Intensities Of Superstructure Reflections Research Articles

Spontaneous formation of an ordered interstratification upon Ni-exchange of Na-fluorohectorite

Interstratified structures are a common phenomenon in layered silicates and the differentiation between random and ordered interstratifications might be in some cases difficult. Atypical basal spacings observed for Ni-exchanged fluorohectorites could be related to such an ordered interstratified structure. Applying powder X-ray diffraction, thermal gravimetric analysis, and various spectroscopic techniques, an ordered interstratification of smectite-like [Ni(H2O)6]2+ and condensed, chlorite-like [Ni(OH)2−y(H2O)y]xy+interlayers, where x refers to the degree of condensation, was observed. By simple ion exchange a corrensite-like structure was obtained with a structural formula of {[Ni(OH)0.83(H2O)1.17]0.371.17+}Int. 1{[Ni(H2O)6]0.282+}Int. 2[Mg5Li] < Si8 > O20F4. Improving the contrast between the two distinct d-spacings and between the electron densities of the interlayers by partial ion exchange with a long chain alkylammonium cation or thermal annealing, helped to increase the intensity of superstructure reflections rendering the ordered interstratified structures more clearly visible.

Orientation control of KNbO3 film grown on glass substrates by Ca2Nb3O10− nanosheets seed layer

ABSTRACT KNbO3 films have been deposited by pulsed laser deposition on glass substrates covered by Ca2Nb3O10− nanosheets used as seed layer, for the purpose of promoting (001) preferential orientation. Nanosheets have been prepared by the exfoliation process of HCa2Nb3O10 phase that is obtained by cation exchange of the Dion-Jacobson KCa2Nb3O10 phase in an acidic solution. Electron diffraction in transmission electron microscopy performed on KCa2Nb3O10 and HCa2Nb3O10 powders, and on Ca2Nb3O10− nanosheets revealed formation of local superstructures that are not detected by X-ray diffraction due to the weakness of superstructure reflection intensities. Nanosheets were deposited on substrates using the Langmuir-Blodgett method. A smooth covering of the surface was shown by atomic force microscopy. 200 nm thick KNbO3 films have been grown by pulsed laser deposition on Ca2Nb3O10−-NS/glass and on (001)SrTiO3 substrates for comparison. Substrates covered with nanosheets induced highly textured (001)-oriented thin film, which possesses a similar microstructure as epitaxial film on single crystalline strontium titanate substrate.

Exploring YSZ/ZrO2 - PuO2 systems: Candidates for inert matrix fuel

The investigations in phase behavior of plutonia-based system are generally carried out by employing surrogates for the ease of handling. However, the curiosity to understand the phase relations by employing PuO2 directly instead of surrogates led to the present work on phase relations in yttria stabilized zirconia (YSZ)–PuO2 and ZrO2–PuO2 systems. These systems have relevance to inert matrix fuel concept. Phase relations have been discussed in both the systems under different conditions. The nominal compositions in YSZ-PuO2 system under vacuum exhibit fluorite structure for low PuO2 content (up to 20 mol %) while the pyrochlore structure was observed for the samples with 30–50 mol% PuO2 content. Annealing under oxidizing conditions did not alter the F-type phases but decreased the ordering in pyrochlore phases. The samples with composition Zr0.8Pu0.2O2 and Zr0.5Pu0.5O2 in ZrO2–PuO2 systems under reduced condition form F-type phase and pyrochlore, respectively. The F-type phase, upon oxidation, transformed to tetragonal phase while the pyrochlore phase retains its identity with concomitant decrease in intensity of superstructure reflections indicating the gradual pyrochlore to fluorite phase transition. The sintering studies on YSZ-PuO2 samples show that no appreciable shrinkage were observed on the samples beyond 1773 K. Linear thermal expansion on the representative nominal compositions was measured in a vertical dilatometer over the temperature range from RT to 1573 K in argon atmosphere. Though the coefficients of linear thermal expansion of 0.6 YSZ-0.4 PuO2 (40 mol% PuO2 containing YSZ) is less compared to YSZ (x = 0.0) at lower temperature, the trend reverses beyond 1473 K.

