Vol.:(0123456789) Brazilian Journal of Physics (2024) 54:187 https://doi.org/10.1007/s13538-024-01554-3 The Growth of  V5S8 Single Crystals by Chemical Vapour Transport C. A. Sonego1,2 · H. Li2 · P. Einarsson Nielsen2 · J. C. Lashley2 · M. A. Avila1 · S. E. Rowley2 Received: 5 June 2024 / Accepted: 8 July 2024 / Published online: 1 August 2024 © The Author(s) 2024 Abstract Single crystals of the d-electron antiferromagnetic metal V5S8 can be prepared by chemical vapour transport with gaseous iodine as a transport agent. We present the outcomes of an endeavour to synthesise high-purity single crystals of V5S8 with reduced crystalline disorder, important to the formation of novel quantum orders. We report results on the residual resistivity ratio of the single crystals as growth parameters are varied including growth temperature, temperature gradient and pre- growth processing of the initial apparatus and reagents. We demonstrate that single crystals of at least a few mm in size can be successfully grown at relatively low temperatures in the range 550–600 °C. The optimisation of this method may imply a better crystallographic organisation, reducing sulphur vacancies and increasing vanadium positional order. The resulting longer electron mean free paths may enhance the probability of finding exotic quantum states of matter at low temperatures. The results presented here may also be of relevance to the development of vanadium sulphide-based energy storage and spintronic devices. Keywords Chemical vapour transport · Transition-metal chalcogenides · Vanadium sulphide · V5S8 · d-electron metallic magnetism · Antiferromagnets · Strongly correlated electron systems · Battery electrodes · Spintronics · Low temperature resistivity · Fermi liquids 1 Introduction Strongly correlated electron systems are known to give rise to a variety of exotic quantum phenomena such as non-Fermi liquids, high-temperature superconductors, heavy-fermion systems and quantum spin liquids. Here, we investigate the synthesis of the transition-metal chalcogenide compound V5S8 [1, 2]. This narrow-band d-electron metal hosts an anti- ferromagnetic phase below a Néel temperature of approxi- mately 33 K [3–9]. It is part of a wider family of compounds VxS8 that hosts a variety of charge and magnetically ordered states at low temperatures upon changes in stoichiometry, x, temperature, magnetic field and pressure. The family may be expanded further with Se or Te in place of S and other transition-metal elements in place of V. They may poten- tially host unconventional forms of superconductivity in pure enough samples, including those anticipated to emerge on the border of magnetism [10] or charge-density-waves in bulk specimens [11, 12]. The vanadium sulphides have also gained significant recent interest as a platform for future energy technologies due to their electrochemical properties and subsequent performance as electrode materials in metal- ion batteries and supercapacitors [13]. V5S8 may also be relevant to the development of thin or quasi two-dimensional spintronic devices when prepared as layers a few nm thick and deposited on SiO2/Si wafers [14]. The success of the development of these technologies depends on the ability to optimise and carefully control the physical and chemical properties of the materials by the particular crystal synthesis procedure. VS2 is a layered material [15] as shown in Fig. 1a and in which transition metals (such as Ag, Ti, Mo, Nb and Ta) may be intercalated within the van der Waals gaps between layers with different degrees of mobility. Indeed, V5S8 may be thought of as a particularly stable phase of VS2 intercalated with additional vanadium between the layers. This results in metal-full layers and metal-deficient layers as seen in Fig. 1b. Previous studies [1–3, 17–19] into the synthesis of vana- dium sulphides by chemical vapour transport showed that * S. E. Rowley ser41@cam.ac.uk 1 CCNH, Universidade Federal do ABC, Santo André, São Paulo, Brazil 2 Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, UK http://crossmark.crossref.org/dialog/?doi=10.1007/s13538-024-01554-3&domain=pdf Brazilian Journal of Physics (2024) 54:187187 Page 2 of 10 Fig. 1 The crystal structures of transition-metal chalcogenides VS2 (trigonal) in (a) and V5S8 (monoclinic) in (b). The vanadium ions are depicted as