Single-crystal Synchrotron X-ray Diffraction Research Articles

Crystal structure of di-chlorido-{2-methyl-2-[(pyridin-2-ylmeth-yl)amino]-propan-1-ol-κ3N,N',O}copper(II) from synchrotron data.

The title compound, [CuCl2(C10H16N2O)], has been synthesized and characterized by synchrotron single-crystal X-ray diffraction and FT-IR spectroscopy. The 2-methyl-2-[(pyridin-2-ylmeth-yl)amino]-propan-1-ol (mpmapOH) ligand, including pyridine, amine and hy-droxy groups, was synthesized by the reaction of 2-amino-2-methyl-propan-1-ol with pyridine-2-carbaldehyde and was characterized by NMR spectroscopy. In its CuII complex, the metal ion has a distorted square-pyramidal coordination geometry with two N and one O atom of the mpmapOH ligand and one chloride anion in the equatorial plane, and the second chloride in an axial position. The bond lengths involving the CuII ion range from 1.9881 (10) to 2.0409 (9) for the Cu-N and Cu-O bonds, and from 2.2448 (5) to 2.5014 (6) Å for the equatorial and axial Cu-Cl bonds, respectively. Inter-molecular hydrogen bonds (N-H⋯Cl and O-H⋯Cl) and face-to-face π-π inter-actions stabilize the mol-ecular structure and give rise to a two-dimensional supra-molecular structure extending parallel to (101).

Atomic structure and phason modes of the Sc-Zn icosahedral quasicrystal

The detailed atomic structure of the binary icosahedrali-ScZn7.33 quasicrystal has been investigated by means ofhigh-resolution synchrotron single-crystal Xraydiffraction and absolute scale measure ...

Carrier concentration dependence of structural disorder in thermoelectric Sn1-x Te.

SnTe is a promising thermoelectric and topological insulator material. Here, the presumably simple rock salt crystal structure of SnTe is studied comprehensively by means of high-resolution synchrotron single-crystal and powder X-ray diffraction from 20 to 800 K. Two samples with different carrier concentrations (sample A = high, sample B = low) have remarkably different atomic displacement parameters, especially at low temperatures. Both samples contain significant numbers of cation vacancies (1-2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400 K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram-Charlier parameters reveals marginal anharmonicity for sample A, whereas sample B exhibits anharmonic effects even at low temperature. For both samples, no indications are found of a low-temperature rhombohedral phase. Maximum entropy method (MEM) calculations are carried out, including nuclear-weighted X-ray MEM calculations (NXMEM). The atomic electron densities are spherical for sample A, whereas for sample B the Te electron density is elongated along the 〈100〉 direction, with the maximum being displaced from the lattice position at higher temperatures. Overall, the crystal structure of SnTe is found to be defective and sample-dependent, and therefore theoretical calculations of perfect rock salt structures are not expected to predict the properties of real materials.

Source assemblage types for cratonic diamonds from X-ray synchrotron diffraction

Three single crystals of clinopyroxene trapped within three different gem-quality diamonds from the Udachnaya kimberlite (Siberia, Russia) were analysed in situ by single-crystal synchrotron X-ray diffraction in order to obtain information on their chemical composition and infer source assemblage type. A non-destructive approach was used with high-energy (≈60keV; λ ≈0.206Å) at I15, the extreme-conditions beamline at Diamond Light Source. A dedicated protocol was used to center the mineral inclusions located deep inside the diamonds in the X-ray beam. Our results reveal that two of the inclusions can be associated with peridotitic paragenesis whereas the third is eclogitic. This study also demonstrates that this non-destructive experimental approach is extremely efficient in evaluating the origin of minerals trapped in their diamond hosts.

