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Fe(Se,Te) system crystallized in molten chlorides flux: The obtained materials and their characterization
Journal of Crystal Growth 528 (2019) 125268
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Fe(Se,Te) system crystallized in molten chlorides flux: The obtained materials and their characterization
T
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A. Masia, , C. Alvanib, A. Angrisani Armenioc, A. Augieric, G. Celentanoc, G. De Marzic, F. Fabbrib, C. Fiamozzi Zignanic, A. La Barberab, F. Padellab, F. Rizzoc, A. Rufolonic, E. Silvaa, A. Vannozzic, A. della Cortec a
Department of Engineering, Università degli Studi Roma Tre, Roma 00146, Italy C.R. Casaccia, ENEA, Roma 00123, Italy c C.R. Frascati, ENEA, Frascati 00044, Italy b
A R T I C LE I N FO
A B S T R A C T
Communicated by P. Rudolph
In this work we report the crystallization of iron chalcogenides compounds starting from a Fe(Se0.5Te0.5) solid pellet. The synthesis process is carried out at 675 °C using a molten chloride salts mixture and permitted to obtain well defined and easily separated hexagonal and tetragonal crystals, characterized by Fe3(Se0.43Te0.57)4 and Fe1.02Se0.41Te0.59 compositions respectively. The obtained tetragonal compound exhibits superconductivity at 14 K with critical current density of ~105 A/cm2 at 5 K.
Keywords: A1. Recrystallization A2. Single crystal growth A2. Growth from molten salts B1. Chalcogenides B2. Superconducting materials
1. Introduction The discovery of superconducting iron compounds [1] led to significant efforts in producing Iron Based Superconductors (IBSCs) with different composition and structures [2,3]. The so called “11” family of IBSCs, namely iron chalcogenides [4], is characterized by a simple structure, consisting in a stack of planes composed by a square lattice of Fe in tetrahedral coordination with chalcogen atoms. Among iron chalcogenides, the mixed selenide-telluride is a very appealing material [5] due to its electric and magnetic characteristics. Properties of compounds falling in the Fe(Se,Te) system have been evaluated by several groups as exhaustively reported in a recent review [6]. The synthesis of superconducting iron chalcogenides can be obtained by using different pathways (such as reactive sintering or melting) and mainly leads to multiple phase polycrystalline solids. Focusing the attention on single crystals, the synthesis is mostly carried out by a properly slow cooling of the melted material (i.e. the so-called self-flux method). However, the self-flux crystallization of the Fe(Se,Te) system often results in multiple crystalline phases coexisting in a single particle body [7–12]. Furthermore, chemical inhomogeneities (i.e. the Fe:Se:Te ratio inside the crystals) are often reported, both on large-scale [13,14] and on nano-scale. This is not uncommon, considering the possibility that the phase can be formed passing through incongruent
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melting and/or solid state phase transformations [15]. Although the phase diagram of the Fe-Se-Te system is unknown, evidences of the occurrence of both a complex solidification pathway and low temperature phase transformations cannot be overlooked [16–19]. An alternative way to obtain chalcogenide crystals consists in promoting the precipitation of the compound from a solution. Chalcogenides possess characteristics that favor their solubilization in polar or ionic solvents such as molten chloride salts [20]. When solubilized, the solute and the solid compound reach an equilibrium condition defined by the solubility product Kps. This means that a balance between solubilization and recrystallization processes is established, and these recurrent phenomena can lead to a refinement of the crystalline particles following Ostwald ripening mechanisms. When a thermal gradient is adopted, different local solubilities of the species can be obtained. The local lower solubility in the cold zone can then promote nucleation of some crystalline germs [21]. Then the subsequent growth of precipitated germs occurs concurrently with the refinement of the pre-existing solid in the hottest zone. In this framework the growth process of single nuclei can be considered as occuring in locally isothermal conditions. For this reason, homogeneous crystalline materials can be obtained [22,23], overcoming the large chemical, microstructural and phase variability deriving from melting syntheses of Fe(Se,Te) mixed chalcogenides [17].
Corresponding author at: Department of Engineering, Università degli Studi Roma Tre, Via Vito Volterra 62, 00146 Roma, RM, Italy. E-mail address: [email protected] (A. Masi).
https://doi.org/10.1016/j.jcrysgro.2019.125268 Received 22 July 2019; Received in revised form 27 September 2019; Accepted 30 September 2019 Available online 01 October 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 528 (2019) 125268
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resistance was measured by means of a four-point contact method, contacting the sample corners with small droplets of silver paste. Infield measurements were carried out applying the magnetic field perpendicularly to the platelets at selected values: 0 T, 0.25 T, 0.5 T, 1 T, 3 T, 5 T, 7 T, 9 T, 12 T. The upper critical magnetic field, Hc2, and the irreversibility field, Hirr, values were calculated evaluating Tc90% and Tc10% as the temperature values corresponding respectively to 90% and 10% of the extrapolated normal state resistance. Magnetic measurements were performed by means of an Oxford Instrument Vibrating Sample Magnetometer (VSM). Magnetization (M) measurements were carried out with the field applied perpendicularly to the platelets. Direct Current (DC) magnetization was measured in Zero Field Cooling (ZFC) conditions as a function of temperature with a 0.02 T magnetic field. The demagnetization factor was estimated considering the crystals as regular prisms [25] after appraising the crystals size by means of optical microscopy analysis. Remanent magnetization was measured as a function of temperature raising the temperature from 5 K after applying and removing a 0.5 T magnetic field. Critical current density Jc was estimated from magnetic hysteresis loops applying the extended Bean model [26] in two different ways: (i) evaluating the difference between the hysteresis loops, with the formula Jc = 20·ΔM/{a[1 − (a/ 3b)]}, where ΔM represents the difference between the hysteresis branches, and a and b are the sample dimensions (a < b); (ii) assessing the amount of the penetration field Hp, using the formula Jc = (Hp/r)/ [2 + ln(t/r)] [27], where t is the crystal thickness and r is the equivalent radius.
The aim of this work is to obtain additional information on chemical composition and structure of iron chalcogenides crystalline phases grown at a temperature value below the melting point of the Fe:Se:Te = 1:0.5:0:5 (at. ratio) system. The crystallization of the different phases was induced into a NaCl/KCl melted mixture flux, to promote the nucleation and growth of homogeneous single crystals at 675 °C. At the end of the experiment, after cooling, dissolution in water of the salts allows an easy recovery of the re-crystallized material. Morphology, chemical composition and crystalline structure of the resolidified materials have been assessed. Due to the well established superconducting properties of the tetragonal Fe(Se,Te) phase, a deeper insight on the obtained tetragonal single crystals was carried out by means of electrical and magnetic measurements.