purple spheres and the sulphur ions as yellow spheres. Vanadium ions reside in sulphur cages whose faces are shaded in red. The conventional unit cells are outlined with black lines. V5S8 can be thought of as being obtained by intercalating additional vanadium atoms between the empty layers in VS2. In c, we show the phase dia- gram of temperature, T, versus composition, z, in VzS1-z of relevance to the synthesis of vanadium sulphide compounds. The phase diagram is consistent with that found in Ref. [16]. Different liquid and solid phases of the compounds are shown in the green shaded areas. The shaded domes labelled with various V:S ratios are stable phases of solid vanadium sulphide compounds. RT and HT stand for room tem- perature and high temperature respectively. V5S8 can be prepared by weighing out the starting elements such that z falls between 0.375 and 0.415 in the low-temperature growing limit. This z window narrows as the growing temperature increases up to a single point at 850 °C Brazilian Journal of Physics (2024) 54:187 Page 3 of 10 187 different stoichiometries could be prepared between the V5S8 and V3S4 members of the phase diagram in Fig. 1c by control- ling the temperature and sulphur pressure inside the growth ampoule. V5S8 has been considered by some researchers as a system for studying disorder. The vanadium atoms have a finite probability of occupying any vacant site in the metal- deficient layers shown in Fig. 1b which, unless particular protocols are followed during synthesis, can result in crystals that are substantially disordered. Moreover, the high volatility of sulphur, particularly during synthesis at high temperatures, can lead to stoichiometries with less sulphur than planned [1]. This has led to residual resistivity ratios, defined as the resistivity at room temperature divided by the resistivity at 4.2 K, RRR = ρ(300 K)/ρ(4.2 K), to be no more than around 4 as reported in the published literature [20] and more typi- cally 2 or less [21] for bulk crystals grown by any method. A low RRR is related to a large residual resistivity, ρ0, defined as the resistivity measured in the zero-temperature limit, itself a gauge of the mean free path of an electron as it trav- els within the material. The ability to find novel quantum states such as finite angular momentum superconductivity, which for example involve non-local electron–electron inter- actions, relies on the electron mean free path well exceed- ing the effective coherence length of the particular quantum state in question. The RRR and mean free path are essentially limited by electron-scattering from impurity atoms, defects within the crystal and other forms of quenched disorder. A key goal of this study was therefore to synthesise V5S8 single crystals with minimal disorder, as measured by the residual resistivity ratio, to open up new possibilities in the search for unconventional quantum phases. The crystal growing temperature-composition phase diagram shown in Fig. 1c shows that crystals of V5S8 may form in a narrow window around 40 atomic per cent vanadium up to around 850 °C. We demonstrate that single crystals can be obtained at lower growth temperatures than those reported previously, meaning that the number of vacancies of the volatile element sulphur may be reduced. We also show that purifying the starting elements by melting vanadium in a vacuum before commenc- ing single crystal growth can lead to an enhancement of the RRR . The chemical vapour transport methods we employed are described in Sect. 2. The results including those of X-ray diffraction, low-temperature resistivity, Néel temperature and RRR as a function of growth temperature are reported in Sect. 3. Concluding remarks are made in Sect. 4. 