On the P-induced behavior of the zeolite phillipsite: an in situ single-crystal synchrotron X-ray diffraction study

The elastic behavior and the structural evolution at high pressure of a natural phillipsite have been investigated by in situ single-crystal X-ray diffraction up to 9.44 GPa, using a diamond anvil cell and the nominally penetrating P-transmitting fluid methanol:ethanol:water (16:3:1) mix. Although no phase transition was observed within the P-range investigated, two different compressional regimes occur. Between 0.0001 and 2.0 GPa, the refined elastic parameters, calculated by a second-order Birch–Murnaghan equation of state (BM-EoS) fit, are V 0 = 1005(1) A3, K 0 = 89(8) GPa for the unit-cell volume; a 0 = 9.914(7) A, K a = 81(12) GPa for the a-axis; b 0 = 14.201(9) A, K b = 50(5) GPa for the b-axis; and c 0 = 8.707(2) A, K c = 107(8) GPa for the c-axis (K a :K b :K c ~1.62:1:2.14). Between 2.0 and 9.4 GPa, a P-induced change in the configuration of H2O molecules, coupled with a change in the tilting mechanisms of the framework tetrahedra, gives rise to a second compressional regime, in which the phillipsite structure is softer if compared to the first compressional range. In the second compressional regime, the refined elastic parameters, calculated by a second-order BM-EoS fit, are V 0 = 1098 (7) A3, K 0 = 18.8(7) GPa for the unit-cell volume; a 0 = 10.07(3) A, K a = 30(2) GPa for the a-axis; b 0 = 14.8(1) A, K b = 11(1) GPa for the b-axis; and c 0 = 8.94(2) A, K c = 21(1) GPa for the c-axis (K a :K b :K c ~2.72:1:1.90). The evolution of the monoclinic β angle with pressure shows two distinct trends in the two compressional regimes: with a negative slope between 0.0001 and 2.0 GPa, and a positive slope between 2.0 and 9.4 GPa. The mechanisms, at the atomic scale, that govern the two compressional regimes of the phillipsite structure are described.

Compressional behavior of omphacite to 47 GPa

Omphacite is an important mineral component of eclogite. Single-crystal synchrotron X-ray diffraction data on natural (Ca, Na) (Mg, Fe, Al)Si2O6 omphacite have been collected at the Advanced Photon Source beamlines 13-BM-C and 13-ID-D up to 47 GPa at ambient temperature. Unit cell parameter and crystal structure refinements were carried out to constrain the isothermal equation of state and compression mechanism. The third-order Birch–Murnaghan equation of state (BM3) fit of all data gives V 0 = 423.9(3) A3, K T0 = 116(2) GPa and K T0′ = 4.3(2). These elastic parameters are consistent with the general trend of the diopside–jadeite join. The eight-coordinated polyhedra (M2 and M21) are the most compressible and contribute to majority of the unit cell compression, while the SiO4 tetrahedra (Si1 and Si2) behave as rigid structural units and are the most incompressible. Axial compressibilities are determined by fitting linearized BM3 equation of state to pressure dependences of unit cell parameters. Throughout the investigated pressure range, the b-axis is more compressible than the c-axis. The axial compressibility of the a-axis is the largest among the three axes at 0 GPa, yet it quickly drops to the smallest at pressures above 5 GPa, which is explained by the rotation of the stiffest major compression axis toward the a-axis with the increase in pressure.

Behavior of cesium cation in zeolite Y (FAU, Si/Al = 1.56) and their single-crystal structures, |Cs75−xNax|[Si117Al75O384]-FAU (x = 35 and 54)

To study the tendency of Cs+ exchange into zeolite Y (Si/Al = 1.56) dependence on Cs+ and Na+ concentration of aqueous solution during exchange, two single-crystals of fully dehydrated, Cs+-and Na+-exchanged zeolites Y were prepared by the flow method using a mixed ion-exchange solution whose CsNO3:NaNO3 mol ratios were 1:1 (crystal 1) and 1:100 (crystal 2), respectively, with a total concentration of 0.1 M, followed by vacuum dehydration at 723 K. Their crystals were determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd $$\overline{ 3}$$ m, respectively, and were refined to the final error indices R 1/wR 2 = 0.084/0.248 and 0.088/0.274 for crystals 1 and 2, respectively. In the structure of |Cs40Na35|[Si117Al75O384]-FAU (crystal 1), 40 Cs+ ions per unit cell occupy five different equipoints; 3, 3, 14, 9, and 11 are at sites I, II′, II, IIIa and IIIb, respectively, whereas, the remaining 35 Na+ ions occupy three different sites: 9, 11, and 15 are at sites I, I′, and II, respectively. In the structure of |Cs21Na54|[Si117Al75O384]-FAU (crystal 2), 21 Cs + ions per unit cell occupy three equipoints; 4, 6, and 11 are at sites II, IIIa, and IIIb, respectively. The residual 54 Na+ ions per unit cell are found at three different sites; 6, 20 and 28 are at sites I, I′, and II, respectively. The degrees of ion exchange are 53 and 28 % for crystals 1and 2, respectively. This result shows that the degree of Cs+ exchange decreased sharply by decreasing the initial Cs+ concentration and increasing the initial Na+ concentration in given ion-exchange solution.

