Crystal Growth and Magnetic Properties of Topological Nodal-Line

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Crystal Growth and Magnetic Properties of Topological Nodal-Line Semimetal GdSbTe with Antiferromagnetic Spin Ordering Raman Sankar,*,†,‡ I. Panneer Muthuselvam,§ K. Ramesh Babu,⊥,∥ G. Senthil Murugan,‡ Karthik Rajagopal,‡ Rakesh Kumar,¶ Tsung-Chi Wu,† Cheng-Yen Wen,# Wei-Li Lee,† Guang-Yu Guo,⊥,∥ and Fang-Cheng Chou*,‡,○ †

Institute of Physics, Academia Sinica, Taipei 10617, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan § Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur 610005, Tamil Nadu, India ⊥ Department of Physics and Center for Theoretical Physics, National Taiwan University, Taipei 10617, Taiwan ∥ Physics Division, National Center for Theoretical Sciences, Hsinchu 30013, Taiwan ¶ Department of Physics, School of Natural Science, Shiv Nadar University, Dadri, Gautam Budh Nagar, UP 201314, India # Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan ○ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

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ABSTRACT: We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 μB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.

I. INTRODUCTION In recent years, 3D topological materials have attracted great attention in quantum physics due to the discovery of many topological phases which may lead to the next generation of spintronic device application, in particular, with the favorable ultrahigh mobility and high magnetoresistance.1−3 The novel topological insulators have an insulating state in the bulk and a gapless surface state of E(k) linear dispersion showing timereversal symmetry (TRS) or crystalline symmetry-protected massless tunneling of Dirac fermions. Additionally, topological semimetals having bulk conduction and valence bands touch at single or discrete points at the Fermi level are categorized as a Dirac semimetal (DSM) or Weyl semimetal (WSM), and for the band touching in a line is called a nodal-line semimetal.4,5 Weyl fermions can be realized as Dirac materials by breaking © XXXX American Chemical Society

the TRS or crystalline inversion symmetry so that a Dirac point splits into a pair of Weyl points. In order to utilize the topological phases for the next generation spintronic device application, the system must have field controllable spins. However, many existing Dirac/Weyl semimetal materials are nonmagnetic, not to mention the lack of long-range magnetic ordering. Recently, a few candidates of Dirac/Weyl semimetal compounds have been discovered having AFM ordering at low-temperature, including GdPtBi, CuMnAs, GdSbTe, CeSbTe, CeSbSe, and some selected Heusler alloys.6−8 GdSbTe crystallizes in a noncentrosymmetric tetragonal structure with space group P4/nmm (No. Received: June 8, 2019

A

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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four Gd atoms in tetrahedral coordination, and the Te atoms are bonded with Gd in quasi-2D square coordination on both sides of the quintuple layer. Weak bonding between neighboring Te−Gd across the quintuple layers is possible judging from the small bond length difference between the intralayer Te−Gd (3.17 Å) and interlayer Te−Gd (3.21 Å), as well as the buckled TeGd4- and GdTe4-squares, instead of a van der Waals type interaction between Te−Te across the quintuple layer in comparison to the known topological insulator Bi2Se3.11 Figure 1c shows the Rietveld refinement of the X-ray powder diffraction results (Bruker D8) using Cu Kα radiation for the pulverized single crystal sample. All of the diffraction peaks can be indexed with the tetragonal system of space group P4/nmm without identifiable impurity phases. EPMA chemical analysis results obtained from the average of five selected points being normalized to Gd indicated a ratio of Gd:Te:Sb = 1:0.997(4):1.003(2), which confirms the chemical stoichiometry of the as-grown crystals. The refined lattice parameters are a = b = 4.3005 (4) Å, c = 9.1725 (5) Å, and volume (V) = 169.644 (7) Å3, which are in good agreement with those reported in the literature.10 The single crystal shows XRD peaks of preferred [00L] orientation (inset of Figure 1c) and the large area of the truncated square pyramid for the as-grown single crystal GdSbTe (Figure 1b) is in agreement with the square symmetry for the tetragonal system. The STEM real and reciprocal images of GdSbTe single crystal is shown in Figure 2 along the [001] and [110] directions, which are

