High Pressure Behavior of Mascagnite from Single Crystal Synchrotron X-ray Diffraction Data

• Main Authors: Paola Comodi, Maximiliano Fastelli, Giacomo Criniti, Konstantin Glazyrin, Azzurra Zucchini
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Crystals
• Subjects: mascagnite; high pressure; crystal structure; ammonium sulphates
• Online Access: https://www.mdpi.com/2073-4352/11/8/976

High-pressure synchrotron X-ray diffraction was carried out on a single crystal of mascagnite, compressed in a diamond anvil cell. The sample maintained its crystal structure up to ~18 GPa. The volume–pressure data were fitted by a third-order Birch–Murnaghan equation of state (BM3-EOS) yielding K₀ = 20.4(7) GPa, K’₀ = 6.1(2), and V₀ = 499(1) ų, as suggested by the F-f plot. The axial compressibilities, calculated with BM3-EOS, were K_0<i>a</i> = 35(3), K’_0<i>a</i> = 7.7(7), K_0<i>b</i> = 10(3), K’_0<i>b</i> = 7(1), K_0<i>c</i> = 25(1), and K’_0<i>c</i> = 4.3(2) The axial <i>moduli</i> measured using a BM2-EOS and fixing K’₀ equal to 4, were K_0<i>a</i> = 52(2), K_0<i>b</i> = 20 (1), and K_0<i>c</i> = 29.6(4) GPa, and the anisotropic ratio of K_0<i>a</i>:K_0<i>b</i>:K_0<i>c</i> = 1:0.4:0.5. The evolution of crystal lattice and geometrical parameters indicated no phase transition until 17.6 GPa. Sulphate polyhedra were incompressible and the density increase of 30% compared to investigated pressure should be attributed to the reduction of weaker hydrogen bonds. In contrast, some of them, directed along [100], were very short at room temperature, below 2 Å, and showed a very low compressibility. This configuration explains the anisotropic compressional behavior and the lowest compressibility of the <i>a</i> axis.