Structural phase transition in irte2: a combined study of optical spectroscopy and band structure calculations

ABSTRACT Ir1−_x_Pt_x_Te2 is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the

spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their

electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe2 in an effort to

elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers

below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the

energy levels of Te (p_x_, p_y_) and Te p_z_ bands. SIMILAR CONTENT BEING VIEWED BY OTHERS STABILIZATION MECHANISM OF MOLECULAR ORBITAL CRYSTALS IN IRTE2 Article Open access 12 December 2022

QUANTIFYING THE ROLE OF THE LATTICE IN METAL–INSULATOR PHASE TRANSITIONS Article Open access 31 May 2022 INCOHERENCE-TO-COHERENCE CROSSOVER OBSERVED IN CHARGE-DENSITY-WAVE MATERIAL

1_T_-TISE2 Article Open access 24 October 2024 INTRODUCTION Transition metal dichalcogenides are layered compounds exhibiting a variety of different ground states and physical properties.

Among the different polytypes, the compounds with 1T and 2H structural forms TX2 (T = transition metal, X = chalcogen) have attracted particular attention. Those compounds consist of stacked

layers of 2D-triangular lattices of transition metals (e.g., Ti, Ta or Nb) sandwiched between layers of chalcogen atoms (e.g., S, Se or Te) with octahedral and trigonal prismatic

coordinations, respectively1,2,3. The low dimensionality of those systems often leads to charge density wave (CDW) instability2,3,4,5,6,7,8,9,10. Yet, a number of those CDW-bearing materials

are also superconducting4,5,6,7. The interplay between the two very different cooperative electronic phenomena is one of the fundamental interests of condensed-matter physics. Recently, the

5d transitional metal ditelleride IrTe2 has added to the interest. The compound has a layered 1T structure consisting of edgesharing IrTe6 octahedra (see Fig. 1 (a)–(c)) and exhibits a

structural phase transition near 270 K11. The transition leads to sharp changes in resistivity and magnetic susceptibility. It was found very recently that both Pt or Pd intercalations and

substitutions could induce bulk superconductivity with T_c_ up to ~3 K12,13,14,15. Since both Ir and Te possess large atomic numbers, strong spin-orbital coupling must be present in those

compounds, which may induce topologically nontrivial state. There is a possibility that the Pt or Pd intercalated or substituted compounds are better candidates for topological

superconductors due to its improved superconductivity in comparison with Cu_x_Bi2Se316,17,18. Understanding the structural phase transition is a crucial step towards understanding the

electronic properties of the system. Up to now, there have been very limited studies on the origin of the structural phase transition near 270 K. Structural characterizations indicated that

the transition is accompanied by the appearance of superstructure peaks with wave vector of Q = (1/5, 0, −1/5), which may match with the Fermi surface (FS) nesting wave vector from

theoretical calculations13. Therefore, the transition was suggested to be a CDW phase transition with involvement of orbital degree13 and the superconductivity competed with the CDW in a

quantum critial point-like manner. On the other hand, a recent Ir 4f XPS study on IrTe2 suggested that the low temperature phase of IrTe2 was accompanied by the establishment of weak

modulation of Ir 5d electron density and the structural phase transition of IrTe2 was argued to be an orbitally-induced Peierls transition14, being similar to the structural transition of

spinel-type CuIr2S419. In this work, we present a detailed optical spectroscopic study on well characterized Ir1−_x_Pt_x_Te2 (_x_ = 0, 0.05) single crystal samples. We find that the

structural phase transition in the undoped compound is associated with the sudden reconstruction of band structure over a broad energy scale up to at least 2 eV. Although the carrier density

is significantly reduced after the phase transition, yielding evidence for a reduced FS area, we could not identify any energy gap associated with the transition with a characteristic

lineshape from the coherent factor of density wave collective phenomenon. In combination with the density function calculations, we elaborate that the structural phase transition is not of a

density wave type but of a different origin. A novel explanation is proposed to explain the structural instability of this interesting material in terms of experimental and band structure

calculation results. RESULTS Figure 1 (a)–(c) show the structural characteristics of IrTe2 compound. Pictures of platelike IrTe2 single crystals with maximum _ab_ plane dimension of 6 × 4 

mm2 are shown in the inset of the figure. Fig. 1 (d) displays the X-ray diffraction patterns of IrTe2 single crystals at room temperature. The (0 0 _l_) diffraction peaks indicate a good

_c_-axis characteristic. Several single crystals were pulverized to measure powder XRD [Fig. 1 (e)]. The major peak patterns confirm a single trigonal phase with the lattice parameters _a_ =