2 The Growth of  V5S8 Single Crystals by Chemical Vapour Transport Chemical vapour transport (CVT) [22] has been widely adopted for the production of advanced materials, with typical arrangements employing a horizontal tube furnace having two zones of different temperatures separated by several centimetres. A typical CVT method involves an evacuated quartz ampoule sealed with high-purity reagents and a transport agent at one of the two ends. The end with the reagents is then placed at one of the two different tem- perature zones of a tube furnace, and the opposite end is placed at the other zone where single crystal formation occurs (see, e.g. Figure 2b). The temperature gradient cre- ated between the hot and cold zones creates a convection current and Stefan flow inside the sealed quartz tube. The direction of the flow depends on the enthalpy of the reac- tion. In the case of V5S8 with an iodine transport agent as used in our experiments, the reaction is endothermic and transport occurs from the hot zone to the cold zone. The gaseous transport agent proceeds by picking up atoms or molecules from the solid state at the hot zone and depositing them at the cold zone to form single crystals, as depicted schematically in Fig. 2b. This paper reports the results of measurements on single crystals selected from five different batches, each batch of crystals having undergone varying preparation and growth conditions as outlined below. Batch 1 The growth parameters for batch 1 are summarized in Table 1 and gave similar results to those previously pub- lished [3, 23]. Vanadium pieces and sulphur powder of nomi- nal purities 99.99% and 99.999%, respectively, were weighed in the ratios shown in Table 1 and sealed in an evacuated quartz ampoule. The ampoule was placed in a chamber fur- nace, uniformly heated up to 700 °C over 24 h and kept at the same temperature for 10 days to allow the reagents to chemi- cally react, forming several grammes of polycrystalline vana- dium sulphide compound. The ampoule was then returned to room temperature and opened. The powdered compound (≈ 5 g) and iodine transport agent (0.61 g, of purity 99.999%) were then loaded into the closed end of a new cylindrical quartz ampoule with a length of ≈ 25 cm and a volume of 30.5 cm3. The ampoule was pumped to a high vacuum and sealed at the opposite (open) end with a hydrogen–oxygen blow torch. The sealed ampoule was placed in a two-zone tube furnace as shown schematically in Fig. 2c, with the rea- gents located at the hot zone and the opposite end located at the cold zone, 23 cm away from the hot zone. The hot and cold zones were raised to 900 °C and 750 °C respectively over a period of a few hours, and crystals were left to grow for 10 days, before cooling over 24 h to room temperature. Batch 2 The following procedures were followed to further minimise the introduction of impurities and disorder, pri- marily by eliminating the separate pre-CVT reaction step. (i) Any quartz tubes to be used were cleaned by sub- merging in 36% concentrated hydrochloric acid for 60 min to etch away surface impurities. The tubes Brazilian Journal of Physics (2024) 54:187187 Page 4 of 10 were then heated to 1100 °C while pumping with an oil-free pump down to < 10−5 mbar for 24 h. This enabled some of the thermally activated impuri- ties trapped inside the walls of the quartz tube to be ejected into the vacuum. The tubes were returned to room temperature and acid etched again, becoming ready for the next steps. (ii) The vanadium starting reagent pieces were etched in 36% concentrated HCl(aq) for 10 min to remove surface impurities. (iii) Vanadium pieces and sulphur powder of nominal purities 99.99% and 99.999% respectively were weighed in the proportions shown in Table 1 and placed inside the closed end of one of the pre-cleaned Fig. 2 a The image shows part of the process of purifying the starting vanadium material as carried out for Batch 5 using a radio-frequency electromagnetic induction furnace. Elemental vanadium of 99.99% nominal purity (labelled (1)) is placed on a water-cooled copper boat (2) and heated to just above its melting point (~ 1910 °C) inside an evacuated quartz tube (3) using an induction coil (4). Volatile impuri- ties are ejected into the vacuum and other impurities migrate to the surface to be removed by chemical surface etching in a later step. Note that the water-cooled copper boat and quartz tube remain rela- tively close to room temperature compared to the heated elements, such that they do not themselves undergo outgassing which could introduce impurities into the melt. b A schematic representation of the apparatus we used for growing single crystals of V5S8 by chemi- cal vapour transport. The distance between the hot and cold zones in our experiments was 23 cm. The purified starting elements were placed at the hot zone with temperature TH and single crystals would nucleate and grow at the cold zone with temperature TC as