BaMxTi1-xO3 (M = Zr, Ce) ceramics: structure-property relationships and comparison with other homovalent-substituted systems

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.