Search

The high relative dielectric constant (?r = 2000-5000) and low losses (?1%) of ferroelectric barium titanate, BaTiO3, has made it the most widely used dielectric ceramic material in the electronic industry for the fabrication of multilayer ceramic capacitors, which are used in most electronic circuits, with an annual production of 2000 billion of devices. Owing to the strong temperature dependence of the dielectric constant, BaTiO3 cannot be used in pure form, but its properties have to be properly modified by the incorporation of suitable dopants. Pure BaTiO3 has 4 crystallographic variants, depending on the temperature. The substitution of Ti4+ (r = 0.605 Å) with a homovalent ion in BaTiO3 ceramics, with formation of BaMxTi1-xO3 (M = Sn, Hf, Zr, Ce) solid solutions, modifies the polar order, with increasing x, from conventional ferroelectric to a so-called relaxor behaviour for high substitution levels via a diffuse transition. Irrespective of M, with increasing x the three phase transitions merge at a tricritical point (xp, Tp), TCP, the so-called pinched transition, where the four polymorphs coexist. For x > xp, the system then directly transforms from the ferroelectric R phase to the C paraelectric phase with a diffuse transition. BaMxTi1-xO3 solid solutions have gained large attention for applications. In fact, they show properties such as a very high (1-2·104) and stable dielectric constant around room temperature, large dielectric tunability and improved piezoelectric activity, of potential interest for a number of applications, such as, respectively, high permittivity dielectrics, microwave devices for telecommunications, and lead-free piezoelectric sensors, actuators and energy harvester. Recently it was also theoretically predicted and experimentally observed for M = Sn, Hf, Zr, that for specific compositions corresponding to the coexistence of different phases, such as the one close to the TCP, BaMxTi1-xO3 develops a large electrocaloric response, that could be used in cooling devices. We present an extensive investigation of BaMxTi1-xO3 (M = Zr, Ce) over a broad range of compositions, combining macroscopic property measurements and average structure information (electric properties, DSC analysis and high resolution XRD) with the local structure analysis (PDF analysis and Raman spectroscopy). Among the four M ions, Ce4+ substitution is the less studied, although it is the largest ion and thus a model system to study the effect of strain on the onset of relaxor behaviour. By comparing our results with the available literature, some general conclusions can be drawn on the origin of relaxor behaviour for the BaMxTi1-xO3 system. In fact, the compositions corresponding to the tricritical point and the ferroelectricto- relaxor crossover in BaMxTi1-xO3 are nearly independent on M, suggesting that, irrespective of the M4+ radius, a critical number of Ti-O-Ti bonds has to be broken before a new "state" is established.

BaTiO3 perovskite and its solid solutions are nowadays the most used ferroelectric materials, being them environmental friendly and with excellent properties. Among the various possible solid solutions with homovalent substitution at Ti site (BaTi1-xMIVxO3), only BaTi1-xZrxO3 was studied extensively from the structural point of view [1]. Other substitutions can be interesting: in particular, the one with cerium, BaCexTi1-xO3 (BCT), which presents a large difference in size between Ti and Ce ions. Therefore, in this work, BCT with different Ce amount (x=0.02-0.20) and various particle sizes (from 80-160 nm to 5 um) was deeply explored both on the average and on the local structure by means of traditional Rietveld method and Pair Distribution Function. BCT in its ceramic form (grain size: 1 to 5 um) presents a polar behaviour variation from conventional ferroelectric to relaxor via diffuse phase transition (DPT) [2, 3]. As the particle size is reduced, the so called "size effect" takes over, with larger variations in the electric properties with composition. It is well known that ferroelectricity is related to titanium displacement within the octahedra, and our Rietveld analysis results find a good agreement between measured material properties and titanium offcenter values in the ceramic materials. The progressive suppression of the spontaneous polarization with x and the cubic phase stabilization is related to the disruption of the correlated Ti displacements engendered by the large Ce4+ incorporated at the octahedral site [4], and a complex model must be used for the local structure. All the described effects are generally more pronounced when the material presents a reduced particle size, as cerium content and reduced size cooperate in the extension of the cubic structure stability field, with respect to the corresponding micrometric materials. The distortions at the local level are therefore confirmed to be largely responsible for the ferroelectric properties of the BaCexTi1-xO3 perovskite, either in the micrometric or in the reduced size samples. [1] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y. Ren, J. Phys. Condens. Matter 26 (2014), 065901. [2] V.V. Shvartsman, D.C. Lupascu, J. Am. Ceram. Soc. 95 (2012), 1. [3] G. Canu, G. Confalonieri, M. Deluca, L. Curecheriu, M.T. Buscaglia, M. Asandulesa, N. Horchidan, M. Dapiaggi, L. Mitoseriu, V. Buscaglia, Acta Mater. 152 (2018), 258. [4] G. Confalonieri, V. Buscaglia, G. Capitani, G. Canu, N. Rotiroti, A. Bernasconi, A. Pavese & M. Dapiaggi, J. Appl. Cryst 51 (2018), 1283.