Diffraction spectra of order–order transition structural states in titanium monoxide

A new class of M(n−i)X(n−i) superstructures (n is an integer constant and i is a rational variable) that are formed after incomplete equilibrium second-order order–order phase transitions in solid solutions and in compounds with atom–vacancy ordering is proposed. New superstructures are superpositions of partially disordered M n −X n superstructures of various symmetries in the matrix of the basic crystal structure. The model parameters affecting the positions and intensities of superstructure reflections on diffraction patterns have been studied by example of the high-temperature ordered phase β-TiO of titanium monoxide. It has been shown that the diffraction spectra of the low-symmetric M(5-i)X(5-i) superstructure (space group P1m1) and the high-symmetric M5X5 superstructure (space group Pm3m) at certain parameters are identical in the positions of superstructure reflections.

Superposition of M5X5 superstructures and X-ray diffraction in TiO1.0 titanium monoxide

The interpretation of diffraction spectra of ordered high-temperature phases of solid solutions and strongly nonstoichiometric compounds is discussed. It has been shown that variations of the intensities of superstructure reflections, which cannot be explained within simple ordering models, can be due to the superposition of superstructures with different symmetries in the matrix of the basis crystal structure. Using an example of atom–vacancy ordering in TiO1.0 titanium monoxide, a model of the order–order transition state formed by the superposition of low-temperature monoclinic (space group A2/m (C2/m)) and high-temperature cubic (space group Pm3m) M5X5 superstructures has been proposed. It has been shown that the transition state is thermodynamically equilibrium and should be implemented instead of the M5X5 cubic superstructure. The transition state model can be considered as an M(5–i)X(5–i) superstructure (i = 1, 14/18, 11/18) with the monoclinic symmetry (space group P1m1).

Cold seep status archived in authigenic carbonates: Mineralogical and isotopic evidence from Northern South China Sea

Cold-seep carbonates are precipitated under high alkalinity conditions created by the anaerobic oxidation of methane in cold-seep sites. Multiple Ca–Mg-carbonate phases are identified, including aragonite, low-Mg calcite (LMC), high-Mg calcite (HMC), protodolomite, and dolomite. These phases result from different conditions that are related with cold-seep activities. Here, we report on the relationship between the Ca–Mg-carbonate phases and the cold-seep status. Authigenic carbonates were sampled from northern slope of South China Sea. Carbon isotopic compositions of samples from Shenhu area are lower than −40‰, indicating methane-derived carbon. The δ13C values of samples from Southwest (SW) Taiwan area range from ~−30‰ to ~−20‰, which is the result of the mixture of methane carbon and seawater carbon. Carbonate phases were identified according to the composition and structure results. Samples from Shenhu area are composed of protodolomite and HMC. Three zones were discovered from the center to the rim of the cross-section of the tube-like sample from SW Taiwan area. From the external to the internal zones, the carbonate phases are HMC; LMC and protodolomite; HMC, respectively. The intensity of superstructure reflections of the protodolomite from Shenhu area is stronger than that from SW Taiwan area, indicating higher MgCO3 content. Based on the formation conditions of Ca–Mg-carbonates from LMC to dolomite, those with higher MgCO3 content are formed in more active cold-seep environment. According to the distribution of carbonate phases in each sample, the cold seep flux was high in Shenhu area and was sustained for a long time. By contrast, the flux in SW Taiwan area was relatively low and not stable. It once became higher, but finally returned to low.

