Neutron Powder Profile Refinement Research Articles

Chromium Site Specific Chemistry and Crystal Structure of LiCaCrF 6 and Related Chromium Doped Laser Materials

The crystal structure of LiCaCrF 6, a fully Cr-substituted lithium metal hexafluoride representative of laser materials with the generic formula Li Me II Me III 1- x Cr x F 6 ( Me II = Ca, Sr; Me III = Al) was determined by X-ray single crystal diffraction and neutron powder profile refinement. LiCaCrF 6 was confirmed as a LiCaAlF 6 (Colquiirite)-type (space group P3̄1 c (No. 163), Z = 2, a = 5.1054(08) Å, c = 9.7786(10) Å, Li in 2 c, Ca in 2 b, Cr in 2 d, and F in 12 i with x, y, z of 0.3652(3), 0.0183(4), 0.1402(1)). Deviation from centrosymmetry ( P31 c; No. 159) and full occupation were examined and appear to be insignificant. We selected LiCaCrF 6 as a model compound for the determination of EXAFS phase shifts to provide Cr site specific information in 3% Cr-doped LiCaAlF 6 laser crystals where Cr occupies a slightly distorted CrF 6 octahedron with a Cr-F distance (1.89(8) Å) practically identical to Cr-F in LiCaCrF 6 (1.901 Å) and CrF 3 (1.899 Å). The pseudobinary laser material LiSrAl 1- x Cr x F 6 forms a true solid solution over the complete composition range 0 ≤ x ≤ 1.

MORGUE, a neutron powder diffraction profile refinement program with control-file facility to include structural and rigid-body thermal-motion constraints

A neutron powder profile refinement program, MORGUE, has been written to facilitate the (possible) inclusion of both structural and rigid-body thermal-motion constraints. MORGUE results from major code modification to the program EDINP [Pawley (1980). J. Appl. Cryst. 13, 630–633] to allow the introduction of constraint conditions from an input parameter file in most practical situations. This replaces the previous need to write constraint-specific sub-routines for each structural and thermal-motion model refined, with the consequent repeated requirement for some recompilation and linkage of the modular parts to form the new executable program. The code is written in (VAX/VMS) Fortran77 and the program has been run on a VAX 11/750 computer. Examples of its use are included.

A neutron powder diffraction study of Rbx(NH4)1-xAlF4 mixed compounds

The neutron powder profile refinement method has been used to determine the structures of mixed compounds Rbx(NH4)1-xAlF4 for x<or=0.10 at 290 and 5 K. At 290 K, the average structure for x <or=0.10 is refined in the space group I4/mcm as is pure NH4AlF4. The introduction of rubidium ions leads essentially to a variation in the cell parameters and to a slight decrease in the fluorine octahedra tilts. At 5 K for x=0.03, the long-range order of the ammonium ions settles as in NH4AlF4. For x=0.07 and x=0.10, it is shown that the low-temperature phase does not exhibit any 3D order in the ammonium network. This result precludes the hypothesis of A3NNI phases and tends to suggest the existence of a structural spin-glass behaviour. Then very small rubidium concentrations are sufficient to destroy the long-range order in the ammonium sublattice.

Structural phase transitions in ferroelastic TlAlF4: DSC investigations and structures determinations by neutron powder profile refinement

The temperature behaviour of TlAlF4 is investigated by DSC and by neutron powder diffraction. Evidence for two structural phase transitions (SPT) is found at Tc1=514 K (a first-order SPT) and at Tc2=435 K. The structures of the three phases are determined by the neutron powder profile-refinement technique at 723, 523, 473, 423, 200 and 4.2 K. They can be described by the space groups P4/mmm above Tc1 (phase I, 'ideal TlAlF4 structure'), I4/mcm between Tc1 and Tc2 (phase II) and I2/a below Tc2 (phase III). It is shown that the transitions are closely related to tilts of AlF6 octahedra with the ion sequences a0a0c0, a0a0c-, aa-aa-c- and can be associated with the freezing of phonons at the A point of the Brillouin zone of the ideal TLAlF4 structure cell. The Tc1SPT is non-ferroic while the Tc2 SPT is of the improper ferroelastic type. The ferroelastic properties of phase III are discussed.

Temperature behaviour of the tetrafluoroaluminates K1− xRbxAlF4. Evolution of the shear structural phase transition

By DSC, X-ray diffraction and neutron powder diffraction the temperature behaviour of the K 1− x Rb x AlF 4 compounds is investigated in the concentration range x = 0 to x = 0.10. For large crystals it is shown that if x is greater than 0.02 the shear transformation of pure KAlF 4 splits into two structural phase transitions. The structures are determined by neutron powder profile refinement and the space groups P4/ mbm, P2 1 and P2 1/ m are attributed to the three successive phases. The high and low temperature phases are the same as in pure KAlF 4. The intermediate phase is derived from the T1AlF 4 type structure by AlF 6 octahedra rotations corresponding to the a 1 pb + pc + tilts system. It is shown that the temperature of the transition leading to the low temperature phase is sample size dependent.

