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Globally, borax is one of the most important borate minerals, both industrially and from a technological viewpoint. Despite its importance, there have been only a few reports on the structural changes of borax upon heating. In this study, we have investigated the thermal behavior and structural characteristics of borax using thermal analysis, in situ high-temperature synchrotron X-ray diffraction, in situ variable-temperature single-crystal X-ray diffraction, and quantum chemical calculations. Differential scanning calorimetry (DSC) curve showed a large endothermic peak at 349 K corresponding to the dehydration reaction of borax. X-ray diffraction (XRD) pattern remained almost unchanged up to 353 K. Above 363 K, the XRD peaks gradually became less intense until they disappeared at 403 K. The unit-cell volume continuously increased with increasing temperature and became constant just before its phase transition to tincalconite. The volumetric thermal expansion coefficient between 113 and 323 K was 7.84 × 10–5K–1. The aand clattice parameters exhibited a slight increase trend with increasing temperature, whereas the blattice parameter increased significantly. Therefore, the thermal expansibilities followed the order b/b0>> c/c0> a/a0. The increasing blattice parameter was closely related to the elongation of the O4···H7B hydrogen-bond interaction along the b-axis. Na-O bond lengths were isotropically expanded with increasing temperature, whereas the B-O bond lengths and angles remained unchanged even after the phase transition to tincalconite. Molecular orbital calculations revealed that an electron cloud shared by two borate tetrahedra led to the formation of a large electron cloud distribution over the B4O5(OH)4cluster. The intramolecular interactions with substantial covalent character made the cluster quite rigid. The existence of borate minerals containing the B4O5(OH)4clusters in their structure evidenced the presence of moderately acidic or alkaline water, wherein the borate minerals grew via the incorporation of B4O5(OH)4clusters. Our results indicate that the connection geometry of the fundamental building block consisting of Bφ3triangles and Bφ4tetrahedra (φ: O2–, OH–) can potentially be used as a palaeoenvironmental indicator.

Globally, borax is one of the most important borate minerals, both industrially and from a technological viewpoint. Despite its importance, there have been only a few reports on the structural changes of borax upon heating. In this study, we have investigated the thermal behavior and structural characteristics of borax using thermal analysis, in situ high-temperature synchrotron X-ray diffraction, in situ variable-temperature single-crystal X-ray diffraction, and quantum chemical calculations. Differential scanning calorimetry (DSC) curve showed a large endothermic peak at 349 K corresponding to the dehydration reaction of borax. X-ray diffraction (XRD) pattern remained almost unchanged up to 353 K. Above 363 K, the XRD peaks gradually became less intense until they disappeared at 403 K. The unit-cell volume continuously increased with increasing temperature and became constant just before its phase transition to tincalconite. The volumetric thermal expansion coefficient between 113 and 323 K was 7.84 × 10–5K–1. The aand clattice parameters exhibited a slight increase trend with increasing temperature, whereas the blattice parameter increased significantly. Therefore, the thermal expansibilities followed the order b/b0>> c/c0> a/a0. The increasing blattice parameter was closely related to the elongation of the O4···H7B hydrogen-bond interaction along the b-axis. Na-O bond lengths were isotropically expanded with increasing temperature, whereas the B-O bond lengths and angles remained unchanged even after the phase transition to tincalconite. Molecular orbital calculations revealed that an electron cloud shared by two borate tetrahedra led to the formation of a large electron cloud distribution over the B4O5(OH)4cluster. The intramolecular interactions with substantial covalent character made the cluster quite rigid. The existence of borate minerals containing the B4O5(OH)4clusters in their structure evidenced the presence of moderately acidic or alkaline water, wherein the borate minerals grew via the incorporation of B4O5(OH)4clusters. Our results indicate that the connection geometry of the fundamental building block consisting of Bφ3triangles and Bφ4tetrahedra (φ: O2–, OH–) can potentially be used as a palaeoenvironmental indicator.