Atomic structure and phason modes of the Sc-Zn icosahedral quasicrystal.

The detailed atomic structure of the binary icosahedral (i) ScZn7.33 quasicrystal has been investigated by means of high-resolution synchrotron single-crystal X-ray diffraction and absolute scale measurements of diffuse scattering. The average atomic structure has been solved using the measured Bragg intensity data based on a six-dimensional model that is isostructural to the i-YbCd5.7 one. The structure is described with a quasiperiodic packing of large Tsai-type rhombic triacontahedron clusters and double Friauf polyhedra (DFP), both resulting from a close-packing of a large (Sc) and a small (Zn) atom. The difference in chemical composition between i-ScZn7.33 and i-YbCd5.7 was found to lie in the icosahedron shell and the DFP where in i-ScZn7.33 chemical disorder occurs on the large atom sites, which induces a significant distortion to the structure units. The intensity in reciprocal space displays a substantial amount of diffuse scattering with anisotropic distribution, located around the strong Bragg peaks, that can be fully interpreted as resulting from phason fluctuations, with a ratio of the phason elastic constants K 2/K 1 = -0.53, i.e. close to a threefold instability limit. This induces a relatively large perpendicular (or phason) Debye-Waller factor, which explains the vanishing of 'high-Q perp' reflections.

High-temperature and high-pressure behavior of carbonates in the ternary diagram CaCO3-MgCO3-FeCO3

We report the thermal expansion and the compressibility of carbonates in the ternary compositional diagram CaCO 3 -MgCO 3 -FeCO 3 , determined by in situ X-ray powder and single-crystal diffraction. High-temperature experiments were performed by high-resolution X-ray synchrotron powder diffraction from ambient to decarbonation temperatures (25–850 °C). Single-crystal synchrotron X-ray diffraction experiments were performed in a variable pressure range (0–100 GPa), depending on the stability field of the rhombohedral structure at ambient temperature, which is a function of the carbonate composition. The thermal expansion increases from calcite, CaCO 3 , α 0 = 4.10(7) ×10 −5 K–1, to magnesite, MgCO 3 , α 0 = 7.04(2) ×10 −5 K −1 . In the magnesite-siderite (FeCO 3 ) join, the thermal expansion decreases as iron content increases, with an experimental value of α 0 = 6.44(4) ×10 −5 K −1 for siderite. The compressibility in the ternary join is higher (i.e., lower bulk modulus) in calcite and Mg-calcite [ K 0 = 77(3) GPa for Ca 0.91 Mg 0.06 Fe 0.03 (CO 3 )] than in magnesite, K 0 = 113(1) GPa, and siderite, K 0 = 125(1) GPa. The analysis of thermal expansion and compressibility variation in calcite-magnesite and calcite-iron-magnesite joins clearly shows that the structural changes associated to the order-disorder transitions [i.e., R 3 ¯ c calcite-type structure vs. R 3 ¯ CaMg(CO 3 ) 2 dolomite-type structure] do not affect significantly the thermal expansion and compressibility of carbonate. On the contrary, the chemical compositions of carbonates play a major role on their thermo-elastic properties. Finally, we use our P-V-T equation of state data to calculate the unit-cell volume of a natural ternary carbonate, and we compare the calculated volumes to experimental observations, measured in situ at elevated pressure and temperatures, using a multi-anvil device. The experimental and calculated data are in good agreement demonstrating that the equation of state here reported can describe the volume behavior with the accuracy needed, for example, for a direct chemical estimation of carbonates based on experimental unit-cell volume data of carbonates at high pressures and temperatures.

