Thermal and dielectric properties of ferroelectric lead germanate single crystals doped with chromium ions (Pb5Ge3O11:Cr3+)

Single crystals of Pb5Ge3O11 (PGO), pure and doped with Cr3+ ions, were grown by means of Czochralski and Kyropoulos method, respectively. Structural, microstructural, thermodynamic and dielectric properties were investigated in terms of Cr3+-doping. The temperature dependences of molar heat capacity in the range 120-700 K at constant pressure (c(p)) for PGO and PGO:Cr3+ (0.2%) crystals were shown. Wide lambda-shape anomalies correspond to the second-order phase transitions from ferroelectric to the paraelectric phase taking place at 453 K and 431 K for PGO and PGO:Cr3+ (0.2%), respectively. The temperature dependences of c(p) for investigated crystals were approximated by the Einstein function and near the Curie temperature the c(p) reaches the classical Dulong-Petit limited value similar to 475 J.mol(-1).K-1. Temperature dependences of dielectric permittivity measured along and perpendicular to the ferroelectric axis in the temperature range 300-550 K are investigated. These studies revealed anisotropic behavior of PGO and PGO:Cr3+ (0.2%) crystals.