Thermal Structural Evolution and Optical Properties of Isosymmetric Nonlinear-Optical Borates α- and β-RbB3O5

Two isosymmetric (sp. gr. P212121) polymorphic modifications of RbB3O5 (α, low-temperature; and β, high-temperature) were obtained by solid-state reactions. Their thermal deformations, mechanism and temperature of polymorphic transition were studied in detail using powder high-temperature X-ray diffraction in the range 30–740°C. Before the transition, the structure of α-RbB3O5 deforms to prepare for the abrupt change in Rb atom position without breaking the boron-oxygen framework. The Kurtz–Perry method is used to evaluate the second-harmonic generation (SHG) responses of the polymorphs and compare them with crystalline quartz. The α-RbB3O5 polymorph exhibits a nonlinear optical (NLO) coefficient more than two times greater than that of β-RbB3O5. A comparison of optical and thermal properties within the series of alkali triborates shows that the low-temperature polymorph possesses superior NLO properties.