3.9322(5) Å, _c_ = 5.3970(8) Å with space group for IrTe2, which is consistent with the previous report11,20. Compared with pure IrTe2, the major peaks of Ir0.95Pt0.05Te2 shift toward

higher 2θ, while they could be indexed by the same structure, resulting in slightly different lattice parameters: longer _a_ = 3.9394(1) Å and shorter _c_ = 5.3890(5) Å. The evolution of the

lattice parameters is quite consistent with the previous report on polycrystalline samples12. The minor extra peak denoted by asterisks were determined to be Te phase, possibly from some Te

flux remaining on the crystal surfaces. Figure 2 presents the physical properties characterizations of Ir1−_x_Pt_x_Te2 single crystals. Figure 2 (a) displays the normalized dc resistivity

ρ/ρ300_K_ for Ir1−_x_Pt_x_Te2 over broad temperature range (2 K ~ 400 K). The most striking phenomenon for pure IrTe2 is the steep jump at 271 K (reaching maximum at 261 K) on cooling, 285 K

(maximum at 275 K) on heating. The significant hysteresis suggests a first order phase transition. The resistivity ρ(T) keeps decreasing with lowering temperature after the phase

transition. An upward curvature is seen below 50 K, reminiscent of ordinary Bloch-Gruneisen lineshape due to electron-phonon interactions. However, after 3% Pt doped into IrTe2, the

transition temperature suddenly descends to 130 K (cooling process). With 5% Pt doping, the phase transition is completely absent. Above the transition temperatures all the ρ/ρ300_K_

resistivity curves could be perfectly scaled together. We find that, while some of the pure IrTe2 crystals just show flat residual resistivity at low temperature as shown in Fig. 2 (a), some

others grown from the same batch exhibit sharp drops at 2.5 K in ρ(T) curve. We applied magnetic field to the latter and found that the transition temperature reduced to 1.5 K under 1 Tesla

with field parallel to _ab_–plane, as shown in Fig. 2 (d). The feature is analogous to the resistivity behavior of superconductivity. Nevertheless, no anomaly could be identified in the

specific heat measurement down to 0.5 K [the inset of Fig. 2 (c)]. The measurement revealed filament superconductivity in some of the crystals, possibly originated from Ir vacancies or

excess Te in the samples. Bulk superconductivity is seen in Pt doped samples, similar to polycrystalline samples12. Figure 2 (e) shows the low temperature magnetic susceptibility data of

Ir0.95Pt0.05Te2, indicating strong superconducting diamagnetic effect. Simultaneously the minor distinction between FC and ZFC processes implies weak superconducting vortex pinning, which,

to a certain extent, indicates the good quality of our single crystals. A significant jump in the specific heat measurement at _T__c_ is observed for Ir0.95Pt0.05Te2, as shown in Fig. 2 (f).

The magnetic susceptibilities of Ir1−_x_Pt_x_Te2 single crystals in an applied magnetic field H = 1 T aligned in _ab_–plane are illustrated in Fig. 2 (b). For samples with Pt contents _x_ =

0 and 0.03, the temperature-independent diamagnetism is observed above their separate structural phase transitions and then sudden drops happen at the transition temperatures, resulting in

more diamagnetic behavior. For Ir0.95Pt0.05Te2, the magnetic susceptibility practically keeps constant except for the upturn at low temperature which is likely from paramagnetic impurities.

The observation of diamagnetic susceptibilities even above the structural phase transition is different from the several recent reports on polycrystalline samples11,12,13. In an earlier

report, diamagnetism was observed in the entirely measured temperature range below 300 K, however no phase transition could be seen in the experimental data20. We emphasize that our

measurements are performed on single crystal samples and are more reliable. As we shall see below from optical measurement, all samples are metallic both above and below structural phase

transitions, the observation of diamagnetism is rather surprising. This is because, for metallic compounds, the diamagnetism is usually a weak effect and the Pauli paramagnetism should

dominate the magnetic susceptibility. As we shall explain below, this peculiar property is related to the valence state of Ir3+ which leads to more closed shells. We performed the specific

heat measurements over broad temperature range (0.5 K ~ 300 K) for the compounds. As illustrated in Fig. 2 (c), a very pronounced sharp peak appears near 280 K for the undoped compound. The

characteristic δ-like shape of the peak reveals a first order phase transition. In the low temperature range, the specific heat follows the relation of _C__p_ = _γT_ + _βT_3. From a plot of

