Rare-earth calcium oxyborate crystals, ReCa₄O(BO₃)₃ (ReCOB, Re = Er, Y, Gd, Sm, Nd, Pr, and La ), are potential piezoelectric materials for ultrahigh temperature sensor applications, due to their high electrical resistivity at elevated temperature, high piezoelectric sensitivity and temperature stability. In this paper, different techniques for ReCOB single-crystal growth are introduced, including the Bridgman and Czochralski pulling methods. Crystal orientations and the relationships between the crystallographic and physical axes of the monoclinic ReCOB crystals are discussed. The procedures for dielectric, elastic, electromechanical and piezoelectric property characterization, taking advantage of the impedance method, are presented. In addition, the maximum piezoelectric coefficients for different piezoelectric vibration modes are explored, and the optimized crystal cuts free of piezoelectric cross-talk are obtained by rotation calculations.

Noncentrosymmetric praseodymium calcium oxyborate single crystals, PrCa₄O(BO₃)₃ (PrCOB), were grown by the Czochralski technique. The monoclinic unit cell parameters were found to be a = 8.177 Å, b = 16.157 Å, c = 3.629 Å and Z = 2 with space group Cm. Crystal density was measured using the Archimedes method, being on the order of 3.47 g cm⁻³. Thermal properties of PrCOB were investigated, where the specific heat was found to be 0.63 J g⁻¹ °C⁻¹ at room temperature, increasing to 0.85 J g⁻¹ °C⁻¹ at 700 °C. The thermal expansion coefficients were measured to be α₁₁ = 7.99, α₂₂ = 4.90 and α₃₃ = 9.46 (10⁻⁶/°C), respectively. In addition, thermal diffusivity λ₂₂ and thermal conductivity κ₂₂ as a function of temperature were studied, where λ₂₂ was observed to decrease from 0.89 to 0.58 mm² s⁻¹, while κ₂₂ was found to maintain the same value, being ∼1.90 W m⁻¹ °C⁻¹ over the temperature range of 20–700 °C.

Mg₂MnO₄ nanoparticles with cubic spinel structure were synthesized by the sol-gel method using polyvinyl alcohol (PVA) as a chelating agent. X-ray powder diffraction, infrared spectrum (IR), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to characterize the crystalline phase and particle size of as-synthesized nanoparticles. The electronic structure of Mg₂MnO₄ spinel was studied by X-ray photoelectron spectroscopy (XPS). The results showed that pure cubic Mg₂MnO₄ spinel nanoparticles were obtained when the annealing temperature was 500–700 °C. The samples had a porous-spongy structure assembled by nanoparticles. XPS studies indicated that Mg₂MnO₄ nanoparticles were mixed spinel structures and the degree of cation inversion decreased with increasing annealing temperature. Furthermore, the performance of Mg₂MnO₄ as lithium anode material was studied. The results showed that Mg₂MnO₄ samples had good cycle stability except for the slight decay in the capacity at 50 cycles. The coulombic efficiency (ratio of discharge and charge capacity) in most cycles was near 100%. The sample annealed at 600 °C exhibited good electrochemical properties, the first discharge capacity was 771.5 mAh/g, and the capacity remained 340 mAh/g after 100 cycles. The effect of calcination temperature on the charge–discharge performance of the samples was studied and discussed.

SHG efficiency of Rb _0.94 Ti _0.94 Ta _0.06 OPO ₄ is about 1.4 times that of RTP.