2. Experimental The starting Fe(Se,Te) material (Fe:Se:Te = 1:0.5:0.5 at. ratio) was obtained by solid state reaction among the elemental reactants (Fe, > 99%, Se, 99.99% and Te, 99.8%, all from Sigma Aldrich), treated at 700 °C in vacuum-sealed quartz vials. By using this powder, cylindrical pellets (diameter ϕ = 5 mm) were prepared by uniaxial pressing. The selection of a pellet coming from the solid state route as source material was preferred with respect to the adoption of molten ingots due to the reported chemical, phase and structural variability observed in the latter case [18,19]. Each pellet was inserted in the bottom of a quartz vial (ϕ = 8 mm) and then covered with a dehydrated NaCl/KCl powder mixture at eutectic composition. The vial was then evacuated and sealed to obtain a final length of approximately 50 mm and placed in a horizontal tubular furnace. The thermal level of the furnace was set to 720 °C in the extremity containing the starting pellet, while the opposite extremity was placed at 675 °C. A schematic representation of the whole experimental setup is reported in Fig. 1. By using the described setup, a first sample was obtained after 450 h of treatment. For comparative purpose a second sample was obtained after 150 h. After the dwell period, the furnace was shut down and cooled. The vials were examined to evaluate the possibility of volatile elements sublimation and re-deposition, and no metallic halos suggesting this event were observed. Finally, the recrystallized iron chalcogenides were recovered by dissolving the chloride mixture in deionized water. After drying in vacuum oven at 80 °C, several crystals were collected for each batch and characterized. The synthesis processes were repeated twice to assess their reproducibility. Back-Scattered Scanning Electron Microscopy (BS-SEM), Energy Dispersive Spectroscopy (EDS) and Electron Back-Scattered Diffraction (EBSD) characterizations were performed using a Leo 1525 field-emission, high-resolution apparatus equipped with an Oxford x-act EDS detector and an Oxford NordlysNano EBSD detector. Powder X-Ray Diffraction (XRD) patterns were collected in θ/2θ geometry on recovered isolated single crystals on a Seifert PAD IV diffractometer (Mo Kα source). Peak profile analysis was carried out by fitting the experimental data by a symmetric Pseudo-Voigt function [24]. Electrical
3. Results and discussion 3.1. Characterization of the recrystallized material In Fig. 2 the well-defined crystals obtained after 450 h of thermal treatment are shown. The crystalline products consist of aggregates of hexagonal (panel a) and squared shaped crystals (panel b). The composition of the hexagonal compound, obtained by EDS analysis, resulted in Fe = 42.6 ± 0.7 At%, Se = 24.9 ± 1.2 At %, Te = 32.5 ± 0.8 At %, corresponding approximately to a Fe3(Se0.43Te0.57)4 stoichiometry. The squared shaped crystals exhibit an average chemical composition of Fe = 50.4 ± 0.3 At%, Se = 20.4 ± 0.6 At %, Te = 29.2 ± 0.3 At %, corresponding to an average stoichiometry of Fe1.02Se0.41Te0.59. The crystalline structure of the obtained crystals was evaluated by EBSD measurements. Similar patterns were obtained for several hexagonal and squared crystals respectively, and representative images are reported in Fig. 3. A pattern resulting from hexagonal crystals is reported in the panel a, while the relative indexing is shown in the panel b. The crystalline structure of the hexagonal phase is ascribable to a P3121 trigonal space group, suggesting a distortion from the ideal P63mmc NiAs compound as commonly occurs in many non-stoichiometric iron selenides and tellurides [28,29]. The results related to the tetragonal phase are reported in panels c and d, reporting the measurements and the indexed pattern respectively. The structure is ascribable to a P4/nmm tetragonal space group, as reported for the superconducting β-Fe(Se,Te) phase. Details on the composition and structure of a tetragonal single crystal are reported in Fig. 4. The absence of chemical fluctuations observed from the BS-SEM image and the EDS maps for Fe, Se and Te (Fig. 4a–d) indicates the microscopic homogeneity of the synthesized crystals. The phase distribution map and the pole figures obtained by means of the EBSD technique shown in Fig. 4e and f assess that the whole crystal structure is ascribable to the tetragonal phase, with a homogeneous orientation within the crystal. The crystals are oriented with the c crystallographic axis perpendicular to the crystal surface. EBSD analysis highlights the absence of inclusions of secondary phases within the crystals. XRD diffraction was carried out on single crystals in θ/2θ geometry to evaluate the lattice parameter c, strictly related to the Se:Te
Fig. 1. Schematic representation of the experimental setup. The material source is placed in the hot end of a sealed quartz vial with a NaCl/KCl eutectic mixture. Crystal nucleation and growth occurs at the cold zone. 2
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Ostwald ripening mechanism [32]. In the samples obtained after 150 h of treatment, a predominance of hexagonal-like structures and 120° angles was observed in the particles. In this view, it is reasonable to suppose that the first solidifying product in the flux is the hexagonallike one. After 450 h of thermal treatment, two kinds of well grown crystals are observed, with hexagonal and square symmetries. This suggests that no energy barrier act towards the development of the expected multi-phase final mixture, containing the tetragonal phase. We can suppose that after 150 h we are still at an initial stage of the refinement process. A longer period is thus needed to promote sufficient grain refinement of both structures, in order to be able to collect aggregates composed by well-grown and easily separable crystals of both phases in the cold end precipitate mixture. Observing that the chemical composition of the different compounds obtained after 150 h and 450 h are all characterized by similar Se:Te ratios, and being the hexagonal phase poorer in iron with respect to the tetragonal phase, it may be supposed that the growth mechanisms of the tetragonal crystals involve Fe introduction from the flux. The recrystallization products are crystals of tetragonal and hexagonal symmetries both characterized by a Se:Te ≈ 0.4:0.6 At% ratio. To assess that the Se:Te ratio inequality of the recrystallized samples is counter-balanced in the feeding material, a fracture of the residue pellet in the hot zone after 450 h of treatment was analyzed by SEM/EDS and compared with the fracture of an analogous pellet not yet treated. The results are summarized in Fig. 6. The pristine sample shows a dense morphology, with no significant chemical composition fluctuations (data not shown). In contrast with the pristine sample, the analysis of the pellet after 450 h at 720 °C (Fig. 6b) reveals a highly porous structure with two clearly visible different zones: smooth, regular crystals and ensembles of corrugated, irregular particles. A refinement of the crystalline grains in the source material is evident when compared with the starting pellet. As expected, also the recrystallization of the starting chalcogenide occurs, concurrentely with the material growth in the cold zone. Of course, the relative entity of the material recrystallized at locally different temperatures will be regulated by a kinetic balance between the difference of local solubilities and the diffusion of the species inside the reactor. The corrugated structures (detail in Fig. 6c) are characterized by a morphology that can be reasonably related to the occurrence of an erosion and dissolution process. EDS line-scans performed on the interfaces between a corrugated and a flat particle reveal a Se enrichment in the eroded structures (see panel c and d in Fig. 6) while the recrystallized material exhibit a Se:Te ratio similar to the collected single crystals. From these results follows that the Te enrichment observed in the recrystallized particles is likely balanced by the depletion observed in these corrugated zones. Starting from the Fe:Se:Te = 1:0.5:0.5, composition the experimental set-up permitted to grow hexagonal and tetragonal crystals in a quasi-equilibrium condition. The obtained chemical compositions of the precipitated phases likely constitute the more stable stoichiometry (i.e. the composition corresponding at the minimum Gibbs energy value). However, not all the elemental source was consumed in the synthesis. This does not allow to unambiguously define the whole equilibrium state of the system at the recrystallization temperature. Work is in progress to advance in this direction. Given the known affinity of these chalcogenides materials for oxygen, it is worth to discuss the possible effects of oxidation of the starting material on the subsequent crystallization process. In our case, experiments were conducted to evaluate the entity of oxidation in similar pellets: the oxygen content of the source remains comparable to the initial oxygen contamination observed in the commercial elemental reactants (less than 0.2 wt%) [33]. Considering the calculated ΔG Gibbs free energy for the reaction of elemental Fe, Se and Te with oxygen [34], the oxidation of iron results largely predominant with respect to Selenium and Tellurium (i.e. at 700 °C ΔG = -96 kcal/mol for 2Fe + O2 = 2FeO, ΔG = −34 kcal/mol for Te + O2 = TeO2, ΔG = −27 kcal/mol for Se + O2 = SeO2 (g), being other common
Fig. 2. SEM images of hexagonal (a) and tetragonal (b) crystals collected after 450 h of treatment. The images evidence the high morphological homogeneity of the collected products.
