Deformation of NaCoF<sub>3</sub> perovskite and post-perovskite up to 30 GPa and 1013 K: implications for plastic deformation and transformation mechanism

Abstract

Abstract. Texture, plastic deformation, and phase transformation mechanisms in perovskite and post-perovskite are of general interest for our understanding of the Earth's mantle. Here, the perovskite analogue NaCoF3 is deformed in a resistive-heated diamond anvil cell (DAC) up to 30 GPa and 1013 K. The in situ state of the sample, including crystal structure, stress, and texture, is monitored using X-ray diffraction. A phase transformation from a perovskite to a post-perovskite structure is observed between 20.1 and 26.1 GPa. Normalized stress drops by a factor of 3 during transformation as a result of transient weakening during the transformation. The perovskite phase initially develops a texture with a maximum at 100 and a strong 010 minimum in the inverse pole figure of the compression direction. Additionally, a secondary weaker 001 maximum is observed later during compression. Texture simulations indicate that the initial deformation of perovskite requires slip along (100) planes with significant contributions of {110} twins. Following the phase transition to post-perovskite, we observe a 010 maximum, which later evolves with compression. The transformation follows orientation relationships previously suggested where the c axis is preserved between phases and hh0 vectors in reciprocal space of post-perovskite are parallel to [010] in perovskite, which indicates a martensitic-like transition mechanism. A comparison between past experiments on bridgmanite and current results indicates that NaCoF3 is a good analogue to understand the development of microstructures within the Earth's mantle.

Deformation and Transformation Textures in the NaMgF3 Neighborite—Post-Perovskite System

“…This indicates that (001) Prv becomes (001) pPrv in the MgGeO 3 system. This orientation relationship is consistent with those documented in this work (on NaMgF 3 ), in the work of Dobson et al [32] on NaNiF 3 , and in the work of Gay et al [33] on NaCoF 3 . In the CaIrO 3 system, a post-transformation texture with (100) planes at high angles to the compression direction has been observed in the pPrv phase [58].…”

“…Alternately, Transformation in our sample seems to follow the mechanism proposed by Tsuchiya et al [28]. It is surprising that Tsuchiya et al [28]'s mechanism is observed here while Dobson et al [32]'s mechanism is reported in NaNiF 3 and NaCoF 3 [32,33]. The major difference between our work and that of Dobson et al [32] and Gay et al [33] is that during their experiments they heated their samples throughout the whole experiment to temperatures of ~800-1000 K, while we only used laser heating to trigger the conversion, once the Prv was already at high pressure.…”

“…It is surprising that Tsuchiya et al [28]'s mechanism is observed here while Dobson et al [32]'s mechanism is reported in NaNiF 3 and NaCoF 3 [32,33]. The major difference between our work and that of Dobson et al [32] and Gay et al [33] is that during their experiments they heated their samples throughout the whole experiment to temperatures of ~800-1000 K, while we only used laser heating to trigger the conversion, once the Prv was already at high pressure. We are unable to determine if differences in heating path may have an impact on the transformation strongly enough to change the transformation mechanisms, and further studies on this subject may be needed to explain these discrepancies.…”

“…In their partially converted NaNiF 3 sample, [001] was preserved between the two structures, and orientation relationships of (010) Prv ~parallel to (110) pPrv were observed [32]. Transformation of Prv into pPrv following that transformation mechanism has also been observed in the NaCoF 3 system [33]. To test if that transformation mechanism could explain our experimental observations, we used the package MTEX to simulate a coherent transformation of our experimental Prv (see Section 2.3 for details) following the crystallographic relationship given by Dobson et al [32].…”

“…MgSiO 3 bridgmanite (Bdm), the dominant phase in much of the lower mantle, undergoes a solid-solid phase transition from a perovskite (Prv) structure to a post-perovskite (pPrv) structure at conditions similar to the top of D" [25][26][27]. For Prv to pPrv phase transitions, computational [28,29] and experimental [30][31][32][33] studies have reported possible coherent transformation mechanisms, i.e., transformation mechanisms where the daughter phase nucleates with a specific crystallographic relationship to the parent phase. The phase transition at the D" discontinuity may hence introduce a transformation texture in the newly nucleated pPrv.…”