Neutron Refinement Research Articles

Understanding color variation with site distribution in inverse spinel structure via neutron diffraction, magnetism, and optical studies

Chromophores at different coordinations can give rise to different colors; usually, chromophores at non-centrosymmetric coordinations are preferred for intense pigments. Different solid solutions M2-xCoxM’O4 (M = Mg/Zn, and M’ = Ti/Sn) with inverse spinel structure were synthesized with the goal of understanding color variation with site distribution, as the chromophore Co2+ in these solid solutions can occupy either the tetrahedral or octahedral sites or both depending on the composition. Another goal was to develop environmentally friendly and cheap blue pigments by reducing the carcinogenic cobalt to obtain a similar color to that of commercially available cobalt blue, which uses a significant amount of Co2+ (33.31 % by mass). For Mg2-xCoxTiO4 series, turquoise blue hues were observed for low cobalt content, and different shades of blue were observed for Mg2-xCoxSnO4 series with a color similar to cobalt blue, including just 4.90% of cobalt by mass. While for Zn2-xCoxTiO4, and Zn2-xCoxSnO4 series, different shades of brown and different shades of green, respectively, were observed. One of the main reasons behind the major difference in color for the Mg and Zn containing solid solutions, regardless of the same chromophore in the same structure is related to the chromophore site distribution in the system. For the Mg-containing solid solutions, different shades of blue are observed as Mg has no preference for any of the sites, Co2+ mostly goes to tetrahedral sites. In contrast, for the Zn-containing solid solutions, no blue shades were observed because of the strong preference of Zn for the tetrahedral sites owing to the sp3 hybridization, which in turn forces Co2+ to occupy the octahedral sites. Neutron refinement proves that Co2+ occupies mainly tetrahedral sites in the Mg-containing solid solutions and mostly octahedral sites in the Zn-containing solid solutions.

Improved joint X-ray and neutron refinement procedure in Phenix.

Neutron diffraction is one of the three crystallographic techniques (X-ray, neutron and electron diffraction) used to determine the atomic structures of molecules. Its particular strengths derive from the fact that H (and D) atoms are strong neutron scatterers, meaning that their positions, and thus protonation states, can be derived from crystallographic maps. However, because of technical limitations and experimental obstacles, the quality of neutron diffraction data is typically much poorer (completeness, resolution and signal to noise) than that of X-ray diffraction data for the same sample. Further, refinement is more complex as it usually requires additional parameters to describe the H (and D) atoms. The increase in the number of parameters may be mitigated by using the `riding hydrogen' refinement strategy, in which the positions of H atoms without a rotational degree of freedom are inferred from their neighboring heavy atoms. However, this does not address the issues related to poor data quality. Therefore, neutron structure determination often relies on the presence of an X-ray data set for joint X-ray and neutron (XN) refinement. In this approach, the X-ray data serve to compensate for the deficiencies of the neutron diffraction data by refining one model simultaneously against the X-ray and neutron data sets. To be applicable, it is assumed that both data sets are highly isomorphous, and preferably collected from the same crystals and at the same temperature. However, the approach has a number of limitations that are discussed in this work by comparing four separately re-refined neutron models. To address the limitations, a new method for joint XN refinement is introduced that optimizes two different models against the different data sets. This approach is tested using neutron models and data deposited in the Protein Data Bank. The efficacy of refining models with H atoms as riding or as individual atoms is also investigated.

Magnetic dilution effect and topological phase transitions in (Mn1−xPbx)Bi2Te4

As the first intrinsic antiferromagnetic (AFM) topological insulator (TI), MnBi$_2$Te$_4$ has provided a material platform to realize various emergent phenomena arising from the interplay of magnetism and band topology. Here by investigating (Mn$_{1-x}$Pb$_x$)Bi$_2$Te$_4$ $(0\leq x \leq 0.82)$ single crystals via the x-ray, electrical transport, magnetometry and neutron measurements, chemical analysis, external pressure, and first-principles calculations, we reveal the magnetic dilution effect on the magnetism and band topology in MnBi$_2$Te$_4$. With increasing $x$, both lattice parameters $a$ and $c$ expand linearly by around 2\%. All samples undergo the paramagnetic to A-type antiferromagnetic transition with the N$\acute{e}$el temperature decreasing lineally from 24 K at $x=0$ to 2 K at $x=0.82$. Our neutron data refinement of the $x=0.37$ sample indicates that the ordered moment is 4.3(1)$\mu_B$/Mn at 4.85 K and the amount of the Mn$_{\rm{Bi}}$ antisites is negligible within the error bars. Isothermal magnetization data reveal a slight decrease of the interlayer plane-plane antiferromagnetic exchange interaction and a monotonic decrease of the magnetic anisotropy, due to diluting magnetic ions and enlarging the unit cell. For $x=0.37$, the application of external pressures enhances the interlayer antiferromagnetic coupling, boosting the N$\acute{e}$el temperature at a rate of 1.4 K/GPa and the saturation field at a rate of 1.8 T/GPa. Furthermore, our first-principles calculations reveal that the band inversion in the two end materials, MnBi$_2$Te$_4$ and PbBi$_2$Te$_4$, occurs at the $\Gamma$ and $Z$ point, respectively, while two gapless points appear at $x = $ 0.44 and $x = $ 0.66, suggesting possible topological phase transitions with doping.