The substitution of Ti4+ (r = 0.605Å) with a homovalent ion in BaTiO3 ceramics, with formation of BaMxTi1-xO3 (M= Sn, Hf, Zr, Ce) solid solutions, has gained large attention for applications as dielectric or piezoelectric materials. Ce4+ (0.87Å) is the largest among these M ions, thus BaCexTi1-xO3 is a model system to investigate the effects of local strain on macroscopic properties. We present an extensive investigation of BaCexTi1-xO3 over compositions in the x=0.02-0.30 range combining macroscopic property measurements and average structure information with the local structure analysis. By comparing our results with the available literature, especially regarding Zr and Sn substitution, some general conclusions can be drawn. In fact, the compositions corresponding to the tricritical point and the ferroelectric-to-relaxor crossover in BaMxTi1-xO3 are nearly independent on M, suggesting that, irrespective of the M4+ radius, a critical number of Ti-O-Ti bonds has to be broken before a new "state" is established.

Barium titanate, BaTiO3, owing to its high relative dielectric constant and low losses is the most widely used dielectric ceramic material in the electronic industry. The substitution of Ti4+ with a homovalent ion with formation of BaMxTi1-xO3 (M = Sn, Hf, Zr, Ce) solid solutions has gained large attention from both fundamental and application point of view. These materials are attractive for applications such as high energy density capacitors, tunable microwave dielectrics and electrocaloric effect-based cooling devices [1-3]. BaCexTi1-xO3 solid solution is a model system to investigate the effects of local strain fields, Ce4+ being a much larger ion (r = 0.87 Å, c.n. 6) than Ti4+ (r = 0.605 Å, c.n. 6), but while an abundant literature exists on the solid solutions in which Ti is substituted by Sn and Zr, the effect of the incorporation of Ce4+ at the Ti site has not been systematically investigated yet. This work is a comprehensive study of BaCexTi1-xO3: in fact, it combines macroscopic property measurements and average structure information with complementary techniques sensitive to the local structure, i.e. X-ray Pair Distribution Function analysis and Raman spectroscopy, to gain comprehensive insight on the structure-property relationships. BaCexTi1-xO3 ceramics have been investigated over a broad composition (x = 0.02 - 0.30, the upper limit being very close to the solubility limit) and temperature (100 - 400 K) range. The data provide a detailed picture of the BaCexTi1-xO3 system and a composition-temperature phase diagram will be proposed. [1] V. V. Shvartsman, D. C. Lupascu, J. Am. Ceram. Soc., 95, 1-26 (2012), [2] X. S. Qian et al., Adv. Funct. Mater., 24, 1300-1305 (2014), [3] V. Buscaglia et al., J. Phys: Condens. Matter, 26, 065901 (2014)