Thickness-dependent cation order and disorder in PbSc0.5Ta0.5O3 thin films grown by pulsed laser deposition

We report on thickness-dependent cation ordering and ferroelectric properties of (001)-oriented epitaxial PbSc0.5Ta0.5O3 (PST) thin films grown by pulsed laser deposition on SrTiO3 (001) and Si (001) substrates. The PST film thickness was varied from 30 to 200 nm. Only films thicker than 40 nm reveal (partial) cation ordering, which increases with thickness as confirmed by the appearance and intensity of superstructure reflections in x-ray diffraction and transmission electron microscopy. In accordance with the two-state thermodynamic model, temperature-dependent dielectric constant investigations showed the presence of two kinds of phase transitions belonging to the normal ferroelectric and the relaxor state, respectively, both being present in the PST films. The influence of cation ordering on the phase transition and ferroelectric properties is discussed in detail.

Extending the S-FFT direct-methods algorithm to density functions with positive and negative peaks. XIV

Some years ago the direct-methods origin-free modulus sum function (S) was adapted to the processing of intensity data from density functions with positive and negative peaks [Rius, Miravitlles & Allmann (1996). Acta Cryst. A52, 634-639]. That implementation used phase relationships explicitly. Although successfully applied to different situations where the number of reflections was small, its generalization to larger problems required avoiding the time-consuming manipulation of quartet terms. To circumvent this limitation, a modification of the recently introduced S-FFT algorithm (that maximizes S with only Fourier transforms) is presented here. The resulting S2-FFT algorithm is highly effective for crystal structures with at least one moderate scatterer in the unit cell. Test calculations have been performed on conventional single-crystal X-ray diffraction data, on neutron diffraction data of compounds with negative scatterers and on intensities of superstructure reflections to solve difference structures.

Magnetic hysteresis properties, Mössbauer spectra and structural data of spherical 250 nm particles of solid solutions [formula omitted]– [formula omitted]- [formula omitted

Magnetic hysteresis, 57Fe Mössbauer and X-ray data are presented for spherical 250 nm particles of solid solutions in the spinel system Fe 3 O 4 – γ - Fe 2 O 3 ( Fe 3 - u □ u O 4 , 0 ⩽ u ⩽ 1 / 3 ( □ = cation vacancy)). For dispersed particles, the composition of u ∼ 0.24 exhibits maxima of coercive force H c ∼ 25 kA / m , relative saturation remanence σ rs / σ s ∼ 0.3 , switching field distribution H SFD ∼ 100 kA / m and of anhysteretic remanence susceptibility at room temperature. The experimental results point to single domain particles with non-homogeneous magnetization mode or to a magnetization mode intermediate between single domain and two domain particles. The maxima are probably a consequence of the superposed action of (i) Fe 2 + —single ion anisotropy and (ii) microstresses. Mössbauer spectra were used to analyse the cation distribution on A- and B-sites of the spinel lattice. The experimental data were found to disagree with the cation distribution according to a formula used in the literature for Mössbauer analysis. From X-ray diffractograms, lattice parameters a 0 and intensity of superstructure reflections, due to ordering of □ in the spinel lattice, were determined as a function of the □ concentration u . The reflections are in agreement with a tetragonal cell with c 0 / a 0 = 3 which has been frequently observed for γ - Fe 2 O 3 ( u = 1 3 ) in the literature.

Synthesis and study of the structural characteristics of a new complex perovskite oxide Sr 2HoHfO 5.5 for its use as a substrate for YBCO superconducting films

A new complex cubic perovskite oxide Sr 2HoHfO 5.5 has been prepared by solid state reaction process. X-ray diffraction studies show that Sr 2HoHfO 5.5 has a A 2BB ′O 6 type complex cubic perovskite structure and presence of the significant intensities of superstructure reflections in the X-ray spectrum reveal that it is an ordered complex cubic perovskite oxide. The experimental value of the lattice parameter of Sr 2HoHfO 5.5, is a=8.1725 Å. Based on the doubling of primitive ABO 3 cubic perovskite unit cell, (1/2) a=4.0862 Å of Sr 2HoHfO 5.5 is comparable to the lattice parameter a=3.8227 Å of YBCO superconductor. Sr 2HoHfO 5.5 has good lattice matching (lattice mismatch ∼6%) with YBCO superconductor. The dc magnetic susceptibility of Sr 2HoHfO 5.5, measured in the temperature range 5–300 K shows a Curie–Weiss behavior. Using a Curie–Weiss law fitting we obtain a effective magnetic moment per Ho 3+ ion in Sr 2HoHfO 5.5 of 9.76 μ B. This value is slightly lower than the spin-only moment (10.60 μ B) expected for an isolated Ho 3+ ion.

