High-temperature Single-crystal X-ray Diffraction Research Articles

Growth, crystal structure and IR luminescence of KSrY1-xErx(BO3)2.

A series of novel KSrY1-xErx(BO3)2 (x = 0-1) phosphors that emit near-infrared radiation was synthesized using solid-state methods. Pure Y and Er crystals were grown using a KF flux via the top-seeded solution growth technique. In situ high-temperature single crystal X-ray diffraction, Raman spectroscopy and DFT calculations were used for characterization. Within the series, a polymorphic phase transition from space group P21/m to R3m was discovered between 550 and 600°C. The concentration dependence of the luminescence intensity was measured for the samples. A strong emission of Er3+ electron transition 4I13/2 → 4I15/2 was detected within the 1529-1549 nm range, with the maximum observed for the KSrY0.4Er0.6(BO3)2 composition.

Thermal behaviour of filatovite – a rare aluminoarsenate mineral of the feldspar group

AbstractThe high-temperature behaviour of a feldspar-group mineral, filatovite (with the simplified formula: K(Al,Zn)2(As,Si)2O8), in which the Al:As:Si ratio is close to 2:1:1), was studied by in situ high-temperature single-crystal X-ray diffraction and in situ high-temperature (hot stage) Raman spectroscopy up to 600°C. In the temperature range studied (25–600°С) filatovite does not undergo any phase transition, whereas at 800°C it decomposes to X-ray amorphous phase(s). The evolution of 12 main Raman bands was traced during heating, which indicates a gradual change in the crystal structure. The thermal expansion coefficients of filatovite demonstrate a sharply anisotropic character of thermal expansion: the maximal expansion is close to the a axis (α11 = 17.7(1) × 10–6 °C–1), whereas along the b and c axes the thermal expansion coefficients are close to zero. Such behaviour is typical for minerals with a similar crystal structure topology; it indicates the dominant role of structure geometry in the thermal behaviour of the mineral.

Novel High-Temperature Modification of Belomarinaite KNaSO4: Crystal Structure and Thermal Order–Disorder Phase Transitions

Belomarinaite KNaSO4 (space group P3m1, a = 5.6072(3), c = 7.1781(4) Å and Z = 2) has been studied by high-temperature single crystal X-ray diffraction. The K and Na atoms are disordered, and M1(1c) and M2(1b) sites merge into an M1(2d)(K50%/Na50%) site with increasing temperature, and novel translationsgleiche phase transition of belomarinaite KNaSO4 (P3m1 → P-3m1) was revealed at 123 °C. Then, the temperature increase leads to phase transition of P3¯m1-polymorph of KNaSO4 to α-KNaSO4 (P63/mmc) at 446 °C, and sodium and potassium atoms in K1(1a) and Na1(1b) sites merged into an M4(2a)(K50%/Na50%) site. Crystal structures of KNaSO4 were refined at 300, 500 and 750 °C to R1 = 0.057, 0.056 and 0.048, respectively. The BVS maps and bond-valence energy landscape (BVEL) have been calculated, and the probability of the Na+ migration has been predicted using structural data for belomarinaite at 25 °C. The Na+ ion migration can occur at 2.14 eV in the ab plane and 2.80 eV along the c axis at 25 °C.

High-temperature behaviour of fedorite, Na2.5(Ca4.5Na2.5)[Si16O38]F2⋅2.8H2O, from the Murun Alkaline Complex, Russia

