Simple Salt Trick Delivers Gram-Scale Crystals of Elusive Metallic 1T′-Phase Materials

Some of the most exciting materials in modern physics and chemistry are also among the most stubborn to make. Transition metal dichalcogenides, a family of layered compounds that includes molybdenum disulfide and tungsten diselenide, can crystallize in several distinct atomic arrangements, and each arrangement confers dramatically different properties. The thermodynamically stable semiconducting 2H phase is easy to obtain and has been studied intensively since the early days of two-dimensional materials research. Its metastable cousin, the 1T′ phase, is a different story entirely. With distorted metal-centered octahedra and a genuinely metallic electronic character, 1T′-phase group VIB transition metal dichalcogenides promise transformative advances in clean-energy electrocatalysis, high-performance electronics and even superconducting devices. Yet for decades, researchers who wanted high-quality 1T′ crystals had to contend with low yields, poor phase purity, tiny crystallites and punishing experimental conditions.

That bottleneck may finally be easing. A team led by Wei Zhai, Zhenyu Shi, Rui Tao, Zijian Li, Zhuangchai Lai and Hua Zhang, working across the City University of Hong Kong, The Hong Kong Polytechnic University and partner institutions, has published a detailed protocol in Nature Protocols describing a salt-assisted method that reliably converts ordinary 2H-phase crystals into phase-pure, highly crystalline 1T′-phase materials. The procedure covers a broad compositional palette, including MoS2, MoSe2, WS2, WSe2 and the mixed-chalcogen alloys MoS2xSe2(1−x) and WS2xSe2(1−x), and it does so at a scale that previous approaches could only dream of: gram quantities per batch, with individual crystals reaching hundreds of micrometers across.

The elegance of the method lies in its simplicity. Rather than building 1T′ crystals from scratch under exotic conditions, the protocol starts with commercially available 2H-phase transition metal dichalcogenides and co-anneals them with elemental chalcogen powders, such as sulfur or selenium, together with common alkali metal salts such as potassium oxalate hydrate or potassium carbonate. The reaction is carried out in a sliding tube furnace under a hydrogen-argon atmosphere. When the mixture is heated, the alkali metal salts act as the trigger for a remarkable structural transformation, coaxing the atomic layers of the stable 2H phase to slide and distort into the metastable 1T′ configuration. The chalcogen vapor maintains the correct chemical environment, preventing decomposition while the phase change proceeds.

Understanding why this works requires a brief tour of the underlying crystallography. In the 2H phase, each metal atom sits in a trigonal prismatic coordination environment, and the layers stack in a specific sequence that renders the material a semiconductor. The 1T′ phase, by contrast, features octahedral coordination with a periodic lateral displacement of the metal atoms, breaking the symmetry in a way that produces metallic conductivity. Because the 1T′ arrangement is metastable, it does not form spontaneously under ordinary synthesis conditions; it must be kinetically trapped. Alkali metal ions appear to facilitate this trapping by intercalating between the layers, weakening the interlayer bonding and lowering the energetic barrier for the intralayer atomic rearrangement. Subsequent deintercalation of these ions leaves behind the transformed lattice, now locked into its new metastable geometry.

The published protocol walks researchers through every stage of this process with unusual care. It specifies the setup of the sliding tube furnace, a configuration that allows the reactants to be rapidly moved into and out of the hot zone, giving precise control over reaction timing. It details the loading of the 2H-phase starting materials, the chalcogen powders and the salt, the purging and establishment of the hydrogen-argon atmosphere, the heating profile and the post-reaction washing steps that remove residual salts and recover the transformed crystals. The entire procedure takes approximately 37 hours and 20 minutes from start to finish, a time investment that is modest compared with the weeks of trial and error that often accompany attempts to grow metastable phases by conventional routes.

Equally important is the protocol’s emphasis on verification. Phase purity is not assumed; it is demonstrated. The authors prescribe a characterization workflow built on three complementary techniques. Raman spectroscopy probes the vibrational fingerprints of the lattice, which differ distinctly between the 2H and 1T′ arrangements. X-ray photoelectron spectroscopy examines the chemical states of the metal and chalcogen atoms, confirming that the transformation is complete and that no residual intercalants or decomposition products remain. X-ray diffraction provides the definitive structural verdict, revealing the characteristic peak positions of the 1T′ lattice and the absence of any detectable 2H-phase remnants. Together, these measurements give researchers the confidence that the material they are studying is genuinely the phase they intended to make.

