Infrared and Raman studies of M2TiO5 compounds (M = rare earths and Y): Isotopic effect and group theoretical analysis

Paques-Ledent, M.Th.

doi:10.1016/0584-8539(76)80177-8

Cited by 29 publications

(15 citation statements)

References 6 publications

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“…One , 1973, 1974Engel, 1976;Kolsi, 1978), this 3 × superstructure had not been reported previously. This is possibly because these powder X-ray diffraction studies have used copper radiation and consequently the weak superstructure reflections were masked by the strong fluorescence of the Co atoms.…”

Section: Description Of the Crystal Structure Of Fl-nacopo4supporting

confidence: 56%

“…The reversible phase transition in NaCoPO4, occurring at 998 K, is well documented and has been reported several times in the literature (Paques-Ledent, 1972, 1973, 1974Engel, 1976;Kolsi, 1978), but no similar phase transition has been reported for NaMnPO4 or NaFePO4. Although Engel did observe an irreversible phase transition between the olivine form of NaMnPO4 (natrophilite) and the maracite form, this is a consequence of the unusual cation arrangement found in natrophilite (Moore, 1972) and is not related to the o~--,fl phase transition in NaCoPO4.…”

Section: Introductionmentioning

confidence: 87%

“…While many of the compounds studied were polymorphic, NaCoPO4 was unique in that the low-temperature ~-phase belonged to the first group and the high-temperature fl-phase was a member of the second. Based on the results of powder Xray diffraction and IR and Raman spectroscopies, Paques-Ledent (1972, 1973, 1974 had previously assigned a phenacite structure to /~-NaCoPO4. This assignment, however, was questionable as no unitcell data were provided for the high-temperature phase and it required that the Na ions would occur in an unusual tetrahedral coordination.…”

Section: Introductionmentioning

confidence: 99%

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Structural chemistry of NaCoPO₄

Hammond¹,

Barbier²

1996

Acta Crystallogr Sect B

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Sodium cobalt phosphate, NaCoPO4, occurs as two different polymorphs which transform reversibly at 998 K. The crystal structures of both polymorphs have been determined by single crystal X-ray diffraction. The lowtemperature form c~-NaCoPO4 crystallizes in the space group Pnma with cell parameters: a = 8.871 (3), b = 6.780 (3), c = 5.023 (1) A, and Z = 4 [wR(F 2) = 0.0653 for all 945 independent reflections]. The o~-phase contains octahedrally coordinated Co and Na atoms and tetrahedrally coordinated P atoms, and is isostructural with maracite, NaMnPO4. The structure of high-temperature fl-NaCoPO4 is hexagonal with space group P65 and cell parameters: a = 10.166 (1), c = 23.881 (5)/~, and Z = 24[wR(F 2) -0.0867 for 4343 unique reflections]. The /3-phase belongs to the large family of stuffed tridymites, with the P and Co atoms occupying tetrahedral sites and the Na atoms located in the cavities of the tetrahedral framework. The long c axis corresponds to a 3 x superstructure of the basic tridymite framework (c _~ 8/~) and is caused by the displacement of the Na atoms, tetrahedral tilts and strong distortions of the CoO4 tetrahedra. A bond-valence analysis of these phases reveals that the polymorphism in NaCoPO4 is due in part to over-/underbonding of the Na atom in the low-/high-temperature structures, respectively.

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Section: Description Of the Crystal Structure Of Fl-nacopo4supporting

confidence: 56%

Section: Introductionmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

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Structural chemistry of NaCoPO₄

Hammond¹,

Barbier²

1996

Acta Crystallogr Sect B

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“…The short Ti-O4 bond distance [1.641 (6) Å in the monoclinic model and 1.612 (5) Å in the triclinic model] is not surprising, and in agreement with the Raman data (Stassen et al, 1998), especially the stretching vibration at 912 cm À1 which is attributed to a pyramidal TiO 5 group with one short Ti-O distance of 1.65 Å (Blasse & van den Heuvel, 1974;Paques-Ledent, 1976;Gabelica-Robert & Tarte, 1981). In this configuration, the apical oxygen O4 is strongly bonded to Ti (via a double Ti O bond) and virtually not bonded to the other cations of the structure.…”

Section: Tablesupporting

confidence: 81%

Structure and stability of BaTiSi₂O₇

Viani

Palermo

Zanardi

et al. 2015

Acta Crystallogr B Struct Sci Cryst Eng Mater

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Due to their optical, photo-luminescence (PL), and afterglow properties, barium titanosilicates are compounds of great interest for functional materials and light-emitting devices. Among them, BaTiSi2O7 (BTS2) is certainly one of the most intriguing; it displays peculiar properties (e.g. PL orange emission) whose exhaustive explanation has been hampered to date by the lack of a structure model. In this work, BTS2 and the related compound BaTiSi4O11 (BTS4) were synthesized through conventional solid-state reaction methods. BTS2 invariably shows complex twinning patterns. Thus, its structure solution and Rietveld structure refinement were attempted using synchrotron powder diffraction. BTS2 was found to be an intergrowth of monoclinic and triclinic crystals. The monoclinic phase has the space group P21/n and unit cell a = 7.9836 (3), b = 10.0084 (4), c = 7.4795 (3) Å, and β = 100.321 (3)°, whereas the triclinic phase has the space group P\bar 1 and unit cell a = 7.99385 (4), b = 10.01017 (5), c = 7.47514 (3) Å, α = 90.084 (8), β = 100.368 (8) and γ = 89.937 (9)°. These lattices can be seen as a distortion of that of tetragonal synthetic β-BaVSi2O7 with Ti in place of V. The structure models obtained from this study confirm the presence of fivefold coordinated Ti atoms in a distorted pyramidal configuration. The proposed solution supports existing theories for the explanation of the PL orange colour in BTS2.

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“…These differences can be explained by the fact, that, in contradiction to crystalline fresnoite, only about 60% of Ti 4þ is fivefold coordinated in the glass, whereas 20% of Ti 4þ are six fold coordinated and another 20% are fourfold coordinated. Consequently, the reflectance is lower for the artificial spectrum in the range between 550 and 750 cm À1 (stretching vibration of fourfold and sixfold coordinated Ti 4þ [26,27]) and measured and artificial spectrum differ markedly in the region of the bending vibrations between 350 and 500 cm À1 . The change of the coordination number of Ti 4þ from 5 to 6 and 4 and the related role swapping from an intermediate to a network modifier and a network former, respectively, is also responsible for differences between measured and artificial spectrum occurring in the high wave number range of the Reststrahlen region (800-1100 cm À1 ).…”

Section: Modeling Glass Spectra Considering Medium Range Ordermentioning

confidence: 99%