Davies, Peter K
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Publication Analysis of phase distributions in the Li2O–Nb2O5–TiO2 system by piezoresponse imaging(2001-02-01) Borisevich, Albina Y; Bonnell, Dawn A; Kalinin, Sergei V; Davies, Peter KThe M-phase solid solutions Li1+x-yNb1-x-3yTix+4yO3) (0.1 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.175) in the Li2O–Nb2O5–TiO2 system have promising microwave dielectric properties. However, these compounds can contain small quantities of ferroelectric impurities that affect the polarization response of the material. Due to their low concentration and their chemical similarity to the host material, the impurities cannot be detected by x-ray diffraction or local elemental analysis. Scanning surface potential microscopy and piezoresponse imaging were used to analyze phase compositions in this system. Piezoresponse imaging demonstrated the presence of thin (<200–300 nm) ferroelectric layers on the grain boundaries oriented along the c-axis of the M-phase. Differences between the surface potential and the piezoresponse of ferroelectric multicomponent systems are discussed.Publication Synthesis and Dielectric Properties of Li1-x+yTa1-x-3yTix+4yO3 M-Phase Solid Solutions(2002-10-01) Borisevich, Albina Y; Davies, Peter KThe synthesis, structure, and dielectric properties of the so-called M-phase solid solutions in the Li2O-Ta2O5-TiO2 system were investigated. Although the range of stability of the tantalate phases is more limited compared with their niobate counterparts, they have identical structures based on intergrowths of LiTaO3-type blocks separated by corundum type layers. The dielectric constants of the tantalate M-phases range from 68 to 52 and they all exhibit a negative temperature coefficient of capacitance. The temperature coefficient of the resonant frequency measured in the microwave region can be tuned to zero, and the system shows quite good quality factors with the highest value reaching a Q x f = 10500 at 6.7 GHz.Publication Structure and Polarization in the High Tc Ferroelectric Bi(Zn,Ti)O3-PbTiO3 Solid Solutions(2007-03-05) Grinberg, Ilya; Suchomel, Matthew R; Dmowski, Wotjek; Mason, Sara E; Wu, Hui; Davies, Peter K; Rappe, Andrew MTheoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x [PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature.Publication A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite(2004-05-01) Tarvin, Rachel; Davies, Peter K(Na1/2La1/2)(Mg1/3Nb2/3)O3 undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (εr ~ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.Publication Potential and Impedance Imaging of Polycrystalline BiFeO3 Ceramics(2002-12-01) Kalinin, Sergei V.; Davies, Peter K; Suchomel, Matthew R.; Bonnell, Dawn AElectrostatic-force-sensitive scanning probe microscopy (SPM) is used to investigate grain boundary behavior in polycrystalline BiFeO3 ceramics. Scanning surface potential microscopy (SSPM) of a laterally biased sample exhibits potential drops due to resistive barriers at the grain boundaries. In this technique, the tips acts as a moving voltage probe detecting local variations of potential associated with the ohmic losses within the grains and at the grain boundaries. An approach for the quantification of grain boundary, grain interior, and contact resistivity from SSPM data is developed. Scanning impedance microscopy (SIM) is used to visualize capacitive barriers at the grain boundaries. In SIM, a dc-biased tip detects the variations of local potential induced by the lateral ac voltage applied to the sample. Unlike the traditional dc and ac transport measurement, both of these techniques are sensitive to the variation of local potential (SSPM) or local voltage oscillation amplitude and phase (SIM), rather than to current. Therefore, special attention is paid to the relationship between SSPM and SIM images and data obtained from traditional impedance spectroscopy and dc transport measurements. For BiFeO3 ceramics excellent agreement between the local SIM measurements and impedance spectroscopy data are demonstrated.Publication Influence of Cation Order on the Dielectric Properties of Pb(Mg1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3 (PMN-PSN) Relaxor Ferroelectrics(2003-11-01) Farber, Leon; Davies, Peter KThe effect of the B-site cation chemistry and ordering on the dielectric properties of solid solutions in the (1-x)Pb(Mg1/3Nb2/3)O3-Pb(Sc1/2Nb1/2)O3 (PMN-PSN) perovskite system was examined in samples with 0.1 ≤ x ≤ 0.9. Thermal annealing treatments were effective in inducing long-range B-site order in the samples within this compositional range. The well-ordered, large chemical domain ceramics exhibit relaxor behavior up to x = ~0.5; for higher values of x, normal ferroelectric behavior was observed. For x ≤ 0.5 reductions in the chemical domain, size had no significant effect on the weak-field dielectric properties, but induced a transition to relaxor behavior for x > ~0.6. The disordered PSN-rich samples undergo a spontaneous zero-field relaxor to ferroelectric transition similar to that reported previously for PSN. The field-dependent properties of compositions lying closest to the relaxor to ferroelectric crossover exhibited the highest sensitivity to alterations in the chemical order. The properties of this system are consistent with a "random site" description of the 1:1 ordered Pb(β′1/2 β1/2)O3 structure with β′ = (Mg(2-2x)/3Nb(1-x)/3Scx) and β″ = Nb.Publication Crystalline Structure and Dielectric Properties of Li1+x-yNb1-x-3yTix+4yO3 M-Phase Solid Solutions(2002-03-01) Borisevich, Albina Y; Davies, Peter KThe structure and dielectric properties of the so-called M-phase solid solutions in the Li2O-Nb2O5-TiO2 system have been investigated. Detailed studies of the lattice parameters of these phases agree well with structure models based on intergrowths of LiNbO3 slabs with a titanium-rich corundum-type layer. The relative permittivity ranges from ~80 to ~55; microwave quality factors increase with the titanium content, reaching values of Q x f = 9000 at 6 GHz. The temperature coefficient of the resonant frequency changes sign within the solid solution region, thus permitting tunability to a zero value. All the compositions can be sintered to high density at temperatures ≤ 1100∘C.Publication Enhanced Tetragonality in (x)PbTiO3-(1-x)Bi(B′B″)O3 systems: Bi(Zn3/4WN1/4)O3(2006-09-28) Stein, David M; Suchomel, Matthew R; Davies, Peter KSolid solutions in the (x)PbTiO3–(1−x)Bi(Zn3/4W1/4)O3 system have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Bi(Zn3/4W1/4)O3increases the tetragonality and Curie temperature of PbTiO3 which reach a value of 1.08 and 530 °C, respectively, at the limit of the single-phase perovskite forming region (x~0.8). The observation of a sustained increase in the tetragonality in this system is similar to the behavior of the (x)PbTiO3–(1−x)Bi(Zn1/2Ti1/2)O3 system and highlights the unique properties of Bi-based systems when the B sites contain high concentrations of highly polarizable cations.Publication Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3–La(Mg2/3Nb1/3)O3 System(1998-08-01) Akbas, Mehmet Ali; Davies, Peter KA complete range of perovskite solid solutions can be formed in the (1 - x)Ba(Mg1/3Nb2/3)O3–xLa(Mg2/3Nb1/3)O3 (BMN–LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.Publication Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics. IV. Tuning τf and the Part Size Dependence of Q × f(2006-07-01) Wu, Hui; Davies, Peter KHigh Q ceramics of Ba3W2O9 (BW)-substituted Ba(Zn1/3Nb2/3) O3 (BZN) were prepared with a zero τf through the partial substitution of Zn by Ni and Co. The small concentrations of B-site vacancies introduced by the substitution of BW accelerated the kinetics and stability of the cation ordering and lowered the sintering temperature. Dense, zero τzf, ordered solid solutions such as 0.99Ba(Zn0.3Co0.7)1/3Nb2/3O3–0.01BW with εr=34.4 and Q×f=82 000 at ~8 GHz could be obtained after sintering at 1380°C for 5 h and annealing at 1300°C for 24 h. Partially ordered ceramics in the Zn/Co and Zn/Ni solid solutions show a large gradient in the ordering throughout the pellets, which produces a resonant frequency dependence of their Q×f value. The ordering gradient is associated with the increased constraints on the growth of the 1:2 ordered structure within the interior of larger and thicker pellets and can be minimized by extended annealing.