Crystal structure and phase transition in noelbensonite: a multi-methodological study

The crystal structure and the phase transition of noelbensonite ((Ba0.72Sr0.31Ca0.01)Σ1.05(Mn1.82Al0.16)Σ1.98[Si2O7](OH)2 ·H2O) were investigated by in situ single-crystal X-ray diffraction, ab initio simulations, and infrared spectroscopy. In contrast to previous assumptions, single-crystal X-ray diffraction data and molecular dynamic simulations at room temperature (RT) displayed the acentric space group P21cn, a = 6.31303(2), b = 9.0977(3), c = 13.5820(4) Å, V = 779.73(4) Å3. This corresponds to the low-temperature (−118 °C) structure of lawsonite (CaAl2[Si2O7](OH)2·H2O) and to the phase of hennomartinite (SrMn+ 32[Si2O7](OH)2·H2O) below 95 °C. At 225 °C, the structure changed to space group Cmcm, which corresponds to that of hennomartinite at >245 °C and of lawsonite above 0 °C. In this structure the oxygen site of the H2O molecule showed positional disorder. Molecular dynamic simulations indicated that the splitting of this site reflects the disordered arrangement of the hydroxyl groups and the H2O molecule in the high-temperature modification. Infrared spectra collected at RT showed similarities with those of lawsonite. The bands at 3566 and 3517 cm− 1 and the two broader bands between 3300 and 2930 cm− 1 agree with the stretching frequencies of the hydrogen bond system as calculated from X-ray diffraction data and theoretical computations. Normal mode analysis of molecular dynamic trajectories allowed to identify the origin of vibration bands and polarization dependence of the IR spectra.