The Jahn-Teller effect at Cu2+in cuprospinel CuFe2O4was investigated using high-pressure single-crystal synchrotron X‑ray diffraction techniques at beamline BL10A at the Photon Factory, KEK, Japan. Six data sets were collected in the pressure range from ambient to 5.9 GPa at room temperature. Structural refinements based on the data were performed at 0.0, 1.8, 2.7, and 4.6 GPa. The unit-cell volume of cuprospinel decreases continuously from 590.8(6) to 579.5(8) Å3up to 3.8 GPa. Least-squares fitting to a third-order Birch-Murnaghan equation of state yields the zero-pressure volume V0= 590.7(1) Å3and bulk modulus K0= 188.1(4.4) GPa with K′ fixed at 4.0. The structural formula determined by electron microprobe analysis and site occupancy refinement is represented as T(Fe3+0.90Cu2+0.10)M(Fe3+1.10Fe2+0.40Cu2+0.50)O4. Most of the Cu2+are preferentially distributed onto the octahedrally coordinated (M) site of the spinel structure. With pressure, the arrangement of the oxygen atoms around the M cation approaches a regular octahedron. This leads to an increase in the electrostatic repulsion between the coordinating oxygen ions and the 3dz2orbital of MCu2+. At 4.6 GPa, a cubic-tetragonal phase transition is indicated by a splitting of the a axis of the cubic structure into a smaller a axis and a longer c axis, with unit-cell parameters a = 5.882(1) Å and c = 8.337(1) Å. The tetragonal structure with space group I41/amd was refined to R1 = 0.0332 and wR2 = 0.0703 using 38 observed reflections. At the M site, the two M-O bonds parallel to the c-axis direction of the unit cell are stretched with respect to the four M-O bonds parallel to the a-b plane, which leads to an elongated octahedron along the c-axis. The cubic-to-tetragonal transition induced by the Jahn-Teller effect at Cu2+is attributable to this distortion of the CuO6octahedron and involves Cu 3dz2orbital, ab initio quantum chemical calculations support the observation. At the tetrahedrally coordinated (T) site, on the other hand, the tetrahedral O-T-O bond angle increases from 109.47° to 111.7(7)°, which generates a compressed tetrahedral geometry along the c-axis. As a result of the competing distortions between the elongated octahedron and the compressed tetrahedron, the a unit-cell parameter is shortened with respect to the c unit-cell parameter, giving a ca′ ratio (a′ = √2 a) slightly greater than unity as referred to cubic lattice (ca′ = 1.002). The ca′ value increases to 1.007 with pressure, suggesting further distortions of the elongated octahedron and compressed tetrahedron.

A structural change in Fe2SiO4spinel (ringwoodite) has been found by synchrotron powder diffraction study and the structure of a new high-pressure phase was determined by Monte-Carlo simulation method and Rietveld profile fitting of X‑ray diffraction data up to 64 GPa at ambient temperature. A transition from the cubic spinel structure to a body centered orthorhombic phase (I-Fe2SiO4) with space group Imma and Z = 4 was observed at approximately 34 GPa. The structure of I-Fe2SiO4has two crystallographically independent FeO6octahedra. Iron resides in two different sites of sixfold coordination: Fe1 and Fe2, which are arranged in layers parallel to (101) and (011) and are very similar to the layers of FeO6octahedra in the spinel structure. Silicon is located in the sixfold coordination in I-Fe2SiO4. The transformation to the new high-pressure phase is reversible under decompression at ambient temperature. A martensitic transformation of each slab of the spinel structure with translation vector generates the I-Fe2SiO4structure. Laser heating of I-Fe2SiO4at 1500 K results in a decomposition of the material to rhombohedral FeO and SiO2stishovite.

The structural phase transition of the titanomagnetite (Fe3-xTixO4) solid solution under pressures up to 60 GPa has been clarified by single-crystal and powder diffraction studies using synchrotron radiation and a diamond-anvil cell. Present Rietveld structure refinements of the solid solution prove that the prefered cation distribution is based on the crystal field preference rather than the magnetic spin ordering in the solid solution.