The incorporation of homovalent M4+ ions at the Ti site of BaTiO3 with formation of the BaMxTi1-xO3 solid solutions (M = Sn, Zr, Hf, Ce) determines significant modifications of the phase transitions, polar order and functional properties. These materials are attractive for applications such as high energy density capacitors, tunable microwave dielectrics and electrocaloric effect-based cooling devices [1-3]. Unlike the systems containing Sn and Zr, the impact of structural disorder and local structure on the evolution of polar order and properties in Ce-doped barium titanate ceramics has not been investigated in detail yet. Dense (97-99% rel. density) ceramic samples with composition BaCexTi1-xO3 (x = 0.02, 0.05, 0.06, 0.10, 0.12, 0.15, 0.175, 0.20 and 0.30) were prepared by the conventional solid-state route starting from fine precursor powders and sintered at 1450-1500 °C. The average crystal structure at different temperatures in the range 100-400 K was determined from the Rietveld refinement of high-energy X-ray diffraction data collected at ESRF. The pair distribution function analysis was used to investigate the local structure and the level of disorder. The phase transitions and the evolution of polar order with composition were studied using dielectric permittivity measurements at 100 Hz-1 MHz between -150 and 150 °C, variable temperature Raman spectroscopy (from -195 to 165 °C) and differential scanning calorimetry (-100 - 150 °C). The ferroelectric hysteresis loops were measured at room temperature. Overall, the data provide a detailed picture of the BaCexTi1-xO3 system and a composition-temperature phase diagram will be proposed. A polar local structure exists irrespective of the long-range average symmetry for all compositions. Differently from the homologous systems BaSnxTi1-xO3 and BaZrxTi1-xO3, a diffuse ferro/para transition is already observed at x = 0.05 as a consequence of the high level of lattice disorder while relaxor behaviour occurs when x >= 0.20. References [1] V. V. Shvartsman, D. C. Lupascu, J. Am. Ceram. Soc., 95, 1-26 (2012) [2] X. S. Qian et al., Adv. Funct. Mater., 24, 1300-1305 (2014) [3] V. Buscaglia et al., J. Phys: Condens. Matter, 26, 065901 (2014)

BaTiO3 perovskite and its solid solutions, as BaTi1-xMIVxO3, are nowadays the most used ferroelectric. These materials are friend-environmental ferroelectrics, being lead-free, and show excellent properties. In this light, they are very useful in very different fields, but unfortunately their characteristics limit their applications in nano technology. Indeed, in these perovskites, the reduced size induces the so called "size effect" which consists in a disappearance of the ferroelectricity under a critical grain size [1]. This degradation of the material properties is then really problematic considering the present tendency in device miniaturization. In order to better understand this phenomenon, many studies have been performed. In pure BaTiO3 the "size effect" and the polarization reduction seem to be linked to the increasing local structural disordered nature of the nano powders as reported by Petkov et al. [2], Page et al. [3], Yoneda et al. [4]. However similar conclusions are not be previously proposed for doped BaTiO3, then the case of nano BaTi1-xCexO3 perovskite is here presented. The purpose is to explore the relation between the reduced size and the structure, as a function of cerium amount. Rietveld method and Pair Distribution Function were applied to give a complete picture of the average and local structure. Data collection was performed at ID22, European Synchrotron Radiation Facility (ESRF), Grenoble, France [5]. In this case study the use of the synchrotron radiation and an appropriate instrumentation is essential. In one hand high resolution data were necessary to recognize phase transitions, observable despite the peak broadness caused by the nano size of the material. On the other hand, in order to obtain suitable Pair Distribution Functions, data has to be collected at very high Qmax. The high quality of the data allowed to obtain good results on both average and local structure. Performed analyses show how the reduced size and the doping amount act at the structural level and how this is linked to the material properties. [1] N. Nuraje, K. Su, Nanoscale 2013, 5(19), 8752-8780. [2] V. Petkov, V. Buscaglia, M.T. Buscaglia, Z. Zhao, Y. Ren, Physical Review B 2008, 78(5). [3] K. Page, T. Proffen, M. Niederberger, R. Seshadri, Chemistry Of Materials 2010, 22(15), 4386-4391. [4] Y. Yoneda, S. Kohara, K. Kato, Japanese Journal Of Applied Physics 2013, 52(9, 2, SI). [5] www.esrf.eu