concentration ratio [5] (shown in Fig. S1). Only reflections ascribable to the 00ℓ planes of the β-Fe(Se,Te) phase are evident, as expected from a highly oriented crystalline solid. The pattern is characterized by sharp and well-defined peaks, with absence of anisotropic broadening. The obtained lattice parameter value is c = 6.04 ± 0.01 Å, in line with results reported for similar chemical compositions [10,13]. 3.2. An insight on the recrystallization process In order to investigate the occurred nucleation and growth processes, the synthesis was carried out also for a shorter time. After 150 h of thermal treatment only crystalline aggregates with heterogeneous morphology and composition were observed. Fig. 5 reports an example of an aggregate. As evidenced by the image, the particle is characterized by a complex morphology. The image suggests an ongoing evolution. Within the particles, regions characterized by different appearance were observed: roughly two average different compositions have been measured: i) Fe = 42 ± 1.5 At%, Se = 26 ± 1.5 At%, Te = 31 ± 1.5 At% and ii) Fe = 49 ± 1.5 At%, Se = 24 ± 1.5 At%, Te = 27 ± 1.5 At%, corresponding to flat and corrugated areas respectively. In crystallization process occurring from a solution, the first nucleating product is often constituted by the compound possessing the structure that is closer to the low-range order initially possessed in the flux [30,31]. Subsequent digestion mechanisms, conducted at constant temperature for a necessarily long time, lead to the structuration of the more stable compounds at the expense of the particles possessing a higher free energy level and promote the crystal growth through the 3
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Fig. 3. Kikuchi pattern of a Fe3(Se0.43Te0.57)4 hexagonal crystal (a) and pattern indexed for a P3121 phase (b); Kikuchi pattern of a Fe1.02Se0.41Te0.59 tetragonal crystal (c) and pattern indexed for a P4/nmm phase (d).
approximately 15 K. At lower temperature, a sharp superconducting transition is evident, with a Tc 90% of about 14 K and a zero-resistance critical temperature of about 13 K. Several samples from different synthesis batches were evaluated, all showing a similar behavior. In Fig. 7b, the resistive transition evaluated at different fields applied parallel to the crystalline c axis is reported. We observe a progressive shift towards lower temperature with the increase of the external fields, with a moderate increase of the transition width. To evaluate the infield behavior, Hirr and Hc2 values were estimated from the resistive transition (Fig. 7c). We observe a steep increase of both curves, testifying the high degree of robustness of the superconducting phase towards the external field, with values comparable to previous results [9]. Results of the magnetic characterization are reported in Fig. 8. The magnetization measured as a function of the temperature is reported in Fig. 8a. The sample is characterized by a diamagnetic signal at low temperature, with the transition temperature Tc at approximately 13.7 K. In contrast with the sharp electrical transition, the magnetic transition is quite broad, even if this is probably related to the relatively large applied field (i.e. 0.02 T), needed to maximize the signal due to the small size of the crystal. Considering the high value of applied field and a possible uncertainty on the demagnetization factor, the measurements indicate a high amount of shielded volume. The remanent magnetization as a function of the temperature is shown in Fig. 8 b. Starting from low temperature, the remanent magnetization is characterized by a gradual decrease when increasing the temperature, vanishing over Tc. The magnetic hysteresis cycle measured at 5 K is reported in the Fig. 8 c. The evaluation of the difference between the magnetization branches allowed to estimate the critical current density resulting in a value Jc ≈ 2.4 ⋅ 105 A/cm2 in self field and Jc ≈ 8 ⋅ 104 A/
oxide compounds characterized by less favorable values). This is in agreement with current literature, where oxidation of Fe(Se,Te) is reported to occur via the formation of surface iron oxides [35]. In the molten salt, an oxide layer could act as a barrier inhibiting the refinement process; however, iron oxide solubility in molten chlorides is not negligible [36], and passivation phenomena are not likely occurring in our case. The pre-existing oxide is therefore expected to be dissolved in the salt, and the same mechanism of dissolution and precipitation involved in the growth of the chalcogenide crystals should be expected. Considering the limited amount of initial oxidation, the absence of evident oxide products in the recovered material suggests that the overall oxide amount is less than its solubility limit or that minor amounts of amorphous silicates are formed in the cold zone [21]. We expect therefore that the effect of a minor oxidation of the starting samples is limited to a slight variation in the overall chemical compositions (due to a capture effect of the starting iron ions), without affecting the chalcogenide crystallization process. Work is in progress to further evaluate this aspect. 3.3. Superconducting characterization of tetragonal crystals The superconducting properties of tetragonal crystals were finally measured. An estimation of the electrical resistivity of the material was carried out at room temperature by means of the Van der Pauw method [37], resulting in a value of approximately ρ ≈ 1.5 * 10−3 Ω cm. The electrical resistance as a function of the temperature of a single Fe1.02Se0.41Te0.59 crystal is reported in Fig. 7a. The normal state resistance is characterized by a non-metallic behavior, with a moderate resistance increase evident decreasing the temperature down to 4
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Fig. 5. SEM image of an aggregate of crystals collected after 150 h of treatment. Flat and corrugated areas are evidenced within the particle. A negligible presence of 90° structural features is observed.