We report in situ Raman spectroscopic studies of ulvöspinel in a diamond-anvil cell under hydrostatic conditions up to 57 GPa at room temperature. Two modes near 493 and 681 cm-1are observed clearly at 1 GPa. In the cubic spinel structure, the lower frequency peak can be assigned to a mode of F2gsymmetry and the higher frequency peak can be assigned to a mode with A1gsymmetry. The remaining three modes could not be observed unambiguously in the measurements, although there are five Raman-active modes (A1g+Eg+3F2g) in the Fd3̅m space group of the spinel structure according to factor group analysis. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequencies of the A1gand F2gmodes increase continuously up to 9 GPa with pressure derivatives of 2.5 and 2.1 cm-1/GPa, respectively. There is no obvious degradation of crystal symmetry or structural change observed within this pressure range. Upon increasing pressure to ~20 GPa, the F2gmode splits into B1g+Eg modes, and then into B1g+B2g+B3gmodes. The intensities of the Raman bands gradually decrease due to the tetragonalorthorhombic phase transition. This mode completely disappears at a pressure of 29 GPa. The most striking characteristic of the Raman spectrum of ulvöspinel is that compression leads to the extinction of the Raman-active mode derived from F2gsymmetry. Only one peak resulting from the A1gmode can be observed continuously up to 57 GPa. The peak shift derived from the A1gmode and its full-width at half maximum (FWHM) values suggests another phase transition occurring around 30 GPa. The Raman spectrum of ulvöspinel is in good agreement with the spectra of ZnCr2O4and ZnFe2O4spinels.

Molecular crystals of an As4S4-xSex(0 ≤ x ≤ 4) solid solution were grown at 400 °C using vacuumsublimation and were characterized using single-crystal X-ray diffraction and electron microprobe analyses. The solid solution crystallizes in the monoclinic space group P21/n with lattice parameters of a = 9.33-9.57 Å, b = 13.57-13.82 Å, c = 6.60-6.74 Å, β = 106.4-106.7°, V = 801-854 Å3, and Z = 4. Substitution of Se for S in the As4X4 molecule causes nearly isotropic unit-cell expansion retaining identical molecular packing to the low-temperature form α-As4S4(natural realgar). Crystal structure refinements show that Se is distributed with a strong preference for the X2 site. The As-X3 bond is slightly elongated in the solid-solution series. The bond length difference between As-X3 and the other As-X bonds increases with the Se content, thereby inducing atomic shift of the As4 bonds to X3 and X4 toward the side of X3. The atomic position of As4 returns to the place on the bisector between X3 and X4 at the As4Se4end-member composition. Although As-As bonds shorten with Se content, the volume of the As4X4molecule and the intercentroid distance between the nearest As4X4 molecules increase continuously, resulting in isotropic expansion of the unit cell in this solid-solution series.

Light-induced degradation in realgar (arsenic sulfide) has been studied by means of four-circle single-crystal X-ray diffraction and X-ray photoelectron spectroscopy. Because of the alteration of realgar exposed to light, the a lattice parameter and c sinβ value increase linearly from 9.327 to 9.385 Å and fro|m 6.320 to 6.364 Å, respectively. In contrast, the b lattice parameter remains substantially constant. Anisotropic variations of the lattice parameters engender a continuous increase of the unit-cell volume from 799.5 to 810.4 Å3. Nevertheless, no correlation exists between the continuous increase of the unit-cell volume and the bond distance variations in As4S4molecules because the As4S4molecule in the unit cell expands very little during light exposure. The most pronounced change was in the distance between centroids UiAs4S4cages. The spread of As4S4intermolecular distances increases continuously from 5.642 to 5.665 Å, which directly affects the unit-cell volume expansion of realgar. In addition, the O1s peak increases rapidly after light exposure. The result substantiates the following reaction proposed by Biudi et al. (2003): 5As4S4+ 3O2-> 4As4S5+ 2As2O5. That is, realgar is transformed into pararealgar if oxygen exists and produces the As4S5molecule. The additional S atom contributes to anisotropic expansion for the a and c axes because the direction of the additional S atom points toward [41̅4] in the unit cell. Furthermore, an S atom in the As4S5molecule is released from one of the equivalent As-S-As linkages in As4S5which becomes the As4S4molecular of pararealgar. After the As4S5molecule is divided into an S atom (radical) and the As4S4(pararealgar type) molecule, the free S atom is re-attached to another As4S4(realgar type) molecule, and reproduces an As4S5molecule. The reproduced As4S5molecule is again divided into an S atom (radical) and an As4S4(pararealgar type) molecule. This cycle whereby realgar is indirectly transformed into pararealgar via the As4S5 molecule is promoted by light and repeated during light exposure.

