The 3.65 Aa phase, MgSi(OH) (sub 6) ; structural insights from DFT-calculations and T-dependent IR spectroscopy

First-principles calculations based on density-functional theory (DFT) and low-T IR spectroscopy were performed to gain more insight into the structure of the so-called 3.65 Aa phase, a high-pressure phase of the composition MgSi(OH) (sub 6) . DFT-calculations predict a monoclinic symmetry with ordered sixfold-coordinated Mg and Si and six unique hydrogen sites as the most stable structure. Adapting the structural parameters of the DFT-determined lowest-energy configuration and assuming (MgSi)-ordering, a new Rietveld refinement of the powder XRD pattern of the 3.65 Aa phase was performed, which resulted in excellent refinement statistics and successful assignment of X-ray reflections that were unassigned in former structural models with orthorhombic symmetry. A configuration with ordered Mg and Si at the octahedral positions causes a small monoclinic distortion of the network of strongly tilted octahedra and thus leads to space group P2 (sub 1) . The structural refinement yields the following unit-cell parameters: a=5.1131(3), b=5.1898(3), c=7.3303(4) Aa, beta =90.03(1) degrees , V = 194.52(2) Aa (super 3) , space group: P2 (sub 1) , Z=2, rho =2.637 g/cm (super 3) . The structure of the 3.65 Aa phase can be considered as a modified A-site defective perovskite with a unique network of corner-sharing alternating Mg(OH) (sub 6) and Si(OH) (sub 6) octahedra and is probably related to the structure of stottite group minerals. Low-T IR spectroscopy confirms the presence of 6 different H-positions in the proposed structure. Measured IR-spectra and computed spectra compare favorably, which further supports the computed structure as the correct model for the 3.65 Aa phase.