AbstractThe thermal behaviour of fedorite from the Murun massif, Russia, has been investigated by means of electron probe microanalysis (EPMA), differential thermal analysis (DTA), thermogravimetry (TG), in situ high-temperature single-crystal X-ray diffraction (HT-SCXRD), ex situ high-temperature Fourier-transform infrared spectroscopy (HT-FTIR). The empirical chemical formula of the sample of fedorite studied is: (Na1.56K0.72Sr0.12)Σ2.40(Ca4.42Na2.54Mn0.02Fe0.01Mg0.01)Σ7.00(Si15.98Al0.02)Σ16.00(F1.92Cl0.09)Σ2.01(O37.93OH0.07)Σ38.00⋅2.8H2O. The TG curve provides a total mass decrease of ~5.5%, associated with dehydration and defluorination processes from 25 to 1050°C. Fedorite crystallises in space group P$\bar{1}$ and has: a = 9.6458(2), b = 9.6521(2), c = 12.6202(4) Å, α = 102.458(2), β = 96.2250(10), γ = 119.9020(10)° and cell volume, V = 961.69(5) Å3. The HT-SCXRD was carried out in air in the 25–600°C range. Overall, a continuous expansion of the unit-cell volume was observed although the c cell dimension slightly decreases in the explored temperature range. Structure refinements indicated that the mineral undergoes a dehydration process with the loss of most of the interlayer H2O from 25 to 300°C. The HT-FTIR spectra confirmed that fedorite progressively dehydrates until 700°C.

Vergasovaite to cupromolybdite topotactic transformation with crystal shape preservation

Abstract Thermal behavior of vergasovaite, ideally Cu3O(SO4)(MoO4), and its synthetic analog has been studied by high-temperature single-crystal X-ray diffraction in the temperature range of 300–1100 K. According to EMPA results, the empirical formulas are (Cu2.36Zn0.61)Σ2.97O[(Mo0.91S0.08V0.04)Σ1.03O4](SO4) for vergasovaite and Cu2.97O[(Mo0.92S0.09)Σ1.01O4](SO4) for its synthetic analog. The mineral is stable up to 950 ± 15 K; at 975 K, the unit-cell parameters and volume increase abruptly due to topotactic transformation of vergasovaite to cupromolybdite, Cu3O(MoO4)2. The transformation is accompanied by loss of sulfur (and excess copper) without destruction of the crystal. The thermal expansion of the vergasovaite structure is strongly anisotropic, being minimal along the [O2Cu6]8+ chains comprised of vertex-sharing OCu4 tetrahedra. This peculiar thermal behavior can be explained by the anisotropy of bond-length evolution in the Cu1O6 and Cu3O6 octahedra and the flexibility of the S-O-Cu and Mo-O-Cu bond angles. Synthetic Zn- and V-free analogs demonstrate negative thermal expansion at 425–625 K and melt at as low temperature as 700 K with no indication of transformation or recrystallization at least below 1200 K. The topotactic transformation observed in vergasovaite may have important implications for the design of novel materials and for understanding the alteration processes of copper minerals.

Re-investigation of ‘minasgeraisite-(Y)’ from the Jaguaraçu pegmatite, Brazil and high-temperature crystal chemistry of gadolinite-supergroup minerals

AbstractThe chemical composition (including B, Be and Li), the Raman spectrum and the crystal-structure evolution (at the temperature range 27–1000°C) of a Mn-bearing, Bi-rich gadolinite-subgroup mineral from the Jaguaraçu Pegmatite, Brazil (type-locality of minasgeraisite-(Y)) was studied. Elemental mapping revealed that the crystal investigated has complex chemical zonation with various Bi (~8–24 wt.% Bi2O3), Ca (~8–10 wt.% CaO) and Y (~11–17 wt.% Y2O3) content. The sample investigated has all the specific features of the chemical composition of minasgeraisite-(Y), except Ca excess and, thus, should be considered as hingganite-(Y). The Raman spectrum of the sample under study has bands at 140, 179, 243, 350, 446, 519, 559, 625, 902, 973, 3224, 3353, 3532 and 3763 cm–1, and is similar to that of hingganite-(Y) / -(Nd). Crystal-structure refinement confirmed that the crystal in question should be considered as hingganite-(Y) and is in line with the previously obtained data on gadolinite-subgroup minerals from the Jaguaraçu Pegmatite. High-temperature single-crystal X-ray diffraction studies revealed that the mineral starts to decompose above 800°C. We can conclude that beryllosilicates are most stable at high-temperature conditions within the gadolinite supergroup and that species with a higher M-site occupancy have higher stability upon heating.