Fig. 4. BS-SEM image of a tetragonal single crystal (a) with EDS distribution maps for Fe (b), Se (c) and Te (d); distribution map of the P4/nmm phase (e) and (1 0 0)/(0 0 1) pole figures (f) as obtained by EBSD analysis (see text for details).
cm2 at 0.4 T. The penetration field was as well estimated from the magnetization loop: in the field-decrease process, the flux is being trapped until it reaches a maximum when the applied field has decreased through the range ΔH = 2Hp. The amount of the penetration field Hp (approximately 0.034 T) was considered to estimate the critical current, resulting in a value Jc ≈ 5.5 ⋅ 104 A/cm2 at the maximum applied field (0.5 T). The measurements were carried out on different crystals from two different synthesis batches, showing reproducible results with Jc in self field of approximately 105 A/cm2, suggesting that in our crystals the whole bulk is superconductive. From a macroscopic point of view, the limitation of iron excess in the tetragonal phase has revealed to be crucial to obtain good superconducting properties in Fe(Se,Te) systems [38–40]. A significant degradation (low critical temperatures, broad superconducting transitions, no magnetic screening, low critical currents and absence of
Fig. 6. (a) SEM image of a fracture of the pristine source pellet; (b) SEM images of the fracture of the source pellet after 450 h at 720 °C ; (c) high magnification image with superimposed position of the EDS line scan; (d) concentration of Fe, Se, Te (At%) along the EDS line.
evident discontinuities in the specific heat) are in fact always observed in Fe-rich samples, and have been associated with the detrimental effect of iron excess in interstitial sites [40]. In this cases, the observed electrical superconducting transitions have been ascribed to the presence of filamentary superconducting paths in a non superconducting bulk [38]. The interstitial iron has also been related to the electrical properties of these chalcogenides. A magnetic coupling between the excess Fe and 5
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currents are obtained and the semi-conducting electrical behavior is converted to a metallic one. The Authors suggest that the annealing treatment induces bulk superconductivity and at the same time metallic behavior by removing the iron excess from the tetragonal lattice. Therefore, limited iron excess has been associated with a metallic resistance behavior. From the synthesis point of view, the easiest and most adopted way to obtain a low Fe excess in the tetragonal phase is to synthesize the compound cooling a melted iron-deficient mixture (i.e. Fe: (Se + Te) = 1:1), as commonly carried out in the self-flux routes. In fact, the binary phase diagrams for FeSe and FeTe systems indicate in both cases that the tetragonal phases exist for a small excess of Fe (e.g. Fe1+δ with δ ≈ 0.03–0.1) [28,29], and a similar feature can be reasonably assumed also for the mixed Fe(Se,Te) compound [13]. The melting routes lead therefore to the structuration of a main tetragonal phase and of a hexagonal/trigonal secondary phase with Fe < 50 At%; the tetragonal phase is thus characterized by the boundary composition with the lowest Fe-excess, corresponding to a Fe-poor line in the unknown ternary phase diagram. The samples obtained in this way are always characterized by metallic behavior and often by good superconducting properties. However, as previously highlighted, syntheses from melts (i.e. self-flux method) do not grant to obtain flawless crystals. As evidenced in the recent literature (e.g. [41,42]), the compounds are frequently characterized by evident chemical and/or structural inhomogeneities, potentially affecting defect overall content and their distribution (e.g. interstitial iron). It is worthy to note that transition metal chalcogenides is a class of compounds which exhibits a rather complex electrical conductivity behavior strongly associated to the nature of the to the metal–chalcogenide bonds, to the local structure, to the compound stoichiometry, to the presence of lattice defects and impurities [43,44]. An indication suggesting that the metallic behavior may not be biunivocally correlated to the iron excess amount can be however found in previous reports of Fe(Se,Te) crystals grown in molten chlorides [22] and through self-flux methods [45,46]. In the first case, similarly as in our work, Ovchenkov et al. described single crystals of Fe1+δSexTe1−x (x up to 0.45), grown in a KCl/NaCl flux at 765/720 °C thermal gradient. These crystals are characterized by low iron excess while exhibiting a non-metallic electrical behavior [22]. In the latter case, the authors assessed the role of a difference in cooling rate during the selfflux synthesis cooling step. Crystals obtained through the slowest procedure are characterized by high homogeneity and lack of secondary phases with respect to the faster process, that leads to inhomogeneity and secondary phase inclusions. No significant difference is evident nevertheless in the chemical composition of the tetragonal phases among the two samples. Despite the same chemical composition, however, the homogeneous samples are characterized by non-metallic electrical behavior, in contrast with the metallic one observed in the inhomogeneous sample [45,46]. The authors concluded ascribing the insurgence of superconductivity in the system to the presence of nanoscale inhomogeneity. An opposed electrical and superconducting behavior is here obtained. The materials have been synthesized by using different growth techniques and show a similar chemical composition. From the here reported XRD characterization is not evidenced the presence of diffracting nano-domains (that could be originated by chemical inhomogeneity). The question whether is defect distribution or chemical homogeneity that affect electrical properties of the material and their association with the superconducting phases remains open.
Fig. 7. (a) electrical resistance normalized with respect to room temperature value (R/R290K) as a function of temperature for a Fe1.02Se0.41Te0.59 single crystal; (b) detail of the superconducting transition measured at different fields up to 12 T; (c) Hirr (solid squares) and Hc2 (empty squares) values estimated from the resistive transitions as a function of the temperature.
Fig. 8. (a) Magnetization, measured in ZFC with a 0.02 T applied field orthogonal to the ab crystal plane, as a function of temperature; (b) remanent magnetization as a function of temperature; (c) hysteresis loop measured at 5 K.
the adjacent Fe square planar sheets has been associated to charge localization effects promoting a semiconducting behavior [38]. It is undoubtful that there is a strict relation between superconducting properties and the normal state electrical behavior of Fe(Se,Te) materials. Sun et al. observed a strict correlation between metallic behavior and good superconducting properties [40]. In their work, Fe1+δSexTe(1−x) samples obtained through a melting method are characterized by a semi-conducting electric behavior, a weak diamagnetic signal below 3 K and low critical currents [14,40]. When the samples are annealed in oxidizing atmosphere, sharp magnetic transitions and large critical
4. Conclusions A flux of molten NaCl/KCl mixture was adopted to synthesize iron chalcogenides single crystals starting from a Fe:Se:Te = 1:0.5:0.5 At% composition. The growth occurred into an extremity of a sealed quartz vial, kept at 675 °C for 450 h. After this period, well-formed and separated single crystals of hexagonal and tetragonal shape were recovered. 6
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The experimental results suggest that in this medium the hexagonal crystal formation is kinetically favored, and that tetragonal crystals nucleation occurs via iron enrichment of the hexagonal pristine compound from the surrounding flux. The composition of the hexagonal and tetragonal crystals corresponds to Fe3(Se0.43Te0.57)4 and Fe1.02Se0.41Te0.59, respectively. The adopted quasi-equilibrium condition suggests that the measured stoichiometries represent equilibrium compositions at the growth temperature. Hexagonal crystals exhibit a P3121 trigonal space group, showing a distortion from the ideal P63mmc NiAs compound as commonly occurs in many iron selenides and tellurides. Tetragonal particles structure is ascribable to a P4/nmm space group, typical of Fe(Se,Te) superconducting compounds. Regarding superconducting properties of the tetragonal phase, the crystals exhibit a non-metallic resistance behavior in normal-state and show a superconducting transition at 14 K. Magnetic measurements allow to estimate critical currents of approximately 105 A/cm2 at 5 K. The unexpected coupling of non-metallic behavior and the high measured critical current suggests a separate role on superconducting and electrical properties of defect distribution or chemical homogeneity in the Fe(Se,Te) system.
