Alla Arakcheeva - Academia.edu (original) (raw)
Papers by Alla Arakcheeva
Zeitschrift für Kristallographie, 1999
ABSTRACT The structure of Cs3Bi2I9 has been refined to R = 0.0165 on the basis of X-ray diffracti... more ABSTRACT The structure of Cs3Bi2I9 has been refined to R = 0.0165 on the basis of X-ray diffraction measurements at T = 290 K (STOE IPDS diffractometer; MoKalpha-radiation; 296 nonequivalent reflection with I > 2 sigma(I)). The absorption correction was accounted for by the Gaussian method. The parameters of the hexagonal unit cell are: a = 8.409(1) Angstrom and c = 21.243(5) Angstrom; space group P6(3)/mmc. The structure consists of identical perovskite-like layer fragments described by the general formula A(3)B(2)X(9) (A = Cs, B = Bi, X = I) With Ordered vacancies in the B-sublattice. These fragments alternate along the c-axis and are related by mirror planes which contains the shared faces of the BiI6 octahedra. The analysis of the intensities in the temperature range 288 K to 190 K indicates a reversible phase transition into a monoclinic phase at T = 218 K which is accompanied by twinning. No satellite or superstructure reflections were observed with the current resolution of the diffractometer. The Xray results are compared wih the studies of the temperature dependence of the linear expansion coefficients, optical characteristics, and the nuclear quadrupole spectra of Cs3Bi2I9. [References: 6]
Acta Crystallographica Section A Foundations of Crystallography, 1993
Crystallography of aperiodic systems is a relatively new field in structural sciences and probabl... more Crystallography of aperiodic systems is a relatively new field in structural sciences and probably one of the most active branch in crystallography both from the theoretical and experimental points of view. Aperiodic crystals are perfectly ordered materials with long range order but lacking the three dimensional periodicity as observed in classical crystals. However, in order to recover periodicity, these structures
The incommensurately modulated structure of K 3 In(PO 4 ) 2 has been solved and re®ned. The origi... more The incommensurately modulated structure of K 3 In(PO 4 ) 2 has been solved and re®ned. The origin of the modulation relates to the ordering of K cations within the hexagonal close packing of the PO 4 anions. The driving forces for the modulation of the other cations are InÐP and KÐP interactions. The modulation of O atoms of rigid PO 4 units follows the cations in order to stabilize the InO 6 octahedron. It is shown that the previously published three-dimensional structure re®ned from powder diffraction data obtained at room temperature is an average structure. Therefore the incommensurately modulated phase of K 3 In(PO 4 ) 2 is the only one that has been unequivocally identi®ed at room temperature. The origin of the modulation is discussed in comparison with the structures of Na 3
Acta Crystallographica Section B-structural Science, 2001
Journal of Solid State Chemistry
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
ChemInform, 2010
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
Acta crystallographica. Section B, Structural science, 2003
The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) a... more The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) at 120 K (low-temperature, LT, structure) and at 293 K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same (or an integer multiple) lattice constants but different space groups. The space groups of both host (H) and guest (G) components cause systematic absences, which result from their intersection. The highest symmetry of a sigma-structure can be described as [H: P4(2)/mnm; G: P4/mbm (c(G) = 0.5c(H)); composite: P4(2)/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In beta-Ta, two components modify their symmetry during the thermal process 293 K (RT-I)=> 120 K (LT)=> 293 K (RT-II): [H: P(-)42(1)m; G: P(-)42(1)m; composite: P(-)42(1)m]=> [H: P(-)4, G: P4/mbm (c(G) = 0.5c(H)), composite: P(-)4]=> [H: P(...
Acta crystallographica. Section B, Structural science, 2002
Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tanta... more Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) A, space group P42(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor approximately 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. Beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.
Chemical Science, 2012
Scheelite related compounds with general formula M n (XO 4 ) m are the subject of hefty interest ... more Scheelite related compounds with general formula M n (XO 4 ) m are the subject of hefty interest owing to their optical properties, stability and relatively simple preparation. Eu 3+ -containing scheelites are considered as redemitting phosphors and the main factors affecting their luminescence are thought to be chemical composition and particle size while the influence of their structure is generally ignored. Here we report eight compounds from the Na x Eu (2Àx)/3 MoO 4 series prepared by conventional solid-state reaction and present a detailed analysis of their crystal structures. Six of them have modulated structures, a common feature of SRCs, in which dopant Eu 3+ ions are orderly distributed. Moreover, different amounts of Eu 3+ dimers are detected in the modulated structures, characterized by weak satellite reflections appearing in the lower angle part of the XRD patterns. These reflections are indexed and incorporated into Rietveld's refinement using superspace (3 + 1)-dimension symmetry. The remarkable feature of the compounds is that the characteristic luminescence parameters, overall (Q Eu L ) and intrinsic (Q Eu Eu ) quantum yields, Eu( 5 D 0 ) lifetimes, and sensitization efficiencies (h sens ), correlate with the number of Eu 3+ aggregates, but not directly with the composition x of the materials. This provides an efficient tool for understanding and controlling the luminescence properties of scheelite related compounds.