_C_/_T_ versus _T_2, shown in the inset of Fig. 2(c), we get γ = 3.96(3) mJK−2mol−1 and Θ_D_ = 151 K. Those values are for the low temperature phase. For the Pt-doped superconducting sample

with _x_ = 0.05, we obtain γ = 7.02(7) mJK−2mol−1 and Θ_D_ = 147 K based on specific heat data just above the superconducting transition temperature [Fig. 2(f)]. It is known that γ is

proportional to the electronic density of states near the fermi level. Taking γ = 3.96 mJK−2mol−1 in the low-T distorted structural phase of pure IrTe2 and assuming that the γ value in the

high-T trigonal phase is approximated to be the same as that of Ir0.95Pt0.05Te2 (i.e. ignore the change of the electronic specific heat coefficient due to Pt doping), we estimate that

roughly 44% fermi surface is removed across the structural phase transition. Below we shall focus on the structural phase transition near 280 K. Fig. 3 (a) presents the reflectance curves

below 27500 cm−1. The inset is the expanded low frequency range below 3000 cm−1. All the spectra approach to unity at zero frequency and good metallic response can be seen both above and

below the phase transition. In both high and low temperature phases, R(ω) shows relatively small change with a variation of temperature. However, a huge spectral change occurs on cooling the

sample across the first order phase transition at 280 K. The reflectance values below roughly 15000 cm−1 in the high temperature phase is significantly higher than those in the low

temperature phase, suggesting much higher Drude spectral weight in the high temperature phase. This is seen clearly from the low frequency optical conductivity spectra shown in Fig. 3 (b).

Since the spectral weight of the Drude component is proportional to the square of the plasma frequency , the sudden reduction of the Drude spectral weight reflects a reduction of the

_n_/_m_*. Besides the Drude component, the interband transitions at higher energies seem to experience some change as well. For example, the clear interband transition peak near 15000 cm−1

at high temperature phase becomes obscured below the phase transition. The overall spectral change reflects a significant band structure reconstruction associated with the first order phase

transition. To quantify the spectral change, particularly the evolution of the Drude component, across the phase transition, we tried to decompose the optical conductivity spectra into

different components using a Drude-Lorentz analysis. The dielectric function has the form8 where is the dielectric constant at high energy and the middle and last terms are the Drude and

Lorentz components, respectively. The Drude component represents the contribution from conduction electrons, while the Lorentz components describe the interband transitions. We found that

the optical conductivity spectra below 25000 cm−1 could be reasonably reproduced by one Drude and four Lorentz components. Fig. 3 (c) shows the conductivity spectra at 300 K and 250 K

together with the Drude-Lorentz fitting components. For clarity, the Lorentz components only at 300 K were shown in the figure. The parameters of Drude components above and below the

structural phase transition, _e.g._ at 300 K and 250 K, are ω_P_ ≈ 39000 cm−1, 29200 cm−1 and 1/τ ≈ 900 cm−1, 1250 cm−1, respectively. The inset of Fig. 3 (c) displays the

temperature-dependent evolution of 1/τ_j_ and . Both parameters are normalized to the values of 350 K. The sudden decease of plasma frequency is seen only at the phase transition. At other

temperatures the plasma frequencies keep roughly unchanged, but the scattering rates decrease with decreasing temperature in both phases, reflecting the expected narrowing of Drude component

for a metallic response. The scattering rate at 250 K is higher than the value at 300 K, which is consistent with the observation of dc resistivity measurement showing a higher resistivity

near 250 K after the jump at the transition. It is noted that the ratio of the square of the plasma frequency at low temperature phase to that at high temperature phase is about 0.56.

Provided that the effective mass of the itinerant carriers remains unchanged, the measurement reveals that roughly 44% itinerant carriers are lost after the first order phase transition.

This is in agreement with our specific heat estimation and the NMR measurement result21. The suppressed Drude spectral weight is transferred to higher energies due to the band

reconstruction. The reduced spectral weight is recovered roughly at the frequency of 25000 cm−1. Fig. 4 displays the reflectance and conductivity spectra for Ir0.95Pt0.05Te2 single crystals

with the variation of temperature from 10 K to 300 K. Because the structural transition is completely suppressed, the huge spectral change seen for the pure IrTe2 across the structural phase

transition is apparently absent for Ir0.95Pt0.05Te2. The low frequency Drude component shows usual narrowing due to the reduced scatterings with decreasing temperature. Weak but visible

intensity change could be identified for the interband transition peaks at high frequencies. Those interband transitions may involve the bands across the Fermi level. Due to the effect of

temperature-dependent Fermi distribution function, the electron occupations near the Fermi level on those bands would show a small change at different temperatures. This may explain the

