Peter K. Davies

Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p–n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol–gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric–electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10−4 to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7–4 electronvolts) of ferroelectric oxides, which allow the use of only 8–20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 − x[BaNi1/2Nb1/2O3 − δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1–3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

The soft chemical synthesis of TiO2 (B) from layered titanates

Abstract Proton exchange and subsequent dehydration of layered titanates with the formula A 2 Ti n O 2 n +1 ( A = Na, K, Cs; 3 ≤ n ≤ 6) yield TiO 2 (B) at temperatures below 350°C. In order to elucidate the mechanism of TiO 2 (B) formation, we have characterized these reactions using thermal analysis and crystallographic techniques, and we have examined the effect of different starting materials on the formation of TiO 2 (B). The dehydration proceeds through three distinct steps: an initial topotactic structural condensation, which is endothermic, followed by an exothermic nucleation and growth step that results in the formation of a TiO 2 (B)-like intermediate, and a final, low-energy transformation, which yields TiO 2 (B). The structures of D 2 Ti 3 O 7 and TiO 2 (B) have been refined from powder neutron diffraction data in order to better understand the relationship between the initial titanate structures and the formation of TiO 2 (B).

Microwave dielectric properties of hexagonal perovskites

The dielectric properties of the hexagonal perovskite oxides, La4BaTi4O15, La4Ba2Ti5O18 and Ba5Nb4O15 have been characterized at microwave frequencies. These systems combine a relatively high permittivity (39 < er < 46) with a low dielectric loss (11,583 < Q.f < 31,839) and small temperature coefficient of resonant frequency (−36 < τf < 79) and may be suitable for application as dielectric resonators. The dielectric polarizability of each compound was compared to values predicted from the sum of tabulated empirical ion dielectric polarizabilities. From these analyses it was concluded that published values for the ion polarizability of La are high; self consistent results for these and other La-containing oxides can be obtained with α(La) = 4.82(A3).

Enhanced tetragonality in (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3 and related solid solution systems

The solid solutions (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3, (x)PbTiO3-(1−x)Bi(Zn1∕2Zn1∕2)O3, and (x)PbTiO3-(1−x)Bi(Zn1∕2Sn1∕2)O3, have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Unlike most PbTiO3-based solid solutions, these systems show enhanced tetragonality with substitution for PbTiO3. In particular, the (x)PbTiO3-(1−x)Bi(Zn1∕2Ti1∕2)O3 system exhibits a high c∕a ratio of 1.11 for x=0.60. Accordingly, the Curie temperature (TC) also increases, exceeding 700°C at the same composition. It is proposed that PbTiO3-Bi(B)O3-type solid solutions which contain only highly polarizable cations on the B site are likely to exhibit similar enhancements in tetragonality and TC.

Predicting the position of the morphotropic phase boundary in high temperature PbTiO3-Bi(B′B″)O3 based dielectric ceramics

Two previously unreported PbTiO3-Bi(B′B″)O3 type solid solution systems possessing a high dielectric transition Curie temperature (TC) at their morphotropic phase boundary (MPB) were investigated. Both Bi(B′B″)O3 type compounds, Bi(Mg1∕2Ti1∕2)O3 and Bi(Mg1∕2Zr1∕2)O3, possess a low tolerance factor and display extensive solid solubility with PbTiO3. Dielectric and structural properties of the MPB systems were measured. Using a relationship presented in this paper, which relates the tolerance factor and the position of the MPB in PbTiO3 based systems, the approximate compositional position of the MPB in these systems was accurately predicted prior to synthesis. It is proposed that this relationship can be used as a guide in the search for PbTiO3 based MPB systems.

Predicting morphotropic phase boundary locations and transition temperatures in Pb- and Bi-based perovskite solid solutions from crystal chemical data and first-principles calculations

Using data obtained from first-principles calculations, we show that the position of the morphotropic phase boundary (MPB) and transition temperature at MPB in ferroelectric perovskite solutions can be predicted with quantitative accuracy from the properties of the constituent cations. We find that the mole fraction of PbTiO3 at MPB in Pb(B′B″)O3–PbTiO3, BiBO3–PbTiO3, and Bi(B′B″)O3–PbTiO3 exhibits a linear dependence on the ionic size (tolerance factor) and the ionic displacements of the B cations as found by density-functional-theory calculations. This dependence is due to competition between the local repulsion and A-cation displacement alignment interactions. Inclusion of first-principles displacement data also allows accurate prediction of transition temperatures at the MPB. The obtained structure-property correlations are used to predict morphotropic phase boundaries and transition temperatures in as yet unsynthesized solid solutions.