Structural refinements of single crystal X-ray diffraction data for synthetic (Sb,Bi)2S3solid solutions revealed structural variations in the stibnite (Sb2S3)-bismuthinite (Bi2S3) series. Coordination environments of the M cations are (3 + 4)-fold for the M1 site and (5 + 2)-fold for the M2 site. For the M1 and M2 polyhedra, the short M-S bond lengths increase constantly with increasing Bi concentration, whereas the long M-S bond lengths decrease continuously. The S-M-S interatomic angles interposing lone-pair electrons increase continuously from stibnite to bismuthinite. Stereochemical activity of the lone-pair electrons induces configurational changes of ligands around the M cations from elongated ellipsoidal coordinations to spheroidal ones. Substitution of Bi3+for Sb3+in the solid solution expands the basic building block, which causes linear increase of the b lattice parameter with slight positive deviation from Vegardʼs law. This feature is ascribed to order-disorder with concentration of Sb at the M1 site and Bi at the smaller M2 site. Furthermore, increased Bi content engenders both expansion of the basic building block and contraction of intervals between these blocks, contributing to smaller changes in the a and c lattice parameters than in the b lattice parameter. The M2 polyhedra expand relative to the M1 polyhedra with increasing Bi content because the large Bi cation is concentrated at the smaller M2 site. One striking characteristic of (Sb,Bi)2S3crystal structures is that geometries of central M cation and ligand atoms can be adapted flexibly to transformation of stereochemical activity from 5s2lone-pair electrons to Bi 6s2lone-pair electrons by altering the centroid-central atom distance and by changing angles of the centroid-central atom to the a axis.

Structural refinements of single crystal X-ray diffraction data for synthetic (Sb,Bi)2S3solid solutions revealed structural variations in the stibnite (Sb2S3)-bismuthinite (Bi2S3) series. Coordination environments of the M cations are (3 + 4)-fold for the M1 site and (5 + 2)-fold for the M2 site. For the M1 and M2 polyhedra, the short M-S bond lengths increase constantly with increasing Bi concentration, whereas the long M-S bond lengths decrease continuously. The S-M-S interatomic angles interposing lone-pair electrons increase continuously from stibnite to bismuthinite. Stereochemical activity of the lone-pair electrons induces configurational changes of ligands around the M cations from elongated ellipsoidal coordinations to spheroidal ones. Substitution of Bi3+for Sb3+in the solid solution expands the basic building block, which causes linear increase of the b lattice parameter with slight positive deviation from Vegardʼs law. This feature is ascribed to order-disorder with concentration of Sb at the M1 site and Bi at the smaller M2 site. Furthermore, increased Bi content engenders both expansion of the basic building block and contraction of intervals between these blocks, contributing to smaller changes in the a and c lattice parameters than in the b lattice parameter. The M2 polyhedra expand relative to the M1 polyhedra with increasing Bi content because the large Bi cation is concentrated at the smaller M2 site. One striking characteristic of (Sb,Bi)2S3crystal structures is that geometries of central M cation and ligand atoms can be adapted flexibly to transformation of stereochemical activity from 5s2lone-pair electrons to Bi 6s2lone-pair electrons by altering the centroid-central atom distance and by changing angles of the centroid-central atom to the a axis.

The crystal structure of getchellite, As0.98Sb1.02S3.00, from the type-locality (the Getchell mine, Humboldt County, Nevada), monoclinic, P21/a, a = 11.949(3), b = 9.028(1), c = 10.130(2) Å, β = 116.15(1)°, V = 980.9(4) Å3, Z = 8, was solved by direct methods and refined with full-matrix leastsquares techniques to R = 0.058 and Rw = 0.062 for 505 observed reflections [Io> 5σ(Io)] collected using MoKα radiation. The structure is made up of (As,Sb)S3trigonal pyramids forming an eight-membered (As,Sb)8S8ring; the sheets formed by the eight-membered rings are connected to each other by S1 and S6 atoms are parallel to (001). The sheet structure is responsible for the cleavage and twin planes of getchellite. The cations predominantly occupy the M2 site in natural getchellite, because the M2S6polyhedron has the most distorted (irregular) geometry compared with other metal polyhedra in the structure. The amount of As incorporated at the M1 sites correlates strongly with M2 site composition. An important feature of As3+sulfide minerals is that the crystal structures are composed of a 3-dimensional network consisting not only of AsS3pyramids but also containing trigonal pyramids of other constituent cations because the structures are governed strongly by the stereochemical requirement of As3+cations for MS3trigonal pyramids

