We present anelastic and dielectric spectroscopy measurements of PbZr(1-x)Ti(x)O(3) with 0.455 < or = x < or = 0.53, which provide new information on the low-temperature phase transitions. The tetragonal-to-monoclinic transformation is first order for x < 0.48 and causes a softening of the polycrystal Young's modulus whose amplitude may exceed the one at the cubic-to-tetragonal transformation; this is explainable in terms of linear coupling between shear strain components and tilting angle of polarization in the monoclinic phase. The transition involving rotations of the octahedra below 200 K is visible both in the dielectric and anelastic losses, and it extends within the tetragonal phase, as predicted by recent first-principle calculations.

The elastic response of (Ba,Ca)(Ti,Zr)O3 at compositions where the piezoelectric effect is maximized has been measured by different methods between 1 Hz and 250 kHz and compared with that of PZT at the middle of its morphotropic phase boundary. In all cases, the compliance is peaked at the border between the tetragonal (T) and the orthorhombic (O) phases, intermediate between the T and the low-temperature rhombohedral phase. The anomalies do not exhibit dependence on frequency and their relative amplitude is 200–300 times larger than the losses, demonstrating that they are intrinsic rather than due to the domain wall motion. This also demonstrates the role of an intermediate O or monoclinic phase in enhancing the transverse instability and piezoelectric coupling at a (morphotropic) phase boundary between R and T phases.

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Among the hybrid metal-organic perovskites for photovoltaic applications FAPbI 3 (FAPI) has the best performance regarding efficiency and the worst regarding stability, even though the reports on its stability are highly contradictory. In particular, since at room temperature the cubic α phase, black and with high photovoltaic efficiency, is metastable against the yellow hexagonal δ phase, it is believed that α−FAPI spontaneously transform into δ−FAPI within a relatively short time. We performed X-ray diffraction and thermogravimetric measurements on loose powder of FAPI, and present the first complete dielectric and anelastic spectra of compacted FAPI samples under various conditions. We found that α−FAPI is perfectly stable for at least 100 days, the duration of the experiments, unless extrinsic factors induce its degradation. In our tests, degradation was detected after exposure to humidity, strongly accelerated by grain boundaries and the presence of δ phase, but it was not noticeable on the loose powder kept in air under normal laboratory illumination. These findings have strong implications on the strategies for improving the stability of FAPI without diminishing its photovoltaic efficiency through modifications of its composition. Graphical TOC Entry1 arXiv:1905.02992v1 [cond-mat.mtrl-sci] 8 May 2019Although MAPbI 3 (MAPI, MA = methylammonium CH 3 NH 3 ) is the most studied hybrid metal-organic perovskite for photovoltaic applications, 1 better performance in terms of photovoltaic efficiency are found in FAPbI 3 (FAPI, FA = formamidinium CH(NH 2 ) 2 ). This is due both to a smaller bandgap of FAPI and to the fact that the FA + ion, in spite of a smaller electric dipole with respect to MA + , has a much larger quadrupole and faster reorientation dynamics that better screen the photoexcited carriers, enhancing their lifetime. 2 FAPI also has a better stability than MAPI at high temperature but its major flaw is that the black cubic α phase, which has the high photovoltaic efficiency, is metastable at room temperature, where instead the stable phase, and the one obtained by standard chemical methods, is the yellow hexagonal δ phase. For these reasons, major efforts are directed now at trying to stabilize the cubic α phase of FAPI through partial substitutions of FA with MA, Cs, etc. or I with Br, although this approach increases the bandgap. 3,4 It has been discussed, based on neutron diffraction measurements and simulations, that the α → δ transformation is complex and occurs through various intermediate stages, requiring to overcome a free energy barrier estimated in the order of hundreds of milli-electron volt. 5 This explains why the α phase of FAPI is kinetically trapped, resulting in a large thermal hysteresis between the δ → α transition at T h δα = 350 K and the α → δ at T c δα = 290 K. 5 Actually, the barrier for the α → δ transition has not been measured, and there is complete uncertainty on the kinetics of this transition. Indeed, also the reported temperatures for the δ → α transition during heati...

Abstract. Anelastic and dielectric spectroscopy measurements on PbZr 1−x Ti x O 3 (PZT) close to the morphotropic (MPB) and antiferroelectric boundaries provide new insight in some controversial aspects of its phase diagram. No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, in agreement with recent structural studies supporting a coexistence of the two phases over a broad composition range x < 0.5, with the fraction of M increasing toward the MPB. It is also discussed why the observed maximum of elastic compliance appears to be due to a rotational instability of the polarisation and therefore cannot be explained by extrinsic softening from finely twinned R phase alone, but indicates the presence also of M phase, not necessarily homogeneous.A new diffuse transition is found within the ferroelectric phase near x ∼ 0.1, at a temperature T IT higher than the well established boundary T T to the phase with tilted octahedra. It is proposed that around T IT the octahedra start rotating in a disordered manner and finally become ordered below T T . In this interpretation, the onset temperature for octahedral tilting monotonically increases up to the antiferroelectric transition of PbZrO 3 , and the depression of T T (x) below x = 0.18 would be a consequence of the partial relieve of the mismatch between the cation radii with the initial stage of tilting below T IT .2

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