129), which is isostructural to the confirmed nodal-line semimetal ZrSiS.9 Recent study of GdSbTe by Hosen et al. revealed that the electrical properties of GdSbTe is similar to that of ZrSiS.10 Besides the reported AFM transition of TN ∼ 13 K, the topological Dirac state identified at the X point is found to be robust in both the paramagnetic and the AFM states, which suggests that TRS is broken. Herein, we report crystal growth, crystal structure, anisotropic magnetic measurements of magnetization M(H, T), AC and DC magnetic susceptibilities χ(T, H), and heat capacity CP(T, H) for field along H||ab and H||c of the GdSbTe single crystal. Chemical vapor transport (CVT) method using TeCl4 and TeBr4 as the precursor for the generation of Cl2 and Br2 transport agent has been shown to grow sizable high quality single crystals successfully. Both χ(T, H) and CP(T, H) studies reveal an AFM long-range spin ordering phase of transition temperature TN ∼ 12 K, and two anomalies indicating a spin reorientation and recovery for the on-site spin anisotropy between Tsr1 ∼ 7 K and Tsr2 ∼ 4 K, respectively. The easy axis for the Gd spins in AFM long-range ordering is identified by the spin flop transition for H||ab with a critical field of Hsf ∼ 2.1 T at 2 K. Additional field-induced metamagnetic phase below TN has also been implied from the magnetization plateau of M(T, H) for field up to ∼5.9 T.

II. RESULTS AND DISCUSSION II.A. Crystal Structure. GdSbTe crystallizes in tetragonal structure of P4/nmm (No. 129) space group, as shown in Figure 1a, which is identical to the fully explored ZrSiS being a prototype topological nodal-line semimetal.10 A 3D crystal structure of GdSbTe can be described as stacked quintuple layers of Te−Gd−Sb−Gd−Te along the c-direction. Each quintuple layer is centered around the Sb, which bonds with

Figure 2. STEM images for GdSbTe projected along the (a) [001] and (d) [110] directions, respectively. The corresponding crystal structure projections (b, c) for [001] projection and (e, f) for [110] projection. The corresponding crystal structure models are overlaid on the enlarged views in parts c and f to demonstrate the agreement on the symmetry assignment as shown in Figure 1a.

consistent to the crystal structure of high quality crystalline nature. In particular, the square symmetry along the [001] direction confirms the tetragonal symmetry described by the indexed space group P4/nmm. Consider the atomic electron configurations of Gd ([Xe] 4f75d16s2), Sb ([Kr] 4d105s25p3), and Te ([Kr] 4d105s25p4), and the respective square pyramid coordination of GdTe5, TeGd5, and the tetrahedral coordination of SbGd4, a proper valence bond model can be constructed via Gd-dsp2, Sb-sp3, and Te-sp2d hybridized orbitals, as shown in the inset of Figure 1a, which is in agreement with the calculated band structure for a semimetal having narrow band overlap.10 Comparing to the

Figure 1. Crystal structure of GdSbTe. (a) Ball and stick representation of GdSbTe. The inset below shows GdTe5, SbGd4, and TeGd5 coordination (from left to right) as a result of implied hybridized orbitals of Gd-dsp2, Sb-sp3, and Te-sp2d. (b) Single crystal of GdSbTe grown with the CVT method using (i) TeCl4 and (ii) TeBr4 as the source of Cl2 and Br2 transport agents, respectively. (c) Rietveld refinements of the crushed crystals of GdSbTe X-ray diffraction data at 300 K. A single crystal of preferred [00L] orientation is shown in the inset. The red open symbols represent the observed data, the black solid line represents the fitted pattern, the blue solid lines represent the difference between the observed and calculated intensities, and the green vertical bars represent the indexed peak positions. B

