Widely spaced planes of magnetic dimers in the ${\mathrm{Ba}}_{6}{\mathrm{Y}}_{2}{\mathrm{Rh}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{17\ensuremath{-}\ensuremath{\delta}}$ hexagonal perovskite

Widely spaced planes of magnetic dimers in the ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ hexagonal perovskite

Loi T. Nguyen, Daniel B. Straus, Q. Zhang, and R. J. Cava

Phys. Rev. Materials 5, 034419 – Published 31 March 2021

Abstract

We report the synthesis and initial characterization of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ , a previously unreported material, to the best of our knowledge, with a hexagonal symmetry structure. Face-sharing $Rh O_{6}$ octahedra form triangular planes of ${Rh}_{2} O_{9}$ dimers that are widely separated in the perpendicular direction. The material displays a small effective magnetic moment, due to the Rh ions present, and a negative Curie-Weiss temperature. The charge transport and optical band gaps are very similar, near 0.16 eV. A large upturn in the heat capacity at temperatures below 1 K, suppressed by applied magnetic fields larger than $μ_{0} H = 2 T$ , is observed. A large T-linear term in the specific heat ( $γ = 166 mJ / {mol f . u K}^{2}$ ) is seen, although the material is insulating at low temperatures. These results suggest the possibility of a spin liquid ground state in this material.

Received 19 December 2020
Revised 19 February 2021
Accepted 17 March 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.034419

Physics Subject Headings (PhySH)

Magnetic coupling Magnetic interactions Magnetic susceptibility

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Loi T. Nguyen¹, Daniel B. Straus ¹, Q. Zhang², and R. J. Cava¹

¹Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
²Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

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Vol. 5, Iss. 3 — March 2021

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Figure 1
(a) The SEM/EDX analysis of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ confirms the metal elemental ratios. (b) The TGA analysis of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ suggests the presence of oxygen vacancies. (c) and (d) Rietveld powder x-ray diffraction and neutron diffraction corefinement of the crystal structure of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ at 300 K in the space group $P 6_{3} / m m c$ (No. 194). For neutron: $wR = 6.50 %, {R_{F}}^{2} = 7.83 %$ . For x-ray: $wR = 11.18 %, {R_{F}}^{2} = 8.91 %$ .
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Figure 2
(a) Crystal structure of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ . (b) Triangular lattice of Rh magnetic dimers in the $a b$ plane. (c) ${Rh}_{2} O_{9}$ dimer (two face-sharing $Rh O_{6}$ octahedra) with one oxygen site depicted in red and the other orange. There are two different Rh-O bond lengths, 2.00 Å and 1.97 Å. The Rh-Rh distance within the dimer is 2.52 Å.
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Figure 3
(a) Temperature dependent magnetic susceptibility in ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ under an applied field of 2 kOe. Inset: Curie-Weiss fitting from 10 to 50 K results in the effective moment of $0.76 μ_{B} / mol$ Rh and the Curie-Weiss temperature of −2.8 K. ZFC/FC magnetic susceptibility of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ under an applied magnetic field of 100 Oe. The absence of bifurcation between ZFC (black squares) and FC (red circles) rules out the possibility of spin glass behavior and atomic disorder in this material. (b) Magnetization as a function of applied magnetic field from 0 to 9 T at different temperatures. There is no sign of magnetic saturation up to 9 T at temperatures down to 1.8 K.
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Figure 4
(a) Heat capacity divided by temperature for ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ measured under different applied magnetic fields below 15 K. At zero field, the presence of a high density of low energy states is evidenced by the presence of a large upturn that persists down to the lowest temperature of the measurement. The small hump at 1 K may come from the Schottky effect, where the spins are populated from one to another energy level. (b) The universal scaling of heat capacity data collapsed at the critical point [46]. (c) The power law fitting of the raw heat capacity data in ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ results in the power of 0.57. Inset: The existence of heavy quasiparticles in insulating ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ is supported by a very large Sommerfeld constant $[γ = 166 mJ mo {l_{f . u}}^{- 1} K^{- 2}]$ . (d) We rule out the possibility that the observed behavior arises from the nuclear spins of the constituent atoms by measuring the heat capacity of ${Ba}_{6} Y_{2} {Ti}_{4} O_{17}$ (no upturn observed). Moreover, the nuclear spins in $Ti O_{2}, Y_{2} O_{3}$ , BaO, and $Rh O_{2}$ result in a low temperature heat capacity orders of a magnitude smaller than the value of $700 mJ / mol K^{2}$ at 0.35 K seen in ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ .
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Figure 5
(a) C/T vs T in ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ measured down to 0.35 K under zero applied magnetic field. Below 1 K, the upturn indicates the presence of spin fluctuations. The heat capacity of ${Ba}_{6} Y_{2} {Ti}_{4} O_{17}$ below 12 K was used to model the phonon contribution. The magnetic entropy is calculated to be $1.86 J / mo l^{- 1} K^{- 1}$ , accounting for about 1/3 of the magnetic entropy of a two state magnetic system. (b) Direct band gap Tauc plot of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ with optical band gap of 0.17 eV. Inset: Resistivity measurement on ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ resulting in a transport gap of 0.15 eV. (c) Raman scattering spectrum of ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ .
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Physical Review Materials

Widely spaced planes of magnetic dimers in the Ba6Y2Rh2Ti2O17−δ hexagonal perovskite

Loi T. Nguyen, Daniel B. Straus, Q. Zhang, and R. J. Cava

Phys. Rev. Materials 5, 034419 – Published 31 March 2021

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Widely spaced planes of magnetic dimers in the ${Ba}_{6} Y_{2} {Rh}_{2} {Ti}_{2} O_{17 - δ}$ hexagonal perovskite