The temperature dependence of the crystal structures of the solid halogens, bromine and chlorine

The neutron powder profile refinement technique has been used to determine the crystal structure of solid Br2 at 5, 80, 170 and 250 K and that of solid Cl2 at 22, 55, 100 and 160 K. The results confirm that the structures of the two halogens are isomorphous, with space group Cmca at all temperatures. For both halogens the lattice parameters a and b increase monotonically with temperature, but the lattice parameter c decreases at the highest temperatures. The data were analysed assuming both isotropic and anisotropic thermal parameters. The intramolecular bond length and its orientation relative to the b-axis show little change with temperature. For Br2 at 5 K the bond length is 2·301(2) A while a = 6·5672(3), b = 4·4678(2), c = 8·6938(4) A. For Cl2 at 22 K the bond length is 1·994(2) A while a = 6·1453(2), b = 4·3954(1), c = 8·1537(2) A, where the errors quoted are those produced by the profile fitting program.

The crystal structures of PbO.PbXO 4 (X = S, Cr, Mo) at 5K by neutron powder profile refinement

The structures of PbO.PbXO 4 monobasic lead oxide- lead sulphate, chromate and molybdate have been determined at 5K by neutron powder profile refinement. They are all isostructural with Lanarkite (the sulphate), being based on the red PbO structure with PbO 2 2− and its lone pair electrons replaced by XO 4 2−. A similar mechanism apparently produces the di-basic and tetra-basic oxides nPbO.PbXO 4. These materials are of relevence to solid state reactions in lead — acid battery electrodes.

The structure of koechlinite bismuth molybdate—a controversy resolved by neutron diffraction

The continuing controversy about the true space group and structure of koechlinite γ-Bi2O3-MoO3 due to uncertainties in the light atoms positions, has been resolved by neutron powder profile refinement. The original structure of Van den Elzen is correct, but contrary to his coordinates there are no unusually short oxygen-oxygen distances.The structure may be related to a series consisting of Bi2O2 layers interleaved with perovskite like layers having the formula Bin-1MnX3n + 1 (Aurivillius, 1952; Wells, 1975). For n = 1, M = Mo and ' = O the perovskite layer is reduced to MoO4, which corresponds to the actual composition of the koechlinite Bi2MoO6. Complex oxides and oxyhalides of bismuth formed according to that principle have a pseudo-tetragonal symmetry. Deviations from tetragonal symmetry are due to MoO6 octahedra tilts. Further distortions are revealed by octahedra rotations and displacement of molybdenum atoms from the centers of the octahedra.

Crystal structures of solid bromoform

The crystal structures of the three solid phases of deuterated bromoform have been determined using the neutron powder profile refinement technique. At 273 K, there are two molecules per unit cell in a hexagonal structure with space group P3̄. The molecules are statistically disordered to give a pseudosymmetry with space group P63/m. Rapid cooling of the sample freezes out the dynamic disorder and results in a metastable structure with trigonal space group P3̄, in which a given molecule is surrounded by molecules of the opposite orientation with C–D bonds in the c direction. This structure can be viewed as being made up of molecular bilayers parallel to the basal plane with the deuterium atoms in the interior of a bilayer. The transformation from this metastable trigonal phase to the stable phase at low temperatures involves mainly a translational motion of one bilayer with respect to the neighboring bilayers. Assuming threefold molecular symmetry, this structure has been determined at 14 and 220 K. It is found to be triclinic (space group P1̄) with two molecules per unit cell.

NH 4AlF 4 : Determination of the ordered and disordered structures by neutron powder profile refinement

NH 4AlF 4 undergoes a structural phase transition in the vicinity of 150 K. The structures of the high and low temperature phases are determined at 300 K and at 5 K respectively, by neutron powder profile refinement. In the high temperature phase the NH 4 + tetrahedra are statistically distributed over two different orientations and the space group is I4 mcm . In the low temperature phase, there is an ordering of the NH 4 + and the space group is P4 2 mbc .