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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c(T) dip with concomitant small χ||ab(T) increase below ∼7 K, and then being recovered below ∼4 K, which seems to indicate the occurrence of a spontaneous spin reorientation behavior below TN. The maximum of dχab/dT (minimum of dχc/dT) near Tsr1 ∼ 7 K suggests the spins are reorienting from the easy axis (ab-direction) to the hard axis (c-direction) for the AFM long-range spin ordering, and the spin system is recovering to the original AFM spins below Tsr2 ∼ 4 K of onsite spin anisotropy along the ab-direction, as reflected on the DC χ(T) measured with H = 0.01 T (see Figure 3b). The exact mechanism that triggers the spin reorientation is unknown, but it is clearly not field-induced as also seen in the zero field AC susceptibilities (next section) and could be closely related to the topological nature of nodal line semimetal, i.e., partial localized spins in 3D spin structure of on-site anisotropy energy K is perturbed by the 2D conduction mechanism described by the band picture in Dirac cone shape.10,13 Similar anomalous behavior has also been observed in the preliminary transport property studies from the same crystal batch, which will be reported separately in the full transport property investigation under various magnetic fields up to 12 T.14 Above TN, a typical Curie−Weiss paramagnetic behavior is shown following the Curie−Weiss law [χ0 + C/(T − Θ)] between 75 and 300 K, where C = NAμ2eff /3kB = 7.41 cm3 K/mol and Θ = −24 K are fitted from the linear 1/χ vs T plot as shown in the lower inset of Figure 3a. The Curie constant is calculated to be μeff ∼ 7.7 μB, which is close and being consistent to the theoretical value of μ = 7.93 μB for Gd3+ having a stable half-filled 4f7 configuration in the inner shell. The Weiss temperature Θ = −24 K indicates a weak AFM coupling among Gd spins, and the small |Θ/TN| ratio is consistent to the existence of 3D long-range spin ordering of weak frustration. For the AFM spin ordering with easy axis along the abdirection, spin flop transition is expected to occur when the applied field along the easy axis is higher than the critical field Hsf, so that the magnetic energy gain is able to overcome the on-site spin anisotropy energy K, as reflected by the easy-axis change from ||ab to || c, as shown in Figure 4b for H = 7 T. Figure 4a shows the M(H) scan at 2 K with field applied along the easy axis (H||ab), two plateaus near ∼2.1 and 5.9 T are identified from the derivative plot of dM/dH(T) as shown in the inset, having magnetization of ∼1 and ∼2.3 μB/fu, respectively. Above the critical field of Hsf ∼ 2.1 T, all spins are flopped into the orientation perpendicular to the field, but the AFM ordering is maintained with canting by the applied field, as illustrated in the inset of Figure 4c−e. For H||ab above the second critical field of ∼5.9 T with a broader transition width, the magnitude of ∼2.3 μB/fu being close to ∼1/3 of the fully aligned Gd spins of S = 7/2 with an expected fully saturated level of Ms = 7.93 μB/fu, which suggests that the metamagnetic transition may have a special ordering of excited triplet state with population about 1/3 of the total spins.15 A detailed modeling and ultrahigh field M(H) is required to test this hypothesis. Similar M(H) plateaus for field along the easy axis have also been observed in the isostructural CeSbSe having AFM easy-axis ||c and a much lower TN ∼ 3 K,7 however, instead of a clear spin-flop transition, an emerging “devil’s staircase” in fractional integer size step has been found, whether a Kondo lattice behavior is involved or a phenomenon due to Se vacancy-induced incommensurable phase remains to be clarified.15 We have carefully examined the M(H) for field along the easy axis (||ab) and confirmed that no similar H-

confirmed topological nodal line semimetal of ZrSiS, the identical nonsymmorphic symmetry of GdSbTe is suggested by the σ−bonds formed by the half-filled Sb-sp3 and Gd-dsp2 orbitals in SbGd4 tetrahedral coordination, so that the square symmetry for the Gd-layer warrants a crystalline symmetryprotected topological order, as revealed by the Dirac cone at the X-point.10,12 II.B. Magnetic Susceptibility. The temperature dependence of DC magnetic susceptibility (χ = M/H) for fields H = 0.01−7 T applied perpendicular (H||c) and parallel (H||ab) to the large square ab-plane of the single crystal sample is shown in Figure 3. A typical temperature dependence of anisotropic

Figure 3. (a) Temperature dependence of the magnetic susceptibility of GdSbTe single crystal for H||ab and H||c. Top inset: Expanded view of dχ(T)/dT. Bottom inset: Inverse magnetic susceptibility (1/χ) for powder GdSbTe measured with magnetic field of H = 0.01 T. (b) Low-temperature magnetic susceptibilities for single crystal GdSbTe along H||ab and H||c measured at low (0.01 T) and high applied fields (7 T). Spin reorientation is suggested by the concomitant magnitude changes as a dip for χ||c and a peak for χ||ab between 7 and 4 K at low field.