High-pressure and high-temperature single-crystal X-ray diffraction of complex garnet solid solutions up to 16\xa0GPa and 823\xa0K

P–V–T equations of state (EoS) of synthetic garnet solid solutions with ternary grossular–almandine–pyrope compositions relevant to the Earth’s upper mantle have been determined in order to examine whether garnet properties can be accurately interpolated from those of the end-members. Volumes have been measured as a function of pressure using single-crystal X-ray diffraction measurements performed inside a diamond anvil cell. Isothermal bulk moduli and first pressure derivatives were obtained by fitting the P–V data using a third-order Birch–Murnaghan equation of state. Two nominally eclogitic garnets (Prp47Alm19Grs31And3 and Prp53Alm19Grs18And3Sps7) were found to have isothermal bulk moduli (KT0) and pressure derivatives (K′T0) of 170(3) GPa, 4.1 (4) and 173 (2) GPa, 3.8 (5), respectively. KT0 and K′T0 for an almandine-rich garnet (Prp26Alm63Grs6And5) were found to be 175 (3) GPa and 3.7 (7), respectively. High-temperature compression experiments at 703 K and 823 K were carried out on sample Prp47Alm19Grs31And3, resulting in the high-temperature EoS term (∂KT/∂T)P = − 0.025 (6) and a thermal expansion (α0) of 2.86 (4) × 10−5 K−1. The results imply that the bulk moduli of aluminous garnet solid solutions stable at upper mantle conditions can be deduced from the properties of the end-members with minimal uncertainty. We show that the difference in the bulk sound velocity determined for a multicomponent eclogitic garnet composition and obtained for the same composition from the end-member properties is better than 0.5% for pressures and temperatures corresponding to Earth’s upper mantle.

Structure and Properties of Cs7(H4PO4)(H2PO4)8: A New Superprotonic Solid Acid Featuring the Unusual Polycation (H4PO4).

We report the discovery of a new superprotonic compound, Cs7(H4PO4)(H2PO4)8, or CPP, which forms at elevated temperatures from the reaction of CsH2PO4 and CsH5(PO4)2. The structure, solved using high-temperature single-crystal X-ray diffraction and confirmed by high-temperature 31P NMR spectroscopy, crystallizes in space group Pm3̅n and has a lattice constant of 20.1994(9) Å at 130 °C. The unit cell resembles a 4 × 4 × 4 superstructure of superprotonic CsH2PO4, but features an extraordinary chemical moiety, rotationally disordered H4PO4+ cations, which periodically occupy one of every eight cation sites. The influence of this remarkable cation on the structure, thermodynamics, and proton transport properties of the CPP phase is discussed. Notably, CPP forms at a temperature of 90 °C, much lower than the superprotonic transition temperature of 228 °C of CsH2PO4, and the compound does not appear to have an ordered, low-temperature form. Under nominally dry conditions, the material is stable against dehydration to ∼151 °C, and this results in a particularly wide region of stability of a superprotonic material in the absence of active humidification. The conductivity of Cs7(H4PO4)(H2PO4)8 is moderate, 5.8 × 10-4 S cm-1 at 140 °C, but appears nevertheless facilitated by polyanion (H2PO4-) group reorientation.