Quantitative correlations of deviations from ideality in binary and pseudobinary solid solutions

Abstract Deviations from ideal mixing in a number of isostructural binary solid solutions are parameterized using regular and subregular thermodynamic mixing models. Linear correlations between calculated interaction parameters and a term representing the volume mismatch of the two end-members are obtained. These correlations apply to a wide variety of structure types and are found to segregate the solid solutions according to the valence of the ions being mixed. Alkali halide systems show smaller relative deviations than oxide and chalcogenide systems. The ratio of the slopes of these correlations agree with predictions made from consideration of the effective charges of the ions being mixed. The correlations are used to predict the variation of critical temperature, and composition, as a function of component volume mismatch. Calculations of the free energies of transformation of rock salt to nickel arsenide structures and of wurtzite/sphalerite (fourfold coordination) to rock salt/nickel arsenide (six-fold coordination) structures are made using the interaction parameters predicted by the correlations and observed terminal solid solubility data.

Chemical order in PMN-related relaxors: structure, stability, modification, and impact on properties

High temperature thermal treatments were used to modify the cation order in several tantalate and niobate members of the Pb(Mg1/3Nb2/3)O3 (PMN) family of relaxors. The observation of complete 1:1 structural order in several compositions, and the refined cation occupancies of well-ordered samples conflict with the predictions of the “space charge” model, and support the “random site” description for the B-site order. In this charge balanced model one of the positions in the ordered structure is solely occupied by Ta (Nb), while the other contains a random distribution of Mg and the remaining Ta (Nb) cations. The stability of the order and magnitude of the domain growth is strongly influenced by solid solution additives. For Pb(Mg1/3Ta2/3)O3 (PMT), ordering-enhancing Zr, Sc, or La substituents increase the cation order–disorder transition temperature (∼1375°C in pure PMT) and promote extensive domain coarsening. For pure PMN a low temperature (<1000°C) order–disorder transition prevents any structure modification, and domain growth could only be realized with additives (Tb, Sc, or La) that stabilize the order to temperatures where the samples are “kinetically active”. The retention of relaxor behavior in all the fully 1:1 ordered, large-domain PMT and PMN-based ceramics suggests that the disorder on the random site is critical in frustrating ferroelectric order. By systematically controlling the concentration of ferroelectrically active cations on this position in fully ordered (1−x)Pb(Mg1/3Ta2/3)O3–(x)Pb(Sc1/2Ta1/2)O3 solid solutions, a crossover from relaxor to normal ferroelectric behavior was induced at x=0.5.

Structure and polarization in the high Tc ferroelectric Bi(Zn,Ti)O3-PbTiO3 solid solutions.

Theoretical 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.

Microwave dielectric properties of Li1+x-yM1-x-3yTix+4yO3 (M = Nb5+, Ta5+) solid solutions

Abstract A series of new low sintering temperature microwave resonator ceramics were investigated in the Li 1+ x – y Nb 1– x –3 y Ti x +4 y O 3 solid solution system. Ceramics with high relative permittivities (78–55), Q × f values up to 9000 (6 GHz), and zero temperature coefficients in the microwave region could be obtained via sintering at 1100°C. Similar phases were identified in the Li 2 O–Ta 2 O 5 –TiO 2 system and their stability region at 1100°C was determined. The microwave dielectric properties of the tantalates were similar to those of their Nb counterparts. The addition of ⩾ 2 mol% V 2 O 5 to the Nb solid solutions was effective in producing a large reduction in the sintering T and dense ceramics could be prepared at T ⩽ 900°C. The addition of V did not induce any significant degradation of the microwave properties and their apparent compatibility with Ag powder may indicate potential applications as co-fired circuit components.