ChemInform, 2010
Praseodymium molybdate Pr 2 (MoO 4 ) 3 was synthesized using the standard ceramic route. The crys... more Praseodymium molybdate Pr 2 (MoO 4 ) 3 was synthesized using the standard ceramic route. The crystal structure of the material has been successfully solved in superspace group I2/b(Rβ0)00 with lattice constants a = 5.30284(4), b = 5.32699(3), c = 11.7935(1) Å , γ = 90.163(1)°, and the modulation vector q = 2/3a* þ 0.88810(2)b*. The deviation of the q vector from a rational value allows a description of the structure in terms of nanosize domains with the La 2 (MoO 4 ) 3 -like structure separated by stacking faults. Under 450 nm excitation, ( 3 P 0 level) Pr 2 (MoO 4 ) 3 exhibits the characteristic red emission, with the most intense band at 649 nm corresponding to a 3 P 0 f 3 F 2 transition. Magnetic susceptibility measurements reveal Curie-Weiss paramagnetism with predominating antiferromagnetic interactions between Pr 3þ -magnetic moments and no evidence of magnetic transitions down to T = 5 K.
Zeitschrift für Kristallographie, 2004
The structure of aperiodic crystals which includes incommensurate, quasi-and composite crystals i... more The structure of aperiodic crystals which includes incommensurate, quasi-and composite crystals is usually described in spaces of higher dimension, the so called superspace. The main advantage of the superspace formalism is that an aperiodic structure in three dimensions recovers its full periodicity in higher dimensions. The symmetry properties of aperiodic crystals are obviously more convenient to describe in superspace too. The origin of the incommensurate nature of structures can often be found in competing interatomic interactions. From molecular dynamics simulation of a simple three dimensional model with close-packed layers and a single degree of freedom for each particle, it is possible to find the existence conditions of commensurate and incommensurate phases. Incommensurate phases can already be predicted on the basis of nearest and next nearest neighbour particle interactions only. We illustrate this principle of interactions with two examples of structures, Na 2 CO 3 and K 3 In(PO 4 ) 2 . These examples shows clearly the importance of non-oxygen interactions i.e.
![Research paper thumbnail of New insight into the pectolite – serandite series: a single crystal diffraction study of Na(Ca1.73Mn0.27)[HSi3O9] at 293 and 100 K](https://attachments.academia-assets.com/41884581/thumbnails/1.jpg)
](Zeitschrift für Kristallographie, 2000
… Section B: Structural …, 2003
Journal of Solid State Chemistry, 2010
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
Inorganic Chemistry, 2010
Inorganic Chemistry, 2006
The structure of room-temperature ZrP 2 O 7 is shown to be orthorhombic by a combination of high-... more The structure of room-temperature ZrP 2 O 7 is shown to be orthorhombic by a combination of high-resolution synchrotron powder diffraction and single-crystal synchrotron diffraction data. Small nontwinned single crystals were obtained by synthesizing the compound using solvothermal methods at temperatures below the cubic to orthorhombic phase transition. The average P−O−P angle is 146°. DFT calculations (B3LYP/AUG-cc-pVDZ) on the isolated P 2 O 7 4anion yield a P−O−P angle of 153.42°and indicate that the barrier to inversion is of the order 3.6 kJ mol -1 .
Ferroelectrics, 2004
Diaqua (15-crown-5) copper (II) dinitrate is an organic salt presenting an one-dimensional commen... more Diaqua (15-crown-5) copper (II) dinitrate is an organic salt presenting an one-dimensional commensurate modulation. The diffraction pattern is characterized by strong Bragg reflections and weaker superstructure reflections (satellites). The structure is described in the superspace formalism as a modulated structure in (3+ 1) dimensions with occupational and displacive modulation functions.
