The high-pressure monazite-to-scheelite transformation in CaSeO₄

Abstract

The high-pressure monaziteÀscheelite structure transition has been observed at P >4.57 GPa in CaSeO 4 by synchrotron X-ray powder diffraction. It is a first-order transition with a 4.5% volume change and is severely hindered kinetically. Scheelite-type CaSeO 4 remains to a maximum experimental pressure of 42.2 GPa and no (002) reflection, specifically indicative of a subgroup transition to a fergusonite-type structure, is observed. Scheelite-type CaSeO 4 remains at ambient conditions, where the tetragonal unit cell has parameters of a = 5.04801(11) c = 11.6644(5) Å and V = 297.21(3) Å 3 with D calc = 4.090 g cm À3 . The diffraction pattern of the recovered material was refined in space group I4 1 /a to R p = 0.98%, wR p = 1.91%, GoF = 0.59, R Fobs = 5.04%, wR Fobs = 4.27%. The oxygen is located on the general 16f site at (0.2578(8) 0.3699(14) 0.5755(4)) and shares four identical bonds with Se (4a: Ý Ý Ý) at 1.644(5) Å . The Ca (4b: 0, 0, Ý) is eight-coordinated via O at 462.440(6) Å and 4 6 2.504(5) Å . This is further evidence of the dissimilarity of sulfate and selenate at high pressure and temperature conditions and the closer resemblance of the selenates to the orthophosphates, arsenates and vanadates, where this type of transition sequence has been described.

High-Pressure Single-Crystal X-ray Diffraction of Lead Chromate: Structural Determination and Reinterpretation of Electronic and Vibrational Properties

“…Before closing this section, we would like to mention that the possible mechanisms of the monazite-scheelite transition have been previously discussed for CaSeO 4 . Here, we would like to add that we noted that the space group of scheelite can be transformed via group–subgroup relationships into space group P 2 1 / n .…”

“…Before closing this section, we would like to mention that the possible mechanisms of the monazite-scheelite transition have been previously discussed for CaSeO 4 . 51 Here, we would like to add that we noted that the space group of scheelite can be transformed via group−subgroup relationships into space ). In addition, in Figure 2, it can be seen that there are significant differences in polyhedral contacts and packing of both polymorphs, which does not make identification of the transition pathway obvious.…”

High‐pressure phase transitions and properties of MTO4 compounds with the monazite‐type structure

“…Typical values of the bulk modulus of monazites are summarized in Table . In this table the results obtained from phosphates , vanadates , chromates , selenates , and sulphates are shown. Notice that chromates and selenates appear to be the most compressible members of the monazite family.…”

Special issue in honour of Mark D. Welch

“…Crichton et al (2012) use synchrotron X-ray powder diffraction to investigate the elastic behaviour and structural stability of CaSeO 4 up to 42 GPa. Dobson et al (2012) provide a description of the relative strengths of perovskite and post-perovskite in the system NaCoF 3 , based on deformation experiments in pure-shear geometry with X-radiographic monitoring of sample strain.…”