Cubic calcite and its structural phase transitions

IF 1.2 4区 地球科学 Q4 MATERIALS SCIENCE, MULTIDISCIPLINARY
Yang Yang, Yixin Lin, Xiangdong Ding, Christopher J. Howard, Ekhard K. H. Salje
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Abstract

Calcite, CaCO3, has been reported to exist in as many as seven different structural forms. The structure at room temperature and pressure (space group R\(\overline{3 }\)c, ‘Phase I’) was established by Bragg many years ago. A phase transition to a higher temperature phase (space group R\(\overline{3 }\)m, ‘Phase V’) was noted to occur at around 1240 K—this may proceed via an intermediate phase (space group again R\(\overline{3 }\)c, referred to as ‘Phase IV’). These phases differ primarily in the disposition of the CO3 groups. Additional phases are found at higher pressures. We report a para-phase (parent phase, virtual prototype, aristotype) which assists in understanding the different phases, the phase transitions, and especially the domain structures and twin wall boundaries associated with these transitions. Molecular dynamics methods were used to study the temperature evolution of an isothermal-isobaric (NPT) ensemble of some 384,000 atoms. These computations reproduced the features of the known structures in R\(\overline{3 }\)c and R\(\overline{3 }\)m and then, at higher temperature, revealed a structure of the sodium chloride type (space group Fm\(\overline{3 }\)m) in which the entities were the Ca2+ cation and the CO32− anion, this latter with effectively spherical symmetry. On this basis we have upon cooling a necessarily first order ferroelastic transition from cubic Fm\(\overline{3 }\)m to rhombohedral R\(\overline{3 }\)m, computed to occur at a simulated temperature of 1900 K, and a possibly continuous transition from the R\(\overline{3 }\)m to rhombohedral (on a doubled cell) R\(\overline{3 }\)c computed to occur at about 1525 K. The computations also allowed us to follow the domain structure and twin walls as a function of temperature, during both heating and cooling. The structure just below the R\(\overline{3 }\)m to R\(\overline{3 }\)c transition shows strong disorder in the orientation of the CO3 groups, and this may be what is sometimes referred to as Phase IV. The domain structure just below the cubic to rhombohedral transition shows twinning of typical ferroelastic character. The doubling of the cell below the R\(\overline{3 }\)m to rhombohedral (on a doubled cell) R\(\overline{3 }\)c leads to a more complicated twin pattern. Indeed, the different structures can be identified from patterns of twinning. Differences between domain structures obtained on heating and cooling indicate extensive thermal metastabilities.

立方方解石及其结构相变
据报道,方解石CaCO3以七种不同的结构形式存在。室温和常压下的结构(空间组R \(\overline{3 }\) c, ' Phase I ')是布拉格多年前建立的。注意到在1240 k左右发生向更高温度相(空间组R \(\overline{3 }\) m,“第五相”)的相变,这可能通过中间相(空间组R \(\overline{3 }\) c,称为“第四相”)进行。这些阶段的不同主要在于CO3基团的分布。在较高的压力下发现了额外的相。我们报告了一个准相(母相,虚拟原型,aristotype),它有助于理解不同的相,相变,特别是与这些相变相关的畴结构和双壁边界。采用分子动力学方法研究了384,000个原子组成的等温-等压系综的温度演化。这些计算再现了R \(\overline{3 }\) c和R \(\overline{3 }\) m中已知结构的特征,然后,在更高的温度下,揭示了氯化钠类型(空间群Fm \(\overline{3 }\) m)的结构,其中实体是Ca2+阳离子和CO32−阴离子,后者具有有效的球对称。在此基础上,在冷却时,我们有一个必然的一阶铁弹性转变,从立方Fm \(\overline{3 }\) m到菱面体R \(\overline{3 }\) m,计算发生在1900 K的模拟温度下,和一个可能的连续转变,从R \(\overline{3 }\) m到菱面体(在双胞上)R \(\overline{3 }\) c,计算发生在大约1525 K。计算还使我们能够在加热和冷却过程中遵循区域结构和双壁作为温度的函数。R \(\overline{3 }\) m到R \(\overline{3 }\) c过渡段下方的结构显示出CO3基团取向的强烈无序性,这可能是有时被称为第四相的结构。立方到菱形过渡段下方的结构显示出典型的铁弹性孪晶特征。R \(\overline{3 }\) m以下的细胞加倍到菱形体(在加倍细胞上)R \(\overline{3 }\) c导致更复杂的双胞胎模式。事实上,不同的结构可以从双胞胎的模式中识别出来。加热和冷却得到的畴结构之间的差异表明广泛的热亚稳态。
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来源期刊
Physics and Chemistry of Minerals
Physics and Chemistry of Minerals 地学-材料科学:综合
CiteScore
2.90
自引率
14.30%
发文量
43
审稿时长
3 months
期刊介绍: Physics and Chemistry of Minerals is an international journal devoted to publishing articles and short communications of physical or chemical studies on minerals or solids related to minerals. The aim of the journal is to support competent interdisciplinary work in mineralogy and physics or chemistry. Particular emphasis is placed on applications of modern techniques or new theories and models to interpret atomic structures and physical or chemical properties of minerals. Some subjects of interest are: -Relationships between atomic structure and crystalline state (structures of various states, crystal energies, crystal growth, thermodynamic studies, phase transformations, solid solution, exsolution phenomena, etc.) -General solid state spectroscopy (ultraviolet, visible, infrared, Raman, ESCA, luminescence, X-ray, electron paramagnetic resonance, nuclear magnetic resonance, gamma ray resonance, etc.) -Experimental and theoretical analysis of chemical bonding in minerals (application of crystal field, molecular orbital, band theories, etc.) -Physical properties (magnetic, mechanical, electric, optical, thermodynamic, etc.) -Relations between thermal expansion, compressibility, elastic constants, and fundamental properties of atomic structure, particularly as applied to geophysical problems -Electron microscopy in support of physical and chemical studies -Computational methods in the study of the structure and properties of minerals -Mineral surfaces (experimental methods, structure and properties)
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