MICROWAVE DIELECTRIC SPECTROSCOPY OF FERROELECTRICS Jonas GRIGAS Vilnius University Lithuania
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.
1) Soft modes near T TT , , (o) 1/ . c c TO TO ε -1 100 cm ? GHz T T -1 c MW. Limit of OS is 10 cm. TO
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
Mode Softening in Displacive (?) FEs i) BaTiO3. ii) SbSI: (0) 60 000 at T IR: 500 ÷ 5000 ( 10% of (0)); mode softens to 10 cm . iii) Sn2P2S6: (0) 60 000 at T IR: 1000 (< 10% of (0)). Speculations: Central peak, Central mode, etc. The main contribution comes from microwave range. c -1 c
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
BaTiO3: Ti position R.Blinc Ba O Ti dynamical disorder Possible titanium atom positions, compatible with the Oh symmetry in the paraelectric phase
ii) SbSI
SbSI: (T) dependence
SbSI: () 1) Resonant or relaxation mode? 2) One mode or more? J.Grigas. Ferroelectrics, v. 226, p.51 (1999).
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).
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).
Soft-mode frequency splits, is the soft mode The phonon Hamiltonian of SbSI Soft-mode frequency splits, is the soft mode 3
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).
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).
Relaxation Modes in: 1) Order-Disorder CsH PO 2 4
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.
CsH PO : (,T). Lines are theoretical, points experimental. 2 4 Proton soft relaxation mode – critical slowing down.
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).
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).
2) PMN- PSN- PZN ceramics PbMg(x)Nb(1-x)O3-PbSc(1/2)Nb(1/2)O3-PbZn(y)Nb(1-y)O3
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).
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.
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.