A critical, source-based review of the available crystallographic literature, known data gaps, technical limitations, and testable hypotheses for electronic devices.
Skip to References & BibliographyHolmium(III) sulfide ($\text{Ho}_2\text{S}_3$), or holmium sesquisulfide, belongs to the lanthanide sesquisulfide family ($\text{Ln}_2\text{S}_3$). Verified literature is sparse and deals mainly with crystal structure, phase equilibria, and the broader electronic structure of rare-earth sesquisulfides [1]–[4].
This report deliberately separates three levels of evidence: reported for Ho₂S₃, known only for related compounds, and proposed but untested. That distinction is essential because no source identified during this review establishes Ho₂S₃ as a memristor, artificial synapse, or CMOS-ready thin-film material.
A chemical catalogue entry is not equivalent to a phase-pure materials characterization. Values should be reported with phase identity, preparation route, uncertainty and measurement method.
The stable ambient phase is monoclinic, space group $P2_1/m$ (No. 11), Pearson symbol mP30 with $Z = 6$ formula units per cell [1], [9], [15]. Schleid and Lissner refined this δ (D-type) structure from single crystals [2].
The structure contains several crystallographically distinct Ho³⁺ sites with unequal sulfur coordination. Computed structure records describe mixed six-, seven- and eight-coordinate Ho sites with Ho–S distances spanning roughly $2.74–2.97\,\text{Å}$ [15]. This complexity is a structural fact; by itself it does not demonstrate ionic mobility, nonlinear conduction, or synaptic behaviour.
Under pressure Ho₂S₃ adopts cubic and orthorhombic structures [13]. A high-pressure diffraction study of heavy rare-earth sesquisulfides reports the transformation of the δ-monoclinic $P2_1/m$ form toward a γ-cubic $I\bar{4}3d$ form [14]. The exact transition pressure should be quoted from that study's own pressure scale and sample conditions.
Three preparation routes are documented in the preparative literature:
All routes require strict control of oxygen, water and sulfur chemical potential to avoid oxide, oxysulfide or off-stoichiometric products. Exact thermal programmes and purification steps should be taken from the primary procedure rather than inferred from another lanthanide sulfide [2].
Prepared Ho₂S₃ can be purified by chemical vapour transport using iodine as the transport agent, a route documented for rare-earth sesquisulfides generally [16]. Single-crystal structure investigation is established [2], but no transferable thin-film recipe was identified.
For electronic devices, the central challenge is different from bulk synthesis: producing a continuous nanometre-scale layer with controlled composition, low roughness, clean interfaces and a thermal budget compatible with the substrate. The $1325\,^\circ\text{C}$ bulk route is far outside any back-end CMOS budget.
Routes involving H₂S or CS₂ require professional gas handling, monitoring, ventilation and abatement. Occupational-health records additionally classify holmium sulfide itself as a skin irritant with fibrogenic potential, so dust control applies to the product as well as the reagents [10]. This document is an assessment, not an operating procedure.
A targeted metadata search found a small body of Ho₂S₃-specific literature. The clearest verified records concern crystal structure, phase diagrams and reference-data compilation; broader papers discuss rare-earth sesquisulfides as a family [1]–[4].
No such demonstration was identified in the sources reviewed for resistive switching, STDP/PPF plasticity, a validated Ho₂S₃ thin-film process, or neural-network hardware. This is an evidence-limited conclusion, not proof that no record exists anywhere; a formal systematic review would require documented databases, search strings, dates and screening criteria.
Using Ho₂S₃ as an artificial-synapse layer is a prospective concept, motivated only by analogies with demonstrated oxide switching and low-dimensional chalcogenide devices [5]–[7]. Analogy generates a testable question; it does not predict performance.
If a continuous, composition-controlled thin film can be produced in a metal–insulator–metal structure, several mechanisms would need to be tested rather than assumed:
Distinguishing these possibilities requires area scaling, electrode controls, temperature dependence, pulse tests and post-mortem microscopy.
Concept status: unbuilt and unverified. The proposals below are research prompts, not existing inventions or performance claims. Every concept depends on properties that have not yet been established for Ho₂S₃. A concept should be abandoned or redesigned if its stated proof test fails.