molecules are transported from hot to cold regions using gaseous iodine (20 mg per volume of the quartz ampoule in cm3). c A typical temperature– time profile for the synthesis procedure for crystal Batches 2–5 as described in the main text with TH and TC values given in Table  1. The ramp profile includes a 1-day warm-up period and a 2-day pre- CVT reaction phase in which both zones are held at the same tem- perature (700 °C), followed by a 14–20-day crystal growing phase in which the cold zone is held 75 °C below the hot zone temperature, creating a temperature gradient. The synthesis is concluded with a combined annealing and cool-down phase by lowering the tempera- ture of both zones to room temperature over 2 days Brazilian Journal of Physics (2024) 54:187 Page 5 of 10 187 quartz ampoules with 0.61 g of iodine of purity 99.999%. The open (opposite) end was then con- nected to an oil-free pump and evacuated to < 10−3 mbar, purged with high-purity argon and pumped down again to < 10−3 mbar. The open end was then sealed with a hydrogen–oxygen blow torch. The use of oil-free pumps and hydrogen–oxygen as opposed to acetylene torches helped to avoid the introduction of hydrocarbons into the crystal growing space. The cylindrical sealed quartz ampoule was approximately 25 cm long and 30.5 cm3 in volume. (iv) The ampoule was placed in the two-zone tube furnace with the reagents at the hot zone and the opposite end located 23 cm away at the cold zone as shown sche- matically in Fig. 2b where single crystals could form. The temperature of the two zones of the tube furnace was then controlled to follow the temperature–time profile as shown in Fig. 2c. That is, for Batch 2, both zones were heated to 700 °C for 2 days to allow a pre-CVT reaction to take place. The hot and cold zone temperatures were then changed as shown in Fig. 2c and Table 1 to create a temperature gradi- ent (3.2 °C/cm) and allow CVT crystal growth to take place for 14 days. Both zones were then slowly cooled to room temperature over 2 days such that the crystals could experience a degree of annealing. This procedure avoided the separate pre-CVT reac- tion stage as in Batch 1, which took place in a differ- ent quartz tube before re-exposing to air and sealing in a new crystal growing ampoule. Avoiding this step helped to minimise the introduction of oxides and other impurities in between stages. Batches 3 and 4 The same procedure was followed as for Batch 2, only with different hot and cold zone temperatures as outlined in Table 1, to test the effect of growth tempera- ture on the measured properties of the crystals. Batch 5 The same procedures were followed for Batch 5 crystals as in Batches 2–4, with the following exceptions. (i) The vanadium starting material of nominal purity 99.99% was further purified before commencing chemical vapour transport growth. This was done by acid etching the vanadium pieces and then placing them on a water-cooled copper boat inside a quartz tube that was continuously pumped under ultra-high- vacuum conditions to < 10−3 mbar. The vanadium was then heated to just above its melting point (1910 °C) for 24 h using a radio-frequency electromagnetic induction coil as shown in Fig. 2a. Once in the liquid phase, impurities migrated to the surface and were ejected into the evacuated quartz tube and pumped away. The vanadium was returned to room tempera- ture and acid etched again to remove any remaining impurities left on the surface. Note that the water- cooled copper boat and quartz tube remain relatively close to room temperature, therefore do not them- selves outgas and introduce impurities. The molten vanadium is able to be levitated and stirred during heating due to the induced currents and emergence of opposing electromagnetic fields. (ii) The acid etching steps of the quartz ampoules prior to use comprised firstly etching with 36% concentrated HCl(aq) for 10 min, followed by submerging the ampoules in an isopropanol and sodium hydroxide solution for 1 h. (iii) Immediately prior to sealing the quartz ampoule for CVT growth, the quartz ampoule with one end closed was heated, or ‘flamed’, close to its melting point along its entire length with a hydrogen–oxygen blow torch while pumping at the open end with an oil-free vacuum pump. This had the effect of fur- ther ejecting trapped impurities from the