Modulated minerals as potential ferroic materials

Salje, Ekhard K. H.

doi:10.1088/0953-8984/27/30/305901

Cited by 7 publications

(6 citation statements)

References 114 publications

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“…The proximity of incommensurate phases in a phase diagram is hence always an indication that flexo-polarity exists in these systems. A compilation of modulated crystal structures, where flexoelectricity is expected to play a key role for the structural modulation, was recently derived [58].…”

Section: Discussionmentioning

confidence: 99%

Flexoelectricity, incommensurate phases and the Lifshitz point

Pöttker

Salje

2016

J. Phys.: Condens. Matter

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The solutions for the minimizers of the energy density f (q, p) = Aq² + Bq⁴ + p² + gA,B + β(q'p - p'q)+ |q'|² +κ|p'|²] describe the flexoelectric effect with a flexoelectric coupling coefficient β. The order parameters q and p can be visualized as strain and polarisation, respectively. The parameter κ denotes the ratio of intrinsic length scales for q and p. We show that the structural ground-states include 3 phases, namely the paraelastic state q = p = 0, the ferroelastic state where polarization exists inside and near twin boundaries, and the incommensurate (modulated) phases with a very rich array of structural modulations ranging from nearly pure sine waves to kink arrays and ripple states. The phases coincide in the multicritical Lifshitz point. Linear flexoelectricity p~q' is encountered only approximately inside the ferroelastic phase and near the phase boundary between the paraelastic phase and the incommensurate phase. The relationship between the polarisation and the strain gradient is highly non-linear in all other states. The spatial profiles and energy distributions are discussed in detail.

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Section: Discussionmentioning

confidence: 99%

Flexoelectricity, incommensurate phases and the Lifshitz point

Pöttker

Salje

2016

J. Phys.: Condens. Matter

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“…It has a large supercell with a doubling of both the a and c edges ( R m , a = 10.91 and c = 39.78 Å) which most probably results from ordering of vacancies in the crystal structure. Note that, palmierite-type (Pb,Ba) 3 (PO 4 ) 2 phases structurally related to embreyite were studied previously as ferroelastic materials exhibiting the R m to C 2/ c ferroelastic transition around T = 180°C, and were considered as model compounds for ferroelastic distortions (Bismayer and Salje, 1981; Aktas et al ., 2013; Salje, 2015).…”

Section: Resultsmentioning

confidence: 99%

Embreyite: structure determination, chemical formula and comparative crystal chemistry

et al. 2018

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Embreyite from the Berezovskoe, Urals, Russia, was studied by the means of powder X-ray diffraction (XRD), single-crystal XRD, infrared spectroscopy and microprobe analysis. The empirical formula of embreyite obtained on the basis of microprobe analysis is Pb1.29Cu0.07Cr0.52P0.43O4 (without taking into account the presence of H2O). An examination of single-crystal XRD frames of the tested crystals cut from embreyite intergrowths revealed split reflection spots of weak intensities, even after a long exposure time. The crystal structure of embreyite (monoclinic, C2/m, a = 9.802(16), b = 5.603(9), c = 7.649(12) Å, β = 114.85(3)o and V = 381.2(11) Å3) has been solved by direct methods and refined to R1 = 0.050 for 318 unique observed reflections. The powder XRD patterns of the holotype embreyite and the fresh material studied are close in both d values and the intensities match the pattern calculated from the structural single-crystal XRD data. The unit-cell parameters were re-calculated for the holotype sample using a new cell setting and corresponding hkl indices. The crystal structure of embreyite is based on layers formed by corner-sharing mixed chromate-phosphate tetrahedra and PbO6 distorted octahedra. The interlayer space is filled by disordered Pb2+ and Cu2+ cations. Generally, the crystal structure of embreyite can be referred to the structural type of palmierite. {Pb[(Cr,P)O4]2]} layers in embreyite are similar in topology to those in yavapaiite-type compounds. The general formula of embreyite can be represented as (Pbx$M_y^{2 +} $□1–x–y)2{Pb[(Cr,P)O4]2}(H2O)n, where M2+ = Cu and Zn and 0.5 ≤ x + y ≤ 1, or, in the simplified form: (Pb,Cu,□)2{Pb[(Cr,P)O4]2}(H2O)n. The simplified formula of embreyite is similar in stoichiometry to vauquelinite and may explain the existence of the solid-solution series. The determination of the crystal structure of embreyite may also help to resolve the crystal chemical nature of cassedanneite. The XRD pattern of cassedanneite contains a distinct reflection with d = 13.9 Å, forbidden for the embreyite unit cell. This feature may indicate the doubling of the c unit-cell parameter of cassedanneite in comparison with embreyite. We assume that cassedanneite has structural similarity to embreyite with, presumably, a disordered distribution of Cr and V.

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“…In addition to point defects and their agglomeration, there are still various kinds of microstructures, including twins, domains, domain walls, phase boundaries and compositional modulations. Moreover, the variation of chemical composition may generate unexpected ferroelectricity in intrinsically uniform compounds or by carefully doping distorted parts of the structure [246]. Polymorphisms and specific microstructures, if any, are listed in Table 2 for the purpose of searching potential elements that could impact the properties of ML compounds beyond their general crystal structure.…”

Section: Mechanoluminescent Compoundsmentioning

confidence: 99%

“…The structure or chemical gradients in modulated structures may generate polarization through flexoelectricity effects [43], as well as by attracting impurities onto domain walls or other structural gradients [246]. The structure fluctuations can impose interesting effects on optical properties which depend heavily on the local atomic environment.…”

Section: Mechanoluminescent Compoundsmentioning

confidence: 99%

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

2018

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Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research.

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Modulated minerals as potential ferroic materials

Cited by 7 publications

References 114 publications

Flexoelectricity, incommensurate phases and the Lifshitz point

Flexoelectricity, incommensurate phases and the Lifshitz point

Embreyite: structure determination, chemical formula and comparative crystal chemistry

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

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