Topotactic, pressure-driven, diffusion-less phase transition of layered CsCoO₂ to a stuffed cristobalite-type configuration

CsCoO₂, featuring a two-dimensional layered architecture of edge- and vertex-linked CoO₄ tetrahedra, is subjected to a temperature-driven reversible second-order phase transformation (α → β) at 100 K, which corresponds to a structural relaxation with concurrent tilting and breathing modes of edge-sharing CoO₄ tetrahedra. In the present investigation, it was found that pressure induces a phase transition, which encompasses a dramatic change in the connectivity of the tetrahedra. At 923 K and 2 GPa, β-CsCoO₂ undergoes a first-order phase transition to a new quenchable high-pressure polymorph, γ-CsCoO₂. It is built up of a three-dimensional cristobalite-type network of vertex-sharing CoO₄ tetrahedra. According to a Rietveld refinement of high-resolution powder diffraction data, the new high-pressure polymorph γ-CsCoO₂ crystallizes in the tetragonal space group I4₁/amd:2 (Z = 4) with the lattice constants a = 5.8711 (1) and c = 8.3214 (2) Å, corresponding to a shrinkage in volume by 5.7% compared with the ambient-temperature and atmospheric pressure β-CsCoO₂ polymorph. The pressure-induced transition (β → γ) is reversible; γ-CsCoO₂ stays metastable under ambient conditions, but transforms back to the β-CsCoO₂ structure upon heating to 573 K. The transformation pathway revealed is remarkable in that it is topotactic, as is demonstrated through a clean displacive transformation track between the two phases that employs the symmetry of their common subgroup Pb2₁a (alternative setting of space group No. 29 that matches the conventional β-phase cell).

Computing details

Program(s) used to refine structure: TOPAS-5 (Bruker AXS 2014).

(I)

Crystal data

CsCoO2	Z = 4
Mr = 223.84	Dx = 5.183 Mg m^−3
Tetragonal, I41/amd:2	Melting point: ???.? K
Hall symbol: -I 4bd 2	Mo Kα1 radiation, λ = 0.70929 Å
a = 5.87111 (13) Å	µ = 18.42 (1) mm^−1
c = 8.32141 (19) Å	T = 303 K
V = 286.84 (2) Å^3

Data collection

??? diffractometer	Data collection mode: transmission
Radiation source: sealed X-ray tube, ???	Scan method: step
??? monochromator	2θmin = 7.000°, 2θmax = 47.056°, 2θstep = 0.011°

Refinement

Rp = 5.412	3641.455 data points
Rwp = 7.405	Profile function: fundamental parameter
Rexp = 6.198	Background function: Chebyshev polynoms
R(F) = ???

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cs1_1	0	0.25	0.375	3.9712*
Co1_1	0	0.75	0.125	0.174421*
O1_1	0	0	0	2.671625*

Geometric parameters (Å, º)

Co1_1—O1_1^i	1.7990 (1)	O1_1—O1_1^iii	2.9356 (1)
Co1_1—O1_1^ii	1.7990 (1)	O1_1—O1_1^vii	2.9356 (1)
Co1_1—O1_1^iii	1.7990 (1)	O1_1—O1_1^viii	2.9388 (1)
Co1_1—O1_1^iv	1.7990 (1)	O1_1—O1_1^ix	2.9388 (1)
O1_1—Co1_1^v	1.7990 (1)	O1_1—O1_1^x	2.9388 (1)
O1_1—Co1_1^vi	1.7990 (1)	O1_1—O1_1^xi	2.9388 (1)
O1_1^ii—Co1_1—O1_1^i	109.5310 (9)	O1_1^ix—O1_1—O1_1^vii	119.9634 (5)
O1_1^iii—Co1_1—O1_1^ii	109.3516 (18)	O1_1^ix—O1_1—O1_1^iii	60.0366 (5)
O1_1^iii—Co1_1—O1_1^i	109.5310 (9)	O1_1^ix—O1_1—Co1_1^vi	35.2345 (4)
O1_1^iv—Co1_1—O1_1^iii	109.5310 (9)	O1_1^ix—O1_1—Co1_1^v	144.7655 (4)
O1_1^iv—Co1_1—O1_1^ii	109.5310 (9)	O1_1^x—O1_1—O1_1^ix	120.0732 (11)
O1_1^iv—Co1_1—O1_1^i	109.3516 (18)	O1_1^x—O1_1—O1_1^viii	59.9268 (11)
Co1_1^vi—O1_1—Co1_1^v	180.000	O1_1^x—O1_1—O1_1^vii	60.0366 (5)
O1_1^iii—O1_1—Co1_1^vi	35.3242 (9)	O1_1^x—O1_1—O1_1^iii	119.9634 (5)
O1_1^iii—O1_1—Co1_1^v	144.6758 (9)	O1_1^x—O1_1—Co1_1^vi	144.7655 (4)
O1_1^vii—O1_1—O1_1^iii	180.000	O1_1^x—O1_1—Co1_1^v	35.2345 (4)
O1_1^vii—O1_1—Co1_1^vi	144.6758 (9)	O1_1^xi—O1_1—O1_1^x	180.000
O1_1^vii—O1_1—Co1_1^v	35.3242 (9)	O1_1^xi—O1_1—O1_1^ix	59.9268 (11)
O1_1^viii—O1_1—O1_1^vii	60.0366 (5)	O1_1^xi—O1_1—O1_1^viii	120.0732 (11)
O1_1^viii—O1_1—O1_1^iii	119.9634 (5)	O1_1^xi—O1_1—O1_1^vii	119.9634 (5)
O1_1^viii—O1_1—Co1_1^vi	144.7655 (4)	O1_1^xi—O1_1—O1_1^iii	60.0366 (5)
O1_1^viii—O1_1—Co1_1^v	35.2345 (4)	O1_1^xi—O1_1—Co1_1^vi	35.2345 (4)
O1_1^ix—O1_1—O1_1^viii	180.000	O1_1^xi—O1_1—Co1_1^v	144.7655 (4)

Symmetry codes: (i) −y−1/4, x+3/4, −z+1/4; (ii) x, y+1, z; (iii) −x, −y+1/2, z; (iv) −y+1/4, x+3/4, z+1/4; (v) x, y−1, z; (vi) −y+3/4, −x+1/4, z−1/4; (vii) −x, −y−1/2, z; (viii) −y−1/4, x−1/4, −z+1/4; (ix) −y+1/4, −x+1/4, −z−1/4; (x) −y+1/4, x−1/4, z+1/4; (xi) −y−1/4, −x+1/4, z−1/4.