The crystal structure of getchellite, As0.98Sb1.02S3.00, from the type-locality (the Getchell mine, Humboldt County, Nevada), monoclinic, P21/a, a = 11.949(3), b = 9.028(1), c = 10.130(2) Å, β = 116.15(1)°, V = 980.9(4) Å3, Z = 8, was solved by direct methods and refined with full-matrix leastsquares techniques to R = 0.058 and Rw = 0.062 for 505 observed reflections [Io> 5σ(Io)] collected using MoKα radiation. The structure is made up of (As,Sb)S3trigonal pyramids forming an eight-membered (As,Sb)8S8ring; the sheets formed by the eight-membered rings are connected to each other by S1 and S6 atoms are parallel to (001). The sheet structure is responsible for the cleavage and twin planes of getchellite. The cations predominantly occupy the M2 site in natural getchellite, because the M2S6polyhedron has the most distorted (irregular) geometry compared with other metal polyhedra in the structure. The amount of As incorporated at the M1 sites correlates strongly with M2 site composition. An important feature of As3+sulfide minerals is that the crystal structures are composed of a 3-dimensional network consisting not only of AsS3pyramids but also containing trigonal pyramids of other constituent cations because the structures are governed strongly by the stereochemical requirement of As3+cations for MS3trigonal pyramids

Thallium aluminosilicate, TlAlSiO4, synthesized hydrothermally is monoclinic with space group P21/n [a = 5.4095(3), b = 9.4232(7), c = 8.2629(6) Å, γ = 90.01(2)°, V = 421.20(6) Å3, Z = 4]. The crystal structure was refined to an R index of 3.8% based on 1852 observed unique reflections. The compound is a unique framework silicate with a topology similar to that of the tridymite structure. The TlO8polyhedron resembles a truncated rectangular pyramid, and shares its edges with three adjacent AlO4tetrahedra, three SiO4tetrahedra, and six TlO8polyhedra. Local understaturation at the Tl position suggested by bond-valence analysis implies that lone-pair electrons are present. The geometrical data indicate that the inert pair causes distortion of the Tl-polyhedron. Polyhedral distortion analysis using the software IVTON places the lone-pair parallel to [010], pointing to the largest base of Tl polyhedron. The rule in the valence shell electron pair repulsion model that a nonbonding pair occupies more space on the “surface” of the central atom than a bonding pair supports the orientation of inert-pair electrons in thallium provided by IVTON. The remarkable structure distortion caused by the inert-pair effect explains the rarity of Tl as a major element in silicate minerals because these cannot accommodate extremely distorted polyhedra. In contrast, about forty species of Tl-sulfide minerals exist because these structures are more flexible. Furthermore this effect probably explains why atoms such as Ge2+, Pb2+, Sn2+, Sb3+, and Bi3+, crystallize not as silicate phases but mainly as sulfide ones in nature.

Thallium aluminosilicate, TlAlSiO4, synthesized hydrothermally is monoclinic with space group P21/n [a = 5.4095(3), b = 9.4232(7), c = 8.2629(6) Å, γ = 90.01(2)°, V = 421.20(6) Å3, Z = 4]. The crystal structure was refined to an R index of 3.8% based on 1852 observed unique reflections. The compound is a unique framework silicate with a topology similar to that of the tridymite structure. The TlO8polyhedron resembles a truncated rectangular pyramid, and shares its edges with three adjacent AlO4tetrahedra, three SiO4tetrahedra, and six TlO8polyhedra. Local understaturation at the Tl position suggested by bond-valence analysis implies that lone-pair electrons are present. The geometrical data indicate that the inert pair causes distortion of the Tl-polyhedron. Polyhedral distortion analysis using the software IVTON places the lone-pair parallel to [010], pointing to the largest base of Tl polyhedron. The rule in the valence shell electron pair repulsion model that a nonbonding pair occupies more space on the “surface” of the central atom than a bonding pair supports the orientation of inert-pair electrons in thallium provided by IVTON. The remarkable structure distortion caused by the inert-pair effect explains the rarity of Tl as a major element in silicate minerals because these cannot accommodate extremely distorted polyhedra. In contrast, about forty species of Tl-sulfide minerals exist because these structures are more flexible. Furthermore this effect probably explains why atoms such as Ge2+, Pb2+, Sn2+, Sb3+, and Bi3+, crystallize not as silicate phases but mainly as sulfide ones in nature.