Transparent ferroelectric crystals with high piezoelectricity are challenging to build because of their complex structure and disordered domains in rhombohedral relaxor ferroelectrics. There are eight domains along the <111> direction, which cause light scattering. In this study, perfect transparency is achieved along the [110] and [001] directions in [110]‐poled rhombohedral 0.72Pb(Mg1/3Nb2/3)O3‐0.28PbTiO3 (PMN‐PT) crystals, which have a high d31 value of 1700 pC N−1 and a high electro‐optic coefficient γ33 of 320 pm V−1. This implies that the [110]‐oriented rhombohedral PMN‐0.28PT crystal can realize the mode of transverse modulation, whereas the [001]‐oriented PMN‐0.28PT crystal is more suitable for the longitudinal mode. Through piezoresponse force microscopy (PFM), it is confirmed that the [110]‐poled rhombohedral PMN‐PT crystals form 71° layered domains, which are similar to the 109° layered domains of the [001]‐oriented transparent crystal. Combined with PFM and birefringence microscopy, the degradation of domains and thickness dependence of piezoelectricity provide clear evidence for the relationship between the engineered domain structures and piezoelectric properties, which should be considered in the design of piezoelectric or electro‐optic devices with excellent performance. This work enriches the research on ferroelectric domain engineering for excellent transparency and high piezoelectricity to provide new ideas for photoacoustic devices.

Optical dispersion properties are indispensable in the thorough design of modern perovskite‐based photonics devices. Based on inverse temperature crystallization (ITC) method, here, CH3NH3PbX3 (CH3NH3═MA, X═Cl, Br, I) bulk single crystals are grown from precursor solution. X‐ray patterns indicate that the perovskite crystals have good crystallinity. MAPbCl3, MAPbBr3, and MAPbI3 crystals are transparent above 427, 574, and 823 nm, respectively. Refractive indices of MAPbX3 crystals are measured with different wavelength at 24 °C and reduced dramatically with the wavelength increase. Sellmeier dispersion equation fitted by least squares can be used to calculate the refractive index above the absorption edges. Refractive indices rank in the order of MAPbI3 > MAPbBr3 > MAPbCl3 at the same wavelength. The halogen elements have great influence on refractive indices. These results play a vital role in studying MAPbX3 nonlinear optical properties as well as in further designing photoelectric devices.

Giant tunability of UC PL by oxygen vacancy control was realized in Er-KNN single crystals grown for the first time.

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