Fine details of crystal structure and atomic vibrations in YbB12 with a metal–insulator transition

2020 ◽  
Vol 76 (6) ◽  
pp. 1117-1127
Author(s):  
Nadezhda Bolotina ◽  
Olga Khrykina ◽  
Andrey Azarevich ◽  
Sergey Gavrilkin ◽  
Nikolay Sluchanko
Keyword(s):  
Crystal Structure ◽  
Structural Model ◽  
Structural Parameters ◽  
Atomic Displacement ◽  
Unit Cell ◽  
Metal Insulator ◽  
Kondo Insulator ◽  
Temperature Range ◽  
Coupled Oscillations ◽  
A Charge

The crystal structure of single-crystal Kondo insulator YbB12 was studied at nine temperatures in the range 85–293 K based on X-ray diffraction data. Very weak Jahn–Teller distortions of the cubic lattice were detected at all temperatures, but did not require a revision of the structural model. Heat capacity and electrical conductivity of YbB12 single crystals were studied in the temperature range 1.9–300 K. It is found that both the structural parameters and the indicated physical properties have some specific features in the temperature range 120–160 K. The unit cell of YbB12 contracts when cooled below 160 K and expands at around 120 K. The temperature dependences of the equivalent atomic displacement parameters U eq(T) are no longer monotonic around 140 K and should be modeled by two Einstein curves for Yb and two Debye curves for boron atoms above and below this temperature. As follows from the temperature behavior of the specific heat, coupled oscillations of Yb ions in a double-well potential lead to the appearance of a charge gap in the density of states and gradual deterioration in conductive properties of the crystal below 150 K. This metal–insulator phase transition is accompanied by a kink in the U eq(T) curves and changes in the unit-cell values.

NQR and Crystal Structure of 4,5-Dichloroimidazole, C3H2N2Cl2

1992 ◽  
Vol 47 (1-2) ◽  
pp. 177-181 ◽  
Author(s):  
Shi-Qi Dou ◽  
Alarich Weiss
Keyword(s):  
Crystal Structure ◽  
Phase Transition ◽  
Temperature Dependence ◽  
Room Temperature ◽  
Unit Cell ◽  
Space Group ◽  
Temperature Range ◽  
Temperature Coefficients ◽  
Group D

AbstractThe two line 35Cl NQR spectrum of 4,5-dichloroimidazole was measured in the temperature range 77≦ T/K ≦ 389. The temperature dependence of the NQR frequencies conforms with the Bayer model and no phase transition is indicated in the curves v ( 35Cl)= f(T). Also the temperature coefficients of the 35Cl NQR frequencies are "normal". At 77 K the 35Cl NQR frequencies are 37.409 MHz and 36.172 MHz and at 389 K 35.758 MHz and 34.565 MHz. The compound crystallizes at room temperature with the tetragonal space group D44-P41212, Z = 8 molecules per unit cell; at 295 K : a = 684.2(5) pm, c = 2414.0(20) pm. The relations between the crystal structure and the NQR spectrum are discussed.

Crystal structure of Al5O3N3 (15R)

2017 ◽  
Vol 32 (S1) ◽  
pp. S2-S5 ◽  
Author(s):  
Jacek Podwórny ◽  
Alicja Pawełek ◽  
Jerzy Czechowski
Keyword(s):  
Crystal Structure ◽  
Structure Determination ◽  
Structural Model ◽  
Stoichiometric Composition ◽  
Unit Cell ◽  
Second Phase ◽  
Space Group ◽  
X Ray ◽  
Cell Dimensions ◽  
Unit Cell Dimensions

Having synthesised an AlON-bonded ceramic corundum material, Al5O3N3 (15R) polytype coexisting with α-Al2O3 was identified. The sample was prepared from an alumina-rich mixture of Al2O3 and AlN substrates and fired at 1650 °C in a nitrogen atmosphere. Using the X-ray external standard quantitative method, one of the reaction products, α-Al2O3, was quantified. From the remaining substrates the stoichiometric composition of the second phase was calculated. The applied method of crystal structure determination consisted of three stages. In the first stage, the Le Bail method of X-ray pattern decomposition was used for the extraction of Al5O3N3 (15R) diffraction lines from a two-phase diffractogram. The space group and unit-cell dimensions from the isostructural SiAl4O2N4 SiAlON phase, producing the same X-ray pattern, were used as input data. Next, the direct structure determination in real space was applied for initial structural model derivation, which was followed by Rietveld refinement. The solved crystal structure of Al5O3N3 (15R), except the stacking sequence, proved to be closely related to the structure of Al7O3N5 (21R) polytype. The Al5O3N3 (15R) is trigonal with space group R-3m, unit-cell dimensions a0 = 3.0128 Å, c0 = 41.8544 Å, and volume V = 329.00 Å3. The model of Al5O3N3 (15R) polytype structure has positional disordering in one of three (6c) Al sites, which leads to stacking faults in six tetrahedral layers. Every third tetrahedron from LR3 and LR4, LR8 and LR9, LR13 and LR14 layers is rotated by 180°.