The Great Barrier Reef Expedition 1928–29: The crystal structure and occurrence of weddellite, ideally CaC2O4·2.5H2O, from the Low Isles, Queensland

Abstract“Envelope crystals” collected during The Great Barrier Reef Expedition in May 1929 have been studied using low-temperature synchrotron single-crystal X-ray diffraction. The crystals are shown to be weddellite, with the largest content of zeolitic water reported to date. A new H2O site has been located within the crystal structure. Study of the crystals show that the end-member formula for weddellite should be reported as CaC2O4·(2.5 – x)H2O, where 0≤x≤ 0.25, instead of CaC2O4·(2H2O or CaC2O4·(2 + x )H2O. This is also the first report of weddellite occurring in a coral reef.

AlPO4-5 zeolite at high pressure: Crystal–fluid interaction and elastic behavior

The high-pressure behavior of the aluminophosphate zeolite AlPO4-5 (AFI-topology) has been investigated by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell, using the “non-penetrating” silicone oil and the “penetrating” 16:3:1 methanol:ethanol:water (m.e.w.) mixture as P-transmitting fluids. The crystals used in this study showed diffraction patterns typical of an incommensurate modulated structure, with modulation vector q ≈ 0.37c*. Compression in silicone oil showed that AlPO4-5 is one of the softest zeolites reported so far, being its refined bulk modulus (KV0 = 1/βV) at ambient conditions only 13.2(11) GPa (βV = 0.076(6) GPa−1), with an elastic anisotropy Ka:Kc = 1.6. The high-pressure experiment performed using m.e.w. showed a significantly lower compressibility of AlPO4-5, if compared to the behavior in silicone oil, suggesting the occurrence of a P-induced intrusion of the fluid molecules into the zeolitic cavities. An indirect evidence for the molecules intrusion is provided by the comparative analysis of the structure deformation mechanisms at the atomic scale. When zeolite AlPO4-5 is compressed in m.e.w., a lower degree of distortion of the secondary building units is observed, with respect to the compression in silicone oil. Overall, this study suggests zeolite AlPO4-5 as a promising microporous material for the pressure-mediated intrusion and hyper-confinement of molecules into the structural micropores.

Disorder and defects are not intrinsic to boron carbide.

A unique combination of useful properties in boron-carbide, such as extreme hardness, excellent fracture toughness, a low density, a high melting point, thermoelectricity, semi-conducting behavior, catalytic activity and a remarkably good chemical stability, makes it an ideal material for a wide range of technological applications. Explaining these properties in terms of chemical bonding has remained a major challenge in boron chemistry. Here we report the synthesis of fully ordered, stoichiometric boron-carbide B13C2 by high-pressure–high-temperature techniques. Our experimental electron-density study using high-resolution single-crystal synchrotron X-ray diffraction data conclusively demonstrates that disorder and defects are not intrinsic to boron carbide, contrary to what was hitherto supposed. A detailed analysis of the electron density distribution reveals charge transfer between structural units in B13C2 and a new type of electron-deficient bond with formally unpaired electrons on the C–B–C group in B13C2. Unprecedented bonding features contribute to the fundamental chemistry and materials science of boron compounds that is of great interest for understanding structure-property relationships and development of novel functional materials.

PKU-20: A new silicogermanate constructed from sti and asv layers

A new silicogermanate (PKU-20) was hydrothermally synthesized using triethylisopropylammonium cation as the structure directing agent in the presence of fluoride. Its structure was determined from a combination of synchrotron single crystal X-ray diffraction and powder X-ray diffraction data. PKU-20 crystallizes in the monoclinic space group C2/m, with the lattice parameters of a = 18.5901(6) Å, b = 13.9118 (4) Å, c = 22.2614 (7) Å and β = 100.1514 (12)°. The framework of PKU-20 is constructed from an alternate stacking of sti and asv layers. The sti layer is exactly the same as that in the STI framework，while the asv layer is a new layer sliced off from the ASV framework parallel to the (112) plane. The take-out scheme of the layer is discussed on the basis of a composite building unit “D4R-lau-D4R”. PKU-20 possesses a two-dimensional channel system, where the 10-ring channels parallel to the [010] direction are intercrossed by 12-ring pockets along the [101] direction.