Idea: combine a Ho₂S₃ resistive element with a local magnetic field so one cell can store an analog conductance and respond to magnetic context.
Proof test: demonstrate repeatable, field-dependent conductance updates that cannot be explained by heating or electrode magnetoresistance.
Idea: use paired Ho₂S₃ devices, one active and one shielded reference, to compensate drift in an analog neural accelerator.
Proof test: show that differential programming reduces long-term weight error across a statistically meaningful device array.
Idea: exploit a reversible resistance state, if one exists, as a field-programmable connection for radiation-tolerant logic.
Proof test: establish non-destructive switching, retention, endurance and radiation response against a qualified fuse technology.
Idea: infer sulfur chemical potential from a calibrated resistance or optical change in an encapsulated Ho₂S₃ microfilm.
Proof test: measure selective, reversible response over known sulfur activities while rejecting humidity and oxygen interference.
Idea: use a temperature-dependent Ho³⁺ magnetic response as the transduction element of a contactless low-temperature thermometer.
Proof test: obtain a monotonic, reproducible calibration curve and quantify self-heating, hysteresis and field sensitivity.
Idea: search for radiation-induced changes in both conductivity and optical absorption, allowing two independent dose readings from one film.
Proof test: demonstrate dose proportionality, spectral selectivity, retention and reset under controlled radiation sources.
Idea: place a thin Ho₂S₃ layer in an optical cavity and tune transmission through a measurable magneto-optical response.
Proof test: detect reversible field-controlled modulation above cavity drift and absorption losses.
Idea: encode a hard-to-copy optical signature using patterned Ho₂S₃ thickness, phase and encapsulation.
Proof test: produce stable, distinguishable spectra after humidity, heat, abrasion and accelerated ageing.
Idea: integrate electrically addressable Ho₂S₃ pixels into a metasurface that changes infrared emissivity on demand.
Proof test: demonstrate reversible emissivity contrast with a complete energy and thermal-cycling budget.
Idea: test whether a Ho₂S₃ heterostructure converts a temperature gradient into an electrically readable spin-dependent signal.
Proof test: separate any spin-caloritronic contribution from ordinary Seebeck voltage and contact artefacts.
Idea: engineer a sulfur-defect gradient across a film to investigate persistent internal fields for low-power energy harvesting.
Proof test: verify sustained power into a load and rule out transient chemistry, trapped charge and electrode reactions.
Idea: use a tiny Ho₂S₃ witness film beside a battery interface to record extreme temperature or sulfur exposure as a permanent state change.
Proof test: correlate the marker state with independent temperature and gas measurements without affecting cell safety.
Idea: isolate dilute Ho-related spin states in a controlled Ho₂S₃ host and test them as addressable memory candidates.
Proof test: measure coherent control and relaxation times before proposing any quantum-memory architecture.
Idea: couple a Ho₂S₃ microcrystal to a microwave cavity to search for controllable collective magnetic excitations.
Proof test: observe reproducible avoided crossings and extract coupling strength, linewidth and temperature limits.
Idea: expose a calibrated Ho-containing precursor coupon beside a production substrate to map sulfur dose and process uniformity.
Proof test: correlate coupon phase and composition with independent chamber metrology across multiple runs.
Idea: pattern several Ho₂S₃ microregions designed to transform at different process conditions, creating a readable thermal-history code.
Proof test: establish distinct, irreversible thresholds with low batch variation and no cross-sensitivity.
Idea: combine magnetic, thermal and radiation-sensitive Ho₂S₃ test pixels on a satellite coupon to retain an exposure history.
Proof test: calibrate each stress independently, then recover the combined exposure after representative vacuum cycling.
Idea: design electronics in which the Ho₂S₃ functional film can be selectively detached and chemically recovered at end of life.
Proof test: demonstrate high holmium recovery and material reuse while comparing energy, hazards and cost with primary supply.
Idea: engineer fast and slow defect responses in adjacent Ho₂S₃ regions so a device can mimic short- and long-term memory in one cell.
Proof test: isolate at least two reproducible decay constants and show they remain stable across devices and temperatures.
Idea: use controlled stochastic switching, if present, as a physical source of noise for Bayesian or sampling-based computation.
Proof test: map output distributions to programming conditions and exclude thermal or measurement noise as the source.