Halogen NQR and Crystal Structure of Ammonium Halides (R-NH3)+X- and (X–) (+H3NR´­NH3+) (X–). R = (HOCH2)3C, R´ = CH2C(CH3)2CH2; X = I,Br

1994 ◽  
Vol 49 (1-2) ◽  
pp. 174-184
Author(s):  
Shi-qi Dou ◽  
Hartmut Fuess ◽  
Helmut Paulus ◽  
Alarich Weiss
Keyword(s):  
Crystal Structure ◽  
Hydrogen Bonds ◽  
Crystal Structures ◽  
Unit Cell ◽  
Space Group ◽  
Temperature Range ◽  
Ammonium Halides

AbstractThe 127I-NQR of(HOCH2)3CNH3+ I- was determined in the range 77 ≤ T/K ≤ 310. At T = 310 K the NQR signal fades out (Tm = 463 K). The 127I spectrum ( T =77 K.): v1 =29.195 MHz, v2 = 14.597 MHz, η(121l)=0, e QΦzz h-1 (127I) = 97.315 MHz, is in agreement with the crystal structure. The 127I NQR spectrum of 1,3-diammonium-2,2-dimethylpropane diiodide, (H3NCH2C(CH3)2CH2NH3)2+ ·2I- , is a quartet within the whole temperature range investigated, and the lines correspond to two crystallographically independent iodines: Space group P21/c, Z = 4, a = 731.2(3) pm, b = 689.0(3) pm, c = 2255.1(8) pm, β = 104.90(1)°. At 7 = 7 7 K the 127I NQR quartet is (MHz): v1 = 34.145, v2 = 32.805, v3 = 22.113, v4 = 16.787; at 295 K (same order, MHz): 30.559, 29.729, 19.810, 15.651. There are two combinations of the NQR frequencies. Considering the coordination of I-, the hydrogen bonds N -H ··· I, eQΦzzQ h-1 and η, we choose for I(1) v1 and v3, for I(2) v2 and v4. At 77 K eQΦzzQ h-1 (I(1))= 118.86 MHz,η (127I(1)) = 0.498, eQΦzzQ h-1 (I(2)) = 109.75 MHz, η(127I(2)) = 0.135 follow for the two iodine atoms. Both, eQΦzzQ h-1 (I(1)) and e eΦzzQ h-1(I(2)) decrease smoothly with increasing T: η I(2)) increases with increasing T whereas η(127I(1)) is almost constant within 77 ≤ T /K ≤ 4 0 6 . The 79,81Br NQR spectrum of l,3-diamino-2,2-dimethylpropane dihydrobromide is also a quartet, showing two crystallographic inequivalent Br atoms in the unit cell. The frequencies are (T =273 K, MHz): v1 (79Br)= 14.303, v2 (79Br)= 12.884, (81Br)= 11.951, v2(81Br) = 10.781; space group C2/c, Z = 8 , a = 2136.4(6) pm, b = 854.6(3) pm, c = 1125.8(3) pm, β = 93.23(1)°. Crystal structures and NQR results are discussed.

Structural parameters: Analysis of results

2010 ◽  
Author(s):  
Jenny Pickworth Glusker ◽  
Kenneth N. Trueblood
Keyword(s):  
Crystal Structure ◽  
Structural Parameters ◽  
Three Dimensional ◽  
Molecular Geometry ◽  
Unit Cell ◽  
Space Group ◽  
Cell Dimensions ◽  
Information File ◽  
The Individual ◽  
Unit Cell Dimensions

The results of an X-ray structure analysis are coordinates of the individual, chemically identified atoms in each unit cell, the space group (which gives equivalent positions), and displacement parameters that may be interpreted as indicative of molecular motion and/or disorder. Such data obtained from crystal structure analyses may be incorporated into a CIF or mmCIF (Crystallographic Information File or Macromolecular Crystallographic Information File). These ensure that the results of crystal structure analyses are usefully archived. There are many checks that the crystallographer can make to ensure that the CIF or mmCIF file is correctly informative. For example, the automated validation program PLATON (Spek, 2003) checks that all data reported are up to the standards required for publication by the International Union of Crystallography. It does geometrical calculations on the structure, illustrates the results, finds if any symmetry has been missed, investigates any twinning, and checks if the structure has already been reported. We now review the ways in which these atomic parameters can be used to obtain a three-dimensional vision of the entire crystal structure. When molecules crystallize in an orthorhombic, tetragonal, or cubic unit cell it is reasonably easy to build a model using the unit-cell dimensions and fractional coordinates, because all the interaxial angles are 90◦. However, the situation is more complicated if the unit cell contains oblique axes and it is often simpler to convert the fractional crystal coordinates to orthogonal coordinates before calculating molecular geometry. The equations for doing this for bond lengths, interbond angles, and torsion angles are presented in Appendix 12. If the reader wishes to compute interatomic distances directly, this is also possible if one knows the cell dimensions (a, b, c, ∝ , β , γ ,), the fractional atomic coordinates (x, y, z for each atom), and the space group.