usually quartz), plus something that may act as a flux (usually feldspars), aiding the melting at high temperature, so that to produce a ceramic material with a low porosity. The use of alternative fluxes (such as NaOH and KOH solutions) has been tested on quartz only, on a mixture of quartz and kaolinite, and on a more traditional mixture encompassing quartz, kaolinite and Na-feldspar. For all the mixtures, quartz with different grain sizes was used (4, 10, and 28 ?m). All the samples were mixed with a fixed amount of a 20%wt solution of NaOH or KOH, in order to produce a paste-like material, treated at 1250°C for 6 hours, and then slowly cooled. The final products were analysed by means of X-ray powder diffraction and the mineralogical composition evaluated with the Rietveld-RIR method, as a non negligible amount of amorphous material was present. The results showed that, for quartz only, the presence of sodium and potassium cations influence the formation of tridymite and cristobalite, as expected. The novelty of this work stands in the in-situ experiments, which showed that tridymite forms at the expense of cristobalite, which is always the first one to form. This is confirmed by ab initio calculations at HF/DFT level, which showed that cristobalite, in presence of sodium and/or potassium, is energetically favoured (Dapiaggi et al., 2015). In the more complex starting mixtures, the equilibrium mullite/amorphous has been carefully studied, using the results of the quantitative determinations. The results show that there is no significant influence of grain size on the equilibrium, and that Na is more effective than K in reducing the residual quartz. Na and K can effectively be used to control the amount of amorphous material present in the final product.

In the past few decades, scientific community has paid great attention on free-lead ferroelectrics. These materials, in comparison to the toxic lead-based ones, have comparable good performances and can be employed for environmental friendly applications. BaTiO3 perovskite in its pure form does not have ideal properties, but by chemical substitutions, it can be tailored to meet a variety of device and performance requirements [1]. The solid solutions BaTi1-xMIV x O3 perovskites (M = Sn, Zr, Hf, Ce) are a classical example: in fact, these materials show an almost continuous variation of the ferroelectric behaviour with composition, from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [2]. In these cases the dipolar behaviour and functional responses seem to depend critically on the order between Ti and its substituents [3]. It is then mandatory to perform deep structural investigations on these materials, in order to clarify the connection between property and structure. In this research work, the solid solution of BaTi1-xCexO3 has been chosen because it represents a limit and interesting case: on one hand Ce4+ (r = 0.87 °A) is much bigger than Ti4+ (r = 0.605 °A) and on the other this kind of substitution does not involve the creation of charge compensating lattice defects. Ceramic solid solutions with x = 0.05, 0.10, 0.20 have been investigated between 100 and 420 K, because, as indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20). Total scattering data have been collected at ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France) and Pair Distribution Function (PDF) refinements, coupled with traditional Rietveld refinements, have been performed with the aim of understanding the induced differences between average and local structure and at the end to describe how the local structure reacts at the substitution between titanium and cerium atoms. As demonstrated by the comparison between local and average analysis, the introduction of cerium in BaTiO3 structure generates a deep local disorder, which is not reflected on the average structure. In order to model it and to understand the local neighbourhood relation between cerium and titanium, lattice energies have been calculated and compared for different structural configurations: from random distribution to a clustered supercell. Different numbers and size of BaCeO3 clusters have been considered. [1] T. Maiti, R. Guo, A.S. Bhalla, J. Am. Ceram. Soc. 91, 6 (2008) [2] V .V. Shvartsman, D.C. Lupascu, J. Am. Ceram. Soc. 95(1), 1-26 (2012) [3] V. Krayzman, I. Levin, J. Appl. Cryst. 41, 386-392 (2008)