observed intensity change. DISCUSSION Understanding the origin of the structural phase transition is a key to the understanding of electronic properties of the system. The transition was

suggested to be a kind of charge density wave with some involvement of Ir 5d orbitals13. However, our present measurement suggests against a density wave type phase transition. It is known

that a hallmark of the density-wave type transition is the formation of an energy gap in the single particle excitation spectrum near the Fermi level22, resulting in the lowering of the

total energy of the system. Within the scheme of the BCS theory for density wave condensate, the opening of energy gap leads to a characteristic peak feature just above the energy gap in

optical conductivity due to the effect of so-called “type-I coherent factor”22. The energy scale of the gap is related to the transition temperature, for example, 2Δ/_T__DW_ ~ 3.5 under the

weak-coupling BCS theory. However, in the present case, no such energy gap at low energy, or in the energy range with much larger gap value, could be identified from the optical conductivity

spectra. As we discussed above, the spectral change occurs over a very broad energy range up to 25000 cm−1, which is attributed to the reconstruction of the band structure associated with

the phase transition. Furthermore, according to all available structural investigation, all Ir sites are still equivalent even in the low temperature low symmetry phase11. As a result, the

structural phase transition is not likely to be driven by the so-called orbital-driven Peierls transition as well. A recent angle-resolved photoemission spectroscopy (ARPES) study is in

agreement with the present conclusion23. To gain insight into the nature of the structural phase transition, we have carefully examined the structural characteristic and its change across

the phase transition and performed local density approximation (LDA) band structure calculations. A very remarkable and characteristic structural feature about IrTe2 compound is that the

average Te-Te bond length d_Te−Te_ = 3.528 Å is noticeably shorter than the usual value of 4.03 Å of regular Te2+ in classic CdI2-like arrangement (e.g. HfTe2), making the valence state of

Ir close to Ir3+ rather than Ir4+ 20. The charge balance of this compound is close to Ir3+[Te−1.5]2, _i.e._ the Iridium ion is close to 5d6. Another piece of important information yielded

from the available structural investigation is that the structural distortion below the phase transition mainly causes a reduction of the Te-Te bond length between the upper and lower planes

of the IrTe6 octahedra slabs from 3.528 Å to 3.083 Å (i.e. d3 in Fig. 1 (c)). The change of Te-Te bond length within the Te layers of each IrTe6 octahedra slab across the transition is much

smaller, from 3.928 Å to 3.934 Å and 3.812 Å (i.e. d1 and d2 in Fig. 1 (c)), respectively11. Those structural characteristics provide a hint on the origin of the structural phase

transition. In the following we shall propose a novel interpretation for the structural phase transition. Since the Ir sits in the octahedral crystal field formed by Te, the Ir 5d level

would split into and manifolds. The six 5d6 electrons would mainly occupy the levels, leading to almost fully filled bands. Although there must be some hybridizations between Ir 5d and Te 5p

orbitals, the states near the Fermi level should be dominantly contributed by the Te 5p orbitals. Indeed, this was confirmed by the local-density approximation (LDA) band structure

calculations as presented in Fig. 5. Therefore, as a simplified picture, we shall only consider the Te 5p (p_x_, p_y_, p_z_) orbitals which accommodate roughly 5.5 electrons. Figure 5 (a)

and (c) show the band dispersions and density of state (DOS) of IrTe2 using the experimentally determined crystal structures at high temperature phase11. In the calculations, we have taken

into account of the spin-orbital coupling and applied the generalized gradient approximation (GGA) for the exchange-correlation potential. There are two bands crossing Fermi level if looking

along Γ-M line. One is closer to Γ which is contributed dominantly from the Te 5p_x_ and 5p_y_ orbitals and is largely filled; the other one, being contributed dominantly from Te 5p_z_

orbtial, is more dispersive and crosses E_F_ at higher momentum position. The resultant two FSs at high temperature phase are plotted in Fig. 5 (e). The results are in agreement with the

recently reported work13. However, our calculated Lindhard response function does not show strong features at any wave vector. The FS itself is not likely to drive a density wave type

instability. Because the structural phase transition mainly causes a suppression of the IrTe6 octahedron along the _c_-axis, it is expected that the structural distortion would further

split/separate the Te p_z_ band from the Te (p_x_, p_y_) band (a schematic picture is illustrated in the inset of Fig. 5 (d)). The Te (p_x_, p_y_) band is further lowered and filled, while

the Te p_z_ band is largely broadened and less occupied. As a consequence, both the inner and outer FSs would become smaller. This is indeed the case in our LDA calculation with a shortened