χ(T) behavior of AFM spin ordering below TN ∼ 12 K and paramagnetic (PM) above TN. No thermal hysteresis is found between zero-field-cool (ZFC) and field-cool (FC) cycles, which is consistent with that reported by Hosen et al. measured with H = 0.1 T.10 In the blow up view of χ(T) below 20 K, as shown in Figure 3b, the χ||c(T) is nearly temperatureindependent and χ||ab(T) decreases below TN for applied fields below ∼2 T, which indicates the easy axis for the AFM ordering is parallel to the ab-plane. However, it is noteworthy that the spin anisotropy for the AFM state is not constant below TN, as suggested by the low field data showing a small χ|| C

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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the AC susceptibility corresponds to the DC susceptibility of low field with zero frequency, which is reflected on the similarity between the real part of powder-averaged AC χ′(T) (Figure 5) and the low field DC χ(T) (Figure 3). On the basis of the AC χ′(T) and the dχ′(T)/dT plot analysis as shown in the inset of Figure 5a, TN ∼ 13 K and Tsr1 ∼ 7.5−8.5 K are better defined to be slightly higher than those identified by the DC susceptibility shown in Figure 3, but the onset Tsr2 ∼ 4 K indicating the spin recovery behavior is not well-defined in the powder-averaged AC measurement results. II.C. Heat Capacity. Figure 6 shows the zero field heat capacity CP as a function of temperature for a GdSbTe single

Figure 4. (a) Field dependent isothermal magnetization of GdSbTe at 2 K for field along H||ab and H||c. Inset shows the derivative of M(H) with respect to H||ab, which indicates the Hsf is near ∼2.1 T and a metamagnetic transition above ∼5.9 T. (b) Proposed AFM spin structure of spins with easy axis along the ab-direction in doubled unit cell. (c−e) Schematic plots to illustrate that spin-flop transition occurs only when the field is applied along the easy-axis and applied higher than the Hsf.

hysteresis and no possible “devil’s staircase” are observed in GdSbTe. In order to check whether the spin reorientation transition below TN is field-induced or not, AC susceptibility without applied DC magnetic field has been performed using AC field Hac = 1 Oe with selected frequencies between 10 to 950 Hz, as shown in Figure 5 (with only 100 Hz data shown for clarity). No frequency dependence up to 950 kHz is found for the AFM transition below TN, which is expected for the long-range ordering of Gd spins with negligible frustration. In principle,

Figure 6. Main panel shows the heat capacity CP vs temperature for GdSbTe crystal in zero field. Inset shows Cmag (T) after the calculated electronic and phonon contributions are subtracted from the total (see text), and the corresponding entropy SM (T) saturates to ∼14.58 J/(mol K) to be ∼84% of the expected R ln(2S + 1) = 17.29 J/(mol K) for S = 7/2.

crystal sample. The CP values at room temperature reaches the Dulong−Petit limit expected for a solid of molecule with three atoms per formula unit, i.e., 74.83 J/(mol K). A λ shaped anomaly at TN ∼ 12 K is shown, but a secondary broad peak of onset near Tsr1 ∼ 7 K can also be identified, which is consistent with the χ(T) measurement to indicate the spin reorientation (Figure 3b). Generally, the total CP of a conducting magnetic material consists of electronic (Celec), magnetic (Cmag), and lattice (Clat) contributions. Since the conventional lowtemperature behavior does not obey the linear dependence of CP/T = γ + βT2 due to the magnetic contribution below TN, we used the Debye model for the lattice heat capacity to fit the CP data between 50 and 250 K without magnetic contribution following16,17 CP = γT + nCV Debye

(1)

and

4 x ji T zy θD/ T x e CV Debye = 9R jjj zzz dx j θD z 0 (e x − 1)2 (2) k { where CVDebye(T) represents the Debye lattice heat capacity at constant volume V, n = 3 is the number of atoms per formula unit, θD is the Debye temperature, γ is the electronic contribution (Sommerfeld coefficient), and R is the molar 3

∫

Figure 5. (a) Real (χ′) and (b) imaginary (χ″) components of the AC susceptibility as a function of temperature measured with ac field of 1 Oe/100 Hz on crushed crystal sample of GdSbTe. dχ′(T)/dT plot is shown in the inset of part a. D