Idea: store both a value and its statistical confidence in coupled Ho₂S₃ conductance states for edge AI hardware.
Proof test: demonstrate two independently addressable states with low cross-talk and measurable retention.
Idea: integrate micrometre-scale Ho₂S₃ elements into an array that converts local field patterns into an electrical image.
Proof test: establish spatially uniform sensitivity and calibrate against a known magnetic test pattern.
Idea: use irreversible sulfide chemistry as a sacrificial witness layer that changes state before a protected metal corrodes.
Proof test: correlate its optical or electrical transition with electrochemical corrosion rates under realistic environments.
Idea: test whether minute oxygen or moisture ingress produces a locked-in Ho₂S₃ signature that can be read after a mission or storage period.
Proof test: identify a low detection threshold, response time and immunity to ordinary temperature cycling.
Idea: exploit a narrow optical resonance, if one can be measured, to slow or delay selected infrared wavelengths in a compact waveguide.
Proof test: measure dispersion, insertion loss and thermal drift before any delay claim is made.
Idea: pattern electrodes over Ho₂S₃ so local refractive-index changes can create an erasable diffractive structure.
Proof test: demonstrate reversible diffraction efficiency changes not caused by electrode deformation or heating.
Idea: combine Ho₂S₃ with a resonant antenna structure to test magnetic control of reflected phase.
Proof test: quantify phase range, switching field, loss and repeatability relative to existing liquid-crystal or MEMS approaches.
Idea: investigate Ho₂S₃ as a temperature-responsive filter that admits useful sunlight while limiting infrared overheating.
Proof test: measure actual spectral switching across operating temperatures and compare net energy performance with established coatings.
Idea: couple a temperature-sensitive Ho₂S₃ magnetic response to a flux guide and coil to scavenge energy from thermal cycling.
Proof test: report delivered energy per cycle, mechanical or magnetic losses and long-term repeatability.
Idea: embed a Ho₂S₃ witness dot in transformers or power electronics to retain evidence of damaging temperature excursions.
Proof test: calibrate an irreversible transition against temperature, duration, atmosphere and electrical stress.
Idea: mount Ho₂S₃ on a piezoelectric substrate to test whether mechanical strain changes magnetic or optical transitions.
Proof test: separate strain coupling from substrate electrical artifacts and verify reversibility.
Idea: use high-purity Ho₂S₃ crystals as a host in which individual defects might show optically addressable spin states.
Proof test: first locate isolated, photostable emission centers; only then test spin initialization and readout.
Idea: embed tiny Ho₂S₃ reference zones into a sputtering or evaporation target to reveal wear and composition drift.
Proof test: show that reference-zone changes predict film non-uniformity earlier than conventional process monitors.
Idea: deposit Ho-containing and sulfur-containing precursors in patterns while an optical sensor adjusts the local ratio in real time.
Proof test: produce phase-pure patterned films over large areas with independently measured composition maps.
Idea: combine Ho₂S₃ pixels with a thermal coating so a spacecraft can alter local heat rejection without moving louvers.
Proof test: validate vacuum cycling, radiation stability, emissivity contrast and fail-safe passive behavior.
Idea: convert recovered holmium from waste magnets into a small, high-value Ho₂S₃ calibration standard rather than a bulk electronic product.
Proof test: meet reference-material purity and traceability requirements with less energy and waste than primary refining.
Idea: seal Ho₂S₃ microparticles inside an inert shell so the strong Ho³⁺ moment can mark an implant position without direct tissue contact.
Proof test: prove shell integrity, absence of leaching and image contrast benefit over approved markers before any biological study.
Idea: use an irreversible Ho₂S₃ transition to confirm that a surgical instrument received a validated radiation or heat dose.
Proof test: match indicator response to certified dosimetry across full sterilization cycles with no false confirmations.
Idea: exploit a low-temperature magnetic or optical signature to record whether a vaccine shipment stayed within range.
Proof test: demonstrate a sharp, irreversible threshold and multi-week stability against existing chemical indicators.
Idea: use narrow Ho³⁺ absorption features as a passive frequency anchor for calibrating optical communication equipment.
Proof test: measure line width, temperature coefficient and long-term drift against certified wavelength standards.