Rare-earth elements in chlorapatite [Ca10(PO4)6Cl2]: Uptake, site preference, and degradation of monoclinic structure

Differences in the inter- and intracrystalline partitioning behavior of rare earth elements (REE) between chlorapatite (ClAp), fluorapatite (FAp), and hydroxylapatite (OHAp) are directly or indirectly related to substitution mechanism and spatial accommodation. The substitution of REE for Ca is charge compensated by Na in ClAp, Na and Si in FAp, and Si in OHAp. Twinned crystals of REE-substituted ClAp [La-ClAp, Nd-ClAp, Sm-ClAp, Dy-ClAp; Ca 10–2y Na y REE y (PO 4 ) 6 Cl 2 , with y = 0.05–0.09; space group P 2 1 / b ] have been grown from SiO 2 -bearing, H 2 O- and Na-rich phosphate-chloride melts, and their hexagonal ( P 6 3 / m ) subcell structures refined at room temperature with single-crystal X-ray intensities to R = 0.020–0.023. The crystal/melt partition coefficients for La, Nd, Sm, and Dy are 0.073, 0.128, 0.122, and 0.101. Thus, uptake of REE is up to two orders of magnitude lower in ClAp than in REE-substituted FAp and OHAp crystallized under equivalent conditions, but remains peaked at Nd. REE site occupancy ratios [(REE-Ca2)/(REE-Ca1)] obtained from direct refinement of electron densities are 0.71, 1.11, 0.21, and 0.09 for La-, Nd-, Sm-, and Dy-ClAp, respectively, and are consistent with relative change in Ca-O bond distances and sizes of Ca polyhedra. Thus, La, Sm, and Dy favor the Ca1 position of ClAp, not Ca2, as in FAp and OHAp; this unusual site preference is attributed to the large increase in size (6–8%) and distortion of the Ca2O 6 X polyhedron on substitution of Cl for (F,OH). The slight preference of Nd for Ca2 and the peaking of REE uptake at Nd are attributed to a 4 f crystal-field contribution, which is revealed by an anomalous decrease in the Ca2-O1 bond length and volume of the Ca2O 6 X polyhedron in Nd-ClAp, Nd-Fap, and Nd-OHAp. The P 2 1 / b structure of La-ClAp has been refined by detwinning the diffraction pattern, confirming that this monoclinic superstructure largely represents ordering of Cl atoms displaced along [001]. The P 2 1 / b structure of Dy-OHAp has been refined similarly. The intensity of superstructure reflections decreases abruptly with increasing substitution of REE for Ca, showing that REE cations interfere with ordering of Cl atoms(and OH − groups) during the P 6 3 / m → P 2 1 / b transition.

Evidence ofc(2×2)antiferromagnetic order of Mn in an ideal monolayer on Ag(001)

We present a dynamical low-energy electron diffraction analysis of the atomic and magnetic structure obtained by deposition of 0.9 Mn monolayers on Ag(001) held at 100 K. We find that the film grows in an essentially flat ideal monoatomic layer with the Mn located in fourfold hollow sites of the Ag(001) substrate that undergoes no sizeable reconstruction of its topmost layers. The Mn-Ag interlayer distance ${d}_{12}=2.00\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ is found to be much larger than \ensuremath{\sim}1.85 \AA{} expected from simple addition of atomic radii in elemental \ensuremath{\alpha}-Mn and face-centered-cubic (fcc) Ag. This is direct evidence of the strong reduction in Mn $3d$ contribution to cohesion due to the atomiclike high-spin state of the Mn in the monolayer structure. Taking into account the exchange scattering by means of a local von Barth-Hedin exchange-correlation potential, we demonstrate that the weak intensity of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ superstructure reflection intensities $I(E)$ observed at low energies only $(El~120\mathrm{eV})$ can be quite well explained in terms of a two-dimensional antiferromagnetic arrangement of the Mn moments. Nonmagnetic origins of these reflections such as out of plane (buckling) or in plane displacive reconstructions of the Mn monolayer as well as surface alloying with Ag or impurity related superstructures can be safely ruled out. Evidence of a $c(2\ifmmode\times\else\texttimes\fi{}2)$ antiferromagnetic to $p(1\ifmmode\times\else\texttimes\fi{}1)$ paramagnetic phase transition is also presented.