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Fe(Se,Te) system crystallized in molten chlorides flux: The obtained materials and their characterization
T
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A. Masia, , C. Alvanib, A. Angrisani Armenioc, A. Augieric, G. Celentanoc, G. De Marzic, F. Fabbrib, C. Fiamozzi Zignanic, A. La Barberab, F. Padellab, F. Rizzoc, A. Rufolonic, E. Silvaa, A. Vannozzic, A. della Cortec a
Department of Engineering, Università degli Studi Roma Tre, Roma 00146, Italy C.R. Casaccia, ENEA, Roma 00123, Italy c C.R. Frascati, ENEA, Frascati 00044, Italy b
A R T I C LE I N FO
A B S T R A C T
Communicated by P. Rudolph
In this work we report the crystallization of iron chalcogenides compounds starting from a Fe(Se0.5Te0.5) solid pellet. The synthesis process is carried out at 675 °C using a molten chloride salts mixture and permitted to obtain well defined and easily separated hexagonal and tetragonal crystals, characterized by Fe3(Se0.43Te0.57)4 and Fe1.02Se0.41Te0.59 compositions respectively. The obtained tetragonal compound exhibits superconductivity at 14 K with critical current density of ~105 A/cm2 at 5 K.
Keywords: A1. Recrystallization A2. Single crystal growth A2. Growth from molten salts B1. Chalcogenides B2. Superconducting materials
1. Introduction The discovery of superconducting iron compounds [1] led to significant efforts in producing Iron Based Superconductors (IBSCs) with different composition and structures [2,3]. The so called “11” family of IBSCs, namely iron chalcogenides [4], is characterized by a simple structure, consisting in a stack of planes composed by a square lattice of Fe in tetrahedral coordination with chalcogen atoms. Among iron chalcogenides, the mixed selenide-telluride is a very appealing material [5] due to its electric and magnetic characteristics. Properties of compounds falling in the Fe(Se,Te) system have been evaluated by several groups as exhaustively reported in a recent review [6]. The synthesis of superconducting iron chalcogenides can be obtained by using different pathways (such as reactive sintering or melting) and mainly leads to multiple phase polycrystalline solids. Focusing the attention on single crystals, the synthesis is mostly carried out by a properly slow cooling of the melted material (i.e. the so-called self-flux method). However, the self-flux crystallization of the Fe(Se,Te) system often results in multiple crystalline phases coexisting in a single particle body [7–12]. Furthermore, chemical inhomogeneities (i.e. the Fe:Se:Te ratio inside the crystals) are often reported, both on large-scale [13,14] and on nano-scale. This is not uncommon, considering the possibility that the phase can be formed passing through incongruent
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melting and/or solid state phase transformations [15]. Although the phase diagram of the Fe-Se-Te system is unknown, evidences of the occurrence of both a complex solidification pathway and low temperature phase transformations cannot be overlooked [16–19]. An alternative way to obtain chalcogenide crystals consists in promoting the precipitation of the compound from a solution. Chalcogenides possess characteristics that favor their solubilization in polar or ionic solvents such as molten chloride salts [20]. When solubilized, the solute and the solid compound reach an equilibrium condition defined by the solubility product Kps. This means that a balance between solubilization and recrystallization processes is established, and these recurrent phenomena can lead to a refinement of the crystalline particles following Ostwald ripening mechanisms. When a thermal gradient is adopted, different local solubilities of the species can be obtained. The local lower solubility in the cold zone can then promote nucleation of some crystalline germs [21]. Then the subsequent growth of precipitated germs occurs concurrently with the refinement of the pre-existing solid in the hottest zone. In this framework the growth process of single nuclei can be considered as occuring in locally isothermal conditions. For this reason, homogeneous crystalline materials can be obtained [22,23], overcoming the large chemical, microstructural and phase variability deriving from melting syntheses of Fe(Se,Te) mixed chalcogenides [17].
Corresponding author at: Department of Engineering, Università degli Studi Roma Tre, Via Vito Volterra 62, 00146 Roma, RM, Italy. E-mail address: [email protected] (A. Masi).
https://doi.org/10.1016/j.jcrysgro.2019.125268 Received 22 July 2019; Received in revised form 27 September 2019; Accepted 30 September 2019 Available online 01 October 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
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resistance was measured by means of a four-point contact method, contacting the sample corners with small droplets of silver paste. Infield measurements were carried out applying the magnetic field perpendicularly to the platelets at selected values: 0 T, 0.25 T, 0.5 T, 1 T, 3 T, 5 T, 7 T, 9 T, 12 T. The upper critical magnetic field, Hc2, and the irreversibility field, Hirr, values were calculated evaluating Tc90% and Tc10% as the temperature values corresponding respectively to 90% and 10% of the extrapolated normal state resistance. Magnetic measurements were performed by means of an Oxford Instrument Vibrating Sample Magnetometer (VSM). Magnetization (M) measurements were carried out with the field applied perpendicularly to the platelets. Direct Current (DC) magnetization was measured in Zero Field Cooling (ZFC) conditions as a function of temperature with a 0.02 T magnetic field. The demagnetization factor was estimated considering the crystals as regular prisms [25] after appraising the crystals size by means of optical microscopy analysis. Remanent magnetization was measured as a function of temperature raising the temperature from 5 K after applying and removing a 0.5 T magnetic field. Critical current density Jc was estimated from magnetic hysteresis loops applying the extended Bean model [26] in two different ways: (i) evaluating the difference between the hysteresis loops, with the formula Jc = 20·ΔM/{a[1 − (a/ 3b)]}, where ΔM represents the difference between the hysteresis branches, and a and b are the sample dimensions (a < b); (ii) assessing the amount of the penetration field Hp, using the formula Jc = (Hp/r)/ [2 + ln(t/r)] [27], where t is the crystal thickness and r is the equivalent radius.
The aim of this work is to obtain additional information on chemical composition and structure of iron chalcogenides crystalline phases grown at a temperature value below the melting point of the Fe:Se:Te = 1:0.5:0:5 (at. ratio) system. The crystallization of the different phases was induced into a NaCl/KCl melted mixture flux, to promote the nucleation and growth of homogeneous single crystals at 675 °C. At the end of the experiment, after cooling, dissolution in water of the salts allows an easy recovery of the re-crystallized material. Morphology, chemical composition and crystalline structure of the resolidified materials have been assessed. Due to the well established superconducting properties of the tetragonal Fe(Se,Te) phase, a deeper insight on the obtained tetragonal single crystals was carried out by means of electrical and magnetic measurements.