Idea: insert a Ho₂S₃ element into a fiber coupler so an external field redirects light without moving parts.
Proof test: quantify insertion loss, cross-talk, switching field and speed relative to standard optical switches.
Idea: test whether Ho₂S₃ permeability shifts can retune a compact antenna across nearby frequency bands.
Proof test: measure real permeability and loss at target frequencies before claiming any tuning range.
Idea: exploit pressure-driven polymorphism in rare-earth sesquisulfides as a passive maximum-depth recorder.
Proof test: locate a reproducible transition pressure in Ho₂S₃ specifically and verify it survives recovery to the surface.
Idea: deploy a ruggedized Ho₂S₃ electrode to log dissolved sulfide chemistry where conventional sensors degrade quickly.
Proof test: establish selectivity, fouling resistance and calibration stability in high-temperature saline conditions.
Idea: cover a secure module with a fragile Ho₂S₃ layer whose electrical signature changes irreversibly if the package is opened.
Proof test: show that realistic attack methods always alter the signature while normal handling never does.
Idea: turn the natural device-to-device variability of Ho₂S₃ films into a hardware fingerprint for key generation.
Proof test: quantify uniqueness, reproducibility and resistance to temperature, ageing and modelling attacks.
Idea: embed patterned Ho₂S₃ regions inside a document or component that only a specific magnetic reader can decode.
Proof test: demonstrate reliable reading through packaging while remaining undetectable to common scanners.
Idea: supply phase-verified Ho₂S₃ as a traceable standard for calibrating diffraction and composition instruments.
Proof test: achieve homogeneity, stability and inter-laboratory reproducibility that meet reference-material standards.
Idea: combine known-thickness Ho₂S₃ regions on one substrate to calibrate optical, magnetic and electrical tools simultaneously.
Proof test: confirm each region's independence and verify values against separately certified specimens.
Idea: circulate sealed Ho₂S₃ samples between laboratories to quantify how much reported property spread comes from measurement rather than material.
Proof test: show sample stability over the full circulation period using before-and-after characterization.
Idea: place a Ho₂S₃ sensing element in an exhaust stream to flag fuel that exceeds sulfur limits.
Proof test: maintain calibration through thermal cycling, soot exposure and vibration for a regulatory service interval.
Idea: mark braking components with a Ho₂S₃ spot that changes permanently after damaging thermal events.
Proof test: correlate the transition with measured rotor temperature and confirm resistance to road salt and abrasion.
Idea: coat a drive shaft with a magnetostrictive Ho₂S₃ composite so applied torque alters a detectable magnetic signal.
Proof test: measure the magnetostrictive coefficient, hysteresis and coating adhesion under realistic loads.
Idea: bury inert-cased Ho₂S₃ sensors that report plant-available sulfur to guide fertilizer application.
Proof test: correlate readings with laboratory soil analysis across soil types and confirm zero release of rare-earth material.
Idea: detect early sulfur-bearing spoilage gases in silos using a low-power Ho₂S₃ element.
Proof test: establish detection limits below spoilage thresholds and reject interference from dust and humidity.
Idea: embed Ho₂S₃ probes in reinforced concrete to signal chemical conditions that precede rebar corrosion.
Proof test: demonstrate decade-scale survival in alkaline concrete and correlate signals with actual rebar condition.
Idea: apply Ho₂S₃ markers across a structure so inspectors can reconstruct peak temperature distribution after a fire.
Proof test: calibrate distinct thresholds and verify readability after smoke, water and firefighting exposure.
Idea: deploy passive Ho₂S₃ badges downwind of facilities to accumulate a record of sulfur-compound exposure.
Proof test: establish a linear accumulation response and validate against reference air-quality monitoring stations.
Idea: monitor sulfide oxidation in tailings ponds, an early driver of acid drainage, with buried Ho₂S₃ probes.
Proof test: prove containment of the probe material itself and correlate output with pH and sulfate measurements.
Idea: produce Ho₂S₃ alongside neighbouring Ln₂S₃ compounds under identical conditions to isolate how the 4f count controls properties.
Proof test: hold synthesis, purity and measurement methods constant so observed differences are attributable to the lanthanide alone.