Investigations of the sublattice magnetizations M (T) in antiferromagnets with fourth-order exchange interactions: Eu Sr Te

We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection intensities in the diamagnetically diluted antiferromagnetic compounds EuxSr1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter. The observed MnO-type diffraction intensities originate in the sublattice magnetizations, \(\), of both order parameters. Due to the different composition dependencies for biquadratic interaction processes \(\) and three-spin interaction processes \(\), the ferromagnetic three-spin interactions dominate for x>xc=0.85, while for x 0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at xc=0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x>0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting \(\) obeys a T2 law until a temperature as large as 0.75TN irrespective of the nature of the transverse order parameter. The T2 law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin dynamics of both completely. From the linear composition dependence of the normalized T2 coefficient the existence of three-spin interactions is again confirmed.

Tungsten Bronze-Type Solid Solutions Ba6−3xSm8+2xTi18O54(x=0.3, 0.5, 0.67, 0.71) with Superstructure

Structural parameters of tungsten bronze-type compounds Ba6−3xSm8+2xTi18O54(x=0.3, 0.5, 0.67, and 0.71) were refined in a space groupPbnmby the Rietveld method using high-resolution synchrotron radiation powder diffraction data. All the specimens crystallized in the orthorhombic system with a superstructure having the doubledcaxis of the reported orthorhombic tungsten bronze-type structure. Unit-cell parameters (in Å) area=12.1715(5),b=22.3772(3),c=7.67523(9) forx=0.3,a=12.1568(1),b=22.3253(2),c=7.66301(7) forx=0.5,a=12.1472(1),b=22.2972(2),c=7.65338(6) forx=0.67, anda=12.1452(1),b=22.3029(2),c=7.65007(6) forx=0.71, andZ=2. The use of two wavelengths allowed the determination of cation and vacancy distributions over two pentagonal and five tetragonal tunnels in the structure. In the compounds withx=0.3 and 0.5, pentagonal tunnel sites were fully occupied by Ba, and the remaining Ba partially substituted Sm at two of five tetragonal tunnel sites. No evidence was found for the substitution of Sm for Ba at the pentagonal sites, and thus vacancies were formed at the pentagonal sites forx=0.71. An amplitude of periodic modulation along thecaxis, which caused superstructure, was increased with increasingxin the compositional range examined. A relationship between observed and calculated intensities of superstructure reflections suggests the presence of local lattice distortion, which takes its minimum atx=0.67.

Structural and magnetic properties of the complex perovskite oxideBa2HoHfO5.5

The complex cubic perovskite oxide ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ in the Ho-Ba-Hf-O rare-earth based oxide system has been prepared by a solid-state reaction process. Powder x-ray-diffraction studies show that ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ has an ${A}_{2}{\mathrm{BB}}^{\ensuremath{'}}{\mathrm{O}}_{6}$ complex cubic perovskite structure. The x-ray-diffraction spectrum of single-phase ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ consists of strong peaks characteristics of a primitive cubic $\mathrm{AB}{\mathrm{O}}_{3}$ perovskite plus a few weak reflection lines arising from the superlattice. The presence of significant intensities of superstructure reflections in the x-ray spectrum of ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ acts as a signature for an ordered complex cubic perovskite oxide. Based on the doubling of primitive $\mathrm{AB}{\mathrm{O}}_{3}$ cubic perovskite unit cells, the lattice parameter of ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ calculated from the experimental x-ray diffraction is $a=8.316$ \AA{}. The real part of the ac magnetic susceptibility of ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5},$ measured in the temperature range 5 to 300 K shows a Curie-Weiss behavior. Using a Curie-Weiss law fitting we obtain an effective magnetic moment per ${\mathrm{Ho}}^{3+}$ ion in ${\mathrm{Ba}}_{2}{\mathrm{HoHfO}}_{5.5}$ of $9.33{\ensuremath{\mu}}_{B}.$ This value is approximately 10% less than the spin-only moment $(10.60{\ensuremath{\mu}}_{B})$ expected for an isolated Ho ion.