Structural phase transitions in ferroelastic RbAlF4. I. DSC, X-ray powder diffraction investigations and neutron powder profile refinement of the structures

The layer compound RbAlF4 is shown to undergo structural phase transitions at 553K and 282K under investigation by DSC and X-ray powder diffraction. The structures of the three phases are determined by neutron powder profile refinement. The high-temperature phase has the ideal TlAlF4 structure P4/mmm which corresponds to the tilt system a0a0c0. The intermediate room-temperature phase is confirmed as P4/mbm, the tilt system is a0a0c+. The low-temperature phase is consistent with the space group Pmmn which corresponds to both ap +b0c+ (near the transition temperature) and ap +bp +c+ (at very low temperature). The 553K phase transition is a non-ferroic phase transition while the 282K transition is ferroelastic. According to Aizu the low-temperature phase is a 4/mmmFmmm species. The temperature behaviour of the structural parameters is discussed, particularly in the framework of ferroelasticity.

Crystal structure and luminescence of compounds A3BC10O 20

In this paper we describe compounds A 3 BC 10O 20 ( A = Sr, Ba, Pb; B = Ti, Ge, Sn; and C = Al, Ga). The crystal structure of Ba 3TiAl 10O 20 has been determined by neutron powder profile refinement. The luminescence of these compounds has been investigated. Apart from the titanate luminescence of Ba 3TiAl 10O 20, these compounds show a semiconductor type of luminescence.

The structures and the improper ferroelastic phase transition of RbAlF4

Abstract The layer compound RbA1F4 investigated by DSC, powder diffraction and linear birefringence is revealed to undergo structural phase transitions at Tc1 = 559.4 K and at Tc2 = 282.1 K. The structures of the three phases are determined by neutron powder profile refinement. The results are consistent with the space group P4/mmm (above Tc1), P4/mbm (from Tc1 to Tc2) and Pmmn (below Tc2). The transition is non-ferroic at Tc1 and improper ferroelastic at Tc2. The linear birefringence measured from 5 K to 630 K showed that the Tc1 phase transition is first order while the Tc2 phase transition is second order. The critical exponent of the order parameter as obtained from Δnab, β = 0.324, does not change on application of uniaxial stress up to 8 MPa.

The crystal structure of deuteroammonia between 2 and 180 K by neutron powder profile refinement

The structure of ND3 has been refined by neutron powder profile analysis at 2, 77 and 180 K to a resolution of sin θ/λ = 0.85 Å-1. No orientational phase transition was found in this range. However, even at 2 K, a strong librational motion exists. The N--D bond length, after correction for libration, is constant with temperature (1.06 Å), as are the D-N-D angles (107.5 ± 0.2°). This bond length is 5% longer than that found in the free molecule, because of hydrogen bonding, but the bond angles are virtually identical.

Structural phase transitions in sodium–potassium niobate solid solutions by neutron powder diffraction

The structures of (Na,K)NbO3 solid solutions have been determined by using the neutron powder-profile refinement technique: phases K and L at room temperature, and G and F at higher temperatures. In each case approximate values have been found for position parameters and temperature factors. The structural models proposed by Ahtee & Glazer [Acta Cryst. (1976), A32, 434-446] based on the lattice parameters and intensities of difference reflexions have been found to be substantially correct. Indications of vibrational modes have been found in the sequential phase transitions of Na0.90K0.10NbO3.

The structures of Na0.98K0.02NbO3and Na0.90K0.10NbO3(phaseQ) at room temperature by neutron powder diffraction

The phase-Q structures of Na0.98K0.02NbO3 and Na0.90K0.10NbO3 have been determined at room temperature by the neutron powder profile refinement technique. Two different models were refined: orthorhombic, space group P21ma (No. 26, in cab setting) and monoclinic, space group Pm (No. 6, in the second setting). The latter is more correct as it takes into account the splitting of the monoclinic ap and cp, but the structures can be described as very nearly orthorhombic. In the 2 mole % structure Nb atoms are all displaced by 0.18 (1) Å in the same direction towards the midpoint of the edge of the surrounding octahedron. The octahedra themselves appear to be regular and are tilted by 6.8 (1)° around ap and cp and by 7.8 (1)° around bp. Na, K displacements [0.23 (4) Å] are almost parallel to those of Nb. For the 10 mole % structure Na, K and Nb displacements are the same but the tilts (5.2° and 7.3°) are somewhat smaller.

Location of hydrogen atoms in ADP by neutron powder profile refinement.

NEUTRON diffraction is a powerful tool for the location of light atoms, especially hydrogen, because the neutron scattering power of these atoms is comparable to that of the heavier elements. In particular, the study of hydrogen bonding and its effect on the physical properties of molecular crystals is well suited to neutron diffraction techniques. Unfortunately, because of the relatively low flux of thermal neutrons available, even from the best reactors, rather large single crystals are required, together with expensively long periods of time for data collection. Even when large crystals can be grown, multiple scattering (extinction effects) introduce systematic errors which limit the precision with which atoms can be located. It would be important then, if some way could be found to eliminate the need for large single crystals and long periods for data collection.