Observation of a reentrant phase transition in incommensurate potassium

Abstract

The phase transitions in the host-guest composite structure of potassium have been determined between 19 and 54 GPa by a combination of x-ray powder and single-crystal diffraction. The guest component of the incommensurate composite phase K-III undergoes an "intraphase" transition at 30.0(7) GPa from a C-centered tetragonal structure to an A-centered orthorhombic form. At 39.7(5) GPa, the guest component undergoes a reentrant transition back to the C-centered tetragonal form. This is the first such reentrant transition observed in the composite structures of the elements. The transition in the guest at 30.0(7) GPa is preceded by a loss of the c-glide symmetry of the host component of the structure, and this is not recovered until some 3 GPa after the reentrant transition at 39.7 GPa.

Structural and electronic properties of the alkali metal incommensurate phases

“…Besides estimates for the optimal formation enthalpy, this procedure also gives the optimal host-guest ratio c H /c G as a function of pressure. As the latter has been measured quite accurately, we can compare the calculated axial ratios to the experimental data, in particular with an interest whether the approximate K-III structures can reproduce the turnaround of c H /c G seen in experiment [9]. The calculated optimal ratio c H /c G is plotted against pressure in Fig.…”

“…The approximants we used included 4g/3h (56 atoms per unit cell for K-IIIa and Rb-IV), 3g/2h (38 atoms), 8g/5h (96 atoms), 5g/3h (58 atoms), 7g/4h (78 atoms), 9g/5h (98 atoms) and 2g/1h (20 atoms); unit cells with the K-IIIb guest structure are twice as large. [9]. Host/guest atoms are denoted by yellow/black spheres.…”

“…The guest atom positions for Na-V, K-IIIa and -IIIb, and Rb-IV were created according to the guest unit cells given in the experimental structure determination [6,9,30]. In Na-V, the guest unit cell is monoclinic (β = 94.7…”

“…Indeed, all alkalis exhibit a phase transition to a close-packed face-centered cubic (fcc) phase under modest compression, see Table I. But compressing further results in a marked departure from this simple picture, and all alkali elements exhibit complex crystal structures demonstrated in a succession of experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. These experiments were accompanied with calculations showing intriguing electronic features including electride behavior (the localization of electrons in interstitial lattice sites) and reconstructions to match the Brillouin zone to the Fermi surface [17][18][19][20][21].…”

“…The guest structure of potassium has three distinct phases [9]. At 23-30 GPa, K-IIIa has a primitive tetragonal guest lattice (space group P 4/mmm).…”