Analysis of the ferroelastic phase transition of (TMMC) by means of x-ray diffraction study

The order - disorder phase transition in the linear chain compound (TMMC) was investigated by means of x-ray diffraction. The hexagonal structure at room temperature (space group with Z = 2) is characterized by an orientational disorder of the organic group TMA. A weakly first-order phase transition occurs at 126 K which stabilizes a monoclinic low-temperature phase (space group with Z = 4) characterized by a doubling of the hexagonal unit cell along the b direction. The cell parameters were determined in a large range of temperature including the two phases (from 5 to 300 K) and the extent of lattice distortion was measured in the ordered monoclinic phase. The temperature dependences of both the spontaneous strain component and the intensity of superstructure reflections were analysed by a Landau type free energy expansion involving two coupled order parameters necessary to account for this `triggered' phase transition.

Biquadratic exchange interactions in the Europium monochalcogenides

We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ3 obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ3 is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ3 > 0). This leads to a conflicting situation in the case of EuTe for which θ1 < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field Bc follows a T2 law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ3 diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range Tc < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve Bc of EuSe.

Twin, superstructure and transient tetragonal-monoclinic transition in electron-doped Nd 2- xCe xCuO 4- δ

The well-known modulation structure in electron-doped Nd 2- x Ce x CuO 4- δ has been found to consist of four-fold twins and two-dimensional superstructures as evidenced by electron diffraction and transmission electron microscopy (TEM). Twins and superstructures are both generated by a second-order tetragonal-monoclinic transition, producing the monoclinic structure in different transient forms and changing the intensity of superstructure reflections and the thickness of twin domains at the same time. This behavior allows us to build Nd(Ce) cation displacement models for the superstructures with the help of their forbidden rules. The influence of the structural properties of the manifestation of superconductivity is also discussed.

On the structure of the superconducting ortho-II phase of YBa2Cu3O6.51

From the intensities of superstructure reflections, the structure of the ortho-II phase in YBa2Cu3O5.51(5) (Tc=55.8K) was determined by single crystal x-ray diffraction. In the basal plane of the 2a×b×c supercell, the oxygen atoms order to alternating Cu−O−Cu and Cu−Cu chains. Due to the oxygen order, the barium and yttrium atoms are displaced in (100) direction in an antiferrodistortive way (Ba-shift: 0.046A toward the Cu−O−Cu chains, Y-shift: 0.008A toward the Cu−Cu chains). The apex oxygen and copper atoms of the superconducting planes are displaced in (001) direction. The Ba displacements are confirmed by anomalous scattering near the Ba−K absorption edge observed by synchrotron x-ray diffraction. A rigid ion model shows that the cation displacements stabilize the ortho-II structure by reducing its lattice energy.

Diffuse X-ray scattering study of sublattice ordering among Group III atoms in In0.5Ga0.5P and In0.5Al0.5P

The intensity of superstructure reflections and associated diffuse scattering from In0.5Ga0.5P and In0.5Al0.5P epitaxic layers grown on (001) GaAs substrates was mapped in reciprocal space. The Warren-Cowley short-range-order parameters were obtained through the usual process for evaluating Fourier coefficients. Varying values for the correlation length in different directions indicate how group III atoms stack up in ordered states. The resultant structure with long-range order confirms the hypothesis made on the basis of electron diffraction and high-resolution transmission electron microscopy studies.