| Summary: | Bredigite with the composition Ca<sub>7</sub>MgSi<sub>4</sub>O<sub>16</sub> (Ca<sub>7</sub>MgSi<sub>4</sub>O<sub>16</sub>-Bre) has been synthesized by a solid-state reaction method at 1.2 GPa and 1373 K for 7 days, and its structure has been determined by single-crystal X-ray diffraction data. Following a relevant genealogy analysis in the literature, we have refined the structure into two space groups, <i>Pnnm</i> and <i>Pnn</i>2, and found that Ca<sub>7</sub>MgSi<sub>4</sub>O<sub>16</sub>-Bre belongs to the space group <i>Pnnm</i>, which can be essentially derived from the space group <i>Pnn</i>2 via an atomic coordinate transformation (with an average deviation of 0.039 Å only). Furthermore, some thermodynamic properties of the Ca<sub>7</sub>MgSi<sub>4</sub>O<sub>16</sub>-Bre have been obtained in this study. Using first-principles simulations based on density functional theory, the isothermal bulk modulus has been determined as 90.6(4) GPa with a pressure derivative of 5.7(1). Using density functional perturbation technique, the phonon dispersions and vibrational density of the states (VDoS) have been calculated. The VDoS has been combined with a quasi-harmonic approximation to compute the isobaric heat capacity (<i>C<sub>p</sub></i>) and standard vibrational entropy (<inline-formula><math display="inline"><semantics><mrow><msubsup><mi>S</mi><mrow><mn>298</mn></mrow><mn>0</mn></msubsup></mrow></semantics></math></inline-formula>), yielding <i>C<sub>p</sub></i> = 8.22(2) × 10<sup>2</sup> − 3.76(6) × 10<sup>3</sup><i>T</i><sup>−0.5</sup> − 1.384(4) × 10<sup>7</sup><i>T</i><sup>−2</sup> + 1.61(8) × 10<sup>9</sup><i>T</i><sup>−3</sup> J mol<sup>−1</sup> K<sup>−1</sup> for the <i>T</i> range of 298-1000 K and <inline-formula><math display="inline"><semantics><mrow><msubsup><mi>S</mi><mrow><mn>298</mn></mrow><mn>0</mn></msubsup></mrow></semantics></math></inline-formula> = 534.1 (22) J mol<sup>−1</sup> K<sup>−1</sup>.
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