A beginner’s guide to neutron macromolecular crystallography

Hydrogen atoms drive biological structure and function, but the lightest element is often unseen in three-dimensional macromolecule structures, hampering our understanding of biochemical processes. This guide will i) present how neutron crystallography uniquely reveals the experimental positions of hydrogen atoms and resolves mechanical controversies, ii) briefly introduce beamlines at neutron facilities, iii) discuss sample requirements and preparation and iv) familiarize the reader with neutron data and refinement statistics.

Hydrogen/deuterium exchange behavior in tetragonal hen egg-white lysozyme crystals affected by solution state.

Neutron diffraction studies of hydrogen/deuterium-exchanged hen egg-white lysozyme were performed by a joint X-ray and neutron refinement to elucidate the hydrogen/deuterium exchange behavior. Large crystals for neutron work, consisting of molecules that were exchanged before crystallization, were obtained by repeatedly adding protein solution to the crystal batch using deuterated precipitant reagent. There are differences in hydrogen/deuterium exchange behavior compared with previous crystallographic or NMR studies, which could be due to intermolecular interactions in the crystal or to different lengths of exchange period.

Post-Hartree-Fock methods for Hirshfeld atom refinement: are they necessary? Investigation of a strongly hydrogen-bonded molecular crystal

Hirshfeld atom refinement (HAR) is a method for refining X-ray crystal structures that is able to provide bond lengths involving hydrogen atoms in statistical agreement with those derived from neutron diffraction data, provided the data reach 0.8 Å resolution. Rather than using tabulated spherical atomic structure factors, HAR uses “tailor made” aspherical atomic structure factors obtained directly from quantum chemical calculations. Despite the very good results obtained so far, which make HAR an emerging refinement method of modern crystallography, until now all the Hirshfeld atom refinements were exclusively based on Hartree-Fock (HF) or density functional theory (DFT) methods, but never on the so-called post-HF techniques of quantum chemistry. Post-HF methods exploit more sophisticated multi-determinant wavefunctions and, consequently, should provide more accurate electron densities for the refinements. For this reason, for the first time we have performed HARs based on two well-known post-HF strategies (MP2 and Coupled Cluster) combined with three different basis-sets (def2-SVP, def2-TZVP and def2-TZVPP). The obtained results have been afterwards analyzed and compared to those resulting from neutron and other Hirshfeld atom refinements, the latter relying on Hartree-Fock and DFT (BLYP and B3LYP) calculations in order to evaluate if the use of more sophisticated and expensive approaches of quantum chemistry can improve the performances of the HAR technique.

Ammonothermal Synthesis, X-Ray and Time-of-Flight Neutron Crystal-Structure Determination, and Vibrational Properties of Barium Guanidinate, Ba(CN3H4)2.

We report the crystal structure of Ba(CN3H4)2 as synthesized from liquid ammonia. Structure solution based on X‐ray diffraction data suffers from a severe pseudo‐tetragonal problem due to extreme scattering contrast, so the true monoclinic symmetry is detectable only from neutron powder diffraction patterns, and structure solution and refinement was greatly aided by density‐functional theory. The symmetry lowering is due to slight deviations of the guanidinate anion from the mirror plane in space group P 4‾ b2, a necessity of hydrogen bonding. At 300 K, barium guanidinate crystallizes in P21/c with a=6.26439(2) Å, b=16.58527(5) Å, c=6.25960(2) Å, and a monoclinic angle of β=90.000(1)°. To improve the data‐to‐parameter ratio, anisotropic displacement parameters from first‐principles theory were incorporated in the neutron refinement. Given the correct structural model, the positional parameters of the heavy atoms were also refinable from X‐ray diffraction of a twinned crystal. The two independent guanidinate anions adopt the all‐trans‐ and the anti‐shape. The Ba cation is coordinated by eight imino nitrogens in a square antiprism with Ba−N contacts between 2.81 and 3.04 Å. The IR and Raman spectra of barium guanidinate were compared with DFT‐calculated phonon spectra to identify the vibrational modes.

