Lueshite crystal structure refinement

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Marco E. Ciriotti
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Lueshite crystal structure refinement

Messaggio da Marco E. Ciriotti » mer 07 feb, 2018 9:50

Referenza:
▪ Mitchell R.H., Kennedy B.J., Knight K.S. (2018): The crystal structure of lueshite at 298 K resolved by high-resolution time-of-flight neutron powder diffraction. Physics and Chemistry of Minerals, 45, 77-83.

Abstract:
Refinement of time-of-flight high-resolution neutron powder diffraction data for lueshite (Na,Ca)(Nb,Ta,Ti)O3, the natural analogue of synthetic NaNbO3, demonstrates that lueshite at room temperature (298 K) adopts an orthorhombic structure with a 2ap × 2ap × 4ap superlattice described by space group Pmmn [#59: a = 7.8032(4) Å; b = 7.8193(4) Å; c = 15.6156(9) Å]. This structure is analogous to that of phase S of synthetic NaNbO3 observed at 753–783 K (480–510 °C). In common with synthetic NaNbO3, lueshite exhibits a series of phase transitions with decreasing temperature from a cubic (Pm3 ¯ m) aristotype through tetragonal (P4/mbm) and orthorhombic (Cmcm) structures. However, the further sequence of phase transitions differs in that for lueshite the series terminates with the room temperature S (Pmmn) phase, and the R (Pmmn or Pnma) and P (Pbcm) phases of NaNbO3 are not observed. The appearance of the S phase in lueshite at a lower temperature, relative to that of NaNbO3, is attributable to the effects of solid solution of Ti, Ta and Ca in lueshite.
Marco E. Ciriotti

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