Dehydration behavior of a natural hydrated Ca-chabazite studied by in-situ high-temperature single-crystal X-ray diffraction

Abstract Dehydration behavior of a natural hydrated Ca-chabazite with composition (Ca1.57Na0.49)(Al3.39Si8.55)O24·12.47H2O has been studied by the structure analyses based on the in-situ high-temperature single-crystal X-ray diffraction data at 100, 200 and 250 °C. Among the five water sites (OW1–OW5) and four exchangeable-cation sites (Ca1–Ca4) found previously in the room-temperature (RT) structure, the present structure analyses revealed the presence of OW1, OW4, Ca3 sites at 100 °C and of OW3, Ca3, Ca4 sites at 200 and 250 °C. The presence of the new extraframework sites was also revealed: OW2’ site at 100 °C and Ca5 site at 200 and 250 °C. From the temperature dependence of the occupation for these extraframework sites, the dehydration and cation migration processes are summarized and rationalized as follows. With heating up to 100 °C, OW3 water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, Ca1 and Ca2 cations lose the large amounts of water molecules as ligands (OW1 and OW5 for Ca1, OW4 for Ca2); this brings about the simultaneous migration of both cations to adjacent Ca3 site, surrounded by more ligands. With further heating up to 250 °C, OW2’ water molecules, having no bonds to exchangeable cations, are completely desorbed. Simultaneously, most of the Ca3 cations migrate to adjacent Ca4 and Ca5 sites by losing the large amounts of water molecules as ligands (OW1, OW4). A part of the water molecules desorbed in the temperature range of 100–250 °C migrate to OW3 site to be bonded to the remaining Ca3 cations. The peculiar temperature dependence of Si/Al−O1, −O2 and −O3 bond lengths in the framework structure can be explained in terms of their bond-valence variations due to such dehydration and cation migration processes. On the other hand, the variation of Si/Al−O4 bond length is dominated by thermal expansion effect rather than the bond valence effect.

Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases

Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565 K) β ↔ (at 637 K) α′ ↔ (at 651 K) α sequence of reversible first-order phase transitions are studied by high-temperature single-crystal X-ray diffraction, high-temperature X-ray powder diffraction, differential scanning calorimetry and impedance spectroscopy. Out of these phases, the structure of γ-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P{\overline 1}. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order–disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637–651 K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = −81 × 10−6 K−1.

High-temperature single-crystal X-ray diffraction study of tetragonal and cubic perovskite-type PbTiO3 phases.

A high-temperature single-crystal X-ray diffraction study of a synthetic PbTiO3 perovskite was carried out over the wide temperature range 298-928 K. A transition from a tetragonal (P4mm) to a cubic (Pm \bar 3 m) phase has been revealed near 753 K. In the non-centrosymmetric P4mm symmetry group, the difference in relative displacement between Pb and O along the c-axis is much larger than that between Ti and O. The Pb and Ti cations contribute sufficiently to polarization being shifted in the opposite direction compared with the shift of O atoms. Deviation from the linear changes in Debye-Waller factors and bonding distances in the tetragonal phases can be interpreted as a precursor phenomenon before the phase transition. Disturbance of the temperature factor Ueq for O is observed in the vicinity of the transition point, while Ueq values for Pb and Ti are continuously changing with increasing temperature. The O site includes the clear configurational disorder in the cubic phase. The polar local positional distortions remain in the cubic phase and are regarded as the cause of the paraelectricity. Estimated values of the Debye temperature ΘD for Pb and Ti are 154 and 467 K in the tetragonal phase and decrease 22% in the high-temperature phase. Effective potentials for Pb and Ti change significantly and become soft after the phase transition.

Origin of the Higher-Tc Phase in the KxFe2−ySe2 System

Single crystals of K$_x$Fe$_{2-y}$Se$_2$ are prepared by quenching at various temperatures. The crystals obtained at higher quenching temperatures have a surface morphology with mesh-like texture. They show a sharp superconducting transition at $T_\rm{c}$ ~32 K with a large shielding volume fraction. On the other hand, the crystals prepared without quenching show an onset superconducting transition at ~44 K and a zero resistivity around ~33 K, and they possess island-like regions on the surface with a larger amount of Fe incorporation. In-situ high-temperature single crystal X-ray diffraction measurements tell us the Fe-vacancy ordered phase is generated at a temperature region around 270 {\deg}C via iron diffusion. The creation of this Fe-vacancy ordered phase may become a driving force of the growth of the higher $T_\rm{c}$ phase. The superconductivity at ~44 K is attributed to a metallic phase with no Fe-vacancy.

