Doping effect on the structure and physical properties of quasi-one-dimensional compounds Ba9Co3(Se1−xSx)15(x=0–0.2)∗

Lei Duan(段磊) Xian-Cheng Wang(望贤成) Jun Zhang(张俊) Jian-Fa Zhao(赵建发)Wen-Min Li(李文敏) Li-Peng Cao(曹立朋) Zhi-Wei Zhao(赵志伟) Changjiang Xiao(肖长江)Ying Ren(任瑛) Shun Wang(王顺) Jinlong Zhu(朱金龙) and Chang-Qing Jin(靳常青)

1School of Materials Science and Engineering,Henan University of Technology,Zhengzhou 450007,China

2Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences,Beijing 100190,China

3School of Physics,University of Chinese Academy of Sciences,Beijing 100190,China

4Department of Physics,Southern University of Science and Technology(SUSTech),Shenzhen 518055,China

5Materials Research Laboratory at Songshan Lake,Dongguan 523808,China

Keywords: doping effect,one-dimensional chain,spin glass,high-pressure synthesis

1. Introduction

Quasi-one-dimensional (1D) systems have attracted intense attention because of their various exotic physical properties such as superconductivity,[1–4]Bose–Einstein condensation,[5]and quantum phase transition.[6–8]Recently,a series of trimerized compounds Ba9M3X15(M=V,Fe,Co,Sn;X=S,Se,Te)with quasi-1D structure have been synthesized and studied actively.[9–13]These compounds crystalize into a hexagonal structure with the space group ofP-6c2, as schematically demonstrated in Fig. 1. The face-sharingMX6octahedrons stack along thecaxis to form–M2–M1–M2–onedimensional chains. These chains are triangularly arranged in theab-plane and separated by Ba atoms. Besides theseMX6chains, there existX-chains located at the center of the triangular lattice. The distance of the nearest neighbor ofMX6chains is very large,given by the lattice constantaabout 9 °A,displaying strongly 1D crystal structure characteristic. These compounds present 1D conducting chain characteristic and complex magnetic ground states owing to their unique structure. For example,Ba9Fe3Te15exhibits a spin-glass behavior with freezing temperature atTf～6.0 K and a hump with the maximum susceptibility at 190 K.[10]Ba9V3Se15undergoes a ferrimagnetic transition at 2.5 K and presents 1D ferromagnetic chains properties,i.e.,T1/2magnetic specific heat above the ordered temperature.[12]Substitution of V by Co, trimerized Ba9Co3Se15hosts a spin glass transition at～3 K,which mainly arises from the magnetic frustration due to the geometrically frustrated triangular lattice.[9]

Fig.. Schematic diagram of the Ba9M3X15 (M=V,Fe,Co,Sn;X =S,Se, Te) in ab-plane (a) and along the c axis (b), and individual MX6 chain(c). Green balls are Ba atoms, deep bule balls are M atoms, and red balls are X atoms.

For a quasi-1D system, the interaction and electronic hopping between the adjacent chains has an important influence on the electronic transport and magnetic properties.[14,15]Chemical doping can effectively shorten the distance and increase the orbital overlap between the atoms, thereby controlling the crystal structure, electronic structure, and even degrees of freedom of orbital and spin.[16]Hence, chemical doping provides an effective way to tune the interchain electronic hopping and spin interaction,and thus helps us to study and regulate the physical properties of quasi-one-dimensional materials. For example, quasi-1D BaVS3displays an insulator behavior with incommensurate anti-ferromagnetic order transition,[17]and the substitution of Se in S site enhances the electronic hoping among chains and then results in a metal to insulator transition.[18]In the series of Ba9Fe3X15(X=S,Se, Te), the variation of anion from S to Te leads to the enhancement of interchain interaction and electronic hopping,which have a significant effect on their transport and magnetic properties.[10,19,20]Besides a spin glass transition at～3 K,iso-structure Ba9Co3Se15also possesses a 1D conducting characteristic.[9]It is interesting to study the effect of chemical doping on the physical properties of Ba9Co3Se15. Here,we report on the synthesis of Ba9Co3(Se1−xSx)15(x=0,0.05,0.1,0.15,0.2)and the influence of S substitution on the crystal structure, electrical transport, and magnetic properties of recently discovered Ba9Co3Se15.