Improving the Performance of Layered Oxide Cathode Materials with Football-Like Hierarchical Structure for Na-Ion Batteries by Incorporating Mg2+ into Vacancies in Na-Ion Layers.

The development of advanced cathode materials is still a great interest for sodium-ion batteries. The feasible commercialization of sodium-ion batteries relies on the design and exploitation of suitable electrode materials. This study offers a new insight into material design to exploit high-performance P2-type cathode materials for sodium-ion batteries. The incorporation of Mg2+ into intrinsic Na+ vacancies in Na-ion layers can lead to a high-performance P2-type cathode material for sodium-ion batteries. The materials prepared by the coprecipitation approach show a well-defined morphology of secondary football-like hierarchical structures. Neutron power diffraction and refinement results demonstrate that the incorporation of Mg2+ into intrinsic vacancies can enlarge the space for Na-ion diffusion, which can increase the d-spacing of the (0 0 2) peak and the size of slabs but reduce the chemical bond length to result in an enhanced rate capability and cycling stability. The incorporation of Mg2+ into available vacancies and a unique morphology make Na0.7 Mg0.05 Mn0.8 Ni0.1 Co0.1 O2 a promising cathode, which can be charged and discharged at an ultra-high current density of 2000 mA g-1 with an excellent specific capacity of 60 mAh g-1 . This work provides a new insight into the design of electrode materials for sodium-ion batteries.

TOPAS and TOPAS-Academic: an optimization program integrating computer algebra and crystallographic objects written in C++

TOPAS and its academic variant TOPAS-Academic are nonlinear least-squares optimization programs written in the C++ programming language. This paper describes their functionality and architecture. The latter is of benefit to developers seeking to reduce development time. TOPAS allows linear and nonlinear constraints through the use of computer algebra, with parameter dependencies, required for parameter derivatives, automatically determined. In addition, the objective function can include restraints and penalties, which again are defined using computer algebra. Of importance is a conjugate gradient solution routine with bounding constraints which guide refinements to convergence. Much of the functionality of TOPAS is achieved through the use of generic functionality; for example, flexible peak-shape generation allows neutron time-of-flight (TOF) peak shapes to be described using generic functions. The kernel of TOPAS can be run from the command line for batch mode operation or from a closely integrated graphical user interface. The functionality of TOPAS includes peak fitting, Pawley and Le Bail refinement, Rietveld refinement, single-crystal refinement, pair distribution function refinement, magnetic structures, constant wavelength neutron refinement, TOF refinement, stacking-fault analysis, Laue refinement, indexing, charge flipping, and structure solution through simulated annealing.

Impact of uncertainties in the uranium 235 cross section resonance structure on characteristics measured in the BFS-79 critical assemblies

The report presents the results of an analysis of benchmark experiments form the international ICSBEP Handbook (HEU-MET-INTER-005) carried out at the the SSC RF – IPPE in cooperation with the Idaho National Laboratory (INL, USA) applicable to the verification of calculations of a wide range of tasks related to safe storage of vitrified radioactive waste. Experiments on the BFS assemblies make it possible to perform a large series of studies needed for neutron data refinement, including measurements of reactivity effects which allow testing the neutron cross section resonance structure. This series of studies is considered as a sample joint analysis framework for differential and integral experiments required to correct nuclea data files of the ROSFOND evaluated neutron data library. Thus, it is shown that despite the wide range of available experimental data, in so far as it relates to the resonance region refinement, the experiments on reactivity measurement make it possible to more subtly reflect the resonance structure peculiarities in addition to the time-of-flight measurement method.

Structural evolution, site ordering and magnetic properties of tetragonal Mn6‐yGa2+y (0 ≤ y ≤ 1.64)

A D022-type super structure with a formula Mn6‐yGa2+y (0≤y≤1.64) is proposed to describe the structural evolution of tetragonal Mn3‐xGa (0≤x≤1.8) intermetallic compounds. Degree of site disorder is obtained based on the 2b-site Mn and 2a-site Ga disordering model. Neutron data refinement indicates that the structure transforms from a completely ordered state (y=0) to the 2a and 2b sites (Mn-Ga) disordered state (y>0.92) with increasing y. The ferrimagnetic ordering structure between Mn (4d) and Mn (2a/2b) has been established, with the magnetic moment retaining almost 2.2μB and ranging from −3.3μB (y→0) to −2.3μB (y→1.64), respectively.

