Structural studies of lead-free piezoelectrics with the fresnoite structure type

Abstract

The lead free piezoelectric fresnoite A2M3O8 (A = Ba, Sr, K, Cs; M = Ti, V, Si, Ge) modulated structure type has been investigated owing to its potential to exhibit excellent piezoelectric response coefficients. Ba2TiSi2O8, Sr2TiSi2O8, and Ba2TiGe2O8 end members in addition to members of the Ba2xSr2-2xTiSi2O8, Ba2xSr2 2xTiGe2O8, Ba2TiGe2ySi2 2yO8, and BaSrTiGe2ySi2 2yO8 series were synthesised and characterised using a combination of variable temperature diffraction techniques. The Ba2TiSi2O8 and Sr2TiSi2O8 modulated structures at ambient temperature were characterised using neutron powder diffraction for the first time. Variable temperature synchrotron X ray diffraction data showed a new means of identifying the incommensurate to prototypic structural phase transition at 433 K. Resonant ultrasound spectroscopy has shown coupling between the elastic moduli and structural changes in fresnoite samples. Polycrystalline Sr2TiSi2O8 samples were shown to undergo a first order phase transition from a two phase mixture of incommensurately modulated tetragonal and orthorhombic phases to a single incommensurately modulated orthorhombic phase that is complete by 567 K. The proportion of the orthorhombic phase Sr2TiSi2O8 samples was shown to slowly decrease on cooling to 125 K. The Sr2TiSi2O8 structure was also shown to undergo an additional phase transition from the incommensurately modulated orthorhombic phase to a tetragonal phase at 1323 K for the first time. The inclusion of barium or germanium into the Sr2TiSi2O8 structure was shown to suppress the formation of the orthorhombic phase at ambient temperature and elevated temperatures. New phase diagrams for the Ba2xSr2-2xTiSi2O8 and Sr2TiGe2ySi2 2yO8 systems summarise the phase transitions investigated. The intrinsic piezoelectric coefficients were calculated to be approximately 5 pm V 1 and 27 pm V 1 for polycrystalline samples of Ba2TiSi2O8 and Sr2TiSi2O8 respectively and compared to common piezoelectric materials.