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ideal gas constant. The fitting of the above equation yields θD ∼ 195(2) K and γ ∼ 7.6(4) mJ/mol K2, which are comparable to those obtained for the isostructural ZrSiS (θD ∼ 493 K, γ ∼ 6.84 mJ/mol K2),18 but the significantly reduced θD suggests a lower high-limit for the lattice vibration normal mode of GdSbTe, presumably as a result of the heavier elements. Magnetic contribution Cmag can be extracted from the experimental total Cp(T) with subtracted calculated electronic and phonon contributions of (γT + nCV Debye) from eq 1 and 2, and then magnetic entropy (SM) is deduced from the integration with respect to T of Cmag/T as shown in the inset of Figure 6. Above TN, the SM saturates to a value of 14. 58 J/(mol K) to be ∼84% of the expected SM ≈ R ln (2S + 1) = R ln (8) = 17.29 J/(mol K) for S = 7/2. CP/T (T) measured at various applied fields of H||ab from 0 to 7 T are shown in Figure 7. With increasing H, the TN ∼ 12

the second broad plateau of M(H) between 3 and 7 T (see inset of Figure 4a). The emerging field-induced phase of lower transition temperature TN2 is presumably coexisting or evolved from the original AFM phase of TN. The reduced TN has been demonstrated in LiFePO4 to signify a finite-size effect of dimension (L) dependence, i.e., 1 − TN(L)/TN(∞) ∼ L1/ν.19 It is likely that the high field induced spin triplet excited state could be distributed in the domain walls of 1D or quasi-1D distribution, so that the TN is suppressed for regions of reduced AFM domain size due to disrupted magnetic correlation length. Preliminary STM studies have shown signature of stripes in incommensurate nature, and the proposed interpretations require more STM and high field neutron diffraction experiments to confirm. II.D. Longitudinal Resistivity. The temperature dependence of longitudinal resistivity ρ(T) for two plate-like samples prepared with different transport agents of TeCl4 (black curve) and TeBr4 (red curve) is shown in Figure 8. The longitudinal

Figure 8. Temperature dependence of the resistivity for two plate-like samples (black, with TeCl4 as the transport agent; red, with TeBr4 as the transport agent).

resistivity ρ(T) was measured by the standard four-probe method with the AC lock-in approach. The contact electrodes were fabricated first by argon ion milling and followed by the deposition of 10 nm titanium and 50 nm gold films, where the contact resistances were found to be less than 10 Ω. Both samples show weak thermally activated conduction behavior below 250 K. While the resistivity of both samples is found to be in the order of mΩcm, the sample using TeBr4 as the transport agent (red curve) shows slightly lower resistivity than that was prepared with TeCl4 agent, presumably due to more halide inclusion or vacancy defects in the latter. Interestingly, the overall behavior in the temperature dependence of longitudinal resistivity of GdSbTe revealed a strike difference with the nonmagnetic semimetal LaSbTe, which was reported to show metallic behavior in the temperature dependence of longitudinal resistivity.8 With the similar crystal structures between GdSbTe and LaSbTe, it indicates that the charge transport in GdSbTe could be heavily affected by the added scattering of Gd spin-7/2 in the ab-plane. More details about the magnetotransport property of GdSbTe will be reported separately.14

Figure 7. CP/T versus temperature for GdSbTe single crystal measured in H||ab between 0 and 7 T. The field-induced TN2 is indicated by the arrow; see discussion in the text.

K signals the AFM transition and the Tsr1 ∼ 7 K corresponding to the spin reorientation transition are nearly constant but with increasing transition width, slight suppression of TN for fields above the critical field of spin flop transition Hsf ∼ 2.1 T (Figure 4) is also observed. In addition, secondary fielddependent TN2(H) is identified for H > Hsf, where higher field induces a phase of lower TN2, which has also been reflected in E

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry II.E. Ab Initio Calculation of Magnetic and Electronic Structures. In order to find out the magnetic ground state of GdSbTe, we consider four different possible magnetic configurations within the 2 × 2 × 1 supercell. The magnetic configurations include an antiferromagnetically coupled ferromagnetic zigzag chain structure (AFM-A), antiferromagnetic coupling in-plane and out of plane structure (AFM-G), ferromagnetic coupling in-plane and antiferromagnetic coupling out of plane structure (AFM-I) and ferromagnetic configuration (FM) as illustrated in parts a−d of Figure 9,

