Metavalent Bonding in GeSe Leads to High Thermoelectric Performance Debattam Sarkar, Prof. Kanishka

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International Winter School 2021

Metavalent Bonding in GeSe Leads to High Thermoelectric Performance

Debattam Sarkar, Prof. Kanishka Biswas’ lab, New Chemistry Unit, JNCASR

Covalent

Metallic Metavalent

Conventional chemical bonds in solids

Ionic bond Weak forces

❖ Bonding in Solid: Mixtures of multiple bonding types

❖ Spectroscopic techniques to identify bonds: Rotational, vibration, NMR, Raman etc.

❖ Quantum Mechanics: VB/MO

Van Arkel–Ketelaar triangle, 1947

Ionicity (r^’) Degree of covalency(r-1 )

Pauling electronegativity

Pauling electronegativity difference

D. Lencer et al. Nat. Mater. 2008, 7, 972

Shared electrons

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International Winter School 2021 ^{GeTe, GeSb}²^Te⁵, PbSe, AgBiTe₂, AgBiSe₂

❖High effective coordination number → Covalent 

❖High optical dielectric constant + moderate electrical conductivity → Metallic 

❖Electron transfer to form a bond is very less → Ionic  Contradictions!!!

Characteristics mechanism



Something between covalency and

metallicity



but distinctly different from

both

Covalent bonding Metallic bonding

Ionic bonding Resonance bonding Metavalent bonding

‘Incipient Metals’

M. Wuttig, V. L. Deringer, X. Gonze, C. Bichara, J. –Y. Raty, Adv. Mater. 2018, 30, 1803777.

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Metavalent Bonding

➢Moderate electrical conductivity (10²–10⁴ S/cm)

➢High optical dielectric constant

➢High effective

coordination number (ECON )

➢High Born effective charges

➢High bonding anharmonicity

➢Electron shared vs.

electron transferred

AgBiTe₂ GeSe

M. Wuttig, C.-F. Schön, M. Schumacher, J. Robertson, P. Golub, E. Bousquet, C.

Gatti, J. –Y. Raty. Adv. Funct. Mater. 2021, 2110166.

Characteristics of metavalent bonding

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AgBiTe₂ Alloying Induces Metavalent Bonding Characteristics in GeSe

Heat flow T_H

T_C R_L

(a)

(b)

0.0 0.3 0.6 0.9 1.2 1.5

GeSe 0.55Te 0.45 (GeSe) 0.9(AgBiTe 2) 0.1

This work

GeAg 0.2Bi 0.2Se 1.4 GeSeAg 0.2Sb 0.2Te 0.4

zT max (GeSe) 0.92(Sb 2Te 3) 0.08

300 400 500 600 700 0.0

0.3 0.6 0.9 1.2 1.5

x = 0 x = 0.08 x = 0.10 x = 0.12 (GeSe)_1-x(AgBiTe₂)_x

zT

T (K)

ZTdev Abundance (ppm)

0.0 0.4 0.8 1.2 1.6 2.0

PbTe SnTe

GeTe

PbSe SnSe

GeSe

This work

Te(0.001)

Se (0.05)

(c)

Non-toxic

zT =

^𝑆²





_{𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑖𝑐}⁺



_{𝑙𝑎𝑡𝑡𝑖𝑐𝑒}

𝑇

Advantages of metavalent bonding in thermoelectrics (TE)

➢ Moderate electrical conductivity (σ ↑)

➢ High Seebeck coeffcieint (S ↓)

➢ High Born effective charges

➢ High bonding anharmonicity (



_{𝑙𝑎𝑡𝑡𝑖𝑐𝑒} ↓) Efficiency

❖ GeSe → structurally similar to SnSe

❖ High theoretical prediction of zT  2.5

❑ Low carrier density (10¹⁴-10¹⁶ cm^-3) and high band gap (1.1 eV)

D. Sarkar, S. Roychowdhury, R. Arora, T. Ghosh, A. Vasdev, B. Joseph, G. Sheet, U. V. Waghmare, K. Biswas, Angew. Chem. Int. Ed. 2021, 60, 10350.

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Conformation of Metavalent Bonding in GeSe-AgBiTe₂

(400)

(011) (111)

(201)

(222)

(002) (022)(110)

(104)

Fm-3m

Pnma R3m

Fm-3m R3m Pnma (GeSe)

1-x(AgBiTe

2)

x

(012)

* * * *

* Ag₂Te

20 30 40 50 60 70 x = 0 x = 0.10 x = 0.30

Intensity (a.u.)

2(^O) (b)

(c) (d)

8 10 12 14 16 18 20 (GeSe)_0.9(AgBiTe₂)_0.1

Ambient Pressure R3m

Intensity (a.u.)