A first-order phase transition in zektzerite, NaLiZrSi6O15: high-temperature single-crystal X-ray diffraction study

The crystal structure of zektzerite, NaLiZrSi6O15, orthorhombic (space group Cmca), consists of six-tetrahedral-repeat corrugated double silicate chains connected into a three-dimensional framework by edge-sharing Li tetrahedral–Zr octahedral chains. At room temperature, the Na+ ions occur at the Na(1) site located within the cylindrical channels created by the corrugation of the silicate double chains. The second sodium site, Na(2), occurs above and below the Zr octahedron and is nominally vacant at room temperature. Above 773 K, vacancies are created at the Na(1) site due to the migration of Na+ ions from the Na(1) to the Na(2) site. Crystal structure refinements at 300, 773, 973, 1223 and 1323 K indicate a progressive increase in the sodium occupancy at the Na(2) site from 0.04 at 773 K to 0.35 at 1323 K. Induced by increasing Na+-ion migration, a first-order high-temperature phase transition occurs above 1123 K as evidenced by a break in the slopes of the thermal expansion curves of the unit cell dimensions and a substantial increase in the electrical conductivity. The unit cell measurements as a function of temperature show marked hysteresis effects during the heating and cooling cycles.

Temperature dependence of crystal structure of CaGeO3high-pressure perovskite phase and experimental determination of its Debye temperatures studied by low- and high-temperature single-crystal X-ray diffraction

Single-crystal X-ray diffraction study of CaGeO 3 perovskite has been conducted over the temperature range of 98 to 1048 K. The crystal begins to deteriorate at a temperature above about 900 K and completely amorphizes by 980 K. The diffraction-intensity distribution and the structure refinements indicate that the Pbnm structure is kept until the occurrence of amorphization. The obtained unit-cell parameters, unit-cell volumes, bond lengths, and displacement parameters increase monotonously with increasing temperature. Thus, no evidence for the existence of the Cmcm high-temperature phase, previously suggested above 520 K, is observed. The Ge-O bond lengths show much smaller thermal expansions than the Ca-O bond lengths; the former ranges between 0.42(2) × 10 −5 K −1 and 0.57(2) × 10 −5 K −1 , and the latter between 1.58(4) × 10 −5 K −1 and 3.96(6) × 10 −5 K −1 . The Debye temperatures and static disorder components for each constituent atom were determined by applying the Debye model to the temperature dependence of mean square displacements (MSDs) of the atoms. Consequently, no significant static disorder components can be detected in each atom. The Debye temperatures averaged over all directions, obtained from the Debye model fitting to U eq , yield the harmonic one particle potential coefficients of 4.76(2) eVÅ −2 for Ca, 11.0(1) eVÅ −2 for Ge, 5.02(2) eVÅ −2 for O1, and 5.33(5) eVÅ −2 for O2. These values become larger in order of Ca < O1 < O2 << Ge, which shows that the one particle potential of Ge is much narrower than that of Ca. This relationship between Ca and Ge is consistent reasonably with bonding stiffness expected from the thermal expansion coefficients of the bond lengths. The anisotropies of MSDs are remarkable in O1 and O2 atoms as a consequence of the strong interaction with adjacent Ge atoms, forming the rigid bonds with these O atoms. In comparison of the three Pbnm orthorhombic perovskites of CaGeO 3 , CaTiO 3 , and MgSiO 3 , all of these have the BO 6 octahedra more rigid than the AO 12 polyhedra (A = Ca or Mg; B = Ge, Ti, or Si) and the tilt angles of BO 6 octahedra are the largest in MgSiO 3 perovskite. These observations indicate that if MgSiO 3 perovskite under high pressures undergoes the same sequence of the high-temperature phase transitions as CaTiO 3 perovskite, the phase boundaries have positive Clapeyron slopes and the phase transition temperatures should become further much higher than those (1512 K for the Pbnm to I4/mcm transition and 1635 K for the I4/mcm to Pm3̅m transition) observed in CaTiO 3 perovskite at ambient pressure. This leads to the conclusion that the high-temperature phase transition to a perovskite phase with different symmetry under high pressures previously suggested in MgSiO 3 perovskite is unlikely.