Figure 10. (a) Electronic band structure and (b) density of states of AFM-A configuration. The top of the valence band is set to zero. The Dirac cone near the X-point is circled in red.10

fluctuation in this system which greatly enhances the quasiparticle mass, although the γ value extracted from Cp measurement is moderate, unlike the heavy-fermion-like transition metal compound Fe2VAl which is also a semimetal with a pseudogap.20 The bands in the vicinity of the Fermi level are Te-p and Sb-p orbital hybridized bands. From the site-resolved density of states, it is clear that the conduction band is dominated by Gd-5d states whereas the valence band is composed of Sb-p and Te-p states. The spin-down Gd-f states lie at above 2 eV from the Fermi level in the conduction band. Therefore, the magnetic structure involves the spin-exchange interactions via Gd−Te−Gd and Gd−Sb−Gd bonding in this compound, as illustrated in the inset of Figure 1a.

Figure 9. Schematic representation of the four possible spin configurations.

respectively. We calculate the total energy of each configuration and the results are summarized in Table 1. Clearly, the

III. SUMMARY We have successfully grown high quality single crystals of GdSbTe using chemical vapor transport method. The crystal structure refinement of GdSbTe confirms the noncentrosymmetric tetragonal symmetry of space group P4/nmm being identical to the known nonmagnetic topological nodal-line semimetal ZrSiS. χ(T, H) and CP(T, H) study results reveal AFM transition of TN ∼ 12 K for the Gd spins of S = 7/2, and two spin reorientation transitions near Tsr1 ∼ 7 K for spin anisotropy turns from ab- to c-direction and recovery to the original below Tsr2 ∼ 4 K. The spin flop transition is confirmed by the M(H) plateau of Hsf ∼ 2.1 T for H||ab applied along the easy axis. Additional field-induced metamagnetic phase transition is observed for H||ab >5.9 T, which could be due to the triplet excited spin state being distributed in domain boundary or having a superlattice of magnetization near onethird of the fully saturated level of all spins aligned by the high field. The peculiar spin reorientation transition below TN could be closely related to the 2D conduction nature for GdSbTe as a topological nodal-line semimetal with confirmed Dirac cone at the X-point.

Table 1. Calculated Total Energy (Relative to the FM Total Energy EFM = −24.4205 eV/fu) and Magnetic Moment of the Gd Atom (msGd) Configuration

ΔE (meV/fu)

msGd (μB/Gd atom)

FM AFM-A AFM-G AFM-I

0.00 −8.08 −5.21 −6.22

7.1 7.1 7.1 7.1

AFM-A configuration has the lowest energy. Therefore, the possible magnetic ground state of GdSbTe is the AFM-A configuration, being consistent with the magnetic susceptibility measurements reported above. The magnetic moment of Gd atom is calculated to be 7.1 μB/atom, which is close to the expected value of 7.93 μB/atom for Gd ([Xe] 4f75d16s2). Next, we calculate the electronic band structure and density of states (DOS) of ground state AFM-A configuration, as shown in parts a and b of Figure 10, respectively. Figure 9 (a) shows that our energy bands near the Fermi level is very similar to that of nonmagnetic GdSbTe reported in ref.10 The calculated band structure reveals that the compound is a semimetal with the density of states at the Fermi level D(EF) = 0.36 states/(eV/fu). Interestingly, this bare D(EF) is nearly 10 times smaller than that deduced from the specific heat measurement reported above. This indicates strong spin-

IV. EXPERIMENTAL AND COMPUTATIONAL SECTION IV.A. Crystal Growth and Experimental Details. The CVT method has been tested using Br2, Cl2, and I2 as the vapor transport agents, which allows an effective and fast vapor transport to produce the necessary supersaturation for the expected final product. GdSbTe single crystals grown in truncated square pyramid shape are shown in Figure 1b. For the preparation of precursor powder materials, F