walls of the quartz ampoules and creating clean and smooth surfaces onto which crystals could precipitate during chemical vapour transport. Batch 5 crystals were pre- pared as described under Batch 2 but with the growth parameters stated in Table 1 and using the purified vanadium and cleaned quartz tubes as described here. 3 X‑ray Diffraction and Low‑Temperature Resistivity Single crystals were grown in all of Batches 1–5 and were shiny hexagonal platelets (see insets of Fig. 3a and c) of up to a few mm in length, in agreement with reports in the published literature. Before any measurements were per- formed, the single crystals were cleaned in acetone for one hour and then etched in 36% concentrated HCl(aq) for ten minutes to remove any surface oxide layers. The results of powder X-ray diffraction (XRD) measurements carried out at room temperature with a Bruker D8 Focus system, on single crystals ground into a fine powder for each batch, are shown in Fig. 3 along with their associated Rietveld refine- ments. For all specimens in which XRD measurements were performed, the samples were found to have crystal- lized with the accepted space group F12/m1 for the mono- clinic V5S8 crystal structure type (sometimes referred to as C12/m1 using an alternative convention). For Batch 3, the lattice parameters were found to be a = 11.38 Å, b = 6.65 Å, Brazilian Journal of Physics (2024) 54:187187 Page 6 of 10 c = 11.31 Å, α = 90°, β = 91.46°, γ = 90° and unit cell vol- ume Vcell = 855.24 Å3 with very little variation among all the batches measured (< 0.2%). As shown in Fig. 3a, Batch 1 crystals were found to have inclusions of a V2O3 phase in addition to V5S8. This may have been because a pre-CVT reaction took place for this batch in a separate quartz tube which was opened exposing the compound to air before seal- ing in a new quartz ampoule for CVT. No impurity phases were detected in the other batches measured that avoided this air-exposing step. The stoichiometries, V5Sy, for each batch were determined from the Rietveld analysis and energy dispersive electron spectroscopy (EDS) measurements, and the results are shown in Table 1. The values of y in each case were moderately less than that expected from the ratios of the weighed starting elements (also shown in Table 1), presumably because sulphur is a volatile element and a pro- portion of it remains in the gaseous phase outside of the solidified crystals. A few grammes of starting material were placed at the hot zone for all of the batches before commenc- ing CVT synthesis. For Batches 1, 2, 3 and 5, a few hundred mg of single crystals had formed after CVT lasting 14–20 days. In the case of Batch 4, however, only a few single crys- tals had grown, presumably because the lower hot and cold zone temperatures had arrested the rate of crystal formation, Fig. 3 X-ray diffraction (XRD) results of the reflected intensity plot- ted against 2θ, obtained from powdered single crystals as grown from different synthesis batches as labelled in the figure and referred to in the main text. θ is the angle between the incident X-ray beam and the crystallographic reflection plane. The black points are the measured data and the red lines are Rietveld refinement results of the V5S8 phase. The magenta vertical bars show the positions of the observ- able peaks of the V5S8 phase. The blue line represents the difference between the observed pattern and the calculated one. Batch 1 crys- tals shown in a contained mostly a V5S8 phase but a V2O3 phase was also observed (green vertical bars). The other batches (b–d) showed single-phase V5S8 crystals forming in the accepted monoclinic space group F12/m1. A scanning electron microscopy (SEM) image and a photograph of a typical single crystal are shown in the insets of a and c, respectively Brazilian Journal of Physics (2024) 54:187 Page 7 of 10 187 Fig. 4 The measured electrical resistivity, ρ(T), is plotted as a func- tion of temperature, T, for crystals taken from the five different batches a–e as described in the main text and labelled in the figures. The residual resistivity ratios, RRR = ρ(300 K)/ρ(4.2 K), indicating the degree of disorder present in the crystals is labelled for crystals from each batch. The antiferromagnetic phase transition temperature (the Néel temperature) is labelled TN and can be observed as a drop in