In-situ high-temperature emissivity spectra and thermal expansion of C2/c pyroxenes: Implications for the surface of Mercury

This work was carried out within the framework of the European Space Agency and Japanese Aerospace Exploration Agency BepiColombo space mission to Mercury and intends to provide valid tools for the interpretation of spectra acquired by the MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board of BepiColombo. Two C 2/ c augitic pyroxenes, with different Mg/Fe ratios and constant Ca contents, were investigated by in situ high-temperature thermal infrared spectroscopy and in situ high-temperature single-crystal X-ray diffraction up to temperatures of about 750 and 770 K, respectively. The emissivity spectra of the two samples show similar band center shifts of the main three bands toward lower wavenumbers with increasing temperature. In detail, with increasing temperature bands 1 and 2 of both samples show a much stronger shift with respect to band 3, which remains almost unchanged. Our results indicate that the center positions of bands 1 and 2 are strong functions of the temperature, whereas the center position of band 3 is a strong function of the Mg# [with Mg# = Mg/ (Mg + Fe 2+ ) atomic ratio]. The analysis of the thermal behavior gives similar thermal expansion volume coefficients, α V, for the Mg-rich and Fe-rich samples, with α V = 2.72(8) and 2.72(7) × 10 -5 K -1 , respectively, using the Berman (1988) equation. This correspondence totally explains the band center shifts similarity between the two samples. Our data suggest that MERTIS spectra will be able to provide indications of C 2/ c augitic pyroxene Mg# and will allow a correct interpretation that is independent on the spectra acquisition temperature.

Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study--Discussion

Article Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study—Discussion was published on April 1, 2013 in the journal American Mineralogist (volume 98, issue 4).

Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study--Reply

Geiger (2013) argues that the large mean square displacement (MSD) of Mg in the dodecahedral site in pyrope, Mg3Al2Si3O12, originates from the dynamic disorder of Mg due to its large anisotropic thermal vibration, including anharmonic contribution, and refutes our findings that the Mg static disorder is present. His argument is based on the diffraction, spectroscopic, thermodynamic, and computational studies made mainly by him and his co-workers. However, these investigations only suggested the large anisotropy of the Mg thermal vibration, and do not necessarily prove that there is no Mg static disorder. What we must obtain is conclusive experimental proof as to whether the Mg static disorder exists or not. In the present paper, a reply to his comment (Geiger 2013), we discuss the validity of our findings (Nakatsuka et al. 2011). A “direct” method to confirm the Mg static disorder is to observe a splitting of electron density maxima around the Mg site form the structure analysis. As shown in Nakatsuka et al. (2011), we successfully observed the residual electron density maxima around the Mg site (24 c site) in the difference Fourier map at a low temperature of 97 K. In addition, the refinements assigning Mg to these electron density maxima (split-atom model refinements) yielded “the Mg positional parameters deviating significantly from the 24 c site” and provided “the atomic displacement parameters (ADPs) of Mg significantly smaller than the ones in the normal-model refinements,” assigning Mg to the 24 c site, as a consequent of the removal of the static disorder components. These are direct evidence for the presence of the Mg static disorder. In the anharmonic refinements, several of the higher-rank tensor coefficients of Mg, Al, Si, and O atoms deviated significantly from …

Static disorders of atoms and experimental determination of Debye temperature in pyrope: Low- and high-temperature single-crystal X-ray diffraction study