Single-crystal neutron diffraction and Mössbauer spectroscopic study of hureaulite, (Mn,Fe)5(PO4)2 (HPO4)2 (H2O)4

The crystal chemistry of hureaulite from the Jocao (Cigana) pegmatite, Conselheiro Pena, Doce Valley, Minas Gerais (Brazil), was investigated by electron microprobe analysis in wavelength-dispersive mode, single-crystal Laue (at 293 K) and monochromatic neutron diffraction (at 2.3 K), and 57 Fe-Mossbauer spectroscopy [ M(1),M(2),M(3) (Mn 2+ 3.61 Fe 2+ 1.21 Ca 0.11 Mg 0.03 ) ∑=4.96 ( P(2) (PO 4 ) 2 (H 1.04 P(1) PO 4 ) 2 (H 2 O) 3.92 , Z = 4, a = 17.603(6), b = 9.087(2), c = 9.404(4) A, β = 96.66(4)°, and V = 1494.1(9) A 3 at 293 K, space group C 2 /c ]. The neutron refinements confirm the general structure model previously reported, showing that five independent H sites (with full site occupancy) occur in the hureaulite structure, one of them as member of the unique hydroxyl group ( i.e. , O(1)-H(1) ) and the other four belonging to two independent H 2 O molecules ( i.e. , H(2)-O(9)-H(3) and H(4)-O(10)-H(5) ). The hydroxyl group is the vertex of one of the two independent P-tetrahedra ( i.e. , HOPO 3 ), whereas the two H 2 O molecules are the vertices of the (Mn,Fe)-octahedra. No zeolitic H 2 O occurs in hureaulite structure. The complex hydrogen bonding scheme in hureaulite is now well defined, with five hydrogen bonds energetically favorable. The element distribution among the octahedral sites, deduced on the basis of the neutron structure refinements, shows that the highest fraction of Fe populates the M(2) site (~36%), whereas lower fractions are observed at M(1) and M(3) ( i.e. , ~13–15 and 11–12%, respectively). The 57 Fe Mossbauer spectrum of hureaulite confirms the absence of ferric iron in the sample. The spectrum shows four well-separated absorption lines which are best fitted with three doublets. The well-resolved doublet (with the largest quadrupole splitting) is here assigned to the M(3) site, in line with the pronounced preference of Mn 2+ for the M(3) site as shown by the neutron structure refinements. The two innermost doublets have almost identical isomer shifts, though with slightly different quadrupole splitting values; the smaller quadrupole splitting value is assigned to the M(2) site, which is expected to have a slightly lower polyhedral distortion than the M(1) site, according to the experimental findings based on the neutron refinements.

The effect of B-site substitution on structural transformation and ionic conductivity in Ho2(ZryTi1−y)2O7

Compounds in the pyrochlore systemHo2(ZryTi1−y)2O7 exhibit an order-disorder transition from pyrochlore to a defect-fluorite type structure. Compositions in this system were prepared via mechanical milling, followed by a two-step sintering process. Structural characterization was carried out via Rietveld refinements using neutron powder diffraction data, supported by X-ray diffraction to determine the phase and location of the pyrochlore-fluorite transformation. Unit-cell parameters were determined for the whole series using Rietveld refinements as well as the Nelson–Riley function. The neutron refinement results confirmed that the cation disorder was independent of the anion Frenkel disorder. The relation between the x-parameter in the oxygen 48f position and anion Frenkel disorder was found to be linear for the pyrochlore structure. The ionic conductivity studies were undertaken via AC impedance analysis to determine the electronic behaviour and its relation to the structural change in the temperature range 300°C–700 °C. The trends in ionic conductivity and activation energy were explained structurally via neutron powder diffraction and X-ray diffraction data. The pyrochlore-fluorite boundary composition (at y = 0.5) exhibited the lowest activation energy and highest ionic conductivity.

Crystallization, neutron data collection, initial structure refinement and analysis of a xyloglucan heptamer bound to an engineered carbohydrate-binding module from xylanase.