the resistivity at around 33 K. The line in d is a guide to the eye and bridges the gap between the data points at low and room tempera- ture. The insets present ρ(T) plotted against T2 below 20 K and show Fermi-liquid behaviour, ρ(T) = ρ0 + AT2, as referred to in the main text Brazilian Journal of Physics (2024) 54:187187 Page 8 of 10 meaning that X-ray diffraction measurements were not pos- sible for that batch. Resistivity as a function of temperature, ρ(T), was measured using a standard four-point resistance technique between 2 and 300 K with either a Quantum Design PPMS system or a Cambridge Cryogenics magnetic refrigeration system employing a voltage-controlled constant current source and lock-in amplifier. The AC excitation current was 100 µA and contacts were made to the samples using silver epoxy. The results are plotted in Fig. 4, and the residual resistivity ratios, RRR = ρ(300 K)/ρ(4.2 K), for each batch are labelled along with the residual resistivities, ρ0. All sam- ples showed a kink at the accepted value of the Néel temper- ature, TN ≈ 33 K, below which ρ(T) dropped to lower values as the temperature is lowered towards absolute zero. The samples with lower crystalline disorder levels as determined by a high RRR value showed a sharper kink at TN. The low temperate resistivity is plotted against T2 in the insets of each figure. Fermi-liquid-like behaviour, ρ(T) = ρ0 + AT2, with A ≈ 0.01 μΩ cm K−2 was observed as shown in the figures, especially for the higher purity samples. The key crystal growing parameters and measurement results are summarized in Table 1 and RRR as a function of growth temperature for samples from each batch are shown in Fig. 5. Based on the results obtained here, the samples with a higher RRR and lower disorder levels seem to occur for batches with moderately lower growth temperatures (cold zone temperatures), TG, until TG reached 550 °C at which point chemical reactions and the rate of single crystal forma- tion seemed to significantly slow down. The crystals with Fig. 5 The residual resistivity ratio, RRR , for different single crystals is plotted against the growth temperature, TG, in a and against y, in V5Sy as determined from energy dispersive X-ray spectroscopy (EDS) measurements (see also Table 1). RRR is the ratio of the sample resis- tivity measured at 300 K divided by the sample resistivity measured at 4.2 K, and TG is the cold zone temperature of the chemical vapour transport synthesis method as described in the main text. The num- bers labelling the data points refer to the batch numbers in Table  1 and the main text. The different shaped data points are a reminder to the reader that the samples were prepared under differing conditions as described in Sect. 2 Table 1 Summary of key growth parameters and results for single crystals of V5S8 taken from different batches of samples prepared as described in Section  2 of the main text. TH and TC are the hot and cold zone temperatures of the tube furnace respectively, e.g. as shown in Fig.  2b. TG is the crystal growing temperature, XRD stands for X-ray diffraction and EDS for energy dispersive X-ray spectroscopy. The values of y in V5Sy as determined from EDS experiments were obtained by measuring y on at least three different and independent small areas of a given crystal for each batch and taking an average. RRR is the residual resistivity ratio, defined as the resistivity meas- ured at 300 K divided by its value measured at 4.2 K. High RRR val- ues indicate lower residual electrical resistivities and lower levels of crystalline disorder. The total mass of single crystals grown in Batch 4 was too small for XRD measurements to be carried out Batch (see main text for growth procedure) TC = TG (°C) ΔT = TH − TC (°C) Crystal growing time (days) y value ± 0.01 of the weighed starting elements for V5Sy y value of grown crystals for V5Sy Crystal structure space group and crystal type from XRD RRR y ± 0.01 from XRD Rietveld refinement y ± 0.01 from EDS 1 750 150 10 8.00 7.68 7.70 F12/m1 V5S8 1.98 2 700 75 14 8.00 7.89 7.90 F12/m1 V5S8 4.14 3 600 75 20 8.00 7.95 7.91 F12/m1 V5S8 4.89 4 550 75 14 8.20 - 7.75 - 1.50 5 700 75 14 8.30 7.95 7.92 F12/m1 V5S8 7.8–10.2 Brazilian Journal of Physics (2024) 54:187 Page 9 of 10 187 a measured value of y closer to 8.00 in V5Sy also seemed to have higher RRR . Values of