diff --git a/Physlib.lean b/Physlib.lean
index 58db66fdb..efa95d934 100644
--- a/Physlib.lean
+++ b/Physlib.lean
@@ -29,6 +29,7 @@ public import Physlib.ClassicalMechanics.Vibrations.LinearTriatomic
 public import Physlib.ClassicalMechanics.WaveEquation.Basic
 public import Physlib.ClassicalMechanics.WaveEquation.HarmonicWave
 public import Physlib.CondensedMatter.Basic
+public import Physlib.CondensedMatter.Thermoelectric.Basic
 public import Physlib.CondensedMatter.TightBindingChain.Basic
 public import Physlib.Cosmology.Basic
 public import Physlib.Cosmology.FLRW.Basic
diff --git a/Physlib/CondensedMatter/Thermoelectric/Basic.lean b/Physlib/CondensedMatter/Thermoelectric/Basic.lean
new file mode 100644
index 000000000..8bbbcf8b6
--- /dev/null
+++ b/Physlib/CondensedMatter/Thermoelectric/Basic.lean
@@ -0,0 +1,213 @@
+/-
+Copyright (c) 2026 Giuseppe Barbalinardo. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Giuseppe Barbalinardo
+-/
+module
+
+public import Mathlib.Data.Real.Basic
+public import Mathlib.Tactic.Ring
+public import Mathlib.Tactic.Positivity
+/-!
+
+# Thermoelectric figure of merit
+
+## i. Overview
+
+A thermoelectric material converts a temperature difference into electrical
+power. In the linear-response (Onsager) regime, the coupled transport of
+charge and heat in an isotropic material is governed by the constitutive
+relations
+
+  J = σ (E − S ∇T),        q = T S J − κ ∇T,
+
+so a thermoelectric material is characterized by exactly four transport
+coefficients: the Seebeck coefficient `S`, the electrical conductivity `σ`,
+and the thermal conductivity `κ = κl + κe`, split into its lattice (phonon)
+and electronic contributions because the two channels respond independently
+to material design. These four coefficients are collected in the structure
+`ThermoelectricMaterial`, and everything in this file is stated on it.
+
+The performance of the material at absolute temperature `T` is captured by
+the dimensionless figure of merit
+
+  zT = σ S² T / (κl + κe),
+
+whose numerator groups into the power factor `PF = σ S²`. Temperature is a
+state variable, not a material property, so `T` enters as an argument rather
+than a field. The four coefficients of a real material are themselves
+temperature-dependent, so a `ThermoelectricMaterial` should be read as a
+material at an operating point, its coefficients evaluated at the same
+temperature at which `zT` is evaluated.
+
+In this file all quantities are real numbers in a fixed consistent system of
+units, following the convention of `Physlib.Thermodynamics.IdealGas.Basic`.
+
+## ii. Key results
+
+- `ThermoelectricMaterial`: the four linear-response transport coefficients
+  of a thermoelectric material, with their physical sign constraints.
+- `powerFactor`: the thermoelectric power factor `σ S²`.
+- `totalThermalConductivity`: the total thermal conductivity `κl + κe`.
+- `figureOfMerit`: the dimensionless figure of merit `zT`.
+- `figureOfMerit_eq`: the flat form `zT = σ S² T / (κl + κe)`.
+- `figureOfMerit_pos`: positivity of `zT` for a material with a nonzero
+  Seebeck coefficient at positive temperature.
+- `figureOfMerit_le_of_le`: lowering the lattice thermal conductivity raises
+  `zT`, the phonon-glass electron-crystal design principle.
+
+## iii. Table of contents
+
+- A. The thermoelectric material
+- B. The power factor
+- C. The total thermal conductivity
+- D. The figure of merit
+  - D.1. Equalities for the figure of merit
+  - D.2. Positivity of the figure of merit
+  - D.3. Monotonicity in the lattice thermal conductivity
+
+## iv. References
+
+- Ioffe, A.F., *Semiconductor Thermoelements and Thermoelectric Cooling*,
+  Infosearch (1957).
+- Snyder, G.J., Toberer, E.S., *Complex thermoelectric materials*,
+  Nature Materials 7, 105–114 (2008).
+
+-/
+
+@[expose] public section
+
+noncomputable section
+
+namespace CondensedMatter
+
+/-!
+
+## A. The thermoelectric material
+
+The four fields are the complete set of linear-response coefficients for
+coupled charge and heat transport in an isotropic material: no further
+material parameter enters the steady-state thermoelectric equations, and
+`zT` is a function of exactly these four together with the temperature.
+The sign constraints are physical: a thermoelectric material conducts
+charge (`0 < σ`) and its lattice conducts heat (`0 < κl`), while the
+electronic heat channel can be negligible but never negative (`0 ≤ κe`).
+The Seebeck coefficient is unconstrained: its sign records the carrier
+type (negative for electrons, positive for holes), and it vanishes at
+compensation points.
+
+-/
+
+/-- A thermoelectric material in the linear-response regime, characterized by
+its four transport coefficients. -/
+structure ThermoelectricMaterial where
+  /-- The Seebeck coefficient `S`: the voltage developed per unit temperature
+  difference. Its sign records the dominant carrier type. -/
+  S : ℝ
+  /-- The electrical conductivity `σ`. -/
+  σ : ℝ
+  /-- The lattice (phonon) contribution `κl` to the thermal conductivity. -/
+  κl : ℝ
+  /-- The electronic contribution `κe` to the thermal conductivity. -/
+  κe : ℝ
+  σ_pos : 0 < σ
+  κl_pos : 0 < κl
+  κe_nonneg : 0 ≤ κe
+
+namespace ThermoelectricMaterial
+
+/-!
+
+## B. The power factor
+
+-/
+
+/-- The thermoelectric power factor `PF = σ S²` of a material. -/
+def powerFactor (M : ThermoelectricMaterial) : ℝ := M.σ * M.S ^ 2
+
+/-- The power factor is positive when the Seebeck coefficient is nonzero. -/
+lemma powerFactor_pos {M : ThermoelectricMaterial} (hS : M.S ≠ 0) :
+    0 < M.powerFactor := by
+  unfold powerFactor
+  -- `positivity` does not look through the projection, so name the field.
+  have hσ := M.σ_pos
+  positivity
+
+/-!
+
+## C. The total thermal conductivity
+
+-/
+
+/-- The total thermal conductivity `κl + κe` of a material, the sum of the
+lattice (phonon) and electronic contributions. -/
+def totalThermalConductivity (M : ThermoelectricMaterial) : ℝ := M.κl + M.κe
+
+/-- The total thermal conductivity is positive. -/
+lemma totalThermalConductivity_pos (M : ThermoelectricMaterial) :
+    0 < M.totalThermalConductivity :=
+  add_pos_of_pos_of_nonneg M.κl_pos M.κe_nonneg
+
+/-!
+
+## D. The figure of merit
+
+-/
+
+/-- The dimensionless thermoelectric figure of merit
+`zT = PF · T / (κl + κe)` of a material at absolute temperature `T`. -/
+def figureOfMerit (M : ThermoelectricMaterial) (T : ℝ) : ℝ :=
+  M.powerFactor * T / M.totalThermalConductivity
+
+/-!
+
+### D.1. Equalities for the figure of merit
+
+-/
+
+/-- The figure of merit in its standard flat form
+`zT = σ S² T / (κl + κe)`. -/
+lemma figureOfMerit_eq (M : ThermoelectricMaterial) (T : ℝ) :
+    M.figureOfMerit T = M.σ * M.S ^ 2 * T / (M.κl + M.κe) := by
+  unfold figureOfMerit powerFactor totalThermalConductivity
+  ring
+
+/-!
+
+### D.2. Positivity of the figure of merit
+
+-/
+
+/-- The figure of merit is positive at positive temperature when the Seebeck
+coefficient is nonzero. -/
+lemma figureOfMerit_pos {M : ThermoelectricMaterial} {T : ℝ}
+    (hS : M.S ≠ 0) (hT : 0 < T) :
+    0 < M.figureOfMerit T :=
+  div_pos (mul_pos (powerFactor_pos hS) hT) M.totalThermalConductivity_pos
+
+/-!
+
+### D.3. Monotonicity in the lattice thermal conductivity
+
+-/
+
+/-- Lowering the lattice thermal conductivity raises the figure of merit: if
+`M.κl ≤ κl'` then the material with lattice conductivity `κl'` (all other
+coefficients equal) has the smaller `zT`. This is the phonon-glass
+electron-crystal design principle: scatter phonons without degrading
+electronic transport. Note the hypotheses: no condition on the Seebeck
+coefficient is needed, since monotonicity only requires the numerator
+`σ S² T` to be nonnegative. -/
+lemma figureOfMerit_le_of_le (M : ThermoelectricMaterial) {κl' T : ℝ}
+    (hpos : 0 < κl') (h : M.κl ≤ κl') (hT : 0 ≤ T) :
+    figureOfMerit { M with κl := κl', κl_pos := hpos } T ≤ M.figureOfMerit T := by
+  unfold figureOfMerit powerFactor totalThermalConductivity
+  have hnum : (0 : ℝ) ≤ M.σ * M.S ^ 2 * T := by
+    have hσ := M.σ_pos
+    positivity
+  have hden : 0 < M.κl + M.κe := add_pos_of_pos_of_nonneg M.κl_pos M.κe_nonneg
+  gcongr
+
+end ThermoelectricMaterial
+
+end CondensedMatter