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 Å phase, a high-pressure phase of the composition MgSi(OH)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 Å 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 P21. The structural refinement yields the following unit-cell parameters: a = 5.1131(3), b = 5.1898(3), c = 7.3303(4) Å, β = 90.03(1)°, V = 194.52(2) Å3, space group: P21, Z = 2, ρ = 2.637 g/cm3. The structure of the 3.65 Å phase can be considered as a modified A-site defective perovskite with a unique network of corner-sharing alternating Mg(OH)6 and Si(OH)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 Å phase.

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