1. MICROWAVE DIELECTRIC SPECTROSCOPY OF FERROELECTRICS
Jonas GRIGAS
Vilnius University
Lithuania

2. Main Goals of MDS: 1) Low-frequency soft modes
close to Tc in displacive FEs;
2) Relaxation modes in order-disorder FEs;
3) Polarization dynamics in relaxors, FE ceramics, nanomaterials, etc.
Broadband MDS (100 MHz-300 GHz) is needed.

3. 1) Soft modes near T TT ,   , (o)  1/ . c  c TO   TO ε -1
100 cm ?
GHz T T -1 c
  MW.
Limit of OS is 10 cm. TO

4. Techniques & Electrodynamics
Frequency-domain:
i) Coaxial: 1 MHz  10 GHz,
ii) Wavequide: 8  120 GHz
(running wave regime).
TE sample
Terahertz time-domain over 120 GHz.
(J.Grigas, Microwave Dielectric Spectroscopy of Ferroelectrics… (Gordon & Breach, 1996). 10

5. Mode Softening in Displacive (?) FEs
i) BaTiO3.
ii) SbSI: (0)  at T
IR:   500 ÷ 5000 ( 10% of (0));
mode softens to 10 cm .
iii) Sn2P2S6: (0)  at T
IR:   1000 (< 10% of (0)).
Speculations: Central peak, Central mode, etc.
The main contribution comes from microwave range. c -1 c

6. tetrahedral  orthorhombic  rhombohedral
i) BaTiO3
R.Blinc
Displacive scenario – freezing of soft TO lattice mode (von Hippel 1943; Cochran 1960)
C4v
C2v
C3v
tetrahedral  orthorhombic  rhombohedral

7. BaTiO3: Ti position R.Blinc Ba O Ti dynamical disorder
Possible titanium atom positions, compatible
with the Oh symmetry in the paraelectric phase

8. ii) SbSI

9. SbSI: (T) dependence

10. SbSI: () 1) Resonant or relaxation mode? 2) One mode or more?
J.Grigas. Ferroelectrics, v. 226, p.51 (1999).

11. In SbSI needle-like single crystals:
The chains consist of few nm long nanodomains elongated in
[001] direction;
The atoms are shifted from the average
positions up (red) or down (blue).
K. Lukaszewitz et al. Acta Cryst. B (2007).

12. SbSI: double-well electronic potential of Sb (of B1u mode)
X-Ray data (2007)
Sb, S and I atoms may occupy two positions shifted from the mirror plane by:
Sb = 0,030,
S = 0,027,
I = 0,028.
J.Grigas, Ferroelectrics v.226, p. 51 (1999).

13. Soft-mode frequency splits,  is the soft mode
The phonon Hamiltonian of SbSI
Soft-mode frequency splits,  is the soft mode 3

14. Anharmonicity splits the soft mode
B1u mode splits near Tc 1
B1u  IR 2 MW 3 Tc T
In MW range reveals 3 – soft mode. It explains (0).
J.Grigas, Ferroelectrics v.226, p. 51 (1999).

15. iii) Sn P S : s =35 (T – Tc) GHz. (1) ; (2) s ; (3) .  = 750 GHz.
1/2 2 2 6 2
Similar soft mode splitting by anharmonicity as in SbSI? It explains (0).
(J.Grigas, Microwave Dielectric Spectroscopy of Ferroelectrics… (Gordon & Breach, 1996).

16. Relaxation Modes in: 1) Order-Disorder CsH PO2 4

17. Only proton dynamics cause PT and the relaxation mode.
The Hamiltonian
The 1st term: short-range configuration interactions of H;
the 2nd term: long-range interactions through the lattice vibrations;
the last term: interactions of H with E- field.

18. CsH PO : (,T). Lines are theoretical, points experimental.2 4
Proton soft relaxation mode – critical slowing down.

19. 2) RbD PO : (,T). 1 – 325 K, 6 – 394 K. Lines theoretical.
Deuteron soft relaxation mode – critical slowing down.
Similar dynamics is in LHP & LDP.
(J.Grigas, Microwave Dielectric Spectroscopy of Ferroelectrics… (Gordon & Breach, 1996).

21. 1) PLZT 8/65/35 (polar nanoclusters)
S.Kamba, V.Bovtun, J.Petzelt, J.Banys, J.Grigas, M.Kosec. J. Phys.: Condens. Matter, v.12, 497 (2000).

22. 2) PMN- PSN- PZN ceramics PbMg(x)Nb(1-x)O3-PbSc(1/2)Nb(1/2)O3-PbZn(y)Nb(1-y)O3

23. 3) Rb1-x(ND4)D2PO4 (DRADP) Dipolar Glass
IR range
J.Banys, J.Grigas, J.Petzelt, S.Kamba, etc. J. Phys.: Condens.Matter v. 14, 3725 (2002).

24. DRADP
The soft relaxation deuteron mode r = 0.75 (T – Tf ) GHz/K above the frustration temperature Tf is responsible for the whole dielectric dynamics.

25. Microwave Dielectric Spectroscopy enables to study:
C O N C L U S I O N S
Microwave Dielectric Spectroscopy enables to study:
Soft modes close to Tc in displacive FEs;
Relaxation modes in order-disorder FEs;
Polarization dynamics in relaxors, FE ceramics, nanomaterials, etc.