2. Experimental The starting Fe(Se,Te) material (Fe:Se:Te = 1:0.5:0.5 at. ratio) was obtained by solid state reaction among the elemental reactants (Fe, > 99%, Se, 99.99% and Te, 99.8%, all from Sigma Aldrich), treated at 700 °C in vacuum-sealed quartz vials. By using this powder, cylindrical pellets (diameter ϕ = 5 mm) were prepared by uniaxial pressing. The selection of a pellet coming from the solid state route as source material was preferred with respect to the adoption of molten ingots due to the reported chemical, phase and structural variability observed in the latter case [18,19]. Each pellet was inserted in the bottom of a quartz vial (ϕ = 8 mm) and then covered with a dehydrated NaCl/KCl powder mixture at eutectic composition. The vial was then evacuated and sealed to obtain a final length of approximately 50 mm and placed in a horizontal tubular furnace. The thermal level of the furnace was set to 720 °C in the extremity containing the starting pellet, while the opposite extremity was placed at 675 °C. A schematic representation of the whole experimental setup is reported in Fig. 1. By using the described setup, a first sample was obtained after 450 h of treatment. For comparative purpose a second sample was obtained after 150 h. After the dwell period, the furnace was shut down and cooled. The vials were examined to evaluate the possibility of volatile elements sublimation and re-deposition, and no metallic halos suggesting this event were observed. Finally, the recrystallized iron chalcogenides were recovered by dissolving the chloride mixture in deionized water. After drying in vacuum oven at 80 °C, several crystals were collected for each batch and characterized. The synthesis processes were repeated twice to assess their reproducibility. Back-Scattered Scanning Electron Microscopy (BS-SEM), Energy Dispersive Spectroscopy (EDS) and Electron Back-Scattered Diffraction (EBSD) characterizations were performed using a Leo 1525 field-emission, high-resolution apparatus equipped with an Oxford x-act EDS detector and an Oxford NordlysNano EBSD detector. Powder X-Ray Diffraction (XRD) patterns were collected in θ/2θ geometry on recovered isolated single crystals on a Seifert PAD IV diffractometer (Mo Kα source). Peak profile analysis was carried out by fitting the experimental data by a symmetric Pseudo-Voigt function [24]. Electrical
3. Results and discussion 3.1. Characterization of the recrystallized material In Fig. 2 the well-defined crystals obtained after 450 h of thermal treatment are shown. The crystalline products consist of aggregates of hexagonal (panel a) and squared shaped crystals (panel b). The composition of the hexagonal compound, obtained by EDS analysis, resulted in Fe = 42.6 ± 0.7 At%, Se = 24.9 ± 1.2 At %, Te = 32.5 ± 0.8 At %, corresponding approximately to a Fe3(Se0.43Te0.57)4 stoichiometry. The squared shaped crystals exhibit an average chemical composition of Fe = 50.4 ± 0.3 At%, Se = 20.4 ± 0.6 At %, Te = 29.2 ± 0.3 At %, corresponding to an average stoichiometry of Fe1.02Se0.41Te0.59. The crystalline structure of the obtained crystals was evaluated by EBSD measurements. Similar patterns were obtained for several hexagonal and squared crystals respectively, and representative images are reported in Fig. 3. A pattern resulting from hexagonal crystals is reported in the panel a, while the relative indexing is shown in the panel b. The crystalline structure of the hexagonal phase is ascribable to a P3121 trigonal space group, suggesting a distortion from the ideal P63mmc NiAs compound as commonly occurs in many non-stoichiometric iron selenides and tellurides [28,29]. The results related to the tetragonal phase are reported in panels c and d, reporting the measurements and the indexed pattern respectively. The structure is ascribable to a P4/nmm tetragonal space group, as reported for the superconducting β-Fe(Se,Te) phase. Details on the composition and structure of a tetragonal single crystal are reported in Fig. 4. The absence of chemical fluctuations observed from the BS-SEM image and the EDS maps for Fe, Se and Te (Fig. 4a–d) indicates the microscopic homogeneity of the synthesized crystals. The phase distribution map and the pole figures obtained by means of the EBSD technique shown in Fig. 4e and f assess that the whole crystal structure is ascribable to the tetragonal phase, with a homogeneous orientation within the crystal. The crystals are oriented with the c crystallographic axis perpendicular to the crystal surface. EBSD analysis highlights the absence of inclusions of secondary phases within the crystals. XRD diffraction was carried out on single crystals in θ/2θ geometry to evaluate the lattice parameter c, strictly related to the Se:Te
Fig. 1. Schematic representation of the experimental setup. The material source is placed in the hot end of a sealed quartz vial with a NaCl/KCl eutectic mixture. Crystal nucleation and growth occurs at the cold zone. 2
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Ostwald ripening mechanism [32]. In the samples obtained after 150 h of treatment, a predominance of hexagonal-like structures and 120° angles was observed in the particles. In this view, it is reasonable to suppose that the first solidifying product in the flux is the hexagonallike one. After 450 h of thermal treatment, two kinds of well grown crystals are observed, with hexagonal and square symmetries. This suggests that no energy barrier act towards the development of the expected multi-phase final mixture, containing the tetragonal phase. We can suppose that after 150 h we are still at an initial stage of the refinement process. A longer period is thus needed to promote sufficient grain refinement of both structures, in order to be able to collect aggregates composed by well-grown and easily separable crystals of both phases in the cold end precipitate mixture. Observing that the chemical composition of the different compounds obtained after 150 h and 450 h are all characterized by similar Se:Te ratios, and being the hexagonal phase poorer in iron with respect to the tetragonal phase, it may be supposed that the growth mechanisms of the tetragonal crystals involve Fe introduction from the flux. The recrystallization products are crystals of tetragonal and hexagonal symmetries both characterized by a Se:Te ≈ 0.4:0.6 At% ratio. To assess that the Se:Te ratio inequality of the recrystallized samples is counter-balanced in the feeding material, a fracture of the residue pellet in the hot zone after 450 h of treatment was analyzed by SEM/EDS and compared with the fracture of an analogous pellet not yet treated. The results are summarized in Fig. 6. The pristine sample shows a dense morphology, with no significant chemical composition fluctuations (data not shown). In contrast with the pristine sample, the analysis of the pellet after 450 h at 720 °C (Fig. 6b) reveals a highly porous structure with two clearly visible different zones: smooth, regular crystals and ensembles of corrugated, irregular particles. A refinement of the crystalline grains in the source material is evident when compared with the starting pellet. As expected, also the recrystallization of the starting chalcogenide occurs, concurrentely with the material growth in the cold zone. Of course, the relative entity of the material recrystallized at locally different temperatures will be regulated by a kinetic balance between the difference of local solubilities and the diffusion of the species inside the reactor. The corrugated structures (detail in Fig. 6c) are characterized by a morphology that can be reasonably related to the occurrence of an erosion and dissolution process. EDS line-scans performed on the interfaces between a corrugated and a flat particle reveal a Se enrichment in the eroded structures (see panel c and d in Fig. 6) while the recrystallized material exhibit a Se:Te ratio similar to the collected single crystals. From these results follows that the Te enrichment observed in the recrystallized particles is likely balanced by the depletion observed in these corrugated zones. Starting from the Fe:Se:Te = 1:0.5:0.5, composition the experimental set-up permitted to grow hexagonal and tetragonal crystals in a quasi-equilibrium condition. The obtained chemical compositions of the precipitated phases likely constitute the more stable stoichiometry (i.e. the composition corresponding at the minimum Gibbs energy value). However, not all the elemental source was consumed in the synthesis. This does not allow to unambiguously define the whole equilibrium state of the system at the recrystallization temperature. Work is in progress to advance in this direction. Given the known affinity of these chalcogenides materials for oxygen, it is worth to discuss the possible effects of oxidation of the starting material on the subsequent crystallization process. In our case, experiments were conducted to evaluate the entity of oxidation in similar pellets: the oxygen content of the source remains comparable to the initial oxygen contamination observed in the commercial elemental reactants (less than 0.2 wt%) [33]. Considering the calculated ΔG Gibbs free energy for the reaction of elemental Fe, Se and Te with oxygen [34], the oxidation of iron results largely predominant with respect to Selenium and Tellurium (i.e. at 700 °C ΔG = -96 kcal/mol for 2Fe + O2 = 2FeO, ΔG = −34 kcal/mol for Te + O2 = TeO2, ΔG = −27 kcal/mol for Se + O2 = SeO2 (g), being other common
Fig. 2. SEM images of hexagonal (a) and tetragonal (b) crystals collected after 450 h of treatment. The images evidence the high morphological homogeneity of the collected products.