scholarly journals Synthesis and crystal structure of a series of incommensurately modulated composite oxohalide compounds

2014 ◽  
Vol 43 (42) ◽  
pp. 15812-15817
Author(s):  
Iwan Zimmermann ◽  
Alexis Corgnet ◽  
Mats Johnsson ◽  
Sven Lidin
Keyword(s):  
Crystal Structure ◽  
Composite Structure ◽  
Oxygen Vacancies ◽  
Unit Cell ◽  
Synthesis And Crystal Structure ◽  
Charge Imbalance ◽  
Composite Nature ◽  
A Charge

The new isostructural oxohalides [Sb4O7+3δX4][Zn3]1+δ (X = Cl, Br, I), δ ≈ 0.2 have a composite structure, in which the Zn atoms are best described in a second-unit cell. The composite nature of the structure leads to a charge imbalance that is compensated by oxygen vacancies.

The crystal structure of tin sulphate, SnSO4, and comparison with isostructural SrSO4, PbSO4, and BaSO4

2012 ◽  
Vol 27 (3) ◽  
pp. 179-183 ◽  
Author(s):  
Sytle M. Antao
Keyword(s):  
Crystal Structure ◽  
High Resolution ◽  
Rietveld Refinement ◽  
Structural Parameters ◽  
Lone Pair ◽  
Unit Cell ◽  
X Ray Diffraction ◽  
Unit Cell Parameters ◽  
X Ray ◽  
Cell Parameters

The crystal structure of tin (II) sulphate, SnSO4, was obtained by Rietveld refinement using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The structure was refined in space group Pbnm. The unit-cell parameters for SnSO4 are a = 7.12322(1), b = 8.81041(1), c = 5.32809(1) Å, and V = 334.383(1) Å3. The average 〈Sn–O〉 [12] distance is 2.9391(4) Å. However, the Sn2+cation has a pyramidal [3]-coordination to O atoms and the average 〈Sn–O〉 [3] = 2.271(1) Å. If Sn is considered as [12]-coordinated, SnSO4 has a structure similar to barite, BaSO4, and its structural parameters are intermediate between those of BaSO4 and PbSO4. The tetrahedral SO4 group has an average 〈S–O〉 [4] = 1.472(1) Å in SnSO4. Comparing SnSO4 with the isostructural SrSO4, PbSO4, and BaSO4, several well-defined trends are observed. The radii, rM, of the M2+(=Sr, Pb, Sn, and Ba) cations and average 〈S–O〉 distances vary linearly with V because of the effective size of the M2+cation. Based on the trend for the isostructural sulphates, the average 〈Sn–O〉 [12] distance is slightly longer than expected because of the lone pair of electrons on the Sn2+cation.

Different Structural Models of YB2C2 and GdB2C2 on the Basis of Single-Crystal X-Ray Data

2014 ◽  
Vol 69 (3) ◽  
pp. 289-293 ◽  
Author(s):  
Olaf Reckeweg ◽  
Francis J. DiSalvo
Keyword(s):  
Crystal Structure ◽  
Single Crystal ◽  
Structural Model ◽  
Induction Furnace ◽  
Unit Cell ◽  
Nominal Composition ◽  
X Ray ◽  
Arc Melting ◽  
Tetragonal Unit Cell ◽  
Ray Methods

Samples of YB2C2 and GdB2C2 were synthesized by arc-melting and subsequent annealing of the products. Single crystals of the title compounds were examined with single-crystal X-ray methods. Five different tetragonal unit cell settings from the literature (obtained on the basis of symmetry considerations) were used for the refinement of the crystal structure of YB2C2, but a converging refinement was only achieved for the structural model in the space group P4=mbm (no. 127, Z = 2) with a = 533:27(3) and c = 354:58(3) pm, without obvious inconsistencies. Nevertheless, the results for the five different unit cell settings are compared and discussed. A refinement of the crystal structure of the isotypic compound GdB2C2 was also performed with the lattice parameters a = 537:46(6) and c = 364:98(11) pm. Neither EuB2C2 nor YbB2C2 were obtained by melting coldpressed pellets with the nominal composition RE1.5B2C2 in an arc-furnace or an induction furnace, or by just heating the pellets in sealed Ta ampoules