y were always < 8.00, likely due to the volatility of sulphur and the difficulty of keeping sulphur atoms inside the solid-state lattice when growing at temperatures well above its boiling point. A substantial increase in RRR from just less than 5 to over 10 was achieved in Batch 5, which involved purifying the vanadium starting element by melting in ultra-high-vacuum (UHV) conditions with an electromagnetic induction furnace and increasing the ratio of sulphur to vanadium of the starting elements before CVT growth. This suggests further optimisation may be possible by following the procedure for Batch 5, but increasing the S:V ratio without breaking the quartz ampoule due to the resulting higher sulphur pressure at hot and cold zone temperatures above 600 °C. Higher pressures might also safely be achieved by using thicker-walled quartz ampoules or stronger sapphire ampoules. 4 Conclusions The chemical vapour transport (CVT) method was used to synthesise V5S8 single crystals with iodine as a transport agent following varying procedures for each of five batches. In all cases, the grown crystals were shiny hexagonal plate- lets of a few mm in length. Crystals from all the batches formed in the accepted F12/m1 monoclinic V5S8 space group type, with slightly varying stoichiometries. Resistiv- ity, ρ, measurements as a function of temperature on crystals from all batches revealed a kink at the Néel temperature of approximately 33 K, below which dρ/dT increased and ρ dropped as the temperature was further lowered. We found that single crystals of a few mm in size could still be grown at relatively low cold-zone temperatures down to 550 °C over a 14–20-day period, where sulphur volatility may be reduced. The highest residual resistivity ratio (RRR ) exceed- ing 10 was found in crystals from Batch 5, which involved purifying the starting vanadium element by melting pieces and pumping under ultra-high-vacuum conditions using an electromagnetic induction furnace before mixing with sul- phur and commencing CVT synthesis. To our knowledge, this value seems to be higher than others reported in the published literature for V5S8, a notoriously difficult system in which to reduce disorder. Such difficulties are potentially due to the finite probability of vanadium atoms randomly occupying vacant sites in the metal-deficient layers and the presence of sulphur vacancies due to sulphur’s high volatil- ity during crystal growing at temperatures well above its boiling point. We believe that single crystals of V5S8 may be synthesized with even lower disorder levels, resulting in higher residual resistivity ratios by further optimising the methods outlined here. Pre-purifying the starting materi- als to levels well in excess of their as-purchased nominal purities (i.e. > 99.99%), using lower growing tempera- tures, increasing the sulphur pressure in the growth tube to increase y, performing re-transportation cycles and anneal- ing for longer times would be suggested areas of focus in future experiments. Samples with lower disorder levels may open up the possibility of finding novel quantum states of matter at low temperatures, such as unconventional forms of superconductivity [10] and other correlated electron states in this magnetic narrow-band d-electron system. Better under- standing and controlling the growth of magnetic vanadium sulphide single crystals with varying thicknesses is relevant to the development of related battery [13] and spintronic devices [14]. Acknowledgements We would like to thank M. N. Ali, R. J. Cava, G. G. Lonzarich and D. Scott for useful help and discussions. We also thank the Royal Society of the United Kingdom, FAPESP (Grant# 2017/10581-1) and CAPES in Brazil for providing financial support. Funding This work was supported by the Royal Society in the United Kingdom; FAPESP (Grant# 2017/10581–1) and CAPES in Brazil. Data Availability The data supporting the findings of this study are available within the paper. Any additional data connected to the study are available from the corresponding author upon reasonable request. Declarations Ethical Approval This manuscript complies with the ethical policies of the journal. Competing Interests The authors declare no competing interests. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. 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