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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).

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CCDC reference: 1907016

Computing details top

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

(I) top
Crystal data top
CsCoO2Z = 4
Mr = 223.84Dx = 5.183 Mg m3
Tetragonal, I41/amd:2Melting point: ???.? K
Hall symbol: -I 4bd 2Mo Kα1 radiation, λ = 0.70929 Å
a = 5.87111 (13) ŵ = 18.42 (1) mm1
c = 8.32141 (19) ÅT = 303 K
V = 286.84 (2) Å3
Data collection top
Data collection mode: transmission
Radiation source: sealed X-ray tube, ???Scan method: step
??? monochromator2θmin = 7.000°, 2θmax = 47.056°, 2θstep = 0.011°
Refinement top
Rp = 5.4123641.455 data points
Rwp = 7.405Profile function: fundamental parameter
Rexp = 6.198Background function: Chebyshev polynoms
R(F) = ???
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
Geometric parameters (Å, º) top
Co1_1—O1_1i1.7990 (1)O1_1—O1_1iii2.9356 (1)
Co1_1—O1_1ii1.7990 (1)O1_1—O1_1vii2.9356 (1)
Co1_1—O1_1iii1.7990 (1)O1_1—O1_1viii2.9388 (1)
Co1_1—O1_1iv1.7990 (1)O1_1—O1_1ix2.9388 (1)
O1_1—Co1_1v1.7990 (1)O1_1—O1_1x2.9388 (1)
O1_1—Co1_1vi1.7990 (1)O1_1—O1_1xi2.9388 (1)
O1_1ii—Co1_1—O1_1i109.5310 (9)O1_1ix—O1_1—O1_1vii119.9634 (5)
O1_1iii—Co1_1—O1_1ii109.3516 (18)O1_1ix—O1_1—O1_1iii60.0366 (5)
O1_1iii—Co1_1—O1_1i109.5310 (9)O1_1ix—O1_1—Co1_1vi35.2345 (4)
O1_1iv—Co1_1—O1_1iii109.5310 (9)O1_1ix—O1_1—Co1_1v144.7655 (4)
O1_1iv—Co1_1—O1_1ii109.5310 (9)O1_1x—O1_1—O1_1ix120.0732 (11)
O1_1iv—Co1_1—O1_1i109.3516 (18)O1_1x—O1_1—O1_1viii59.9268 (11)
Co1_1vi—O1_1—Co1_1v180.000O1_1x—O1_1—O1_1vii60.0366 (5)
O1_1iii—O1_1—Co1_1vi35.3242 (9)O1_1x—O1_1—O1_1iii119.9634 (5)
O1_1iii—O1_1—Co1_1v144.6758 (9)O1_1x—O1_1—Co1_1vi144.7655 (4)
O1_1vii—O1_1—O1_1iii180.000O1_1x—O1_1—Co1_1v35.2345 (4)
O1_1vii—O1_1—Co1_1vi144.6758 (9)O1_1xi—O1_1—O1_1x180.000
O1_1vii—O1_1—Co1_1v35.3242 (9)O1_1xi—O1_1—O1_1ix59.9268 (11)
O1_1viii—O1_1—O1_1vii60.0366 (5)O1_1xi—O1_1—O1_1viii120.0732 (11)
O1_1viii—O1_1—O1_1iii119.9634 (5)O1_1xi—O1_1—O1_1vii119.9634 (5)
O1_1viii—O1_1—Co1_1vi144.7655 (4)O1_1xi—O1_1—O1_1iii60.0366 (5)
O1_1viii—O1_1—Co1_1v35.2345 (4)O1_1xi—O1_1—Co1_1vi35.2345 (4)
O1_1ix—O1_1—O1_1viii180.000O1_1xi—O1_1—Co1_1v144.7655 (4)
Symmetry codes: (i) y1/4, 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, y1, z; (vi) y+3/4, x+1/4, z1/4; (vii) x, y1/2, z; (viii) y1/4, x1/4, z+1/4; (ix) y+1/4, x+1/4, z1/4; (x) y+1/4, x1/4, z+1/4; (xi) y1/4, x+1/4, z1/4.

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