Zeitschrift für Kristallographie, 1999
ABSTRACT The structure of Cs3Bi2I9 has been refined to R = 0.0165 on the basis of X-ray diffracti... more ABSTRACT The structure of Cs3Bi2I9 has been refined to R = 0.0165 on the basis of X-ray diffraction measurements at T = 290 K (STOE IPDS diffractometer; MoKalpha-radiation; 296 nonequivalent reflection with I > 2 sigma(I)). The absorption correction was accounted for by the Gaussian method. The parameters of the hexagonal unit cell are: a = 8.409(1) Angstrom and c = 21.243(5) Angstrom; space group P6(3)/mmc. The structure consists of identical perovskite-like layer fragments described by the general formula A(3)B(2)X(9) (A = Cs, B = Bi, X = I) With Ordered vacancies in the B-sublattice. These fragments alternate along the c-axis and are related by mirror planes which contains the shared faces of the BiI6 octahedra. The analysis of the intensities in the temperature range 288 K to 190 K indicates a reversible phase transition into a monoclinic phase at T = 218 K which is accompanied by twinning. No satellite or superstructure reflections were observed with the current resolution of the diffractometer. The Xray results are compared wih the studies of the temperature dependence of the linear expansion coefficients, optical characteristics, and the nuclear quadrupole spectra of Cs3Bi2I9. [References: 6]
Acta Crystallographica Section A Foundations of Crystallography, 1993
Crystallography of aperiodic systems is a relatively new field in structural sciences and probabl... more Crystallography of aperiodic systems is a relatively new field in structural sciences and probably one of the most active branch in crystallography both from the theoretical and experimental points of view. Aperiodic crystals are perfectly ordered materials with long range order but lacking the three dimensional periodicity as observed in classical crystals. However, in order to recover periodicity, these structures
The incommensurately modulated structure of K 3 In(PO 4 ) 2 has been solved and re®ned. The origi... more The incommensurately modulated structure of K 3 In(PO 4 ) 2 has been solved and re®ned. The origin of the modulation relates to the ordering of K cations within the hexagonal close packing of the PO 4 anions. The driving forces for the modulation of the other cations are InÐP and KÐP interactions. The modulation of O atoms of rigid PO 4 units follows the cations in order to stabilize the InO 6 octahedron. It is shown that the previously published three-dimensional structure re®ned from powder diffraction data obtained at room temperature is an average structure. Therefore the incommensurately modulated phase of K 3 In(PO 4 ) 2 is the only one that has been unequivocally identi®ed at room temperature. The origin of the modulation is discussed in comparison with the structures of Na 3
Acta Crystallographica Section B-structural Science, 2001
Journal of Solid State Chemistry
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
ChemInform, 2010
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
Acta crystallographica. Section B, Structural science, 2003
The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) a... more The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) at 120 K (low-temperature, LT, structure) and at 293 K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same (or an integer multiple) lattice constants but different space groups. The space groups of both host (H) and guest (G) components cause systematic absences, which result from their intersection. The highest symmetry of a sigma-structure can be described as [H: P4(2)/mnm; G: P4/mbm (c(G) = 0.5c(H)); composite: P4(2)/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In beta-Ta, two components modify their symmetry during the thermal process 293 K (RT-I)=> 120 K (LT)=> 293 K (RT-II): [H: P(-)42(1)m; G: P(-)42(1)m; composite: P(-)42(1)m]=> [H: P(-)4, G: P4/mbm (c(G) = 0.5c(H)), composite: P(-)4]=> [H: P(...
Acta crystallographica. Section B, Structural science, 2002
Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tanta... more Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) A, space group P42(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor approximately 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. Beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.
Chemical Science, 2012
Scheelite related compounds with general formula M n (XO 4 ) m are the subject of hefty interest ... more Scheelite related compounds with general formula M n (XO 4 ) m are the subject of hefty interest owing to their optical properties, stability and relatively simple preparation. Eu 3+ -containing scheelites are considered as redemitting phosphors and the main factors affecting their luminescence are thought to be chemical composition and particle size while the influence of their structure is generally ignored. Here we report eight compounds from the Na x Eu (2Àx)/3 MoO 4 series prepared by conventional solid-state reaction and present a detailed analysis of their crystal structures. Six of them have modulated structures, a common feature of SRCs, in which dopant Eu 3+ ions are orderly distributed. Moreover, different amounts of Eu 3+ dimers are detected in the modulated structures, characterized by weak satellite reflections appearing in the lower angle part of the XRD patterns. These reflections are indexed and incorporated into Rietveld's refinement using superspace (3 + 1)-dimension symmetry. The remarkable feature of the compounds is that the characteristic luminescence parameters, overall (Q Eu L ) and intrinsic (Q Eu Eu ) quantum yields, Eu( 5 D 0 ) lifetimes, and sensitization efficiencies (h sens ), correlate with the number of Eu 3+ aggregates, but not directly with the composition x of the materials. This provides an efficient tool for understanding and controlling the luminescence properties of scheelite related compounds.