Carbohydrate-binding modules (CBMs) are discrete parts of carbohydrate-hydrolyzing enzymes that bind specific types of carbohydrates. Ultra high-resolution X-ray crystallographic studies of CBMs have helped to decipher the basis for specificity in carbohydrate-protein interactions. However, additional studies are needed to better understand which structural determinants confer which carbohydrate-binding properties. To address these issues, neutron crystallographic studies were initiated on one experimentally engineered CBM derived from a xylanase, X-2 L110F, a protein that is able to bind several different plant carbohydrates such as xylan, β-glucan and xyloglucan. This protein evolved from a CBM present in xylanase Xyn10A of Rhodothermus marinus. The protein was complexed with a branched xyloglucan heptasaccharide. Large single crystals of hydrogenous protein (∼1.6 mm(3)) were grown at room temperature and subjected to H/D exchange. Both neutron and X-ray diffraction data sets were collected to 1.6 Å resolution. Joint neutron and X-ray refinement using phenix.refine showed significant density for residues involved in carbohydrate binding and revealed the details of a hydrogen-bonded water network around the binding site. This is the first report of a neutron structure of a CBM and will add to the understanding of protein-carbohydrate binding interactions.

Visualization of Pronation States in PLP Dependent Enzymes

Pyridoxal phosphate (PLP) dependent enzymes are essential proteins that catalyze a diverse number of reactions: transamination, racemization, phosphorylation, decarboxylation, aldol cleavage, elimination, and replacement. Despite completing a myriad of chemical reactions, these proteins have a unifying Schiff base catalytic intermediate but the overall mechanisms remain ambiguous. Based upon previous X‐ray structures and abundant literature of the different families of PLP dependent enzymes, one question must be answered: How do the different families of PLP dependent enzymes utilize different protonation profiles of pyridoxal phosphate that enable them to promote catalysis of a specific reaction? The objective of this research will be development of detailed model mechanisms for the different classes of PLP dependent enzymes, based on the protonation state of the coenzyme. Aspartate aminotransferase (AAT) and tryptophan synthase (TS) are two model PLP dependent enzymes that have shown to grow neutron diffraction quality crystals (1mm3). Neutron crystallography is an unparalleled tool to determine the atomic coordinates of the hydrogen/deuterium in macromolecules undetectable by other means. Initial neutron data collection and refinement of these proteins gives insight to the protonation state of PLP during catalysis.

A single-crystal neutron and X-ray diffraction study of a Li, Be-bearing brittle mica

AbstractThe crystal chemistry of a meso-octahedral Li,Be-bearing mica from the Harding pegmatite (Dixon, Taos County, New Mexico, USA) has been investigated by constant-wavelength single-crystal neutron diffraction at 20 K, single-crystal X-ray diffraction at 100 K and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The chemical composition based on ICP-AES analysis leads to the following chemical formula (calculated on the basis of 12 oxygen atoms):Ca(Na0.26K0.04Ca0.69)∑0.99M(Li0.29Mg0.03Fe0.023+Al1.78)∑2.12T(Al1.73Be0.16Si2.11)S4.00O12H2.53. The apparent excess of H is probably due to the fact that the fraction of H2O was assumed by difference to 100 wt.%, and slightly overestimated. On the basis of the previous experimental findings on Li,Be-bearing mica, X-ray (at 100 K) and neutron (at 20 K) structure refinements were performed in the space groupsCcandC2/c. The neutron structure refinement in the space groupCcoffers a view about the (Al,Be,Si)-tetrahedral ordering: the best fit of the refinement was reached with theT1 andT4 sites occupied by (Be + Al) andT2 andT3 fully occupied by Si. This leads to a final population ofT(Al1.88Be0.12Si2.00)∑4.00p.f.u., in reasonable agreement with the chemical analysis. The neutron refinement provides unambigous evidence of the occurrence of Li at theM1 site. The refined fraction of Li at theM1 site ranges between 0.27 and 0.29 a.p.f.u., in excellent agreement with the chemical analysis. The presence of Li, at least at a significant level, at theM2 (andM3) site can be ruled out, as a full site occupancy with the scattering length of Al was obtained. The location of the H sites and the complex hydrogen-bonding scheme are described. A comparison between the structure features of this Li,Be-mica and other brittle micas is carried out.