The significance of gram-scale availability is difficult to overstate. Much of the early literature on metastable transition metal dichalcogenides relied on chemically exfoliated nanosheets produced through lithium intercalation, a method dating back to the 1980s that yields small, often defective flakes with variable phase content. Colloidal synthesis routes later offered better control but typically produced nanometer-scale particles rather than the large, well-ordered crystals needed for fundamental physics experiments and device fabrication. Hydrothermal and solution-phase approaches to 1T′ WSe2 and related compounds expanded the toolkit further, yet crystal size and phase purity remained persistent limitations. With crystals hundreds of micrometers wide available in gram batches, experiments that were previously impractical, such as building van der Waals heterostructures from 1T′ layers, performing detailed transport measurements, or loading industrial-scale electrocatalyst electrodes, become routine possibilities.

The applications that stand to benefit span an impressive range. In electrocatalysis, the metallic character of 1T′-phase materials makes them natural candidates for the hydrogen evolution reaction, the cathodic half of water splitting that underpins green hydrogen production. Previous work by members of the same team demonstrated that the phase of the support material dramatically influences how platinum catalysts grow and perform on molybdenum disulfide, achieving highly efficient hydrogen evolution when the metal deposited on metallic-phase surfaces. In electronics, 1T′ phases offer low-resistance contacts to semiconducting 2H layers, a strategy known as phase engineering that has been used to build better transistors. In photonics and sensing, 1T′ monolayers stabilized on gold nanowires have enabled ultrasensitive surface-enhanced Raman scattering detection. And in condensed matter physics, the metallic 1T′ phases of certain dichalcogenides host superconductivity, making high-purity crystals essential for studying emergent quantum phenomena.

The protocol also reflects a maturing view of what its authors call phase engineering of nanomaterials, a discipline that treats crystal phase as a design parameter on par with composition and dimensionality. Over the past decade, reviews in Chemical Reviews and Nature Reviews Chemistry have charted the rapid growth of this field, cataloguing unconventional phases across two-dimensional materials and the synthetic strategies developed to access them. The salt-assisted approach described here joins other recent innovations, including photoredox phase transformation driven by light, as part of a growing arsenal for navigating the energy landscapes of metastable matter. By codifying the method into a reproducible, step-by-step protocol, the team has effectively lowered the barrier to entry for laboratories worldwide.

For the broader materials community, the message is one of cautious optimism. Metastable phases have long been viewed as fragile curiosities, accessible only to specialists with finely tuned equipment and generous patience. A protocol that transforms shelf-stable commercial powders into gram quantities of phase-pure 1T′ crystals using a standard tube furnace and inexpensive salts suggests that these materials are ready to move from the margins of feasibility into the mainstream of research practice. Whether the next breakthrough comes in hydrogen catalysis, quantum devices or entirely unforeseen directions, the raw ingredients are now, quite literally, available by the gram.

Subject of Research: Salt-assisted synthesis of high phase-purity metastable 1T′-phase group VIB transition metal dichalcogenide crystals

Article Title: Salt-assisted synthesis of high phase-purity metastable 1T′-phase group VIB transition metal dichalcogenides

Article References: Zhai, W., Shi, Z., Tao, R., Li, Z., Dong, D., Tang, J., Wang, W., Wang, L., Yang, H., Zhai, L., Ruan, X., Wu, Y., Wang, J., Yao, Y., Zhao, C., He, Q., Lai, Z., & Zhang, H. (2026). Salt-assisted synthesis of high phase-purity metastable 1T′-phase group VIB transition metal dichalcogenides. Nature Protocols. https://doi.org/10.1038/s41596-026-01429-8

Image Credits: AI Generated

DOI: 10.1038/s41596-026-01429-8

Keywords: transition metal dichalcogenides, 1T′ phase, phase engineering, salt-assisted synthesis, metastable materials, two-dimensional materials, hydrogen evolution, electrocatalysis, MoS2, WS2, crystal growth, Nature Protocols