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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stoichiometric amount of 5−6 N pure elements in molar ratio of Gd:Sb:Te = 1:1:1.1 with 10% excess of Te was sealed into an evacuated quartz ampule and heated for 2 days at 650 °C. For the CVT method crystal growth, about 10 g of the prereacted GdSbTe powder was placed together with variable amounts of transport agent (purity 4 N) of either TeBr4, TeCl4, or solid I2 at one end of the silica ampule (40 cm long, i.d./o.d. = 1.8/2.0 cm). To prevent oxygen contamination, all preparation steps before flame-sealing were carried out in an Ar gas filled glovebox with oxygen and water level kept below ∼1 ppm. The loaded ampule was evacuated and flame-sealed before loading into the tube furnace for the CVT growth. A temperature profile of gradient near 2.5 °C/cm is maintained, where the prereacted material is held at 1050 °C and the growth end is maintained at 980 °C; thin plate-like GdSbTe single crystals were obtained with I2 transport agent. For the growth of thick crystals of GdSbTe in a truncated square pyramid shape, sizes up to 3.5 × 2.5 × 1 mm3 and 2 × 2 × 2 mm3 were obtained using transport agents of TeCl4 and TeBr4, respectively. The optimal temperature profile is kept at 980−880 °C with a small temperature gradient near 2.5 °C/ cm for about a week. It is found that the vapor pressure controlled at ∼7 mg of TeBr4 per cm3, ∼5 mg of TeCl4 per cm3, and ∼6 mg of solid I2 per cm3 would yield growth at high transport rate about ∼150, ∼120, and ∼175 mg per day, respectively. Powder X-ray diffraction (XRD) measurements using crushed single crystal sample were performed at room temperature using a Rigaku diffractometer with Cu Kα radiation and a graphite monochromator. Single crystals of GdSbTe in truncated square pyramid shape with size up to 2.7 × 2.7 × 2.9 mm3 was employed to measure the magnetic properties. Lattice parameters were obtained by Rietveld refinements. In order to check the actual chemical composition of the samples, energy dispersive X-ray (EDX) spectroscopy analysis was performed on a set of crystals. Chemical analysis was performed using electron probe microanalysis (EPMA). STEM (scanning transmission electron microscopy) of the grown crystals was done in 200 kV JEOL 2100F equipped with a probe corrector for the spherical aberration. The magnetic measurements were performed using a SQUID-vibrating sample magnetometer (QD-USA). The heat capacity and resistivity measurements were carried out using the physical property measurement system PPMS (QD-USA). IV.B. Computational Details. We perform theoretical calculations based on density functional theory with the generalized gradient approximation (GGA). The GGA is taken care by Perdew− Burke−Ernzerhof parametrization.21 To describe the electron− electron interactions of 4f states of Gd, the on-site Coulomb repulsion energy U is considered within the GGA + U scheme22 with an effective Ueff = (U − J) = 6 eV.23 The crystal structure of GdSbTe contains two formula units per unit cell. In order to get the magnetic ground state of the system, we consider 2 × 2 × 1 supercell that contains 8 Gd atoms. All the calculations are carried out with experimental lattice constants. The accurate projector-augmented wave method as implemented in Vienna Ab initio simulation package24,25 together with a large plane wave cutoff energy of 540 eV is used. Integrations over the Brillouin zone are performed with a Γ-centered k-point grid of 8 × 8 × 4. The convergence criterion for the total energy is set to be 10−6 eV.

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ACKNOWLEDGMENTS R.S. and F.-C.C. acknowledge the support provided by the Academia Sinica research program on Nanoscience and Nanotechnology under Project Number NM004. F.-C.C. acknowledges the support provided by the Ministry of Science and Technology in Taiwan under Project Number MOST106-2119-M-002-035-MY3. R.S. acknowledges the support by the Development of Novel Thermoelectric Materials for Sustainable Energy Academia Sinica in Taiwan AS-SS-10601-1. I.P.M thanks Department of Science and Technology in India for the support of INSPIRE Faculty Award No. DST/ INSPIRE/04/2016/002275 (IFA16-PH171).

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AUTHOR INFORMATION

Corresponding Authors

*(R.S.) E-mail: [email protected]. *(F.-C.C.) E-mail: [email protected]. ORCID

Raman Sankar: 0000-0003-4702-2517 I. Panneer Muthuselvam: 0000-0002-7763-5915 Cheng-Yen Wen: 0000-0002-9788-4329 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b01698 Inorg. Chem. XXXX, XXX, XXX−XXX