1 Single crystal growth and magnetism of Sr3NaIrO 6 and Sr 3AgIrO 6 tracking the J 0 ground state of Ir5

2025-04-30 1 0 1.13MB 21 页 10玖币

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Single crystal growth and magnetism of Sr3NaIrO6 and Sr3AgIrO6: tracking

the J = 0 ground state of Ir5+

Peng-Bo Song (宋鹏博)1,2, Zhiwei Hu3, Su-Yang Hsu4, Jin-Ming Chen4, Jyh-Fu Lee4,

Shan-Shan Miao (苗杉杉) 1, You-Guo Shi (石友国) 1,2,5*, Hai L. Feng (冯海)1,5*

1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics,

Chinese Academy of Sciences, Beijing 100190, China

2Center of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100190, China

3Max Planck Institute for Chemical Physics of Solids, Dresden 01187, Germany

4National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic

of China

5Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

*Corresponding authors:

ygshi@iphy.ac.cn (YGS)

hai.feng@iphy.ac.cn (HLF)

Abstract. Single crystals of Sr3NaIrO6 and Sr3AgIrO6 have been successfully grown

using hydroxides flux. Sr3NaIrO6 and Sr3AgIrO6 crystallize in the K4CdCl6-type

structure with the space group R-3c (No.167). Sr3NaIrO6 and Sr3AgIrO6 are electrically

insulating with estimated activation gaps of 0.68 eV and 0.80 eV, respectively.

Sr3NaIrO6 and Sr3AgIrO6 show paramagnetic behavior down to 2 K. In this work, the

observed effective moments, μeff, for Sr3NaIrO6 single crystals are 0.31 μB for H⊥c and

0.28 μB for H||c, which are much smaller than that of 0.49 μB previously reported for

the polycrystalline Sr3NaIrO6 samples. For Sr3AgIrO6 single crystals, a much larger

value of μeff = 0.57 μB is observed as compared with Sr3NaIrO6 single crystals. The x-

ray absorption spectroscopy and low-temperature specific heat data indicate that the Ir

in Sr3NaIrO6 has an almost pure Ir5+ valence state, while the Ir in Sr3AgIrO6 is slightly

lower than +5. The estimated low limits of magnetic impurity Ir4+ are about ~1.7% and

~9.2% for Sr3NaIrO6 and Sr3AgIrO6, respectively. These magnetic impurities are likely

to fully explain the observed μeff values for Sr3NaIrO6 and Sr3AgIrO6 single crystals,

supporting the J = 0 ground state of Ir5+ in Sr3NaIrO6 and Sr3AgIrO6.

Introduction

In 3d transition metal oxides, the valence electrons are strongly correlated and the

Hubbard U plays an important role [1–5]. In comparison with 3d electrons, the orbitals

of 5d electrons are more extended and the U in the 5d system becomes weaker while

the spin-orbit coupling (SOC) becomes much stronger due to their larger atomic

number [6,7]. In the strong SOC regime, the SOC can split three t2g orbitals in the

octahedral crystal field into an upper j = 1/2 doublet and a lower j = 3/2 quadruplet [8,9].

For example, in tetravalent iridate Sr2IrO4 (Ir4+: 5d5) the SOC-assisted Mott-insulating

state is explained with the J = 1/2 ground state [9]. Resonant inelastic x-ray scattering

measurements on pentavalent osmates (Os5+: 5d3) reveal the SOC controlled J = 3/2

ground state [10]. In such a scenario, if there are four 5d electrons filling the lower

quadruplet, the ground state should be j = 0. Long-range magnetic orders reported in

Ir5+ (5d4) double perovskite oxides Sr2YIrO6 and Ba2YIrO6 with effective moment (μeff)

of 0.91 μB/Ir and 1.44 μB/Ir, respectively, raise concerns about the ground state of 5d4

oxides [11,12]. These results have been challenged by other studies reporting that no

magnetic order was found in Ba2YIrO6 [13] and Sr2YIrO6 [14] down to ~430 mK.

Studies on A2YIrO6 (A = Sr, Ba) and other Ir5+ double perovskite oxides generally

reveal a weak paramagnetic behavior with small μeff values of 0.19 – 0.63 μB/Ir [13–25]

which are much lower than the theoretical spin-only μeff = 2.83 μB/Ir demonstrating a

SOC dominated ground state. The origin of these finite magnetic moments is still

ambiguous. Quench of the J = 0 state for Ir5+ due to IrO6 octahedra distortion in

Sr2YIrO6 was proposed by Cao et al [11]. However, this scenario cannot explain the

paramagnetic moment observed in cubic Ba2YIrO6 where is no structural distortion,

and the studies on Ba2-xSrxYIrO6 studies do not find correlations between μeff values

and the degree of structural distortions [19,26]. The existence of magnetic impurities

has been suggested by studies in Sr2YIrO6 [14] and Ba2YIrO6 [17]. Fuchs et al.

confirmed the existence of Ir4+ and Ir6+ magnetic defects which are responsible for the

magnetism in Ba2YIrO6 [18]. The antisite disorder in double perovskites has also been

suggested to play an important role [16,20]. Laguna-Marco et al suggest that the Ir4+

and Ir6+ magnetic impurities may locate in the antisite disorder region [20].

