First observation of the reversible O3 reversible P2 phase transition Crystal structure of the quenched high-temperature phase Na_(0.74)Ni_(0.58)Sb_(0.42)O_2

摘要

According to thermal expansion data, O3-type phase Na_xNi_((1+x)/3)Sb_((2-x)/3)O_2 (x ≈ 0.8) undergoes at ca. 1270 K a reversible transition to a less dense form. The high-temperature phase quenched to liquid nitrogen belongs to P2 type, space group P6_3/mmc (no. 194), a = 3.0123 A(2), c = 11.2264 A(7) for x ≈ 0.74 at 298 K. The stabilisation of P2 versus O3-type structure at high temperatures seems to be due to alkali distribution over greater number of sites thus increasing entropy and decreasing Na~+-Na~+ repulsion.