/pdf

Rights Holder

Omar Salas

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