- The document summarizes the century-long journey of ferroelectric molecular crystals from their discovery in 1920 to recent developments. - Rochelle salt was the first discovered ferroelectric crystal in 1921. KH2PO4 was another important early discovery from 1935-1940. - Barium titanate (BaTiO3) was the first man-made perovskite ferroelectric discovered in 1942, triggering interest in their military applications. - Lead zirconium titanate (PZT) was a breakthrough perovskite discovered in 1952-1953, but concerns about heavy metal toxicity grew. - Recent work investigates metal-free molecular ferroelectrics like diisopropy

1. A Century’s Journey of Ferroelecric
Molecular Crystals
(1920-2019)
Dr. Muklesur Rahman
Department of Physics
ALIAH UNIVERSITY, Kolkata

2. 2
1920: Discovery
Rochelle salt (Potassium sodium tartrate tetrahydrate)
Prepared by Pierre Seignette, of La Rochelle,
France in 1675
Sir David Brewster demonstrated piezoelectric
effect in Rochelle salt in 1824
Alexander McLean Nicolson used it in
microphones and speakers in Bell Labs during
1919
Rochelle salt shows an electric hysteresis in P
analogous to the magnetic hysteresis in the
case of ferromagnet.
J. Valasek, Phys. Rev 17, 47 (1921)

3. 1920-1930: Rochelle salt period
๏The only ferroelectric crystal.
๏A molecular Crystal with Tc = 24oC
Monopotassium phosphate KH2PO4
1935- Busch and Scherer

4. 1930-1940: KH2PO4 (KDP) age๏In 1935 Bausch and Schere reported
ferroelectricity in potassium dihydrogen
phosphate with Tc = -130oC and in iso-
structural dihydrogen arsenate.
๏ Hydrogen bonds between adjacent
oxygen atoms to be responsible for
ferroelectricity
A parallel war also began ……
September 1st, 1939

5. Development of perovskite ferroelectric (BaTiO3)
This triggered the interest in ferroelectrics, mainly for
sonar systems to detect submarines and other military
applications.
Barium Titanate, BaTiO3, the first man-made perovskite
ferroelectric was discovered in 1942 and 1944 in the
United States, Russia and Japan
Dielectric constant
7000
Highest polarization 
26C/cm2
Curie temperature 
120oC – 130oC

6. lead zarconium titanate PbZrTiO3 (PZT): perovskite ferroelectric
1952-1953- Shirane, Sawaguchi
Ps23.0C/cm2
1920-Rochelle Salt
1930-KDP
1940-BTO
1950-PZT
1960- Many ferroelectric
crystal
1970- ferroelectric liquid
crystal
1990- Packages, composites

8. No Liquid Crystal so far is ferroelectric

9. PZT and other heavy metal
perovskite ferroelectric materials
are the key market holder.

10. •Pb or other heavy metal based material
- major environmental concern
•High temperature processing
Molecular ferroelectrics
•Aqueous solution processing
•Easy disposal
•Metal free

11. Ps8.0C/cm2

12. Single-component (polar) organic molecules
Horiuchi and Tokra Nature Materials 70 357 (2008)

13. Horiuchi and Tokra Nature Materials 70 357 (2008)

14. Molecular ferroelectrics are less in number-
WHY?
Organic substances synthesized in large numbers
and almost 30% of them posses’ polar symmetries.
But ferroelectric found rarely!!!
 The dipole-dipole interaction usually cancels out
neighboring molecular dipoles and hence loses
ferroelectricity.
 Even if molecules crystalize to polar solids
without cancelling out their dipoles but their low
energy barrier for the molecular reorientations to
overcome in the crystalline solid.

15.  Molecules should
necessarily possess
symmetry higher
than C1 to exhibit
ferroelectricity in
crystalline state.
Non-centrosymmetric crystal classes
3m(C)
4mm(C4n)
6mm(C4n)
422 (D4)
222 (D2)
622 (D6)
32 (D3)
23 (T)
1(C1)
2(C2)
3(C3)
4(C4)
5(C5)
6(C6)
4(S4)
m(C1)
42m(D2d)
6(C3d)
62m(C3d)

16. IUPAC Name: N-propan-2-ylpropan-1-amine
MF: C6H15N; MW: 101.19 g/mol
Diisopropylammonium cations (dipa), has an
approximate two fold symmetry, are known as
stabilizing counter-ions for various anionic systems.

17. Philip et. al. Acta Cryst. C46 336 (1990); Fu et. al. Adv. Mat. 23 5658 (2011)
Synthesis:
Block-like colorless crystals of 1 were easily obtained by reaction of diisopropyl
amine with 30% aqueous HCl and re-crystallized in methanol.
Monoclinic P21 Monoclinic P21/n

18. Kuhn et. al. Acta Cryst. C52 2309 (1996); Fu et. al. Science 339 425 (2013)
Polarization Ps 23.0C/cm2
Br is more electronegative than chlorine, hence replacing Cl with Br
could increase the dipole-moment and as well as the polarization of
the compound.

19. Polarization Ps 33.0C/cm2

20. Diisopropylammonium based organic ferroelectric
dipaX (X= Cl, Br, I)
DIPAC
DIPAB
DIPAI
1.81 1.96 2.2
8.9
23
33
0
5
10
15
20
25
30
35
DIPAC DIPAB DIPAI
Ionic Radius in Å Polarization in μC/Cm2Polarization in 𝛍C/cm2
And the story begins here!!!!

21. C Jiang et. al.
…………dipaI is not ferroelectric……….
Inorganic
Chemistry

22. Ps values
dipaCl= 7 𝛍C/cm2
dipaBr= 6 𝛍C/cm2
dipaI= 5 𝛍C/cm2

24. 𝜎(𝜔 = 𝜎 𝑑𝑐 + 𝐴𝜔 𝑛(0 ˂ 𝑛 ˂ 1
𝜎 𝑑𝑐 = 𝜎0 𝑒𝑥𝑝 −
𝐸 𝜎
𝐾 𝐵 𝑇𝜔ℎ = 𝜔ℎ0 𝑒𝑥𝑝 −
𝐸ℎ
𝐾 𝐵 𝑇
The dc and ac conductions are correlated and governed by the
same transport mechanism, namely thermally activated nearest
neighbour hopping.

25. Journey of Ferroelecric Molecular
Crystals Continues……..
Thank you for your kind attention
Dr. Muklesur Rahman
mrahman.phys@gmail.com