Condensation of J = 1 triplon excitations of 5d4 oxides is also a possible source for the

observed magnetic moments [27,28]. Chen et al. proposed that the condensation is

unlikely in Sr2YIrO6 and Ba2YIrO6 with the ideal crystal structure, but the antisite

disorder between Y3+ and Ir5+ can break down the local nonmagnetic singlets [16].

Recent studies on A2BIrO6 (A = Ba, Sr; B = Lu, Sc) also support the J = 0 ground state

for these Ir5+ oxides and indicate the magnetic signals are from extrinsic sources, such

as magnetic impurities and antisite disorder [21].

To narrow down the possible explanations, it is better to studies on Ir5+ oxides

with less antisite disorder. Recently, studies on K4CdCl6-type polycrystalline Ir5+ oxide

Sr3NaIrO6 have been reported and indicate a possible quantum spin liquid ground state

(reported μeff = 0.49 μB/Ir) [22]. In comparison with A2YIrO6 (A = Sr, Ba), where the

Ir5+O6 octahedra are separated by Y3+O6, the Ir5+O6 octahedra are separated with Na1+O6

octahedra in the Sr3NaIrO6. The larger charge difference would reduce the antisite

disorder between Na1+ and Ir5+ in Sr3NaIrO6 as compared with Sr2YIrO6 and Ba2YIrO6.

Thus, the K4CdCl6-type iridate is a good platform to investigate the ground state of Ir5+

ions. To track the J = 0 gound state for Sr3NaIrO6, it is better to grow single crystals to

minimize any by-phases and lattice defects. In this work, we successfully grow single

crystals of K4CdCl6-type iridate oxides Sr3NaIrO6 and Sr3AgIrO6. Magnetic

measurements reveal that the μeff for Sr3NaIrO6 single crystals is about 0.31 μB for H⊥

c and 0.28 μB for H||c which are smaller than that of 0.49 μB reported for the

polycrystalline Sr3NaIrO6 [22]. The presence of a few percent of magnetic Ir4+ impurity

is indicated by the analysis of low-temperature specific heat data which is likely to fully

explain the observed μeff, supporting the J = 0 ground state of Ir5+ in Sr3NaIrO6.

Experiment

Single crystal samples of Sr3NaIrO6 and Sr3AgIrO6 were both prepared by

flux method using NaOH-KOH and KOH, respectively. The SrOH·8H2O, NaOH, KOH,

and Ir with a molar ratio of 3:50:40:1 (for Sr3NaIrO6) and SrOH·8H2O, Ag2O, KOH,

and Ir with a molar ratio of 3:0.5:40:1 (for Sr3AgIrO6) were weighted, respectively. The

mixtures were placed into Al2O3 crucibles with lids, then heated to 873 K in 1 h and

annealed for 12h before cooled to room temperature by stopping the heating. Single

crystals were separated by washing with deionized water.

Single-crystal x-ray diffraction measurements were conducted on a Bruker D8

Venture diffractometer at 300K using Mo Kα radiation (λ = 0.71073 Å). The frames

were integrated with the Bruker SAINT software package using a narrow-frame

algorithm. Data were corrected for absorption effects using the multi-scan method

(SADABS). The crystalline structures were refined by the full-matrix least-squares

method on F2 using the SHELXL-2018/3 program.

Single crystals of selected samples were used for magnetic susceptibility (χ),

longitudinal resistivity ρ, specific heat, and x-ray absorption spectroscopy (XAS). The

magnetic properties were measured under different applied magnetic fields in Field-

Cooling (FC) and Zero-Field-Cooling (ZFC) modes using SQUID-VSM device in a

magnetic properties measurement system (MPMS). Isothermal magnetization (M-H)

was measured at several fixed temperatures. These transport measurements (both

resistivity and specific heat) were measured by a physical property measurement system

(Quantum Design inc) using the standard DC four-probe technique and a thermal

relaxation method, respectively. XAS spectra at the Ir-L3 edges were studied at the

Taiwan Light Source (TLS) beamline 17C of the National Synchrotron Radiation

Research Center (NSRRC).

RESULTS AND DISCUSSION

Single crystals for Sr3NaIrO6 with dimensions of ~2mm×0.3mm×0.3mm and

Single crystals for Sr3AgIrO6 with ~0.3mm×0.3mm×0.3mm were obtained as shown in

Figs. 1(c) and 1(d), respectively. Analysis of the room temperature SCXRD data

confirms that Sr3NaIrO6 and Sr3AgIrO6 crystallize in the K4CdCl6-type structure with

the space group R-3c (No.167). The refined lattice parameters were a = 9.6408(3) Å,

and c = 11.5508(5) Å for Sr3NaIrO6 and a = 9.5996(3) Å, and c = 11.9032(6) Å for

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1SinglecrystalgrowthandmagnetismofSr3NaIrO6andSr3AgIrO6:trackingtheJ=0groundstateofIr5+Peng-BoSong(宋鹏博)1,2,ZhiweiHu3,Su-YangHsu4,Jin-MingChen4,Jyh-FuLee4,Shan-ShanMiao(苗杉杉)1,You-GuoShi(石友国)1,2,5*,HaiL.Feng(冯海)1,5*1BeijingNationalLaboratoryforCondensedMatterPhysicsandInstituteofPhysics,ChineseAcademy...

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1 Single crystal growth and magnetism of Sr3NaIrO 6 and Sr 3AgIrO 6 tracking the J 0 ground state of Ir5

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