2. Experimental methods

Commercially available lumps of Ba(Alfa, immersed in oil,>99.2%pure)and powders of Co(Alfa,99.995%pure),Se (Alfa, 99.999% pure), and S (Alfa, 99.999% pure) were used as starting materials for sample synthesis. The precursors BaSe were prepared by the reaction of the Ba blocks and Se powders in an alumina crucible sealed in an evacuated quartz tube at 700°C for 20 h. The obtained BaSe powder,Se,S,and Co powder were mixed according to the elemental ratio of stoichiometric Ba9Co3(Se1−xSx)15(x=0, 0.05, 0.1,0.15,0.2),and pressed into a pellet with a diameter of 6 mm,and then loaded to a cubic anvil high-pressure apparatus under 5 GPa and 1000°C for 30 min,of which the details have been reported in Refs. [21,22]. After the high-pressure and hightemperature process, the black polycrystalline samples were obtained.

Room temperature powder x-ray diffraction (XRD) was conducted on a Huber diffractometer with CuKαradiation(λ=1.54060 °A).The XRD data were collected with a scanning rate of 1°/min and a scanning step of 0.02°. The Rietveld refinements on the diffraction patterns were performed using GSAS software packages. The dc magnetization measurements were carried out using a superconducting quantum interference device (SQUID) in the temperature range of 2–300 K under the field of 0.1 T.The temperature dependence of electrical resistivityρ(T)in the temperature range of 2–300 K was measured by four-probe electrical conductivity methods in a physical property measuring system(PPMS).The ac magnetic susceptibility was also measured by PPMS at different frequencies ranging from 133 Hz to 6333 Hz.

3. Results and discussion

Polycrystalline samples of Ba9Co3(Se1−xSx)15(x= 0,0.05,0.1,0.15,0.2)were synthesized under high pressure and high temperature conditions.Powder x-ray diffraction patterns measured at room temperature for all the samples with nominal composition Ba9Co3(Se1−xSx)15(x=0, 0.05, 0.1, 0.15,0.2)are shown in Fig.2(a).It can be seen that all these diffraction patterns can be indexed into a hexagonal structure with the space groupP-6c2.The enlarged views are shown in Fig.2(b).

Fig.2. (a)XRD patterns of Ba9Co3(Se1−xSx)15 (x=0,0.05,0.1,0.15,0.2)measured at room temperature. (b)The enlarged view between 29°and 38°.

We observed that the peaks shift monotonously towards a high-angle direction with the increase of content in the S dopants. This result demonstrates that the S atoms are successfully doped into Ba9Co3Se15and implies that the lattice shrinks with S doping. Then the structure of Ba9Co3Se15was adopted as the initial model to carry out the refinements for the XRD data of Ba9Co3(Se1−xSx)15(x=0, 0.05, 0.1, 0.15,0.2). The refinement for the XRD of Ba9Co3(Se1−xSx)15withx=0.2 is shown in Fig. 3(a), and the others are displayed in Figs. S1(a)–S1(d). The lattice parametersaandccan be obtained from the refinements. The obtained lattice constantsa=9.6994(1) °A andc=18.9842(3) °A of Ba9Co3Se15agree with those reported in the previous work.[9]The doping dependence of the lattice parameters is shown in Fig.3(b).

These lattice constants decrease linearly as the doping levelxincreases as expected due to the smaller ionic radius of S compared to Se, implying that the solid solutions follow Vegrad’s law. The summary of the crystallographic data for Ba9Co3(Se1−xSx)15withx=0.2 is obtained and listed in Table 1, and the others are exhibited in Tables S1–S4. For the compound Ba9Co3Se15,there are six atomic sites for Se,two sites in the CoSe6chain and the other four sites in the Sechain. From Table 1,we can see that when S is doped,S ions prefers to first occupy the atomic sites of Se/S in the CoSe/S6octahedral chain. The situation is the same for the other Sdoping levels. Within the S-doping level less than 20%, the occupation of S on the Se/S sites in the Se/S6chain is zero.When increasing the S-doping level, the occupation of S on Se sites in CoSe6chain increases gradually, as shown in Table 2. The similar site-selected doping phenomena have also been observed in the Ba9Sn3(SexTe1−x)15series.[13]

Fig.3. (a)XRD patterns and the refinements with a hexagonal structure and the space group of P-6c2 for Ba9Co3(Se1−xSx)15 with x=0.2. (b)Variation of the lattice parameter as a function of S content.

