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  • Springer  (5)
  • 1985-1989  (5)
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  • 1
    Electronic Resource
    Electronic Resource
    A new phase transition in MnTiO3: LiNbO3-perovskite structure (1989)
    Springer
    Physics and chemistry of minerals 16 (1989), S. 621-629 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The stable polymorph of MnTiO3 at room temperature and pressure has the ilmenite structure. At high temperatures and pressures, MnTiO3 ilmenite transforms to a LiNbO3 structure through a cation reordering process (Ko and Prewitt 1988). Single crystals of both phases have been studied with X-ray diffraction to 5.0 GPa. We have obtained the first experimental verification of the close relationship between the LiNbO3 and perovskite structures, first postulated by Megaw (1968). MnTiO3 with the LiNbO3 structure (MnTiO3 II) transforms directly to an orthorhombic perovskite structure (MnTiO3 III) between 2.0 and 3.0 GPa. The transition involves a change of volume of -5%, is reversible and has pronounced hysteresis. Only pressure is required to drive the transition because it involves no breaking of bonds; it simply involves rotation of the [TiO6] octahedra about their triad axes accompanied by displacement of the Mn cations to the distorted twelve-coordinated sites formed by the rotations. An unusual aspect of this transition is that twinned MnTiO3 II crystals transform to untwinned MnTiO3 III crystals with increasing pressure. The twin plane of MnTiO3 II, $$\left( {10\bar 1\bar 2} \right)$$ , corresponds to the (001) mirror plane of the orthorhombic perovskite structure. MnTiO3 III examined at 4.5 GPa is very distorted from the ideal cubic perovskite structure. The O(2)-O(2)-O(2) angle describing the tilting in the ab plane is 133.3(7)°, in contrast to 180° for a cubic perovskite and the O(2)-O(2)-O(2) angle describing the tilting in the ac plane is 109.3(4)°, as opposed to 90° in a cubic perovskite.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00223309
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  • 2
    Electronic Resource
    Electronic Resource
    High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure (1988)
    Springer
    Physics and chemistry of minerals 15 (1988), S. 355-362 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract Crystals of a high-pressure phase of MnTiO3 have been synthesized at pressures of 60 kbar using the SAM-85 cubic-anvil high pressure apparatus. Although all crystals examined were twinned on (10 $$\bar 1$$ $$\bar 2$$ ), a set of diffraction intensities that are essentially unaffected by the twinning were obtained. Three possible structure models were considered: (1) the corundum (completely disordered Mn and Ti), (2) the partially-disordered ilmenite, and (3) the LiNbO3 structures. The R factors of the corundum and the disordered ilmenite models were much larger than that of LiNbO3. Using structure factors unaffected by twinning, the final LiNbO3-type refinement gave R w=0.037 and R=0.034. The averaged bond lengths for Mn-O and Ti-O were consistent with ones calculated using Shannon and Prewitt's (1969) radii. The study concludes that MnTiO3 II actually has an ordered LiNbO3-type structure rather than the disordered one as reported previously. From the analysis of the two MnTiO3 structures, the transition can be related to a cation reordering process, in which half of the cations participate, accompanied by the rotation of oxygens to accommodate the cations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00311040
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  • 3
    Electronic Resource
    Electronic Resource
    Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite (1989)
    Springer
    Physics and chemistry of minerals 16 (1989), S. 727-733 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00209693
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  • 4
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    Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite (1989)
    Springer
    Details
    Publication Date: 1989-11-01
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Springer
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  • 5
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    High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure (1988)
    Springer
    Details
    Publication Date: 1988-03-01
    Print ISSN: 0342-1791
    Electronic ISSN: 1432-2021
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Springer
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