Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles

Piezoelectric and elastic properties of new layered ferroelectric-semiconductor materials of Sn2_{\bf 2}P2_{\bf 2}S6_{\bf 6} family

In this contribution, we present review of elastic and piezoelectric properties of new polar layered materials of Sn$_{2}$P$_{2}$S$_{6}$ family such as: CuInP$_{2}$S$_{6}$, CuInP$_{2}$Se$_{6}$, CuCrP$_{2}$S$_{6}$ and SnP$_{2}$S$_{6}$. These, recently obtained compounds, exhibit rich variety of piezoelectric properties, photosensitivity and ionic conductivity. Two investigation methods: resonance-antiresonance and ultrasonic pulse echo were used in order to measure the elastic and piezoelectric constants. The temperature dependences of longitudinal ultrasonic velocity and attenuation revealed clear anomalies near phase transitions in these layered materials. We observed large elastic nonlinearity in CuInP$_{2}$S$_{6}$ samples across layers in c-direction. The elastic nonlinnearity along layerss was much smaller. By direct pulse-echo experiments in low temperature phases, it was shown that the thin plates of CuInP$_{2}$S$_{6}$, CuCrP$_{2}$S$_{6}$, CuInP$_{2}$Se$_{6}$ and SnP$_{2}$S$_{6}$ layered compounds could effectively excite and detect ultrasonic waves

Ultrasonic Studies of Incommensurate Phase Transitions

The temperature dependencies of the ultrasonic velocities and attenuation coefficients are studied in the vicinity of the incommensurate phase transitions of BCCD and AHSe crystals. The largest contribution to the longitudinal ultrasonic anomalies is due to the coupling to the amplitude mode, whose relaxation time τ diverges as the transition temperature Ti is approached with the temperature dependence given by : τ=τoTi/T-Ti). The amplitudon relaxation time for AHSe crystal (τo=5.1.10-12s) was found to be considerably longer than for BCCD (τo=1.3.10-13s). From the measurements of shear ultrasonic wave velocity in incommensurate phase below Ti = 164 K the critical exponent that describes the order parameter temperature behaviour has been calculated for BCCD. The value β is consistent with three dimensional X - Y model. The substitution of Cl by Br in BCC1-xBrxD crystals with x<0.24 manifests itself in smearing of the elastic anomalies and shifting Ti to lower temperatures with increase of Br concentration