Te-Te bond length of d3 in terms of the available crystal structure at low temperature phase11, as shown in Fig. 5 (b), (d) and (f). Our calculations also indicate a reduction of the density

of sate (DOS) at Fermi level from N(E_F_) = 2.161 to 1.335 states/eV f.u., _i.e._ roughly a 40% loss, which could qualitatively interpret the specific heat and optical results. The

increased occupation of the further lowered Te (p_x_, p_y_) energy levels would result in a decrease of the kinetic energy of the electrons, which should be the driving force for the

transition. Finally, we comment on the striking diamagnetism observed in magnetic susceptibility measurement for those samples. As mentioned above, the valence state of Ir is Ir3+, the 5d6

electrons of Ir would fully fill the bands which are split from the e_g_ bands in the octahedral crystal field. The conducting electrons are mainly from the Te 5p orbitals. It is well-known

that the closed shells (or fully occupied bands) would contribute to the diamagnetism (Larmor diamagnetism). Because both Ir and Te are relatively heavy, many closed shells are present for

the compound. As a result, the Pauli paramagnetism contributed from the Te 5p orbitals could not overcome the diamagnetism resulted from the closed shells. Below the structural phase

transition, the Pauli paramagnetism is reduced due to a reduction of density of state near E_F_ , which results in further enhanced diamagnetism. To conclude, we have successfully grown

single crystal samples of Ir1−_x_Pt_x_Te2 (_x_ = 0, 0.03 and 0.05) and characterized their electronic properties. In particular, we performed a combined optical spectroscopy and first

principle calculation study on the undoped sample of IrTe2 in an effort to understand the origin of the structural phase transition at 280 K. The measurement revealed a sudden reconstruction

of band structure over broad energy scale and a significant removal of conducting carriers below the transition. The study indicated that the first order structural phase transition was not

driven by the density wave type Fermi surface instability, but caused by the crystal field effect which further split/separated the energy levels of Te (p_x_, p_y_) and Te p_z_ and resulted

in a reduction of the kinetic energy of the electronic system. METHODS Single crystals of Ir1−_x_Pt_x_Te2 have been successfully grown via self-flux technique. The mixtures of Ir (Pt)

powder and Te pieces in an atomic ratio of 0.18:0.82 were placed in an Al2O3 crucible and sealed in an evacuated quartz tube. The mixture was heated up initially to 950°C and kept for

several hours, then to 1160°C for one day and finally cooled down slowly to 900°C at a rate of 2°C/_h_. The flux Te was separated from single crystals by using a centrifuge. The dc

resistivity data were measured with a commercial Quantum Design physical properties measurement system (PPMS) by a four-probe method. The magnetic susceptibility was performed in a quantum

design superconducting quantum interference device vibrating sample magnetometer system (SQUID-VSM). The specific-heat measurements were conducted by a relaxation-time method using PPMS. The

temperature-dependent optical reflectance measurements were performed on Bruker 113v, Vertex 80v and a grating spectrometers on freshly cleaved surfaces of IrTe2 single crystals. An _in

situ_ gold and aluminum over-coating technique was used to get the reflectivity R(ω). The real part of conductivity σ1(ω) is obtained by the Kramers-Kronig transformation of R(ω). REFERENCES

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references ACKNOWLEDGEMENTS This work was supported by the National Science Foundation of China and the 973 project of the Ministry of Science and Technology of China

(2011CB921701,2012CB821403). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,

Beijing, 100190, People's Republic of China A. F. Fang, G. Xu, T. Dong, P. Zheng & N. L. Wang Authors * A. F. Fang View author publications You can also search for this author

inPubMed Google Scholar * G. Xu View author publications You can also search for this author inPubMed Google Scholar * T. Dong View author publications You can also search for this author

inPubMed Google Scholar * P. Zheng View author publications You can also search for this author inPubMed Google Scholar * N. L. Wang View author publications You can also search for this

author inPubMed Google Scholar CONTRIBUTIONS A.F.F. grew single crystals and carried out measurements. G.X. performed density function calculations. T.D. and P.Z. helped with transport and

specific heat measurement. A.F.F. and N.L.W. wrote the paper. N.L.W. supervised the project. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests.

RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit

http://creativecommons.org/licenses/by-nc-nd/3.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Fang, A., Xu, G., Dong, T. _et al._ Structural phase transition in IrTe2: A

combined study of optical spectroscopy and band structure calculations. _Sci Rep_ 3, 1153 (2013). https://doi.org/10.1038/srep01153 Download citation * Received: 11 October 2012 * Accepted:

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