The crystal-structure and the high-pressure behavior of a natural cubic melanophlogite (Sp. Gr. Pm3n at room-conditions), a type-I SiO2 clathrate containing only methane as guest gas, was investigated by X-ray single-crystal diffraction at room-conditions and in situ high-pressure synchrotron powder diffraction with a diamond anvil cell and methanol:ethanol:water = 16:3:1 (MEW) mixture and silicon-oil (Si-oil) as pressure transmitting media up to about 6 GPa. A cubic-to-tetragonal phase-transition was observed at P ≥ 1.14 GPa, in both MEW and Si-oil runs. The tetragonal polymorph is stable up to the highest pressure in the run with MEW, but it switches back to cubic symmetry at P > 3.12 GPa in Si-oil. The different elastic behavior is ascribable to the non-hydrostatic conditions in Si-oil medium at P > 1 GPa. The elastic behavior of melanophlogite was described fitting the P-V data with a Birch-Murnaghan Equation-of-State, showing similar bulk modulus values for the low-pressure cubic and high-pressure tetragonal phases [i.e. K0,L-cub = 23.4(2.4), K0,tetr = 22.2(9)]. The spontaneous strain for the cubic-to-tetragonal transition is almost completely governed by a compression along the c axis, with negligible contribution along a. An inverse behavior is found with respect to the tetragonal-to-cubic high-temperature phase-transition previously described for Mt. Hamilton melanophlogite. A Landau fit of the spontaneous strain (εss), related to the order parameter (Q) of the transition as εss ∝ Q, showed a first-order character of the P-induced phase-transition and a Pc = 1.2(3) GPa. Symmetry breaking spontaneous strain prevails respect to the volume strain, which is however present and significant.
Bibliographical noteFunding Information:
Experiments at PAL were supported in part by Ministry of Science and Technology (MOST) of the Korean Government and Pohang University of Science and Technology (POSTECH). Y. Lee thanks the support by the Korea Science and Engineering Foundation through the Nuclear R&D Program (Grant No. M2AM06-2008-03931 ). Careful revision by two anonymous reviewers is acknowledged. Thanks are also due to Danilo Bersani for providing the studied sample.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials