Tellurium

Tellurium (Te, atomic number 52) is a Group 16 chalcogen metalloid with a trigonal crystal structure analogous to gray selenium, a melting point of 449.5 °C, and a narrow bandgap of ~0.33 eV — making it an intrinsic semiconductor with the highest Seebeck coefficient (~500 µV/K) of any elemental solid and the best thermoelectric figure of merit (zT ~1 near room temperature) among elemental materials. Tellurium is one of the rarest stable elements in Earth's crust (~1 ppb, comparable to platinum-group metals), occurring primarily in gold and copper telluride minerals (calaverite AuTe₂, petzite Ag₃AuTe₂, sylvanite AgAuTe₄) and recovered almost entirely as a byproduct of copper anode slime processing in electrolytic copper refining — global production is only ~500 tonnes/year, heavily constrained by copper output. Te has eight naturally occurring isotopes spanning mass 120–130, six of which are suspected or confirmed double beta decay candidates (Stable*); the most abundant, ¹³⁰Te (34.1%), is the target of the CUORE experiment searching for neutrinoless double beta decay. Tellurium compounds produce a distinctive garlic odor (dimethyltelluride) at trace exposures; even nanogram-level skin contact causes persistent breath odor lasting weeks.

The dominant use of tellurium is in cadmium telluride (CdTe) thin-film solar cells, which represent the second-largest photovoltaic technology by installed capacity and the lowest-cost solar module at scale — consuming ~40% of global Te production. CdTe solar cells (First Solar Series 6/7 modules, ~22% record cell efficiency, ~19% commercial) use a ~3–4 µm CdTe absorber layer deposited by close-space sublimation (CSS) at ~500 °C; the 1.45 eV direct bandgap of CdTe is nearly ideal for single-junction solar energy conversion under AM1.5. Te supply constraints are a recognized bottleneck for CdTe PV scaling — at ~40–100 tonnes Te per GW of CdTe modules, the entire ~500 tonnes/year Te production supports only 5–12 GW/year of new CdTe capacity. Bismuth telluride (Bi₂Te₃) and its alloys (Bi₂Te₃–Sb₂Te₃ for p-type, Bi₂Te₃–Bi₂Se₃ for n-type) are the dominant commercial thermoelectric materials for solid-state cooling (Peltier coolers in CPU thermal management, laser diode temperature stabilization, portable cooling devices) and waste heat recovery at near-ambient temperatures.

Beyond photovoltaics and thermoelectrics, tellurium is essential in phase-change optical storage media, free-machining steel and copper alloys, and as the chalcogenide component of the topological insulator Bi₂Te₃ — a model system for topologically protected surface states and proximity-effect topological superconductivity research. Phase-change materials based on GeTe and Ge-Sb-Te (GST) alloys exploit the rapid, reversible amorphous↔crystalline phase transition triggered by laser pulses to store data in rewritable optical discs (CD-RW, DVD-RW, Blu-ray) and, more recently, in phase-change memory (PCM/PCRAM) non-volatile semiconductor memory chips (Intel Optane, now discontinued but historically significant). In metallurgy, Te additions of 0.02–0.1 wt% to free-machining copper (ASTM C14500, 0.4–0.7% Te) and steel dramatically improve machinability and chip breakage without the toxicity concerns of Pb additives; this application consumes ~25% of global Te. Tellurium nanowires and nanostructures have emerged as a research platform for piezoelectricity, chirality-driven photoconductance, and quantum confinement effects.