All 60 concepts first require a reproducible Ho₂S₃ film, verified phase and stoichiometry, basic optical/electrical/magnetic data, air-stability measurements and suitable control materials. Without that foundation, device-level optimization would produce ambiguous results.
A numerical comparison is not scientifically valid without matched device geometry, electrode materials, compliance current, pulse protocol and statistical population. The table therefore compares evidence status, while the metrics required for a future like-for-like benchmark are listed explicitly.
| Evidence criterion | TiO₂ / HfO₂ RRAM | 2D chalcogenide devices | Ho₂S₃ |
|---|---|---|---|
| Resistive switching literature | Extensive Reviews and device studies [5], [6] | Demonstrated in research [7] | Not identified |
| Synaptic functions | Reported for multiple oxide-device architectures | Reported for selected MoS₂ and related structures [7] | Not measured |
| Thin-film process | Established deposition and integration routes | Research-scale transfer/growth routes | No validated process identified |
| Required quantitative metrics | $R_{OFF}/R_{ON}$, forming/set/reset voltage, pulse energy, speed, endurance, retention, variability and failure mode | All remain to be measured | |
Ranges from unrelated publications were intentionally removed: comparing best-case values reported under different test conditions would create false precision.
Advancing $\text{Ho}_2\text{S}_3$ beyond basic laboratory research faces severe physical, chemical, and economic constraints:
Holmium is a minor constituent of rare-earth mineral streams and is recovered through complex separation rather than mined as a stand-alone commodity. Spot catalogue prices were removed because purity, form, quantity, date and contract terms can change the figure by orders of magnitude. A credible cost model must specify those parameters and include separation yield, target utilization and recycling.
Bulk thermal sulfidation relies on hazardous gases ($\text{H}_2\text{S}$, $\text{CS}_2$) at temperatures $>1000\,^\circ\text{C}$. $\text{H}_2\text{S}$ is highly toxic, corrosive, and flammable, imposing strict environmental, health, and safety (EHS) containment requirements that increase industrial operating capital.
Air, humidity and thermal stability of nanometre-scale Ho₂S₃ films have not been established. Accelerated ageing under controlled oxygen, humidity and temperature should precede any choice of encapsulation.
Because Ho₂S₃ is not reported as a bulk industrial commodity, upstream availability would depend on separated holmium compounds, principally holmium oxide (Ho₂O₃). The USGS 2026 heavy-rare-earth summary provides price and trade indicators, but does not publish a separate world-production tonnage for Ho, Ho₂O₃ or Ho₂S₃ [8].
| Indicator | 2021 | 2022 | 2023 | 2024 | 2025 estimate |
|---|---|---|---|---|---|
| Ho₂O₃ price, minimum purity 99.5% FOB, US$/kg |
$140 | $180 | $91 | $67 | $70 |
| Total U.S. imports of heavy-rare-earth compounds and metals tonnes REO equivalent; all covered elements combined |
71 t | 70 t | 70 t | 74 t | 100 t |
| U.S. net import reliance heavy-rare-earth compounds and metals, aggregated |
100% | 100% | 100% | 100% | 100% |
U.S. minerals containing heavy rare earths were mined, and several companies produced small-scale quantities of heavy-rare-earth compounds or metals. However, the USGS reports no sustained commercial-scale U.S. production and gives no Ho-specific tonnage.
Import source, 2021–2024: shipping records attribute 100% of U.S. imports of holmium compounds and metals to China. The USGS cautions that no individual customs code exists for each heavy rare earth and that finished products are excluded [8].
Using molar masses $M(\text{Ho}_2\text{O}_3)=377.86\,\text{g mol}^{-1}$ and $M(\text{Ho}_2\text{S}_3)=426.04\,\text{g mol}^{-1}$:
The table below concerns all rare-earth mine production, not holmium production and not Ho₂S₃ production. It is retained only as a broad indicator of geographic concentration and cannot be used to calculate holmium availability by country.