Competing Jahn-Teller distortions and hydrostatic pressure effects in the quasi-one-dimensional quantum ferromagnetCuAs2O4

${\mathrm{CuAs}}_{2}{\mathrm{O}}_{4}$ is a $S=1/2$ quasi-one-dimensional ribbon chain compound which orders ferromagnetically at 7.4 K under ambient conditions. ${\mathrm{CuAs}}_{2}{\mathrm{O}}_{4}$ features nearest- and next-nearest-neighbor spin-exchange interactions along the spin chains with a ratio $\ensuremath{\alpha}={J}_{\mathrm{nn}}/{J}_{\mathrm{nnn}}$ in close proximity to the quantum critical point at $\ensuremath{\alpha}=\ensuremath{-}4$. We apply hydrostatic pressure up to $\ensuremath{\sim}11.5$ GPa and examine the structural and magnetic properties using Raman spectroscopy, single-crystal synchrotron x-ray diffraction, and magnetic susceptibility measurements. External pressure severely reduces the axial Jahn-Teller elongations of the oxygen octahedra surrounding the ${\mathrm{Cu}}^{2+}$ cations and stabilizes the ferromagnetic ground state. At 9.2(2) GPa, we detect a structural phase transition leading to an increased twisting of the ${\mathrm{CuO}}_{2}$ ribbon chains and a large drop of the magnetic ordering temperature. Ab initio density functional theory calculations of the spin-exchange parameters, using the structural parameters as a function of pressure, support the experimental findings.

High-Pressure Crystal Structures of an Insensitive Energetic Crystal: 1,1-Diamino-2,2-dinitroethene

Understanding the insensitivity/stability of insensitive high explosive crystals requires detailed structural information at high pressures and high temperatures of interest. Synchrotron single-crystal X-ray diffraction experiments were used to determine the high-pressure structures of 1,1-diamino-2,2-dinitroethene (FOX-7), a prototypical insensitive high explosive. The phase transition around 4.5 GPa was investigated and the structures were determined at 4.27 GPa (α′ phase) and 5.9 GPa (e phase). The α′ phase (monoclinic, P21/n), structurally indistinguishable from the ambient α phase, transforms to the new e phase (triclinic, P1). The most notable features of the e phase, compared to those of the α′ phase, are formation of planar layers and flattening of molecules. Density functional theory (DFT-D2) calculations complemented the experimental results. The results presented here are important for understanding the molecular and crystalline attributes governing the high-pressure insensitivity/stability of ...

High thermal durability of a layered Cs4CoII[WV(CN)8]Cl3framework: crystallographic and133Cs NMR spectroscopic studies

Temperature-variable single-crystal synchrotron X-ray diffraction and 133Cs NMR studies were performed for a cyanido-bridged layered metamagnet, Cs4CoII[WV(CN)8]Cl3 (1), to elucidate the origin of its unusually high thermal durability up to ca. 523 K (250 °C). 1 reveals a layered structure composed of negatively charged two-dimensional cyanido-bridged Co–W layers ({CoII[WV(CN)8]Cl3}4−) and cesium ions between and within the layers, where there is a lack of both non-coordinated and coordinated solvent molecules that is an important and characteristic feature of the crystal structure. The cesium ions are surrounded by chlorides and cyanides of the coordination layers, and some Cs+⋯Cl−, Cs+⋯N, and Cs+⋯C distances are shorter than the estimated sums of an ionic radius of Cs+ and ionic or van der Waals radii of Cl−/CN−. These short distances suggest strong interactions between Cs+ and Cl−/CN−, which stabilize the crystal structure by connecting the layers. The temperature-dependent single-crystal structural analyses show that there is no significant structural change upon heating up to 473 K, and indicate that the short distances between Cs+ and Cl−/CN− still remain. Thus, the high thermal durability of 1 is due to not only the absence of solvent molecules but also the existence of strong Cs+⋯Cl− and Cs+⋯−NC interactions. The 133Cs NMR spectrum shows four peaks assignable to the four independent Cs+ ions in the crystal structure. The observed peaks were located in high ppm owing to the hyperfine interactions between the nuclear spin of Cs and electron spins from the magnetic centers CoII and WV. As temperature increased, the peaks moved to low ppm, and the shifts of the peaks are expressed by the Curie–Weiss law indicating no large structural changes around Cs+ ions, that is, the Cs+ ions are fixed at the same positions. This proves the presence of strong interactions involving Cs+ and terminal ligands of cyanido-bridged layers of 1 being responsible for its high thermal stability.