Table 1. Crystallographic data for Ba9Co3(Se0.80S0.20)15.

Table 2. The occupation of S atoms on Se sites for Ba9Co3(Se1−xSx)15.

Table 3 shows some selected important distances and angles of Ba9Co3(Se1−xSx)15. In a trimer unit of Ba9Co3(Se1−xSx)15, there are two Co atoms on the Co2 site and one on the Co1 site, as shown in Fig. 1(c). Upon increasing the S content, the distance of the adjacent Co1–Co2(d12) decreases, while the distance of the adjacent Co2–Co2(d22) increases. The radio of (|d22−d12| :|d22+d12|) can well show the degree of trimerization.[11]According to the distances of adjacent Co1–Co2 and Co2–Co2 in Table 3, we can obtain that the trimerization degree increases continuously from 7.4 % forx=0 to 14.7% forx=0.20, which indicates that substitution of S tends to the formation of trimerization in this system. The bond angle of Se(S)1–Co1–Se(S)1 of Ba9Co3(Se1−xSx)15is also shown in Table 3. It can be observed that all the angles deviate from the value of 180°in a regular octahedron, which suggests that the CoSe(S)6octahedrons are slightly compressed along thecaxis. The averaged intra-chain distancedintraof Co–Co can be calculated using the lattice constantcdivided by six and the inter-chain distancedinteris equal to the lattice constanta, as shown in Fig.4. We can obviously observe that with the replacement of S,the averaged intra-chain distancedintraand inter-chain distancedinterboth gradually decrease as expected. For a quasione-dimensional system, thedintraanddinterhave a key influence on the physical properties. Hence the replacement of S on Se should affect the properties of Ba9Co3(Se1−xSx)15, as we will discuss below.

Table 3. Selected distances(°A)between adjacent atoms and angles(°).

Fig.4. The S doping dependence of the dintra and dinter. dintra denotes the averaged intra-chain distance of Co–Co; while dinter denotes the inter-chain distance.

The dc magnetic susceptibility measurement as a function of temperature for Ba9Co3(Se1−xSx)15samples was carried out with an applied field of 1000 Oe in both zero-field-cooled(ZFC)and field-cooled(FC)modes to explore the doping effect on the magnetic properties of Ba9Co3(Se1−xSx)15. The magnetic susceptibility curves of Ba9Co3(Se1−xSx)15samples in the temperature range 2–60 K are shown in Fig. 5(a). It is seen that the ZFC and FC curves of Ba9Co3Se15are overlapped in high temperature region and then begin to bifurcate at about 5 K,which demonstrates aλ-shape agreed with the previous report. Meanwhile, the spin glass behavior of Ba9Co3Se15with frozen temperatureTfabout 3 K is confirmed by ac susceptibility curves.[9]The temperature dependence of magnetization of all doped samples also exhibits clear cusps at low temperature, which is similar to that of Ba9Co3Se15(shown in Fig.5(a)),suggesting that all the doped samples also display a spin-glass-like behavior like Ba9Co3Se15. To verify the spin glass feature,we measured the ac magnetic susceptibility for all the doped samples. The temperature dependence of the real part of the ac susceptibility for the doped sample withx=0.2 is shown in Fig. 5(b), and the others are exhibited in Figs. S2–S4. The results show that all the ac susceptibility curvesχ′display a peak at the frozen temperatureTf,which is similar to that of the ZFC curve of the dc susceptibility. As the frequency increases, the maximum value ofχ′decreases,while the frozen temperatureTfincreases,confirming the spin-glass ground state of all the doped samples. In addition,Fig.5(c)shows the frozen temperatureTfas a function of doping level of S.We can obviously observe that with the doping level increasing,Tfgradually increases from 3.1 K to 5.2 K. The spin glass behavior of the parent compound Ba9Co3Se15mainly arises from the magnetic frustration.[9]We speculate that the doping of S enhances the magnetic frustration and then causes the increase of the frozen temperatureTf.

