The structural properties of calcite-type orthoborates ABO(3) (A = Al, Fe, Sc, and In) have been investigated at high pressures up to 32 GPa. They were studied experimentally using synchrotron powder X-ray diffraction and theoretically by means of ab initio total-energy calculations. We found that the calcite-type structure remains stable up to the highest pressure explored in the four studied compounds. Experimental and calculated static geometries (unit-cell parameters and internal coordinates), bulk moduli, and their pressure derivatives are in good agreement. The compressibility along the c axis is roughly three times that along the a axis. Our data clearly indicate that the compressibility of borates is dominated by that of the [AO(6)] octahedral group and depends on the size of the trivalent A cations. An analysis of the relationship between isomorphic borates and carbonates is also presented, which points to the potentiality of considering borates as chemical analogues of the carbonate mineral family.
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