Fm-3m

2.21 GPa 3.14 GPa 4.64 GPa 6.70 GPa (a)

Ge/Ag/Bi Se/Te

12 15 18 21 24 27 30 33 R3m

473 K 563 K

300 K 423 K

Intensity (a.u.) Fm-3m

(GeSe)

0.9(AgBiTe

2)

0.1

2(^O) 2(^O)

12 15 18 21 24 27 30 33 R3m

473 K 563 K

300 K 423 K

Inyensity (a.u.)

2 (o)

Fm-3m (GeSe)

0.9(AgBiTe

2)

0.1

8 10 12 14 16 18 20 (GeSe)

0.9(AgBiTe

2)

0.1

Ambient Pressure

R3m

Intensity (a.u.)

2 (^o) Fm-3m

2.21 GPa 3.14 GPa 4.64 GPa 6.70 GPa

GeSe (orthorhombic covalent )

❖ ECON 2

❖ Orthorhombic → Cubic 920 K or more than 82 GPa GeSe-AgBiTe₂(Metavalent)

✓ High ECON 6

✓ Rhombohedral → cubic (563 K or 6.7 GPa)

Rhombohedral (R3m)

Crystal structure of metavalent GeSe-10 mol% AgBiTe₂

0.00 0.05 0.10 0.15 0.20 0.2

0.4 0.6 0.8 1.0 1.2

Band gap E g (eV)

x (GeSe)

1-x(AgBiTe

2)

x

300 400 500 600 700 0.0

0.4 0.8 100 200 300 400

(GeSe)

1-x(AgBiTe

2)

x

x = 0.12 x = 0.10 x = 0.08

x = 0

Electrical conductivity  (S/cm)

T (K)

❑ Band gap reduction

❑ Narrow band gap semiconductor

❑ Carrier density improved to

~10¹⁹ cm^-3

❑ Improved σ Ag⁺ Ge²⁺

Advantages of AgBiTe₂alloying

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Thermal Transport in GeSe-AgBiTe₂

❖ Low optical phonon modes (50 cm^-1)

❖ Coupled with acoustic phonon modes

❖ Scatters the acoustic phonons

Low energy optical phonons in rhombohedral GeSe

Anomalous Born effective charges, Bi: 7.96, Ag: 2.51, Ge: 7.63, Se: −5.60 and Te: 4.04

In collaboration with Prof. U. V. Waghmare, JNCASR, India

Low energy modes in rhombohedral GeSe-10%

AgBiTe₂

Heat carriers

0 200 400 600 800 1000 0.0

0.1 0.2 0.3

0.4 Experimental Debye + 2E

C p (J/mol.K2 )

T² (K²) 𝐶_𝑝

= 𝛾 + 𝛽𝑇𝑇 ²

+ ෍

𝑛

𝐴_𝑛 Θ_𝐸_𝑛 ². 𝑇^{2 −}²³. 𝑒

Θ_𝐸𝑛 𝑇

𝑒

Θ_𝐸𝑛 𝑇 − 1

2

Experimental verification of low energy optical phonons

Ө_E1~ 29 cm^-1 Ө_E2 ~ 68 cm^-1

Debye – Einstein Model

300 400 500 600 700 0.3

0.6 0.9 1.2 1.5 1.8 2.1

2.4 (GeSe)_1-x(AgBiTe₂)_x x = 0 x = 0.08 x = 0.10 x = 0.12

 lattice (W/mK)

T (K)

_glass

_diff

Ferroelectric Instability !!!

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Thermoelectric Performance of Metavalent GeSe-AgBiTe₂

Piezoresponse force microscopy (PFM)

Ferroelectric hysteresis Butterfly loop shape Local ferroelectric domains

-15 -10 -5 0 5 10 15

0 50 100 150 200 250

Phase (°)

Bias (V)

-15 -10 -5 0 5 10 15

0 30 60 90 120

Amplitude(pm)

Bias (V)

In collaboration with Prof. G. Sheet, IISER Mohali, India

Experimental verification of ferroelectric

instability

Light and dark contrast



Oppositely polarized domains

Metavalent bonding

300 400 500 600 700 0.0

0.3 0.6 0.9 1.2

(GeSe)

1-x(AgBiTe

2)

x

x = 0 x = 0.08 x = 0.10 x = 0.12

T (K)

zT

Highest reported TE figure of merit among the GeSe based thermoelectrics

Ferroelectric instability induced ultra-low

_lattice

D. Sarkar, S. Roychowdhury, R. Arora, T. Ghosh, A. Vasdev, B. Joseph, G.

Sheet, U. V. Waghmare, K. Biswas, Angew. Chem. Int. Ed. 2021,60, 10350.

D.S acknowledges CSIR for fellowship