Thermodynamics, elasticity and phase stability of grossite (CaAl4O7) at high pressure and temperature

Although grossite (CaAl4O7) is an important constituent phase of high alumina cements (HAC) and a common refractory phase in calcium-rich inclusions (CAIs) found in primitive chondritic meteorites, its thermophysical properties are poorly constrained and its thermodynamic behaviour mostly unknown. In particular, the knowledge of phase stability relations up to high pressure and temperature conditions (HP-HT) is concealed by the lack of informations on elasticity and P-V-T equation of state parameters. Thermodynamics, equation of state and elasticity of grossite (space group C2/c) have been investigated in this work by first principles theory (using a WC1LYP hybrid functional and CRYSTAL program) and experimental methods (synchrotron radiation high-pressure single crystal X-ray diffraction, using an ETH-type Diamond Anvil Cell and M.E.W. as pressuretransmitting fluid, at Xpress beamline at Elettra, Trieste, λ = 0.4957 Å) in a broad range of P-T conditions (i.e. 0-10 GPa and 0-2000 K). HP experiments reveal a displacive first order phase transition, which has been observed between 6.1 and 7 GPa by ca. a 3% change in lattice volume and the violation of C-centring extinction conditions. In fact, the symmetry changes by losing the centring of the lattice and the centre of symmetry, passing from C2/c to Pc space group. The phase transition is marked by a dramatic change of the isothermal bulk modulus (Ko) from 123(4) GPa in the low-pressure regime to 35(8) GPa in the high-P phase, which shows a very soft and anisotropic behaviour. First principles calculations provide static EoS parameters [e.g. K0 = 128.8(1) GPa and K′0 = 4.0(1) for the C2/c phase], which are in remarkable agreement with experiments. Furthermore, ab initio calculation of the full elastic tensor (with 13 independent components) permits to define the shear modulus (i.e. GVRH = 52.1 GPa) and seismic anisotropy of the C2/c phase, for which no experimental data exist so far. Seismic anisotropy of P-, S1- and S2-waves turns out to be equal to AP=16.0%, AS1=24.7% and AS2=36.3%. AP and AS1 both decrease with pressure, while AS2 increase up to a huge value (ca. 58.8%) at 9 GPa. Thermodynamic properties have been computed in the framework of quasi-harmonic approximation (QHA) by phonon dispersion calculations on 2×2×2 supercells to reach numerical convergence. The calculated heat capacity and standard-state entropy values shows a good agreement with the available calorimetric data. P-V-T relations have been obtained by a first-principles Mie-Gruneisen EoS [1] and permitted to calculate Gibbs free energy in the invetigated P-T range. Due to high computational cost, thermodynamic properties of the Pc phase have been defined by a modified Kieffer’s model approach splitting acoustic and optic contributions and defining the former from the ab initio elastic constant tensor, the latter from vibrational frequencies calculated at Γ-point [2]. A temptative phase transition boundary for the C2/c → Pc reaction is thus predicted by Gibbs free energy minimization. Finally, ab initio thermodynamic properties of grossite have been used to constrain its phase stability field in the CaO-Al2O3-SiO2 (CAS) ternary system at HP-HT along the guidelines defined in a previous study [3]. [1] D. Belmonte Minerals. 2017, 7, 183. [2] D. Belmonte, C. Gatti, G. Ottonello, P. Richet J. Phys. Chem. A. 2016, 120, 8881. [3] G. Ottonello, M. Attene, D. Ameglio, D. Belmonte, M. Vetuschi Zuccolini, M. Natali Chem. Geol. 2013, 346, 81.