Water-triggered macroscopic structural transformation of a metal–organic framework

A millimeter-long metal–organic framework fiber has been successfully constructed. The fiber structure was unexpectedly found via the solvent vaporization process of a dispersion of a Zn-based MOF crystal. Detailed experiments and structural analysis indicate that a humid environment is a key to the growth of the fiber structure, inducing drastic transformation of the MOF crystal morphology from the nanoscopic to macroscopic scale. Synchrotron X-ray single-crystal diffraction analysis reveals that the complete crystal transformation from the Zn-MOF sheet to Zn-MOF fiber is accompanied by water coordination and ligand rearrangement. The highly anisotropic MOF fiber is easily converted to an electro-conductive fiber while maintaining the long-fiber morphology.

Theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena

AbstractPre-martensitic phenomena, also called martensite precursor effects, have been known for decades while yet remain outstanding issues. This paper addresses pre-martensitic phenomena from new theoretical and experimental perspectives. A statistical mechanics-based Grüneisen-type phonon theory is developed. On the basis of deformation-dependent incompletely softened low-energy phonons, the theory predicts a lattice instability and pre-martensitic transition into elastic-phonon domains via ‘phonon spinodal decomposition.’ The phase transition lifts phonon degeneracy in cubic crystal and has a nature of phonon pseudo-Jahn–Teller lattice instability. The theory and notion of phonon domains consistently explain the ubiquitous pre-martensitic anomalies as natural consequences of incomplete phonon softening. The phonon domains are characterised by broken dynamic symmetry of lattice vibrations and deform through internal phonon relaxation in response to stress (a particular case of Le Chatelier’s principle), leading to previously unexplored new domain phenomenon. Experimental evidence of phonon domains is obtained by in situ three-dimensional phonon diffuse scattering and Bragg reflection using high-energy synchrotron X-ray single-crystal diffraction, which observes exotic domain phenomenon fundamentally different from usual ferroelastic domain switching phenomenon. In light of the theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena, currently existing alternative opinions on martensitic precursor phenomena are revisited.

High-pressure synthesis of skiagite-majorite garnet and investigation of its crystal structure

Skiagite-rich garnet was synthesized as single crystals at 9.5 GPa and 1100 °C using a multi-anvil apparatus. The crystal structure [cubic, space group Ia 3 d , a = 11.7511(2) A, V = 1622.69(5) A3, D calc = 4.4931 g/cm3] was investigated using single-crystal synchrotron X-ray diffraction. Synchrotron Mossbauer source spectroscopy revealed that Fe2+ and Fe3+ predominantly occupy dodecahedral (X) and octahedral (Y) sites, respectively, as expected for the garnet structure, and confirmed independently using nuclear forward scattering. Single-crystal X-ray diffraction suggests the structural formula of the skiagite-rich garnet to be Fe32+(Fe2+0.234(2)Fe3+1.532(1)Si4+0.234(2))(SiO4)3, in agreement with electron microprobe chemical analysis. The formula is consistent with X-ray absorption near-edge structure spectra. The occurrence of Si and Fe2+ in the octahedral Y-site indicates the synthesized garnet to be a solid solution of end-member skiagite with ~23 mol% of the Fe-majorite end-member Fe32+(Fe2+Si4+)(SiO4)3.

The MnCO3-II high-pressure polymorph of rhodocrosite

We investigated the behavior of MnCO3 in the pressure range 0–50 GPa and ambient temperature by synchrotron X-ray single-crystal diffraction technique. MnCO3 maintains the calcite-type structure ( R 3 c symmetry) up to 44 GPa. Above this pressure we observed a phase transition. The high-pressure phase, MnCO3-II, is triclinic, with cell parameters a = 2.928(2), b = 4.816(4), c = 5.545(4) A, α = 101.71(6)°, β = 94.99(6)°, γ = 89.90(6)°, and V = 76.28(10) A3 at 46.8 GPa. The structure is solved with the charge flipping algorithm. MnCO3-II is isostructural with CaCO3-VI. The density increase on phase transition is 4.4%. The occurrence of CaCO3-VI structure in MnCO3 composition indicates that CaCO3-VI structure is also adopted by carbonates with cations smaller than calcium.