Fyzikální ústav Akademie věd ČR

We focus on study of dielectric spectra of ferroelectrics in the form of single crystals, ceramics, thin films and multilayers in a very broad spectral and temperature range (1 mHz – 150 THz, 5 – 950 K).

People

Head
Stanislav Kamba
Staff
Viktor Bovtun	microwave and broad-band dielectric spectroscopy
Elena Buixaderas	Raman and infrared spectroscopy
Ivan Gregora	Raman spectroscopy, PFM microscopy connected with Raman spectroscopy
Martin Kempa	microwave and broad-band dielectric spectroscopy
Dmitry Nuzhnyy	infrared and terahertz spectroscopy
Tetyana Ostapchuk	infrared spectroscopy
Jan Petzelt	infrared and broad-band dielectric spectroscopy, structural phase transitions
Jan Pokorný	Raman spectroscopy
Maxim Savinov	low-frequency dielectric spectroscopy
Vladimír Vorlíček	Raman spectroscopy
PhD. students
Fedir Borodavka	Raman spectroscopy
Veronica Goian	infrared spectroscopy of multiferroics
Elizabeth J. Vakkechalil	infrared and Raman spectroscopy

What is original and unique in our lab?

• We developed unique experimental methods for dielectric measurements of high-permittivity and high-loss ferroelectric materials (bulk and thin films) in the microwave range from 10 to 400 K.
• Time domain THz transmission spectra (0.1-2.4 THz) can be measured between 5 and 900 K, THz reflectance below 300 K.
• Infrared dielectric response can be obtained not only on bulk samples but also on ultra-thin films with thickness down to 20 nm.
• Micro-Raman spectrometer (488 nm excitation) in combination with AFM microscope allows us to study phonons with high spatial resolution together with AFM topography. Additional ultraviolet laser (325 nm excitation) is suitable for measurements of ultrathin films.
• In collaboration with other groups from our Institute we can study magnetoelectric and magnetic properties of multiferroics.

Our phonon studies are frequently supplemented by inelastic neutron scattering studies in Institute of Laue-Langevin and inelastic X-ray scattering in ESRF (both in Grenoble, France). Infrared spectroscopy with external magnetic fields we conduct in GHMFL (Grenoble, France). Magnetic and magnetocapacitive experiments are performed in the Departement of magnetic and low-temperature physics in our Institute (equipped with PPMS14 and SQUID Magnetometer Quantum Design).

Equipment

• Dielectric analyzer Alpha_AN (Novocontrol), frequency range 3x10^-6Hz–20 MHz, 10–900K
• Impedance analyzer HEWLETT-PACKARD 4192A, frequency range 100 Hz – 5 MHz, 10 – 900 K
• Impedance analyzer AGILENT 4291B, frequency range 1 MHz – 1.8 GHz, coaxial technique, 100 – 550 K
• Network analyzer AGILENT E8364B, frequency range 50 MHz – 50 GHz, coaxial technique suitable for dielectric measurements of high-permittivity high loss thin films and bulks, temperature range 10 – 400 K
• Custom made setup for time-domain THz spectroscopy; spectral range: 5 – 80 cm^-1 (0.1 – 2.5 THz), temperature range 10 – 950 K (in collaboration with the THz spectroscopy group)
• Fourier spectrometer BRUKER IFS113v, spectral range 15-10.000 cm^-1, temperature range 5 – 950 K, transmission and specular reflection measurements (two instruments)
• Micro-Raman spectrometer RM 1000 (RENISHAW), multichannel detection, temperature range 10 – 750 K, polarizing microscope
• In-Via Reflex Raman Microscope (RENISHAW) combined with NTEGRA Spectra AFM Upright Microscope (NT-MDT).

Some recent results

Relaxor ferroelectrics

Relaxor ferroelectrics exhibit broad dielectric relaxations, therefore we use a broad-band dielectric spectroscopy (100 Hz – 100 THz) for investigation of these materials. Recently we have investigated coarse-grain and fine-grain ceramics of PbMg_1/3Nb_2/3O₃-35%PbTiO₃ [V. Bovtun et al., Phys. Rev. B 79, 104111 (2009)]. ε^*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below T_C, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation and therefore the macroscopic ferroelectric phase transition does not occur.

Dynamics of ferroelectric phase transitions in piezoelectrics

Detailed temperature dependence of ferroelectric soft modes and a broad dielectric relaxations were investigated not only in lead based piezoelectrics [e.g. PLZT - see E. Buixaderas et al., Appl. Phys. Lett. 94, 052903 (2009)], but mainly in lead-free piezoelectrics like KNN [see E. Buixaderas et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 56, 1843 (2009)] and NBT-BT [Hlinka et al., submitted to Ferroelectrics]

New ferroelectric phase transition

V oddělení dielektrik byl objeven feroelektrický přechod v Sr_9-xPb_xCe₂Ti₁₂O₃₆ (x=0-9). Bylo ukázáno, že vzorky s nízkou koncentrací olova (x<3) jsou tzv. incipientní feroelektrika, tj. že se s ochlazováním blíží k feroelektrickému stavu, ale kvantové fluktuace mu zabrání vzniknout. Vzorky s vyšší Pb koncentraci se stávají feroelektrické a jejich kritická teplota lineárně roste s koncentrací olova. Komplexní dielektrická, terahertzová, infračervená a Ramanova spektra ukázala, že fázové přechody jsou čistě posuvného typu, protože se pozoroval jasný měkký feroelektrický mód fononového původu. Strukturní měření ukázala, že paraelektrická fáze má trigonální strukturu, zatímco feroelektrická krystaluje v monoklinické struktuře. Obsáhlý článek byl publikován v prestižním časopise Chemistry of Materials [S. Kamba et al., Chem. Mat., 21, 811 (2009)].

Teplotní závislost permitivity v Sr_9-xPb_xCe₂Ti₁₂O₃₆ (x=0-9). Teploty maxim odpovídají teplotám feroelektrického fázového přechodu.