concentration ratio [5] (shown in Fig. S1). Only reflections ascribable to the 00ℓ planes of the β-Fe(Se,Te) phase are evident, as expected from a highly oriented crystalline solid. The pattern is characterized by sharp and well-defined peaks, with absence of anisotropic broadening. The obtained lattice parameter value is c = 6.04 ± 0.01 Å, in line with results reported for similar chemical compositions [10,13]. 3.2. An insight on the recrystallization process In order to investigate the occurred nucleation and growth processes, the synthesis was carried out also for a shorter time. After 150 h of thermal treatment only crystalline aggregates with heterogeneous morphology and composition were observed. Fig. 5 reports an example of an aggregate. As evidenced by the image, the particle is characterized by a complex morphology. The image suggests an ongoing evolution. Within the particles, regions characterized by different appearance were observed: roughly two average different compositions have been measured: i) Fe = 42 ± 1.5 At%, Se = 26 ± 1.5 At%, Te = 31 ± 1.5 At% and ii) Fe = 49 ± 1.5 At%, Se = 24 ± 1.5 At%, Te = 27 ± 1.5 At%, corresponding to flat and corrugated areas respectively. In crystallization process occurring from a solution, the first nucleating product is often constituted by the compound possessing the structure that is closer to the low-range order initially possessed in the flux [30,31]. Subsequent digestion mechanisms, conducted at constant temperature for a necessarily long time, lead to the structuration of the more stable compounds at the expense of the particles possessing a higher free energy level and promote the crystal growth through the 3
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Fig. 3. Kikuchi pattern of a Fe3(Se0.43Te0.57)4 hexagonal crystal (a) and pattern indexed for a P3121 phase (b); Kikuchi pattern of a Fe1.02Se0.41Te0.59 tetragonal crystal (c) and pattern indexed for a P4/nmm phase (d).
approximately 15 K. At lower temperature, a sharp superconducting transition is evident, with a Tc 90% of about 14 K and a zero-resistance critical temperature of about 13 K. Several samples from different synthesis batches were evaluated, all showing a similar behavior. In Fig. 7b, the resistive transition evaluated at different fields applied parallel to the crystalline c axis is reported. We observe a progressive shift towards lower temperature with the increase of the external fields, with a moderate increase of the transition width. To evaluate the infield behavior, Hirr and Hc2 values were estimated from the resistive transition (Fig. 7c). We observe a steep increase of both curves, testifying the high degree of robustness of the superconducting phase towards the external field, with values comparable to previous results [9]. Results of the magnetic characterization are reported in Fig. 8. The magnetization measured as a function of the temperature is reported in Fig. 8a. The sample is characterized by a diamagnetic signal at low temperature, with the transition temperature Tc at approximately 13.7 K. In contrast with the sharp electrical transition, the magnetic transition is quite broad, even if this is probably related to the relatively large applied field (i.e. 0.02 T), needed to maximize the signal due to the small size of the crystal. Considering the high value of applied field and a possible uncertainty on the demagnetization factor, the measurements indicate a high amount of shielded volume. The remanent magnetization as a function of the temperature is shown in Fig. 8 b. Starting from low temperature, the remanent magnetization is characterized by a gradual decrease when increasing the temperature, vanishing over Tc. The magnetic hysteresis cycle measured at 5 K is reported in the Fig. 8 c. The evaluation of the difference between the magnetization branches allowed to estimate the critical current density resulting in a value Jc ≈ 2.4 ⋅ 105 A/cm2 in self field and Jc ≈ 8 ⋅ 104 A/
oxide compounds characterized by less favorable values). This is in agreement with current literature, where oxidation of Fe(Se,Te) is reported to occur via the formation of surface iron oxides [35]. In the molten salt, an oxide layer could act as a barrier inhibiting the refinement process; however, iron oxide solubility in molten chlorides is not negligible [36], and passivation phenomena are not likely occurring in our case. The pre-existing oxide is therefore expected to be dissolved in the salt, and the same mechanism of dissolution and precipitation involved in the growth of the chalcogenide crystals should be expected. Considering the limited amount of initial oxidation, the absence of evident oxide products in the recovered material suggests that the overall oxide amount is less than its solubility limit or that minor amounts of amorphous silicates are formed in the cold zone [21]. We expect therefore that the effect of a minor oxidation of the starting samples is limited to a slight variation in the overall chemical compositions (due to a capture effect of the starting iron ions), without affecting the chalcogenide crystallization process. Work is in progress to further evaluate this aspect. 3.3. Superconducting characterization of tetragonal crystals The superconducting properties of tetragonal crystals were finally measured. An estimation of the electrical resistivity of the material was carried out at room temperature by means of the Van der Pauw method [37], resulting in a value of approximately ρ ≈ 1.5 * 10−3 Ω cm. The electrical resistance as a function of the temperature of a single Fe1.02Se0.41Te0.59 crystal is reported in Fig. 7a. The normal state resistance is characterized by a non-metallic behavior, with a moderate resistance increase evident decreasing the temperature down to 4
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Fig. 5. SEM image of an aggregate of crystals collected after 150 h of treatment. Flat and corrugated areas are evidenced within the particle. A negligible presence of 90° structural features is observed.