Geiger (2013) argues that the large mean square displacement (MSD) of Mg in the dodecahedral site in pyrope, Mg3Al2Si3O12, originates from the dynamic disorder of Mg due to its large anisotropic thermal vibration, including anharmonic contribution, and refutes our findings that the Mg static disorder is present. His argument is based on the diffraction, spectroscopic, thermodynamic, and computational studies made mainly by him and his co-workers. However, these investigations only suggested the large anisotropy of the Mg thermal vibration, and do not necessarily prove that there is no Mg static disorder. What we must obtain is conclusive experimental proof as to whether the Mg static disorder exists or not. In the present paper, a reply to his comment (Geiger 2013), we discuss the validity of our findings (Nakatsuka et al. 2011). A “direct” method to confirm the Mg static disorder is to observe a splitting of electron density maxima around the Mg site form the structure analysis. As shown in Nakatsuka et al. (2011), we successfully observed the residual electron density maxima around the Mg site (24 c site) in the difference Fourier map at a low temperature of 97 K. In addition, the refinements assigning Mg to these electron density maxima (split-atom model refinements) yielded “the Mg positional parameters deviating significantly from the 24 c site” and provided “the atomic displacement parameters (ADPs) of Mg significantly smaller than the ones in the normal-model refinements,” assigning Mg to the 24 c site, as a consequent of the removal of the static disorder components. These are direct evidence for the presence of the Mg static disorder. In the anharmonic refinements, several of the higher-rank tensor coefficients of Mg, Al, Si, and O atoms deviated significantly from …

Fast ion conduction character and ionic phase-transition in silver sulfosalts: The case of fettelite [Ag6As2S7][Ag10HgAs2S8

The mineral fettelite, [Ag 6 As 2 S 7 ][Ag 10 HgAs 2 S 8 ], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag 6 As 2 S 7 ] 2- and a second module layer (labeled B) with composition [Ag 10 HgAs 2 S 8 ] 2+ . Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chañarcillo, Copiapó Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P3̄m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.

HT P21/c–C2/c phase transition and kinetics of Fe2+–Mg order–disorder of an Fe-poor pigeonite: implications for the cooling history of ureilites

A natural Ca-poor pigeonite (Wo6En76Fs18) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P21/c, was annealed up to 1,093°C to induce a phase transition from P21/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P21/c phase) showed a displacive phase transition P21/c to C2/c at a transition temperature T Tr = 944°C, first order in character. The Fe–Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in a range of temperatures between the closure temperature of the Fe–Mg exchange reaction and the transition temperature. Isothermal disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression of ln k D versus 1/T yielded the following equation: $$ \ln \,k_{\text{D}} = - 3717( \pm 416)/T(K) + 1.290( \pm 0.378);\quad (R^{2} = 0.988) $$ . The closure temperature (T c) calculated using this equation was ∼740(±30)°C. Analysis of the kinetic data carried out taking into account the e.s.d.'s of the atomic fractions used to define the Fe–Mg degree of order, performed according to Mueller’s model, allowed us to retrieve the disordering rate constants C 0 K dis + for all three temperatures yielding the following Arrhenius relation: $$ \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = \ln \,K_{0} - Q/(RT) = 20.99( \pm 3.74) - 26406( \pm 4165)/T(K);\quad (R^{2} = 0.988) $$ . An activation energy of 52.5(±4) kcal/mol for the Fe–Mg exchange process was obtained. The above relation was used to calculate the following Arrhenius relation modified as a function of X Fe (in the range of X Fe = 0.20–0.50): $$ \ln \left( {C_{0} K_{\text{dis}}^{ + } } \right) = (21.185 - 1.47X_{\text{Fe}} ) - {\frac{{(27267 - 4170X_{\text{Fe}} )}}{T(K)}} $$ . The cooling time constant, η = 6 × 10−1 K−1 year−1 calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late cooling history of the ureilite parent body after impact excavation.