ChemInform, 2010
Praseodymium molybdate Pr 2 (MoO 4 ) 3 was synthesized using the standard ceramic route. The crys... more Praseodymium molybdate Pr 2 (MoO 4 ) 3 was synthesized using the standard ceramic route. The crystal structure of the material has been successfully solved in superspace group I2/b(Rβ0)00 with lattice constants a = 5.30284(4), b = 5.32699(3), c = 11.7935(1) Å , γ = 90.163(1)°, and the modulation vector q = 2/3a* þ 0.88810(2)b*. The deviation of the q vector from a rational value allows a description of the structure in terms of nanosize domains with the La 2 (MoO 4 ) 3 -like structure separated by stacking faults. Under 450 nm excitation, ( 3 P 0 level) Pr 2 (MoO 4 ) 3 exhibits the characteristic red emission, with the most intense band at 649 nm corresponding to a 3 P 0 f 3 F 2 transition. Magnetic susceptibility measurements reveal Curie-Weiss paramagnetism with predominating antiferromagnetic interactions between Pr 3þ -magnetic moments and no evidence of magnetic transitions down to T = 5 K.
Zeitschrift für Kristallographie, 2004
The structure of aperiodic crystals which includes incommensurate, quasi-and composite crystals i... more The structure of aperiodic crystals which includes incommensurate, quasi-and composite crystals is usually described in spaces of higher dimension, the so called superspace. The main advantage of the superspace formalism is that an aperiodic structure in three dimensions recovers its full periodicity in higher dimensions. The symmetry properties of aperiodic crystals are obviously more convenient to describe in superspace too. The origin of the incommensurate nature of structures can often be found in competing interatomic interactions. From molecular dynamics simulation of a simple three dimensional model with close-packed layers and a single degree of freedom for each particle, it is possible to find the existence conditions of commensurate and incommensurate phases. Incommensurate phases can already be predicted on the basis of nearest and next nearest neighbour particle interactions only. We illustrate this principle of interactions with two examples of structures, Na 2 CO 3 and K 3 In(PO 4 ) 2 . These examples shows clearly the importance of non-oxygen interactions i.e.
Zeitschrift für Kristallographie, 2000
… Section B: Structural …, 2003
Journal of Solid State Chemistry, 2010
a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction ... more a-PbO 2 related structure Superspace approach Supercell (3+ 1)D structure type X-ray diffraction Transmission electron microscopy a b s t r a c t A novel lithium zinc niobium oxide LiZnNb 4 O 11.5 (LZNO) has been found in the Nb-rich part of Li 2 O-ZnO-Nb 2 O 5 system. LZNO, with an original a-PbO 2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a =17.8358(9)Å, b=15.2924(7)Å, c=5.0363(3)Å and g=96.607(5)1 or as a-PbO 2 -like with lattice constants a=4.72420(3)Å, b=5.72780(3)Å, c=5.03320(3)Å, g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO 2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P112 1 /n(ab0)00) as well as a supercell (space group P2 1 /b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO 2 -Z (3 + 1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li 2 O-ZnO-Nb 2 O 5 system are predicted on the basis of this detailed HRTEM analysis.
Inorganic Chemistry, 2010
Inorganic Chemistry, 2006
The structure of room-temperature ZrP 2 O 7 is shown to be orthorhombic by a combination of high-... more The structure of room-temperature ZrP 2 O 7 is shown to be orthorhombic by a combination of high-resolution synchrotron powder diffraction and single-crystal synchrotron diffraction data. Small nontwinned single crystals were obtained by synthesizing the compound using solvothermal methods at temperatures below the cubic to orthorhombic phase transition. The average P−O−P angle is 146°. DFT calculations (B3LYP/AUG-cc-pVDZ) on the isolated P 2 O 7 4anion yield a P−O−P angle of 153.42°and indicate that the barrier to inversion is of the order 3.6 kJ mol -1 .
Ferroelectrics, 2004
Diaqua (15-crown-5) copper (II) dinitrate is an organic salt presenting an one-dimensional commen... more Diaqua (15-crown-5) copper (II) dinitrate is an organic salt presenting an one-dimensional commensurate modulation. The diffraction pattern is characterized by strong Bragg reflections and weaker superstructure reflections (satellites). The structure is described in the superspace formalism as a modulated structure in (3+ 1) dimensions with occupational and displacive modulation functions.