Fig. 4. BS-SEM image of a tetragonal single crystal (a) with EDS distribution maps for Fe (b), Se (c) and Te (d); distribution map of the P4/nmm phase (e) and (1 0 0)/(0 0 1) pole figures (f) as obtained by EBSD analysis (see text for details).
cm2 at 0.4 T. The penetration field was as well estimated from the magnetization loop: in the field-decrease process, the flux is being trapped until it reaches a maximum when the applied field has decreased through the range ΔH = 2Hp. The amount of the penetration field Hp (approximately 0.034 T) was considered to estimate the critical current, resulting in a value Jc ≈ 5.5 ⋅ 104 A/cm2 at the maximum applied field (0.5 T). The measurements were carried out on different crystals from two different synthesis batches, showing reproducible results with Jc in self field of approximately 105 A/cm2, suggesting that in our crystals the whole bulk is superconductive. From a macroscopic point of view, the limitation of iron excess in the tetragonal phase has revealed to be crucial to obtain good superconducting properties in Fe(Se,Te) systems [38–40]. A significant degradation (low critical temperatures, broad superconducting transitions, no magnetic screening, low critical currents and absence of
Fig. 6. (a) SEM image of a fracture of the pristine source pellet; (b) SEM images of the fracture of the source pellet after 450 h at 720 °C ; (c) high magnification image with superimposed position of the EDS line scan; (d) concentration of Fe, Se, Te (At%) along the EDS line.
evident discontinuities in the specific heat) are in fact always observed in Fe-rich samples, and have been associated with the detrimental effect of iron excess in interstitial sites [40]. In this cases, the observed electrical superconducting transitions have been ascribed to the presence of filamentary superconducting paths in a non superconducting bulk [38]. The interstitial iron has also been related to the electrical properties of these chalcogenides. A magnetic coupling between the excess Fe and 5
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currents are obtained and the semi-conducting electrical behavior is converted to a metallic one. The Authors suggest that the annealing treatment induces bulk superconductivity and at the same time metallic behavior by removing the iron excess from the tetragonal lattice. Therefore, limited iron excess has been associated with a metallic resistance behavior. From the synthesis point of view, the easiest and most adopted way to obtain a low Fe excess in the tetragonal phase is to synthesize the compound cooling a melted iron-deficient mixture (i.e. Fe: (Se + Te) = 1:1), as commonly carried out in the self-flux routes. In fact, the binary phase diagrams for FeSe and FeTe systems indicate in both cases that the tetragonal phases exist for a small excess of Fe (e.g. Fe1+δ with δ ≈ 0.03–0.1) [28,29], and a similar feature can be reasonably assumed also for the mixed Fe(Se,Te) compound [13]. The melting routes lead therefore to the structuration of a main tetragonal phase and of a hexagonal/trigonal secondary phase with Fe < 50 At%; the tetragonal phase is thus characterized by the boundary composition with the lowest Fe-excess, corresponding to a Fe-poor line in the unknown ternary phase diagram. The samples obtained in this way are always characterized by metallic behavior and often by good superconducting properties. However, as previously highlighted, syntheses from melts (i.e. self-flux method) do not grant to obtain flawless crystals. As evidenced in the recent literature (e.g. [41,42]), the compounds are frequently characterized by evident chemical and/or structural inhomogeneities, potentially affecting defect overall content and their distribution (e.g. interstitial iron). It is worthy to note that transition metal chalcogenides is a class of compounds which exhibits a rather complex electrical conductivity behavior strongly associated to the nature of the to the metal–chalcogenide bonds, to the local structure, to the compound stoichiometry, to the presence of lattice defects and impurities [43,44]. An indication suggesting that the metallic behavior may not be biunivocally correlated to the iron excess amount can be however found in previous reports of Fe(Se,Te) crystals grown in molten chlorides [22] and through self-flux methods [45,46]. In the first case, similarly as in our work, Ovchenkov et al. described single crystals of Fe1+δSexTe1−x (x up to 0.45), grown in a KCl/NaCl flux at 765/720 °C thermal gradient. These crystals are characterized by low iron excess while exhibiting a non-metallic electrical behavior [22]. In the latter case, the authors assessed the role of a difference in cooling rate during the selfflux synthesis cooling step. Crystals obtained through the slowest procedure are characterized by high homogeneity and lack of secondary phases with respect to the faster process, that leads to inhomogeneity and secondary phase inclusions. No significant difference is evident nevertheless in the chemical composition of the tetragonal phases among the two samples. Despite the same chemical composition, however, the homogeneous samples are characterized by non-metallic electrical behavior, in contrast with the metallic one observed in the inhomogeneous sample [45,46]. The authors concluded ascribing the insurgence of superconductivity in the system to the presence of nanoscale inhomogeneity. An opposed electrical and superconducting behavior is here obtained. The materials have been synthesized by using different growth techniques and show a similar chemical composition. From the here reported XRD characterization is not evidenced the presence of diffracting nano-domains (that could be originated by chemical inhomogeneity). The question whether is defect distribution or chemical homogeneity that affect electrical properties of the material and their association with the superconducting phases remains open.
Fig. 7. (a) electrical resistance normalized with respect to room temperature value (R/R290K) as a function of temperature for a Fe1.02Se0.41Te0.59 single crystal; (b) detail of the superconducting transition measured at different fields up to 12 T; (c) Hirr (solid squares) and Hc2 (empty squares) values estimated from the resistive transitions as a function of the temperature.
Fig. 8. (a) Magnetization, measured in ZFC with a 0.02 T applied field orthogonal to the ab crystal plane, as a function of temperature; (b) remanent magnetization as a function of temperature; (c) hysteresis loop measured at 5 K.
the adjacent Fe square planar sheets has been associated to charge localization effects promoting a semiconducting behavior [38]. It is undoubtful that there is a strict relation between superconducting properties and the normal state electrical behavior of Fe(Se,Te) materials. Sun et al. observed a strict correlation between metallic behavior and good superconducting properties [40]. In their work, Fe1+δSexTe(1−x) samples obtained through a melting method are characterized by a semi-conducting electric behavior, a weak diamagnetic signal below 3 K and low critical currents [14,40]. When the samples are annealed in oxidizing atmosphere, sharp magnetic transitions and large critical
4. Conclusions A flux of molten NaCl/KCl mixture was adopted to synthesize iron chalcogenides single crystals starting from a Fe:Se:Te = 1:0.5:0.5 At% composition. The growth occurred into an extremity of a sealed quartz vial, kept at 675 °C for 450 h. After this period, well-formed and separated single crystals of hexagonal and tetragonal shape were recovered. 6
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The experimental results suggest that in this medium the hexagonal crystal formation is kinetically favored, and that tetragonal crystals nucleation occurs via iron enrichment of the hexagonal pristine compound from the surrounding flux. The composition of the hexagonal and tetragonal crystals corresponds to Fe3(Se0.43Te0.57)4 and Fe1.02Se0.41Te0.59, respectively. The adopted quasi-equilibrium condition suggests that the measured stoichiometries represent equilibrium compositions at the growth temperature. Hexagonal crystals exhibit a P3121 trigonal space group, showing a distortion from the ideal P63mmc NiAs compound as commonly occurs in many iron selenides and tellurides. Tetragonal particles structure is ascribable to a P4/nmm space group, typical of Fe(Se,Te) superconducting compounds. Regarding superconducting properties of the tetragonal phase, the crystals exhibit a non-metallic resistance behavior in normal-state and show a superconducting transition at 14 K. Magnetic measurements allow to estimate critical currents of approximately 105 A/cm2 at 5 K. The unexpected coupling of non-metallic behavior and the high measured critical current suggests a separate role on superconducting and electrical properties of defect distribution or chemical homogeneity in the Fe(Se,Te) system.
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[39]
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