| Rank | Country | Relative Mine Production | Share of Global Output |
|---|---|---|---|
| 1 | China | ~ 69 % (~270,000 t REO) | |
| 2 | United States | ~ 12 % (~45,000 t REO) | |
| 3 | Myanmar | ~ 8 % (~31,000 t REO) | |
| 4 | Australia | ~ 3 % (~13,000 t REO) | |
| 5 | Thailand | ~ 3 % (~13,000 t REO) | |
| 6 | Nigeria | ~ 3 % (~13,000 t REO) | |
| 7 | India | ~ 1 % (~2,900 t REO) | |
| 8 | Russia | ~ 1 % (~2,500 t REO) | |
| — | Others (Vietnam, Madagascar, etc.) | ~ 1 % |
Mine output, separation capacity and refined-element output are different indicators. A proper Ho₂S₃ supply study must quantify holmium content in feedstocks, separation capacity, refining location, inventory, trade flows and recycling. The USGS now publishes a separate heavy-rare-earth summary, which should be preferred for future updates [8].
Any Ho₂S₃ programme inherits the constraints of the holmium supply chain that sits above it. Holmium is never mined directly: no holmium-dominant mineral is known, and the element is recovered only as a minor co-product of rare-earth processing [17].
| Indicator | Reported value | Interpretation |
|---|---|---|
| Crustal abundance | ≈ 1.3 mg/kg (1.3 ppm) [17] | Comparable to tungsten; scarcity is not the binding constraint, separation is. |
| Rank among lanthanides | Most abundant odd-numbered heavy lanthanide [17] | Follows the Oddo–Harkins rule: rarer than its neighbours Dy and Er. |
| Host minerals | Gadolinite, monazite, xenotime, ion-adsorption clays [17] | Always a dilute passenger in a mixed rare-earth matrix. |
| Grade in monazite sand | ≈ 0.05 % Ho [17] | Roughly 2000 t of monazite per tonne of contained holmium. |
| Principal current source | Ion-adsorption clays of southern China; yttrium ≈ 2/3 of the rare-earth mass, holmium ≈ 1.5 % [20] | Geographic concentration is a policy risk, not only a geological one. |
| Estimated reserves | ≈ 400,000 t holmium [17] | An order-of-magnitude estimate, not a reported reserve statement. |
| Annual holmium metal production | ≈ 10 t/year [18] | A trade-association estimate; USGS publishes no Ho-specific tonnage [8]. |
| Stage | Operation | Principal difficulty |
|---|---|---|
| 1. Mining & concentration | Recovery of monazite, xenotime or leaching of ion-adsorption clays | Thorium and uranium in monazite create a radioactive residue stream. |
| 2. Cracking | Acid or alkaline decomposition into a mixed rare-earth solution | High reagent consumption and large volumes of process effluent. |
| 3. Separation | Ion exchange and multi-stage solvent extraction [19] | The core bottleneck. Ho sits between Dy and Er, whose ionic radii differ by only picometres, so many stages are needed for high purity. |
| 4. Oxide production | Precipitation and calcination to Ho₂O₃ | This is the actual traded commodity form and the practical entry point for buyers. |
| 5a. Metal route | Conversion to anhydrous HoF₃ or HoCl₃, then calciothermic reduction [9] | Not required for Ho₂S₃ if the oxide route is used; adds cost and oxygen-contamination risk. |
| 5b. Sulfide route | $\text{Ho}_2\text{O}_3 + 3\,\text{H}_2\text{S} \rightarrow \text{Ho}_2\text{S}_3 + 3\,\text{H}_2\text{O}$ at $1325\,^\circ\text{C}$ [11] | Toxic gas handling plus a thermal budget incompatible with CMOS back-end processing. |
Derived from molar masses alone, assuming 100 % recovery at every stage. Real yields are lower, so these are best-case floors:
At roughly 10 t/year of holmium metal worldwide [18], the entire global holmium output corresponds to at most about 12.9 t/year of Ho₂S₃.
Holmium already has established competing uses — magnet pole pieces, Ho:YAG surgical lasers, optical calibration standards, nuclear burnable poisons [17]. A new electronic application would compete with these, not draw on spare capacity.
Older USGS material quotes holmium metal near US$1000/kg [17], while the 2026 USGS summary reports Ho₂O₃ at about US$70/kg [8]. These differ by chemical form, purity, quantity and year, and must never be treated as a price trend. Any cost model should state form, purity, date and contract basis explicitly.
Canada produces no holmium. No Canadian operation was identified that separates, refines or sells holmium, holmium oxide or holmium sulfide at any scale. Canada holds significant heavy-rare-earth resources, which is a different statement from production capacity.