Structure Investigations of Molecular Crystals Containing the Ring System Cyclo-tris(2,6-pyridylformamidine) by Means of X-ray Powder Diffraction and Force-Field-Constrained Rietveld Refinement

1998 ◽  
Vol 31 (6) ◽  
pp. 874-880 ◽  
Author(s):  
P. Friedel ◽  
J. Tobisch ◽  
D. Jehnichen ◽  
J. Bergmann ◽  
T. Taut ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Force Field ◽  
Diffraction Pattern ◽  
Powder Diffraction ◽  
Structural Model ◽  
Structure Refinement ◽  
Geometry Optimization ◽  
Cyclic Compound ◽  
Unit Cell ◽  
X Ray

A new heterocyclic compound, cyclo-tris(2,6-pyridylformamidine), was synthesized as a highly crystalline powder-like substance from which very small whiskers were produced by a sublimation technique. By rotating-crystal and powder diffraction measurements, a set of structural data were obtained which were fitted mathematically to a structural model. The fitting procedure included geometry optimization byab initiocalculations, crystal structure and X-ray diffraction pattern simulation as well as structure refinement based on the newly developed Rietveld algorithmBGMN®under force-field constraints. A single-crystal structure refinement could not be carried out because of the small size of the whiskers. In order to obtain the atomic coordinates a new procedure incorporating a powder diffraction pattern refinement was used. Cyclo-tris(2,6-pyridylformamidine) crystallizes in a monoclinic unit cell witha = 28.18 (1),b = 14.67 (1),c = 4.428 (1) Å, β = 90.10 (2)°. The symmetry was determined asP21/a. The expected threefold symmetry of the cyclic compound was disturbed in the crystallized state, in agreement with solid-state NMR investigations. The best approximation was obtained when water molecules were additionally packed into the unit cell.

Electron density distribution and crystal structure of lithium barium silicate, Li2BaSiO4

2010 ◽  
Vol 25 (4) ◽  
pp. 336-341 ◽  
Author(s):  
Tatsunari Kudo ◽  
Yoshinori Hirano ◽  
Koichi Momma ◽  
Koichiro Fukuda
Keyword(s):  
Crystal Structure ◽  
Electron Density ◽  
Structural Model ◽  
Rietveld Method ◽  
Three Dimensional ◽  
Direct Methods ◽  
Atomic Displacement ◽  
Entropy Method ◽  
Reliability Indices ◽  
Atomic Displacement Parameters

Crystal structure of Li2BaSiO4 was reinvestigated by laboratory X-ray powder diffraction. The title compound was hexagonal with space group P63cm, Z=6, unit-cell dimensions a=0.810 408(2) nm, c=1.060 829(4) nm, and V=0.603 370(3) nm3. The initial structural model was successfully derived by the direct methods and further refined by the Rietveld method, with the anisotropic atomic displacement parameters being assigned for all atoms. The reliability indices calculated from the Rietveld refinement were Rwp=6.72%, S=1.17, Rp=5.06%, RB=1.86%, and RF=0.98%. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the structural model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The final reliability indices calculated from MPF were Rwp=6.74%, S=1.17, Rp=5.10%, RB=1.49%, and RF=0.69%. Atomic arrangements of the final structural model were in excellent agreement with the three-dimensional electron-density distributions determined by MPF.

Crystal structure of layered perovskite compound, Li2LaTa2O6N

2011 ◽  
Vol 26 (1) ◽  
pp. 4-8 ◽  
Author(s):  
Motoaki Kaga ◽  
Hirokazu Kurachi ◽  
Toru Asaka ◽  
Bing Yue ◽  
Jinhua Ye ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Structural Model ◽  
Rietveld Method ◽  
Direct Methods ◽  
Atomic Displacement ◽  
Layered Perovskite ◽  
Reliability Indices ◽  
Layered Perovskite Structure ◽  
Atomic Displacement Parameters ◽  
Unit Cell Dimensions

The crystal structure of Li2LaTa2O6N was determined from laboratory X-ray powder diffraction data (Cu Kα1) using the Rietveld method. The title compound is tetragonal with space group I4/mmm, Z=2, and unit-cell dimensions a=0.395 049(4) nm, c=1.850 97(3) nm, and V=0.288 869(6) nm3. The initial structural model was successfully derived by the direct methods and further refined by the Rietveld method, with the anisotropic atomic displacement parameters being assigned for all atoms. The final reliability indices were Rwp=5.73%, S=1.46, Rp=4.33%, RB=1.13%, and RF=0.53%. Li2LaTa2O6N has a layered perovskite structure similar to that of Li2LaTa2O7.

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