Intense Turquoise and Green Colors in Brownmillerite-Type Oxides Based on Mn5+ in Ba2In2–xMnxO5+x

Brownmillerite-type oxides Ba(2)In(2-x)Mn(x)O(5+x) (x = 0.1-0.7) have been prepared and characterized. Magnetic measurements indicate that manganese in as-prepared samples is substituting predominantly as Mn(5+) for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Electron paramagnetic resonance measurements indicate that this Mn(5+) is present in a highly distorted tetrahedral environment. Neutron diffraction structure refinements show that Mn(5+) occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ≥ 0.3 samples, neutron refinements show that the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x = 0.1) to green (x = 0.2, 0.3) or to dark green (x ≥ 0.4). Under reducing conditions, Mn(5+) is reduced to Mn(3+), and Ba(2)In(2-x)Mn(x)O(5+x) phases become black Ba(2)In(2-x)Mn(x)O(5) phases still with the brownmillerite structure.

A single-crystal neutron and X-ray diffraction study of elbaite

The crystal chemistry and crystal structure of gem-quality elbaite from the Pederneira mine, Sao Jose da Safira, Minas Gerais, Brazil, were investigated by electron microprobe analysis in wavelength dispersive mode, un-polarized Fourier transform infrared spectroscopy, single-crystal neutron and X-ray diffraction. The average chemical formula of the sample resulted X(Na0.73Ca0.06□0.21)Y(Al1.24Li1.02Fe0.41Mn0.18Mg0.07)ZAl6TSi6O18B(BO3)3V(OH)3W(OH0.46F0.54) and the chemical analysis showed an almost homogeneous distribution of the principal elements over the crystal, with significant amounts of Fe (wt% FeO ~ 3.1) and Mn (wt% MnO ~ 1.3) which substitute YAl. The infrared spectrum over the diagnostic range of fundamental hydroxyl stretching vibrations is characterized by three strong absorption bands at 3,591 (ν1), 3,562 (ν2), and 3,491 (ν3) cm−1, respectively. The neutron structure refinement showed: (a) the presence of boron at the B site only and (b) the absence (at a significant level) of aluminum at the T site, which appears to be fully occupied by silicon. Only one proton site was successfully located. The H bond configuration is described with O3 as “donor” and O5 as “acceptor.” The O3–H bond distance corrected for “riding motion” is 0.9834 A, and H···O5 = 2.238(7) A, O3···O5 = 3.152(4) A and O3–H···O5 = 157.7(5)°. The reasons of the positional disorder at the O1 and O2 sites, observed in both neutron and X-ray refinements, are discussed.

Response of intergrown microstructure to an electric field and its consequences in the lead-free piezoelectric bismuth sodium titanate

We investigate the R3c average structure and micro-structure of the ceramic Bi0.5Na0.5TiO3 (BNT) in situ under applied electric fields using diffraction techniques. Electron diffraction implies the presence of significant octahedral tilt twin disorder, corresponding to the existence of a fine scale intergrown microstructural (IGMS) ‘phase’ within the R3c rhombohedral average structure matrix. A careful neutron refinement suggests not only that the off-centre displacements of the cations relative to the oxygens in the R3c regions increases systematically on application of an electric field but also that the phase fraction of the IGMS regions increases systematically. The latter change in phase fraction on application of the electric field enhances the polar displacement of the cations relative to the oxygen anions and affects the overall strain response. These IGMS regions form local polar nano regions that are not correlated with one another, resulting in polarisation relaxation and strain behaviour observed in BNT-containing materials.

Thermal expansion and high temperature structure evolution of zoisite by single-crystal X-ray and neutron diffraction

The thermo-elastic behaviour and the temperature-induced structure evolution of a natural Fe-free zoisite have been investigated by in situ single-crystal X-ray and neutron diffraction. Neither discontinuities in volume expansion nor changes in symmetry have been observed up to 1,023 K. Zoisite shows a negative thermal expansion along [100] at T > 700 K, while a continuous positive expansion occurs on the (100) plane. Two different regimes in the anisotropic thermal behaviour of zoisite can be distinguished (i.e. at T < 700 K and T > 700 K), corresponding to an increase in the volumetric thermal expansion at T > 700 K. The structure evolution with temperature has been described by a series of X-ray and neutron refinements at different temperatures. In particular, the M(3) polyhedra show a significant octahedral flattening and expansion in the equatorial plane with T. All [SiO4] tetrahedra show a regularization with increasing T. The neutron refinements show no change in the configuration of the hydrogen bonding at least up to 873 K. The effects of the T-induced main deformation mechanisms on the anisotropic elastic behaviour of zoisite are discussed. A comparison with the thermal behaviour of epidote has been carried out.