Natural Resources Canada reports rare-earth resources on the order of 15.2 Mt of total rare-earth oxides, hosted mainly in peralkaline silicate complexes and carbonatites [21], [22]. Rare earths are listed on Canada's critical minerals list, but no published Canadian figure isolates holmium [21].
| Project | Location | Relevance to holmium | Stage |
|---|---|---|---|
| Strange Lake Torngat Metals |
Quebec / Labrador border | Peralkaline complex with a high heavy-rare-earth proportion. Company disclosure emphasises dysprosium and terbium; holmium is not marketed as a product [24]. | Pre-construction |
| Nechalacho Avalon / Vital Metals |
Northwest Territories | Contains both light and heavy rare earths plus Y, Zr, Ta and Nb; the closest Canadian analogue to a holmium-bearing feedstock [25]. | Development |
| Wicheeda | British Columbia near Prince George |
Carbonatite, light-rare-earth dominant. Low holmium fraction expected [22]. | Development |
| Kipawa Matamec |
Quebec Témiscamingue |
Peralkaline complex; REE–Y–Zr hosted in eudialyte and mosandrite, genuinely heavy-rare-earth enriched. The most holmium-relevant Quebec geology identified [29]. | Suspended |
| Ashram / Eldor Commerce Resources |
Quebec Nunavik |
Carbonatite-hosted, light-rare-earth dominant but noted in the literature for unusual heavy-rare-earth enrichment for its deposit class [30]. | Development |
| Montviel Géoméga |
Quebec Abitibi |
Carbonatite REE–Nb deposit, bastnäsite-dominant. Light-rare-earth deposit — a poor holmium candidate despite frequent listing as an “HREE” source [28]. | Undeveloped |
| Hoidas Lake | Saskatchewan near Uranium City — not NWT |
Apatite–allanite vein system with some heavy-rare-earth content; unusual mineralogy for its class [27]. | Exploration |
| SRC Rare Earth Processing Facility | Saskatoon, Saskatchewan | Midstream separation and metals plant, reported as North America's first at commercial scale. Focused on magnet-metal output rather than holmium [23]. | Operating midstream |
The single most useful screen when reading Canadian project claims is the deposit class, because it controls the light-to-heavy rare-earth ratio before any processing decision is made [22], [28]:
Bastnäsite- and monazite-bearing carbonatites are strongly light-rare-earth enriched (La, Ce, Nd, Pr). Canadian examples: Wicheeda, Montviel, Ashram. Holmium is present but at a low fraction of total oxides.
Eudialyte-, zircon- and mosandrite-bearing peralkaline rocks concentrate heavy rare earths and yttrium. Canadian examples: Strange Lake, Nechalacho, Kipawa. Nechalacho's heavy fraction exceeds roughly 20 % of total oxides [26].
Holmium typically represents on the order of 1–2 % of total rare-earth oxides in heavy-rare-earth-enriched deposits [20]. Applied naively to the 15.2 Mt national figure, that would imply a few hundred thousand tonnes of contained Ho₂O₃ in place.
This number must not be quoted as a reserve. Light-rare-earth carbonatites carry a far lower holmium fraction, and in-place metal is not recoverable metal.
For a research-scale Ho₂S₃ programme, the practical route in Canada is purchasing separated Ho₂O₃ on the international market and converting it domestically. Building a sovereign holmium supply chain is not justified by the volumes any electronic application in this document would consume.
The programme should be stage-gated: each phase has a falsifiable question and a stop criterion, preventing device claims from outrunning the evidence.
Holmium(III) sulfide is a legitimate but sparsely documented member of the rare-earth sesquisulfide family. Verified records support interest in its crystal chemistry and phase behaviour; they do not establish a competitive electronic device.
However, an objective scientific assessment confirms that:
Bibliographic correction: two DOI previously listed in this document were removed after Crossref verification showed that they described ytterbium nitrate and neptunium fluoride, not Ho₂S₃. References 9–30 were added to close data gaps previously marked as unresolved. They include handbooks, compilations, a trade-association estimate and company disclosures; these are secondary or self-reported sources and are labelled as such at each point of use. Company project statements have not been independently verified against filed technical reports.