Perovskite-like compounds exhibit a variety of exploitable properties that can be chemically tuned using substitutions on the A, B or both cation sites of the ABO3 perovskite structure [1]. BaTiO3 is considered an environmental-friend ferroelectric: chemical substitutions at the Ba2+ and/or Ti4+ sites are in fact usually made to tailor properties for a variety of device and performance requirements [2]. A classical example is the solid solution BaTi1-xMIVxO3, where M could be Sn, Zr, Hf, Ce etc., whose ferroelectric behavior shows an almost continuous variation with composition [3]. The reason for this is, usually, that distinct B cations mixed on the octahedral sites form ordered arrangements, and the properties of complex perovskites are strongly affected by the type, degree and spatial extent of this order [4]. In this work attention has been focused on BaTi1-xCexO3 ceramic solid solutions. It represents a limit and interesting case due to a large difference in the Ti and Ce ionic radius ( rCe4+ = 0.87 Å and rTi4+= 0.605 Å), and it is a substitution that does not involve the creation of charge compensating lattice defects. Three samples with different cerium amount and three different polar behavior has been chosen: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20). In order to model the local structure and in particular to explore the local neighbourhood relation between cerium and titanium, refinements have been performed using Pair Distribution Function technique. With the aim of confirming PDF results, lattice energies have been furthermore calculated and compared for different structural configurations: from random distribution to a clustered supercell. Different numbers and size of BaCeO3 clusters have been considered and discussed. [1] Krayzman V., Levin I. and Tucker M.G. Journal of Applied Crystallography, 2008, 41, 4. [2] Maiti T., Guo R. and Bhalla A.S. Journal of the American Ceramic Society, 2008, 91, 6. [3] Shvartsman V.V., Lupascu D.C., Journal of the American Ceramic Society, 2012, 95, 1. [4] Krayzman V. and Levin I. Journal of Applied Crystallography, 2008, 41, 2.

Elastic and anelastic properties of La0.5Ca0.5MnO3 determined by resonant ultrasound spectroscopy in the frequency range ∼100-1200 kHz have been used to evaluate the role of grain size in determining the competition between ferromagnetism and Jahn-Teller/charge order of manganites which show colossal magneto resistance. At crystallite sizes of ∼75 and ∼135 nm the dominant feature is softening of the shear modulus as the charge order transition point, Tco (∼225 K), is approached from above and below, matching the form of softening seen previously in samples with 'bulk' properties. This is consistent with a bilinear dominant strain/order parameter coupling, which occurs between the tetragonal shear strain and the Jahn-Teller (Γ3(+)) order parameter. At crystallite sizes of ∼34 and ∼42 nm the charge ordered phase is suppressed but there is still softening of the shear modulus, with a minimum near Tco. This indicates that some degree of pseudoproper ferroelastic behaviour is retained. The primary cause of the suppresion of the charge ordered structure in nanocrystalline samples is therefore considered to be due to suppression of macroscopic strain, even though MnO6 octahedra must develop some Jahn-Teller distortions on a local length scale. This mechanism for stabilizing ferromagnetism differs from imposition of either an external magnetic field or a homogeneous external strain field (from a substrate), and is likely to lead both to local strain heterogeneity within the nanocrystallites and to different tilting of octahedra within the orthorhombic structure. An additional first order transition occurs near 40 K in all samples and appears to involve some very small strain contrast between two ferromagnetic structures.

Evaporitic minerals, such as gypsum, within sedimentary sequences play an important role in localizing deformation, especially in thrust tectonics, implying that their strength is generally lower than that of other rocks. To study the rheological and microstructural evolution of gypsum with strain, a set of experiments was performed on natural gypsum samples from Volterra (Italy). To reach high shear strain (up to γ = 5), deformation tests were performed in torsion at 300 MPa confining pressure, at temperatures up to 127 °C, and at shear strain rates between 10-3 and 10-5 s-1. All samples were studied by optical microscopy, to investigate the evolution of the microstructure with strain, and by X-ray diffraction (XRD) analyses, to determine whether and to what extent gypsum dehydrated during deformation. The shear stress increased with shear strain rate and decreased with temperature. A peak stress was usually reached at γ between 0.5 and 1.5. After the peak, 30-40% of weakening occurred but mechanical steady-state conditions were never reached. The microstructure evolved from a plastic deformation microstructure, where grains changed shape according to the bulk strain imposed, into a recrystallization-dominated microstructure, where grains were more equant. The shear stress sensitivity to strain rate increases with progressive strain, thus a meaningful constitutive flow law can only be determined from experiments in which steady-state flow is eventually reached. These results imply that gypsum in nature will flow plastically at shear stress levels lower than those expected from previous experimental studies due to the strain weakening associated with dynamic recrystallization, which can occur at temperatures even lower than gypsum dehydration. © The Geological Society of London 2005.

High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals Cu2O and Ag2O as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to 240 K for Cu2O, up to 470 K for Ag2O) and anisotropic thermal displacements of M atoms (M=Cu,Ag). EXAFS measures a positive expansion of the nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 M-M next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid M-O rods.