Acta Crystallographica Section B

Structural Science

Volume 65, Part 3 (June 2009)

research papers

Acta Cryst. (2009). B65, 326-333 [ doi:10.1107/S010876810901444X ]

Characterization of the pressure-induced second-order phase transition in the mixed-valence vanadate BaV₆O₁₁

K. Friese, Y. Kanke and A. Grzechnik

Abstract: The pressure dependence of the structure of the mixed-valence vanadate BaV₆O₁₁ was studied with single-crystal X-ray diffraction in a diamond-anvil cell. The compressibility data could be fitted with a Murnaghan equation of state with the zero-pressure bulk modulus B₀ = 161 (7) GPa and the unit-cell volume at ambient pressure = 387.1 (3) Å (B' = 4.00). A phase transition involving a symmetry reduction from P6₃/mmc to P6₃mc can be reliably detected in the high-pressure data. The estimated transition pressure lies in the range 1.18 < P_c < 3.09 GPa. The transition leads to a breaking of the regular Kagomé net formed by part of the V ions. While in the ambient pressure structure all V-V distances in the Kagomé net are equal, they split into inter-trimer and intra-trimer distances in the high-pressure phase. In general, these changes are comparable to those observed in the corresponding low-temperature transition. However, the pressure-induced transition takes place at a lower unit-cell volume compared with the temperature-induced transition. Furthermore, overall trends for inter-trimer and intra-trimer V-V distances as a function of the unit-cell volume are clearly different for datapoints obtained by variation of pressure and temperature. The behavior of BaV₆O₁₁ is compared with that of NaV₆O₁₁. While in the latter compound the transition can be explained as a pure volume effect, in BaV₆O₁₁ an additional degree of freedom related to the valence distribution among the symmetrically independent vanadium sites has to be taken into account.

Keywords: pressure dependence; phase transition; mixed-valence vanadate.


	PDF file (2853.9 kbytes) [ doi:10.1107/S010876810901444X/eb5003sup9.pdf ] Plots of observed versus calculated structure factors

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