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Gillespite, a barium-iron sheet silicate (BaFeSi₄O₁₀), was one of the first examples for which single-crystal X-ray diffraction in a diamond-anvil cell was used to investigate a pressure-induced structural phase transition. The tetragonal-to-orthorhombic transition, which occurs at relatively moderate pressures between 1.2 and 1.8 GPa, is known to be reversible and first-order in character. The relative size of the eight-fold coordinated Ba cation was thought to control the transition as the coordination number of Ba changes from 8 to 10. Moreover, the planar four-fold coordination of the Fe²⁺ transforms to a uniaxially elongated octahedron, which explains the characteristic colour change from red to blue observed for this pressure-induced transition. Even though both the low and the high-pressure polymorphs are structurally well characterized, the mechanism of the transition is poorly understood; symmetry analysis suggests that the transition could be a two-step process involving an intermediate phase.

In order to understand the mechanisms driving the transition on a microscopic scale, the gillespite-type analogue BaCrSi₄O₁₀ was chosen for high-pressure investigations, because it was found to approach gillespite closest from the point of view of atomic size (see also Annual Report 1995). High-pressure investigations by means of single-crystal X-ray diffraction and optical absorption spectroscopy were performed in diamond-anvil cells to quasihydrostatic pressures of ~10 and ~13 GPa. The optical absorption spectra to a maximum of 12.5 GPa are characterized by spin-allowed d-d transition bands as typical for square-planar coordinated Cr²⁺ cations. The changes within the spectra are continuous with band shifts of about 63 cm^-1/GPa. The crystal field stabilisation energy rises from 13600 to 13850 cm^-1 at maximum pressure whereas the t_2g splitting (~3530 cm^-1) remains virtually unchanged. For pressures in excess of 12.5 GPa we observe a color change from pink to blue thus indicating a transition from a square-planar to a distorted six-fold co-ordination. The observed phase change is irreversible and appears to be due to amorphization.

P-V data collected by single-crystal diffraction are suggestive of a change in the pressure-dependence of the unit-cell parameters between ~2 and ~3 GPa, although the lattice symmetry remains tetragonal at all pressures up to 10 GPa. The changes, which are indicative of a tetragonal-to-tetragonal phase transition, appear to be related to a P4/ncc-P42₁2 transition as indicated by preliminary refinements of the crystal structure at high pressures. The high-pressure polymorph, which appears to be an intermediate phase between the two gillespite polymorphs, retains the square-planar configuration around the Cr²⁺ atoms which explains why the absorption spectra are basically unaffected by the transition. The high-pressure investigations reveal that the structure of the Cr analogue of gillespite, although being so similar to gillespite itself, does not undergo the I-II transition reported for gillespite, at least not to a maximum pressure of 12.5 GPa. It appears that the different electronic d-configurations control the phase stability rather than the relative size of the Ba cations as previously suggested. Such a model appears to be very likely as the square-planar geometry is more favoured by the Cr²⁺ d⁴configuration than by the d⁶ configuration of Fe²⁺ in gillespite. The example demonstrates the major influence of the electronic structure of transition-metal cations in a silicate structure, which reveals a shift of the transition pressure in this example by at least 10 GPa, and has to be considered to be important even for other silicate systems at high pressures.