For a 1D conducting chain system, the electron hopping among the chains plays a key role in the transport properties.In Ba9Co3(Se1−xSx)15system, as the content of S doping increases,the inter-chain distance is reduced gradually as shown in Fig. 4. However, the decrease of the inter-chain distance could enhance the 1D conducting characteristic,as will be discussed below. In order to study the fluence of S content on the electronic transport properties,resistivity measurements were performed on the polycrystalline Ba9Co3(Se1−xSx)15(x=0,0.05, 0.1, 0.15, 0.2) samples. The temperature dependence of resistivity is shown in Fig. 6(a). For all samples, the resistivity increases with decreasing temperature and displays a semiconducting behavior,exhibiting a quasi 1D conducting chain characteristic. Moreover, the resistivity increases more rapidly when the Se atoms are substituted by S.The curves of lnρversus 1/Tare shown in Fig.6(b). The straight line in the whole experiment temperature range suggests that the semiconducting behavior can be described based on the Arrhenius law for thermally activated conduction. By using the formulaρ∝exp(Δg/2kBT),whereΔgis the semiconducting band gap andkBis the Boltzmann’s constant, the resistivity curves are well fitted(shown in Fig.6(b))and the band gap could be obtained(shown in Fig.6(c)). We can observe that as the doping content of S increases,the band gap is gradually enlarged from 0.75 eV forx=0 to 0.86 eV forx=0.20.

Indeed,the increase of band gap in Ba9Co3(Se1−xSx)15as the substitution of Se by S is a typical phenomenon observed in the quasi-one-dimensional conducting chain system,where the transport properties depend on the strength of the electron hopping between the chains.[23,24]Since the S-3p electrons are more localized than Se-4p ones, the replacement of Se by smaller size S should reduce the electron hopping between CoSe/S6chains and give rise to an increase of the band gap. Therefore, although the interchain distance of Co decreases as the substitution of S into Se sites,the 1D conducting characteristic of Ba9Co3(Se1−xSx)15becomes stronger. Similar phenomena have occurred in Ba9Fe3X15(X=S, Se, Te)or Ba9Sn3(SexTe1−x)15series.[10,13,19]Besides chemical doping, high pressure also provides an effective way to change the inter-chain distance,then enhances the inter-chain electric hopping,and thus changes the 1D conducting chain system to a three-dimensional metal. In this process, some interesting physical properties have been observed like metal to insulator transition in Ba9Fe3Se15[19]and even possible superconductivity in Ba9Fe3Te15.[11]Ba9Co3Se15possesses the same structure as Ba9Fe3Se/Fe15and a 1D conducting chain characteristic,which deserves to be further studied under high pressure.

Fig. 5. (a) The magnetic susceptibility as a function of temperature of Ba9Co3(Se1−xSx)15 (x=0, 0.05, 0.1, 0.15, 0.2) samples in the range of 2–60 K.(b)The temperature dependence of the real part of ac magnetic susceptibility(χ′)for the doped sample with x=0.20 at different frequencies. (c)The frozen temperature Tf of all the samples vs. the doping level of S.

Fig.6. (a)Temperature dependence of the resistivity for Ba9Co3(Se1−xSx)15 (x=0,0.05,0.1,0.15,0.2);(b)lnρ versus 1/T. The red line is the linear fitting.(c)The S doping dependence of the band gap.

4. Conclusion and perspectives

A series of Ba9Co3(Se1−xSx)15(x=0, 0.05, 0.1, 0.15,0.2) samples were successfully synthesized under high pressure and high temperature conditions. For all doped samples,the S atoms tend to occupy the site of Se atoms in the CoSe6octahedron. In physical properties, all the samples are semiconductors and display spin glass behavior,similar to the parent compound Ba9Co3Se15. The frozen temperatureTffor the doped samples gradually increases with the doping S increasing. Furthermore, the S-3p electrons are more localized than Se-4p ones,which results in the decrease of the electronic hopping among chains. Hence, although the inter-chain instance decreases as the substitution of S increases, the band gap obviously increases.